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A review: nutrition and process attributes of corn
in pet foods
Isabella Corsato Alvarenga, Amanda N. Dainton & Charles G. Aldrich
To cite this article: Isabella Corsato Alvarenga, Amanda N. Dainton & Charles G. Aldrich (2021):
A review: nutrition and process attributes of corn in pet foods, Critical Reviews in Food Science and
Nutrition, DOI: 10.1080/10408398.2021.1931020
To link to this article: https://doi.org/10.1080/10408398.2021.1931020
© 2021 The Author(s). Published with
license by Taylor & Francis Group, LLC
Published online: 03 Jun 2021.
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A review: nutrition and process attributes of corn in pet foods
Isabella Corsato Alvarenga, Amanda N. Dainton, and Charles G. Aldrich
Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas, USA
Corn is one of the largest cereal crops worldwide and plays an important role in the U.S. econ-
omy. The pet food market is growing every year, and although corn is well utilized by dogs, some
marketing claims have attributed a negative image to this cereal. Thus, the objective of this work
was to review the literature regarding corn and its co-products, as well as describe the processing
of these ingredients as they pertain to pet foods. Corn is well digested by both dogs and cats and
provides nutrients. The processing of corn generates co-products such as corn gluten meal and
distillers dried grains with solubles that retain quality protein, and fibrous components that dilute
dietary energy. Further, corn has much functionality in extrusion processing. It may yield resistant
starch under certain processing conditions, promoting colonic health. Carotenoids in corn may
enhance immune support in companion animals if concentrated. Mycotoxin contamination in
grains represent a health hazard but are well controlled by safety measures. Genetically modified
(GM) corn is still controversial regarding its long-term potential for mutagenicity or carcinogenicity,
thus more long-term studies are needed. In conclusion, the negative perception by some in the
pet food market may not be warranted in pet foods using corn and its co-products.
Animal health; co-products;
Global pet food sales have enjoyed a nearly 6% annual
growth rate since 2013 and were estimated to be U.S. $91.1
billion in 2018 (Philipps-Donaldson 2019). The majority of
dogs and cats are fed dry foods as whole meals or treats.
Dry foods, like extruded kibbles, generally contain between
30% and 60% starch ingredients, which provide kibble bind-
ing and expansion properties due to starch when cooked in
the presence of water and heat. Corn is an example of a
starch ingredient that can be used in dry pet foods due to
its physical properties. It also provides energy and nutri-
tional value to pets, which will be discussed in this review.
Although corn has been used in pet foods for decades
with few issues or concerns regarding pet health, recent
marketing claims have contributed to a negative image of
this cereal. One claim is that corn and other grains cause
allergies. Veterinary reports have shown that the main cause
of allergies is peptides or glycoproteins, and that grains
cause less than 1.5% of all food allergy cases (Laflamme
et al. 2014). Some pet owners also have the perception that
corn has no nutritional value, and that it is added to the
food as a “filler”or as an ingredient to lower production
cost. A review of the existing literature should be conducted
to determine whether there is validity to the claims or not.
The research question posed was what the effect of corn
in dog or cat diets may have on animal health, diet utiliza-
tion, and potential physiological implications. Thus, the goal
of this review was to explore the breadth of published
research regarding corn in pet foods, and to summarize
research regarding this cereal along with its co-products for
their nutritional value, antioxidant potential, and functional
properties as it pertains to pet nutrition and health. A litera-
ture search was conducted utilizing scientific search engines
and key words to identify all peer-reviewed publications
with corn and pets as common themes. Specifically, Google
Scholar and Scopus were used with key words including
original research featuring dogs or cats that had a corn com-
ponent as a dietary treatment were selected. Additional sup-
porting literature and published reviews were utilized where
appropriate. Only publications evaluating corn inclusions of
30% or more were considered in this review. In cases where
corn inclusion level in experimental diets was insignificant
and (or) not the research target of the particular manuscript
it was excluded from the summary tables.
Corn ingredient: US production, usage and
Corn is a major crop for farmers in the United States. In
2018, U.S. corn growers produced 14.626 billion bushels of
corn (USDA ERS 2018), which at 56 pounds per bushel is
approximately 366 million US tons (1 US ton ¼2000
pounds). From this supply, 5.5 billion bushels (154 million
CONTACT Charles G. Aldrich email@example.com 201 Shellenberger Hall, 1301 Mid Campus Drive, Manhattan, KS 66502, USA.
ß2021 The Author(s). Published with license by Taylor & Francis Group, LLC
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.
0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION
US tons) were used for animal feed and residual use, 0.2 bil-
lion bushels (5.85 million US tons) for cereals and related
products, 0.39 billion bushels (10.92 million US tons) for
glucose and dextrose uses, 0.46 billion bushels (12.88 million
tons) for high-fructose corn syrup production, 0.03 billion
bushels (0.87 million US tons) for use as seeds, and 0.24 bil-
lion bushels (6.72 million US tons) were used for starch pro-
duction (USDA ERS 2018).
Unfortunately, there is very little information about the
amount of corn that is used in the United States pet food
industry. But one could extrapolate from a few assumptions.
The pet food market in North America is approximately 8.5
million metric tons (approximately 9.4 million US tons).
Around 80% of pet foods have grains (Corsato Alvarenga
and Aldrich 2020), which comprise 30–60% of the food
(consider 45%). Corn is approximately 25% of these grains,
so it might contribute to 0.67 million metric tons (approxi-
mately 0.74 million US tons) of the pet food market. That
remains a substantial portion, but corn has lost market share
to the grain-free segment since the beginning of the millen-
nium (nearly 20% of the market). This resulted from dispar-
aging information and the lack of consolidated information
that contradicted incorrect statements.
Corn (Zea mays L.) is considered as one of the most
important cereals in the world. Kernel types can be grouped
as dent, flint, flour, sweet, pop, and pod (Watson and
Ramstad 1991). Dent hybrids are grown in the US corn belt
and certain countries in Europe, and the yellow endosperm
is the predominant type. Flint kernels are grown in South
America and Northern Europe, floury corn is one of the
oldest types produced by the Aztecs and Incas, and sweet
corn is an important crop consumed as fresh, frozen, or
canned by humans (Gy}
Corn used in pet foods is usually dent hybrids, or co-
products of other corn varieties from the human food chain.
Its nutritional composition may benefit both health and dif-
ferent food processes that transform corn ingredients into a
final product. The whole kernel can be divided into three
anatomical parts: the pericarp, endosperm, and the germ
ori 2010). Some authors also consider the tip as an ana-
tomical part of corn. The corn endosperm is divided into
the horny endosperm, which contains protein and starch,
and the floury endosperm, with mostly starch granules. The
germ (embryonic tissue) contains high amounts of lipids
and protein, and the bran, which includes the pericarp and
hull, is the fibrous fraction of corn with primarily cellulose,
hemicelluloses, lignin, and pectic substances (Gy}
Corn varieties used in pet foods have on average 88.0% DM,
1.42% ash, 9.79% CP, 6.01% fat, 77.2% starch, and 10.8%
TDF (Table 1).
The largest nutrient fraction of the corn kernel is starch,
which is located in the endosperm. The endosperm has large
(15–18 lm) and small (5–7lm) starch granules with non-
smooth cuboid or rounded shapes (Singh and Singh 2003).
The starch granule is composed of the polymers amylose
and amylopectin. Amylose is a linear molecule composed of
anhydroglucose units linked by a-1,4 bonds, whereas amylo-
pectin is a branched glucose polymer with a-1,4 and a-1,6
bonds. While amylose has a low molecular weight, amylo-
pectin is a much larger polymer (Bul
eon et al. 1998). There
are certain cultivars within species that differ in their ratio
of amylose to amylopectin, such as waxy maize with almost
100% amylopectin, or high-amylose corn.
Effects of corn on pet health and nutrition
Corn utilization by dogs and cats
Corn is included in many pet food recipes due to its struc-
ture-forming properties, economics, and history of nutri-
tional utility. Nutritional value is commonly measured by
difference from the fecal disappearance of nutrients, from
which one can calculate the apparent total tract digestibility
(ATTD). Dogs and cats can obtain glucose from dietary sug-
ars or synthetize it from other substrates through a process
called gluconeogenesis (Tanaka et al. 2005). A moderate
intake of starch ingredients like corn adds energy to the
diet, besides providing some fiber, protein, and fatty acids.
When corn starch is digested by pancreatic alpha-amylase
in the duodenum, the end products are maltotriose, alpha-
limit dextrins, and maltose. These are further digested by
maltase-glucoamylase, sucrase, and isomaltase into glucose.
Monosaccharides are absorbed in the SI, mainly in the
jejunum region, enter the bloodstream, and are then trans-
ported into cells with the aid of insulin and other hormones.
Table 1. Nutritional composition of whole corn used in various pet food studies.
Nutrient, dry matter basis
Authors Corn type
DM, % Ash, % CP, % AHF, % Starch, % TDF, %
Murray et al. (1999) Corn 90.1 0.6 5.6 3.2 88.3 3.0
Bednar et al. (2001) Corn 86.8 1.4 12.8 4.9 NR
Gajda et al. (2005) Conventional corn 87.0 1.6 8.5 6.8 78.1 12.0
Gajda et al. (2005) High-protein corn 87.5 1.3 10.2 5.8 79.1 10.7
Gajda et al. (2005) High protein, low phytate corn 87.8 1.5 10.1 6.2 79.1 11.2
Gajda et al. (2005) High-amylose corn 86.9 1.7 11.6 7.2 69.1 18.8
Carciofi et al. (2008) Corn 88.8 1.9 10.5 5.2 78.4 4.1
Cutrignelli et al. (2009) Corn 88.4 1.6 8.8 1.2
Fortes et al. (2010) High oil maize 88.2 1.3 9.5 8.5 71.7 8.7
Bazolli et al. (2015) Maize 88.3 1.3 9.1 6.3 71.9 11.2
Averages –88.0 1.4 9.8 6.0 77.2 10.8
DM, dry matter; CP, crude protein; AHF, acid-hydrolyzed fat; TDF, total dietary fiber.
Corn variety as described in each study.
Not reported in the study.
Measured by ether extract. This fat value was not included in the average.
2 I. C. ALVARENGA ET AL.
The extent of processing, inclusion of starch ingredients and
type of starches all influence the extent to which glucose is
absorbed. Highly digestible starches rapidly increase plasma
glucose and insulin (Carciofi et al. 2008). Intake of these
diets chronically can lead to insulin resistance (Andr
e et al.
2017) and eventual development of type II diabetes. Carciofi
et al. (2008) reported that dogs fed a diet with 53.5% corn
as the only starch ingredient led to peak blood glucose and
insulin levels within a few minutes postprandial, in a man-
ner similar to dogs fed a brewer’s rice diet.
For ground whole corn or corn starch included in a pet
food and properly cooked, the starch is almost entirely
digestible by the end of the ileum (89.9–99.5%; Gajda et al.
2005; Murray et al. 1999; Walker et al. 1994). Any remain-
ing undigested starch that reaches the colon is fermented.
unemann et al. (1989) demonstrated that canine ileal
digestion of raw corn was increased after cooking, and
Moore et al. (1980) found no difference in starch ATTD
between cooked and uncooked corn. This is evidence that
the portion of raw corn starch that escaped small intestinal
digestion was fermented in the colon, and thereby absent
in the feces. When comparing different corn hybrids,
Gajda et al. (2005) found that DM ileal digestion of
extruded high protein corn was higher than conventional,
low protein and low phytate corn, high-amylose corn and
amylomaize (64.6% vs. average 57.8%). These authors con-
cluded that high-amylose corn should not be used in dog
diets because it had low ATTD and lower in vitro
The DM ATTD in corn-based diets has been reported
to be high (average 80.23%; Table 2) and comparable to
other grains like sorghum, and similar or slightly lower
than rice (Bazolli et al. 2015; Carciofi et al. 2008; Fortes
et al. 2010; Kore et al. 2009; Twomey et al. 2002). Another
study found that DM ATTD of a corn-based diet was
similar to rice, wheat, and potato-based foods, and higher
than barley and a sorghum-based diets by 2.9 and 5.7 per-
cent units, respectively, when fed to dogs (Murray et al.
1999). Dogs fed corn-based diets were also reported in sev-
eral studies to produce firm, high quality feces (Kore et al.
2009; Murray et al. 1999; Twomey et al. 2002; Walker
et al. 1994). So, the notion that corn is an indigestible filler
does not appear to be valid based on these results.
However, controlling glucose absorption in rapidly digested
corn-based diets is beneficial for animal health. This can
be accomplished by combining different starch sources in
the diet (Sunvold 2002) and/or modifying the process to
favor the retention of slowly digestible or resistant starches
(Ribeiro et al. 2019; Peixoto et al. 2018).
Impact of corn on nutrient digestibility
Mean values for corn content in the diet and the resulting
ATTD by dogs were extracted from peer-reviewed papers
where conventional corn comprised at least 30% of the diet
formulation (Table 2). This yielded 11 data points with DM
digestibility, 10 data points with both crude protein and fat
digestibility, nine data points with gross energy and starch
digestibility. Organic matter and total dietary fiber digestibil-
ity were excluded from analysis due to the low number of
data points (4 and 3, respectively). Pearson correlation coef-
ficients between dietary corn content and each nutrient
digestibility were determined with statistical analysis soft-
ware (SAS 9.4, Cary, NC). Correlations were considered sig-
nificant at pvalues less than .05 and the absolute rvalue
was used to determine the strength of the association
(0–0.4 ¼weak; 0.4–0.7 ¼moderate; 0.7–0.9 ¼strong;
1¼perfect). Only starch digestibility exhibited a correlation
(p¼.0189) with dietary corn content; a strong linear associ-
ation (r¼0.754) was observed, meaning that canine ATTD
of starch increased with increased corn in the diet.
Digestibility of DM (r¼0.395, p¼.2291), crude protein
(r¼0.332; p¼.3490), fat (r¼0.033; p¼.9282), and gross
energy (r¼0.244; p¼.5263) did not exhibit significant
correlations with corn content. While this analysis cannot
determine causation, specifically if increasing dietary corn
content is detrimental to nutrient digestibility, it does show
that dietary corn content does not have a significant rela-
tionship with most macro-nutrients except for a strong posi-
tive linear relationship with starch digestibility.
Table 2. Dry matter (DM) apparent total tract digestibility (ATTD) by adult dogs fed diets containing at least 30% corn.
Authors Corn ingredient
; inclusion, % Dog breed Diet type Digestibility method
DM ATTD, %
Wolter et al. (1978) Corn; 50.0 Beagle Wet food TFC 91.9
Walker et al. (1994) Maize; 67.0 Mature mongrel Extruded kibbles TFC 87.2
Murray et al. (1999) Corn flour; 43.6 Hound bloodline Extruded kibbles CO 85.4
Gajda et al. (2005) Conventional corn; 31.9 Hound bloodline Extruded kibbles TFC 77.1
Gajda et al. (2005) High-protein corn; 32.0 Hound bloodline Extruded kibbles TFC 75.5
Gajda et al. (2005) High-protein, low phytate corn; 32.4 Hound bloodline Extruded kibbles TFC 77.6
Gajda et al. (2005) High-amylose corn; 33.2 Hound bloodline Extruded kibbles TFC 60.6
Gajda et al. (2005) Amylomaize, 26.0 Hound bloodline Extruded kibbles TFC 66.9
Twomey et al. (2002) Maize; 53.5 Mixed-breed Extruded kibbles Celite 92.0
Carciofi et al. (2008) Corn; 53.5 Mixed-breed Extruded kibbles CO 78.6
Kore et al. (2009) Maize; 70.5 Spitz Pressure cooked TFC 83.8
Fortes et al. (2010) Maize; 66.7 Mixed-breed Extruded kibbles TFC 84.9
Bazolli et al. (2015) Maize ground at 360 um ; 53.5 Beagle Extruded kibbles CO 80.5
Bazolli et al. (2015) Maize ground at 451 um ; 53.5 Beagle Extruded kibbles CO 82.1
Bazolli et al. (2015) Maize ground at 619 um ; 53.5 Beagle Extruded kibbles CO 75.9
Schauf et al. (2018) Maize; 34.0 Beagle ND
Domingues et al. (2019) Corn; 30 Beagle Extruded kibbles TFC 81.7
As described by each author.
TFC, total fecal collection; CO, chromic oxide marker method; Celite, celite marker method.
ND, not defined.
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 3
The starch contained in the corn endosperm can be classi-
fied into digestible or resistant to digestion. Digestible
starches completely disappear by the end of the ileum, and
the undigested fraction, called resistant starch (RS), is read-
ily fermented by bacteria in the colon. The most studied fer-
mentation products are short chain fatty acids (SCFA)
acetate, propionate, and butyrate. These SCFA can shift the
microbiota toward more beneficial bacteria in the colon,
promote trophic effects on the gastrointestinal tract (GIT)
mediated by GLP-2 (NRC 2006), and support the GIT
immune function and homeostasis (Corr^
ea-Oliveira et al.
2016). Butyrate is almost entirely used by colonocytes as an
energy source, whereas acetate and propionate are trans-
ported to the liver through the portal vein (Haenen et al.
2013) and converted to energy substrates. Cutrignelli et al.
(2009) found that corn fermentation pattern using dog fecal
inoculum was similar to most carbohydrate sources (rice,
potato, spelt) in acetate, propionate, and butyrate production
(approximated ratio 5:5:1 acetate, propionate, and butyrate).
Cooking disrupts the compact crystalline structure of raw
granules and improves starch digestion (Murray et al. 2001).
Cereals like corn do not contain a high level of naturally
occurring RS (type II) like legumes and some tubers (Dhital
et al. 2017), but some may be preserved or developed upon
processing. Pet food extrusion is considered a medium
moisture/high energy cooking process (Rokey 2000) which
fully gelatinizes starch (Murray et al. 1999) and yields low
RS in the kibble (Corsato Alvarenga and Aldrich 2020).
When extruded at low mechanical energy, corn-based pet
foods might retain a small amount of RS and benefit the
colonic health of both dogs (Peixoto et al. 2018; Ribeiro
et al. 2019; Jackson, Waldy, Cochrane, et al. 2020) and cats
(Jackson, Waldy, and Jewell 2020). Other less intensive
cooking processes like baking were observed to have lower
starch gelatinization compared to extrusion (Gibson and
Alavi 2013). Thus, baked kibbles should retain more RS
than the same extruded recipe, but this must be confirmed.
The extent of corn grinding (before extrusion) also affects
its utilization. Bazolli et al. (2015) found that kibbles with
coarse maize (ground at 521 mm mean geometric diameter)
tended to be less digested by dogs. They noted that the por-
tion that escaped digestion was fermented and produced
feces with lower pH and more butyrate in comparison to
the same diet extruded with fine corn (277 mm mean geo-
metric diameter). Thus, these authors found benefits from
under-processing corn. Peixoto et al. (2018) also found that
a diet produced with coarser corn ground at 312 mm mean
geometric diameter (vs. 224 mm mean geometric diameter of
fine corn) and a lower extruder specific mechanical energy
(SME) of 11.6 kW h/ton (vs. high SME of 21.5 kW h/ton)
benefited colonic health of geriatric Beagle dogs.
Additionally, the diet with less processing had a tendency to
increase gastrointestinal mucosa crypt depth, which suggests
an improvement in nutrient absorption. Ribeiro et al. (2019)
produced diets in a similar manner as Peixoto et al. (2018)
and confirmed that dogs fed a diet with coarsely ground
corn (312 mm mean geometric diameter) extruded at
11.4 kW h/ton SME had greater fecal SCFA.
Some marketing claims suggest that high protein diets
are better for pets, but this depends on factors such as pro-
tein quantity, quality, and the amino acid profile. Hang
et al. (2013) reported benefits in feeding a high corn diet vs.
high protein greaves-meal to dogs. Dogs fed the high corn
diet produced firm feces with low ammonia (835 mg/g of wet
feces) and neutral fecal pH (7.2). These were similar to dogs
fed a dry commercial diet, and lower than those fed the
high protein greaves-meal diet (fecal ammonia 1191 mg/g of
wet feces and pH 7.5; Hang et al. 2013). A low fecal ammo-
nia and lower pH could indicate that less protein reached
the colon for putrefaction. Other indicators of protein putre-
faction like polyamines may be attenuated by the addition of
a fiber bundle to the diet (Jackson and Jewell 2019). Dogs
fed a corn-rich diet also produced 30% more fecal SCFA
than a diet rich in greaves-meal protein (Hang et al. 2013).
Thus, from a gastrointestinal health perspective, a diet rich
in fiber and resistant starches might be preferred over a
high protein food resulting in undigested proteins reaching
the large intestine, which are fermented into unhealthy
nitrogen compounds like indoles and phenols.
Corn and some foods derived from it are considered to be
high in the carotenoids lutein and zeaxanthin (Masisi et al.
2015). Lutein and zeaxanthin are xanthophylls that have
been related to eye protection in humans against age-related
macular degeneration due to their antioxidant capacity.
Perry, Rasmussen, and Johnson (2009) measured xantho-
phylls in corn and corn products and found that yellow
corn was high in trans zeaxanthin (531 ppm) and was com-
parable to cooked egg yolks (587 ppm). Conversely, Moreau,
Johnston, and Hicks (2007) reported that lutein and zeaxan-
thin in whole ground corn was much lower (2.63 and
4.59 ppm, respectively). Carotenoids like lutein have been
found beneficial to dogs and cats. For example, Kim et al.
(2000a) fed dogs crystalline lutein at 5, 10, and 20 mg/d and
detected a significant improvement in the immune system
after just two weeks. Kim et al. (2000b) also found that crys-
talline lutein improved humoral and cellular immune
responses in cats.
Corn alone would likely not reach the target xanthophyll
levels necessary to improve the immune response. A level of
5 mg of lutein would be required and whole corn was
reported to have 2.63 ppm lutein, which would mean that a
dog would need to consume >19 kg of corn per day to have
an immune effect. While concentrations for a measurable
effect are not there today, some co-products of corn may
provide a future benefit.
The process of hydrolyzing cereal protein to enhance
antioxidant activity has been well explored in the last dec-
ade. Zhuang, Tang, and Yuan (2013) hydrolyzed corn gluten
meal (60% crude protein) with alkaline protease and meas-
ured antioxidant activities of different hydrolysate sizes.
They observed that free radical scavenging, metal ion chelat-
ing activities and lipid peroxidation inhibition increased as
4 I. C. ALVARENGA ET AL.
hydrolysate size decreased, and was greatest with hydrolysate
molecular weight less than 10 kDa. Zhou et al. (2015) also
found that the antioxidant activities of corn gluten meal
hydrolyzed proteins was highly correlated to small peptide
molecules and antioxidative amino acids such as tyrosine,
lysine, histidine, and methionine. More specifically, Li, Han,
and Chen (2008) reported that the highest antioxidant activ-
ity of corn gluten meal hydrolysates occurred when peptides
were in the range of 500–1500 kDa. Hence, corn has the
potential to produce functional food ingredients high in
antioxidants derived from hydrolyzing proteins from corn
gluten meal, and(or) from concentrating carotenoids.
Consumers may perceive that pet foods produced with corn
and other grains are allergens. Interestingly, the prevalence
is very low. Verlinden et al. (2006) compiled data from
seven studies and concluded that most food allergies were
caused by animal protein (36% beef, 28% dairy, 10% egg,
9.6% chicken, 4% pork, 1% rabbit, and 1% fish), 15% were
caused by wheat, and none by corn. More recently,
Laflamme et al. (2014) reported that animal proteins were
involved in most cases of food allergy in dogs, while wheat
had a prevalence of 15%, and no cases (N¼198) of food
allergenicity were due to corn consumption. In France,
Carlotti, Remy, and Prost (1990) gathered veterinarian
reports of 33 cases of canine food allergy and, again, no dog
on the research reviewed was allergic to corn. Clearly the
public perception does not match the data.
Mycotoxins & genetically modified corn
Corn is a functional ingredient that provides nutrition to
pet foods, but some issues exist that cannot be overlooked.
Mycotoxins are naturally occurring secondary metabolites of
fungal metabolism that can easily grow in cereals under cer-
tain conditions. These toxins are known to cause severe
acute or chronic health conditions in both animals and
humans, even at low concentrations. In pet foods, the most
problematic mycotoxins are aflatoxin, fumonisin, and deoxy-
nivalenol, among others (Atungulu, Mohammadi-Shad, and
Wilson 2018). The Food and Drug Administration (FDA)
issued guidelines and allowances for aflatoxins in corn used
in pet foods, for a maximum legal limit of 20 ppm. A recent
study found that mycotoxins were present in dry commer-
cial grain-based diets but were below the threshold estab-
lished by the FDA (Tegzes, Oakley, and Brennan 2019).
They concluded that more long-term studies should be con-
ducted to assess the effect of chronic low-dose intake of
There have been pet food recalls due to mycotoxin con-
tamination around the world, but none in the US since 2001
(Atungulu, Mohammadi-Shad, and Wilson 2018). This is
mainly because pet food companies are more aware of safety
measures needed to prevent fungal growth and, therefore,
the release of mycotoxins to the food. These control points
can be implemented during pre-harvest, post-harvest and
storage of grains, which are well described by Atungulu,
Mohammadi-Shad, and Wilson (2018). It is not the focus of
this review to provide a full description of mycotoxins, but
to acknowledge that this concern exists and that mycotoxins
are controlled by pet food and ingredient companies
through testing in order to be in compliance with the FDA
and Food Safety Modernization Act (FSMA) regulations.
Although genetically modified (GM) corn is viewed as a
negative by some consumers, GM corn may have additional
benefits when compared to traditional corn. For instance,
the most common GM corn in the US, the Bt corn, has a
gene that encodes for a protein derived from Bacillus thurin-
giensis that controls lepidopteran insect infestations
(Hammond et al. 2004). The GM Bt corn also has a lower
incidence of mycotoxin contamination and, therefore, con-
tributes to a reduction in crop losses and positively impacts
the economy (Hammond et al. 2004).
The controversial topic of GM cereals was explored in
depth by Domingo (2016). According to this review, the
vast majority of studies conducted in the last decade regard-
ing GM corn showed no adverse health effects in rats, mice,
or miniature pigs. The few studies that reported issues with
GM corn consumption were suspect, which led Domingo
(2016) to suggest a need for more long-term safety studies
assessing mutagenicity, carcinogenicity and teratogenicity.
Secondary products from corn: application in pet
foods and their benefits to pet health
Many sources of corn used in dog and cat foods are second-
ary products from corn processing for human food or bio-
fuels. Corn as an industrial input is processed by three
methods with different primary and secondary finished
products: wet milling, dry milling, and dry grind. The nutri-
ent composition (Table 3) of the secondary products and
utilization by the animal will be heavily influenced by these
Starch is the primary product from the wet milling pro-
cess and can be converted into fuel ethanol, high-fructose
corn syrup, or modified starches. The purified starch can
also be included in a pet food formulation. Compared to the
whole corn kernel, corn starch is nearly 100% nitrogen free
extract (NFE) with minute amounts of ash, crude protein
(CP), and acid-hydrolyzed fat (AHF; Bednar et al. 2001). In
their study, Bednar et al. (2001) reported that the starch
component of the ingredient was 70.0% rapidly digestible,
20.0% slowly digestible, and 7.9% resistant to digestion. This
translates into very high starch digestibility with a rapid
release of energy in the form of glucose. As such, corn
starch could be included in diets to provide rapidly avail-
Secondary products from corn wet milling include corn
gluten meal, corn germ, and corn gluten feed. Corn gluten
meal is a concentrated protein source with low amounts of
ash, CP, AHF, and TDF and moderately low amounts of
NFE (de Godoy et al. 2009; Smith 2018;Table 3). Corn glu-
ten meal has been included in extruded experimental diets
(34.6% of the diet) for cats (Funaba et al. 2005). Dry matter
ATTD was lower than for cats fed meat meal or chicken
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 5
meal (Table 4). Their feces were more moist, which suggests
corn gluten meal may have a higher water holding capacity
than the animal proteins. However, cats fed the corn gluten
meal diet had more acidic urine (pH 6.08), and fewer stru-
vite crystals. One item of note –corn gluten meal as the pri-
mary protein source may require supplementation
Compared to corn gluten meal, corn germ meal is lower
in CP and gross energy (GE), but higher in TDF and ash
and similar in AHF. An assay utilizing cecetomized roosters
to measure amino digestibility found that corn germ meal
was less digestible than corn gluten meal in terms of total
amino acids (79.0% vs. 95.4%), essential amino acids (80.4%
vs. 94.3%), and non-essential amino acids (76.8% vs. 97.0%)
and could be influenced by the greater fiber content (de
Godoy et al. 2009). However, greater protein efficiency for
chicks fed corn germ meal compared to corn gluten meal
(2.83 vs. 0.76, respectively) suggests that corn germ meal
may have a more favorable amino acid profile (de Godoy
et al. 2009).
Corn gluten feed is similar to corn germ meal in that it
contains moderate amounts of TDF and CP. Increasing the
level of corn gluten feed in an extruded diet for dogs
decreased the digestibility of all nutrients (Kawauchi et al.
2011;Table 4), indicating this ingredient could act as a fiber
source in complete diets. Very little research has compared
the use of corn gluten feed to similar ingredients in pet
diets. However, research has been published assessing the
benefits of using the fiber stream often incorporated into
corn gluten feed. Depending on the process used to obtain
the corn fiber product, TDF can vary from 63.0% to 88.2%
on a DMB (Guevara et al. 2008). Adult Beagles dogs fed
diets containing 7% of beet pulp, native corn fiber with or
without fines, hydrolyzed corn fiber, or hydrolyzed extracted
corn fiber did not show differences in food intake, fecal
excretion, stool quality, AHF ATTD (83.8–94.7%), and CP
ATTD (81.2–83.5%). Using native corn increased DM
ATTD (82.3%; Table 4) compared to the other fiber sources,
including beet pulp. Additionally, TDF ATTD by dogs of
the native corn fiber diet was higher than the native corn
fiber with fines and hydrolyzed corn fiber diets (30.9%,
19.1%, and 17.8%, respectively), with the beet pulp and
hydrolyzed extracted corn fiber diets intermediate (average
23.9%). While minor differences were observed, these corn
Table 4. Dry matter (DM) apparent total tract digestibility (ATTD) by adult pets fed diets containing a corn co-product.
Authors Corn ingredient; inclusion, % Species, breed Diet type Digestibility method
DM ATTD, %
Guevara et al. (2008) Corn fiber, hydrolyzed; 7.0 Dog, beagle Extruded kibbles CO 94.2
Guevara et al. (2008) Corn fiber, hydrolyzed extracted; 7.0 Dog, beagle Extruded kibbles CO 96.4
Guevara et al. (2008) Corn fiber, native; 7.0 Dog, beagle Extruded kibbles CO 92.1
Guevara et al. (2008) Corn fiber, native with fines; 7.0 Dog, beagle Extruded kibbles TFC 87.4
Kawauchi et al. (2011) Corn gluten feed; 7.0 Dog, beagle Extruded kibbles TFC 79.4
Kawauchi et al. (2011) Corn gluten feed; 14.0 Dog, beagle Extruded kibbles TFC 75.2
Kawauchi et al. (2011) Corn gluten feed; 21.0 Dog, beagle Extruded kibbles TFC 72.8
Funaba et al. (2005) Corn gluten meal; 34.6 Cat, mixed-breed Extruded kibbles TFC 77.7
Smith (2018) Corn gluten meal; 20.5 Dog, beagle Extruded kibbles TD 83.4
Silva et al. (2016) Distillers dried grains with solubles; 6.0 Dog, beagle Extruded kibbles TFC 85.2
Silva et al. (2016) Distillers dried grains with solubles; 6.0 and xylanase Dog, beagle Extruded kibbles TFC 84.1
Silva et al. (2016) Distillers dried grains with solubles; 12.0 Dog, beagle Extruded kibbles TFC 82.6
Silva et al. (2016) Distillers dried grains with solubles; 12.0 and xylanase Dog, beagle Extruded kibbles TFC 81.5
Silva et al. (2016) Distillers dried grains with solubles; 18.0 Dog, beagle Extruded kibbles TFC 83.2
Silva et al. (2016) Distillers dried grains with solubles; 18.0 and xylanase Dog, beagle Extruded kibbles TFC 80.6
Smith (2018) Next-generation distillers dried grains; 25.0 Dog, beagle Extruded kibbles TD 78.2
CO, chromic oxide marker method; TFC, total fecal collection method; TD, titanium dioxide marker method.
Table 3. Nutritional composition of corn co-products used in various pet food studies.
Nutrient, dry matter basis
Authors Corn co-product DM, % Ash, % CP, % AHF, % TDF, % NFE
, % GE, kcal/kg
Guevara et al. (2008) Corn fiber, hydrolyzed 94.2 0.5 12.0 6.8 79.9 0.8 4900.0
Guevara et al. (2008) Corn fiber, hydrolyzed extracted 96.4 0.5 10.8 2.4 88.2 0.0 4700.0
Guevara et al. (2008) Corn fiber, native 92.1 1.0 12.0 5.6 71.1 10.3 4800.0
Guevara et al. (2008) Corn fiber, native with fines 87.4 0.7 14.1 4.9 63.0 17.3 4800.0
de Godoy et al. (2009) Corn germ meal 90.1 3.9 28.4 6.0 45.0 16.7 4559.0
Kawauchi et al. (2011) Corn gluten feed 89.4 6.7 24.9 4.5 42.7 12.5
de Godoy et al. (2009) Corn gluten meal 90.5 1.8 73.9 7.8 0.3 16.2 5743.0
Smith (2018) Corn gluten meal 89.8 1.1 74.7 1.8
Bednar et al. (2001) Corn flour 92.1 0.6 11.2 2.6 2.8 84.3
Bednar et al. (2001) Corn starch 91.2 0.2 0.6 0.9 0.0 102.5
de Godoy et al. (2009) Distillers dried grains with solubles 91.8 4.3 27.6 15.2 30.5 22.4 5175.0
Silva et al. (2016) Distillers dried grains with solubles 90.8 2.0 30.1 9.0 9.3
Smith (2018) Next-generation distillers dried grains 93.3 4.6 54.4 4.2
DM, dry matter; CP, crude protein; AHF, acid-hydrolyzed fat; TDF, total dietary fiber; GE, gross energy.
Calculated as 100 –(ash þCP þfat þfiber).
Reported as starch content.
Reported as crude fat.
Not reported in the study.
Reported as crude fiber.
6 I. C. ALVARENGA ET AL.
fibers appear to have a similar value in pet foods as a fiber
option to beet pulp, a readily accepted and well researched
The primary products from dry milling are flaking grits,
which are commonly used in the human food industry as
corn flakes. Secondary products from this process include
corn flour and hominy feed. Corn flour is predominantly
used in baked products with high NFE, and low CP (Bednar
et al. 2001). Hominy feed contains germ and bran and is
commonly included in livestock diets (MacGregor, Sniffen,
and Hoover 1978). There is no information regarding the
use of corn flour or hominy feed in pet diets. There might
be potential for them in extruded or baked pet
Dry grind corn processing is primarily directed toward
ethanol production and produces distillers dried grains with
or without solubles as secondary products. Solubles are the
product resulting from the evaporation of water from the
thin stillage distillation process effluent. These solubles are
usually added to the distillers wet grains before drying.
Because distillers dried grains with solubles (DDGS) is a sec-
ondary product, the composition fluctuates between 27.6%
and 30.1% CP, 9.0% and 15.2% AHF, and 2.0% and 4.3%
ash, on a DMB (de Godoy et al. 2009; Silva et al. 2016).
Some evaluations in pet food have been reported, but the
acceptance by pet owners has been limited and the greatest
use occurs in value-based pet foods.
The protein in DDGS is similar to that in corn germ
meal, with comparable amino acid digestibility and protein
quality rankings (2.63 vs. 2.83, respectively; de Godoy et al.
2009). Increasing the level of DDGS from 0% to 18%
resulted in reduced ATTD (Table 4; Silva et al. 2016). Fecal
pH was more acidic when DDGS were included in the diet.
This suggests that the diet containing DDGS provided sub-
strate for colonic fermentation and supported gastrointes-
tinal health. Interestingly, as the level of DDGS increased,
dogs preferred the diet over the control, indicating ready
acceptance. Clearly, DDGS could be a viable protein source
for pet foods with the potential to provide fermentable fibers
and improve pet gut health if consumer attitudes were
Recently, modified DDGS with elevated protein has
become available. This “next-generation”distillers dried
grains (NG-DDGS) were incorporated into a dog food at
25% and fed to adult Beagle dogs (Smith 2018). They
observed that dogs fed NG-DDGS had lower DM (Table 4),
organic matter, crude fat, crude fiber, and GE digestibility
than those fed a diet containing soybean meal or corn glu-
ten meal. Although the feces of these dogs were of accept-
able firmness and consistency and palatability was similar to
soybean meal. Cats preferred the diet with NG-DDGS or
soybean meal relative to corn gluten meal. These results sug-
gest that NG-DDGS may be used with success in diets that
traditionally contain soybean meal or corn gluten meal.
Even though secondary products from corn processing
have been stigmatized as low-quality by-products, published
research suggests otherwise. Depending on the specific
ingredient, secondary products from corn can be quality
sources of concentrated protein and/or fiber. They can also
contribute more targeted benefits, such as high quality pro-
tein, energy, or fiber. They may also add to the sustainability
Processing of pet foods with corn
The majority of dogs and cats are fed extruded dry kibbles,
with market estimates of $72.64 billion worldwide by 2022
(Research and Markets 2017). Pet food extrusion is a com-
plex and versatile process that involves cooking under pre-
determined conditions of pressure, moisture, mechanical
and thermal energies. Nearly all extruded pet food is formu-
lated to be nutritionally complete and balanced to meet the
animals’nutrient requirements by combining ingredients
with different physical and nutritional attributes.
The macro-ingredients in a diet have an impact on extru-
sion performance. According to the Guy Classification
System (Guy 2001), ingredients can be structure-forming,
dispersed phase fillers, or plasticizers. Structure-forming
ingredients like starches and (or) vegetable proteins promote
binding, homogenization, and structuring of the dough.
Starchy ingredients like ground corn or corn flour are the
most common structure-forming ingredients. At the other
end of the spectrum, ingredients classified as dispersed
phase fillers or plasticizers do not contribute to the physical
matrix formation and can negatively affect the structural
quality of the dough. Thus, corn by-products high in fiber
or high in overly processed proteins like corn gluten meal
tend toward dispersed phase fillers.
Prior to extrusion processing, corn, along with the other
dry ingredients, must be ground. The particle size may
influence the outcome. For example, coarsely ground corn
using a sieve size 3.0 mm was reported to produce less
expanded, denser kibbles (438 g/L) due to low particle sur-
face area to mass compared to a finely ground corn using a
sieve size 0.8 mm (Bazolli et al. 2015). Mathew, Hoseney,
and Faubion (1999b) also reported lower kibble expansion
and a 6% decrease in SME from when corn was ground
using a hammermill screen size of 1.5 mm when compared
to 0.75 mm sieve size. During pre-conditioning and cooking
in the extruder barrel, water and heat (energy) promote
starch gelatinization, which increases cold water solubility
and starch viscosity, and releases amylose during pasting
(Cheftel 1986). The increase in viscosity due to gelatiniza-
tion aids in binding and homogenization. The coarser the
corn, the less gelatinized the kibble will be (Ribeiro et al.
2019; Bazolli et al. 2015). Phase transition of the starch poly-
mer from glassy to rubbery and, finally, melt occurs inside
the extruder barrel (Moraru and Kokini 2003). When the
extrudate reaches the end of the barrel, the air bubbles were
got entrapped during the process grow as the melt leaves
the die due to moisture vaporization, where the high pres-
sure of the water nuclei overcomes the mechanical resistance
of the melt, and expansion occurs (Moraru and Kokini
2003). Liu et al. (2006) found that as low as 15% corn flour
increased expansion of a corn and oat flour blend. Some
expansion is required in pet foods to produce a kibble with
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 7
the right appearance and texture. Conversely, kibbles with
less starch and high dietary fiber tend to expand less
(Alvarenga and Aldrich 2018) and to be less palatable
(Koppel et al. 2015).
The amylose:amylopectin ratio plays an important role in
extrusion processing. Waxy starches (low amylose, high
amylopectin) have a tendency to form a sticky paste with a
light, elastic, and homogeneous texture, whereas starches
with higher amylose content form a harder and less
expanded extrudate (Moraru and Kokini 2003).
Chinnaswamy and Hanna (1988) reported that the expan-
sion ratio of corn starch almost doubled as amylose content
increased from 0% to 50%, but decreased with further
increases in amylose. Thus, around 50% amylose in the
starch granule led to the greatest expansion. Different corn
genotypes may also vary in fiber and protein, which can fur-
ther influence extrusion. In a study evaluating three different
corn samples with similar grinding and extrusion parame-
ters, corn variety significantly affected expansion, breaking
strength, and bulk density of a pet food (Mathew, Hoseney,
and Faubion 1999a). Thus, the corn variety, growing condi-
tions, the amylose:amylopectin ratio, and the amounts of
each nutrient may influence pet food processing.
Medium moisture and high-heat extrusion, which is com-
mon for pet foods, results in near complete gelatinization of
the starch with a significant increase in its digestibility (Dust
et al. 2004; Murray et al. 2001). Murray et al. (2001)
reported that high temperature relative to low temperature
extrusion increased the rapidly digested starch fractions of
corn, and decreased slowly digested and resistant starch
fractions. This finding means that the more starch is cooked,
the easier it is digested. Milder cooking processes like baking
and pelleting of kibbles have been reported to result in less
starch gelatinization when comparing the same extruded
Inal et al. 2018; Gibson and Alavi 2013). Pelleting is
not a common process for dog food and pellets have been
reported to be less palatable than extruded kibbles (
et al. 2018). This is likely because of the lower starch gelat-
inization and different texture of pellets. Wolter et al. (1978)
reported that the extent of cooking and gelatinization of the
corn starch improved palatability.
Wet or canned pet foods are another common product in
the market, but studies exploring different aspects of corn
in canning are scarce. Corn starch can be used as a binder
in canned loaf format products or to create structured pieces
in chunks and gravy recipes. Similarly, a study with buffalo
meat nuggets used corn starch as a binder and reported that
it produced a more stable emulsion than wheat semolina,
wheat flour, or tapioca starch (Devadason, Anjaneyulu, and
Babji 2010). This same study found that corn starch contrib-
uted to a firmer texture and more chewiness of the product,
resulting in higher sensory scores and overall acceptability
by humans. In canned pet foods, stronger binders like egg
white or porcine plasma are often used instead.
Studies using ground corn as the main starch source in
baked pet foods or treats are nonexistent. Corn does not
contain functional gluten and, therefore, does not bind as
efficiently as wheat flour, but might produce quality baked
kibbles if a complementary binding ingredient were identi-
fied to replace the gluten. The high starch content that pro-
motes particle binding and matrix formation in corn-based
pet foods has been evaluated extensively and reported to
provide a quality functional ingredient.
Corn has many benefits when included in pet foods. It has
been reported to be nutritionally available, lead to quality
stools and regular elimination, promote palatability, and
possess functional benefits to extrusion. But research is lim-
ited to extruded products. Work describing corn’s impact
on processing of treats, raw foods, thermally processed wet
foods, or alternatively processed foods is very limited. While
extruded products represent the majority of pet food prod-
ucts sold in the US, these alternative formats are popular
and may represent new markets for corn.
This work was commissioned by the Kansas Corn Commission.
This work was supported by the Kansas Corn Commission.
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