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Animal Feed Science and Technology 165 (2011) 258–264
Contents lists available at ScienceDirect
Animal Feed Science and Technology
journal homepage: www.elsevier.com/locate/anifeedsci
Effects of drying temperature and time of a canine diet extruded with a
4 or 8 mm die on physical and nutritional quality indicators
Q.D. Tran1, W.H. Hendriks, A.F.B. van der Poel∗
Animal Nutrition Group, Department of Animal Sciences, Wageningen University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands
article info
Article history:
Received 4 July 2008
Received in revised form 24 February 2011
Accepted 11 March 2011
Keywords:
Extrusion
Drying
Canine diet
Amino acids
Reactive lysine
Fatty acids
abstract
Two factorial experiments (4 temperatures ×2 durations) were carried out to test the effect
of drying variables on nutritional and physical quality indicators of extruded canine diets
produced using a 4 and 8 mm die (kibble size). The diet was extruded using a single screw
extruder at 130 ◦C and 300g moisture/kg. The drying temperatures used were 80, 120, 160
and 200 ◦C and each diet was dried to 90 or 60 g moisture/kg diet (drying duration). Dry-
ing of the diets was conducted in draught-forced ovens and each sample was analysed for
dry matter, nitrogen, amino acids (including reactive lysine) and fatty acid content. Hard-
ness and specific density of the tested diets were not affected by drying temperature or
time. Kibble durability was affected (P<0.05) by drying temperature: the highest tempera-
ture (200 ◦C) resulted in a decreased durability compared to 80 ◦C. The drying time had no
effects on the level of individual or total amino acids or on the proportion of reactive lysine.
In 4 mm kibbles, drying temperature of 200 ◦C lowered (P<0.05) only proline, total lysine
and reactive lysine concentrations: the reactive to total lysine ratio in kibbles dried at 120 ◦C
was higher than that of kibbles dried at 200 ◦C. Drying temperature of 200◦C decreased the
concentration of linolenic and linoleic acid and increased that of oleic acid (P<0.05), a find-
ing that might be indicative for lipid oxidation of 4 mm kibbles during the drying process. In
8 mm kibbles, only reactive lysine concentrations were significantly lower with a concomi-
tant decrease of the kibble durability (P<0.05). Drying of pet foods at high temperatures
(160–180 ◦C) can significantly reduce nutrients or nutrient reactivity.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The pet food production process includes a number of stages where heat treatments such as preconditioning, extrusion
cooking, sterilisation and drying are employed. Pet food companies mainly use extrusion cooking technology to produce dry
pet foods because of a combination of benefits including better pasteurisation, maintenance of nutritional value, flexibility
and density control. Dry extruded diets for cats and dogs are commonly produced at a moisture level between 200 and
300 g/kg (Lankhorst et al., 2007) and must be dried afterwards to reduce moisture content to less than 60–90 g/kg (Tran
et al., 2008) in order to increase shelf-life of the final product. Drying time depends among others on the drying temperature
employed, dryer design, dryer air speed, kibble size and bulk density. Two recent studies (Lankhorst et al., 2007; Tran et al.,
2007) have investigated the effects of the extrusion cooking process on the nutritional quality of pet foods. This nutritional
Abbreviations: OMIU, O-methylisourea; DM, dry matter; Lys, lysine; FA, fatty acid.
∗Corresponding author. Tel.: +31 317 484082; fax: +31 317 484260.
E-mail address: Thomas.vanderpoel@wur.nl (A.F.B. van der Poel).
1Current address: Human and Animal Physiology Group, Biology Faculty, Vinh University, 182 Le Duan Street, Vinh, Viet Nam.
0377-8401/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.anifeedsci.2011.03.009
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Table 1
Ingredient and analysed (n= 2) nutrient composition of the untreated experimental diet.
Ingredient Inclusion (g/kg diet) Nutrient Amount (g/kg DMa)
Wheat 245.0 Dry matter 932.4
Maize 215.2 Crude protein 188.1
Rice (dehulled) 150.0 Crude fat 109.1
Chicken meat MDMb100.0
Poultry meal 53.6 Arginine 9.5
Barley 50.0 Histidine 3.6
Pork bone fat 43.3 Isoleucine 7.2
Fish meal (>680 g/kg CP) 35.0 Leucine 13.6
Sugar beet pulp 30.0 Lysine 9.1
Whole egg powder 20.0 Methionine 3.2
Dicalcium phosphate 16.6 Phenylalanine 7.9
Brewers yeast 15.0 Threonine 6.5
Linseed 10.0 Valine 9.0
Salt 6.7
Potassium chloride 2.6 Stearic acid (C18:0) 5.5
Premixc5.0 Oleic acid (C18:1 n-9 cis) 34.6
Limestone 1.7 Linoleic acid (C18:2 n-6 cis) 25.0
Inulin 0.3 Linolenic acid (C18:3 n-3 cis) 4.6
aDry matter.
bMechanically deboned meat.
cComposition of premix (g/kg feed for minerals and mg/kg for vitamins, unless otherwise stated): Ca: 22; Mg: 0.02; Fe: 73 mg/kg; Mn: 35mg/kg; Cu:
5.0 mg/kg; Zn: 75mg/kg; I: 1.8mg/kg; Co: 2.0mg/kg; Se: 0.20 mg/kg; vitamin A: 17,500IU/kg; vitamin D3: 2000 IU/kg; vitamin E: 100; vitamin K3: 2.0;
vitamin B1: 10.0; vitamin B2: 10.0; niacin: 50.0; pantothenic acid: 25.0; vitamin B6: 7.5; vitamin B12: 50.0 g/kg; biotin: 300 g/kg; choline chloride: 475;
folic acid: 1.25; vitamin C: 100.
quality is not only affected by the agglomeration process itself but also by the down-stream drying process or the stability
(shelf life) when stored inappropriate. Drying, however, may also cause chemical changes in the product (Davenel and
Marchal, 1995). Excessive heating, for instance, destabilizes fat which can lead to a sticky mass and cause evaporation of
fine flavour volatiles (Acquistucci, 2000). The extrusion process results in a high degree of bound moisture which is more
difficult to remove than the moisture in pellets which is present as free moisture (Dexter et al., 1981). Moderate drying
temperatures of approximately 75 ◦C for 50 or 90 min can result in heat damage in pasta as indicated by an increase in
furosine concentration (Acquistucci, 2000). Research into commercial canine and feline diets has shown a large difference
between the content of O-methylisourea-reactive and total lysine (Williams et al., 2006; Tran et al., 2007; Rutherfurd et al.,
2007) which is an indicator of heat damage to lysine. Recently, Lankhorst et al. (2007) determined the effect of the extrusion
process on lysine reactivity, thereby explaining some of the variation in reactive lysine content which can be observed in
commercial pet foods (Williams et al., 2006; Rutherfurd et al., 2007). There is, however, a lack of information in the literature
on the contribution of the drying process to the large variation observed in lysine reactivity of commercial pet foods.
The present study investigated the effect of both drying temperature and drying time on a number of nutritional and
physical quality parameters of a canine diet. Since drying efficacy depends on the surface area of a kibble, a 4 and 8 mm
extruder die opening during extrusion were examined. The hypothesis tested was that drying temperatures of 120 ◦C will
cause minor and acceptable lysine damage in extruded canine diets.
2. Materials and methods
2.1. Experimental ingredients and diets
The ingredients and nutrient composition of the experimental formulation used to produce the extrudates for drying is
presented in Table 1. All vegetable ingredients were supplied by Research Diet Services (Wijk bij Duurstede, the Netherlands).
Poultry meal, chicken meat, whole egg powder, fish meal and pork bone fat were obtained from International Quality
Ingredients BV (Ermelo, the Netherlands). All intact vegetable ingredients were ground over a 1.5-mm sieve in a hammer
mill (Condux LHM, Hanau, Germany). The diet ingredients were mixed for 180 s in a F60 paddle mixer (Forberg AG, Larvik,
Norway). Prior to mixing, the chicken meat and pork bone fat were heated (60 ◦C) using a water bath to enhance mixing
properties. After mixing, the meal mixture (density, 0.65 g/cm3; initial moisture level, 115 g/kg) was transported to the
storage bin above the extruder.
2.2. Experimental design, extrusion cooking and sampling
Two 4 ×2 factorial experiments were carried out using drying temperature and drying time as variables. The temperatures
used were 80, 120, 160 and 200 ◦C and the drying times (used to dry the samples to a target moisture content of 60 or 90 g/kg,
respectively) were referred to as t60 and t90. The only difference between the two experiments was the size of the die used
(4 mm in Exp. 1 and 8 mm in Exp. 2). The experiments were carried out at consecutive days at the Wageningen Feed Processing
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Centre, Wageningen University (Wageningen, the Netherlands). Diets were extruded at 130 ◦C in a single screw Almex AL150
extruder (Zutphen, the Netherlands) with a length to diameter ratio of 8 and with moderate shear screw configuration using
forwarding screws and paddles. At the conditioner phase, water was added to obtain a content of 300 g/kg to guarantee a
good kibble durability after extrusion (Lankhorst et al., 2007). Two die sizes of 4 and 8 mm, each with four orifices, were
used. A die face cutter was operated to cut the extrudates to approximately 12 or 8 mm length (longitudinal expansion) for
the 4 and 7 mm die, respectively. All parameters, such as extruder throughput temperature and feed rate, were controlled
by a program logic controller, monitored and kept constant per die size throughout the experiment.
After extrusion, batches of feed were collected from each experimentand transported to the laboratory. There, each batch
was divided into 16 identical subsamples and dried in duplicate using eight identical draught-forced ovens (WTB Binder
Labortechnik, Tuttlingen, Germany) at temperatures of 80, 120, 160 and 200 ◦C. The actual temperatures were electronically
displayed on the oven screen. The drying time required to obtain end moisture contents of 60 and 90 g/kg was recorded for
each sample as follows: the sample moisture content before drying was used to calculate the expected sample weight after
drying for each target end-moisture. During drying, samples were weighed approximately every 15 or 30 min depending
on temperature until the sample reached the expected weight. Two control samples (from die sizes of 4 or 8 mm) were
dried at 40 ◦C for 15 h. After drying, each of the 32 samples were divided into two parts. One part was used for physical
analysis (durability, hardness and specific density) of whole kibbles as described by Lankhorst et al. (2007). The other
part was ground in a laboratory mill (ZM 100, Retsch BV, Ochten, the Netherlands), fitted with a cyclone for cooling to
avoid excessive heat generation, over a 1-mm sieve and then stored in airtight plastic containers at 4 ◦C prior to chemical
analysis.
2.3. Analytical methods
The nutrient composition of the experimental diets was determined by the proximate analysis methods (AOAC, 1990)
with dry matter (DM) analysed by drying samples to a constant weight at 103 ◦C. Long-chain fatty acids were analysed by lipid
extraction according to Folch et al. (1957) followed by methylation (sodium methanolate in absolute methanol) of the fatty
acids. Methylated fatty acid samples were separated by gas chromatography (Carlo Erba Instruments HRGC Mega 2, Milan,
Italy). Amino acids (except cysteine and tryptophan) were determined on 5-mg samples by hydrolyzing with 1 ml of 6mol/l
glass distilled HCl for 24 h at 110±2◦C in glass tubes, sealed under vacuum. The tubes were opened and 200 lof2.5mol
norleucine was added to each tube as an internal standard, thereafter the tubes were dried under vacuum (Savant SpeedVac
Concentrator SC210A, Savant Instruments Inc., Farmingdale, NY, USA). Reactive lysine was determined according to the
developed O-methylisourea (OMIU)-method as described by Moughan and Rutherfurd (1996). Amino acids were loaded
onto a water ion exchange HPLC system (Biochrom 20 Plus, Amersham Pharmacia Biotech, Staffanstorp, Sweden) employing
postcolumn derivatisation with ninhydrin and detection at 570 nm. Proline was detected at 440 nm. The chromatograms
were integrated using specific software (Chrom-Card version 2.3.3, Thermo Scientific, Waltham, MA, USA) with amino acids
identified and quantified by retention time against a standard amino acid mixture. All chemical analyses were conducted in
duplicate. Durability and hardness of the extrudates were measured using the Holmen (Holmen Chemical Ltd., Borregaard
group, Norsolk, UK that simulate pneumatic transport during 60 s) and an automatic Kahl device (Amandus Kahl, Reinbek,
Germany), respectively, as described by Thomas and van der Poel (1996). Specific density was calculated as mean (n= 12)
quotient of kibble weight to kibble volume.
2.4. Statistical analysis
The effects of drying (temperature and time) parameters on the nutritional and physical quality indicators for each die
size were statistically analysed by analysis of variance (SPSS, 2007; SPSS 15.0.1.1 for Windows) using the following model:
Yij =m+Ti+tj+(T∗t)ij +eij
where Yij = quality indicator, = overall mean, Ti= drying temperature (i=80, 120, 160 or 200◦C), tj= drying duration (j=t60
or t90), (T*t)ij = interaction between drying temperature i and drying duration j,eij = residual error term.
Homogeneity of variance was checked with ˛= 0.05. The Student’s t-test was used to compare differences between
temperature and the drying times.
3. Results
The duration of drying time recorded ranged between 43 (200 ◦C and t90) and 539min (80◦C and t60) depending on the
drying temperature, kibble size and the desired moisture level of the end product. As expected, the time required to dry the
kibbles to 60 g/kg moisture was longer compared to drying to 90 g/kg moisture for all samples. The 4 mm kibbles required
a longer drying time (497 and 139 min) compared to the 8 mm kibbles (260 and 107 min), especially at the two lower (80◦
and 120 ◦C, respectively, for t90) temperatures.
The hardness of the kibbles was not affected by the drying temperature or by residence time. Kibble durability was
reduced by higher drying temperature: extrudates produced with a 8 mm die size, dried at 80 ◦C had a higher (P<0.05)
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Table 2
Effects of drying temperature and drying time on physical characteristics (mean ±SEM) of the experimental diets extruded witha4or8mmdiesize.
4 mm (Exp. 1) 8 mm (Exp. 2)
HardnessaDurabilitybHardness Durability
Drying temperature (◦C)
80 4.9 ±0.3 87.7 ±1.1 7.9 ±0.5 95.8d ±0.4
120 4.1 ±0.3 88.3 ±1.1 6.3 ±0.5 94.4de ±0.4
160 4.3 ±0.3 86.1 ±1.1 7.7 ±0.5 94.1de ±0.4
200 4.0 ±0.3 83.6 ±1.1 6.6 ±0.5 93.1e ±0.4
Pooled SEM 0.3 1.1 0.5 0.4
Drying timec(min)
t60 4.3 ±0.2 86.8 ±0.8 6.8 ±0.3 94.2 ±0.4
t90 4.3 ±0.2 86.1 ±1.1 7.0 ±0.4 94.5 ±0.5
Pooled SEM 0.2 0.4 0.2 0.3
Different letters denote (d and e) significant differences (P<0.05) between means within a column.
aKibble hardness (force in kg to break a kibble).
bKibble durability (% fines formed after controlled mechanical stress).
cDrying time up to 60 (t60) or 90 (t90) g/kg moisture in the end product.
Table 3
Effects of drying temperature on long-chain fatty acids concentrations (FA; g/kg feed dry matter) of 4 and 8 mm die size extrudates.
FA Die size
4 mm (Exp. 1) 8 mm (Exp. 2)
Temperature (◦C) Pooled SEMaTemperature (◦C) Pooled SEMa
80 120 160 200 80 120 160 200
C12:0 0.5 0.6 0.6 0.6 0.04 0.4 0.5 0.7 0.5 0.12
C14:0 0.8 0.8 0.7 0.8 0.04 1.0 0.8 0.7 0.8 0.12
C14:1 n-5 cis 0.0 0.0 0.0 0.0 0.03 0.1 0.1 0.0 0.0 0.05
C16:0 21.6b 22.0a,b 22.3a 22.4a 0.12 21.7b 21.8a,b 22.1a 22.0a,b 0.10
C16:1 n-7 cis 3.5 3.5 3.5 3.5 0.06 3.5 3.5 3.5 3.5 0.08
C18:0 5.4b 5.3b 5.4b 5.8a 0.07 5.4 5.4 5.5 5.6 0.09
C18:1 n-9 trans 0.1 0.0 0.1 0.3 0.07 0.0 0.1 0.0 0.1 0.05
C18:1 n-9 cis 34.6 34.1 33.8 34.4 0.26 34.3 34.2 34.4 34.3 0.19
C18:1 n-7 cis 2.0 2.0 1.9 1.9 0.05 1.9 2.0 2.0 2.0 0.07
C18:2 n-6 cis 25.4a 25.4a 25.4a 24.2b 0.19 25.4 25.3 25.6 24.8 0.20
C18:3 n-3 cis 4.5a 4.5a 4.5a 4.2b 0.06 4.7a 4.6a,b 4.4b 4.4b 0.05
C20:0 0.1 0.1 0.1 0.2 0.05 0.0b 0.1a,b 0.0b 0.2a 0.03
C20:1 n-9 cis 0.5 0.3 0.4 0.5 0.08 0.4 0.5 0.2 0.4 0.06
C20:5 n-3 0.3 0.3 0.3 0.2 0.05 0.3 0.3 0.1 0.3 0.05
C22:6 n-3 0.2 0.3 0.3 0.3 0.05 0.3 0.3 0.2 0.3 0.09
Different letters (a and b) denote significant differences (P<0.05) between means within a row.
aSEM: n=3.
durability than those dried at 200 ◦C(Table 2). Specific density of kibbles was not influenced by drying temperature. Drying
time had no effect on the physical characteristics of the kibble.
Drying time had no effects on the content of long-chain fatty acids in the diets (Table 3). The concentration of stearic
acid (C18:0), linoleic acid (C18:2) and linolenic acid (C18:3) were decreased by drying temperature especially at 200 ◦Cin
the 4 mm kibbles. Palmitoleic acid (C16:0) (in 4 mm kibbles) and eicosanoic acid (C20:0) (in 8mm kibbles) were significant
(P<0.05) influenced by the drying temperature, although the differences were of small practical interest.
Statistical analysis showed that the drying time had no significant effects on the concentration of any of the amino acids
including reactive lysine and as such the data were pooled only per drying temperature. The total lysine content of the
control sample (dried at 40 ◦C; 4 mm die size) was 8.61g/kg DM; the reactive lysine content, 7.79 g/kg DM and the reactive
to total lysine ratio, 0.90. The total lysine content of the control sample (dried at 40 ◦C; 8mm die size) was 9.06g/kg DM; the
reactive lysine content, 7.77 g/kg DM and the reactive to total lysine ratio, 0.86.
In the experiment using a 4 mm die, a drying temperature of 200 ◦C increased (P<0.05) proline content. Both total and
reactive lysine were affected (P<0.05) by drying temperature: extrudates dried at 200 ◦C had a lower total and reactive
lysine content compared to extrudates dried at the other temperatures. The reactive to total lysine ratio in extrudates dried
at 200 ◦C was significantly lower (P<0.05) than that in extrudates dried at 120◦C(Fig. 1). For diets produced with the 8 mm
die, only reactive lysine was affected by drying temperature. In this experiment, extrudates dried at 200 ◦C had a lower
(P<0.05) reactive lysine content compared to extrudates dried at the other temperatures. However, the reactive to total
lysine ratio of extrudates dried at 160 ◦C was higher than that of extrudates dried at all other temperatures (Fig. 1).
No interactions were found between drying temperature and drying time on the various quality indicators except for
reactive lys (P<0.001) and the ratio of reactive to total lys (P<0.01) for the kibbles produced with a die size of 8 mm. Drying
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6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
20016012080
Temperature (ºC)
Lysine content (g/kg DM)
Ratio
Total lysine
Reactive lysine
20016012080
Temperature (ºC)
0.75
0.80
0.85
0.90
0.95
1.00
Reactive/total lysine ratio
Ratio
Total lysine
Reactive lysine
Ratio
Total lysine
Reactive lysine
Fig. 1. Effect of drying temperature on total and reactive lysine content in a diet extruded using a die opening of 4mm (left) or 8mm (right).
temperature of 200 ◦C as well as a longer time of drying caused increased damage to reactive lysine and, as a result, produced
lower ratios of reactive to total lysine.
4. Discussion
Most pet food recipes include a mixture of cereal products, vegetable and animal proteins and lipids. These recipes are
commonly extruded at moisture levels between 200 and 300 g/kg. Excessive product moisture after extrusion cooking is
removed to a level of 90 or even 60 g/kg (Tran et al., 2008) for final packaging and sale. The removal of this excess moisture is
an aim of the drying process so that after cooling the kibble can be sprayed or vacuum coated with a palatability enhancing
solution. A high temperature drying process permits control of the growth of micro-organisms and allows a shortening
of drying time. This has economic benefits (Casiraghi et al., 1992) but it may also positively affects lysine content of the
product (Arrage et al., 1992). The latter authors reported a significant increase in (dye-binding) reactive lysine content after
drum-drying of whole wheat at 152 ◦C (untreated whole wheat, 3.01; extruded, 2.54 and drum-dried, 3.17 g lysine/100 g
protein). However, this increase in reactive lysine has to be viewed with caution due to potential inaccuracies with the dye
binding lysine methodology (Hendriks et al., 1994).
During drying, the surface moisture of the product is absorbed by the dry process air and carried away. The surface
moisture evaporates first, thereafter the moisture at the core of the kibble is driven to the surface where it evaporates. Free
moisture is the first to evaporate, followed by internally bound moisture upon further heating. The range of the drying times
found in the present study (43–539 min for 200 and 80◦C, respectively) is similar to drying time used by Acquistucci (2000)
in pasta of 40, 65 or 75 ◦C for 30–600 min. The drying time for larger (8 mm) kibbles was less than for small 4 mm kibbles.
This seems logical since in staples or layers, there is more space between larger kibbles and this results in an increased
ventilation during drying compared to small kibbles (4 mm). Physical quality of extrudates has been traditionally associated
with durability and hardness. A kibble durability index of 90% or more is generally used today as a target in the feed industry. A
durable kibble is less likely to break during handling and transportation and as a result, a lower amount of fines are produced.
The results of the present study showed that the durability of the diets produced with a 4 mm die at 80 ◦C was similar for
those obtained at higher drying temperatures. For the 8 mm diets, a lower durability (P<0.05) was observed in kibbles dried
at 200 ◦C. This may be because of the retrogradation of starch during drying and cooling. At low drying temperatures, drying
time is longer, causing more starch retrogradation (Svihus et al., 2005). After drying to the desired moisture content, the
diets in this study were allowed to cool down to the room temperature before sampling. The hardness of kibbles in the
present study was not affected by drying temperature and residence time. Hardness of the 4 and 8 mm kibbles in the present
experiments was lower compared to kibbles in commercial canine diets (Tran et al., 2007), possibly due to the additional
spray- or vacuum coating employed in the production of the latter.
Lipid oxidation is the major chemical challenge for preservation of pet food. This oxidation can reduce the nutritive
quality by decreasing the content of essential fatty acids such as C18:3 and C18:2, long-chain, unsaturated fatty acids. High
extrusion temperatures may increase the pro-oxidant transition metal concentration, particularly iron, due to the metal wear
on extruder parts (Lin et al., 1998). Neutral, inorganic form of minerals, e.g. iron, has been reported to promote oxidation
(O’Keefe and Steward, 1999). In the present study, fatty acids were only affected at high drying temperatures: in particular,
the C18 fatty acids (especially C18:3 n-3, an unsaturated fatty acid) were most affected (P<0.05) by the highest drying
temperatures (Table 3).
For extrudates produced using a 8 mm die size, the total lysine content was unaffected by both drying temperature and
residence time. For extrudates produced using a 4 mm die size, the total lysine content of extrudates dried at the highest
temperature (200 ◦C) was lower (P<0.05) than extrudates dried at other temperatures. This is in line with the study of
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Table 4
Effects of drying temperature on amino acid concentrations (AA; g/kg feed dry matter) of 4 and 8 mm die size extrudates.
Amino acid Die size
4 mm (Exp. 1) 8 mm (Exp. 2)
Temperature (◦C) Pooled SEMaTemperature (◦C) Pooled SEMa
80 120 160 200 80 120 160 200
Indispensable amino acid
Arginine 8.9 8.7 8.9 8.4 0.3 8.7 8.3 8.6 7.4 0.6
Histidine 3.5 3.5 3.7 3.4 0.1 3.5 3.5 3.4 2.8 0.3
Isoleucine 7.0 6.9 7.0 7.0 0.1 6.8 6.8 6.7 5.9 0.4
Leucine 13.3 13.0 13.1 13.4 0.2 13.0 12.9 12.7 10.9 1.0
Lysine 9.1a 8.9a 9.0a 7.5b 0.3 8.9 8.9 8.6 8.4 0.1
Reactive lysine 7.8a 8.4a 8.3a 6.1b 0.2 8.0a 8.3a 8.3a 7.5b 0.1
Methionine 3.2 3.1 3.1 3.2 0.1 3.5 3.0 2.9 2.7 0.3
Phenylalanine 7.6 7.5 7.5 8.1 0.1 7.4 7.4 7.6 6.4 0.7
Valine 9.1 8.7 8.7 8.9 0.2 8.9 8.6 8.4 7.6 0.5
Dispensable amino acid
Alanine 10.0 9.8 9.6 9.9 0.2 9.7 9.7 9.2 8.5 0.4
Aspartic acid 13.9 13.7 14.1 13.9 0.3 13.1 13.4 13.3 11.2 1.0
Glutamic acid 27.4 27.4 27.7 28.1 0.4 29.4 27.3 26.9 21.4 3.0
Glycine 10.8 10.5 10.7 10.8 0.2 10.4 10.2 10.1 9.1 1.3
Proline 12.0b 14.5a 14.6a 13.9a,b 0.5 13.9 13.4 14.0 11.7 0.6
Serine 7.7 7.5 7.6 7.4 0.1 8.7 7.4 7.5 6.0 0.9
Threonine 6.4 6.3 6.4 6.4 0.1 6.5 6.2 6.2 5.1 0.6
Tyrosine 5.6 5.5 5.5 5.5 0.1 5.4 5.5 5.4 4.4 0.5
Different letters (a and b) denote significant differences (P<0.05) between means within row.
aSEM: n=3.
Håkansson et al. (1987) where the effects of thermal processes on whole-grain wheat and its derived flour were reported.
Iwe et al. (2001) have reported that lysine and arginine in particular as well as tryptophan, cysteine, methionine and histidine
are the amino acids that are most affected by reactions taking place during heating (e.g. extrusion and drying). However, only
lys is routinely used as an indicator for the evaluation of protein quality deterioration by the Maillard reaction (Erbersdobler
and Hupe, 1991; Hendriks et al., 1999). In the present study, the contents of amino acids lysine and proline in the 4 mm
kibbles were significantly decreased by the high drying temperature. No other amino acids were significantly lowered
although there were some lower values for methionine, histidine and arginine at the 8 mm diet samples (see Table 4).
Apart from Lankhorst et al. (2007), who found an increase in reactive lysine due to an initial increase in temperature,
several authors have reported a reduction in the quality of diet proteins/reactive lysine during manufacturing, due to exces-
sive heating during extrusion or drying as a result of Maillard reactions and emphasizes the use of special chemical assay
methodology (Moughan, 2003; Moughan and Rutherfurd, 1996). These reactions involve binding of free amino groups to the
carbonyl group of reducing sugars. Free amino groups exist in all crystalline amino acids and at the end of protein molecules.
Amino acids that are bound to protein are not reactive because they contribute their amino group to the peptide bond; lysine
is therefore a unique amino acid since it has two amino groups making protein-bound lysine also reactive.
The OMIU-reactive lysine contents, validated in heat-treated foodstuffs (Rutherfurd, 2010), in samples dried at 200 ◦C
were shown to be significantly lower (6% for 8 mm kibbles; 20% for 4 mm kibbles) compared to samples dried at lower
temperatures. The reactive lysine content after drying at temperatures of 120 and 160 ◦C was numerically higher than
after drying at 80 ◦C. This is in agreement with results of Arrage et al. (1992) who reported that reactive lysine of whole
wheat increased when dried at 152 ◦C compared to 79 and 93 ◦C. According to these authors, the moisture content of
the extruded products is rapidly reduced from 300 to 150 g/kg by oven-drying. This means that the extruded product
was held only a few seconds at the optimum water activity level required for Maillard browning. In a study into the
Maillard reaction and its influence on protein modification at different drying temperatures, Acquistucci (2000) reported
that major changes were observed when sample moisture ranged between 180 and 150 g/kg after the extrusion pro-
cess.
5. Conclusions
Drying can significantly affect the nutrient content of pet foods. At high drying temperatures (160–200◦C), lysine dam-
age occurs with reactive lysine decreasing faster compared to total lysine. No other amino acids were affected by drying
temperature in the present study except for proline in the 4 mm kibbles, which was increased by drying temperature of
200 ◦C. In addition to amino acids, most unsaturated long-chain fatty acids were observed to be decreased in the 4 and 8 mm
kibble dried at 200 ◦C. Hardness and specific density of kibbles were not affected by drying temperature. Durability of diets
produced with a 4 mm die size was numerically lower, while durability of diets produced with a 8 mm die size was signifi-
cantly decreased with increasing drying temperature. The drying of extruded pet foods can affect the nutrient composition
Author's personal copy
264 Q.D. Tran et al. / Animal Feed Science and Technology 165 (2011) 258–264
especially at high drying temperatures. Drying pet foods below 160 ◦C minimises the negative effects of the drying process
on nutrients.
Acknowledgements
The authors thank Mr. T. Zandstra, Mr. S. Alferink and Miss M. Domenis of the Wageningen University, Animal Nutrition
group for assistance in operating the extruder and in the drying experiment.
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