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Effects of extrusion processing on nutrients in dry pet food


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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.
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Journal of the Science of Food and Agriculture J Sci Food Agric 88:14871493 (2008)
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
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.
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
(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.
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 lipidprotein 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 88:14871493 (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 amyloselipid 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.
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 proteinprotein, proteinlipid and
proteincarbohydrate 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,
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 2050% 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
Total lysine (%)
OMIU-reactive lysine (%)
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.
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 88:14871493 (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– 135C.
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
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 ¨
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
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 mg1DM); CTI, chymotrypsin inhibitors
(IU mg1DM); α-AI, α-amylase inhibitors (IU mg1DM); HgA,
haemagglutinating activity (HU mg1DM); PA, phytic acid (g kg1
DM); CT, condensed tannins (g equivalent catechin kg1DM); Pph,
polyphenols (g kg1DM).
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
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.
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
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.
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-
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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).
orck I and Asp NG, The effects of extrusion cooking on
nutritional value, a literature review. J Food Eng 2:281 308
6 Harper JM, Food extrusion. Crit Rev Food Sci 11:155– 215
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.
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... In almost all cases, pet food processing extends the shelf life of foods by destroying microorganisms or removing moisture to slow spoilage reactions during shelf life. At the same time, heat processing can degrade nutrients, particularly vitamins present in the foods (Tran et al., 2008). Processing often improves both the flavor and texture of foods but may also result in the formation of flavors and colors that are less desirable than the preprocessed versions. ...
... Another potential advantage of a gentle cooking process is that more nutrients may be retained during the process. Vitamins are highly sensitive to heat, light, oxygen, and mixing and are subsequently often oxidized or degraded during pet food processing (Tran et al., 2008;Dainton et al., 2021). As such, pet food formulators must add additional supplementation to compensate for the losses that occur during processing and ensure that the finished product meets the nutritional requirements for the animal. ...
... When considering shelf life, product developers have to consider 3 aspects: nutrients, aroma/appearance, and spoilage microorganisms. During processing, vitamins are particularly sensitive to temperature, light, and oxygen during storage; losses may also occur if the food is not stored properly (Ilo and Berghofer, 2008;Tran et al., 2008;Dainton et al., 2021;Morin et al., 2021). The aroma and appearance of fresh, high-moisture pet foods can also change rapidly during storage because the fats in the foods are prone to oxidation when exposed to air (Koppel et al., 2014). ...
Full-text available
Commercialized petfood has been evolving since the late 1800s. From rock-hard, flour-based biscuits to the adaptation of extrusion andcanning, or the most recently developed “fresh-cooked” processes, thedifferentiation between foods for pets and humans is becoming increasinglyblurred. Today, the U.S. pet food industryis worth $42 billion and continues to rapidly grow year after year. This growth is, in part, driven by anincreasing pet population which was accelerated by the pandemic (APPA, 2022; Euromonitor,2022); however, much of the growth and evolution of the pet food industry isarguably in response to a growing bond between humans and their pets. As petparents are increasingly looking for new ways to nourish and delight their “fur-babies”,the development of new, alternative formats of pet foods offers many advantagesto pet parents including more variety of food choices that typically have amore pleasant appearance and aroma. Benefitsfor the pet from these innovative foods include exceptional palatability andnutrient digestibility as compared to conventional pet foods, although barriersto the growth of new, alternative pet food formats such as “fresh-cooked” foodsdo exist. Most notably, the cost to feedtheir pets a diet solely consisting of “fresh-cooked” foods may be prohibitivefor many pet parents; furthermore, these novel foods bring unique technical challengesto manufacturers that do not exist for conventional kibble and canned petfoods. From securing ingredient supplyto ensuring the safety and shelf life of products, manufacturers of “fresh-cooked”pet foods have been challenged to adapt new formulations, processing methods,and supply chain strategies that have not previously existed in the pet foodindustry. This paper will exploreconsumer insights that have led to the evolution of the pet food industry aswell as the unique opportunities that pet food brands face as they seek tosatisfy consumer demand for high quality, humanized foods for pets withoutcompromising the nutritional adequacy, safety, and sustainability of commercialpet food diets.
... substances, all processes that can alter the final characteristics of the product, such as the color, flavor, and aroma [22,30]. Industrial processes, therefore, affect the lipid component, subject to phenomena of hydrogenation, isomerization, and, in particular, oxidation, a process that can also occur during product storage and which is strictly dependent on the nature of the lipids and the degree of humidity [22,34]. ...
... substances, all processes that can alter the final characteristics of the product, such as the color, flavor, and aroma [22,30]. Industrial processes, therefore, affect the lipid component, subject to phenomena of hydrogenation, isomerization, and, in particular, oxidation, a process that can also occur during product storage and which is strictly dependent on the nature of the lipids and the degree of humidity [22,34]. ...
... The protein content is certainly a key ingredient of the final product for pets; in fact, the composition of AAs and the bioavailability of proteins define an important nutritional value of the final product [22,35], but it can be altered by production processes [22,29,32,36]. As far as the proteins are concerned, during the extrusion treatment, high temperatures can alter their structure; however, this event is not exclusively negative because slight protein denaturation can favor their subsequent digestion by the body and therefore improve their digestibility [22,36,37]. ...
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Dry pet food, made of fresh meats and especially meat meals, represents one of the main types of complete food available on the market by virtue of its practicality and long shelf life. The kibble production process includes mixed thermal and mechanical treatments that help to improve the palatability and durability of the final product but may have undesirable effects on nutrient bioavailability and digestibility. An analysis of the protein and lipid content of different dry pet food formulations, together with an in vitro digestibility analysis, can reveal which formulation can provide a more nourishing diet for pets. In this study, a quantitative and qualitative analysis was performed on three different formulations of chicken-based dry pet food, consisting of fresh meats, meat meals, or a mix of these two. The soluble protein concentration was determined by the Bradford assay, while the crude protein content was assessed through the Kjeldahl method. Quadrupole time-of-flight liquid chromatography/mass spectrometry (Q-TOF LC/MS) was used to analyze the amino acid (AA) and lipid compositions. Finally, a gastric and small intestinal digestion simulation was used to determine the in vitro digestibility. The results show that dry pet food consisting only of chicken fresh meats has the highest content of soluble protein; it also contains more Essential AAs, Branched-Chain AAs, and Taurine, as well as a greater quantity of monounsaturated and polyunsaturated fatty acids. In addition, its in vitro digestibility was the highest, exceeding 90% of its dry weight, in agreement with the soluble protein content. These findings thus make the fresh-meat-based formulation a preferable choice as dry pet food.
... Dry extruded pet foods are mainly composed of finely-ground dry ingredients from plant and to a lesser extent animal origin that are cooked and shaped into a kibble which is coated to enhance palatability. The extrusion process increases the digestibility of the food (9) and lowers the amount of undigested fractions. Therefore, mainly plant fibres and undigested proteins reach the distal intestine where they are used by the microbial population (10) . ...
Feeding whole prey to felids has shown to benefit their gastrointestinal health. Whether this effect is caused by the chemical or physical nature of whole prey is unknown. Fifteen domestic cats, as a model for strict carnivores, were either fed minced mice (MM) or whole mice (WM), to determine the effect of food structure on digestibility, mean urinary excretion time of 15N, intestinal microbial activity and fermentation products. Faeces samples were collected after feeding all cats a commercially available extruded diet (EXT) for 10 days before feeding for 19 days the MM and WM diets with faeces and urine collected from d11-15. Samples for microbiota composition and determination of mean urinary excretion time were obtained from d16-19. The physical structure of the mice diet (minced or not) did not affect large intestinal fermentation as total SCFA and BCFA, and most biogenic amine (BA) concentrations were not different (P>0.10). When changing from EXT to the mice diets, the microbial community composition shifted from a carbolytic (Prevotellaceae) to proteolytic (Fusobacteriaceae) profile and led to a reduced faecal acetic to propionic acid ratio, SCFA, total BCFA (P<0.001), NH3 (P=0.04), total BA (P<0.001) and para-cresol (P=0.08). The results of this study indicate that food structure within a whole-prey diet is less important than the overall diet type, with major shifts in microbiome and decrease in potentially harmful fermentation products when diet changes from extruded to mice. This urges for careful consideration of the consequences of prey-based diets for gut health in cats.
... While manufacturing commercial canine food products, processing methods positively or negatively affect the nutritional value (8). For instance, extrusion cooking positively influences palatability, digestibility, and destruction of undesirable factors but can also have a potentially negative impact on protein quality and vitamin availability (9). The inclusion of vegetablebased ingredients may add some anti-nutritional factors to the canine diet (10). ...
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Digestibility and nutrient availability are important parameters when estimating the nutritional quality of pet food. We have developed a simulated semi-dynamic in vitro canine digestion model to evaluate the digestibility of dry extruded canine food. Canine food was assessed for digestible energy, dry matter digestibility, protein digestibility, non-fibrous carbohydrate (NFC) digestibility, and total antioxidant capacity (TAC) in the absence and presence of an enzyme blend (DigeSEB Super Pet). Enzyme blend supplementation in canine food was found to increase the dry matter digestibility (18.7%, p < 0.05), digestible energy (18.1%, p < 0.05), and protein digestibility (11%, p < 0.1) and reducing sugar release (106.3%, p < 0.005). The release of low molecular weight peptides (48.7%) and essential amino acids (15.6%) increased within 0.5 h of gastrointestinal digestion due to enzyme blend supplementation. Furthermore, the TAC of the digesta was also increased (8.1%, p < 0.005) in the canine food supplemented with enzyme blend. Overall, supplementation of enzyme blend in canine food is an effective strategy to enhance the food digestibility and nutrient availability for absorption.
... It is, therefore, not clear if the above mentioned palatability issues were due to the level of inclusion or to some intrinsic characteristics of the meal used. Indeed, if not properly conducted, processing methodology can lead to the production of unpleasant flavours or Maillard reactions, which can, in turn, decrease the overall palatability of the feed (Kim et al., 2016;Tran et al., 2008). Moreover, it is important to recall that insects can uptake unpleasant flavours from the rearing substrate. ...
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Insect meals are promising alternative feed ingredients although their application is still not commonplace. Their inclusion requires the consideration of various factors to optimise growth, animal welfare, and feed costs. The insect meal form (whole or defatted) impacts the level of inclusion, in particular in feeds where low amount of lipids is needed (e.g. poultry). From a nutritional point of view, the factors that influence the insect meal characteristics include insect species, rearing substrates and production processes. Processing (drying, defatting) can dramatically influence the nutrient digestibility and availability that requires assessment through in vivo or in vitro trials, with differences being observed in relation to the entity of the defatting process as well. The inclusion of full-fat or defatted meal may impact the final product quality (fatty acid profile). Low digestibility of chitin is also a limiting factor. Studies to increase the digestibility of insect meals using additives are ongoing. For these reasons, when different insect protein suppliers are used for feed production, chemical analyses need to be performed. In addition to the nutritional aspect, in some species (i.e. fish), a physical evaluation of the feed is necessary. In particular, the high fat content of whole larvae meal may increase the mixture viscosity and decrease the pellet stability, resulting in nutrient loss. Palatability affects feed ingestion; though insect meals seem well accepted, some palatability issues have been reported at high inclusion levels. It is however not clear if these issues are due to the level of inclusion or to some intrinsic characteristics of the meal used. Finally, the crucial factor for the future practical incorporation of insect meals in animal feeds is the availability and consistency of the supply. Without large and consistent quantities, it will be difficult for feed producers to incorporate these alternative ingredients within their production processes.
... In 2008, extruded dog foods were estimated to make up approximately 95% of dry pet foods [8]. The process of extrusion includes forcing raw ingredients under high pressure and temperature through a die for shaping and trimming. ...
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Starch gelatinization in pet food may be affected by moisture, retention time, and ingredients used. Starch gelatinization has been associated with changes in digestibility but is not well studied using non-traditional ingredients in canine diets. The objective of this research was to examine differences in starch content and gelatinization associated with changes in ingredient profile (traditional vs. non-traditional) and nutrient content requirements associated with differing life stages. Traditional diets (n = 10) utilizing protein sources including chicken, chicken by-product meal, meat and bone meal and plant-based ingredients including rice, barley, oats, and corn were examined in comparison with non-traditional diets (n = 10) utilizing protein sources including alligator, buffalo, venison, kangaroo, squid, quail, rabbit, and salmon along with plant-based ingredients including tapioca, chickpeas, lentils, potato, and pumpkin. Total starch and gelatinized starch (as percent of total diet) were measured with variation due to ingredient type assessed using Student’s t-test in SAS 9.4. Significance was set at p < 0.05. Total starch (as a percent of diet) was higher in traditional diets compared to non-traditional diets formulated for maintenance (p < 0.0032) or all life stages (p < 0.0128). However, starch gelatinization as a proportion of total starch was lower in traditional diets formulated for maintenance (p < 0.0165) and all life stages (p < 0.0220). Total starch and gelatinized starch had a strong negative correlation (r = −0.78; p < 0.01) in diets utilizing traditional ingredients. These novel data reveal important differences between starch content and gelatinization and may impact selection of various ingredient types by pet food manufacturers.
... The extrusion process induces structural and chemical changes in ingredients, which affects kibble shape and texture and improves the nutritional value of the final product (Riaz 2007;Tran et al. 2008). Variables, such as humidity, pressure, temperature, time, and ingredients affect extrusion and directly impact kibble quality. ...
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Pet food companies often use fibres in extruded diets as a strategy to improve intestinal functionality and reduce energy density, but studies evaluating the effect of fibres on the extrusion process, kibble characteristics, and palatability of dog diets are scarce. Thus, the objective of this study was to evaluate the effect of cassava fibre (CA) levels and conventional fibre sources on the extrusion process, kibble characteristics, and palatability of diets. Seven diets were evaluated: control diet (CO), without the inclusion of fibre sources; three diets with increasing levels of CA—4, 8, and 12% (totalling 6.1, 7.2, and 8.3% total dietary fibre—TDF, respectively); diet with 3.8% cellulose (CE); diet with 6% beet pulp (BP); and diet with 3.8% lignocellulose (LC). Diets 12% CA, CE, BP, and LC presented approximately 8.0% TDF. Diet palatability was evaluated in 16 adult beagle dogs in a completely randomised design. Seven paired tests were conducted, with two consecutive days per test, totalling 32 repetitions. Diets with fibre sources had lower kibble density than the CO diet (p = .004). The inclusion of increasing dietary CA levels resulted in a linear increase in the kibble expansion index (p = .0001). Dogs preferred the 12% CA diet to the CO (p = .032) or BP diets (p = .0001). The evaluated insoluble fibre sources resulted in greater expansion and lower density kibbles than the CO diet. Furthermore, 12% of CA positively affected diet palatability in dogs. • HIGHLIGHTS • Cassava fibre linearly increased the expansion of kibbles. • 12% cassava fibre positively affected diet palatability. • Diets with fibre sources changed the physical characteristics of kibbles.
In this paper, we review the physicochemical phenomena occurring during the structuring processes in the manufacturing of plant-based meat analogs via high-moisture-extrusion (HME). After the initial discussion on the input materials, we discuss the hypotheses behind the physics of the functional tasks that can be defined for HME. For these hypotheses, we have taken a broader view than only the scientific literature on plant-based meat analogs but incorporated also literature from soft matter physics and patent literature. Many of these hypotheses remain to be proven. Hence, we hope that this overview will inspire researchers to fill the still-open knowledge gaps concerning the multiscale structure of meat analogs.
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Dietary fibre substances in dog foods are composed of cell wall polysaccharides, non-cellulose polysaccharides, and structural non-polysaccharides. The purpose of this study was to determine the effects of dog age and heat-steam pressure (extruding process) on the in vitro fermentation of fibrous feedstuffs (sugar beet pulp, tomato pomace, wheat bran, corn bran, and rice bran) to be included in dog foods. The fibrous feedstuffs were incubated with a fermentation medium mixture and faeces inoculum, which were collected from two 6 month old (puppy), two 2 year old (adult), and two 8 year old (geriatric) dogs. The in vitro cumulative gas production (at 6, 12, 18, 24, 36, and 48 h), true-organic matter disappearance (T-OMd) (at 6, 12, 24, and 48 h), and molarities of short chain fatty acids (SCFA) (acetic, propionic, and butyric acids; AA, PA, and BA) in fermentation fluid (at 24 h) of fibrous feedstuffs were determined. The extruding process increased the in vitro cumulative gas production of rice bran (at 12, 18, 24, and 48 h) and wheat bran (at 6, 12, 24, and 36 h) (P < 0.05). In addition, in vitro cumulative gas production values of tomato pomace (at 12, 18, 24, 36, and 48 h) were reduced by the extruding process (P < 0.05). The extruding process increased the in vitro T-OMd values of sugar beet pulp (at 6, 12, and 24 h), wheat and rice bran (at 6 and 12 h), and the molarities of AA, BA, and total SCFA of the in vitro fermentation fluid of sugar beet pulp and wheat bran (P < 0.05). The in vitro cumulative gas production values of tomato pomace from faecal inoculums of the puppy and adult dogs were higher than that of the geriatric dog (P < 0.05). The in vitro T-OMd of sugar beet pulp (at 48 h) and extruded corn bran (at 6 h) with faecal inoculums of adult dog was higher than those of inoculum from puppy faeces (P < 0.05). The molarity of AA of tomato pomace with adult dog’s faecal inoculum was higher than those from puppy and geriatric dog (P < 0.05). The molarities of AA, BA, and total SCFA with corn bran from faeces inoculums of the puppy and adult dog was higher than that of the geriatric dog (P < 0.05). The molarities of AA and total SCFA of the in vitro fermentation fluid of extruded rice bran with faecal inoculums from the geriatric dogs was higher than those with faecal inoculums from the puppy and adult dogs (P < 0.05). As a result, the extruding process positively affected the in vitro fermentation values of sugar beet pulp, wheat bran, and rice bran. Furthermore, the organic acid molarities of the in vitro fermentation fluid of extruded rice bran from the geriatric dog was higher than those from the puppy and adult dogs. Sugar beet pulp, tomato pomace, wheat bran, and corn bran can be used as a source of fibre in food for puppy, adult, and geriatric large dogs.
Two mathematical models were used to determine the moisture adsorption isotherm curves of dry pet foods. Ten extruded commercial diets for dogs (n = 6) and cats (n = 4) were tested within 7 days of manufacture. The equilibrium moisture content of each food at 30 and 40 °C was determined by gravimetry. Six saturated saline solutions (lithium chloride, potassium acetate, sodium nitrite, magnesium chloride, sodium chloride, and potassium chloride) were prepared and used to obtain pet food samples at different water activities. Solutions were placed in airtight flasks containing food samples (without direct contact) and oven-dried to constant weight. The relationship between water activity and equilibrium moisture content was modeled by the exponential Peleg and Guggenheim–Anderson–de Boer (GAB) models. All pet foods exhibited a type II isotherm. Akaike information criterion, R², and standard deviation values were −29.79, 0.914, and 3.89, respectively, at 30 °C by the Peleg model; −28.86, 0.876, and 6.36 at 40 °C by the Peleg model; −31.17, 0.937, and 4.34 at 30 °C by the GAB model; and −29.63, 0.888, and 7.71 at 40 °C by the GAB model. At 0.60 water activity, equilibrium moisture contents by the Peleg and GAB models were 12.04 ± 0.84 and 11.67 ± 0.78 g H2O/g dry matter, respectively, at 30 °C and 7.83 ± 1.31 and 8.09 ± 0.81 g H2O/g dry matter at 40 °C. The GAB model also allowed estimating monolayer moisture content (5.81 ± 0.81%). Both models provided similar results and may be useful for determining quality parameters for pet foods. Adsorption isotherm studies can provide practical information about the moisture ranges of pet foods, contributing to the optimization of food safety, palatability, and processing conditions.
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Extrusion processing of vegetable ingredients such as soyabean (Glycine max) characteristically depends on associating process conditions that influence the product qualities. The process parameters were optimized for extrusion of steam-conditioned material in order to obtain the maximum nutritive value by inactivating the anti-nutritional factors such as urease, trypsin inhibitors and lipase. The processing conditions such as moisture content, temperature and time were precisely controlled to avoid over heating or under heating which otherwise would result in a product of lower nutritional quality. The urease activity and Trypsin inhibitors were measured in steam conditioned extruded soyabean material and test results indicated that urease activity units and trypsin inhibitory units were reduced from 2.0 and 50 to 0.3 and 5 respectively in extruded soyabean during the extrusion process. These results were used for comparative evaluation of 'inactivation levels of anti-nutritional factors' for optimizing the processing conditions of the steam conditioner and extruder for producing soyabean products for pet food applications. Stability studies were done on pet food samples and free fatty acid content and peroxide values were used as the criteria to evaluate the lipase activity in extruded soyabean samples. Extrusion of steam-conditioned soyabean material inactivated the antinutritional factors making extruded soyabean an ideal ingredient for pet food and its stability.
The effect of steam treatment on the protein quality and antinutritional factors in beans (Phaseolus vulgaris L) have been evaluated. The thermal inactivation of total and functional lectins and trypsin inhibitor activity as well as total and available lysine during steam treatment at 102, 119 and 136°C can be described by first order reaction kinetics. Inactivation of trypsin inhibitor factors occurred in two stages with different reaction rates, the initial stage having a higher rate of inactivation. The effect of steaming temperature on the rate constants can be predicted by an Arrhenius-type relation. Part of the total lysine was lost on heating but the amount of available lysine was reduced to a greater extent. The results of the present investigation indicate that steam treatment at 119°C for 5 or 10 min seems to be a good compromise in terms of antinutritional factor inactivation and protein damage as measured by total and available lysine.
Effects of fat type (beef tallow and poultry fat), fat content (0–0.75 g kg−1) and processing conditions (initial moisture: 1.6–2 g kg−1; screw speed: 200–400 rpm) on lipid oxidation of extruded dry pet food were studied. The results showed that the lipid oxidation rate constant of the extruded dry pet food was a function of fat types, added fat content and feed moisture content. Control (no fat) samples had the highest lipid oxidation rate, as compared to the samples with fat addition. Samples with poultry fat had a higher lipid oxidation rate than those with beef tallow addition. Samples containing a higher fat content resulted in a lower rate of lipid oxidation during storage. Feed moisture content had the same effect as the fat content; the extrudates with a higher moisture content resulted in a lower lipid oxidative rate. Pet foods extruded at 300 rpm had a significantly higher lipid oxidation rate than the ones produced at 200 and 400 rpm. Lipid oxidation of the extrudates appeared to be affected mainly by the degree of extrudate expansion. Products with a higher degree of expansion, which had larger cells and thinner cell walls, were more susceptible to oxidation.
The effect of basic bacterial protein meal (BPM) and bacterial protein meal homogenate (HOM) on length, expansion, density, sinking rate, fat leakage, durability, and breaking force of extruded dog food and salmon feed exposed to mild and moderate processing conditions was evaluated. The treatment consisted of a control diet and four test diets where high-quality (low temperature dried; LT) fish meal was partly replaced with either 25 or 50g BPM or HOMkg−1. The differences in processing characteristics were obtained by a combination of conditioner and extruder adjustments. Fat was added to the extruded diets by vacuum coating. In the dog diets, the inclusion of BPM and HOM resulted in shorter pellets with increased diametric expansion, and reduced dust percentage, sinking rate and breaking force, with the effect being in general greatest with the highest concentration. In general, moderate feed processing resulted in increased pellet length and expansion while sinking rate and fat leakage decreased in both the BPM and HOM diets. Neither BPM nor HOM affected fat leakage, but fat leakage decreased by moderate processing. In the salmon diets, dietary BPM and HOM increased density and breaking force, but decreased durability and had no effect on sinking rate or fat leakage of the extruded salmon pellets. A significant interaction between feed processing and bacterial protein source was found for pellet length and diameter. Moderate feed processing increased pellet length in the BPM diets but reduced pellet length and increased pellet expansion in the HOM diet. Moderate processing decreased durability and sinking rate of the pellets. Coefficients of total tract apparent digestibility (CTTAD) of the control, 50gkg−1 BPM and 50gkg−1 HOM dog diets exposed to mild and moderate feed processing were determined in mink. There was no significant effects of type of diet or feed processing on CTTAD of major dietary components. The results demonstrate that low amounts of BPM and HOM influenced the technical quality of extruded diets for dogs and salmon. The effect of BPM and HOM was different in dog food with higher starch content compared to the salmon feed with lower starch content, and was dependent on extrusion conditions.
Starch is organized in concentric alternating semi-crystalline and amorphous layers in granules of various sizes within the endosperm. The amount of amylose in starch normally varies between 200 and 300g/kg, but waxy cereals may contain negligible amounts and starch from high-amylose varieties may contain up to 700g amylose/kg. High amylose content is associated with reduced digestibility. Fat and protein are found on the surface of starch granules, and these components may act as physical barriers to digestion. Heat treatment with sufficient water present will cause gelatinisation that will increase susceptibility for starch degradation in the digestive tract, although a linear relationship between extent of gelatinisation due to processing and digestibility has not been found. The low water content during feed processing limits the extent of gelatinisation, but gelatinisation temperature and extent of gelatinisation will be affected by properties of the starch, which in turn may affect digestibility. The effect of starch properties and feed processing on digestion in non-ruminant animals and ruminants are discussed.
The reactions that occur during the cooking, baking, and preservation of foods of all kinds are of great importance for the production of aroma, taste, and color. However, more recently it has been shown that these reactions may be accompanied by a reduction in nutritive value and the formation of toxic compounds. For these reasons, the very complex reactions between reducing sugars and the free amino groups of amino acids or proteins, known as non-enzymatic browning or the Maillard reaction, have again caught the interest of chemists. The Maillard reaction came to be seen in a new light as it was realized that it actually occurred in the human body. As a general rule, the longer the half-life of a protein, the larger the amount of its Maillard products found, i.e., important factors are the ‘age’ or persistence of the protein in the body and the glucose concentration, particularly in diabetics. Many of the symptoms developed by diabetics resemble those of premature aging, which leads to the possibility that glucose, because of its reactivity towards proteins, is fundamentally involved in the normally slow progress of aging.
The influence of extruding vs pelleting of a feed mixture on nitrogen, dry matter, fat and ash digestibility was investigated in six mature dogs. Extrusion decreased nitrogen digestibility, increased ash absorption and had no effect on dry matter and fat digestibility. After feeding the extruded diet, the pH of faeces was lowered and the faecal water content increased, suggesting that this diet stimulated bacterial fermentation in the intestine.
Under mild heat treatment, the interaction of the protein and reducing sugars occurs with ε-deoxyfructosyllysine as the product of reactions between free or protein-bound lysine and reducing sugars. Furosine, the acid hydrolysis product of ε-deoxyfructosyllysine, was demonstrated to be a good indicator for expressing the extent of the Maillard reaction. A simple isocratic HPLC technique was developed to quantitatively determine furosine. A reversed-phase Lichrosorb RP8 column, a 0.5% NaOAc-HOAc buffer (pH 4.3) mobile phase, and UV detection at 280 nm were utilized. The method was applied on storage samples of dry pet food, powdered meal replacer, formed meal bar, diet bar, dry milk, cheese sauce, and dry gravy. Good correlation between the storage conditions and furosine levels was obtained for all the samples tested.