Colour stability and lipid oxidation of fresh beef. Development of a response surface model for predicting the effects of temperature, storage time, and modified atmosphere composition
Introduction
Modified atmosphere packaging (MAP) is widely used to extend the shelf life and quality of chill stored beef. Colour, microbial growth, and lipid oxidation are important factors for the shelf life and consumer acceptance of fresh meat. Shelf life and quality of fresh beef are strongly influenced by initial meat quality, package parameters, and storage conditions (Zhao, Wells & McMillin, 1994). It is well known that elevated levels of carbon dioxide inhibit microbial growth (Gill & Tan, 1980, Marshall et al., 1991, Wimpfheimer et al., 1990), whereas elevated levels of oxygen prolong colour stability (Asensio et al., 1988, Bartkowski et al., 1982, Taylor, 1972). The gas composition normally used for modified atmosphere packaged beef is 20–30% CO2 and 70–80% O2 (Blakistone, 1998, Taylor, 1996).
Lipid oxidation causes a rancid off-flavour and off-odour in meat. Numerous factors affect lipid oxidation including light, oxygen concentration, temperature, presence of anti- and prooxidants, degree of unsaturation of the fatty acids and the presence of enzymes (Skibsted, Mikkelsen & Bertelsen, 1998). Lipid oxidation is normally not considered to be a limiting factor for shelf life of aerobic packed chill stored meat, as lipid oxidation occurs at a slower rate than discoloration or microbial growth (Zhao, Wells & McMillin, 1994). However, when modified atmosphere packaging represses the other deteriorative mechanisms in meat, lipid oxidation might limit shelf life (McMillin, 1993). While an elevated oxygen level is known to prolong colour stability, it is also expected to increase the rate of lipid oxidation (Zhao et al., 1994). Increased lipid oxidation has been reported for meat stored at elevated oxygen concentrations (Jackson et al., 1992, Jensen et al., 1997, Taylor, 1985), although other researchers did not find any increase in lipid oxidation under similar conditions (Asensio et al., 1988, Lopez-Lorenzo et al., 1980, Ordonez & Ledward, 1977).
Mathematical techniques can be of help in predicting the effects of environmental variables, such as temperature and initial gas composition, on deterioration reactions in meats. Attempts have been made to develop predictive equations to describe the quantitative relationships between bacterial growth and the combined effects of storage time, temperature and initial gas composition on modified atmosphere stored beef (Zhao et al., 1992). However, corresponding models for predicting chemical changes such as discoloration and lipid oxidation are not available.
The object of the present study was to develop mathematical models describing the relationship between discoloration and lipid oxidation in beef and the combined effect of time, temperature and partial pressure of oxygen. These models should form the basis for proposing an optimal MAP gas composition to maintain the bright red colour preferred by the consumer and at the same time keeping lipid oxidation at a minimum.
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Meat samples
Longissimus dorsi muscles containing approximately 3% intramuscular fat (IMF) were used. The muscles were removed from the animal before 4 days post slaughter at less than 7°C and matured for one week in vacuum bags at 2°C. Subsequently, the muscles were trimmed of external fat and cut into 1.5 cm thick steaks. Prior to packaging the samples were allowed to bloom for approximately one hour at 5°C. Meat from four different animals was used. The meat from the different animals had been handled
Results
Development of lipid oxidation during chill storage varied significantly between muscles at all storage temperatures, the differences being most pronounced at 8°C. As can be seen from Fig. 1(a), lipid oxidation was most pronounced in animal A followed by animals B, C and D. The differences may partly be explained by varying levels of endogenous vitamin E, lipid content and fatty acid composition of the lipids (Table 2), parameters all known to affect lipid oxidation (Skibsted et al., 1998).
Discussion
The developed models can be used to predict the effect of different packaging (oxygen content) and storage parameters (time and temperature) on the meat quality parameters. The models can also identify the most important factors for these meat quality parameters. The most important factor for maintaining the red oxymyoglobin colour and keeping lipid oxidation to a minimum is the temperature. A low temperature (below approximately 4°C) almost prevents lipid oxidation, regardless of oxygen level (
Acknowledgements
This work was sponsored by the FØTEK program through LMC—Centre for Advanced Food Studies. The authors are grateful to Bente Sørensen for excellent technical assistance. Also thanks to AGA A/S, Danisco Flexible ON and Danish Crown, Hvidovre for providing materials for this experiment.
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