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Food and Bioprocess Technology
https://doi.org/10.1007/s11947-024-03726-0
REVIEW
Management Strategies fortheAnti‑nutrient Oxalic Acid inFoods:
AComprehensive Overview ofIts Dietary Sources, Roles, Metabolism,
andProcessing
AhmedZayed1· GhadaM.Adly2· MohamedA.Farag3
Received: 19 July 2024 / Accepted: 17 December 2024
© The Author(s) 2025
Abstract
Oxalic acid isamong the most abundant organic acids found in different biospheres, including plants, as an end product of
metabolism. It forms either soluble or insoluble salts with monovalent or divalent cations, respectively. Then, consumption of
oxalic acid-rich foods in human diets, particularly leafy vegetables (e.g., spinach, tea, and rhubarb), affects minerals absorption
such as calcium. Meanwhile, its high level in blood is associated with many diseases such as hyperoxaluria systemic oxalosis and
is thus classified among potential anti-nutrients. Various factors have affected oxalic acid levels in foods, including agricultural
traits and consumption practices. Hence, the current review aimed at rediscovering oxalic acid dietary sources, metabolism,
and the various processes employed to reduce its content in foods, and consequently, health harmful effects. Among them are
physical/cooking, chemical, fermentation, and biotechnological processing. Recent biotechnological approaches have been
attempted to produce transgenic crops remodeling oxalate metabolism, particularly its degradation. The soluble form of oxalate
seems to be better absorbed and more harmful than insoluble salts in foods aiding in kidney stones formation. Cooking (e.g.,
boiling, microwaving, and steaming) appears as a useful management strategy to reduce soluble oxalate and, therefore, lowering
oxaluria. The present review provides new perspectives on different processing methods to lower oxalate in essential vegetables
highlighting their advantages or any limitations to aid improve these foods nutritional value and consumption.
Keywords Dietary sources· Food processing· Metabolism· Oxalic acid· Oxaluria
Introduction
Oxalic acid (C2O42−), chemically known as ethanedioic
acid, is among the most abundant low molecular weight
dicarboxylic organic acids identified in the rhizosphere
beside to lactic, malic, and citric acids (Xiang etal., 2020;
Zhao etal., 2017). It can be found in soil, minerals, bacte-
ria, fungi, and in Planta. It exhibits strong acidity, chelat-
ing, and reducing abilities. It forms either soluble (often as
salts of sodium and potassium or as free acid) or insoluble
salt (produced with divalent metals like calcium or copper)
(Grąz, 2024). Oxalic acid production and roles have been
widely investigated in fungi, animals, and bacteria com-
pared to Planta (Jiao etal., 2022; Palmieri etal., 2019).
For example, it is known that fungi excrete a significant
amount of oxalic acid, as a byproduct of carbohydrates
metabolism (Grąz, 2024). Considering that it is produced
by a number of phytopathogenic fungi during plant colo-
nization, oxalic acid is also regarded as a pathogenesis
factor in plants resulting in a potential loss of crop produc-
tion (Kumar etal., 2019). It facilitates the breakdown of
plant cell walls by maximizing the impact of the enzymes
released by these phytopathogens that function best at an
acidic pH (Kumar etal., 2019). Nevertheless, it is also
involved in various positive functions in plants, including
regulation of plant growth and development, response to
biotic and abiotic stresses such as heavy metal detoxifica-
tion and plant defense, and food quality (Li etal., 2022b).
A significant portion of the world’s oxalic acid production
* Mohamed A. Farag
mohamed.farag@pharma.cu.edu.eg
1 Pharmacognosy Department, College ofPharmacy, Elguish
Street (Medical Campus), Tanta University, Tanta31527,
Egypt
2 Chemistry Department, American University inCairo, Cairo,
Egypt
3 Pharmacognosy Department, College ofPharmacy, Cairo
University, Kasr El Aini St, P.B. 11562, Cairo, Egypt
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Food and Bioprocess Technology
is left behind on soil surfaces as leaf litter, where microbes
oxidize it, raising pH levels and changing carbonate equi-
libria. Carbon dioxide is stored in soils as insoluble cal-
cite (CaCO3) as a result of calcium oxalate metabolism, a
process known as the oxalate-carbonate pathway (Cowan
etal., 2024).
Different plant species have variable natural levels of
oxalate which is produced as an end product of metabolism
accounting for 3–80% of the dry mass (Akhtar etal., 2011).
Oxalic acid is typically deposited in the form of calcium
oxalate crystals, oxalic acid exists as intracellular or extra-
cellular deposits in angiosperms’ tissues. Examples include
the vacuoles of specialized cells known as crystal idioblasts.
Nonetheless, most crystals in gymnosperms are found in the
cell wall playing a role in controlling plants’ levels of free
calcium (Kumar etal., 2019; Li etal., 2022b). Furthermore,
numerous factors affect oxalate content in the same plant,
including agricultural practices (Ghanati etal., 2024) such
as genotypes (e.g., black tea vs. green tea leaves (Ferraro
etal., 2020) and taro leaves cultivar (Du Thanh etal., 2017),
season of harvesting (e.g., tea leaves harvested in autumn
vs. spring) (Hönow etal., 2010), and maturity stage (e.g.,
58% total oxalates in mature leaves of silver beet (Beta vul-
garis) versus up to 89% in young counterpart) (Simpson
etal., 2009).
Both exogenous sources, consumed in dietary sources
especially green leafy vegetables alongside endogenous
sources as byproducts of amino acid metabolism, provide
oxalic acid or oxalates inside the body. A balance between
the body’s excretion, exogenous supply, and endogenous
sources is required to maintain oxalate homeostasis (Bal-
tazar etal., 2023; Ermer etal., 2023). Its presence in a
variety of food sources may have detrimental effects on
the body’s ability to absorb nutrients putting it under the
classification of “anti-nutrient” alongside phytates, tannins,
glycoalkaloids, proteinase inhibitors, nitrites, nitrates, and
cyanogenic glycosides. Anti-nutrient effect of oxalic acid is
mostly attributed for its capacity to chelate minerals such as
sodium, potassium, calcium, iron, and magnesium, reducing
their bioavailability (Dagostin, 2017; Gupta, 2018). High
oxalate levels have consequently been linked to many dis-
eases such as hyperoxaluria systemic oxalosis in extreme
cases resulting in nephropathy and acute renal failure due
to kidney stones formation which are composed mainly
of calcium phosphate and calcium oxalate, cardiovascular
disease, and osteoporosis (Dassanayake & Gnanathasan,
2012; Ermer etal., 2016; Petroski & Minich, 2020; Rendina
etal., 2020; Salgado etal., 2023). It is important to high-
light, though, that hyperoxaluria can be primary or second-
ary, with the former being attributed to genetic disorders in
oxalate metabolism, and the latter by non-genetic factors
related to diet, gut microbiota, renal, and metabolic diseases
(Baltazar etal., 2023).
It is therefore imperative to monitor oxalate consumption
and to consider food processing or agricultural practices that
can limit its level in dietary sources below the hazardous
levels (Ghanati etal., 2024; Huynh etal., 2022). Example
of processing method impact on oxalate include rhubarb
stalks cooking (e.g., boiling, steaming, and blanching)
(García-Herrera etal., 2020; Ghosh Das & Savage, 2013).
Employing cooking techniques that greatly reduce soluble
oxalate presents a potential strategy for lowering oxaluria
in those prone to the development of kidney stones since
soluble forms of oxalate seem to be more bioavailable than
insoluble sources (López-Moreno etal., 2022). Moreover,
the addition of calcium salts (e.g., calcium carbonate and
calcium chloride to the oxalates-rich food source (e.g., spin-
ach leaves,Spinacia oleracea L.) upon cooking could also
lead to oxalate reduction, particularly by converting solu-
ble oxalates into insoluble form (Bong etal., 2017; Faudon
& Savage, 2014; Nguyen & Savage, 2020). Since various
genes have been reported to affect oxalate biosynthesis (e.g.,
SoGLO2, SoGLO3, SoOXACs, SoMLS, SoMDH1, SoMDH2,
and SoMDH4) (Cai etal., 2018), other processing includes
development of genetically engineered crops manipulating
its metabolism, and hence, reduce production mostly derived
via three main precursors namely, glyoxylate/glycolate,
oxaloacetate, and ascorbate (Kumar etal., 2019). Further-
more, the human gut is home to symbiotic organisms such
as Oxalobacter and Lactobacillus, which act synergistically
to modify certain metabolic pathways and aid in the break-
down of oxalates, thereby preventing the formation of stones
(Sadaf etal., 2017). Patients with enteric hyperoxaluria and
those who have recurring calcium oxalate stones have been
shown to have reduced intestine colonization of Oxalobacter
formigenes (Wigner etal., 2022).
This thorough review aims to analyze the various aspects
of oxalate content in foods, including its occurrence, bio-
synthesis, metabolism, and processing techniques to lower
its level in dietary sources and subsequently harmful health
effects, as well as its versatile applications as an agrochemi-
cal and food preservative. Physical, chemical, biological,
and biotechnological processing techniques are explored in
an effort to find the best methods for reducing oxalates with-
out compromising food values.
Searching Criteria
The following databases were used to choose the articles:
Web of Science, PubMed, Google Scholar, and Scopus.
The search phrases “biosynthesis” OR “metabolism” OR
“Dietary sources” OR “degradation” OR “cooking” OR
“fermentation” OR “maturity” OR “harvesting season” OR
“transgenic crops” AND “oxalate” OR “hyperoxaluria” were
used to find the relevant articles. Only English-language and
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Food and Bioprocess Technology
published articles in impacted journals with full accessible
text were included in this review. The used articles were not
limited to a specific kind, while the timing of publishing has
focused on the last 15years. These criteria have resulted
in more than thousands of articles in Google Scholar.
Excluding articles discussing bacterial and fungal sources
and related pathophysiology to hyperoxaluria has reduced
the number to about 3700. Also, extraction and analytical
determinations were to finally about 130 articles that were
suitable candidates for the objectives of the current review
article.
Dietary Sources ofOxalates
The oxalate content of different foods is key to regulat-
ing the exogenous oxalate supply inside the body. Several
studies have measured total oxalate levels in foods and
consequently labeled them as high oxalate-containing
foods such as spinach, taro, and rhubarb (Han etal., 2015;
Ruan etal., 2013; Salgado etal., 2023). The average daily
consumption of oxalate in English diets has been estimated
to range from 70 to 150 mg, with tea appearing to account
for the largest amount of oxalate (Siener etal., 2017). Raw
legumes are common constituents in various dishes world-
wide and also rich sources of oxalate, where the highest
amount (370 mg/100 g DW) is found in soybean, followed
by common bean (98–117 mg/100 g DW), chickpea (192
mg/100 g DW), lentil and peas (168–293 mg/100 g DW),
and bean (Petroski & Minich, 2020). Generally, increased
vegetable consumption by vegetarians can result in a
higher intake of oxalates, which may decrease the avail-
ability of calcium, particularly women, who need higher
dietary calcium intake especially at certain life stages
(Craig, 2010; Galchenko etal., 2023). For vegetarians,
avoiding foods high in oxalate is more challenging than
for omnivores. For omnivores, the typical daily oxalate
intake is between 70 and 930mg; however, for vegetar-
ians, the range is between 80 and 2000mg (Vanhanen &
Savage, 2015).
It should be noted that the total oxalate content of a
food is a combination of the soluble and insoluble oxa-
late, with the excess soluble oxalate to play a much more
significant role in diseases and chronic kidney disease
(CKD) progression (Ermer etal., 2016; Salgado etal.,
2023). A comparison of the soluble oxalate content in
particular would give a more accurate representation of
the risk degree. Table1 lists differences in soluble oxalate
content in different foods commonly consumed by house-
holds. For instance, oxalate content in spinach is distrib-
uted in a nearly normal range: 400–1700 mg/100 g on a
fresh weight (FW) basis or 5–15% on a dry weight basis;
of which, 20–80% were found to be insoluble (Bong etal.,
2017). Very high-risk foods were designated as those that
contain more than 100 mg/100 g FW, while high-risk
foods were assigned to those that encompass more than
50 mg/100g FW. For all types of foods presented, the sam-
ples analyzed were raw, suggesting more needed work to
indicate changes in oxalate levels with different food pro-
cessing methods for results to be conclusive.
Table 1 Average soluble and insoluble oxalate contents of 100-g fresh weight (FW) or dried mass (DM) samples in different dietary sources
Food Soluble oxalate
(mg/100g FW)
Insoluble oxalate (mg/100g FW) Ref
Very high risk (> 100 mg soluble oxalate/100 g FW)
Spinach 648.0 258.1–1486.9 (Bong etal., 2017) (Mirahmadi etal., 2022)
Parsley 782.0 320.0 (Morosanova etal., 2018)
Taro 143mg/100g DM for
raw taro corms
110–147 (Savage & Mårtensson, 2010)
Rhubarb 380.0 208.9 (Nguyễn & Savage, 2013b)
Mushroom 320.0 17.0 – 88.0mg/100 DM (Chamjangali etal., 2009) (Sharma & Gorai, 2018)
Swiss chard 252.3 230.0 (Abdel-Moemin, 2014)
Soybean 155.0 343.0 (Akhtar etal., 2011)
Starfruit 138.9 156.5 (Salgado etal., 2023)
High risk (> 50 mg soluble oxalate/100 g FW)
Sweet potato 76.7 17.0 (Abdel-Moemin, 2014) (Siener etal., 2020)
Beetroot 45.0 22.0 (Akhtar etal., 2011)
Okra 56.3 0.0 (Siener etal., 2020) (Abdel-Moemin, 2014)
Eggplant 53.7 13.3–313.4mg/100g DM (Morosanova etal., 2018) (Azid etal., 2021)
White bean 52.0 7.0–106 (Abdel-Moemin, 2014; Akhtar etal., 2011)
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Biosynthesis andDegradation ofOxalic Acid
inPlants
Three mechanisms, including the oxaloacetate, ascorbic
acid, and glycolate/glyoxylate pathways, have been linked
to the production of oxalate in plants, Fig.1. According to
previous studies, oxalate has three main precursors includ-
ing ascorbate, oxaloacetate, and glyoxylate/glycolate. Oxa-
late builds up during photorespiration and the glyoxylate
cycle in plants as a result of the oxidation of glyoxylate/
glycolate (Goldsmith etal., 2022; Yu etal., 2010). Oxa-
late and acetate can also be produced via the oxidation
of oxaloacetate.
l
-Ascorbic acid is another important
precursor of oxalate found in many plant species (Kumar
etal., 2019). Oxalate in spinach may be regulated by the
glyoxylate cycle, ascorbate degradation, and photorespira-
tory pathway, according to the expression profiles of genes
linked to various physiological processes. Such hypothesis
is supported with the roles that acyl-activating enzyme 3
(AAE3), ascorbate metabolism-related genes, and isoci-
trate lyase (ICL) aiding oxalate homeostasis in spinach
(Cai etal., 2018; Joshi etal., 2021). Moreover, transcrip-
tome sequencing (RNA-Seq) of spinach leaves and root
tissues represented by two genotypes led to the annotation
of a total of 2308 leaf- and 1686 root-specific differentially
expressed genes (DEGs) to be involved in oxalate metabo-
lism (Joshi etal., 2021).
However, with regards to oxalate catabolism in Planta,
three distinct processes, i.e., oxidation, decarboxylation,
and acetylation are known to break down excess oxalate,
with acetylation pathway as the most well-studied pathway.
Oxalate is initially converted by oxalyl-CoA synthetase
into oxalyl-CoA, which is subsequently broken down via
oxalyl-CoA decarboxylase into formyl-CoA and CO2.
Next, formyl-CoA hydrolase catalyzes the breakdown of
formyl-CoA into formate. Lastly, formate dehydrogenase
converts formate into CO2 and H2O (Li etal., 2022a; Lou
etal., 2016). Annotation of these genes was further uti-
lized to create transgenic tomato (Solanum lycopersicum)
that expressed an oxalate decarboxylase (OXDC) leading
to a possible metabolic remodeling of oxalic acid in the
transgenic fruit (Chakraborty etal., 2013).
Fig. 1 Biosynthesis (blue text) and catabolism/fate (red text) path-
ways of oxalic acid in plants from its biochemical precursors and
breakdown products, respectively. Ascorbic acid is the principal
substrate to produce oxalic acid in plants, while in most other organ-
isms, glycolic, glyoxalic, isocitric, oxaloacetic, and ascorbic acid are
also involved precursors following the findings of Chakraborty, etal.
(Chakraborty etal., 2013) and Ul Hasan, etal. (Hasan etal., 2023)
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Dierences inOxalate Levels: Agricultural
andConsumption Practices
While the data presented in Table1 is categorized per
food source, it is important to consider a multitude of
other factors that could impact oxalate levels even for the
same food source. Other variables that have been shown
to influence oxalate levels in foods include mostly agri-
cultural variables (e.g., cultivar, region, and harvesting
season). Consumption practices/habits were also found to
affect oxalate levels to include the part of the plant that
is consumed (stems/leaves/stalks, etc.), the serving size
of the dietary source (given that different food items are,
most commonly, consumed with differing frequencies and
portions), and lastly any relevant processing of the food
before consumption (to be discussed in more detail in the
following sections). The combination of all the aforemen-
tioned variables is thus crucial when evaluating oxalate
levels in food. Figure2 shows the different agricultural
and consumption practices that affect oxalate levels in
dietary sources, with the next subsections to discuss these
variables/practices in detail from different food sources.
Agricultural Practices
Cultivar/Variety Type
It has been documented that oxalate content varies signifi-
cantly in context to plant genotypes/cultivars (Freidig & Gold-
man, 2011; Hang etal., 2013; Kristl etal., 2021). A study per-
formed by Cai etal. on 2 spinach varieties cultivated in China
(SP14 and SP104) revealed SP14 showed higher soluble
(567.0mg/100g FW) and total oxalate (1164.0mg/100g FW)
content compared to SP104 at 2.3- and 2.6-fold increase in
the case of mature leaf lamina, respectively (Cai etal., 2018).
Also, the total oxalate contents in green (Actinidia deliciosa
L.) and golden (Actinidia chinensis L.) kiwifruit varied from
12.7–84.3 and 7.8–45mg/100g FW, respectively (Nguyễn
& Savage, 2013c).
Harvesting Season
Green tea leaves are another example that showed vari-
able oxalate content with regard to harvesting season, with
mature leaves collected in autumn to encompass more
Fig. 2 Agricultural and consumption practices that affect oxalate levels in dietary sources
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Food and Bioprocess Technology
oxalate than small and young leaves harvested in spring.
Same pattern was observed in case of Chinese green tea
leaves collected in autumn found to encompass higher oxa-
late level of 66.0–89.1 versus 24.2–28.6 mg/L in spring
(Brzezicha-Cirocka etal., 2016). Although comparison of
the soluble oxalate content in different foods appears to
be more logical approach than total oxalate content, it is
still merely a generic comparison which does not account
for other factors, e.g., extraction and analytical methods
(Filho etal., 2023; Nguyễn & Savage, 2013a; Salgado
etal., 2023).
Plant Maturity Stage
It has been reported that oxalate accumulation varies among
numerous plant species at different phases of plant develop-
ment. For instance, tea younger leaves are richer in oxa-
late than older ones. This effect could explain the higher
oxalate levels in black and oolong teas produced from old
mature leaves compared to the green counterpart (Brzezicha-
Cirocka etal., 2016). Nevertheless, cocoa beans (Theobroma
cacao L.) investigated regarding oxalate content at four dif-
ferent phases of maturity: immature, mature, half-ripe, and
fully ripe showed comparable total oxalate levels ranging
from 600 to 670mg/100g dry matter (DM) (Nguyễn etal.,
2018) suggestive of plant type effect and not to be general-
ized for all dietary food. Similarly, the effect of maturity
was also found to affect oxalate content in taro (Colocasia
esculenta var. Schott) leaves grown in New Zealand. The
study was performed on young (100–200mm long) and
older leaves (200–300mm long). The young taro leaves had
again a greater total oxalate concentration (589mg/100g
FW) than the older leaves (433mg/100 Fw), of which, solu-
ble oxalates represented 74% (Du Thanh etal., 2017).
Nitrogen Fertilizers
The exposure of plants to nitrogen containing chemicals
in the soil, specifically nitrate, has been found to stimulate
oxalate accumulation. Many studies have shown that plants
treated with nitrate fertilizers generally accumulated higher
oxalate levels as compared to untreated, or even those treated
with other nitrogen fertilizers such as ammonium, urea, and
organic nitrogen fertilizers (Liu etal., 2015). Since it is well
known that nitrate can efficiently lead to oxalate accumula-
tion in vegetables (Al Daini etal., 2013), a study investigat-
ing the effect of nitrate fertilizer on spinach leaves reported
that spinach leaves were significantly affected by nitrate
treatments. The results showed that the total oxalate content
in the leaves of 2 genotypes of spinach treated with 20.0
mmol L−1 nitrate increased by 21.3% and 23.6%, compared
to those treated with 2 mmol L−1 nitrate (Liu etal., 2015).
Similar findings were reported in rice leaves revealing the
same positive correlation between nitrate treatment and
oxalate biosynthesis exists (Miyagi etal., 2020). Although
the exact molecular mechanism behind such interaction
remains unclear and requires further examination at tran-
script and protein levels. It has been suggested that increase
in nitrate ions within plants inhibits oxalate oxidase, result-
ing in higher oxalate levels (Ghaly etal., 2017), but other
studies showed that nitrate reductase activity was shown to
be positively linked with oxalate levels in rice leaves (Al
Daini etal., 2013). The reported relationship presented in
the aforementioned studies suggest that controlling nitrate
levels in plantation could be an effective method for total
oxalate reduction.
Consumption Practices
Aside from the individual habits (e.g., smoking, opium
consumption, and alcohol drinking (Zainodini etal., 2023)
which may affect oxalate bioavailability and consequent
kidney stone formation, other factors such as the consumed
part and serving size shall be mainly considered. A daily
fluid intake of at least 2.5 to 3.0 L is advised to ensure an
appropriate urine volume. The oxalate content of beverages
should be taken into consideration, as a high dietary oxalate
consumption can significantly contribute to urinary oxalate
excretion, a key risk factor for the formation of calcium oxa-
late stones (Siener etal., 2017).
Consumed Part ofthePlant
The edible plant part is also a factor affecting the oxalate
level suggestive for variable metabolic mechanisms among
organs (Kang etal., 2019). For example, the study of Cai
etal. performed in 2 Chinese cultivars of spinach, the mature
leaf lamina and mature leaf petiole of SP14 showed solu-
ble oxalate levels at 56.7 and 27.3mg/g FW, respectively,
which were 1.7 and 2.3 times higher than those of SP104.
In contrast, juvenile leaf lamina and leaf petiole of SP14
showed comparable soluble oxalate levels of 32–34mg/g
FW. Although there were minor variations between juvenile
leaf laminas and petioles, soluble oxalate levels in the leaf
laminas were still noticeably larger than those in the petioles
(Cai etal., 2018). In addition, different parts of kiwifruit,
i.e., pulp, skin, and seeds, showed a comparable oxalates
profile. For instance, the golden cultivar showed soluble oxa-
late levels at 8.5, 11.9, and 12.6mg/100g FW in pulp, skin,
and seeds, respectively. However, the total oxalate contents
varied significantly with 15.7, 55.4, and 97.3mg/100g FW
in pulp, skin, and seeds, respectively (Nguyễn & Savage,
2013c). This might indicate that insoluble oxalate repre-
sented higher percentages especially in seeds which need
further genetic investigations to determine action mecha-
nisms responsible for such variability.
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Moreover, investigation of some edible Mediterranean
non-cultivated vegetables such as Silybum marianum (L.)
Gaertn., Beta maritima L., and Sonchus oleraceus L. showed
high oxalic acid levels at 1030, 581, and 539mg/100g FW,
respectively. These contents were determined in their basal
leaves which are traditionally consumed in Spain. Hence,
these species are recommended to be avoided by individu-
als who are prone to form kidney oxalate calculus (Morales
etal., 2014).
Serving Size
With regards to serving size as a factor, while parsley, a very
high-risk food in reference to its soluble oxalate content, it is
not typically consumed at large amounts, making it far less
of a risk food. This is opposed to sweet potatoes or white
beans, which although have a lower soluble oxalate content
than parsley, are typically consumed at much higher levels,
potentially presenting a higher risk food than that of parsley
(Crivelli etal., 2020). In a typical western diet, dietary oxa-
late ranges from 70 to 930 mg/day depending on food choice
and serving sizes (Nguyen, 2012).
Oxalic Acid Roles andIndustrial Applications
Roles inFungi, Bacteria, Plants, andAnimals
Oxalic acid is a chemically versatile substance that is widely
present in ecosystems, suggesting that it may be important
for interactions and ecosystem function (Hasan etal., 2023).
Roles of oxalic acid in fungi, bacteria, plants, and animals
are summarized by Palmieri etal. (2019) and shown in
Fig.3.
Aside from roles in fungi, bacteria, and animals, oxalic
acid plays pivotal functions for plants, Fig.3. It is hypoth-
esized that oxalic acid is important for calcium homeosta-
sis, ionic equilibrium, heavy metal detoxification, and plant
defense against herbivores (Li etal., 2022b) (Khan etal.,
2024). Through biochemical control of cellular processes as
well as signal transduction, calcium plays a significant role
in plant metabolism. However, calcium is poisonous to cells
at concentrations of 10−6–10−8M (Palmieri etal., 2019).
As a result, insoluble calcium oxalate complexes represent
a sink for controlling calcium levels to maintain appropri-
ate cell function. Furthermore, calcium oxalate crystals can
Fig. 3 Proposed roles of oxalic acid in different organisms, including fungi, bacteria, plants, and animals following the findings of Palmieri etal.
(2019)
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be used as a calcium storage container. More crystals will
form when the amount of calcium in cells is too high, but if
there is not enough calcium, crystals will disintegrate and
release the calcium and carbon needed for cellular activities
(Gómez-Espinoza etal., 2020).
Moreover, plants lack a skeleton, with maintenance of
hydrostatic pressure necessary for a plant to remain erect
depends on ionic equilibrium. Since oxalic acid can balance
the charge of the positive ions and amino acids in a cell, it
may therefore be important for maintaining ion homeosta-
sis. Furthermore, oxalic acid contributes to the detoxifica-
tion of heavy metals. For instance, a significant issue for
plants grown in acidic soil is aluminum poisoning. Different
mechanisms have evolved by plants against aluminium tox-
icity under these circumstances. The formation of oxalate
is linked to both the internal tolerance of aluminum and its
exclusion from plant roots. Whether in the rhizosphere or
inside the plant, aluminum will be contained as non-toxic
aluminum-oxalate salts (Lou etal., 2016; Zhang etal.,
2019). Oxalate may also offer protection against herbivores
as crystals can form in leaves in reaction to herbivory or as
a preventative measure to fortify themselves (Paiva, 2021).
Industrial andAgricultural Applications ofOxalic
Acid
Due to acidity, metal chelating, and antioxidant-stimulating
activities, oxalic acid exhibits diverse applications in both
industry and agriculture. It can be used as a pre- or post-harvest
treatments, deactivation of preservatives containing copper,
detoxification of aluminum toxicity, and remediation of organic
contaminants (Hasan etal., 2023; Serna-Escolano etal., 2021)
as explained in more details in the next subsections.
Pre‑ andPost‑harvest Treatments
The use of pre-harvest treatments with natural elicitor
chemicals, which trigger a tree response and delay the fruit’s
maturation process during cold storage, is one of the com-
mon approaches in agronomy being helpful to reduce losses
and improve the functional qualities of fruits and vegetables
during storage (Barberis etal., 2019; Cefola & Pace, 2015;
Moreno-Escamilla etal., 2018; Viacava etal., 2018). Among
these agrochemicals, oxalic acid is one elicitor which has
been reported to be applied either as pre- or post-harvest
treatments to inhibit physiological processes such as respira-
tion, ethylene production, and water loss, delaying ripening
and senescence. By reducing oxidative stress, oxalic acid
treatment for banana fruit prevented unwanted storage effects
such as chilling injury, enzymatic browning, and flesh sof-
tening. This treatment included dipping in 20mM solutions
for 10min, then keeping at room temperature (23 ± 2°C)
and 75–90% relative humidity (Huang etal., 2013).
It was also reported that applying oxalic acid as pre-har-
vest treatment to fruits specifically aided to postpone aging
and to maintain the quality of sweet cherries, peaches, and
pomegranates (García-Pastor etal., 2020). Moreover, pre-
harvest treatment of lemon (Citrus limon (L.) Burm. F) trees
with 1mM of oxalic acid resulted in fruits with maintained
firmness, total soluble solids (TSS), and total acidity (TA)
compared with the control while reducing weight loss (WL)
after storage for 35days at 10°C. Moreover, the flavedo
of citrus fruit showed increased activity of the antioxidant
CAT, APX, and POD compared to the controls. Likewise,
fruit from the oxalic acid-treated trees had a higher total
phenolic content (TPC) in the flavedo and juice than in the
control presenting an added value especially if fruits are
targeted for increased phenolics to be used in nutraceuticals.
Profiling of flavonoids in response to oxalic acid treatment
should now be followed to determine changes in individual
compounds. Hence, the incidence of deterioration after har-
vest was decreased by preharvest oxalic acid treatments that
strengthened lemon fruits’ antioxidant system (Serna-Escol-
ano etal., 2021), which has yet to be tested using invivo
assay.
While the exact action mechanisms of oxalic acid is
unknown, post-harvest treatments in lemon and banana fruits
have been linked to scavenging reactive oxygen species
(ROS) buildup (Huang etal., 2013; Serna-Escolano etal.,
2021), and maintaining high ascorbic acid and ATP levels in
post-harvest oxalic acid-treated kiwifruit (Chunqiang etal.,
2017). Additionally, post-harvest oxalic acid treatment of
muskmelon (Cucumis melo L. cv. Yindi) fruit at 50mM con-
taining 0.05% Tween 80 for 10min. against pink rot caused
by Trichothecium roseum resulted in an average infection
incidence and lesion diameter on days three through seven
following T. roseum inoculation that were 25.1% and 16.9%
lower than those of the control, respectively (Deng etal.,
2015). Also, jujube (Ziziphus jujuba Mill. cv Dongzao) fruit
treatment at 5mM oxalic acid solution for 10min, then dried
in air at 20°C for 2h, was linked to a decrease in decay dur-
ing storage (Wang etal., 2009). Such effect was associated
with reduction of ethylene production in jujube fruit treated
by oxalic acid by about threefold as opposed to control fruit
These effects were explained by increased activity of anti-
oxidant enzymes associated with disease resistance, such
as catalase (CAT), ascorbate peroxidase (APX), superoxide
dismutase (SOD), and peroxidase (POD) (Deng etal., 2015;
Wang etal., 2009).
Food Preservation
Previous reports have been carried out on the application of
organic acids and their salts for decontamination and extend-
ing shelf life of beef and poultry, including oxalic acid,
based on its antimicrobial action as typical for acidulants
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Food and Bioprocess Technology
(Gonzalez-Fandos etal., 2020; Kumar etal., 2018). Addi-
tionally, commercial pectin preparations containing oxalic
acid were able to prevent the browning of fresh, raw Granny
Smith apple juice due to enzymatic reaction. These prepa-
rations when added to fresh, unrefrigerated apple juice at a
concentration of 0.5% (w/v), totally stopped browning after
24 h. This effect was confirmed, where the anti-browning
action of the commercial pectin was eliminated or signifi-
cantly diminished upon the removal of oxalic acid, which
has yet to be tested in other fruits. Additionally, it was shown
that pure oxalic acid at the same concentration of 0.5% pre-
vented fresh juice from browning confirming the previous
finding. However, it should be considered that oxalic acid
has not been approved officially as a food additive yet by the
US Food and Drug Administration (FDA).
Oxalate Absorption andMetabolism
inHumans
Oxalate is introduced to the body in one of two forms:
endogenous oxalate and exogenous oxalate as summarized
in Fig.4. Endogenous oxalate is the oxalate produced by the
body as a metabolite from the breakdown of substances such
as ascorbic acid, glyoxylate, hydroxyproline, glycine, and
protein (Ermer etal., 2023; Franceschi & Loewus, 2020).
The liver is the primary producer of endogenous soluble
oxalate, and oxalate produced by the liver enters the blood-
stream (Petroski & Minich, 2020). On the other hand, exog-
enous oxalate is obtained from high oxalate dietary sources,
mostly being plant foods (specifically leafy greens), whole
grains, and nuts (Han etal., 2015).
Transcellular anion transporters, particularly the solute-
linked carrier (SLC)−26 family, and paracellular fluxes
through tight junctions are responsible for oxalate absorp-
tion from the digestive tract (Crivelli etal., 2020). It seems
that the quantity of oxalate that is consumed and absorbed
by the intestine ranges from 2 to 20%, depending on the
co-consumption of fiber, calcium, and magnesium (Criv-
elli etal., 2020), where the peak blood level happens 1–6
h after consumption of foods high in oxalate content (Sav-
age & Mårtensson, 2010). Malabsorptive intestinal dis-
orders in case of surgical removal of intestinal segments,
such as after Roux-en-Y gastric bypass (RYGB) surgery
operated for weight loss, have been linked to hyperab-
sorption of oxalate, i.e., enteric hyperoxaluria with urine
oxalate excretion ≥ 45 mg/day that resulted in urinary cal-
cium oxalate supersaturation (Kumar etal., 2011; Witting
etal., 2021). So, dietary oxalate may not be recognized
as a great problem since absorption is limited and based
on several factors, including co-consumed foods (Crivelli
etal., 2020).
Fig. 4 Sources of oxalate and pathways to excretion from the body. Red arrows indicate pathways ending in urinary excretion. Blue arrows indi-
cate pathways ending in fecal excretion. Double asterisks (**) indicate associated health risk or disease
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Food and Bioprocess Technology
Dietary oxalate contains both, soluble and insoluble oxa-
late, with insoluble oxalate to be excreted from the body
as feces, whereas, soluble oxalate is absorbed in multiple
organs of the gastrointestinal tract such as the stomach,
small intestine, and large intestine (Salgado etal., 2023).
Absorbed oxalate can chelate minerals such as calcium and
magnesium, forming insoluble fecal oxalate salts that can
be excreted from the body. However, oxalate remaining in
the soluble form is absorbed in the intestines and to reach
the blood stream (Ermer etal., 2016; Petroski & Minich,
2020). Therefore, it is recommended to increase the intake
of calcium supplements or calcium-rich foods (e.g., dairy
products as hard cheese (1% w/w) and nuts containing up to
0.6% w/w) (Cormick & Belizán, 2019) together with foods
high in oxalic acid in order to eliminate calcium oxalate from
the stomach and lower blood oxalate levels (Duraiswamy
etal., 2023). It should be possible to counteract any potential
inhibitory effects from dietary oxalates with a typical cal-
cium diet of 800–1,000 mg/day (Petroski & Minich, 2020).
However, it has been reported that it is recommended that
oxalic acid/calcium ratio should be monitored and be not
more than 2.5 (9:4 as the optimal limit) in foods to prevent
the consequent harmful effects, i.e., formation of kidney
stones and decrease of calcium bioavailability (Lee & Lin,
2011).
While bacteria, fungi, and plants all have a mechanism
for the catabolism of oxalate, vertebrates, including humans,
do not exhibit such machinery (Chakraborty etal., 2013).
Oxalate is broken down by O. formigenes, which also main-
tains a vital symbiotic relationship with its hosts by con-
trolling the intestinal absorption of oxalic acid, i.e., oxalic
acid homeostasis (Lee etal., 2014). There is no known gas-
trointestinal route for the excretion of oxalate; thus, once
it is consumed, it must be eliminated through the kidneys
(Karamad etal., 2022; Miller etal., 2017). Nonetheless, the
least lethal dose of oxalic acid for an adult is approximately
5 g, indicating its comparatively low toxicity (Morales etal.,
2014). Serum oxalate, obtained from either the endogenous
or exogenous pathway, is processed by the kidney and is
either excreted in urine, or is chelated with calcium ions in
the kidney leading to the formation of calcium oxalate crys-
tals, and contributing to serious health risks such as kidney
stone formation (nephropathy) (D’Alessandro etal., 2019).
Processing Methods toReduce Total Oxalate
Levels inDietary Sources
Aside from factors discussed before affecting oxalate levels
inside the body, processing methods appear to play a more
critical role in reducing oxalate (particularly soluble oxa-
late). Such methods can be classified as physical, chemical,
biological, and/or combinations. Alongside the preparation
method, it is vital to consider all other relevant variables,
as illustrated in Fig.2. Nevertheless, for the sake of generic
comparison, many studies were included in this review,
while taking note of such variables. The next subsections
shall provide application of each technique, highlighting its
advantages and or any limitations for comparison among
these approaches to present best methods to reduce oxalate
in different dietary sources. In addition, Table2 and Table3
summarized and compared relevant literature that reported
the use of processing methods for oxalate level reduction
in foods.
Physical/Cooking Processing Methods
Oxalate levels are mostly reduced in cooked foods. Some
physical procedures that have been associated with a
decrease in oxalate include, but are not limited to, common
household cooking methods to include boiling, steaming,
roasting, dehydration, soaking, blanching, and frying attrib-
uted to oxalate water solubility (Mashitoa etal., 2021; Pet-
roski & Minich, 2020; Salgado etal., 2023). While the afore-
mentioned physical processing methods have been linked to
a reduction in oxalate, the oxalate content of roasted peanuts,
cashews, and almonds was not significantly affected (Pet-
roski & Minich, 2020). Whether such an effect is common
for all roasting procedures or identified only in nuts should
be examined in other dietary sources such as coffee for
results to be conclusive. Moreover, it was believed that the
physical binding of oxalates to food fiber, and consequently,
eliminated during juice processing and the amount of oxa-
lates in the juice might be considerably lower. Vanhanen and
Savage, however, demonstrated that large amounts of soluble
oxalates were added regardless of the presence of pulp fiber,
depending on the type and quantity of vegetables used to
prepare green juices (Vanhanen & Savage, 2015).
One study in pumpkin leaves revealed that boiling
showed the largest reduction at 82% of total oxalate of the
sample, while microwaving, frying, and steaming, showed
reductions of 76.5%, 62.1%, and 51.5% respectively, clearly
presenting boiling as the most effective technique among
different physical methods (Mashitoa etal., 2021). Another
study on Galega kale leaves showed similar results using
microwaving that showed a 35.8% reduction on total oxalate,
while boiling and steaming, showed reduction at 34.2% and
12.7% respectively, marking that both microwaving (at 900
W for 20 or 30 min) and boiling (for 20 or 30 min) as almost
equally effective (Armesto etal., 2019). In contrast, a study
on sweet potato varieties showed that while boiling (until
the root was easily accessible with a fork) was most effec-
tive for Whitesp and Yellowsp sweet potato, dehydration in
an air oven at 70 °C for 12 h was a more efficient technique
for Kabode sweet potato (Abong’ etal., 2021), and sugges-
tive for matrix effect on processing method impact towards
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Food and Bioprocess Technology
oxalate reduction. Such results warrant comparing each tech-
nique in different cultivars or varieties within each food type
for results to be conclusive. Moreover, while frying reduced
oxalate in pumpkin leaves by 62.1%, it increased oxalate
levels in the bitter melon or Bitter Guord fruit (Momordica
charantia) in the Indian and Vietnamese cultivar by 6.5%
and 23.4%, respectively (Bong & Savage, 2018), suggestive
for a food matrix effect. However, frying reduced oxalate by
15.6% in case of the fruits of Malaysian Bitter Guord (Bong
& Savage, 2018).
Table2 summarizes the effect of different physical pro-
cessing methods (e.g., boiling, steaming, microwaving, fry-
ing, and dehydration) on oxalate levels (mg/100 g) of some
dietary sources. A study showed that boiling spinach and
carrots resulted in a total decrease in oxalate that ranged
from 30 to 87%, while steaming resulted in a much less
reduction level. Similarly, after boiling 12 of the 13 fresh
Thai vegetables for 30 min, soluble oxalate was reduced
by 30.4–65.0%. Among these, the soluble oxalates in taro
corms were reduced by 34.2% (Juajun etal., 2012). Several
studies have attributed the general effectiveness of boiling
in particular to the water solubility of oxalate making it
susceptible to degradation using boiling (Sotelo etal., 2010).
In fact, previously mentioned increase in oxalate in case of
Indian and Vietnamese Bitter Guord frying was attributed by
the loss of moisture during frying, which prohibits oxalate
from leaching onto the water (Bong & Savage, 2018). Con-
clusively, the effectiveness of any processing method involv-
ing high heat has been attributed to likewise the thermal
instability of oxalate. Also, heat treatment has been found
to cause changes in cell permeability which increases the
chemical extraction of some organic compounds from plant
tissue and decrease in foods, including anti-nutrients (e.g.,
nitrate, phytate, and oxalate) (Singh etal., 2015).
Soaking of legumes seed (e.g., soybean, lentils, fava
beans, peas, and chickpeas) was found to be an effective
strategy to reduce oxalate content. Seeds germination/
sprouting is induced by soaking which decreased not only
oxalate level, but also lectins. Soaking in distilled water,
(1:5 seed:water ratio) for 4h at room temperature, and then
cooked, at 95°C in water for 1 h, succeeded in decreas-
ing the total oxalate and soluble oxalate by 17.4–51.9% and
26.7–56.3%, respectively in various legume seeds. Par-
ticularly, the raw soybean showed the highest total oxalate
Table 2 The effect of different physical processing methods (e.g., boiling, steaming, microwaving, frying, and dehydration) on oxalate levels
(mg/100 g) in different dietary sources
■▯ ↑ increase in oxalate content (%)
■▯ ↓ significant reduction in oxalate content (> 30%)
Method Time (min) Dietary Source Part of plant Initial oxa-
late (mg/100
g)
Final oxalate
(mg/100 g)
% Reduc-
tion/
Increase
Ref
Boiling 10 Pumpkin Leaves 92.4 16.7 ↓ 81.8 (Mashitoa etal., 2021)
20 Galega kale Leaves + stems 314.54 238.51 24.17 (Armesto etal., 2019)
30 206.84 ↓ 34.24 (Abong' etal., 2021)
20 Kabode sweet potato Leaves 1820.56 1399.67 23.12
Whitesp sweet potato 1737.20 1468.80 15.45
Yellowsp sweet potato 1347.66 1341.63 0.45
- Kabode sweet potato Roots 217.35 164.48 24.32
- Whitesp sweet potato 181.36 129.83 28.41
- Yellowsp sweet potato 223.97 184.27 17.76
Steaming 15 Pumpkin Leaves 92.43 44.86 ↓ 51.47 (Mashitoa etal., 2021)
20 Galega kale Leaves & stems 314.54 299.81 4.68 (Armesto etal., 2019)
30 274.70 12.67 (Armesto etal., 2019)
Microwaving 10 Pumpkin Leaves 92.43 22.01 ↓ 76.19 (Mashitoa etal., 2021)
20 Galega kale Leaves & stems 314.54 217.60 ↓ 30.82 (Armesto etal., 2019)
30 201.90 ↓ 35.81 (Armesto etal., 2019)
Frying 2 Pumpkin Leaves 92.43 35.00 ↓ 62.13 (Mashitoa etal., 2021)
10 Bitter Guord—Indian Whole fruit 83.32 88.72 ↑ 6.48 (Bong & Savage, 2018)
Bitter Guord—Vietnamese 72.33 89.26 ↑ 23.41
Malaysian 102.06 86.19 15.55
Dehydration 720 Kabode sweet potato Leaves 1820.56 1321.90 27.39 (Abong’ etal. 2021)
Whitesp sweet potato 1737.20 1690.56 2.68
Yellowsp sweet potato 1347.66 1341.63 0.45
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Food and Bioprocess Technology
Table 3 A summarized comparison between relevant literature used salts addition, fermentation, enzymatic treatment, and transgenic crop development methods to reduce soluble oxalate con-
tent
Method Food % change in soluble oxalate Ref
Salts addition method
Cooking with addition of calcium chloride (25.6%) Spinach leaves ↓ 98.25 (Bong etal., 2017)
Addition of calcium chloride 500mg/100g to the
fresh juice
Green juices, made from common fruit and
vegetables available in New Zealand
↓ 98.3% (Vanhanen, 2018)
Soaking in a calcium chloride solution Spinach leaves ↓ 50% (Vanhanen, 2018)
Pairing with a calcium source using milk Rhubarb stalks ↓ 74.5% (Nguyen & Savage, 2020)
Ultrasonication, followed by the addition of calcium
sulphate, and lastly pairing with high calcium milk
Cocoa beans ↓ 92.8% (Huynh etal., 2020)
Soaking in brine has been reported in a ratio of 1:2 in
10% salt water for 2h at room temperature, then 9h
in the fridge
Silver beetroot leaves ↓ 86% in total oxalate content (Wadamori etal., 2014)
Soaking in calcium chloride solution (5%) for 60min Taro corm chips ↓ 88.0% (Saleh, 2019)
Fermentation method
Lactobacillus acidophilus-mediated fermentation or
in combination with Saccharomyces cerevisiae for
12days
Sesame seed ↓ 69% (Hajimohammadi etal., 2020)
Fermentation of soaked seeds in calabash pots lined
uniformly with banana leaves inside the incubator
(30°C) for 4days
Green amaranth seed ↓ 20.8% (Olawoye & Gbadamosi, 2017)
Controlled fermentation by inoculation with spore
suspension of Aspergillus niger at room temperature
(29 ± 3°C) for 2days
Vigna racemosa seed ↓ 96.9% (Difo etal., 2015)
Cooking and fermentation (cooking for 2h at 100°C
followed by fermentation in a calabash at 30°C for
4days)
Sandbox tree seed ↓ 65.5% (Osungbade etal., 2016)
Enzymatic treatment
Treatment with oxalate oxidase produced by Fusarium
oxysporum RBP3
- 4% (w/v) taro flour was dispersed in 100 U mg−1 of
oxalate oxidase in succinate/NaOH buffer (pH 3.8)
at 55°C,
- Steep for 150min with continuous shaking at
150rpm
Taro tuber flour ↓ 98.3% in total oxalate (Kizhakedathil etal., 2022)
Treatment with oxalate oxidase produced by Ochro-
bactrum intermedium CL6
- 0.125g of taro tuber flour mixed with oxalate oxidase
enzyme solution and incubated at 55°C
- The enzymatic solution was prepared in 50mM
sodium succinate buffer (pH 3.8)
Taro tuber flour ↓ 97% in total oxalate (Kumar & Belur, 2018)
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Food and Bioprocess Technology
(370.5mg/100g DM) and soluble oxalate (200.7mg/100g)
levels. However, the total oxalates decreased to 178.2
5mg/100g DM (51.9%) and 125.4 5mg/100g DM (66.2%)
in soaked and cooked seeds, respectively. Also, soaked and
cooked seeds showed soluble oxalate reduction to 87.7
mg/100g DM (56.3%) and 52.3 5mg/100g DM (74%),
respectively (Shi etal., 2018). In addition, legumes’ total
and soluble oxalate content is potentially reduced by soak-
ing overnight followed by boiling or autoclaving (Bhat &
Karim, 2009; Sharma, 2021). Also, germination of cereals
such as pearl millet (Pennisetum typhoideum) resulted in
oxalate reduction, where the total oxalate content decreased
significantly (p < 0.05) by as much as 48% for Maharashtra
Rabi Bajra and as much as 24% for the Kalukombu cultivar.
This decrease may have been caused by the soluble oxalate
content leaching during steeping (Suma & Urooj, 2014).
Addition ofSalts andChange ofpH
One of the most examined chemical processing methods
to reduce oxalate entails processing the food sourc 2e with
a calcium source, most typically a calcium salt such as
calcium chloride, calcium carbonate, and calcium sulfate
(Bong etal., 2017; Huynh etal., 2020, 2022; Vanhanen,
2018; Wang etal., 2019). The addition of calcium salt to
food upon cooking as a processing method can be classified
as a physio-chemical one. One study investigated the effect
of adding calcium carbonate (with a calcium content 29.3%
as determined by IPC analysis), calcium chloride (25.6%),
calcium citrate (24.7%), and calcium sulphate (33.7%) at 0,
50, 100, 200, 300, 400, and 500 mg/100 g of homogenized
spinach leaves and measured the amount of soluble oxalate
remaining after treatment. The study not only reported an
inverse relationship between the mass of salt and remaining
oxalate in spinach, but also revealed that among the differ-
ent tested calcium salts, i.e., calcium chloride exerted the
highest percentage reduction in soluble oxalate with only
1.75% remaining oxalate (Bong etal., 2017). Likewise, a
different study on green juices, made from common fruit
and vegetables available in New Zealand, reported calcium
chloride to reduce soluble oxalate content by 98.3% (Van-
hanen, 2018). The same study also investigated the effect of
soaking spinach leaves in a calcium chloride solution and
reported a 50% and 28% reduction in soluble and insoluble
oxalate, respectively (Vanhanen, 2018).
A similar approach was reported by another study that
measured reduction in oxalate in rhubarb stalks by pairing
them with a calcium source during cooking but using milk
rather than calcium salt. This study reported a soluble oxalate
reduction of 65.9% and 74.5% upon cooking rhubarb with trim
and standard milk, respectively (Nguyen & Savage, 2020), and
posing milk as good alternative considering its common use
Table 3 (continued)
Method Food % change in soluble oxalate Ref
Treatment with phytase extracted from wheat
- 0.1g seeds mixed with sodium acetate buffer (pH
5.5), magnesium sulfate solution and deionized water
- The solution was mixed and equilibrated at 55°C for
a few min, and phytase enzyme solution (0.4mL)
was added
Legumes and cereal seeds - Phytate degradation and reduction of soluble oxalate
has been found significantly in wheat and oat bran
samples
- The exact % was not determined
(Israr etal., 2017)
Biotechnology/development of transgenic crops
Development of transgenic tomato plants (E8.2-OXDC
tomato plants) expressing oxalate decarboxylase
FvOXDC gene derived from fungus Flammulina
velutipes
Tomato fruit ↓ 90% (Chakraborty etal., 2013)
Expression of 43 KD-sized OXDC from yvrK gene of
Bacillus subtilis by recombinant E. coli and aerobic
incubation with sodium oxalate in the presence of
MnCl2
in vitro and in vivo modeled degradation study ↓ 50% (Lee etal., 2014)
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Food and Bioprocess Technology
as ingredient in many recipes. Another study also measured
the oxalate content of taro leaves after baking them either
alone or combined with cow milk or coconut milk prior to
cooking. This study also reported a decrease in oxalate when
food was combined with milk (Savage & Mårtensson, 2010).
Comparison of different milk types and at different fat levels
should follow to identify best milk composition to be added
to reduce oxalate levels especially considering that it can be
readily adopted in home prepared foods.
Moreover, another study investigating oxalate in cocoa
beans studied the effects of six calcium sources (three of
which were calcium salts and the remaining three were milk
types) and likewise assessed the effectiveness of ultrasound
assistance in reducing oxalate (Huynh etal., 2020). This
physio-chemical study reported the top three effective meth-
ods to include ultrasonication (39.6% reduction), followed
by the addition of calcium sulphate (31.8% reduction), and
lastly pairing with high calcium milk (21.4% reduction)
(Huynh etal., 2020). The rationale behind the effectiveness
of adding a calcium source in oxalate reduction is attributed
to oxalate binding to the free calcium ions from the added
source (Bong etal., 2017; Huynh etal., 2022). This conse-
quently converts soluble oxalate into insoluble oxalate to
be excreted in feces as opposed to bloodstream, as shown
in Fig.4. An analogous approach is adding a magnesium
salt enabling the oxalate to bind to magnesium ions, follow-
ing the same pathway as previously discussed (Riley etal.,
2013).
The pairing of calcium sources, although frequently
reported as effective, is heavily influenced by pH (Huynh
etal., 2022; Vanhanen, 2018). Therefore, in considering
such processing method in preparation of oxalate rich foods,
pH levels must be adjusted accordingly. A study exploring
potential strategies for reduction of soluble oxalate content
in green juices reported that the optimum conditions for
extracting oxalate from green juice included a pH of 0.93
and showed that the least amount of oxalate extracted was
at a pH of 4.6 (Vanhanen, 2018). Note however that the
temperature values were not consistent across both sets of
conditions. Nevertheless, this suggested that slightly acidic
pH values are optimum for oxalate reduction. This can be
explained by the findings of another study on the oxalate
content of silver beet leaves (B. vulgaris), which showed
that at pH values less than 6 in the presence of low fat milk,
the proportion of fully deprotonated divalent oxalate ion
(C2O42−) sharply decrease (Simpson etal., 2009). As a
result, there is a reduced potential for the ion to bind with
other mineral cations such as calcium to form insoluble oxa-
lates. Therefore, upon pairing an oxalate-rich food with a
source of calcium intended to convert the soluble oxalate
into insoluble oxalate, it is vital to ensure neutral pH values
as this will guarantee the highest amount of C2O42− that can
readily bind to the calcium ion (Simpson etal., 2009).
Finally, soaking in brine has been applied with some
oxalate-rich foods such as the silver beetroot leaves (B.
vulgaris var. cicla) has been reported. The study soaked
the leaves in a ratio of 1:2 in 10% salt water for 2 h at
room temperature, then 9 h in the fridge. The salted silver
beet leaves showed a decrease in the amount of total oxa-
late content from 4.3 ± 0.2 mg/100 g to 3.7 ± 0.2 mg/100
g FW (Wadamori etal., 2014). Table3 summarizes the
effect of salts addition processing method of some dietary
sources.
Fermentation Processing Method
Microorganisms are helpful in breaking down plant pol-
ysaccharides and polyphenols being able to improve the
nutrients’ bioavailability and digestibility (Difo etal.,
2013). Fermentation is one of the most discussed bio-
logical processing procedures shown to reduce oxalate in
food sources (Difo etal., 2015; Hajimohammadi etal.,
2020; Olawoye & Gbadamosi, 2017), especially consid-
ering fermentation positive effects on improving food
shelf life and sensory attributes presenting an added
value.
One study on sesame seed showed that fermentation
decreased oxalate level by 69% at a time period of 12 days
(Hajimohammadi etal., 2020), and likewise observed in
case of fermentation, in calabash pots lined uniformly with
banana leaves, of green amaranth (Amaranthus viridis L.)
seed as manifested by 20.8% reduction in total oxalate
level (Olawoye & Gbadamosi, 2017). Further investigation
of different fermentation setups, viz open versus controlled
fermentation in Vigna racemosa seeds, revealed that con-
trolled fermentation led to large reduction of 96.9% com-
pared with 59.1% in case of open fermentation process
(Difo etal., 2015). Screening of different microorganisms,
especially those reported in food fermentation, i.e., Lacto-
bacillus acidophilus, and Aspergillus niger should now be
followed to identify the best strain type and exact underly-
ing mechanism. Reduction of oxalate with fermentation
in foods has been attributed to microorganisms arising in
the fermentation process degrading oxalate-mineral com-
plex (Sahoo etal., 2023). It is therefore reasonable that
controlled fermentation to be more effective than open
fermentation as it includes more than one microorganism.
While fermentation alone stands as an effective process-
ing method for oxalate reduction, pairing it with another
processing method can enhance efficiency further due to a
synergistic action. A study investigating anti-nutrient con-
tent of sandbox tree (Hura crepitans) seed studied effects
of cooking and fermentation separately and in combina-
tion as what is often termed hurdle technology in food.
The study revealed that cooked fermented seeds had less
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Food and Bioprocess Technology
oxalate than separate fermented or cooked unfermented
seeds (Osungbade etal., 2016). Further studies into indi-
vidual and combined effects of the different processing
methods would aid identify best measures to reduce oxa-
late, while maintaining other food health benefits espe-
cially in case of oxalate rich foods. Recently, the use of
modified microbes has emerged as a novel approach to the
management and prevention of kidney stones (Wan etal.,
2024). Table3 summarizes the effect of fermentation pro-
cessing method of some dietary sources.
Enzymatic Treatment Method
The addition of exogenous enzymes for oxalate reduction
has also been extensively examined as potential approach
for oxalate reduction considering its specificity targeting
oxalate without affecting other phytochemicals in food,
as possible (Tharifkhan etal., 2021). One study exploring
oxalate in taro flour found that addition of oxalate oxi-
dase (produced by Fusarium oxysporum RBP3) resulted
in a 98.3% removal of total oxalate (Kizhakedathil etal.,
2022). Similarly, a different study investigating taro
though of a different cultivar, reported a 97% reduction
in total oxalate upon treatment with oxalate oxidase pro-
duced by Ochrobactrum intermedium CL6 (Kumar &
Belur, 2018). The advantage of such processing method
is primarily a preservation of properties of the food source
and almost with zero changes in characteristics of food
composition and nutritional value (Kizhakedathil etal.,
2022; Kumar & Belur, 2018). Moreover, expression of the
various oxalate-degrading enzymes has been employed in
probiotic dairy and pharmaceutical preparations for oxa-
late degradation (Abratt & Reid, 2010; Youssef, 2024).
In addition, decrease in the amount of oxalate excreted
in urine using an animal model was achieved through the
administration of liposome-encapsulated oxalate oxidase
taken from Bougainvillea glabra (Dahiya & Pundir, 2013).
Encapsulation can aid protect enzyme and extend its appli-
cation for commercial purposes (Xu etal., 2024).
Furthermore, exogenous addition of phytase to oxalate-
rich foods was applied as beans (e.g., red kidney bean
and white bean) and bran (e.g., wheat, oat, and barley)
to decrease soluble oxalate content. The application of
phytase resulted in a rise in phosphate content; however,
the effect on the concentration of soluble oxalate differed.
When compared to bean samples, wheat and oat bran
shown a substantial decrease (p < 0.05) in soluble oxa-
late. It has been found that the concentration of calcium
ions affects the correlation between the calcium:phosphate
molar ratio and phosphate release, which in turn affects
the concentration of soluble oxalate (Israr etal., 2017).
Table3 summarizes the effect of enzymatic treatment of
some dietary sources.
Biotechnology/Development ofTransgenic Crops
Methods
Aside from physicochemical and biological processing
methods, genetic manipulation and development of trans-
genic crops has been investigated with advances in plant
biotechnology especially with anti-nutrients rich crops,
including oxalic acid (Duraiswamy etal., 2023). Metabolic
remodeling of oxalic acid metabolism has been reported
in transgenic crops found capable to breakdown oxalic
acid (Chakraborty etal., 2013). Kumar etal. have recently
reviewed the various genes and enzymes involved in oxa-
late breakdown in bacteria, fungi, and plants. The enzymes
oxalate oxidase (EC 1.2.3.4), oxalate decarboxylase (EC
4.1.1.2), oxalyl-CoA synthetase (EC 6.2.1.8) or oxalate-
CoA ligase, and oxalyl-CoA decarboxylase (EC 4.1.1.8)/
formyl-CoA transferase (EC 2.8.3.16) are responsible for
breaking down oxalate (Kumar etal., 2019). Therefore,
different oxalate-metabolizing enzymes, and in particular
oxalate-degrading enzymes, have been targeted by genetic
engineering to control oxalate level. The genes encoding
the oxalate-metabolizing enzymes, derived from microbes
or plants, have found extensive application in various bio-
technological fields, including crop enhancement, the crea-
tion of oxalate-free genetically engineered food, and human
diagnostic and therapeutic uses (Kumar etal., 2016; Li etal.,
2022a).
Examples include OXDC which catalyzes the substrate-
specific decarboxylation breakdown of oxalic acid, resulting
in the formation of carbon dioxide and formic acid. OXDC
is a multimeric manganese-containing enzyme that belongs
to the cupin protein superfamily (Mäkelä etal., 2010).
Development of transgenic tomato (Solanum lycopersicum)
plants (E8.2-OXDC tomato plants) that express an OXDC
(FvOXDC) derived from the fungus Flammulina velutipes
that selectively expressed in the fruit. The oxalate concen-
tration of these E8.2-OXDC fruits was reduced by up to
90%, concomitant with increases in calcium, iron, and cit-
rate (27–42%). The breakdown product formic acid content
was confirmed to be increased to 30–67% suggestive for
its catalytic efficiency. Also, oxalic acid precursor showed
a 10–12% reduction. At this experiment, neither carbon
dioxide assimilation rates nor any apparent morphologi-
cal changes in transgenic tomato were identified by OXDC
expression, which has yet to be examined in other oxalate-
rich food such as spinach (Chakraborty etal., 2013; Pereira
Menezes Reis etal., 2022).
The heterologous expression of a 43 KD-sized OXDC
was successfully performed from YvrK gene of Bacillus
subtilis by recombinant E. coli (pBy). The purified OXDC
degraded more than 50% of oxalate when it was treated
with MnCl2 and sodium oxalate at a pH of 5 and a tem-
perature of 28°C. Also, invivo investigations carried out
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Food and Bioprocess Technology
in the hyperoxaluric rat model showed that the recombinant
enzyme was able to lower urine oxalate levels significantly
following the oral administration of sodium oxalate (Lee
etal., 2014). Besides, development of recombinant Lac-
tobacillus plantarum expressing OXDC (gene taken from
Bacillus subtilis) was able to break down oxalate in probi-
otic dairy and pharmaceutical products (Anbazhagan etal.,
2013). Furthermore, treatment for calcium oxalate stone
disease involves expressing heterologous OXDC in human
embryo kidney cell line (HEK293 cells) to guard against oxi-
dative damage caused by oxalate is possible which shielded
cells from oxidative damage and may therefore be a useful
therapeutic approach for preventing calcium oxalate stone
disease (Albert etal., 2017). Table3 summarizes some
reported examples for the use of biotechnology/develop-
ment of transgenic crops methods of some dietary sources,
in addition to invitro and invivo application.
Challenges andFuture Work
Management of anti-nutrients, including phytic acid, lec-
tins, tannins, protease inhibitors, and oxalates by soaking,
sprouting, heating, boiling, and fermentation has attracted
increasing attention in previous literature considering that
they compromise the food value. Despite few research to
investigate the impact on other nutrient contents such as
minerals, the effect on the other nutraceuticals, which may
be heat-sensitive, have not been commonly assessed for
these foods after reduction of oxalate levels. Hence, moni-
toring of not only minerals, but also vitamins, essential fatty
and amino acids are highly recommended to assess how
removal approaches can impact main macro- and micronu-
trients in these food sources. Similarly, low nitrogen levels,
i.e., nitrates, in soil fertilizers is recommended to decrease
the oxalate content, but the crops growth and quality may
be worsened associated with nitrogen deficiency. Nitrogen-
deficient plants produce smaller-than-normal fruit, leaves,
and shoots. Also, they showed more crimson than usual
leaves in Autumn and drop earlier than usual. Then, this
challenge should be considered carefully and nitrogen level
optimization in used fertilizers is a priority in oxalate-rich
crops. Besides, younger leaves are usually richer in oxalate
contents, despite their desirable and fine taste for most con-
sumers. Hence, application of metabolomics approach based
on various analytical platforms (e.g., LC–MS, NMR, and
IR) for oxalate screening and metabolic profiling, in addi-
tion to simple quantification method, is suggestive for future
work monitoring of different quality aspects concomitantly,
and in all similar studies to remove oxalate from dietary
sources. Evaluation of the gut microbiome using multi-
omics has been recently applied in uncommon hyperoxaluric
circumstances.
In addition, development of transgenic less oxalate-pro-
ducing crops is still in early stages and, therefore, requires
more studies to investigate if they are nutritional food can-
didate and can replace the natural counterparts, alongside
their safety as genetically modified foods (GMF). Moreover,
similar enzymes to OXDC derived from probiotics, such as
oxalate oxidase, should be targeted in other research after
thorough investigations of biosynthetic/degradation path-
ways. Oxalate oxidase belongs to the cupin superfamily of
proteins, which exhibits significant similarities to OXDC at
the amino acid level. Attempts to decrease oxalate content
in foods may lead to higher calcium bioavailability result-
ing in hypercalcemia, and therefore, it is better to moni-
tor the oxalic acid:calcium ratio to be not more than 2.5 in
foods and should be assessed in all studies to identify best
methods. Finally, development of drugs or nutraceuticals
which have high affinity to oxalate and consequently are
able to adsorb oxalates in human gastrointestinal tract or
blood may be recognized as a potential solution to get rid of
most exogenous dietary and endogenous sources, especially
for people who are at risk for formation of kidney stones.
Among biological methods for removal of oxalates, develop-
ments of drugs containing degrading enzymes produced by
probiotics, i.e., microbial enzymes-derived, may be potential
for future studies considering the rich metabolic machin-
ery in that niche and several health benefits of probiotics
to improve food value. Aided by advances in analytics and
bacterial culturing, identification of active bacterial strains
should be expedited. Recently, the use of modified microbes
has emerged as a novel approach to the management and
prevention of kidney stones. With all these challenges and
future perspective, optimization of these methods is highly
recommended for efficacy assurance alongside reduction of
the anti-nutrient content.
Conclusion
The current review highlighted different aspects about
oxalic acid in foods as a potential anti-nutrient. It can be
found either in soluble or insoluble forms in various dietary
sources, mostly in vegetables. Also, its inclusion in various
applications in industry and agriculture has aided in increas-
ing its food contents. Despite oxalic acid’s key functions for
plants, the soluble form was found to be harmful due to its
effect on minerals bioavailability as calcium. In addition, its
intestinal absorption results in chelation of calcium in blood
and formation of kidney stones due to lack of a degrada-
tion mechanism in vertebrates, including human, warranting
for developing low oxalate foods or processing methods for
its removal in food maximizing their nutritional attributes.
Interestingly, previous literature showed that levels in foods
vary in context to several factors, including agricultural
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Food and Bioprocess Technology
practices (e.g., plant genotype, harvesting season, and region
of cultivation) and consumption habits (e.g., edible parts and
serving size). Despite a lack of consensus on the most effec-
tive management strategies, physical processing method,
boiling and microwaving seem to be especially effective in
reducing oxalate levels compared to steaming and baking.
Additionally, calcium or magnesium salts or milk reduced
oxalate levels through ion-pairing in relatively low pH pos-
ing administration of calcium as potential adjunct treatment
for reducing absorption of dietary oxalate. Nitrate levels in
soil are a determining factor for oxalate accumulation in a
direct relationship. So, low soil nitrates are recommended
to reduce oxalate in foods, but growth and quality attrib-
utes shall be affected. Moreover, the oxalate-metabolizing
enzymes' genes such as oxalate decarboxylase, which are
derived from microorganisms or plants, have been recently
used in a variety of biotechnological disciplines, including
crop improvement, the development of genetically altered
foods free from oxalate, and human diagnostic and thera-
peutic applications. Further studies should now follow to
generate drugs and probiotics containing and producing
oxalate-metabolizing enzymes, considering its higher safety
and less likely to affect food nutritional value. Also, more
genetic research is required to investigate plausible molecu-
lar mechanisms underlying the varying levels of variability
among plant organs.
Author contributions M.F.: Conceptualization, writing-review and
editing, and Supervision. A.Z. and G.A.: data collection, analysis,
writing-original draft. A.Z.: preparing figures, writing-review, and
editing. All authors have approved the final version that was submitted.
Funding Open access funding provided by The Science, Technology &
Innovation Funding Authority (STDF) in cooperation with The Egyp-
tian Knowledge Bank (EKB).
Data Availability No datasets were generated or analysed during the
current study.
Declarations
Conflict of Interest The authors declare no competing interests.
Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format,
as long as you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons licence, and indicate
if changes were made. The images or other third party material in this
article are included in the article’s Creative Commons licence, unless
indicated otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons licence and your intended
use is not permitted by statutory regulation or exceeds the permitted
use, you will need to obtain permission directly from the copyright
holder. To view a copy of this licence, visit http://creativecommons.
org/licenses/by/4.0/.
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