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W. H. Hendriks, M. M. Emmens, B. Trass and J. R. Pluske
rat bioassay
Heat processing changes the protein quality of canned cat foods as measured with a
1999, 77:669-676.J ANIM SCI
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669
1We thank S. M. Rutherfurd for assistance with the OMIU-
reactive lysine analysis.
2To whom correspondence should be addressed.
3Current address: Schoolstraat 214, 6014 GV, Roermond, The
Netherlands.
4Heinz Wattie’s Australasia, Hastings, New Zealand.
Received May 13, 1998.
Accepted September 12, 1998.
Heat Processing Changes the Protein Quality of Canned Cat Foods as
Measured with a Rat Bioassay
1
W. H. Hendriks2, M.M.A. Emmens3, B. Trass4, and J. R. Pluske
Monogastric Research Centre, Institute of Food, Nutrition and Human Health, Massey University,
Private Bag 11-222, Palmerston North, New Zealand
ABSTRACT: The purpose of this study was to
determine the influence of increasing heat treatment
on the protein quality of a canned cat food. A standard
recipe cat food was canned and heat-treated for
different times in a standard laboratory autoclave to
obtain experimental diets containing different lethal-
ity values. Estimates of the lethality value of the
different diets were calculated using the temperature-
time relationship recorded with a data logger posi-
tioned at the center of the can. The experimental diets
were analyzed for crude protein, amino acids, and
reactive lysine (fluorodinitrobenzene and O-
methylisourea) and were used in a rat bioassay for
the determination of the true ileal digestibility of
amino acids. The heat treatment of the cat food
resulted in experimental diets with lethality values of
5.3, 8.6, 17.2, and 24.3 min. There was no decrease in
the amino acid content of diet with increasing heat
treatment. The reactive lysine content of the diets also
showed no change with heat treatment. There were
significant ( P< .05) changes in the true ileal
digestibility of all amino acids and amino acid
nitrogen, and the digestibility of most amino acids
decreased with increasing heat treatment.
Key Words: Heat Treatment, Protein Quality, Lysine, Digestibility, Cats
1999 American Society of Animal Science. All rights reserved. J. Anim. Sci. 1999. 77:669–676
Introduction
Diets for companion animals such as cats and dogs
are extensively heat-processed to increase shelf life,
achieve a desired physical form, and(or) increase
palatability. The heat treatments most often used
include expansion, extrusion, baking, pasteurization,
and sterilization. There is a plethora of information
concerning the influence of various heat treatments on
the protein quality of human foods and on feeds for
production animals (e.g., Bender, 1978; Somogyi and
Mu
¨ller, 1989; van Barneveld, 1993; van der Poel et al.,
1993; Voragen et al., 1995). However, little informa-
tion is available on the effects of heat treatment on the
nutritive value of diets for companion animals.
Heat processing generally has a negative impact on
the nutritive value of pet foods (NRC, 1986; Lewis et
al., 1987; Heinicke, 1995). Loss of vitamins during the
production of pet foods has been documented (Roche,
1981), along with clinical signs of deficiency (Baggs et
al., 1978). Hickman et al. (1992) showed that heat
processing of a canned cat food changed the taurine
status of cats, which was shown later by Kim et al.
(1996a) to be a result of microbial deconjugation of
bile acids. To the authors’ knowledge, there is no
information available in the literature concerning the
effects of heat processing on the protein quality of
diets for cats.
The present work was undertaken, therefore, to
determine the influence of heat treatment on the
protein quality of a canned moist food for cats. The
diet was autoclaved for different times, and protein
quality was measured using in vitro assays and a rat
ileal digestibility assay.
Materials and Methods
Heat Processing of Cat Food. A standard recipe cat
food (Table 1) comprising 60% meat (from fish,
mutton, beef, and poultry offals), 37% water, 1.3% soy
protein concentrate, .6% gelling and stabilizing agents
(carageenan, locust bean gum, and guar gum), .4%
shell powder, and .7% minor ingredients such as
K2PO4, and vitamin premix was canned in 20
standard 700-g cans (diameter 83 ×143 mm). The
cans were cold-filled to 705 to 710 g using steam flow
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HENDRIKS ET AL.
670
Table 1. The nutrient composition of the unprocessed
cat food
Concentration,
Nutrient g/kg dry matter
Crude protein 567.3
Lipid 288.1
Ash 77.5
Amino acids
Arginine 35.1
Histidine 14.5
Isoleucine 18.6
Methionine 12.0
Leucine 44.0
Lysine 36.2
Phenylalanine 25.4
Threonine 24.2
Valine 28.3
Alanine 38.8
Aspartic acid 50.8
Glutamic acid 69.3
Proline 33.0
Glycine 48.1
Serine 26.2
Tyrosine 17.4
Table 2. Ingredient composition (g/kg air-dry diet)
of the cat food-based (CF) and enzymatically
hydrolyzed casein-based (EHC) experimental diets
aProcessed to contain lethality values of 0, 5.3, 8.6, 17.2, and
24.2. Nutrient composition of the unprocessed cat food is given in
Table 1.
bNew Zealand Pharmaceuticals Ltd., Palmerston North, New
Zealand.
cFielders Wheaten Cornflour, Starch Australasia Ltd., Tam-
worth, Australia.
dAhaki Chemical Industry Co Ltd., Osaka, Japan.
eProvided (g/kg diet): Ca, 4.6; P, 3.5 (CaHPO4·2H2O); K, 3.6
(K2SO4); Cl, 1.0; Na, .7 (NaCl); choline, .9 (choline chloride); Mg,
.5 (MgSO4·7H2O); (mg/kg diet); Fe, 36 (FeSO4·H2O); all-rac-a-
tocopherol, 33; Zn, 26 (ZnSO4·H2O); nicotinic acid, 21; Mn, 13
(MnSO4·H2O); pantothenate, 10; Cu, 6.5 (CuSO4·5H2O); pyridox-
ine, 6.0; thiamine, 4.0; riboflavin, 3.5; cholecalciferol, 1.7; folic acid,
1.0; phylloquinone, 1.0; retinol, .7; cobalamin, .5; I, .3 (CaIO3);
biotin, .2; and Se, .1 (Na2SeO4).
Diet
Ingredient CF EHC
Cat fooda185.8 —
Enzymatically hydrolyzed
caseinb— 123.8
Wheat starchc572.9 634.9
Sucrose 100.0 100.0
Purified cellulosed50.0 50.0
Soybean oil 50.0 50.0
Vitamin-mineral mixe36.3 36.3
Chromic oxide 5.0 5.0
closure to give a vacuum of 17 to 34 kPa. Four cans
were immediately frozen after canning, and the
remaining 16 cans were heat-treated at 121 ±1°Cina
laboratory-scale autoclave in batches of four cans. One
can in each batch contained a Tracksense data logger
(ELLAB A/S, Roedovre, Denmark) that recorded the
temperature at the center of the can at 5-min
intervals. The autoclave temperature was monitored
using a standard thermometer positioned directly
above (.1 m) the cans being processed. The four
batches of cans were heat-treated for different periods
of time, ranging from approximately 80 to 120 min, so
that the center of the can was exposed to a calculated
lethality value of 5, 10, 15, or 25. Lethality value of a
thermal process is designated by the symbol F0and
represents the time equivalent (minutes) of a heating
process to destroy microorganisms at the reference
temperature of 121.1°C. After processing, the cans in
each batch were cooled for 30 min in running water,
and the data logger was recovered. A Tracksense
interface station (ELLAB A/S) was used to read the
stored temperature data, and the lethality value for
each batch of cans was calculated using Tracksense
PCSOFT92/PCLINK92 software (ELLAB A/S).
Rat Digestibility Study. Approval for this experi-
ment was granted by the Animal Ethics Committee at
Massey University, Palmerston North, New Zealand.
Sprague Dawley male rats (n = 36) weighing 157 g
(SE 4.0) were obtained from the Small Animal
Production Unit at Massey University. The rats were
randomly allocated to one of six diets such that there
were six rats on each diet. The rats were housed
individually in stainless steel wire-bottomed cages at
22°C(±2.0) with a 12-h reverse light/dark cycle. A
basal diet was formulated to exceed the requirements
of growing rats (NRC, 1995) for all nutrients except
protein. The protein source in the experimental diets
was included at 10% and was supplied by the cat food
processed to different lethality values. A diet based on
enzymatically hydrolyzed casein (EHC) was also
formulated to allow determination of endogenous ileal
amino acid flows (Butts et al., 1991). The ingredient
composition of the experimental diets is shown in
Table 2.
The rats were acclimatized for 3 d before the start
of the study, during which time they were fed a
normal rat chow to appetite. The rats received their
respective experimental diets during nine equal meals
served hourly between 0800 and 1600. The hourly
feeding regimen was employed to ensure a constant
flow of material at the terminal ileum. At each
mealtime the diets were available for a 10-min period.
The rats were weighed on the first and final day of the
study, and food intake was recorded hourly. Fresh
water was available at all times.
On the 9th d of the study, from 6 to 8 h after the
start of the hourly feeding regimen, the rats were
asphyxiated in carbon dioxide gas and then decapi-
tated. The body cavity of the rat was opened, and 20
cm of the ileum (approximately 20% of the length of
the small intestine) immediately anterior to the
ileocecal junction was dissected out. The dissected
ileum was washed with distilled deionized water to
remove any blood and hair and then carefully blotted
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PROCESSING CHANGES CAT FOOD PROTEIN QUALITY 671
on an absorbent paper towel. The digesta were then
gently flushed from the ileal section using a syringe
with distilled deionized water. The digesta of the rats
fed the diets containing cat food were freeze-dried,
thoroughly mixed, and analyzed for dry matter,
chromium, and amino acids. The digesta of the rats
fed the EHC-based diet were adjusted to approxi-
mately pH 3 with 6 MHCl to minimize protease
activity and then centrifuged and ultrafiltered accord-
ing to the procedure of Butts et al. (1991) before
chemical analyses.
The experimental diets containing cat food were
analyzed for dry matter, chromium, and amino acids.
The cat foods with different lethality values were
analyzed for dry matter, nitrogen, amino acids, and
reactive lysine determined with the fluorodinitroben-
zene and O-methylisourea method. The unprocessed
cat food was also analyzed for crude fat and ash.
Chemical Analyses. Total nitrogen was determined
in duplicate using the Kjeldahl method (AOAC,
1995), and crude protein was calculated by multiply-
ing total nitrogen by 6.25. Dry matter was determined
in duplicate by drying samples at 105°C to constant
weight, and ash was determined by heating the
samples at 550°C for 16 h. Lipid was determined by
petroleum ether extraction of duplicate freeze-dried
samples (AOAC, 1995). The chromium contents of the
diet and ileal digesta samples were determined in
duplicate on a GBC 902 AA absorption/emission
spectrophotometer (GBC Scientific NZ Ltd, Auckland,
New Zealand) following the method of Costigan and
Ellis (1987).
Amino acids were determined on duplicate
5-mg samples by hydrolyzing with 1 mL of 6 Mglass-
distilled HCl (containing .1 g phenol/L) for 24 h at
110°C(±2.1) in glass tubes, sealed under vacuum.
The tubes were opened, norleucine was added to each
tube as an internal standard, and the tubes were then
dried under vacuum (Savant Speedvac Concentrator
AS 290, Savant Instruments, Farmingdale, NY).
Amino acids were dissolved in 2 mL of sodium citrate
buffer (pH 2.2) and loaded onto a Waters ion-
exchange HPLC system (Millipore, Milford, MA)
employing postcolumn derivatization with ninhydrin
and detection at 570 nm. Proline was detected at 440
nm. The chromatograms were integrated using dedi-
cated software (Maxima 820, Waters, Millipore) with
amino acids identified by retention time against a
standard amino acid mixture (Pierce, Rockford, IL).
Tryptophan and cysteine were not determined. No
corrections were made for loss of amino acids during
acid hydrolysis. Amino acid concentrations were cor-
rected for recoveries of norleucine and converted to a
weight basis using free amino acid molecular weights.
1-Fluoro-1, 4-dinitrobenzene (FDNB)-reactive ly-
sine was determined according to the method of
Carpenter (1960) using the modifications described
by Booth (1971). The method involves reacting the
lysine with FDNB in ethanol and NaHCO3at room
temperature for 2 h followed by hydrolysis of the
formed dinitrophenyl (DNP)-protein in 5.8 MHCl for
16 h, extraction with diethyl ether to remove interfer-
ing components, and measurement of the DNP-lysine
by absorbance at 435 nm. A blank value is obtained by
treatment of a paired hydrolyzed sample with methox-
ycarbonyl chloride followed by extraction with diethyl
ether. Correction factors for the loss of DNP-lysine
during acid hydrolysis were determined according to
the method of Booth (1971).
O-Methylisourea (OMIU)-reactive lysine was de-
termined according to the method described by
Rutherfurd et al. (1997). This involved incubation of
the sample (5 mg) with .5 to 1.0 mL of .6 MOMIU in
a shaking water bath at 21°C for 3 d. The formed
homoarginine was measured in the dried sample
according to the amino acid analysis procedure
described previously.
Materials. The FDNB, DNP-lysine, and OMIU were
obtained from Sigma Chemical (St. Louis, MO).
Barium hydroxide octahydrate was obtained from
BDH Laboratory Supplies (Poole, U.K.). Centriprep
10 disposable ultrafiltration devices were obtained
from Amicon (Beverly, MA).
Data Analysis. Amino acid flows at the terminal
ileum for each rat were calculated using the following
equation (units are mg/g DMI): Ileal amino acid flow =
amino acid concentration in ileal digesta ×(diet
chromium/ileal chromium). True ileal amino acid
digestibility was calculated using the following equa-
tion ( mg/g DMI): True ileal amino acid (AA) digesti-
bility (%) = [dietary AA intake −(ileal AA flow −
endogenous ileal AA flow)]/dietary amino acid intake)
×100.
The true ileal digestibility coefficient for each amino
acid was tested for homogeneity of variance using
Bartlett’s test (Snedecor and Cochran, 1980). When
the variance was heterogeneous, the data were
transformed (log10). True ileal amino acid digestibili-
ties were subjected to ANOVA (SAS, 1985) with
lethality value as the variable. The mean daily food
intake data of the rats receiving the cat food
experimental diets were subjected to repeated meas-
ures ANOVA (SAS, 1985) with diet as the variable
and time (d) as the repeated factor.
Results
The measured lethality values for the differently
processed batches of cat food, as calculated using the
temperature data recorded with the data logger at the
center of the can, were 5.3, 8.6, 17.2, and 24.2.
The rats consumed the experimental diets readily
and appeared healthy throughout the study. The rats
gained weight during the trial; they weighed 157 g
(SE 4.0) on d 1 and 187 g (SE 4.3) on d 9. There was
no difference (P> .05) in food intake of the rats
receiving the different cat food diets as analyzed by
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HENDRIKS ET AL.
672
Table 3. Amino acid composition (g/16 g N) of a canned cat food heat-treated to
contain different lethality values (min)
Lethality value
Amino acid 0 5.3 8.6 17.2 24.2
Arginine 6.2 6.3 6.5 6.5 6.4
Histidine 2.6 2.6 2.7 2.6 2.7
Isoleucine 3.3 3.4 3.5 3.5 3.5
Methionine 2.1 2.2 2.3 2.3 2.3
Leucine 7.8 7.9 8.1 8.1 8.1
Lysine 6.4 6.8 6.9 6.9 6.9
Phenylalanine 4.5 4.7 4.8 4.8 4.9
Threonine 4.3 4.3 4.5 4.4 4.5
Valine 5.0 5.1 5.2 5.2 5.2
Alanine 6.8 6.9 7.0 7.0 7.0
Aspartic acid 9.0 9.1 9.4 9.4 9.4
Glutamic acid 12.2 12.5 13.1 13.0 13.1
Proline 5.8 5.9 6.2 6.3 6.5
Glycine 8.5 8.8 9.0 8.8 9.0
Serine 4.6 4.8 4.9 4.9 4.9
Tyrosine 3.1 3.2 3.4 3.3 3.4
Table 4. The concentration (g/kg dry matter) of crude protein, total lysine, fluorodinitrobenzene (FDNB)- and
O-methylisourea (OMIU)-reactive lysine of a cat food heat-treated to contain different lethality values (min)
aThe value in parentheses is the percentage of the crude protein content.
bThe correction factor used for the FDNB method was 1.05.
Lethality value
Chemical component 0 5.3 8.6 17.2 24.2
Crude protein 567 564 563 568 565
Total lysine 36.2 (6.4)a38.1 (6.7) 39.1 (6.9) 39.2 (6.9) 39.2 (6.9)
FDNB-reactive lysineb32.9 (5.8) 31.2 (5.5) 32.3 (5.7) 34.1 (6.0) 32.4 (5.7)
OMIU-reactive lysine 31.9 (5.6) 32.9 (5.8) 34.1 (6.1) 32.7 (5.8) 32.4 (5.7)
repeated measures analysis. There was an effect (P<
.001) of time on the food intake of the rats; food intake
increased as the study proceeded. The food intake of
rats receiving the experimental diets containing cat
food on the first (total of nine meals) and last (total
of six meals) days of the study were 14.4 (SE, .39)
and 11.7 g (SE, .50), respectively. The food intake of
rats fed the EHC-based diet on the first (total of nine
meals) and last (total of six meals) days were 12.7
(SE, .38) and 13.5 g (SE, .11), respectively. Feces
were not detected in the gastric content at slaughter,
indicating that coprophagy had not occurred.
The amino acid composition of the unprocessed
(F0) and processed (F0= 5.3, 8.6, 17.2, or 24.2) cat
food is shown in Table 3. There was no apparent
decrease in the concentration of amino acids due to the
processing of the cat food.
The crude protein and total and reactive lysine
content of the cat food containing different lethality
values is given in Table 4. The differences in crude
protein, total lysine, and reactive lysine content
among the five cat foods were small. The crude protein
content varied between 563 (F0= 8.6) and 568 g/kg
dry matter (F0= 17.2), and the total lysine content
varied from 36.2 (F0= 0) to 39.2 g/kg dry matter (F0
= 17.2 and 24.2). The FDNB-reactive and OMIU-
reactive lysine content of the cat foods with different
lethality values, when expressed on a dry matter or
crude protein basis, were consistently lower than the
corresponding total lysine values. The differences
between the FDNB-reactive lysine content and the
OMIU-reactive lysine content were small. There was
no significant change in the reactive lysine content
due to the processing of the cat food.
The true ileal amino acid digestibility and amino
acid nitrogen digestibility of the unprocessed cat food
and the processed cat foods are shown in Table 5.
There was an effect (P< .05) of lethality value on the
true ileal digestibility of all amino acids, and the
majority of the effects were significant at P< .001. In
general, the true ileal amino acid digestibility coeffi-
cients were high in the unprocessed cat food and
decreased with increasing lethality value. The true
ileal digestibility of arginine, histidine, alanine, pro-
line, and glycine, however, increased as a result of the
first heat treatment (F0= 5.3), whereafter the
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PROCESSING CHANGES CAT FOOD PROTEIN QUALITY 673
Table 5. Mean true ileal amino acid digestibility coefficients (%) of a canned cat food heat-treated
to contain different lethality values (min)a
aValues are means for six rats.
Lethality value Overall
SE
Amino acid 0 5.3 8.6 17.2 24.2 P<
Arginine 87.8 90.0 86.9 84.8 84.5 .81 .001
Histidine 69.4 75.7 67.0 69.6 64.2 1.95 .01
Isoleucine 85.2 80.8 76.5 73.5 72.0 1.31 .001
Methionine 85.9 81.9 78.2 75.2 75.0 1.12 .001
Leucine 87.7 83.2 79.7 77.6 76.7 1.04 .001
Lysine 84.2 84.1 81.1 78.3 77.4 .94 .001
Phenylalanine 83.4 79.2 75.9 73.8 71.6 1.45 .001
Threonine 77.1 75.3 70.5 66.9 64.5 1.61 .001
Valine 84.2 80.2 75.4 73.1 71.9 1.34 .001
Alanine 77.8 80.0 75.7 71.9 71.7 1.25 .001
Aspartic acid 78.3 58.7 49.0 43.0 40.2 2.43 .001
Glutamic acid 81.3 80.2 76.1 72.5 71.6 1.25 .001
Proline 64.2 72.4 69.2 66.2 64.7 1.89 .05
Glycine 44.8 62.3 52.9 46.4 45.0 2.78 .001
Serine 75.7 74.2 69.8 64.7 62.6 1.62 .001
Tyrosine 87.6 80.9 77.1 74.5 73.2 1.62 .001
Amino acid nitrogen 76.7 77.4 72.5 69.3 67.9 1.37 .001
digestibilities of these amino acids decreased similarly
to the other amino acids.
Discussion
Animal models are often used for ethical or
practical reasons because direct measurements on the
animal species of interest may be difficult to obtain.
The rat is a convenient animal model, and is often
used for determination of protein quality in human
diets (FAO/WHO, 1990). Recently the rat was shown
to be an accurate animal model for estimating the ileal
digestibility of amino acids of protein sources used in
diets for pigs (Donkoh et al., 1994; Pearson et al.,
1998) and chinook salmon (Wright, 1996). In the
present study, besides various in vitro assays, an in
vivo rat ileal digestibility assay was used to measure
protein quality. The laboratory rat, however, has not
been validated as an animal model for the digestion of
protein in diets for domestic cats. An indication of the
suitability of the rat as an animal model for the
digestion of protein in cats can be obtained from the
anatomy of the digestive tract of both animals. The
ratios of mucosal to serosal area of the intestine of cats
have been found to be as follows: jejunum 15:1, ileum
12:1, and colon 1.1, and these values for rats are 6:1,
4:1, and 1:1, respectively (Wood, 1944). Even though
the mucosal area to serosal area is greater in cats, the
ratios of mucosal area of the entire small intestine to
body weight (absorbable surface area of amino acids
per unit body weight) in cats and rats are almost
identical (Wood, 1944). Gross anatomy of the diges-
tive tract, therefore, indicates that the rat may be a
suitable animal model for the digestion of protein in
cats. However, many other factors also determine the
in vivo digestibility of protein, and the rat as an
animal model for cats, will need to be validated
experimentally.
The effectiveness of a heating process in achieving
product sterility can be evaluated by determination of
the lethality value (F0) of the process. The F0
represents the time equivalent of a heating process to
destroy microorganisms at the reference temperature
of 121.1°C, and serves as a standard to compare
sterilization values for different processes. A product
processed to an F0of 10, theoretically, gives 100%
sterilization, because this is equivalent to processing
for 10 min at 121.1°C (Ball and Olson, 1957). Rather
than measuring the external thermal energy applied
to the product, F0provides a measure of the effective-
ness of the heating applied at the coldest site within
the product. The lethality value of a process can be
calculated using heat transfer equations with the
appropriate boundary conditions or can be determined
from the relationship between time and temperature
(Holdsworth, 1985). The data logger used in the
present study recorded the temperature at the center
of the can at 5-min intervals and provided accurate
information on the time-temperature relationship at
the coldest point of the can during processing. The F0
values calculated using the time-temperature relation-
ship for the various batches (5.3, 8.6, 17.2, and 24.2)
agreed closely with the targeted F0values of 5, 10, 15,
and 25 (calculated based on previous heat penetration
data), although one batch was slightly underprocessed
(F0= 8.6) and another batch was slightly over-
processed (F0= 17.2). Canned pet foods are normally
heat-processed to an F0value of 12 to 14, and a value
of 8 is accepted as the minimum. Heat processing the
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HENDRIKS ET AL.
674
center of a can to an F0value lower then 8 to 10
increases the risk of spoilage of the product by
microorganisms during storage and may ultimately
result in cans exploding due to a build-up of pressure.
Heat processing of the canned cat food in the
present study did not alter the amino acid content of
the diet. These results indicate that heat treatment of
canned cat foods does not result in destruction of
amino acids. The side chain of lysine readily combines
with reducing sugars (Maillard reaction), oxidizing
lipids, and polyphenolic acids during processing and
storage to give covalent complexes or oxidation
products (Hurrell, 1989). It has been suggested
(NRC, 1986; Morris and Rogers, 1994; Heinicke,
1995) that lysine in diets for companion animals may
undergo Maillard-type reactions during the heat-
processing step in their manufacturing, thereby reduc-
ing lysine availability. In the present study, the total
lysine content (expressed both on a dry matter and
16-g N basis) was slightly higher in the heat-treated
samples than in the unheated sample, indicating that
there was no significant destruction of lysine as a
result of heat processing. Additionally, heat treatment
did not result in a reduced reactive lysine content. The
FDNB-reactive and OMIU-reactive lysine content
remained relatively constant with increasing heat
treatment of the diet. These results indicate that the e-
amino group of lysine did not react with other dietary
components during heat processing of the moist cat
food. This was unexpected, because lysine has been
shown to undergo chemical reactions with other
compounds present in a complex feed and feed
ingredients under mild heating conditions (Ruther-
furd and Moughan, 1997; Rutherfurd et al., 1997).
Some lysine in the diet, however, was chemically
unreactive, because the FDNB- and OMIU-reactive
lysine contents were approximately 85% of the total
lysine content. This unreactive or bound lysine, which
reverted back under the conditions of acid hydrolysis,
likely originated from the ingredients and(or) a heat
processing step used in the formulation of the diet,
because the reactive lysine content in the unprocessed
diet was also lower than the total lysine content.
Meat offals contain relatively large amounts of
connective tissue that consist of collagen and elastin
(Davey and Winger, 1979). Collagen naturally con-
tains covalent cross-links involving lysine to maintain
the native three-dimensional structure of the protein
(Asghar and Henrickson, 1982; Singh, 1991). Colla-
gen is, therefore, a natural source of bound lysine,
which most likely explains the results found in the
present study. The diet did not contain any blood meal
or meat and bone meal, two ingredients that have
been shown to contain bound lysine (Rutherfurd et
al., 1997).
Heat treatment affected the true digestibility of all
amino acids as measured at the terminal ileum of the
rats, with a general tendency for true ileal amino acid
digestibility coefficients to be high in the unprocessed
cat food and to decrease with increasing lethality
value. The digestibility coefficient of aspartic acid
showed the largest change, decreasing approximately
38 percentage units. For glycine and proline there was
a marked increase in digestibility due to the first heat
treatment (F0= 5.3). As mentioned previously, meat
offals contain relatively large amounts of connective
tissue that consist of collagen and elastin, two types of
protein high in glycine and proline (Asghar and
Henrickson, 1982). Upon heat treatment, collagen is
gelatinized (Hamm, 1974; Asghar and Henrickson,
1982) and becomes more digestible. The relatively
mild heat treatment employed to obtain the diet with
an F0value of 5.3 is likely to have gelatinized the
connective tissue, making it more digestible and, as a
result, increased the digestibility especially of glycine
and proline.
There are several possible explanations for the
reduction in amino acid digestibility with increasing
heat treatment. The food intake of rats on the
different diets was similar, indicating that the differ-
ences in amino acid digestibility seen in the present
study were not caused by differences in food intake.
Changes in digesta viscosity have been shown to affect
the digestibility of nitrogen and amino acids in other
animals, such as chickens (Smits et al., 1997) and
pigs (van Barneveld et al., 1995). Guar gum, a soluble
nonstarch polysaccharide (NSP), is a common ingre-
dient in canned cat foods and has been shown to
decrease the digestibility of protein in diets for cats
(Harper and Siever-Kelly, 1997). The diet used in the
present study was formulated using guar gum, locust
bean gum, and carrageenan; all are soluble NSP
sources. It is possible, therefore, that the different
heat treatments affected the viscosity of the diet,
thereby causing differences in digesta viscosity and
affecting amino acid digestibility. The viscosity of the
five diets, when analyzed using a cone-plate viscome-
ter (Smits et al., 1997), was similar (data not
shown), suggesting that the reduction in amino acid
digestibility with increasing heat treatment was
caused by differences other than digesta viscosity.
A likely cause of the decrease in amino acid
digestibility with increasing heat treatment may have
been the formation of cross-linkages between various
amino acids within and between proteins. Cross-
linking reduces the rate of protein digestion by
preventing enzyme penetration or by masking the
sites of enzyme attack (Hurrell and Finot, 1985) and
can occur between many amino acids. However, lysine,
cysteine, and phosphoserine seem to be the most
susceptible (Bender, 1978; Singh, 1991). Wiseman et
al. (1991) subjected commercially processed fish meal
to additional heating at 130°C for 3 h or 160°C for 1.5
h and measured the ileal digestibility of amino acids
in pigs. There was no effect of the additional heating
on the level of amino acids (including lysine) in the
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PROCESSING CHANGES CAT FOOD PROTEIN QUALITY 675
fish meal. However, the 2,4,6-trinitrobenzene sul-
phonic acid reactive lysine level and the ileal digesti-
bility of all amino acids decreased with increasing
heat treatment, indicating that cross-linking had
occurred. In the present study, the reactive lysine
content did not decrease with increasing heat treat-
ment of the diet, indicating that cross-linkages involv-
ing lysine were not formed. The latter casts doubt on
cross-linking being the mechanism responsible for the
reduction in ileal digestibility of amino acids in the
present study. However, other amino acids such as
cysteine may have formed cross-linkages, thereby
reducing the digestibility of amino acids.
Another possible mechanism to explain the
decrease in true ileal digestibility of amino acids with
increasing heat treatment is increased gut endogenous
amino acid excretions with increasing heat treatment.
The EHC/ultrafiltration method used in the present
study determines the endogenous amino acid losses,
which are due to the ingestion of the N-free basal
mixture plus protein source (Boisen and Moughan,
1996). It is possible that other components that may
have formed in increasing amounts with increasing
heat treatment in the cat food caused “additional”
endogenous amino acid excretions. Because these
additional endogenous amino acid losses would not
have been measured in the present study, these
increased losses may have lowered the digestibility of
the amino acids. However, in order to explain the
reduction in aspartic acid digestibility, the endogenous
excretions of the rats fed the experimental diet
containing cat food with an F0value of 24.2 must have
been increased by a factor of five.
In addition to the changes in protein quality found
in the present study, other effects of heat processing
on moist canned cat foods have been observed.
Heinicke (1995) noted that a longer processing time
negatively affects the palatability of canned cat foods,
an observation that we have also seen. Furthermore,
processing of canned cat foods has been shown to
increase the endogenous taurine loss from the gas-
trointestinal tract of cats (Hickman et al., 1992). Kim
et al. (1996a) showed that the cause of the increased
endogenous taurine loss was due to the increased
deconjugation of bile acids by microorganisms. These
authors hypothesized that a greater level of available
substrate at the lower intestine is responsible for the
increase in microbial activity and, subsequently,
causes the increased deconjugation of bile acids in cats
(Kim et al., 1996b). The present study supports this
hypothesis; it was shown that the ileal amino acid
digestibility of a canned cat food decreases with
increasing heat treatment, resulting in more un-
digested amino acids at the terminal ileum.
Heat treatment of canned cat foods results in
changes in the digestibility of amino acids, and most
amino acids show a decrease in digestibility. The
likely cause of this decrease in digestibility is at
present unknown, and further studies are required.
Although the digestibility of lysine is reduced due to
the heat treatment of canned cat foods, the e-amino
group of lysine does not seem to react with other
dietary components. The heat treatment of canned cat
foods, furthermore, does not result in destruction of
amino acids.
Implications
The processing of canned moist diets for cats results
in changes in the digestibility of amino acids, and the
majority of amino acids show a decrease in digestibil-
ity as heat processing time increases. Overprocessing
above the minimum time required for sterilization of
the product results, therefore, in an avoidable loss of
amino acids. Accurate control on heat processing
conditions of canned moist diets for cats is important
to minimize the reduction in amino acid digestibility,
to maintain a high palatability, and potentially to
maintain adequate taurine status.
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