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International Journal of Food Science and Nutrition
188
International Journal of Food Science and Nutrition
ISSN: 2455-4898
Impact Factor: RJIF 5.14
www.foodsciencejournal.com
Volume 3; Issue 4; July 2018; Page No. 188-193
Glycoalkaloids, bitter tasting toxicants in potatoes: A review
Deepthi Inoka Uluwaduge1
1 Senior Lecturer, Department of Allied Health Sciences, Faculty of Medical Sciences, University of Sri Jayewardenepura,
Gangodawila, Nugegoda 10250, Sri Lanka
Abstract
Potatoes make popular dishes among humans. Currently the fresh potato consumption is decreasing and most of the potatoes are
converted to value added products to meet consumer demand. Glycoalkaloids are natural bitter tasting, heat stable toxicants present
in potatoes. In the edible tuber, majority of these compounds are confined to the peel. High content of glycoalkaloids impart an off
flavour to the potatoes and shown to be toxic to humans and animals. There have been food safety concerns linked with the
potatoes and potato based products owing to the unacceptable glycoalkaloid content in the past. Thus the glycoalkaloid content is
the major determinant of the quality and safety of edible potatoes. This review highlights major areas relevant to glycoalkaloids in
potatoes such as distribution and accumulation in edible tubers, factors enhancing formation, effect of various cooking methods,
toxic effects and measures to minimize the content to ensure consumer safety.
Keywords: potato, glycoalkaloids, α-solanine, α-chaconine, bitter toxicants
Introduction
Potato (Solanum tuberosum L.), a member of the Solanaceae
family, is a popular crop among different cultural back
grounds. It is well grown in majority of countries and
worldwide production stands in the fourth place among other
major crops wheat, maize and rice [1, 2]. From year 2005 the
potato production in developing countries exceeded that of the
developed countries [3]. At present China contributes to the
highest amount of the world’s production followed by India,
Russian Federation and United States [2]. It serves as a major
inexpensive low fat food source rich in energy [4]. Other
nutritional importance are supplementation of high quality
protein, fibre and vitamins [5, 6].
The potato tuber contains a natural bitter-tasting steroidal
toxicant known as glycoalkaloids [7]. These are nitrogen
containing secondary metabolites present in some members of
the Solanaceae family including potatoes, tomatoes and egg
plants [6, 8]. Low amounts of glycoalkaloids present in
commercial varieties impart the flavour to the potatoes [9].
However, potato has a bitter taste when the levels exceed
14mg/100g [5, 10].
Potato glycoalkaloids have shown to possess many health
benefits such as anticancer, antimalarial, anti-inflammatory,
hypoglycaemic and hypocholesterolaemic activities [5, 11, 12].
Glycoalkaloids have fungicidal and pesticidial properties and
it is one of the plants natural defences [4]. Synthesis is
significantly increased under unfavourable conditions, and it
may serves as a stress metabolite in the potato plant [5].
This review aims to provide a comprehensive summary of the
scientific literature on potato glycoalakaloids and their
biological role. Since there had been reports on incidental
poisoning of potatoes due to intolerable glycoalkaloid levels,
practical measures documented in this review are important to
improve the quality of edible tubers.
Chemical nature of glycoalkaloids in potatoes
The most important glycoalkaloids found in potato tubers are
α -solanine and α-chaconine, (Figure 1) comprising about 95%
of total glycoalkaloid content [1, 4, 13]. Solanine and chaconine
are nitrogen containing steroidal alkaloids, bearing the same
aglycone, solanidine but differ in the trisaccharide side chain
[1, 9, 14]. The trisaccharide in α - solanine is galactose, glucose
and rhamnose and that in α-chaconine is glucose and two
rhamnose residues [1, 14]. A minor percentage of glycoakaloids
(5%) contain β- solanine, γ- solanine, β-chaconine and γ-
chaconine [14].
Fig 1: The structure of glycoalkaloids α -solanine and α-chaconine
(R1= β-D-galactose, R2= β-D-glucose, R3= α-L-rhamnose for
α-solanine and R1= β-D-glucose, R2=R3=α-L- rhamnose for
α-chaconine)
Distribution of glycoalkaloids in potato plants
Glycoalkaloid content in various parts of the plant
These compounds are distributed throughout the potato plant
and the concentrations vary significantly depending on the
anatomical part or the genetic variety [1, 3, 15]. A broad range
for total glycoalkaloids for a given part of the plant was
reported by many studies owing to considerable variation of
the compound among potato plants [1, 13, 16, 17, 18]. Synthesis of
International Journal of Food Science and Nutrition
189
glycoalkaloids occur in all parts of the plant and the highest
concentrations have been reported in those parts with high
metabolic rates. Flowers (215-500 mg/100g), sprouts (200-
730 mg/100g) and young leaves (23-100 mg/100g) are
comparatively rich in glycoalakaloids [1, 16, 17, 18].
Glycoalkaloid content in the edible tuber
Most of the edible tubers contain low amount of
glycoalkaloids (less than 10 mg/100g fresh weight) [10]. The
profile of glycoalkaloids in each part of the tuber was not
revealed by most of the past records and the total
glycoalkaloid content was reported by many such studies [1, 13,
16, 17, 18, 19, 20]. However the content of α-chaconine is reported
to be slightly higher than α –solanine [13].
Glycoalkaloid content in commercial potato tubers are
comparatively lower and the distribution within the tuber is
not uniform. The highest levels are confined to the skin and
the peel while less amounts could be observed towards the
pith [13, 21]. The pith had undetectable levels of glycoalkaloids
indicating that the tubers are safe to consume though there had
been few reported occasions with banned levels of
glycoalkaloids in tubers [3, 22, 23]. These compounds are mainly
concentrated in the “eye” regions of the tuber and
consumption of potatoes rich in those parts may cause
potential health risks [13].
Small tubers are reported to be rich in glycoalkaloids on
weight for weight basis than larger ones [1]. Green potatoes
often show bitterness and this off flavour is due to the
accumulation of excess amounts of glycoalkaloids in the peel
[10]. Table 1 illustrates the distribution of glycoalakaloids in
various tuber tissues.
Table 1: Distribution of total glycoalkaloid content in various tuber
tissues of potato [16, 17, 18]
Part Total glycoalkaloid content (mg/100g fresh weight)
Whole tuber
1-15
Skin (2-3% of tuber) 30-64 Peel (10-12% of tuber)
15-107
Flesh
1.2-10
Bitter tasting tuber
25-80
Peel from bitter tuber
150-220
Modern cultivars vs. wild progenitors
The glycoalkaloid levels in modern cultivars are much lower
than in wild progenitors and this information is useful for
commercial potato breeders [1]. These compounds are not
transported between various parts of the plant and therefore
the amount present in each part is propionate to synthesis [3].
Genetic engineering approaches could be useful for the
manipulation of the level of glycoalkaloids according to
commercial needs such as to reduce the levels in tubers to
enhance the edibility and the safety and to increase the content
in leaves to ensure the protection from diseases and predators [24].
Factors affecting levels of glycoalkaloids in the tuber
Both genetic and environmental factors have been shown to
affect the levels of glycoalkaloid in potato tubers.
External factors
Environmental factors during pre-harvest period
Several external factors during pre-harvest period (soil
composition and climate) and post-harvest events (effect of
light, temperature, storage time, humidity, mechanical injury
and sprouting) may increase the glycoalkaloid content [4, 16].
Exposure to light may stimulate chlorophyll synthesis leading
to ‘greening’ and those tubers are reported to be rich in
glycoalkaloids [25]. Extreme temperatures and dry or wet
growing seasons may influence the synthesis of glycoalkaloids
during growth of the plant [3, 25]. Water- logging and drought
stress are other environmental factors which could enhance
the production of glycoalkaloids in significant amounts in
some cultivars [26, 27]. It has been reported that a double
nitrogen rate during the cultivation increased glycoalkaloid
content by 10% in some varieties [28]. A perusal of literature
indicates that early harvested tubers may show cultivar
specific impacts on glycoalkaloid accumulation than late
harvested tubers [10, 23, 29].
Biodynamic conditions vs. classic cultivations
Studies conducted by Norgia et al., (2008) have revealed that
potato breeds grown in biodynamic conditions were rich in
glycoalkaloids and solanidine compared to the breeds grown
in classic conditions (5-15% increase) [14]. Biodynamic
conditions provide the natural environment in which potatoes
could develop healthy and be able to fight against detrimental
agents and therefore those varieties may have synthesized
more phytorepelents. Classic cultivation of potatoes involves
use of fertilizers and other chemicals according to the need
and therefore those varieties are protected artificially. Hence
the need of natural phytorepellents such as glycoalkaloids is
less. Home Guard potato tubers had higher glycoalkaloid
content and showed a little increase in response to adverse
environmental conditions [26].
Factors to be considered during post-harvest period
Tubers subjected to post- harvest stress factors such as
physical damage (cutting and bruising during harvest or
transit), microbial or herbivore attack, improper handling and
inadequate storage conditions are known factors which may
influence the content of these compounds in potato tubers [30].
Tubers which are exposed to the mentioned factors if used for
skin-on or peel based products have higher glycoalkaloid
levels and therefore cause potential health risks. However,
safe handling and monitoring of other storage conditions such
as temperature and light are important to improve the quality
of potatoes in commercial varieties.
Other contributing factors for glycoalkaloid synthesis
Ability to produce glycoalkaloid is inherited to potato
cultivars [10, 15, 31]. This information is useful for commercial
production of edible potato varieties with low content through
breeding and biotechnological methodologies, while potato
genotypes with high glycoalkaloid content may be developed
for the pharmaceutical purposes [15]. It is recommended to
grow potatoes with inherently low glycoalkaloid and to
protect them from other inducers of glycoalkaloid synthesis to
enhance the quality of the commercial potato destined for
consumption [10]. Breeding for new varieties to obtain
characteristics such as disease resistance and withstand with
cold and other major changes in agricultural practices should
be accompanied by careful control of the glycoalkaloid levels.
International Journal of Food Science and Nutrition
190
Regulatory control of the level of glycoalkaloids
The current safe level of glycoalkaloid in potatoes had set at
20mg/100g of fresh weight by several leading authorities and
if the threshold value is exceeded by any cultivar, those
varieties are not recommended for human consumption [32, 33].
Though the standard potato varieties with the acceptable
steroidal glycoalkaloid content have released to the
commercial producers for growing, occasionally the safe level
could be exceeded by several environmental, physical and
storage conditions as pointed out in the text. Two such
reported occasions are withdrawal of potato cultivars ‘Lenape’
and ‘Magnum Bonum’ from United State and Swedish
markets due to unacceptable glycoalkaloid levels. The
reported average in ‘Lenape’ was 30mg/100g and that of
‘Magnum Bonum’ was 25.4/100g [3, 22, 23]. Therefore it is
advisable to monitor the steroidal glycoalkaloid content when
they have faced to adverse environmental conditions during
the tuber bulking and also to screen the amount of these
compounds in those batches before releasing them to the
market as a safety precaution. Further it is recommended to
assess the glycoalkaloid levels in potatoes generated from
breeding programs due to the genetic transmission of
undesirable levels of glycoalkaloids from wild species to
hybrid progeny without which it may result in wasted effort,
time and money [34].
Effect of processing on glycoalkaloid content of potatoes
Levels of glycoalkaloids in potato based products
Consumption of fried potato chips and crisps are being on the
rise in most of the countries due to its attractive flavour, easy
preparation and affordable price [21, 35]. Therefore recently
more attention has paid to evaluate the safety of potato based
products. According to Smith et al., (1996) potato chips (US
French fries) and potato crisps (US potato chips) normally
contain glycoalkaloid levels of 0.04-0.8 and 2.3-18 mg/100g
product respectively [1]. Further it is reported that the skin
based products such as fried skins and crisps are
comparatively rich in glycoalkaloids (56.7-145 and 9.5-72
mg/100g product respectively) [1]. Jacket potatoes and more
recently skin based preparations (potato crisps) have shown a
relatively high content of glycoalkaloids [1] According to the
reports by Mondy and Gosselin, (1988) “salt potatoes” (small
whole potatoes) are popular dishes among the consumers of
the North Eastern United State due to the belief that the peel
contains more nutrients [22]. Further, according to the
documentary evidence by, potatoes are processed with peel
into fried snack foods such as potato chips and kettle chips by
some Asian countries and may lead to ingestion of toxic
glycoalkaloids [33, 36].
Loss of glycoalkaloids during production of potato based
Products
Production of French fries from potatoes involve several steps
such as cutting, blanching, drying and frying 35. During the
production chain, the highest amount of glycoalkaloids were
removed during peeling, blanching and frying and the French
fries ready for consumption contained 3-8% of glycoalkaloids
compared to raw material [35]. Similar findings have been
obtained by a study carried out to evaluate the quality of
French fries by Tajner-Czopek et al., (2008) and reported that
the highest decrease in glycoalkaloids was caused by frying,
which is the final step of processing (97.5% loss when
compared to raw unpeeled potatoes) [37]. The average loss
during other preliminary steps were 50%, 53% and 58% in
peeling, cutting and blanching respectively [38]. The level of
glycoalkaloids in potato granules were evaluated by Rytel
(2012) and reported that the highest decrease in glycoalkaloids
were caused by peeling (50%) and blanching (63%) and the
finished product had only 14% of the initial quantity [38].
Effect of in-home cooking methods on glycoalkaloid
content
Several studies suggest that peeling usually removes most of
the glycoalkaloids in the edible tubers. Sinden and Deahl
(1976) reported that up to 60% of the total glycoalkaloids in
whole tubers were removed with the peel [10]. Mondy and
Gosellin reported that potatoes cooked with peel were bitterer
than unpeeled potatoes due to the migration of glycoalkaloids
into the cortex during the cooking process, though they are
less mobile [22]. Similar findings have been obtained by
Tajner-Czopek et al., (2014) and the authors further claimed
that peeling removes higher amount of glycoalkaloids
(approximately twice) than cooking [37]. Bushway and
Ponnampalam (1981) investigated the stability of
glycoalkaloids during four cooking procedures; frying,
baking, microwaving and boiling [39]. A slight loss of
glycoalkaloids was shown by frying whereas the other
cooking methods which were tested did not significantly
reduce the glycoalkaloid content. In a similar study, it was
reported that the potato glycoalkaloids are relatively stable
under normal home cooking conditions revealing that only
little reduction was shown by boiling and microwave
treatment [40]. The study further suggested that the critical
temperature for decomposition of glycoalkaloids in potatoes
may be around 170° C based on the results obtained by using
deep frying at different temperatures (157°C, 170 °C and
260°C) [40].
Studies done to evaluate the effect of various processing
methods suggest that the glycoalkaloids are relatively stable
under conventional cooking procedures. In typical household
preparations, many Asians including Sri Lankans peel off the
potatoes after boiling. These compounds migrate into the
internal tissue of the tuber during boiling and therefore
conventional method of boiling reduces the quality of edible
potatoes. Thus it is advisable to remove the peel of the tubers
before any cooking process to reduce glycoalkaloids and to
improve the quality [40].
Toxicity caused by glycoalkaloids in potatoes
It is important to be aware on toxicity caused by potato
glycoalkaloids due to following reasons.
1. The potato makes a part of the regular diet of majority of
the people worldwide.
2. The concentration of glycoalkaloids present in potato has a
major economic impact on potato breeders since the potato
varieties exceeding the level of 20mg/100g fresh weight is
probably a rejection criteria in marketing [3, 22, 23].
3. According to the reported data by Smith et al. (1996),
modern preparations such as skin based products (crisps
and fried skins) may contain unacceptable levels of
International Journal of Food Science and Nutrition
191
glycoalkaloids (up to 72 mg/100g in crisps and 145
mg/100g in skin based products) and ingestion may elicit
health problems in humans [1]. A review of toxicological
literature by Zeiger (1998) reported that baked and fried
potato peels are a major source of large quantities of α -
solanine and α-chaconine in the diet [41].
Published toxicological findings on humans
The documented history of the steroidal glycoalkaloid
poisoning from potatoes extend up to year 1917 from Britain
on an outbreak of solanine poisoning by a hotel proprietor [42].
Baked potatoes with skins were identified as the causative
agent and the victims showed vomiting, diarrhoea and
abdominal pain [42]. A clinical observation was recorded by
Unverricht (1937) on an outbreak of sickness among
agricultural workers in a village near by Beriln [43]. Excluding
potatoes from the diet resolved the symptoms confirming that
the potatoes could be the causative agents for the illness. Mc
Millan and Thompson (1979) have reported an incidence of
poisoning among school boys caused by ingestion of potatoes
which had high amounts of α-solanine and α- chaconine [44].
Cumulative assessment of toxicological data suggests that
these compounds are toxic to humans at a very lower dose
when compared with other animal models [4].
Biological activities of glycoalkaloids
General symptoms of glycoalkaloid poisoning in humans are
nausea, vomiting, diarrhoea, stomach and abdominal cramps,
headache, fever, rapid and weak pulse, rapid breathing and
hallucination [3, 14]. Coma and death has been resulted in more
serious cases [1, 3]. There are two main biological functions of
glycoalkaloids [1, 3]. The first is their ability to bind with the
membrane sterols and there by cause the disruption of the
membrane architecture leading to leakage of cellular contents
raising gastrointestinal disturbances such as abdominal
cramps, vomiting and diarrhoea [1, 4, 45]. Differential diagnosis
of glycoalkaloid poisoning is complicated since the symptoms
of acute intoxication share common features of other gastro
intestinal disorders [1]. The other major biological action of
glycoalkaloid is the inhibition of acetylcholine esterase, the
enzyme involved in the hydrolysis of the neurotransmitter
acetylcholine at the cholinergic synapses. The anti-
acetylcholine esterase activity of glycoalkaloids is manifested
by neurological symptoms such as weakness, confusion and
depression [46].
Absorption vs. excretion
Absorption of potato glycoalkaloids in humans are apparently
proportional to the amount ingested [47]. An overview on the
toxicology of solanine (the major glycoalkaloid in potato)
compiled by Dalvi and Bowie (1983) revealed that solanine
when ingested is less toxic compared to the parenterally
administered solanine due to its poor absorption, rapid
excretion and conversion into less toxic secondary metabolites
in the stomach [48]. A controversial opinion regarding the
clearance of glycoalkaloids from the human body was
reported by Mensinga et al. in 2005 [49]. However, this
ascending dose study with human volunteers showed that the
clearance of glycoalkaloids from the body takes more than 24
hours and further suggested that there is a possibility of
accumulating the toxicants in case of daily consumption. The
observation by Mensinga et al. (2005) was further confirmed
by Nishie et al. (1971) reporting that once it is in the blood
stream, excretion appears to be low indicating that the
compounds might accumulate in various organs in the body
including liver by using animal models [49, 50].
Safe level of intake
Poisoning caused by glycoalkaloids on humans is subjected to
individual variations [1]. The toxic dose of glycoalkaloids in
humans is 1-5 mg/Kg body weight and lethal dose is 3-6
mg/Kg body weight when administered through the oral route
[1]. Therefore the USDA and other leading authorities have
defined a glycoalkaloid level of 20 mg/100g fresh weight and
100mg/100g dry weight as the safe limit in edible tubers [1, 3,
33]. Analysis of the toxicological data on human subjects failed
to establish a safe level of intake and further indicated that a
considerable effort is required to work out on such a cut off
value [13]. Due to variations of the glycoalkaloid content
according to pre harvest and post-harvest factors and the
individual variations of the toxic dose, it has been proposed
that the safety limit has to be brought down to a level less than
the recommended [20]. Commercial varieties tend to have
glycoalkaloid content less than the accepted safety limit of 20
mg/100g fresh weight [1]. It has been shown that the potato
tubers exceeding the glycoalkaloid level of 14mg/100g had
bitterness while a burning sensation in throat and mouth was
caused by tubers exceeding 22mg/100g [32]. Therefore the
quantity of glycoalkaloids present in edible tubers has a direct
impact on the quality of tubers. Since the off flavours caused
by high glycoalkaloid content will reduce the commercial
value of tubers, routine testing for glycoalkaloids are
necessary for the edible tubers and for potato based products
to ensure the safety of the consumer.
Published findings on animal experiments
Several laboratory experiments have shown that the
glycoalkaloids are toxic to animal models such as Syrian
Golden hamsters, rabbits, rats and mice [50, 51, 52, 53].
Experimental studies by Nishie et al. (1971) further confirmed
that the toxicological potency of the agyclone (solanidine) was
less when compared that with the solanine revealing that the
potential toxicity is mediated by the carbohydrate side chains
of the two compounds, α -solanine and α-chaconine [50]. The
acute toxicity studies have revealed that the LD50 for solanine
in mice is 32.3 mg/Kg BW and that of chaconine is 19.2
mg/Kg BW [13]. Oral administration of solanine to mice
showed less toxic effects (oral LD50 ≥1000 mg/KgBW) when
compared with that of the parenteral administration [47]. Rats
exhibited a comparatively higher toxic dose for solanine and
chaconine (65.6-107.5 mg/KgBW) when administered intra
peritonially [14]. Animal experiments suggest that α-chaconine
is more toxic than α-solanine [20, 50].
Measures to optimize the safety of edible tubers
Based on the scientific evidences, following recommendations
(Table 2) would be helpful for farmers and commercial
producers, retail sellers and consumers to improve the quality
of edible potatoes.
International Journal of Food Science and Nutrition
192
Table 2: Strategies for controlling glycoalkaloid formation/accumulation in potatoes and potato products
Intervention
Farmers and
commercial producers
Selection of cultivars low in glycoalkaloids Careful manipulation of environmental factors (low temperature,
desired soil nitrogen content) Minimization of damage to tubers during post- harvest handling Screening for
glycoalkalod content of new varieties prior to market release
Retail sellers
Packing in opaque plastic films or paper bags (to protect from light) Rotate the stocks in retail displays Store in a
shaded, cold environment
Consumers
Selection of intact tubers of moderate/large size Peel off the tuber before any processing method Eliminate use of
tubers with bitter taste and green colour
Conclusion
Potatoes, as a staple for humans have shown to be safe
throughout the long history of consumption despite the
presence of bitter tasting toxicants. Fortunately most of the
commercial potato varieties contain a glycoalkaloid level less
than 20mg/100g fresh weight (the acceptable upper safety
limit) in edible tubers. Processed potato products have
increased in popularity and therefore, from a food safety
perspective it is important that farmers and retailers review
their cultural and marketing practices in order to ensure that
the tubers contain a safe level of glycoalkaloids from the field
through storage and retail outlets to the table. In addition
selection criteria and processing methods adopted by
consumers are important to ensure the safety of edible tubers.
Declaration of interest
The author reports no conflicts of interest.
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