4 Meat refrigeration Meat is considered spoiled by bacteria when the products of their meta- bolic activities make the food offensive to the senses of the consumer(Gill 1983). Therefore, the perception of a state of spoilage is essentially a sub- jective evaluation that will vary with consumer expectations. Few, however, would not acknowledge that the appearance of slime, gross discoloration and strong odours constitute spoilage Off odours are due to an accumulation of malodorous metabolic prod ucts, such as esters and thiols. Several estimations have been made of the number of bacteria on meat at the point at which odour or slime becomes evident and the mean is about 3 cm(Shaw, 1972). When active growth occurs, the number of bacteria increases exponentially with time. Therefore, a convenient measure of the growth rate is the time required for doubling of numbers, often called the generation time. If this, for example, were one hour, the number would increase two-fold in 1h, four-fold in 2h, eight-fold in 3h. and so on. The bacterial safety and rate of spoilage depends upon the numbers and types of micro-organisms initially present, the rate of growth of those micro- organisms, the conditions of storage(temperature and gaseous atmosphere) and characteristics(pH, water activity aw )of the meat. Of these factors, tem- perature is by far the most important 1.1 Factors affecting the refrigerated shelf-life of meat 1.1.1 Initial microbial levels l.. Tissue sterility or many years microbiologists believed that the tissues of healthy animals normally contained bacteria(Reith, 1926; Ingram, 1972). These intrinsic bacteria were the cause of phenomena such as ' bone taint. The cause of bone taint is still questioned and will be discussed later. The prevailing view of the majority of textbooks(Banwart, 1989: Varnam and Sutherland, 1995) based in part on the work of Gill(Gill, 1979, 1980)is that the meat of a healthy animal is essentially sterile. Low numbers of specific micro- organisms, which have reached the tissues during the life of the animal, may occur in the viscera and associated lymph nodes from time to time(Gill, 1979: Roberts and Mead, 1986). These are often pathogenic species, such as Salmonella, and clostridia spores. The absence of bacteria appears to be due to the continued functioning of the immune system in slaughtered animals. Experiments with guinea pigs showed that the antibacterial defences of live animals persisted for an hour or more after death and could inactivate bacteria introduced during slaughter (Gill and Penney, 1979). Clearly, if bacteria are thus inactivated there can be no multiplication, in deep tissue, during carcass chilling irrespective of cooling rates.Meat is considered spoiled by bacteria when the products of their meta￾bolic activities make the food offensive to the senses of the consumer (Gill, 1983). Therefore, the perception of a state of spoilage is essentially a sub￾jective evaluation that will vary with consumer expectations. Few, however, would not acknowledge that the appearance of slime, gross discoloration and strong odours constitute spoilage. ‘Off’ odours are due to an accumulation of malodorous metabolic prod￾ucts, such as esters and thiols. Several estimations have been made of the number of bacteria on meat at the point at which odour or slime becomes evident and the mean is about 3 ¥ 107 cm-2 (Shaw, 1972).When active growth occurs, the number of bacteria increases exponentially with time.Therefore, a convenient measure of the growth rate is the time required for doubling of numbers, often called the generation time. If this, for example, were one hour, the number would increase two-fold in 1 h, four-fold in 2 h, eight-fold in 3 h, and so on. The bacterial safety and rate of spoilage depends upon the numbers and types of micro-organisms initially present, the rate of growth of those micro￾organisms, the conditions of storage (temperature and gaseous atmosphere) and characteristics (pH, water activity aw) of the meat. Of these factors, tem￾perature is by far the most important. 1.1 Factors affecting the refrigerated shelf-life of meat 1.1.1 Initial microbial levels 1.1.1.1 Tissue sterility For many years microbiologists believed that the tissues of healthy animals normally contained bacteria (Reith, 1926; Ingram, 1972). These ‘intrinsic’ bacteria were the cause of phenomena such as ‘bone taint’. The cause of bone taint is still questioned and will be discussed later.The prevailing view of the majority of textbooks (Banwart, 1989;Varnam and Sutherland, 1995), based in part on the work of Gill (Gill, 1979, 1980) is that the meat of a healthy animal is essentially sterile. Low numbers of specific micro￾organisms, which have reached the tissues during the life of the animal, may occur in the viscera and associated lymph nodes from time to time (Gill, 1979; Roberts and Mead, 1986). These are often pathogenic species, such as Salmonella, and clostridia spores.The absence of bacteria appears to be due to the continued functioning of the immune system in slaughtered animals. Experiments with guinea pigs showed that the antibacterial defences of live animals persisted for an hour or more after death and could inactivate bacteria introduced during slaughter (Gill and Penney, 1979). Clearly, if bacteria are thus inactivated there can be no multiplication, in deep tissue, during carcass chilling irrespective of cooling rates. 4 Meat refrigeration