The Effect of Fat-enriched Diets on the Perirenal Fat Quality and

MearScience,Vol.47, No. l/2, 9s-103, 1997
0 1997Elsevier Science Ltd
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ELSEVIER
The Effect of Fat-enriched Diets on the Perirenal Fat Quality
and Sensory Characteristics of Meat from Rabbits
MaAngels Oliver,“* Luis Guerrero,a Isabel Diaz,” Marina Gispert,”
Marcia1 Plab & Agustin Blascob
OInstitut de Recerca i Tecnologia Agroaliment&ies (IRTA), Centre de Tecnologia de la Carn,
Granja Camps i Armet, 17121, Monells, Girona, Spain
bDepartamento de Ciencia Animal, Universidad Politecnica de Valencia, Camino de Vera, 14,
46020, Valencia, Spain
(Received 5 August 1996; revised version received 14 April 1997; accepted 20 April 1997)
ABSTRACT
Two hundred and eighty four medium sized young rabbits were fed ad libitum with two
fat-enriched experimental diets (V: vegetable fat, A: animal fat) and a control diet (C).
Diet had a significant effect on the ultimate pH (pHu) of the Longissimus dorsi muscle
and on the colour, weight and fatty acid composition of the perirenal fat. The carcasses
from diet C had significantly less perirenal fat (14.2~) compared to the carcasses from
group A and V (24.4 and 23.Og) using carcass weight and age of the animals as covariables. Supplementing the diet with animal fat produced perirenal fat which was richer in
oleic acid (41%) than In groups V (20.9O%) and C (27.9%). The P:S ratio was higher in
group V (2.45) compared with groups A and C (0.53 and 0.42). Diet also affected the
organoleptic quality of the loin meat. The meat from groups V and C was considered to
have a more aniseedfiavour than that from group A. In contrast the loin meat from group
A had more liverjlavour. Both groups V and A were considered juicier than group C. No
d@erences were found in hardness, chewiness and fibrousness. These results suggest than
from a human health point of view, the use of a diet supplemented with animalfat is useful
on the basis of the nutritional index, P:S of the perirenal fat. However the amount of fat
added in the diets V and A should be less than used in this study if the carcass is sold whole
in order to avoid excessive perirenal fat. From an organoleptic point of view, the greater
aniseed and grass Jlavour of group V compared to group A may give meat of better sensory
quality. 0 1997 Elsevier Science Ltd
INTRODUCTION
In Europe, the most important
countries producing
rabbit meat are Italy, France and
Spain. Average consumption
in Spain is around 2.5 kg per person per year according to
1992 data (Carnica 2000, 1996). Traditional
shops sell over 50% of the total rabbit meat
*To whom correspondence
should be addressed.
95
A. Oliver et al.
96
in Spain, usually as whole carcasses. Supermarkets sell about 15% as cuts. Where rabbit
meat is sold as whole carcasses, excessive amounts of perirenal fat is a drawback from
both appearance and dietary considerations, since it is the largest deposit of fat in the
rabbit (Ouhayoun et al., 1987). Also, in monogastric animals including the rabbit, it has
been demonstrated that the quantity and composition of the fatty acids in the fat and
meat can be manipulated by diet. Many studies have confirmed this and it appears that
the majority of ingested fatty acids are incorporated directly into the fat (Raimondi et al.,
1975; Fraga et al., 1983; Ouhayoun et al., 1987).
One of the main aims in producing healthy meat is to reduce the saturated fatty acids
and increase the unsaturated fatty acids in the fat. The nutritional quality of fat has been
assessed by the ratio of P:S (C18:2+C18:3/C14:0+C16:0+C18:0).
For a whole diet P:S
ratios of 0.45 or higher are recommended (Enser et al., 1996). High levels of oleic acid in
the diets of rabbits leads to an increase of this acid in the fat and a decrease in palmitic
acid, which is regarded as beneficial from the point of view of human health (Ouhayoun et
al., 1987). Feeding rabbits with diets containing considerable amounts of linoleic acid
brings about an increase in the amount of this acid in the perirenal fat, though only up to
a certain point (Ouhayoun et al., 1987; Cobos et al., 1993).
Various studies suggest that rabbit meat has less fat and calories than beef, chicken, lamb
and pork and also a lower cholesterol content (Lee and Ahn, 1977; Rao et al., 1978;
Holmes et al., 1984; Lukefahr et al., 1989; Youssef et al., 1994). These characteristics,
together with the possibility of manipulating the composition of the fatty acids through
diet, means that rabbit meat could be valuable in human nutrition. With this in mind, the
use of industrial by-products (soya full-fat, soya oil and animal or vegetable fat) which
reduce feeding costs is of great interest, provided these do not impair the organoleptic or
nutritional quality of the meat. Cobos et al. (1995) found no effect on the composition of
the fatty acids of rabbit meat using 15% sugar beet pulp instead of barley in the feed.
However other authors have shown that diet may affect the organoleptic quality of rabbit
meat (Ouhayoun et al., 1987).
The aim of this study was to examine the effect of adding vegetable and animal fat to
the diet of rabbits, on the quantity and nutritional quality of the perirenal fat and the
organoleptic quality of the meat.
MATERIALS
AND METHODS
Two hundred and eighty-four medium sized young rabbits of the same genetic type (a
three-way cross, males and females) were divided into three groups at the weaning stage
and fed ad libitum on three different diets. Two experimental and one control diet were
used. The control diet (C) was a commercial diet (ether extract 2.6%). The experimental
diets contained either vegetable fat (V, ether extract, 9.9%) or animal fat [a commercial
mixture, 65% lard, 25% tallow and 10% poultry fat (Chillinach and Ballester, Carpesa,
Valencia) (A), ether extract 11.7%]. At the age of 65 &3 days, 60 animals from each
group, weighing 1.75-2.25 kg, were slaughtered on the same day at the farm, thereby
avoiding stress caused by transportation. No fasting procedure was used.
Chemical analysis of the diet
The diets were formulated to include all essential nutrients to ensure the proper growth of
the animals (Lebas, 1979). The feed ingredients used for the various diets in the experiment are shown in Table 1 together with their chemical and fatty acid compositions and
digestible energy. Moisture, ash, crude fibre, crude protein, ether extract and digestible
97
Effect of fat enriched diets
TABLE 1
Main Ingredients (% of Diet), Chemical Composition (% DM), Main Fatty Acid Composition (%
of Fatty Acids) and Digestible Energy (MJ kg-’ MS) of Experimental Diets (V and A) and Control
Diet (C)
Diet”
A
c
24
50
2.5
0.1
2.8
0.4
0.2
20
18
50
8.5
0.1
2.8
0.4
0.2
35
12
50
0.1
2.8
0.4
0.2
Chemical composition
Moisture
Ash
Crude protein
Ether extract
Crude fibre
9.28
10.7
20.0
9.9
16.9
9.30
10.7
18.9
11.7
16.9
9.24
10.7
18.9
2.6
16.9
Fatty acids
14:o
16:0
16:l
18:O
18:l
18:2
18:3
0.2
13.0
0.3
4.3
18.7
53.7
9.8
1.9
23.0
2.8
12.4
36.9
18.7
4.3
0.7
24.4
1.2
4.0
11.6
43.6
14.5
Digestible energy
Ratio P:S*
Ratio U:S*
Total 18:2*
12.7
2.42
3.36
5.31
12.4
0.52
1.77
2.18
11.2
0.42
1.16
1.14
V
Ingredients
Barley
Soya meal
Soya full-fat
Alfalfa hay
Soya oil
Animal fat
DL-metionine
Bicalcium phophat
Salt
Mixture
20
“V =experimental diet by addition of vegetable fat (ether extract 9.9%). A=experimental
addition of animal fat (ether extract 11.7%). C =control diet (ether extract 2.6%).
*P:S=(C18:2+C18:3/C14:0+C16:0+C18:0).
*U:S=(C18:1 +C18:2+C18:3/C14:0+C16:0+C18:0).
*Total 18:2: expressed as a percentage of DM.
diet by
by the method of Santoma et al. (1987). The digestible energy was
similar for diets V and A. For fatty acid composition, the fat was extracted by the method
of Folch et al. (1957). Methyl esters of fatty acids (FAMES) were obtained by the following procedure: an aliquot (100mg extractable fat) was evaporated and 1 ml of 14%
BF3/methanol added and the mixture heated at 70°C for 20min. After cooling, lOm1 of a
saturated solution of NaCl was added and FAMES were extracted with 2ml of hexane;
one ~1 of this solution was injected and analysed by capillary GC under the following
conditions: FSOT capillary column 50% cyanopropylsilicone (25 m x 250 pm, width film
energy were determined
A. Oliver et
98
al.
0,250 CL);temperature program: 150”C-3”Cmin-‘-21O”C
(5 min), injector and detector
temperature 250°C injection mode split (40mlmini),
carrier gas helium at 30cm s-i,
detector FID. A Dani HR-3800 gas chromatrograph was used.
Meat quality measurements and fatty acid composition
The ultimate pH at 24 h (pHu) post mortem (p.m.) was determined using a Crison pH
meter with a combined electrode by penetrating 3 mm into the longissimus dorsi (LD)
muscle. The colour was assessed in a transversal cut of loin and at the surface of the
perirenal fat at 24 h p.m., using a MINOLTA CR200 calorimeter.
The perirenal fat was collected from the carcass, weighed and frozen at -20°C until
analysed. The fatty acid composition was determined as described above.
Sensory evaluation of the rabbit meat loin
A quantitative descriptive analysis (Stone et al., 1974) was carried out by eight trained
tasters of rabbit meat in 16 sessions. First four sessions of generation, selection and
description of attributes were undertaken. The parameters retained were: aniseed flavour,
grass flavour, liver flavour, sweet taste, toughness, juiciness, chewiness and fibrousness.
The sensory analysis was carried out on samples of the longissimus dorsi muscle following
a complete block design (Steel and Torrie, 1980). Samples were cooked in an electric oven
for 22min at 180°C cut into four pieces and distributed in such a way to the panellists to
eliminate any location effect within the loin. To prevent cooling, samples were served on
preheated plates.
Statistical analysis
A least squares analysis was performed using the General Linear Model program of the
SAS statistical package (Statistical Analysis Systems Institute, SAS, 1988). The effect of
the interaction diet xsex was analysed and found not to be significant for any of the variables studied, so the definitive model was as follows:
Y,,
Y,,
=
=
_
p+Di+Sj+bW+CA+egk
where:
ijkth observation;
general average;
1 diet effect (i= V, A, CT);
sex effect (j = 1,2,);
carcass weight covariable;
W=
A = age at slaughter covariable;
residual random term.
Li
Sj
=
The carcass weight and age at slaughter were used as covariables.
RESULTS
Analysis of diets
With regard to the chemical composition of the diets (Table l), the ether extract was the
variable that differed most. As far as the fatty acid composition was concerned, experimental diet V contained 5.31% linoleic acid (C18:2), corresponding to 53.7% of all the
99
Effect of fat enriched diets
fatty acids analysed. The unsaturated: saturated fat ratio (U:S = Cl 8: 1 + Cl 8:2 + Cl 8:3/
C14:O + C16:O + C18:O) was very high (3.36). Diet A, which had animal fat incorporated,
had a linoleic acid content amounting to 2.18%, corresponding to 18.7% of the total fatty
acids and its U:S ratio was 1.77. The C diet had a similar level of palmitic acid (C16:O) as
diet A (24.4% and 23.0% respectively). The linoleic acid of C diet was 1.14%, which was
43.6% of the total amount of fatty acids and its US ratio was 1.16.
The effect of the diet on the meat and perirenal fat
The sex of the rabbits did not affect any of the variables studied. The results of pH (pHu)
and colour measurements on the LD muscle and perirenal fat measurements are shown in
Table 2. Diet had a significant effect on the pHu of the LD muscle. Carcasses of the animals fed with diet A displayed a significantly higher pHu (5.77) than carcasses of animals
fed diets V (5.70) and C (5.66). No significant differences in L* value of the LD were
observed between the three groups. The a* and b* values were higher in group V. Significant differences in the lightness of the perirenal fat was observed. Perirenal fat of the
carcasses from group V(L* = 63.97) was darker than from groups A and C (70.18 and
69.12) . The b* values of the fat in the carcasses from group A were lower.
Table 3 shows the weight in grams of the perirenal fat and the fatty acid composition of
the total lipids of the perirenal fat of rabbits fed with the three diets. Significant differences
were observed in the amounts of perirenal fat between the carcasses of the three different
groups using the carcass weight and age of the animals as covariables. Carcasses from
group C had less fat (14.15 g) compared to carcasses from groups A and V (24.36 g and
22.99 g respectively). There was a dietary effect on the fatty acids of the perirenal fat. The
perirenal fat of the carcasses from group V had a significantly lower proportion of myristic (C14:0), palmitic (C16:0), palmitoleic (C16:1), stearic (C18:O) and oleic acid (C18:l)
and a higher proportion of linoleic acid (C18:2, 48.91%) and linolenic acid (C18:3,
6.36%), in comparison to groups A and C. As a result the P:S ratio was higher in group V
(2.45) compared to groups A and C (0.53 and O-42).
The perirenal fat in the carcasses from group C had a significantly higher percentage of
myristic acid (4.23) palmitic acid (35.72) and palmitoleic acid (6.27) compared with the
TABLE 2
Meat Quality Measurements (Least-squares Means and Standard Errors) of Longissimus dorsi (LD)
and Perirenal Fat of Rabbits Fed Three Different Diets
V
LSM
pH LD
Colour LD
L*
*
;*
Colour perirenal fat
L*
a*
b*
SE
Group (I)
A
LSM
SE
LSM
C
SE
5.70b
0.014
5.77a
0.015
5.66b
0.015
51.71
3.86a
3.35a
0.259
0.148
0.116
52.11
3.38b
2.7313
0.287
0.164
0.129
51.53
3.39b
2.76b
0.259
0.148
0.116
63.97b
3.42b
4.21b
0.378
0.227
0.202
70.18a
2.94b
3.14c
0.417
0.251
0.223
69.12a
5.80a
5.06a
0.378
0.228
0.203
Least squares means with different letters are significantly different, p < 0.05.
1-Group V = rabbits fed with diet added vegetal fat. Group A = rabbits fed with diet added animal
fat. Group C = rabbits fed on control diet.
A. Oliver et al.
100
TABLE 3
Least-squares Means and Standard Errors of the Fatty Acid (%) Composition of the Total Lipids
of Perirenal Fat and Perirenal Fat (g) from Rabbits Fed with Three Different Diets
Fatty acid
c-14:0
C-16:0
C-16:1
C-18:0
C-18:1
C-18:2
C-18:3
Ratio US
Ratio P:S
Perirenal fat
Group (I)
A
V
1.33b
16.8lb
0.92~
4.72~
20.94c
48.91a
6.36a
3.40a
2.45a
22.99a
0.06
0.28
3.42~
25.97~
3.95b
6.73a
41 .OOa
16.20b
2.88~
1.78b
0.53b
24.36a
0.06
0.28
0.10
0.09
0.23
0.24
0.08
0.03
0.02
0.59
C
4.23a
35.72a
6.27a
6.26b
27.90b
15.65b
3.92b
1.17c
0.42b
14.15b
0.10
0.09
0.24
0.24
0.08
0.03
0.02
0.61
0.06
0.28
0.10
0.09
0.24
0.24
0.08
0.03
0.02
0.60
Least squares means with different letters are significantly different, p < 0.05.
l-Group V = rabbits fed with diet added vegetable fat. Group A = rabbits fed with diet added animal
fat. Group C = rabbits fed on control diet.
others groups. The highest percentage of oleic acid (41%) was found in the perirenal fat
from the carcasses from group A although the percentage of linoleic acid in the fat did not
differ significantly between carcasses from groups A and C.
The effect of the diet on the sensory characteristics of the loin meat
Table 4 shows the least squares means and standard errors of the sensory characteristics
of cooked loin. There was a significant effect of diet on the occurrence of aniseed flavour,
liver flavour and grass flavour in the cooked loin meat. The meat from groups V and C
TABLE 4
Least Squares Means and Standard Errors of Sensory Characteristics of Cooked Loin from Rabbits
Fed Three Different Diets
Group (I)
c
V
A
Flavour
Aniseed
Grass
Liver
sweet
2.3a
3.4a
1.2b
2.0
1.5b
2.8b
1.9a
1.8
2.3a
3.4a
I.lb
2.1
0.188
0.168
0.207
0.213
Texture
Hardness
Juiciness
Chewiness
Fibrousness
2.9
3.0a
3.8
2.5
3.6
2.9a
3.9
2.7
3.4
2.lb
4.0
2.6
0.273
0.224
0.122
0.120
Sensory evaluation
SE
Least squares means with different letters are significantly different, p < 0.05.
l-Group V = rabbits fed with diet added vegetable fat. Group A = rabbits fed with diet added animal
fat. Group C = rabbits fed on control diet.
Effect of fat enricheddiets
101
was considered to have more aniseed and grass flavour than that from group A. In contrast the loin meat from group A had more liver flavour. Both groups V and A were
considered juicier than group C. No differences were found in hardness, chewiness and
fibrousness between the groups.
DISCUSSION
The effect of the diet on the meat quality and perirenal fat
The diets used to feed the rabbits had a significant effect on the muscle pH (Table 2)
though these differences are within the limits considered normal for pHu. These values are
similar to those found by Holmes et al. (1984), Blasco and Piles (1990) and Xiccato et al.
(1994). In the study carried out by Xiccato et al. (1994) the L*, a* and b* values were
slightly higher than recorded in our study, probably because the results are an average
colour of two muscles Biceps femoris and Longissimus dorsi and also the rabbits were
older. As the colour of rabbit meat is affected mainly by pre-slaughter treatment, stress
and age we did not expect differences in L* value (Battaglini et al., 1994; Xicatto et al.,
1994). However, fat colour was affected by diet being lighter in group V (lower L* values)
as a consequence of the different fatty acid compositions of the fat between the three
groups (Ouhayoun et al., 1987). This significant difference in the colour of the fat could
affect consumer acceptance at the point of purchase because the colour of the fat is used
as an index of freshness. This subject has been discussed recently by Issanchou (1996) in
relation to meat and meat products.
The perirenal fat is the main source of dissectable fat in the rabbit (Ouhayoun et al.,
1987). Rabbit meat is normally consumed in Spain, as in other countries, by cooking the
whole rabbit, that is, with the perirenal fat included. There is an effect of diet on the
weight of the perirenal fat, which was significantly higher in the carcasses of the animals
fed with diets supplemented with fat (V and A). The greater quantity of perirenal fat
found in groups V and A could be a disadvantage from the dietary point of view and
could also downgrade the appearance of the rabbit meat. However, when rabbit meat is
sold as joints, the problem does not arise.
The effect of the diet on the fatty acid composition of the perirenal fat and the sensory
quality of the loin meat
It is of particular interest to consider what level and composition of fat can be included in
the diet of rabbits, in order to avoid human health and organoleptic problems. In our
study, as expected, supplementing the diet with soya full-fat (Diet V) caused the production of additional perirenal fat with a greater degree of unsaturation than obtained using
animal fat. A percentage of lincleic acid of 48.91% of the total fatty acids (Table 3) has
been observed in group V. These results agree with those of Ouhayoun et al. (1981) who
found that soya full fat can increase the proportion of linoleic acid in the adipose tissue to
30% of the total fatty acids. Therefore, the fat in groups A and C can be considered fat
with a good nutritional P:S ratio (0.53 and 0.42 respectively). The P:S ratio recently
reported by Enser et al. (1996) for retail samples of beef, lamb and pork muscle were 0.11,
0.15 and O-58, respectively. Cobos et al. (1993) noted that enriching the diet of rabbits
with soya, sunflower oils or soya bean oil increased the proportion of unsaturated fatty
acids compared to those obtained using conventional diets, this was considered an
important nutritional benefit to the consumer. Xiccato et al. (1994) also observed a high
level of unsaturation in rabbit fat.
A. Oliver et al.
102
Diet also affected the organoleptic quality of the loin meat; meat from rabbits fed diets
with animal fat (A) was considered to have more liver taste. From the sensory point of
view the increase in liver flavor in the meat from group A could be explained by the
incorporation of an aromatic liposoluble compound associated with the animal fat. Font
et al. (1995) discovered the same descriptor in beef. The greater intensity of this attribute,
though significant, appears to be of little importance since it is a common descriptor in
meat flavour. The same can be said about aniseed flavour or grass flavour. However we
think that these slightly aromatic notes (positive at this level of intensity) together with a
reduction of the liver flavour could have a positive effect on consumer acceptability. To
confirm the practical significance of this effect it would be necessary to undertake a consumer study. The more aniseed flavour (found in groups V and C) may be an important
qualitative attribute from the consumers point of view. With regard to the texture attributes diets V and A produced an increase in juiciness, which could be attributed to the
greater lipid content of these carcasses. No significant differences were observed between
diets for the other texture parameters.
CONCLUSION
This work confirms that there is a clear effect of the composition of the diet on the fatty
acid composition of rabbit fat. Fat colour was affected by diet, being less white in the
group V with lower L* values. From a human health point of view, the use of the diet
supplemented with animal fat could be considered on the basis of the nutritional index
P:S of the perirenal fat. However, we feel that the amount of fat added to these diets
(vegetable and animal fat) should be lower if the carcass is sold whole in order to avoid
excessive amounts of perirenal fat. From an organoleptic viewpoint, these results suggest
that diet V could produce meat with better sensory characteristics (greater aniseed and
grass flavour) than diet A. It would be advisable to select the by-products used in the
rabbit’s diet on the basis of the composition of fatty acids in order to achieve rabbit meat
of optimal quality.
ACKNOWLEDGEMENTS
This work was supported in part by the Ministerio de Education y Ciencia (CICYT). The
authors wish to acknowledge Ma Angels Rius and Dolors Guardia for technical assistance.
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