Meat Spoilage and Evaluation of the Potential Storage Life of Fresh Meat

Abstract

Microbiological proccsses by which meat develops qualities unacceptable to consumers vary with the composition of the meat and spoilage microflora. Composition of the spoilage microflora is affected by meat composition and storage conditions. Aerobic spoilage microfloras are usually dominated by pseudomonads. With this type of microflora, spoilage occurs when glucose in meat is no longer sufficient for the requirements of the spoilage microflora and the bacteria start to degrade amino acids. When meat is deficient in glucose, spoilage becomes evident while bacterial numbers are relatively small. Anaerobic microfloras are usually dominated by lactobacilli which produce spoilage by the slow accumulation of volatile organic acids. Meat of high utimate pH packaged anaerobically spoils rapidly because the high pH allows anaerobic growth of bacterial species of higher spoilage potential than the lactobacilli. Before overt spoilage develops, the spoilage status of meat can be accurately assessed from the bacterial numbers on meat only when there is assumption or knowledge of meat composition, storage conditions and the types of bacteria present. Methods for estimating spoilage which depend upon detection of products of amino acid degradation have little predictive value as such products will only be present after attack on amino acids has commenced and are irrelevant to spoilage under anaerobic conditions. Estimation of the concentrations of other spoilage products may be the only method applicable to asseSSment of incipient spoilage of meat stored anaerobically. It is, therefore, unlikely that any single test can give unequivocal information on meat quality under all circumstances. but rapid tests for meat quality could be of value for specific commercial purposes. provided such tests are appropriate to the circumstances and the inherent limitations of any test are reeognized. Meat is spoiled by bacteria when the products of their metabolic activities make the food offensive to the senses of the consumer. The perception of a state of spoilage is. therefore, essentially a subjective evaluation which will vary with consumer expectations, but there would be few who do not acknowledge that the appearance of slime, gross discoloration and strong odors constitute spoilage of meat. It would be desirablc to identify the status of meat with respect to spoilage before its condition becomes evident to the senses. This would allow more accurate assessment of the meat's shelf-life. thus enabling appropriate action to be taken to prevent the loss of meat when spoilage was incipient but not overt, and could provide a basis for regulations controlling meat quality. For such a test to be of any practical value, it would have to be both simple and comparatively rapid. Numerous objective tests for spoilage have been suggested and claims made for their practical value. Most of this work has been concerned with establishing empirical relationships between spoilage and some readily measurable change in the composition of muscle foods. Such tests have been the subject of previous reviews, which have generally concluded that the reliability of most of the proposed methods has not been established (8, 37, 67). This conclusion is not surprising as the spoilage of meat is a variable phenomenon whose course is determined by both the composition of the meat and the types and relative numbers of bacterial species present in the spoilage micro-flora, The composition of the spoilage microflora is affected by the meat composition and also by storage conditions. No one method is likely to be applicable under all circumstances and attempts to assess the merits of any proposed test will be confused by differing circumstances unless the relationships between the factors being analysed and the spoilage processes are known. Since processes that are critical for spoilage development have now been more clearly defined, it is possible to understand at least some of these relationships. It is the objective of this review to use the current understanding of spoilage processes to describe the circumstances under which methods of estimating meat spoilage are likely to give results of practical value. THE SPOILAGE PROCESS Although there are recent reviews which give substantial accounts of current understanding of mcat spoilage processes (24,50,59), some description of these processes is necessary for any discussion of tests for spoilage. Accounts of earlier work and other aspects of meat spoilage have been the subjects of previous reviews (2,33,37).