Cooking ways on composition of intramuscular phospholipid fatty acids of inra rabbit

Influência do cozimento na composição de ácidos graxos fosfolipídos intramusculares do coelho inra

ABSTRACT:

The effect of boiling, microwaving and aluminium (Al) foil-baking on composition of intramuscular phospholipid fatty acids of Inra rabbit was evaluated. Results showed that, the proportion of polyunsaturated fatty acids (PUFA)(e.g. C18:2n-6, C20:4n-6, C20:5n-3, C22:5n-3 and C22:6n-3) and monounsaturated fatty acids (MUFA)(e.g. C18:1n-7 and C18:1n-9) of treated longissimus dorsi muscle (LD) decreased, whilst the proportion of saturated (SFA)(e.g. C16:0 and C18:0) and n-6/n-3 value increased during cooking. Among the three treatments, microwaving can do better to stop the unsaturated fatty acids (UFA) being destroyed than boiling and Al foil-baking. However, boiling treatment did more serious damage to PUFA portion. Even so, the n-6/n-3 values of all of the cooked LD were within the recommended range. By analysis of partial least squares regression (PLSR), the microwaving treatment was more suitable in reserving UFA of intramuscular phospholipids from inra rabbit.

Key words:
Inra rabbit; cooking ways; intramuscular phospholipid fatty acids; composition; partial least squares regression

RESUMO:

Avaliou-se o efeito do cozimento da fervura, emprego de microondas e de preparo da carne envolvida em alumínio (Al) de três formas de cocção na composição de ácidos graxos fosfolipídicos intramusculares de coelho Inra. Os resultados mostraram que, a proporção de ácidos graxos poliinsaturados (AGPI) (por exemplo, C18: 2n-6, C20: 4n-6, C20: 5n-3, C22: 5n-3 e C22: 6n-3) e ácidos graxos monoinsaturados (MUFA) (por exemplo, C18: 1n-7 e C18: 1n-9) de longissimus dorsimuscle (LD) tratado, diminuem, enquanto a proporção de saturado (SFA) (por exemplo, C16: 0 e C18: 0) e n-6 / n -3 valor aumentado durante o cozimento. Entre os três tratamentos, o micro-ondas parace ser o melhor por impedir a destruição dos ácidos graxos insaturados (UFA) do que a fervura e o cozimento em folha Al. No entanto, o tratamento de ebulição causou danos mais sérios à porção de PUFA. Mesmo assim, o valor n-6 / n-3 de todas as amostras de LD cozidas estava dentro da faixa recomendada. Por análise de regressão por mínimos quadrados parciais (PLSR), o tratamento com microondas foi mais adequado para preservar UFA de fosfolipídios intramusculares de Inra coelho.

Palavras-chave:
Inra coelho; formas de cozinhar; ácidos gordos fosfolipicos intramusculares; composição; regressão parcial por mínimos quadrados.

INTRODUCTION:

For the past few years, not only the amount of intramuscular phospholipids, but also the composition of phospholipid fatty acids was widely concerned. The amount of intramuscular lipid significantly influenced the palatability of meat (HOCQUETTE et al., 2010). Muscle lipids comprise polar lipids and triacylglycerols, the main component of polar lipid is phospholipids, which located in the cell membranes, and the triacylglycerols are along the muscle fibers, which had high level of saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA).(RAES et al., 2004). In the intramuscular fat, PUFA is mainly derived from the contribution of phospholipids (WOOD et al., 2004). Thus, the content of intramuscular phospholipid is an important index to reflect the nutrition of meat (GRAY et al., 1996). Phospholipids contain quite many long-chain fatty acids attached to the phosphoryl group. But, the unsaturated fatty acids (UFA) portion, especially PUFA are easily oxidized (ALFAIA et al., 2010), thus the oxidation reaction during heat treatment significantly affects the intramuscular fatty acids composition.

Functional food plays an important role in human nutrition. Rabbit meat is characterized by high levels of PUFA, high levels of protein with essential amino acids; high digestibility value; lower cholesterol contents; high quality of vitamin B family etc (DALLE ZOTTE and SZENDRÖ, 2011; SIMITZIS et al., 2014). Moreover, the dietary rabbit meat can provide people with various nutrients needed to maintain health, since the nutrition of meat can be improved by adding essential fatty acids to its diet, such as n-3 PUFA (DAL BOSCO et al., 2004; KOUBA et al., 2008). Since the rabbit meat is characterized relatively abundant n-3 PUFAs and lower n-6/n-3 value (7～12) than a variety of other meats (DALLE ZOTTE, 2002). Inra rabbit is a French rabbit, which has high breeding efficiency. However, there are few reports about the effects of different cooking ways on its composition of intramuscular phospholipid fatty acids.

Since the 1970s, in some European countries rabbit meat production has progressively become a highly specialized industry, which has made Europe the second (after China) largest rabbit meat producer in the world (CULLERE et al., 2018). Chinese people have been consuming rabbit meat for a long period of time due to its delectable texture and taste. Several advanced processed rabbit meat products, including smoked, roasted, canned, cured, dried, sauce-picked, and sausage products, are currently available in China (LI et al., 2018; ZENG et al., 2016). As far as we know, three household cooking ways of boiling, microwaving and Al foil-baking were effective to realize palatable and safe product (TORNBERG, 2005). Besides, by the heat processing, not only the pathogenic microorganisms can be killed or inhibited, the flavor and tenderness can also be enhanced (BRONCANO et al., 2009; RODRÍGUEZ-ESTRADA et al., 1997). The cooking type, heating rate, cooking time and cooking temperature can modify the chemical composition of meat, which may lead to a massive loss of nutrients, especially the fatty acid profiles (CLAUSEN and OVESEN, 2005). Fatty acid composition and the content of intramuscular phospholipid of the meat were afected by the cooking methods, and the report found microwave processing to be better than stewing and baking. (XUE et al, 2015). WANG et al. (2015) reported that the drip loss, shear force, and textural properties of rabbit meat varied significantly with central temperatures during the cooking process. To sum up, diferent processing methods form diferent sensory characteristics by afecting the food components. Consequently, the efect of diferent processing methods on the quality of rabbit meat should be analyzed to further develop the rabbit industry as a specialty meat source.

Thus, the aim of this study was to investigate the change of intramuscular phospholipid fatty acid composition of Inra rabbit cooked by three household treatments, in order to provide theoretical basis for deep-processing and dietary nutrition guidance of rabbit meat. This study has not been previously reported.

MATERIALS AND METHODS:

Materials

Inra standardized farming and a total of 30 male Inra rabbits were tested as samples. The facilities rabbits of the abattoir met the requirements of the Institute of Animal Care and Use Committee (IACUC), which is funded by the United States National Institutes of Health. Processing of raw materials was the same with XUE et al (2016a). After 24 h post-mortem, the longissimus dorsi muscle (LD) of the carcass was removed and immediately vacuum-packed and frozen at -20 °C until analyzed.

Experimental methods

Cooking treatment

The material processing method referred to XUE (2016b): The LD sample was thawed, and removed visible fat and connective tissue. The meat was cut into about 30×20×20 mm portions. All samples were divided into four groups, one of which represented the raw reference, whereas the other groups were randomly cooked by boiling, microwaving and Al foil-baking, respectively (Table 1). Every process of cooking was divided into three stages (Level 1, Level 2, Level 3), and the final stage had reached a final internal temperature of 75 ^oC as ascertained by an iron-constantan (type J) wire thermocouple connected to a digital potentiometer (Mod. Microtemp 2, Eurotron Italiana, S.r.l., Italia).

Table 1
The sample processing group.

Processing method	---------------------------------------------Heat treatment time (min)-----------------------------------------
	Level 1	Level 2	Level 3
boiling treatment	3	6	9
microwaving treatment (2450 MHz)	1	2	3
Al foil-baking treatment (180 oC)	5	10	15
The control group	------------------------------------------------------No processing----------------------------------------------

The analysis of intramuscular phospholipid

The chloroform - methanol solvent (2:1 v/v) was used for extraction intramuscular lipids (FOLCH, 1957). Total lipid content was measured by weighing after solvent evaporation. Fractions of intramuscular phospholipids were separated with silica cartridges (Sep-Pack, Waters, Milford, MA, USA) (JUANEDA and ROCQUELIN, 1985). Phosphorous determination was used to quantify the phospholipids by the method of BARTLETT (1959). Phospholipids were methylated with boron fluoride-methanol (Sigma Aldrich) according to XUE (2016a).

The fatty acid methyl esters were analyzed by a QP-2010 gas chromatograph (Shimadzu, Japan) equipped with a flame ionization detector and a split injector. One microliter of fatty acid methyl esters was injected in split mode (5:1) onto a Rtx-Wax capillary column (Restek, USA; 30 m × 0.25 mm id × 0.25 µm film thickness). Temperature of the column was programmed as follows: 1 min at 140 °C, increments of 8 °C/min to 180 °C and held at 180 °C for 2 min; increments of 3 °C/min to 210 °C; and increments of 5 °C/min to 230 °C and held at 230 °C for 10 min. Temperature of injector and the detector were both 250 °C. Flow rate of the carrier gas (N₂) was 1.5 mL/min. Fatty acids were identified by comparing the retention time of the samples with those of the standards (Sigma). Results were expressed as percent of the total fatty acid methyl esters (XUE, 2016a).

Statistical analysis

The means and standard errors were determined by the SAS^® 9.0 (SAS Institute Inc., Cary, USA). Difference among means was compared by Duncan’s multiple range test.

The results were performed by ANOVA-partial least squares regression. Seven 0/1 indicators variables (raw sample, boiling, microwaving and Al foil-baking, level 1, level 2, level 3), SFA+MUFA, and PUFA/SFA in the X-matrix. The 21 kinds of intramuscular phospholipid fatty acids were represented in the Y-matrix. Ellipses represent R²=0.5 (50%) and 1.0 (100%). The Unscrambler Software was used to make the analysis of PLSR.

RESULTS AND DISCUSSION:

Influence of cooking ways on content of intramuscular phospholipid

Table 2 represented the effect of processing conditions on the intramuscular phospholipid content of Inra rabbit. The content of total intramuscular lipids (cooked muscle %) of LD were all significantly increased (P<0.05), while the intramuscular phospholipid (total intramuscular lipid %) were all significantly decreased after cooking (P<0.05). The significantly increasing of absolute content of phospholipid (cooked muscle %) were observed in all cooked samples (P<0.05). Absolute content of phospholipids in microwaved- and boiled- samples were obviously higher than foil-baking, and the increase of absolute content of phospholipids in microwaved- and boiled- samples were relatively rapid. Comparing the three cooking methods, the Al foil-baking accounted for the largest degradation of phospholipid proportion (total intramuscular lipid %), followed by boiling and microwaving.

Table 2
The effect of processing conditions on the intramuscular lipid content of Inra rabbit^a,b.

Content	Cooking methods	Raw material	Level 1	Level 2	Level 3
	boiling	0.96±0.11c	2.05±0.08b	2.20±0.05b	2.98±0.05a
intramuscular lipid (cooked muscle %)	microwaving	0.96±0.11d	1.39±0.03c	2.09±0.04b	2.75±0.02a
	Al foil baking	0.96±0.11d	1.27±0.02c	1.92±0.06b	2.54±0.09a
	boiling	32.28±1.02a	29.67±0.94b	28.63±0.66b	26.91±0.92c
intramuscular phospholipid (total intramuscular lipid %)	microwaving	32.28±1.02a	31.07±0.69a	30.75±0.85a	28.54±0.70b
	Al foil baking	32.28±1.02a	30.69±0.66b	28.76±0.81c	26.43±0.53d
	boiling	0.31±0.03c	0.61±0.04b	0.63±0.03b	0.80±0.04a
intramuscular phospholipid (cooked muscle %)	microwaving	0.31±0.03d	0.43±0.01c	0.64±0.02b	0.81±0.02a
	Al foil baking	0.31±0.03d	0.39±0.01c	0.55±0.03b	0.67±0.03a
	boiling	1.24±0.03a	1.06±0.08b	0.84±0.04c	0.73±0.04d
intramuscular lipid (dry weight %)	microwaving	1.24±0.03a	1.11±0.03b	1.05±0.02c	0.94±0.03d
	Al foil baking	1.24±0.03a	0.97±0.02b	0.89±0.05c	0.78±0.04d

• ^aResults were expressed as means ± SE, data were means of three replicates (n=3).
• ^bValues in the same row with different letters were significantly different (P<0.05).

The cooking loss of raw material and heated samples under different processing conditions were showed in table 3. Among the processing, the cooking loss of microwaving was higher, and changed severer with the cooking gradient increase (time), whilst the cooking loss of Al foil-baking was lower, and changed more smoothly with the processing. Thus, considering the different cooking loss extent of heated samples, the dry matter of the intramuscular phospholipids (dry weight %) would be more accurate to present the results. During the processing, the intramuscular phospholipids (dry weight %) significantly decreased (P<0.05). The longer the cooking time was, the higher the reduction would be. Among the cooking methods, microwaving showed the least change in the content of intramuscular phospholipids; whilst the biggest change was observed in boiling treatment.

Table 3
The cooking loss (%) under different processing conditions^ab.

Cooking methods	Level 1	Level 2	Level 3
boiling	33.35±0.16c	36.18±2.09b	40.79±0.27a
microwaving	31.55±0.31c	38.75±2.35b	48.13±0.85a
foil baking	32.97±0.10c	35.15±1.86b	37.49±0.16a

• ^aResults were expressed as means ± SE, data were means of three replicates (n=3).
• ^bValues in the same row with different letters were significantly different (P<0.05).

At level 1, the content of intramuscular lipid in Al foil-baking group decreased more obviously than other groups, which may be due to the higher baking temperature (180 ^oC) and longer cooking time (5 min). To our knowledge, heat treatment may cause lipid oxidation, and the temperature and time are key factors (BYRNEA et al. 2002). According to RODRÍGUEZ-ESTRADA et al. (1997), baking treatment can produces more obvious lipid oxidation because of the high-temperature and long-time cooking condition. Hence, the aluminium foil is often used for wrapping heat-sensitive raw food for protection against direct heat, during storage and preparation of food (RANAU et al, 2001), and there are no evident risks to the health of the consumer from using aluminium foil to cook meats (TURHAN, 2006). Besides, the research of RODRÍGUEZ-ESTRADA et al. (1997) showed that the microwave treatment can also cause lipid oxidation, though the time is shorter and the temperature is lower. Anyway, boiling, microwaving, Al foil-baking are all basic and familiar thermal processes of food preparation, which may change the fatty acid profiles, sensory characteristics and nutritional value of food (MARGARET et al, 2014, FELLOWS, 2017).

Effect of cooking ways on composition of intramuscular phospholipid fatty acids in LD

The fatty acids composition of intramuscular phospholipid significantly changed under different cooking methods (boiling, microwaving and Al foil baking) and cooking extent (time) (table 4,5,6). The SFA proportion in cooked-LD significantly increased during the processing (P<0.05), corresponding to the significant decrease of UFA, including the PUFA and MUFA proportion (P<0.05). Among the cooking methods, the maximum increase of SFA was reported in boiled samples, while the minimum in microwaved-samples. Therefore, the microwaving treatment was more effective to protect the UFA (especially the MUFA) than baking and boiling. At the same time, the boiling treatment did the worst in maintaining the stability of UFA, especially the PUFA fraction. The change of single major fatty acid was represented in table 7.

Table 4
Effect of boiling conditions on the composition of intramuscular phospholipid fatty acids (%) of Inra rabbit^a.

Fatty acids	Raw material	Level 1	Level 2	Level 3
C12:0b	0.12±0.03a	0.12±0.01a	0.13±0.01a	0.15±0.02a
C14:0	0.60±0.17b	1.04±0.03a	1.10±0.04a	1.14±0.04a
C14:1	0.11±0.02a	0.08±0.01b	0.08±0.02b	0.07±0.01b
C15:0	1.81±0.22d	2.34±0.03c	3.13±0.02b	4.15±0.14a
C16:0	22.37±0.11d	23.05±0.06c	24.87±0.13b	26.04±0.08a
C16:1n-7	0.34±0.03a	0.25±0.04b	0.25±0.02b	0.31±0.05ab
C17:0	0.38±0.02c	0.45±0.03c	0.57±0.03b	0.78±0.08a
C17:1	0.92±0.07a	0.74±0.01ab	0.66±0.02b	0.64±0.06b
C18:0	12.56±0.13d	13.56±0.05c	14.52±0.06b	15.61±0.08a
C18:1n-9	15.01±0.03a	14.30±0.03b	13.86±0.03c	13.35±0.12d
C18:1n-7	1.00±0.03a	0.58±0.06b	0.26±0.06c	0.24±0.08c
C18:2n-6	22.67±0.02a	22.87±0.04b	22.64±0.06c	22.37±0.10d
C18:3n-3	0.48±0.02a	0.43±0.03ab	0.38±0.03b	0.32±0.03c
C20:0	0.11±0.03a	0.09±0.03a	0.09±0.01a	0.07±0.02a
C20:1n-9	0.18±0.06a	0.17±0.04a	0.16±0.02a	0.18±0.08a
C20:2n-6	1.19±0.02a	1.23±0.04a	0.84±0.06b	0.70±0.02c
C20:3n-6	1.26±0.03a	1.23±0.02a	0.80±0.05b	0.70±0.02c
C20:4n-6	9.72±0.14a	9.47±0.04b	8.27±0.04c	6.37±0.06d
C20:5n-3	4.60±0.04a	4.39±0.07b	4.09±0.06c	3.80±0.06d
C22:5n-3	2.34±0.02a	2.23±0.02b	2.01±0.02c	1.82±0.03d
C22:6n-3	1.40±0.03a	1.36±0.04a	1.29±0.03b	1.19±0.04c
SFAc	37.96±0.11d	40.65±0.06c	44.42±0.08b	47.94±0.28a
PUFA	43.68±0.17a	43.23±0.02b	40.31±0.04c	37.28±0.04d
MUFA	18.36±0.18a	16.12±0.05b	15.27±0.04c	14.79±0.30d

• ^aResults were expressed as means ± SE, data were means of three replicates (n=3).
• ^bValues in the same row with different letters were significantly different (P<0.05).
• ^cSFA, total saturated fatty acids (C12:0, C14:0, C15:0; C16:0, C17:0, C18:0, C20:0); MUFA, total monounsaturated fatty acids (C14:1, C16:1n-7, C17:1, C18:1n-9, C18:1n-7, C20:1n-9); PUFA, total polyunsaturated fatty acids (C18:2n-6, C18:3n-3, C20:4n-6, C20:5n-3, C22:5n-3, C22:6n-3).

Table 5
Effect of microwaving conditions on the composition of intramuscular phospholipid fatty acids (%) of Inra rabbit^a.

Fatty acids	Raw material	Level 1	Level 2	Level 3
C12:0b	0.12±0.03a	-	-	-
C14:0	0.60±0.17c	1.04±0.03b	1.17±0.02ab	1.32±0.06a
C14:1	0.11±0.02a	0.07±0.02b	0.06±0.02b	0.06±0.02b
C15:0	1.81±0.22b	2.34±0.03a	2.42±0.02a	2.54±0.04a
C16:0	22.37±0.11d	23.02±0.02c	23.60±0.07b	24.47±0.05a
C16:1n-7	0.34±0.03a	0.25±0.04b	0.26±0.02b	0.32±0.03a
C17:0	0.38±0.02d	0.75±0.03c	0.83±0.04b	0.97±0.03a
C17:1	0.92±0.07a	0.74±0.01b	0.69±0.02bc	0.66±0.04c
C18:0	12.56±0.13d	13.56±0.05c	14.07±0.03b	14.82±0.03a
C18:1n-9	15.01±0.03a	14.30±0.03b	14.16±0.05c	13.91±0.03d
C18:1n-7	1.00±0.03a	0.58±0.06c	0.64±0.02c	0.77±0.02b
C18:2n-6	22.67±0.02a	22.86±0.05a	22.39±0.06a	21.69±0.06a
C18:3n-3	0.48±0.02a	0.43±0.03a	0.35±0.02b	0.22±0.03c
C20:0	0.11±0.03b	0.07±0.01b	0.11±0.02b	0.18±0.03a
C20:1n-9	0.18±0.06b	0.24±0.03ab	0.24±0.02ab	0.25±0.03a
C20:2n-6	1.19±0.02b	1.27±0.03a	1.17±0.05b	1.03±0.03c
C20:3n-6	1.26±0.03a	1.23±0.02a	1.08±0.03b	0.88±0.04c
C20:4n-6	9.72±0.14a	9.35±0.04b	9.17±0.04c	8.86±0.05d
C20:5n-3	4.60±0.04a	4.35±0.02b	4.20±0.02c	4.00±0.06d
C22:5n-3	2.34±0.02a	2.23±0.02b	2.10±0.04c	1.89±0.06d
C22:6n-3	1.40±0.03a	1.33±0.02b	1.27±0.03b	1.16±0.03c
SFAc	37.96±0.11d	40.78±0.03c	42.19±0.11b	44.30±0.07a
PUFA	43.68±0.17a	43.04±0.01b	41.76±0.03c	39.73±0.08d
MUFA	18.36±0.18a	16.17±0.03b	16.06±0.13b	15.97±0.09b

• ^aResults were expressed as means ± SE, data were means of three replicates (n=3).
• ^bValues in the same row with different letters were significantly different (P<0.05).
• ^cSFA, total saturated fatty acids (C12:0, C14:0, C15:0; C16:0, C17:0, C18:0, C20:0); MUFA, total monounsaturated fatty acids (C14:1, C16:1n-7, C17:1, C18:1n-9, C18:1n-7, C20:1n-9); PUFA, total polyunsaturated fatty acids (C18:2n-6, C18:3n-3, C20:4n-6, C20:5n-3, C22:5n-3, C22:6n-3).

Table 6
Effect of Al foil-baking conditions on the composition of intramuscular phospholipid fatty acids (%) of Inra rabbit^a.

Fatty acids	Raw material	Level 1	Level 2	Level 3
C12:0b	0.12±0.03a	-	-	-
C14:0	0.60±0.17c	1.54±0.04b	1.95±0.03a	1.96±0.02a
C14:1	0.11±0.02a	0.09±0.02ab	0.06±0.02b	0.07±0.01b
C15:0	1.81±0.22c	2.20±0.10b	2.40±0.04b	2.84±0.08a
C16:0	22.37±0.11c	23.21±0.05b	24.11±0.56a	24.08±0.06a
C16:1n-7	0.34±0.03a	0.23±0.02b	0.25±0.04b	0.23±0.02b
C17:0	0.38±0.02c	0.47±0.06bc	0.54±0.04b	0.73±0.07a
C17:1	0.92±0.07a	0.78±0.08a	0.62±0.03b	0.66±0.04b
C18:0	12.56±0.13d	13.32±0.09c	14.45±0.08b	15.66±0.12a
C18:1n-9	15.01±0.03a	14.23±0.05b	14.09±0.03c	13.86±0.07d
C18:1n-7	1.00±0.03a	0.61±0.05b	0.51±0.05c	0.41±0.04d
C18:2n-6	22.67±0.02b	22.86±0.05a	22.37±0.06c	21.88±0.08d
C18:3n-3	0.48±0.02a	0.43±0.03a	0.35±0.02b	0.19±0.04c
C20:0	0.11±0.03b	0.09±0.03b	0.15±0.04b	0.20±0.02a
C20:1n-9	0.18±0.06a	0.17±0.03a	0.14±0.02a	0.14±0.02a
C20:2n-6	1.19±0.02a	1.18±0.06a	1.04±0.03b	1.03±0.03b
C20:3n-6	1.26±0.03a	1.20±0.05a	1.07±0.05b	0.93±0.09c
C20:4n-6	9.72±0.14c	9.41±0.09b	8.46±0.12c	8.25±0.11c
C20:5n-3	4.60±0.04a	4.40±0.07b	4.12±0.08c	3.99±0.06d
C22:5n-3	2.34±0.02a	2.23±0.02b	2.10±0.04c	1.88±0.06d
C22:6n-3	1.40±0.03a	1.36±0.05ab	1.21±0.05bc	1.04±0.17c
SFAc	37.96±0.11d	40.82±0.16c	43.59±0.43b	45.46±0.10a
PUFA	43.68±0.17a	43.07±0.05b	40.74±0.32c	39.19±0.09d
MUFA	18.36±0.18a	16.11±0.13b	15.67±0.12c	15.35±0.01d

• ^aResults were expressed as means ± SE, data were means of three replicates (n=3).
• ^bValues in the same row with different letters were significantly different (P<0.05).
• ^cSFA, total saturated fatty acids (C12:0, C14:0, C15:0; C16:0, C17:0, C18:0, C20:0); MUFA, total monounsaturated fatty acids (C14:1, C16:1n-7, C17:1, C18:1n-9, C18:1n-7, C20:1n-9); PUFA, total polyunsaturated fatty acids (C18:2n-6, C18:3n-3, C20:4n-6, C20:5n-3, C22:5n-3, C22:6n-3).

Table 7
The change of single major fatty acid effected by different cooking.

Category	Main single fatty acid	Change	The most influential cooking method	significance
SFA	C16:0	increase	Boiling	Yes
	C18:0	increase	Foil baking	Yes
MUFA	C18:1n-7	decrease	Boiling	Yes
	C18:1n-9	decrease	Boiling	Yes
PUFA	C18:2n-6	decrease	microwaving	Yes
	C20:4n-6	decrease	Boiling	Yes
	C20:5n-3	decrease	Boiling	Yes
	C22:5n-3	decrease	Boiling	Yes
	C22:6n-3	decrease	Boiling	Yes

The unique biological effects of fatty acids vary with different species and chemical structures (HOSSEINI et al., 2014). According to LARSEN et al. (2011), long-chain PUFA (e.g. C20:4n-6, 20:5n-3 and 22:6n-3) have markedly different physiological properties and biological functions compared to the shorter chain PUFAs, such as C18:2n-6 and C18:3n-3. Indeed, the PUFA portion was more susceptible to oxidation during cooking than their saturated analogues. The change of intramuscular phospholipid fatty acids (n-6 PUFA, n-3 PUFA, n-6/n-3 value, PUFA/SFA) was showed in figure 1. With the extension of processing time, the n-6 PUFA and n-3 PUFA portion decreased significantly (P<0.05), and the microwaving treatment did the minimal impact than boiling and baking, whilst the boiling treatment did more damage on n-6 PUFA, the Al foil-baking did more damage on n-3 PUFA. The increase trend of n-6/n-3 value was reported from level 1 to level 3 (P<0.05). Boiling, microwaving and Al foil-baking treatment changed the n-6/n-3 values from 3.95 (raw material) to 4.13～4.23, 4.16～4.46, 4.11～4.52, respectively. In the early stage of cooking, the three processing methods had little effect on the value, but in the later stage, the value of microwave and baking increased significantly. As for the PUFA/SFA value, a downward trend was observed (P<0.05), and the most decrease was reported in boiled samples. To sum up, the microwaving did less significant effect on the proportion of n-6 PUFAs, n-3 PUFAs, and PUFA/SFA value than boiling and baking. This may be due to the lower temperature and shorter time of microwaving treatment than boiling and baking, resulting in the less oxidation of fatty acids, especially the proportion of PUFAs.

Figure 1
Comparison of degradation proportion of intramuscular fatty acids of Inra rabbit under different processing conditions.

According to WHO, the SFA have been recognized as the main goal of weight loss by the international dietary authorities. In contrast, the increase of PUFA intake, especially the n-3 PUFA portion, may have a significant impact on the improvement of public health and nutrition. Based on FAO/WHO, the recommended ratio of n-6 PUFA/n-3 PUFA in a healthy daily diet was 5～10, and a lower ratio is more desirable in reducing the risk of many of chronic diseases, and the requirements for optimal ration vary by different disease (SIMOPOULOS, 2002). Nutritionists recommend that n-6 PUFA/n-3 PUFA should be less than 5 (KOUBA et al., 2003; WOOD et al., 2004), and the ratio in the diet should be less than 4.0 to combat “lifestyle diseases”, such as cancer and coronary heart disease (SIMOPOULOS, 2004; WILLIAMS, 2000). Therefore, the microwaved-LD of Inra rabbits may characterize excellent intramuscular fatty acids profile, which has potential nutritive value.

Analysis of partial least squares regression (PLSR)

The PLSR is a new method of multivariate statistical data analysis, which makes a combination of the advantages of three analytical methods including principal component analysis, canonical correlation analysis and multiple linear regression analysis. (MARINI, 2010). This regression has been applied in analysis the change of total intramuscular fatty acids of cooked Hyla rabbit (XUE et al, 2016b), the composition of intramuscular phospholipid fatty acids of Inra rabbit during growth (XUE, 2016a), discriminating low-fat and full-fat yogurts (CRUZ et al., 2013), and the discrimination of Brazilian artisanal and inspected pork sausages (MATERA et al., 2014) and so on.

The change of intramuscular phospholipid fatty acids of Inra rabbit was showed in figure 2. Results showed that the first and second main ingredients explained Y variables as 70% and 8%, respectively. In the first principal, both cooking ways and processing time (level) affected the composition of intramuscular fatty acids significantly. Besides, the processing time (level) affected the fatty acids composition more obviously than the cooking ways did.

Figure 2
The PLSR correlation loadings plot for first two Principal Components (PCs), Main design variables: seven 0/1 indicator variables (raw material, three cooking methods and three processing levels), SFA/PUFA and SFA+MUFA value in the X-matrix and change in the composition of intramuscular phospholipid fatty acid in the Y-matrix. Ellipses represent r²=0.5 (50%) and 1.0 (100%).

As shown in the figure 2, On the first principal component, “microwaving” is much closer to “raw material”, indicating that the microwaving treatment was more effective to preserve the composition of phospholipid fatty acids than the boiling and Al foil baking, whilst the biggest influence were reported in boiled-sample. Besides, the “short-time-cooking” (Level 1) located closer with raw material, and the long-time-cooking (Level 2 and Level 3) located further with raw material; therefore, we can easily know that the longer the processing time, the more dramatic changes in the composition of fatty acids. With the extention of cooking time, the significantly decreasing of PUFA and significantly increasing of SFA in intramuscular phospholipid were observed from Level 1 to Level 3 (P<0.05). Based on the figure, the level 1 samples contained more PUFA (e.g., 16 (C20:2n-6), 18 (C20:4n-6), 19 (C20:5n-3), 20 (C22:5n-3) and 21 (C22:6n-3)) and some of MUFA (e.g., 10 (C18:1n-9), 11 (C18:1n-7)). The long-time-cooking (Level 3) was closely related to most of the SFA (e.g., 4(C15:0), 5(C16:0), 9(C18:0)). Thus the shorter processing time was more conducive to retain the PUFA in intramuscular phospholipids, especially the long chain PUFAs (C20-22). From what has been discussed above, comparing the three cooking ways, microwaving treatment showed the relatively superiority in reserving the composition of intramuscular phospholipid fatty acids of Inra rabbit, especially the long chain PUFAs (n-6 PUFA and n-3 PUFA) portion, while boiling did the most serious damage to intramuscular phospholipid fatty acids of Inra rabbit. This result was consistent with the previous study in this paper.

CONCLUSION:

After heat treatment of boiling, microwaving and Al foil-baking, the proportion of intramuscular phospholipids (dry weight %) of Inra rabbit were significantly decreased (P <0.05). The microwaving treatment did the minimal impact on the change of intramuscular phospholipids content, whilst the foil baking did the greatest impact on the content. Despite the composition of intramuscular phospholipid fatty acids varied according to different treatment, the n-6/n-3 ratio in each of our experimental groups was all within the recommended range. Due to the analysis of PLSR, the microwaving showed the superiority in keeping stable on the phospholipid fatty acids composition of Inra rabbit, especially the UFA portion, which was consistent with the experimental results in this paper, and may be instructive to the processing of rabbit meat.

ACKNOWLEDGEMENT

This study was funded by the middle-aged teachers in Fujian Province (Grant No. JT180307); Postdoctoral program of Damin Food Co., Ltd. & Fujian Agriculture and Forestry University; College outstanding youth research talent cultivation program in Fujian Province.

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• 0
  CR-2019-0007.R1

Publication Dates

• Publication in this collection
  13 May 2019
• Date of issue
  2019

History

• Received
  03 Jan 2019
• Accepted
  20 Mar 2019
• Reviewed
  17 Apr 2019