3.1 Introduction Much poultry and red meat is sold in a chilled unprocessed state. However, an increasing proportion is used as a basic raw material for chilled meat products and ready meals. A growing trend is the development of added-value convenience meals, especially ethnic products, many of which are pre- or part-cooked and necessitate chilled storage. Many of these products contain meat as a key ingredient. This meat for further processing can be supplied chilled, as boneless blocks of frozen material or increasingly as minced or diced material. The dice or mince can be chilled or increasingly it will be supplied as bags of individually quick frozen (IQF) product. This chapter discusses a number of issues which influence the quality of meat as a raw material in high added-value chilled foods. The quality of meat is judged by its bacterial condition and appearance. Bacterial condition is subjectively assessed by the presence or absence of odour or slime. Quantitative tests can also carried out to determine the total viable counts and the presence of specific pathogens or indicator organisms. Appearance criteria are primarily; colour, percentage of fat and lean and amount of drip exuding from the meat. Any unacceptable change in the microbial or appearance criteria will limit the shelf-life of the meat. After cooking its eating quality is partially judged by its appearance but mainly by its tenderness, flavour and juiciness. Red meat and poultry are very perishable raw materials. If stored under ambient conditions, 16–30oC, the shelf-life of both can be measured in tens of hours to a few days. Under the best conditions of chilled storage, close to the initial freezing point of the material, the storage life can be extended to 3 Raw material selection: meat and poultry S. J. James, Food Refrigeration and Process Engineering Research Centre approaching six weeks for some red meat. Even under the best commercial practice (strictly hygienic slaughtering, rapid cooling, vacuum packing and storage at super chill (C01C60.5oC)) the maximum life that can be achieved in red meat is approximately 20 weeks, however, freezing will extend the storage life of meat to a number of years. In a perfect world, red meat and poultry would be completely free of pathogenic (food poisoning) micro-organisms when produced. However, under normal methods of production pathogen-free meat cannot be guaranteed. For example, salmonella contamination of chilled and frozen poultry carcasses has been significantly reduced in the UK (Fig. 3.1). However, over one-third was still contaminated in 1994. 1 While the internal musculature of a healthy mammal or bird is essentially sterile after slaughter, all meat animals carry large numbers of different micro-organisms on their skin/feathers and in their alimentary tract. Only a few types of bacteria directly affect the safety and quality of the finished carcass. Of particular concern are food-borne pathogens such as Campylobacter spp., Clostridium perfringens, Salmonella spp., and pathogenic serotypes of Escherichia coli. The minimum and optimum growth temperatures for some of the pathogens associated with red and poultry meat are shown in Table 3.1. Inevitably, small numbers of pathogens will be present on meat and cooking regimes are designed to eliminate their presence. Most red meat and poultry food poisoning is associated with inadequate cooking or subsequent contamina- tion after cooking and poor cooking and storage. Normally it is the growth of spoilage organisms that has the most important effect in limiting the shelf-life of meat. The spoilage bacteria of meats stored in air under chill conditions include species of Pseudomonas, Brochothrix and Acinetobacter/Moraxella. Varnam and Sutherland state that in general, there is Fig. 3.1 Percentage of chilled and frozen poultry carcasses contaminated with salmonella found in UK surveys carried out in 1979–80 to 1994. 64 Chilled foods little difference in the microbial spoilage of beef, lamb, pork and other meat derived from mammals. The presence of exudate or ‘drip’, which accumulates in the container of pre- packaged meat, or in trays or dishes of unwrapped meat, substantially reduces its sales appeal. 3 Drip can be referred to by a number of different names including ‘purge loss’, ‘press loss’ and ‘thaw loss’ depending on the method of measurement and when it is measured. Drip loss occurs throughout the cold chain and represents a considerable economic loss to the red meat industry. Poultry meat is far less prone to drip. The potential for drip loss is inherent in fresh meat and is influenced by many factors. Some of these, including breed, diet and physiological history, are inherent in the live animal. Others, such as the rate of chilling, storage temperatures, freezing and thawing, occur during processing. Meat colour can be adversely affected by a variety of factors, including post-mortem handling, chilling, storage and packaging. 4 In Australia, CSIRO 5 stated that ‘Toughness is caused by three major factors – advancing age of the animal, ‘cold shortening’ (the muscle fibre contraction that can occur during chilling) and unfavourable meat acidity (pH).’ There is general agreement on the importance of these factors, with many experts adding cooking as a fourth equally important influence. 3.2 The influence of the live animal Some of the factors that influence the toughness or meat are inherent in the live animal. Church and Wood 4 state that it is now well established that it is the properties of the connective tissue proteins, and not the total amount of collagen in meat, that largely determines whether meat is tough or tender. As the animal Table 3.1 Chilled storage life of meat and meat products at different storage temperatures Storage time (days) in temperature range: C04.1 to C01.1oC C01to2oC 2.1 to 5.1oC 5.2 to 8.2oC Food Mean sd Mean sd Mean sd Mean sd Bacon 45 6 15 3 42 20 Beef 40 26 34 32 10 8 9 9 Lamb 55 20 41 46 28 34 Pork 50 58 22 30 16 16 15 18 Poultry 32 18 17 10 12 11 7 3 Veal 21 10 6 49 49 Rabbit 9 7 13 6 Offal 7 7 6 14 7 Bacon 45 6 15 3 42 20 Sausage 80 43 21 16 36 28 24 10 Raw material selection: meat and poultry 65 grows older the number of immature reducible cross-links decreases. The mature cross-links result in a toughening of the collagen and this in turn can produce tough meat. Increasing connective tissue toughness is probably not commer- cially significant until a beast is about four years old. 6 The pigment concentration in meat which governs its colour is affected by many factors affecting the live animal. These include species – beef, for example, contains substantially more myoglobin than pork; breed and age – pigment concentration increases with age; sex – meat from male animals usually contains more pigment than that from female animals; muscle – muscles that do more work contain more myoglobin. There are also two specific meat defects; dark, firm, dry (DFD) and pale, soft, exudative (PSE) associated with the live animal that result in poor meat colour. DFD meat has a high ultimate pH and oxygen penetration is low. Consequently, the oxymyoglobin layer is thin, the purple myoglobin layer shows through, and the meat appears dark. In PSE meat the pH falls while the muscle is still warm and partial denaturation of the proteins occurs. An increased amount of light is scattered and part of the pigment oxidised so that the meat appears pale. 3.2.1 Between species and breeds In all species the range of storage lives found in the literature is very large (Table 3.1) and indicate that factors other than species have a pronounced effect on storage life. Overall, species has little effect on the practical storage life of meat. In general, beef tends to lose proportionately more drip than pork and lamb. Unfrozen poultry meat looses little if any drip. Since most of the exudate comes from the cut ends of muscle fibres, small pieces of meat drip more than large intact carcasses. In pigs, especially, there are large differences in drip loss from meat from different breeds and between different muscles. Taylor 7 showed that there was a substantial difference, up to 2.5 fold, in drip loss between four different breeds of pig (Table 3.2). He also showed that there was a 1.7 to 2.8 fold difference in drip between muscle types (Table 3.3). Although there is a common belief that breed has a major effect on meat quality CSIRO 8 state ‘although there are small differences in tenderness due to Table 3.2 Drip loss after two days storage at 0oC, from leg joints from different breeds of pig cooled at different rates Breed Drip loss (% by wt.) Slow Quick Landrace 0.47 0.24 Large White 0.73 0.42 Wessex X Large White 0.97 0.61 Pietrain 1.14 0.62 66 Chilled foods breed, they are slight and currently of no commercial significance to Australian consumers.’ That said, there are substantial differences in the proportion of acceptable tender meat and toughness between Bos indicus* and Bos taurus* cattle. The proportion of acceptable tender meat has been found to decrease from 100% in Hereford Angus crosses, to 96% in Tarentaise, 93% in Pinzgauer, 86% in Brahman and only 80% in Tsahiwal. 9 Toughness of meat increases as the proportion of Bos indicus increases. 10 3.2.2 Animal to animal variation There is little data on any relationship between animal to animal variation and chilled storage life. However, it is believed to cause wide variations in frozen storage life; differences can be as great as 50% in the freezing of lamb. 11, 12 Differences would appear to be caused by genetic, seasonal or nutritional variation between animals, but there is little reported work to confirm this view. Variations were found between the fatty acids and ratio of saturated/unsaturated fatty acids in lambs from New Zealand, America and England. 13 Differences related to sex, geographical area and cut were mainly a reflection of fatness, with ewes having a greater percentage of body fat than rams. However, differences between areas were found to produce larger variations between animals than sex differences. A number of other trials have detailed differences between animals. There can also be significant differences in texture within a breed. Longissimus dorsi shear force values for double muscled Belgium Blue bulls were significantly higher than those of the same breed with normal conformation. 14 Calpain I levels at 1 h and 24 h post mortem were also much lower. It was suggested that the lower background toughness in the double muscle was compensated for by reduced post mortem proteolytic tenderisation. Sex of the animal appears to have little or no influence on tenderness. Huff and Parrish 15 compared the tenderness of meat from 14-month-old bulls and Table 3.3 Drip loss after two days storage at 0oC from four muscles from two breeds of pig cooled at different rates Drip (as % muscle weight) Cooling Semi- Semi- Adductor Biceps Combined rate tendinosus membranous femoris (4 muscles) Pietrain Quick 2.82 4.40 5.52 2.69 3.86 Slow 3.99 6.47 6.61 4.11 5.30 Large White Quick 1.69 2.01 2.92 1.04 1.92 Slow 1.95 3.50 5.07 2.32 3.21 * Bos indicus are tropical and semitropical breeds of cattle primarily Brahman and Bos taurus are temperate breeds such as Hereford or Aberdeen Angus. Raw material selection: meat and poultry 67 steers, and cows (55 to 108 months old). No differences were found between the tenderness of bulls and steers. Tenderness decreased with the age of the animal. Hawrysh et al. 16 reported that beef from bulls may be less tender than that from steers. Sex can have a substantial influence on flavour. For example, cooking the meat from entire male pigs can produce an obnoxious odour known as ‘boar taint’. Problems can also occur with meat from intact males of other species. However, they can still be attractive to industry because of their higher rate of growth and lower fat content. 3.2.3 Feeding The way in which an animal is fed can influence its quality and storage life. It has been reported that chops from pigs fed on household refuse have half the frozen storage life of those fed on a milk/barley ration. 17, 18 Again, pork from pigs that had been fed materials containing offal had half the practical storage life (PSL) and higher iodine numbers in the fat than that of pigs which had not been fed this type of diet. 19 Conversely, Bailey et al. 20 did not find any differences between meal- and swill-fed pigs after 4 and 9 months at C020oC. Rations with large amounts of highly unsaturated fatty acids tend to produce more unstable meat and fat. The type of fatty acid composition of ‘depot fat’ in poultry and its stability have been shown to be directly related to the fatty acid composition of ingested fats. 21, 22 The feeding of fish oils or highly unsaturated vegetable oils (such as linseed oil) to poultry is known to produce fishy flavours in the meat. 21, 23 The use of vitamin E supplements is recommended for both beef and turkey. This will ‘result in delayed onset of discoloration in fresh, ground and frozen beef and in suppression of lipid rancidity, especially in fresh, ground and frozen beef and less so in cooked beef’. 24 With turkey, vitamin E supplements have been shown to improve oxidative stability of cooked and uncooked turkey burgers during six months frozen storage at C020oC. 25 3.2.4 Variations within an animal Reports of variations in the storage life of different cuts of meat are scarce and primarily deal with dark and light meat. Both Ristic 26 and Keshinel et al. 27 have found that poultry breast meat stores better than thigh meat. Ristic states that frozen breast meat will store for 16 months while thigh meat can be stored for only 12 months due to its higher fat content. Judge and Aberle 28 also found that light pork meat stored for a longer time than dark meat. This was thought to be due to either higher quantities of haem pigments in the dark muscle (which may act as major catalysts of lipid oxidation), or to higher quantities of phospholipids (which are major contributors to oxidised flavour in cooked meat). 68 Chilled foods 3.3 Pre- and post-slaughter handling 3.3.1 Red meat The way animals are handled and transported before slaughter affects meat quality and its storage life. Increased stress or exhaustion can produce PSE (pale soft and exudative) or DFD (dark firm and dry) meat, which is not recommended for storage, mainly due to its unattractive nature and appearance. Jeremiah and Wilson 29 found that the use of PSE muscle produced low yields after curing and it was concluded that PSE meat was unsuitable for further processing. Experiments designed to determine the effect of treatments immediately before or at the point of slaughter appear to show that they have little effect on meat texture. Exercising pigs before slaughter has been shown to have no effect on texture parameters, i.e. muscle shortening and shear force. 30 The use of different stunning methods (both electrical and carbon dioxide) does not seem to have a significant effect on the quality of pork. 31 Consumers’ surroundings influence their appreciation of tenderness. 32 Consumers were more critical of the tenderness of beef steaks cooked in the home than those cooked in restaurants. The Warner-Bratzler force transition level for acceptable steak tenderness was between 4.6 and 5.0 kg in the home and between 4.3 and 5.2 kg in the restaurants. Overall, there appears to be little correlation between chilling rates or chilling systems and bacterial numbers after chilling. The microorganisms that usually spoil meat are psychrotrophs; i.e. bacteria capable of growth close to 0oC. Only a small proportion of the initial microflora on meat will be psychrotrophs; the majority of microorganisms present are incapable of growth at low temperatures. As storage temperature rises the number of species capable of growth will increase. The growth rate of microorganisms also accelerates with increasing temperature. In the accepted temperature range for chilled meat, C01.5 to 5oC, there can be as much as an eightfold increase in growth rate between the lower and upper temperature. For any particular treatment the maximum chilled storage life will be obtained by holding the meat at C01.5oC. Chilled storage life is halved for each 2–3oC rise in temperature. Odour and slime cause by the growth of microorganisms will be apparent after approximately 14.5 and 20 days respectively with beef sides stored at 0oC (Fig. 3.2). At 5oC the respective times are significantly reduced to 8 and 13 days, respectively. Rapid chilling reduces drip loss (Table 3.2 & 3.3). However, chilling has serious effects on the texture of meat if it is carried out rapidly when the meat is still in the pre-rigor condition, that is, before the meat pH has fallen below about 6.2. 33 In this state the muscles contain sufficient amounts of the contractile fuel, adenosine triphosphate (ATP), for forcible shortening to set in as the temperature falls below 11oC, the most severe effect occurring at about 3oC. This is the so-called ‘cold-shortening’ phenomenon, first observed by Locker and Hagyard 34 and its mechanism described by Jeacocke. 35 The meat ‘sets’ in the shortened state as rigor comes on, and this causes it to become extremely Raw material selection: meat and poultry 69 tough when it is subsequently cooked. 36 If no cooling is applied and the temperature of the meat is above 25oC at completion of rigor then another form of shortening ‘rigor’–or ‘heat-shortening’ will occur. 37 Electrical stimulation (ES) of the carcass after slaughter can allow rapid chilling without much of the toughening effect of cold shortening. However, Buts et al. 38 reported that in veal ES followed by moderate cooling affected tenderness in an unpredictable way and could result in tougher meat. Electrical stimulation will hasten rigor and cause tenderisation to start earlier at the prevailing higher temperature. In meat from carcasses given high or low voltage stimulation and slow cooling, adequate ageing in beef can be obtained in about half the time of non-stimulated beef 38 This will therefore reduce the requirement and cost of storage. When meat is stored at above freezing temperatures it becomes progressively more tender. This process, known as ageing, conditioning or maturation is traditionally carried out by hanging the carcass for periods of 14 days or longer. The rate of ageing differs significantly between species and necessitates different times for tenderisation. Beef, veal and rabbit age at about the same rate and take about ten days at 1oC to achieve 80% of ageing (Table 3.4). Lamb ages slightly faster than beef but more slowly than pork. The ultimate tenderness will depend on the initial ‘background’ tenderness of the meat and the tenderisation that has occurred during chilling. In veal acceptable tenderness can be obtained after five days at 1oC compared with 10 days for beef. Red colour is more stable at lower temperatures because the rate of oxidation of the pigment decreases. At low temperatures, the solubility of oxygen is greater and oxygen-consuming reactions are slowed down. There is a greater penetration of oxygen into the meat and the meat is redder than at high temperatures. The major improvement in tenderness has been shown to occur in less than 14 days. In a study by Martin et al., 39 in which more than 500 animals were examined, it was concluded that for beef carcasses, a period of six days is Fig. 3.2 Time for odour and slime to develop on beef carcasses at different storage temperatures. 70 Chilled foods sufficient for a consumer product of satisfactory tenderness. Buchter 40 also showed that no significant increase in tenderness occurs after 4–5 days for calves and 8–10 days for young bulls at 4oC. The ageing process can be accelerated by raising the temperature, and the topic was well studied in the 1940s, 50s and 60s. Ewell 41 found that the rate of tenderising more than doubled for each 10oC rise. Meat from a three-year-old steer requiring ten days at 0oCto reach the same tenderness as two days at 23oC. Sleeth et al. 42 showed that the tenderness, flavour, aroma and juiciness of beef quarters and ribs aged for 2–3 days at 20oC were comparable to those aged 12–14 days at 2oC. Busch et al. demonstrated that steaks from excised muscles held at 16oC for two days were more tender than those stored at 2oC for 13 days. The microbiological hazards of high-temperature ageing were well recognised and several investigators used antibiotics and/or irradiation to control bacterial growth. 42, 44, 45 Although high-temperature ageing in conjunc- tion with ultraviolet (UV) radiation has been used in the US its use has not expanded owing to its high cost. 46 With the use of irradiation gaining more acceptance in the US its use to accelerate ageing in conjunction with modified atmosphere packaging and high-temperature storage has been investigated by Mooha Lee et al. 47 Irradiated steaks stored for two days at 30oC were more tender than unirradiated controls stored at 2oC for 14 days (Table 3.5). In red meat, there is little evidence of any relationship between chilling rates and frozen storage life. However, there is evidence for a relationship between Table 3.4 Time taken to achieve 50 and 80% ageing at 1oC for different species 8 Time (d) taken to achieve Species 50% 80% Beef 4.3 10.0 Veal 4.1 9.5 Rabbit 4.1 9.5 Lamb 3.3 7.7 Pork 1.8 4.2 Chicken 0.1 0.3 Table 3.5 Shear values for steaks after post mortem ageing Treatment Storage time (d) Unirradiated 2oC Irradiated 15oC Irradiated 30oC 1 4.47C61.76 4.89C61.40 4.24C61.65 2 – 4.81C60.73 3.44C61.34 3 – 4.07C61.03 3.33C61.21 7 3.62C61.00 –– 14 3.52C61.51 Raw material selection: meat and poultry 71 storage life and the length of time that elapses before freezing occurs. Chilled storage of lamb for one day at 0oC prior to freezing can reduce the subsequent storage life by as much as 25% when compared to lamb which has undergone accelerated conditioning and 2 hours storage at 0oC. 12 It has been shown that pork which had been held for seven days deteriorated at a faster rate during storage than carcasses chilled for one and three days. 48 Ageing for periods greater than seven days was found by Zeigler 49 to produce meat with high peroxide and free fatty acid values when stored at C018oCorC029oC. Although shorter ageing times appear to have a beneficial effect on storage life there is obviously a necessity for it to be coupled with accelerated conditioning to prevent any toughening effects. Whilst there has been significant research in such areas as these, little appears to be known about the relationship between the frozen storage life of meat as a raw material and the chilled life of the product in which it is used. 3.3.2 Poultry After bleeding and death, poultry carcasses are scalded by immersing them in hot water for approximately three minutes. Scalding loosens the feathers so that they can be easily removed. Carcasses can either be soft scalded at 52–53oCor hard scalded at 58oC. Hard scalding removes the cuticles on chicken skin, which gives an unattractive appearance after air chilling. Generally if the poultry is marketed in a chilled state it will be soft scalded and air or spray chilled. Spray washing is used at numerous points during processing to remove visual contamination. It also has some small role in reducing bacterial contamination. The EU Poultry Meat Directive 50 requires poultry to be washed inside and out immediately prior to water chilling. The amount of water to be used (i.e. 1.5 litres for a carcass up to 2.5 kg) is defined in the directive. Water chillers are designed to operate in a counter-current manner to minimise cross-contamination. The carcasses exit from the chillers at the point where the clean, chilled water enters the system. Again, the Directive defines a minimum water flow through the chiller, i.e. at least one litre per carcass for carcasses up to 2.5 kg in weight. Chicken breast muscle ages ten times faster than beef (Table 3.4). Hence, ageing in poultry carcasses occurs during processing and is usually accomplished before they reach the chiller/freezer. Pool et al. 51 have shown that there were no detectable flavour differences over an 18-month period between turkey that had been frozen immediately and turkey that had been held at +2oC for 30 hours. 3.4 Conclusions Even with the best practice, the maximum shelf-life of chilled meat can be measured in weeks. Freezing will extend the storage life of meat to a number of years. If the frozen storage life is not exceeded, freezing and frozen storage of 72 Chilled foods meat has little (if any) effect on the main quality parameters. Drip is the only exception, as it is substantially increased by freezing. Numerous experiments and blind testings have shown that consumers cannot tell the difference between frozen and chilled meat. However, they are still willing to pay up to twice as much for the equivalent chilled product. Despite a large price differential in favour of the frozen product it is the ‘fresh’ chilled market that is growing fastest. For example, there is a growing trend for retailers to sell bulk packs of chilled chicken breasts specifically for home freezing. The breasts are individually packed and the consumer is happy to pay a premium price and slowly freeze them in a domestic freezer. There are no obvious technical developments that are likely to change the trend towards chilled products with poultry carcasses and portions. Chilled product is more convenient for the consumer even if there are no quality advantages over frozen poultry. If food safety becomes much more of an issue then freezing directly after decontamination may become more common. The low temperature will stop any growth of pathogenic or spoilage organisms that survive the decontamination process. In the red meat industry there is an increasing demand for meat of a consistent, guaranteed high eating quality. Specifications already take into account, breed, age, feed and handling of the live animal. Slaughter, chilling and ageing conditions are also carefully controlled. However, at the end of this carefully controlled process we have a quality-chilled product with a very limited shelf-life. 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