Lipid Oxidation and Color Changes of Fresh Camel Meat Stored Under Different Atmosphere Packaging Systems

Extension of the shelf-life of meat was one of the technological necessities to meet the demands of consumers. In this respect, increasing attention was put on packaging techniques. Modified atmosphere packaging (MAP) is the recent innovation that has been gaining importance as preservation technique to improve the shelflife of meat. Retention of meat color was better in MAP than in either vacuum packaging or in air [1]. Modified atmosphere packing has been used for increased distribution range and longer shelf-life. The effects and roles of the gases normally used in the modified atmospheres (O2, CO2 and N2) have been extensively reported [2-4]. Hood and Mead (1995) indicated that the effects which the mixture of gas produces in meat quality, such as color and shelf life, are the principal factors that should be considered when choosing the gas mixture [5]. In addition, Gill affirmed that the principal factors to be addressed in the preservation of chilled meat are the retention of an attractive, fresh appearance for the product displayed, and the retardation of bacterial spoilage [3]. Several studies have been carried out on the physical, chemical composition, sensory properties and nutritive values of camel meat [6-11]. No data has been published on the preservation of fresh camel meat by modified atmosphere packaging. Our objective was to investigate the color and lipid oxidation changes of fresh camel meat using modified atmosphere packaging under refrigeration.


Introduction
Extension of the shelf-life of meat was one of the technological necessities to meet the demands of consumers. In this respect, increasing attention was put on packaging techniques. Modified atmosphere packaging (MAP) is the recent innovation that has been gaining importance as preservation technique to improve the shelflife of meat. Retention of meat color was better in MAP than in either vacuum packaging or in air [1]. Modified atmosphere packing has been used for increased distribution range and longer shelf-life. The effects and roles of the gases normally used in the modified atmospheres (O 2 , CO 2 and N 2 ) have been extensively reported [2][3][4]. Hood and Mead (1995) indicated that the effects which the mixture of gas produces in meat quality, such as color and shelf life, are the principal factors that should be considered when choosing the gas mixture [5]. In addition, Gill affirmed that the principal factors to be addressed in the preservation of chilled meat are the retention of an attractive, fresh appearance for the product displayed, and the retardation of bacterial spoilage [3]. Several studies have been carried out on the physical, chemical composition, sensory properties and nutritive values of camel meat [6][7][8][9][10][11]. No data has been published on the preservation of fresh camel meat by modified atmosphere packaging. Our objective was to investigate the color and lipid oxidation changes of fresh camel meat using modified atmosphere packaging under refrigeration.

Sampling preparation and packaging
Camel meat samples were obtained at a slaughter house (Tehran, Iran). Any visible fat was removed from the muscle tissues. A Turbovac packaging machine, model A 200, (Henkelman, Netherlands) was used for packing. Meat samples were randomly assigned to one of the three types of different atmospheres packaging (AP: Air packaging, VP: Vacuum packaging, MAP: 60% CO 2 +40% N 2 ) using sterile polyester polyethylene (PET/Poly) pouches (thickness -62 lm).

Lipid oxidation
Lipid oxidation was evaluated by the determination of thiobarbituric acid reactive substances (TBARS) using the extraction method described by Witte, et al. [12]. Twenty grams of the minced meat were blended with 50 mL of cold solution containing 20% trichloroacetic acid in 2M phosphoric acid for 2 min. The resulting slurry was then transferred into a 100 mL volumetric flask. The slurry was diluted to 100 mL with double-distilled water, homogenized by shaking and filtered through Whatman no. 1 filter paper. 5 mL of the filtrate was then pipetted into a test tube and 5 mL of fresh chilled 2-thiobarbituric acid (0.005 M in double distilled water) was added. The test tube was shaken well and placed in the dark at room temperature (25°C) for15 h to develop the color reaction. The resulting color was measured in a spectrophotometer at 530 nm to calculate the TBARS value. The results were expressed as mg malonaldehyde/kg meat.

Color measurement
Color was recorded using a Minolta Chroma meter CR-400 KON made in Japan. Readings at per sample, in the center of the steak was taken. CIELAB system, L*(lightness), a* (redness) and b* (yellowness) were measured [13].

Sensory analysis
Camel meat samples were evaluated by eight semi-trained panelists. The panelists consisted of staff members in the Dept. of Meat Science, University of Tehran. Panelists were given an orientation for 30 min about appearance (color), odor, texture and overall quality of fresh camel meat. Acceptability of raw meat was evaluated using a 9-point hedonic scale, where 9=like extremely, 8=like very much, 7=like moderately, 6=like slightly, 5=neither like nor dislike, 4=dislike slightly, 3=dislike moderately, 2=dislike very much, and 1=dislike extremely [14]. Scores from 6 to 9 were considered acceptable [15]. Evaluation was performed under cool white fluorescent light in the sensory laboratory. The same meat samples were evaluated over storage times. The shelf life limit was defined as the point when 50% of the panelists rejected the sample.

Statistical analysis
The data were analyzed using analysis of variance to determine the effects of MA type (1, 2, 3, and 4) on the parameters of meat quality: color, lipid oxidation and SF. When the differences among types of MA were significant (P<0.05), Tukey's test was carried out to check the differences between pairs of groups. The effect of storage for each treatment packing on meat quality was analyzed using Tukey's test at a significance level of P<0.05. Data were analyzed using the SAS (1988) statistical package [16].  [17]. It could be supposed that the low intensity of oxidative processes was due to the raising mode of the studied animals which provided natural antioxidants, such as vitamin E, carotenoids, etc. [18]. It was shown that pasture [19] increased significantly the content of vitamin E in bovine muscles and hence reduces the development of oxidation in meat.

Color
Results of color measurement are shown in (Table 2). Initial values for L*, a*, b* and Chroma were 34.49, 20.12, 7.59 and 22.74 respectively. The L* value increase by 21 days with time in all groups and reached significant levels (P<0.05) in AP, with this usually being attributed to the oxidation of heme pigments [20]. The lowest L values after 21 days corresponded to samples under Vacuum packaging which showed significant differences with samples under Air. Parameter b* (yellowness) increased by 21 days for of storage only in for camel meat during storage under Air, but no significant differences were found among samples packed under vacuum and MAP at the end of 21 days for yellowness. Differences in b* along the storage period could be related to the intensity of the oxidation process that takes place during storage and might tend to increase yellowness of samples by rancidity, although no measures of oxidation intensity are available to support this hypothesis. The a* (redness) value in Air-Pakaging decreased significantly (P<0.01) at the same storage time. On the other hand, a decrease in a* values due to oxygen content in AP would reflect myoglobin oxidation. Mercier et al. (1998), have observed an increase in the hue angle (arc tan b*/a*) of stored turkey pectorals muscle, suggesting a degree of change from red to yellow, an indication of increased oxidation with time [21]. In the present study, calculation of hue angle values (not reported) showed an increase for Air-Packaged camel meat during storage, whereas in Vacuum-Packaged samples, values for camel meat remained relatively stable. The more rapid changes in L*, a* and b* value of Air Packaged samples suggest that this gas is responsible for the determination of colour. Moore and Gill (1987) also found increases in L* and b* values with time, in agreement with our results [22]. The increase in b* may be associated with the transformation of the meat pigment and the formation of meat myoglobin, which is faster at relatively low oxygen concentration [23]. Our results show that a mixture with 30% CO 2 and 70% N2 maintains a good colour for up to 21 days at 4 ± 1°C in the absence of O 2 . Chroma showed an opposite co-variation with L*. In both parameters there were significant differences among groups only from 21 days onward. The forward stepwise logistic regression model of acceptance was statistically significant (P<0.01) and showed that this acceptance was affected (P<0.01) by Chroma, time storage and MAP. Samples stored under MAP gases were accepted for a longer time than the other groups. Gas composition in packs (Table 1) was associated with the changes in colour and the probability of being accepted. In agreement with other authors [24] our study found a slower discoloration of samples stored with higher proportions of CO 2 , this being more evident in MAP treatment.

Sensory analysis
The camel meat was evaluated for changes in surface color, texture, and odor by semi-trained panelists. By the end of the storage time (at day 21), MAP were acceptable (scores >6) and significant differences (P<0.05) were found between other packaging system for all sensory attributes. The surface color of the samples in MAP was not severely discolored and remained acceptable even after 21 days storage. Storage time effect within treatment indicated that surface discoloration increased (P<0.05) especially at day 14 in Air-Packaged samples ( Table  2). At day 21, surface colour of samples packed with MAP remained unchanged (P>0.05). The data suggest that the MAP with high CO 2 protected the surface color. The colour and odor changes in meats are highly dependent upon packaging condition [25]. Panelists rejected Air-Packaged samples after 14 days storage at 4°C but MAP increased the shelf life of fresh camel meat refrigerated at 4°C by more than 21 days.

Conclusion
In this study we have observed the evolution of the main parameters that affect camel meat quality (colour, lipid oxidation and shear force) when preserved in modified atmospheres with different mixtures of gas. For colour, however, values obtained indicated that MAP was the best of those tested. Modified atmosphere packaged fresh camel meat with high CO 2 reduced the increasing rate of lipid oxidation during storage. Our study showed that even though oxidative rancidity (TBARS) increased with storage time in all packed samples, it did not result the deterioration of sensory quality in MAP. This indicates that lipid oxidation is not a major problem in MA-packaged fresh camel meat stored at 4°C up to 21 days. In summary, packaging with MAP (60% CO 2 +40% N 2 ) of fresh camel meat accompanied by refrigeration storage enhanced product shelf life at least for 3 weeks without undesirable and detrimental effects on its sensory acceptability.