Effects of extrusion processing on nutrients in dry pet food

Abstract

Extrusion cooking is commonly used to produce dry pet foods. As a process involving heat treatment, extrusion cooking can have both beneficial and detrimental effects on the nutritional quality of the product. Desirable effects of extrusion comprise increase in palatability, destruction of undesirable nutritionally active factors and improvement in digestibility and utilisation of proteins and starch. Undesirable effects of extrusion include reduction of protein quality due to e.g. the Maillard reaction, decrease in palatability and loss of heat-labile vitamins. Effects of extrusion processing on the nutritional values of feeds for livestock have been well documented. Literature results concerning effects of extrusion on dry pet foods, however, are scarce. The present review discusses the results of studies investigating the impact of extrusion cooking on the nutritional quality of dry pet foods. (c) 2008 Society of Chemical Industry.

Full text

Journal of the Science of Food and Agriculture J Sci Food Agric 88:1487–1493 (2008)
Mini-review
Effects of extrusion processing on
nutrients in dry pet food
Quang D Tran,1,2Wouter H Hendriks1and Antonius FB van der Poel1∗
1Animal Nutrition Group, Department of Animal Sciences, Wageningen University, Marijkeweg 40, NL-6709 PG Wageningen,
The Netherlands
2Human and Animal Physiology Group, Biology Faculty, Vinh University, 182 Le Duan street, Vinh, Vietnam
Abstract: Extrusion cooking is commonly used to produce dry pet foods. As a process involving heat treatment,
extrusion cooking can have both beneficial and detrimental effects on the nutritional quality of the product.
Desirable effects of extrusion comprise increase in palatability, destruction of undesirable nutritionally active
factors and improvement in digestibility and utilisation of proteins and starch. Undesirable effects of extrusion
include reduction of protein quality due to e.g. the Maillard reaction, decrease in palatability and loss of heat-
labile vitamins. Effects of extrusion processing on the nutritional values of feeds for livestock have been well
documented. Literature results concerning effects of extrusion on dry pet foods, however, are scarce. The present
review discusses the results of studies investigating the impact of extrusion cooking on the nutritional quality of
dry pet foods.
2008 Society of Chemical Industry
Keywords: extrusion; protein denaturation; reactive lysine; starch gelatinisation; pet food; palatability
INTRODUCTION
Extrusion cooking technology is commonly used for
the manufacture of commercial dry canine and feline
diets: about 95% of dry pet foods are extruded.1In
this processing technology a mixture of ingredients
is steam conditioned, compressed and forced through
the die of the extruder.2The reason for the widespread
use of extrusion cooking to produce pet diets is
the versatility of this technology to mix diets and
functionally improve, detoxify, sterilise and texturise a
large variety of food commodities and food ingredients.
A combination of moisture, pressure, residence time,
temperature and mechanical shear is used for these
reactions and transformations.2,3
Extensive reviews3–6 on the effects of extrusion
cooking on product quality have been published. As
a thermomechanical treatment, extrusion cooking can
affect characteristics of extruded products (extrudates
or kibbles) by changing the digestibility or utilisation
of nutrients such as proteins, carbohydrates, lipids
and vitamins. In addition, denaturation of proteins,
alteration of carbohydrate structure, oxidation of
lipids and Maillard reactions between different food
components can alter the nutritional quality of
extrudates. Published reviews discussing the effects
of extrusion cooking technology on product quality
have been mostly restricted to dietary ingredients,
weaning pig feeds, meat replacers, livestock feeds and
dietetic foods. Although the effects of process variables
during extrusion have been widely recognised,3the
precise effects of extrusion cooking for its application
to companion animal foods are not well documented.
The present contribution discusses results of studies
investigating the effects of extrusion processing on
the nutritional quality of dry pet foods and provides
recommendations for further studies to control the
extrusion process variables in order to optimise the
nutritional quality of dry pet foods.
EFFECTS OF EXTRUSION ON STARCH
Companion animal diets may contain up to 50%
starch, which is derived mainly from cereal grains.1
The starch in cereal grains is organised in concentric
layers of semicrystalline or amorphous regions in the
endosperm. The structural features and components
that are associated with the starch granule, such as
lipids, minerals, proteins and non-starch components,
have been clearly reviewed.4Extrusion cooking causes
swelling and rupture of the granules, modification of
the crystalline spectra, increase in cold water solubil-
ity, reduction in viscosity of the starch and (partial to
complete) release of amylose and amylopectin.3When
extruded at low moisture content, starch granules
are partially transformed through the application of
heat (loss of crystalline structure) and shear (granular
fragmentation), leading to formation of a homoge-
neous phase called a starch melt or ‘gelatinisation’.4,7
∗Correspondence to: Antonius FB van der Poel, Animal Nutrition Group, Department of Animal Sciences, Wageningen University, Marijkeweg 40, NL-6709 PG
Wageningen, The Netherlands
E-mail: thomas.vanderpoel@wur.nl
(Received 14 November 2007; revised version received 6 February 2008; accepted 11 February 2008)
Published online 12 May 2008;DOI: 10.1002/jsfa.3247
2008 Society of Chemical Industry. J Sci Food Agric 0022–5142/2008/$30.00

QD Tran, WH Hendriks, AFB van der Poel
Physical and chemical characteristics of extrudates are
affected by the viscosity of the food mixture in the
barrel,7where the viscosity is related to the degree
of starch gelatinisation (SGD) of the food mixture.
A high screw speed (400 rpm) during the produc-
tion of pet foods has been shown to decrease SGD,
where interactive effects have been found with the
initial lipid content.7During extrusion of starches,
factors such as temperature, moisture level before
extrusion, amylose content and lipid content may all
lead to structural modifications of starch granules.
These changes, however, may differ between cereal
and potato starches. Gelatinisation improves faecal
and ileal digestibility of tapioca starch but has no effect
on wheat starch digestibility.8In addition, digestible
starch increased in barley and corn (Table 1) but
was not changed in oat bran after extrusion.9High-
moisture and high-temperature extrusion results in
complete gelatinisation and a significant increase in
in vitro9,10 and in vivo4starch availability.
The phenomenon of retrogradation of starch as
a result of extrusion and downstream processes
has not been studied intensively. Retrogradation of
starch is the crystallisation of gelatinised starch in an
amorphous matrix, whereby amylose, as opposed to
amylopectin, has been found to be the most important
starch component. The level of retrograded starch
depends on the initial starch concentration, the starch
source and its resistance to digestion in the small
intestine.4In the large bowel, retrograded starch may
be fermented or excreted and thus displays fibre-like
properties (starch that is not accessible to pancreatic
α-amylase but is fermentable to produce short-chain
fatty acids and gases) in companion animals. Studies
have shown that certain retrograded starch sources
are readily fermented in the large bowel, producing
short-chain fatty acids. Meanwhile, other retrograded
starches are less fermentable, resulting in laxation
properties (improvement in bowel health, reduction in
transit time and increase in stool weight) in companion
animals.1For example, feeding dogs a diet with high
retrograded starch content increased faecal bulk1and
increased excretion of microbial matter. The starch
digestion rate and that of retrograded starch in vivo
vary considerably among diet ingredients commonly
used in pet food manufacturing.1,11 Since retrograded
starch is not only caused by the extrusion itself,4
research should be focused not only on extrusion
but also on downstream processes (drying, cooling)
during which retrograded starch can be formed.
This modification of a diet ingredient after extrusion
to contain more retrograded starch/soluble dietary
fibre can increase the production of short-chain fatty
acids (especially butyric acid), compounds known to
increase the colonic health of dogs.9,10 Murray et al.10
studied the factors that influence starch retrogradation
after gelatinisation of starches from cereals and potato.
These authors provide a basic procedure to obtain
retrograded starch after gelatinisation in an excess of
water and subsequent cooling and drying which can
be used in the manufacture of pet foods.
Extrusion may enhance the formation of complexes
of starch with lipids: the hydrophobic core of the
amylose molecule can trap the hydrocarbon chain of
lipid molecules to form a lipid – amylose complex.7
Complex formation with monoglycerides for example
may inhibit its digestion by amylase. Similarly,
complexes with other nutrients, e.g. proteins or amino
acids, can limit hydrolysis. Murray et al.,11 using ileal
chyme of dogs, reported that amylose –lipid complexes
were resistant to in vitro digestion when amylose was
complexed with long-chain saturated monoglycerides.
After extrusion, complex compositions in which
molecular interactions among different ingredients are
formed can have beneficial (e.g. palatability) as well as
adverse (e.g. nutrient loss) affects. Literature on the
separate effects of these interactions as a result of the
extrusion process is generally not well documented.
The extrusion variables to control retrograded starch
formation and its effect on glycemic index in relation
to canine and feline health should be subjects of
further research. Moreover, the nutritional effects of
interactions between starch and other nutrients during
extrusion in pet foods need further study.
EFFECTS OF EXTRUSION ON LIPIDS
The nutritional value of lipids from sources such as
tallow, poultry fat, vegetable oil, marine oil and various
blends can be affected during extrusion as a result
of hydrogenation, isomerisation, polymerisation and
lipid oxidation.2Lipid oxidation is a major challenge
to pet food preservation. Oxidation rate is affected by
many factors such as fat type, fat content, moisture
content and expansion degree, where the unsaturation
in fats increases the preservation challenge.12,13 In
addition, trace minerals, iron in particular, and the
use of biological antioxidants may play a significant
role in oxidation post-extrusion.12,13
Under specific extrusion conditions, complexes
of lipid–protein or lipid –starch can be formed.
For example, high-moisture and high-temperature
Table 1. Effects of ingredient extrusion on total, digestible and resistant starch (% DM)9
Total starch Digestible starch Resistant starch
Ingredient Control 80– 90 ◦C 120– 130 ◦C Control 80– 90 ◦C 120– 130 ◦C Control 80– 90 ◦C 120– 130 ◦C
Barley 78.4 78.4 81.2 27.7 47.5 50.4 50.7 23.1 20.4
Corn 80.2 82.8 83.0 36.7 45.6 58.9 43.5 37.2 24.1
Oat bran 67.2 65.1 66.4 42.7 39.9 37.6 24.5 25.1 28.8
1488 J Sci Food Agric 88:1487–1493 (2008)
DOI: 10.1002/jsfa

Effects of extrusion processing on nutrients in dry pet food
conditions can increase the hydrolysis of lipids, which
increases potential interactions with the side chains
of amino acids in proteins. Free fatty acids and polar
lipids are especially reactive in these situations. If
formation of amylose–lipid complexes does not occur
to a large extent, it will not impair the utilisation of
the fat. For example, amylose – lysolecithin complexes
were almost completely digested in rats.5
Literature concerning the effects of extrusion
on crude fat and fatty acids, especially in pet
food diets, is sparse. Extrusion of a feed mixture
showed no effect on the digestibility of nitrogen,
dry matter, fat and ash in six mature dogs.14 This
indicates that, if lipid complexes are formed during
extrusion of pet food, these complexes may be readily
digested. This is in accordance with the high lipid
digestibility commonly found in canine and feline
diets.15 Extrusion inactivated lipase and lipoxidase
present in foods, resulting in less oxidation of fatty
acids during storage.13 In particular, interactive effects
between process variables and lipids and effects on
lipid complexation during extrusion are emphasised
for future research.
EFFECTS OF EXTRUSION ON PROTEIN
The protein component in pet foods can constitute
between 25 and 70% of the dry matter (DM).2
This relatively high proportion is required as dogs
and cats are carnivorous by nature. The amino acid
composition, digestibility and subsequent availability
of amino acids in the protein define its nutritional
quality.15,16 Vegetable protein sources alone may not
supply sufficient essential amino acids, e.g. taurine and
other sulfur amino acids, in comparison with proteins
of animal origin.17 Addition of animal proteins to the
cereal-based ingredient is therefore often necessary to
provide a balanced dietary amino acid profile for cats
and dogs.
Effects of extrusion on the protein component can
be either beneficial or detrimental for the physical
and nutritional characteristics of the food mixture.
The thermal treatment during extrusion cooking can
inactivate protein-based nutritionally active factors by
destroying the integrity of their structure and hence
prevent their activities.18,19 Mild denaturation of pro-
teins can make them more susceptible to digestive
enzymes and therefore improve the digestibility of
these proteins.20 Enzymes (e.g. lipoxigenase, perox-
idase) present in pet foods can cause deteriorative
effects during storage and can be inactivated as well by
extrusion cooking.3The latter contributes to storage
stability and increases the shelf-life of dry pet foods.
Undesirable effects of heat treatment involve destruc-
tion of amino acids, racemisation of amino acids, inter-
and extra-peptide linkages and a number of chemical
reactions such as Maillard reactions and crosslink-
ing reactions of protein–protein, protein–lipid and
protein–carbohydrate complexes.5
Extrusion can result in an increase in protein
digestibility. Ega ˜
na et al.21 reported that diet extrusion
improved the digestibility of crude protein in growing
dogs compared with pelleting the diet. However,
Hull´
ar et al.15 and Øverland et al.16 found that the
digestibility of crude protein in cat and dog foods
is not affected by the extrusion process. In soybeans
and many other legumes, protein digestibility and
availability of (limiting) sulfur amino acids increase
via (i) thermal unfolding of the major seed globulins
and (ii) thermal inactivation of trypsin inhibitors
and lectins.18 An increase in temperature during
extrusion enhances the degree of inactivation of
protease inhibitors in wheat flour and thus increases
the protein digestibility of legume protein. In pet
foods, Bednar et al.22 studied the effect of processing
of various vegetable and animal protein sources on the
ileal nutrient digestibility in dogs and reported that
extruded and pelleted diets with different vegetable
protein sources provided adequate levels of highly
digestible protein and amino acids.
One of the main mechanisms responsible for a
reduction in protein quality as a result of extrusion is
the Maillard reaction, a non-enzymatic browning and
flavouring reaction involving the amino acid lysine
in particular. The Maillard reaction can be divided
into three stages: early, advanced and final. The early
stage is chemically a well-defined step where there is
no formation of coloured components. The advanced
stage comprises a variety of reactions leading to the
production of volatile or soluble substances, while
insoluble brown polymers (melanoidins) are formed
during the final stage. These products can provide
some flavouring and browning of foods. Several studies
have related the loss of lysine to physical process
parameters during extrusion of a model mixture.23
In general, parameters that promote the Maillard
reaction are temperature, moisture content, thermal
duration and pH value.24 Extrusion temperature and
duration appear to be the most important process
parameters for the Maillard reaction, with the reaction
rate increasing with an increase in both variables.
The temperature dependence of chemical reactions
is expressed as the activation energy in the Arrhenius
equation. With high activation energy the reaction rate
becomes more temperature-dependent. The product
temperature should be kept below 180 ◦C to minimise
losses in pet foods and other animal feeds.3
Lysine in particular can undergo several reactions,
including the classical Maillard reaction, during extru-
sion owing to its free ε-amino group. Reactive lysine,
a lysine molecule with a free ε-amino group as deter-
mined in the laboratory, can be used as a predictor for
the availability of lysine in vivo and can also serve as
an indicator for protein damage during extrusion.25,26
Recently, Williams et al.27 showed that there was a
large difference (up to 58%) between the total lysine
and O-methylisourea (OMIU)-reactive lysine content
of canine foods, indicating that the extrusion and sub-
sequent drying process may cause significant lysine
J Sci Food Agric 88:1487 – 1493 (2008) 1489
DOI: 10.1002/jsfa

QD Tran, WH Hendriks, AFB van der Poel
binding. In feline foods, Rutherfurd et al.28 measured
the difference between the total and OMIU-reactive
lysine in moist and dry diets and found a difference
of 20–50% for the dry diets, similar to the result
of Williams et al.27 Figure. 1 presents analysed data
on the relationship between total and OMIU-reactive
lysine content in canine and feline dry diets. Feline
diets appear to have a larger difference between total
and reactive lysine, which may be due to the smaller
kibble size of feline diets. Moreover, digestibility and
palatability are very important for cats and less so
for dogs. For that reason, pet food producers tend to
process more intensively to ensure that cat food has
achieved target digestibility and palatability. This may
be a main reason why reactive lysine and sulfur amino
acids (cysteine and methionine) are more limited in
cat food compared with dog food (Fig. 1). Ruther-
furd et al.28 determined the true ileal digestible total
and reactive lysine content using a rat bioassay and
observed a large overestimation of the available lysine
content such that the amino acid pattern relative to
lysine in these diets may not be optimal to promote
health. In addition to lysine, other amino acids such as
arginine, tryptophan, cysteine and histidine can also
be affected by the extrusion process. Of particular
importance may be the sulfur amino acids (cysteine
and methionine), which are often limiting in diets for
cats, as these amino acids are susceptible to oxidation.
A change in protein reactivity may also include the
racemisation or formation of non-nutritive D-amino
acids from their naturally occurring Lconfiguration.
This racemisation of amino acids impairs protein
nutritional quality (Table 2). It also enables the
formation of bonds that resist in vivo hydrolysis.
Also, limited attention has been paid in research to
differences in amino acid utilisation as a result of
protein interaction. During extrusion, proteins can
react with carbohydrates, lipids and their oxidation
products such as oxidised polyphenols, vitamin B6
and other additives.25
0
0
0.5
0.5
1
1
1.5
1.5
2
2
2.5
2.5
Total lysine (%)
OMIU-reactive lysine (%)
Feline
Canine
Figure 1. Total and OMIU-reactive lysine content in dry canine and
feline foods (after Williams et al.,27 Rutherfurd et al.28 and Hendriks
WH (unpublished)).
Table 2. Racemisation of amino acids upon expansion processinga
Aspartic acid form Glutamic acid form
Feed treatment LDD/LbLDD/Lb
Untreated (mash) 13.4 1.08 8 28.4 0.46 2
Expanded 10.9 1.21 11 25.1 0.46 2
Expanded/pelleted 10.8 1.23 11 24.7 0.46 2
aVan der Poel AFB, Fledderus J and Beumer H (unpublished).
bRatio of Dto L=D/L×100.
In conclusion, amino acid metabolism in dogs is
a very important issue. At least in cereal/soy-based
mixtures, loss of reactive lysine depends on extrusion
conditions,29 but hardly any literature has been found
on extrusion effects on animal proteins in dog diets.
In addition to protein interaction, the research focus
should emphasise the effects of extrusion on total and
reactive lysine, on other important amino acids such
as arginine, tryptophan, cysteine and histidine and on
the sensitivity to the damage of free lysine.
EFFECTS OF EXTRUSION ON VITAMINS
A number of vitamins are sensitive to physical and
chemical treatments. Vitamin stability depends on the
chemical structure of the vitamin in question and
can be decreased by exposure to heat, light, oxygen,
moisture and minerals. In general, literature on the
effects of extrusion on vitamins in animal diets is not
abundant. In an extensive review on the effects of
extrusion on (especially B group) vitamins in food
and feed products, Killeit30 showed a large variety
of extrusion effects on vitamin retention. As vitamins
differ greatly in chemical structure and composition,
their stability during extrusion is variable.29 The effects
of extrusion, however, were mainly destructive for
vitamins from the B group, vitamin A and vitamin E;
no data on the retention of vitamin D and vitamin K
were presented.30
In pet foods, extrusion cooking has been shown
to be detrimental to vitamin concentrations, with
oxidation being a main mechanism of degradation3
and the iron content of the food mixture catalysing
this oxidation. A summary of vitamin losses during
pet food manufacturing is presented in Table 3. It
is noted that the losses of vitamins are a result of
the extrusion process, including downstream processes
such as drying. From Table 3 it can be deduced that
the reported vitamin losses in pet foods appear to
be highly variable depending on the conditions used
during extrusion. Extrusion temperature may be a
decisive factor for vitamin retention, since an extrusion
temperature of 130– 135 ◦C32 led to higher losses
than an extrusion temperature of 107 ◦C31 (Table 3).
However, it is not elucidated whether a short retention
time during high-temperature extrusion results in
higher retention of vitamins. The results imply that
further studies on the effects of extrusion variables on
vitamins in the target foods for companion animals
1490 J Sci Food Agric 88:1487–1493 (2008)
DOI: 10.1002/jsfa

Effects of extrusion processing on nutrients in dry pet food
Table 3. Vitamin losses during extrusion of dry canine foods
Vitamin loss (%)
Vitamin Reference 31aReference 32bReference 44c
Vitamin A 20.0 65.0 9.5
Vitamin E 0.0 16.0 15.4
Thiamin (B1) 7.0 20.0 4.0
Riboflavin (B2)26.0 8.0 0.0
Vitamin B12 0.0 11.0 0.0
Folic acid 14.0 30.0 8.5
Pyridoxine (B6) 7.0 21.0 0.0
Niacin 21.0 30.0 0.0
Biotin 14.0 31.0 0.0
aExtrusion at 107 ◦C.
bExtrusion at 131– 135◦C.
cExtrusion temperature not given.
are needed. These studies should take into account
different forms of vitamins, which have been shown to
have different stabilities.32 The results of the vitamin
retention studies can be used in a strategy to enrich
dry pet foods with vitamins either before extrusion
(overdosing during mixing or use of improved stability
forms to compensate for processing and storage losses)
or after extrusion (downstream extrusion coating).30,33
EFFECTS OF EXTRUSION ON NUTRITIONALLY
ACTIVE FACTORS
The nutritional quality of certain dietary ingredients,
especially grain legumes, is such that their inclusion
levels in diets are limited owing to the presence
of nutritionally active factors (NAFs) that hamper
nutrient digestion or utilisation. Examples of these
NAFs are enzyme inhibitors, lectins and tannins.
Reduction or inactivation of these factors by means
of processing technology requires knowledge of their
type, distribution, chemical reactivity and thermal
sensitivity within the matrix of the seed. The
principles, adequacy and process optimisation for
these factors have been described previously.34
As a heat treatment, extrusion cooking inactivates
the activity of NAFs, especially those of a pro-
teinaceous structure.18,19 Extrusion cooking is the
most effective method to reduce the activity of
trypsin inhibitors,35 chymotrypsin inhibitors and α-
amylase inhibitors (Table 4). According to Bj ¨
orck
and Asp,5an extruder barrel temperature in the
range 133 –139 ◦C is sufficient to inactivate 95% or
more of trypsin inhibitors. At a constant tempera-
ture the inactivation of these factors increased with
a longer residence time and higher moisture con-
tent. Compared with traditional processing methods,
extrusion showed the largest effect in reducing levels of
several enzyme inhibitors and lectins,19,36 with a con-
comitant improvement in in vitro starch and protein
digestibility.19
Information on the content of other NAFs in
pet foods is limited. A number of studies have
reported high concentrations of phytoestrogens37,38
Table 4. Effects of extrusion cooking and soaking on nutritionally
active factorsain faba and kidney beans19
Treatment TI CTI α-AI HgA PA CT Pph
Vicia faba
Raw seeds 4.47 3.56 18.9 49.3 21.7 1.95 3.92
Soaking 4.27 3.41 16.1 49.3 14.6 1.02 3.73
Extrusion 0.05 1.68 0.0 0.2 15.9 0.89 2.80
Phaseolus vulgaris
Raw seeds 3.10 3.97 248 74.5 15.9 3.59 2.07
Soaking 2.93 3.37 220 74.5 15.0 2.72 1.64
Extrusion 0.43 0.00 0.0 0.2 12.6 0.58 1.12
aTI, trypsin inhibitors (IU mg−1DM); CTI, chymotrypsin inhibitors
(IU mg−1DM); α-AI, α-amylase inhibitors (IU mg−1DM); HgA,
haemagglutinating activity (HU mg−1DM); PA, phytic acid (g kg−1
DM); CT, condensed tannins (g equivalent catechin kg−1DM); Pph,
polyphenols (g kg−1DM).
and mycotoxins39 in canine and feline diets. Both
phytoestrogens and mycotoxins have been shown to be
physiologically active in cats and dogs. Routine extru-
sion technology, however, does not inactivate these
thermally stable NAFs,40 and other means such as
ingredient selection or use of absorption clays would
thereforebemoreappropriatewaystoreducetheir
concentration in pet foods.
Additional research should focus on elucidating
the fate of relatively heat-stable active factors after
extrusion and on studying co-processes such as
combined extrusion and application of enzymes.36
For example, as more pets become companions to
humans, their foods, especially dog foods, must be free
from oligosaccharides. However, apart from ingredient
processing methods such as cooking in water, no
literature was found on the inactivation of these
flatulence compounds such as stachyose, raffinose and
verbascose during extrusion of pet foods. In addition,
the contribution of single NAFs to nutritional effects
should be assessed properly, since several NAFs in
dietary ingredients are present simultaneously and act
synergistically to exert negative effects.36 Although the
inactivation of undesired factors during the thermal
processing of separate ingredients is well documented,
their inactivation when processing a complete pet diet
requires further research.
EFFECTS OF EXTRUSION ON DIET
PALATABILITY
In pet food production, palatability, which deals with
factors such as taste, aroma and mouthfeel (texture,
shape and particle size), is typically referred to as
a measured value of food preference and ingestive
behaviour. Diet palatability is a key factor in the
acceptance of a diet by a dog or cat and may be
influenced by a number of factors, including diet
nutrient composition, e.g. fat/carbohydrate ratio,41
and its processing.15
According to Kvamme and Phillips,42 extrusion may
play a role in affecting palatability by controlling the
J Sci Food Agric 88:1487 – 1493 (2008) 1491
DOI: 10.1002/jsfa

QD Tran, WH Hendriks, AFB van der Poel
level of specific mechanical energy (SME). Energy
added to the extrusion processing comprises two main
forms: thermal (from steam and water) and mechanical
(from the main drive motor). The mechanical energy
can be adjusted by hardware tools such as screw
configuration, die configuration and extruder speed,
and, with extra SME, palatability for cats increases.42
Dogs seemed to favour a more thermally cooked
product.43 As the thermal energy was increased, the
palatability increased for dogs.
Loss of palatability may be caused by risk factors
such as microbial growth, autoxidation and changes in
aroma and texture.12 Control of the expansion degree
thus seems important.13 In addition, shelf-life studies
should also include palatability testing, once a target
palatability is first accomplished after extrusion, drying
and other downstream processes.12
CONCLUSIONS
Extrusion cooking is a complex process involving
interrelations between process and product parameters
that affect the nutrient reactivity of product quality.
The most important process variables are temperature,
residence time, moisture and pH, which can be
controlled to achieve desired results.
Recent research on the effects of extrusion on
nutrients such as starch, proteins and lipids in pet
foods has been considered; in general, it appears
that relatively little is known about the effects of the
extrusion process (variables) on the quality of pet diets.
Among the effects of extrusion on pet foods are starch
gelatinisation, protein denaturation, vitamin loss and
inactivation of nutritionally active factors.
The effects of extrusion parameters on starch ret-
rogradation, amino acid reactivity and lipid oxidation
as well as on nutrient utilisation by pets should be stud-
ied. For example, limited research has been published
on the effects of resistant starch in companion animal
nutrition and its effects on intestinal health. Based on
existing in vitro and in vivo research, it has been indi-
cated that certain resistant starch sources are readily
fermented in the large bowel, producing copious quan-
tities of short-chain fatty acids, whereas others are less
fermentable, resulting in excellent laxation properties.
The qualitative and quantitative effects of (modified)
starch sources, however, have still to be elucidated.
Moreover, nutrient interaction may be a reason for
different utilisation of nutrients by pets upon extru-
sion and storage of foods. In addition, it is important
to study the effects of extrusion downstream processes
such as drying on nutrient retention. Quantification of
nutrient modification and its interactions after extru-
sion and storage will provide more possibilities to
control the nutritional value of pet foods.
REFERENCES
1 Spears JK and Fahey Jr GC, Resistant starch as related to
companion animal nutrition. JAOACInt87:787– 791 (2004).
2 Rokey G and Plattner B, Process description: pet food pro-
duction. Wenger Mfg, Inc., Sabetha, KS USA, pp. 1 – 18
(1995).
3 Cheftel JC, Nutritional effects of extrusion cooking. Food Chem
20:263– 283 (1986).
4 Svihus B, Uhlen AK and Harstad OM, Effect of starch granule
structure, associated components and processing on nutritive
value of cereal starch: a review. Anim Feed Sci Technol
122:303– 320 (2005).
5Bj
¨
orck I and Asp NG, The effects of extrusion cooking on
nutritional value, a literature review. J Food Eng 2:281 – 308
(1983).
6 Harper JM, Food extrusion. Crit Rev Food Sci 11:155– 215
(1978).
7 Lin S, Hsieh F and Huff HE, Effects of lipids and processing
conditions on degree of starch gelatinisation of extruded dry
pet food. Lebensm Wiss Technol 30:754 – 761 (1997).
8 Wolter R, Socorro EP and Houdre C, Faecal and ileal digestibil-
ity in the dog of diets rich in wheat or tapioca. Rescueil Med.
Vet. 174:45– 55 (1998). (in French).
9 Dust JM, Gajda MA, Flickinger AE, Burkhalter TM, Merchen
NR and Fahey Jr GC, Extrusion conditions affect chemical
composition and in vitro digestion of selected food ingredients.
J Agric Food Chem 52:2989–2996 (2004).
10 Murray SM, Flickinger AE, Patil AR, Merchen NR, Brent Jr JL
and Fahey Jr GC, In vitro fermentation characteristics of
native and processed cereal gains and potato starch using
ileal chyme from dogs. JAnimSci 79:435–444 (2001).
11 Murray SM, Patil AR, Fahey Jr GC, Merchen NR, Wolf BW,
Lai C-S, et al, Apparent digestibility of a debranched
amylopectin– lipid complex and resistant starch incorporated
into enteral formulas fed to ileal-cannulated dogs. JNutr
128:2032– 2035 (1998).
12 Deffenbaugh L, Optimizing pet food, aquatic and livestock
feed quality, in Extruders and Expanders in Pet Food, Aquatic
and Livestock Feeds, ed. by Riaz MN. Agrimedia, Clenze,
pp. 327–342 (2007).
13 Lin S, Hsieh F and Huff HE, Effects of lipids and processing
conditions on lipid oxidation of extruded dry pet food during
storage. Anim Feed Sci Technol 71:283 – 294 (1998).
14 Stroucken WPJ, van der Poel AFB, Kappert HJ and Bey-
nen AC, Extruding vs pelleting of a feed mixture lowers
apparent nitrogen digestibility in dogs. J Sci Food Agric
71:520– 522 (1996).
15 Hull´
ar I, Fekete S and Szocs Z, Effect of extrusion on the
quality of soybean-based catfood. JAnimPhysiolAnimNutr
80:201– 206 (1998).
16 Øverland M, Romarheim OH, Ahlstrøm Ø, Storebakken T and
Skrede A, Technical quality of dog food and salmon feed
containing different bacterial protein sources and processed
by different extrusion conditions. Anim Feed Sci Technol
134:124– 137 (2007).
17 Hickman MA, Rogers QA and Morris JM, Taurine balance is
different in cats fed purified and commercial diets. JNutr
122:553– 559 (1992).
18 Van der Poel AFB, Blonk J, van Zuilichem DJ and van
Oort MG, Thermal inactivation of lectins and trypsin
inhibitor activity during steam processing of dry beans
(Phaseolus vulgaris) and effects on protein quality. J Sci Food
Agric 53:215– 228 (1990).
19 Alonso R, Aguirre A and Marzo F, Effects of extrusion and
traditional processing methods on anti-nutrients and in vitro
digestibility of protein and starch in faba and kidney beans.
Food Chem 68:159–165 (2000).
20 Hendriks WH and Sritharan K, Apparent ileal and fecal
digestibility of dietary protein is different in dogs. JNutr
132:1692– 1694 (2002).
21 Ega ˜
na JI, L´
opez A and Quezada Q, Effect of extrusion on
acceptability and digestibility of growing dog diets. Arch
Latinoam Nutr 41:111 – 120 (1991).
22 Bednar GE, Murray SM, Patil AR ,Flickinger AE, Merchen NR
and Fahey Jr GC, Selected animal and plant protein sources
1492 J Sci Food Agric 88:1487–1493 (2008)
DOI: 10.1002/jsfa

Effects of extrusion processing on nutrients in dry pet food
affect nutrient digestibility and fecal characteristics of ileally
cannulated dogs. Arch Anim Nutr 53:127 – 140 (2000).
23 Ledl F and Schleicher E, New aspects of the Maillard reaction
in foods and in the human body. Angew Chem Int Ed Engl
29:565– 594 (1990).
24 Chiang GH, A simple and rapid high-performance liquid chro-
matographic procedure for determination of furosine, lysine-
reducing sugar derivative. J Agric Food Chem 31:1373 –1374
(1983).
25 Hurrell RF and Carpenter KJ, The estimation of available lysine
in foodstuffs after Maillard reaction. Prog Food Nutr Sci
5:159– 175 (1981).
26 Hendriks WH, Emmens MMA, Trass B and Pluske JR, Heat
processing changes the protein quality of canned cat food as
measured with a rat bio-assay. JAnimSci77:669 –676 (1999).
27 Williams BA, Hodgkinson SM, Rutherfurd SM and Hen-
driks WH, Lysine content in canine diets can be severely
heat damaged. JNutr 136:1998S–2000S (2006).
28 Rutherfurd SM, Rutherfurd-Markwich KJ and Moughan PJ,
Available (ileal digestible reactive) lysine in selected pet foods.
J Agric Food Chem 55:3517 –3522 (2007).
29 Singh S, Gamlath S and Wakeling L, Nutritional aspects of food
extrusion: a review. Int J Food Sci Technol 42:916 –929 (2007).
30 Killeit U, Vitamins retention in extrusion cooking. Food Chem
49:149– 155 (1994).
31 Hoffmann-LaRoche, Rationale for Roche Recommended Vitamin
Fortification – Dogs and Cats. Hoffmann-LaRoche, Nutley, NJ
(1981).
32 BASF, Vitamin Stability in Premixes and Feeds: a Practical
Approach. BASF, Mount Olive, NJ (2001).
33 Engelen GMA and van der Poel AFB, Post-pelleting Application
of Liquid Additives. Wageningen Pers, Wageningen (1999).
34 Melcion JP and van der Poel AFB, Process technology and
antinutritional factors: principles, adequacy and process
optimization, in Recent Advances of Research in Antinutritional
Factors in Legume Seeds, ed. by van der Poel AFB, Huisman J
and Saini HS. Wageningen Pers, Wageningen, pp. 419–434
(1993).
35 Purushotham B, Radhakrishna PM and Sherigara BS, Effects
of steam conditioning and extrusion temperature on some
anti-nutritional factors of soyabean (Glycine max) for pet food
applications. Am J Anim Vet Sci 2:1 – 5 (2007).
36 Hajos G and Osagie AU, Technical and biotechnological
modifications of antinutritional factors in legume and oilseeds,
in Recent Advances of Research in Antinutritional Factors in
Legume Seeds and Oilseeds, ed. by Muzquiz M, Hill GD,
Burbano C, Cuadrado C and Pedrosa MM. Wageningen
Academic, Wageningen, pp. 293 –301 (2004).
37 Cerundolo R, Court MH, Hao Q and Michel KE, Identification
and concentration of soy phytoestrogens in commercial dog
foods. Am J Vet Res 65:592–596 (2004).
38 Bell KM, Rutherfurd SM and Hendriks WH, The isoflavone
content of commercially available feline diets in New Zealand.
NZ Vet J 54:103 – 108 (2006).
39 Leung MCK, Diaz-Llano G and Smith TK, Mycotoxins in pet
food: a review on worldwide prevalence and preventative
strategies. J Agric Food Chem 54:9623–9635 (2006).
40 Hughes DM, Gahl MJ, Graham CH and Grieb SL, Overt signs
of toxicity to dogs and cats of dietary deoxynivalenol. JAnim
Sci 77:693– 700 (1999).
41 Case LP, Carey DP, Hirakawa DA and Daristotle L, Canine and
Feline Nutrition: a Resource for Companion Animal Professionals.
Mosby, St Louis, MO (2000).
42 Kvamme JL and Phillips TD, Petfood Technology. Watt Publish-
ing, Mount Morris, IL (2003).
43 Dunsford B, Plattner B, Greenbury B and Rockey G, The
influence of extrusion processing on petfood palatability, in
Proceedings of Petfood Forum, Chicago, Illinois, USA. Watt
Publishing, Inc., Mt. Morris, Illinois, USA (2002).
44 Frye T, Vitamin Stability in Canned and Extruded Pet Food. Hill’s
Pet Nutrition Science and Technology Center, Topeka, KS
(1995).
J Sci Food Agric 88:1487 – 1493 (2008) 1493
DOI: 10.1002/jsfa