10 Chilled and frozen storage Theoretically, there are clear differences between the environmental con- ditions required for cooling, which is a heat removal/temperature reduction process, and those required for storage where the aim is to maintain a set product temperature. However, in many air-based systems, cooling and storage take place in the same chamber and even where two separate facil- ities are used, in many cases not all the required heat is removed in the cooling phase. This failure to remove the required heat can be due to a number of causes: ? insufficient time allowed ? insufficient refrigeration capacity to cater for high initial product load ? overloading ? variability in size of products ? incorrect environmental conditions. Extensive data are available on the optimum storage conditions and attainable chilled and frozen storage lives for many products (IIR, 2000; IIR, 1986; ASHRAE, 1998). 10.1 Storage life terms There are a wide range of rather confusing definitions used to define storage life. The EC directive (Commission of the European Community, 1989) states simply that frozen storage must ‘preserve the intrinsic characteristics’ of the food. Although this is probably every food technologist’s aim, many different criteria can be used to measure these characteristics. The IIR recommendations (1986) define frozen storage life as being ‘the physical and biochemical reactions which take place in frozen food products leading to a gradual, cumulative and irreversible reduction in product quality such that after a period of time the product is no longer suitable for consump- tion or the intended process’. This definition tends to indicate that a frozen product may deteriorate until it is in a very poor condition before storage life ends, and so rather contradicts the EC definition. IIR (1986) recommendations also include the term of practical storage life (PSL). PSL is defined as ‘the period of frozen storage after freezing during which the product retains its characteristic properties and remains suitable for consumption or the intended process’. B?gh-S?rensen (1984) describes PSL as ‘the time the product can be stored and still be acceptable to the consumer’. Both of these definitions of PSL depend on the use of sensory panels, leading to the difficulty of defining acceptability and select- ing a panel that represents consumers. Another term referred to is high quality life (HQL). This concept was developed in the ‘Albany’ experiments started in 1948. HQL is ‘the time elapsed between freezing of an initially high quality product and the moment when, by sensory assessment, a statistically significant difference (P < 0.01) from the initial high quality (immediately after freezing) can be established’ (IIR, 1986). The control is stored at -40°C or colder to mini- mise quality changes. Although well suited to research work, some draw- backs have been noted. The actual definition of storage life and the way it is measured has there- fore been widely left to the assessment of individual authors. In some cases sensory assessment has been coupled with chemical or instrumental tests, which although probably more repeatable than human judgements, are again used at the author’s discretion. Food technologists have no standard way of estimating shelf-life. Researchers have used many different methods of assessing samples, often with little thought of the initial quality, pre- freezing treatment or size of their samples. This deficiency has led to poor conclusions and recommendations that can be misleading to users of the data. The IIR (2000) definition of chilled storage is very similar to that of frozen storage life. Expected or practical storage life is ‘the greatest length of time for which the bulk of the produce may be stored either with maximum commercially acceptable loss of quality and nutritive value or with maximum acceptable wastage by spoilage’. 10.2 Chilled storage Extensive data are available on the attainable chilled storage lives for many products (Table 10.1). In most cases the limiting factors that control the chilled storage life of meat are based on bacterial growth. ‘Off’ odours and slime caused by microorganisms are detected when populations reach ca. 208 Meat refrigeration 10 7 –10 8 organisms cm -2 . Temperature is the principal factor affecting the rate of microbial growth and hence the shelf-life of chilled meat. 10.2.1 Unwrapped meat Temperature is the prime factor controlling storage life of wrapped meat. Odour and slime will be apparent after ca. 14.5 and 20 days, respectively, with beef sides stored at 0°C (Fig. 10.1). At 5°C, the respective times are significantly reduced to 8 and 13 days. Chilled and frozen storage 209 Table 10.1 Chilled storage times Storage time (days (sd)) in temperature range (°C) -4.1 to -1.1 -1–2 2.1–5.1 5.2–8.2 Bacon 45 (6) 15 (3) 42 (20) Beef 40 (26) 34 (32) 10 (8) 9 (9) Cold meat 14 (9) 20 (17) 8 (0) Lamb 55 (20) 41 (46) 28 (34) Meals 34 (18) 15 (7) 21 (38) 18 (4) Offal 7 7 (6) 14 (7) Pork 50 (58) 22 (30) 16 (16) 15 (18) Poultry 32 (18) 17 (10) 12 (11) 7 (3) Rabbit 9 (7) 13 (6) Sausage 80 (43) 21 (16) 36 (28) 24 (10) Veal 21 10 (6) 49 49 Source: IIR, 2000. 20 18 16 14 12 10 8 6 4 2 0 T ime (days) 20 10 5 0 Storage temperature (°C) Odour Slime Fig. 10.1 Time (days) for odour or slime to be detected on beef sides with average initial contamination stored at different temperatures (source: Ingram and Roberts, 1976). The initial level of bacterial contamination will of course affect the storage life. Over 40 years ago Ayres (1955), in his comprehensive review of microbiological contamination in slaughtering, concluded that an aerobic population of 4.0–5.0log 10 cfucm -2 and an anaerobic population of between 3.7 and 4.7log 10 cfug -1 would be reasonable for wholesale cuts of meat. Surveys from the mid-1970s have shown that in general levels of between 1 and 4log 10 cfug -1 can be expected on red meat carcasses (Table 10.2). Specific surfaces of the carcass can have very high levels of initial con- tamination. Beef subcutaneous fat has been shown to have a high initial microbial load and a capacity to support extensive bacterial growth (Lasta et al., 1995). Initial values of total viable counts increase from an initial value of 5.4 to 10.0 log 10 cfucm -2 after 11 days in a moist environment at 5°C (Fig. 10.2). No noticeable deterioration in appearance of the sample was found after 14 days which was worrying.This type of material is often incor- porated in manufactured products or could provide a cross contamination source. The above results were obtained on the surface of samples stored in air nearly saturated with water vapour. There is much industrial belief that the surface of meat carcasses must be allowed to dry or storage life will be com- 210 Meat refrigeration Table 10.2 General levels of microbiological contamination reported on meat carcasses throughout the world Type of Country APC* Reference meat (log 10 organism) Beef UK 1.9–3.7 Ingram and Roberts (1976) Sweden 2.2–3.4 Ingram and Roberts (1976) New Zealand 1.3–4.3 Ingram and Roberts (1976) New Zealand 1.4–2.2 Newton et al. (1978) Norway 1.3–3.9 Johanson et al. (1983) EU 2.3–3.9 Roberts et al. (1984) UK 3.4–3.8 Hudson et al. (1987) New Zealand 0.4–3.3 Bell et al. (1993) Australia 3.2 Anon (1997) Canada 1.5–3.2 Gill et al. (1998b) UK 2.45–4.29 Hinton et al. (1998) Lamb/ New Zealand 2.5–2.9 Newton et al. (1978) sheep Spain 4.96 Prieto et al. (1991) New Zealand 2.3–4.1 Bell et al. (1993) New Zealand 3.9–4.6 Biss and Hathaway (1995) Australia 3.9 Anon (1997) Pig UK 2.5–3.3 Ingram and Roberts (1976) Norway 2.6–3.9 Johanson et al. (1983) Denmark 1.6–3.8 Christensen and S?rensen (1991) Italy 4.3–5.0 Barbuti et al. (1992) * Values are not directly comparable since different sampling techniques and incubation temperatures have been used. promised. There appear to be no clear scientific studies that store carcasses under a range of industrial conditions to prove or disprove this belief. Inves- tigations on pork chilling (Greer and Dilts, 1988) have shown that while conventional chilling significantly reduces the level of mesophilic bacteria, this does not occur when spray chilling. However, this work found that after boning there was no significant difference in bacterial counts on loins pro- duced by either treatment. Other studies found no difference in off-odours during storage and retail display of pork chops from pork cooled under either of the two methods, though the appearance of the spray chilled samples deteriorated slightly faster than those treated conventionally (Jeremiah and Jones, 1989). 10.2.2 Wrapped meat TTT (time–temperature–tolerance) and PPP (product–process–packaging) factors significantly influence the storage life of chilled meat (B?gh- S?rensen et al., 1986). In some cases the initial processing stage can have more effect than the subsequent storage conditions. After manufacture, sausages made from hot-boned pork had higher total bacterial counts (4.1log 10 cfug -1 ) than those from cold-boned meat (2.7 log 10 cfug -1 ) (Bentley et al., 1987). When they were stored at 4 or -1°C for 28 days there were no differences between counts at the two temperatures. However, the counts were very high, 8.7 and 8.9 log 10 cfug -1 , in both cases. In vacuum-packaged primals, Egan et al. (1986) have shown that the tem- perature of storage and pH determines both the storage life and the nature of the changes during storage (Table 10.3). Flora on high pH (>6.0) beef cuts, vacuum packaged in polyvinylidene chloride (PVDC) reached maximum levels in 6 weeks at 1°C compared Chilled and frozen storage 211 10 8 6 4 2 0 02479114 Days at 5 °C Log cfu cm –2 Total Gram negs Pseudomonas Fig. 10.2 Growth of bacteria on naturally contaminated beef-brisket fat stored at 5 °C (source: Lasta et al., 1995). with 12 weeks for normal pH beef (Gill and Penney, 1986). In metalized polyester or aluminium foil laminate vacuum packs, times were 9 and 15 weeks respectively. At a lower temperature Jeremiah et al. (1995a, b) have shown that off-flavour development, coinciding with lactic acid bacteria reaching maximum numbers, currently restricts the storage life of carbon dioxide (CO 2 ) or vacuum-packaged pork at -1.5 °C to 9 weeks. Based on appear- ance, CO 2 -packaged and vacuum-packed pork loin had storage lifes of over 15 weeks and slightly over 12 weeks, respectively. Only small differences were found between pork loins from dark, firm, dry (DFD); pale, soft, exuding (PSE) and normal quality groups. They believed that reducing the current levels of microbial contamination would allow storage life to be extended to meet all domestic and export requirements. Bell et al. (1996) detected no major off odours after 98 days at -0.1 °C from hot-boned bull beef that had been cooled and stored in vacuum or CO 2 packs. On opening, the appearance of the strip loins was also acceptable. However, overageing was believed to have reduced the retail display life of the meat. The authors thought that the process could produce high quality beef for catering use with a storage life of 70 days. The effect of temperature and packaging was clearly demonstrated by Lee et al. (1985) and Gill and Harrison (1989). Only small changes in micro- bial numbers (Fig. 10.3), pH, drip and off-odour were found in vacuum or vacuum plus gas flushed packs of pork after 49 days storage at -4°C (Lee et al., 1985), whilst green discolouration was significant after 14 days at 3 and 7 °C and 28 days at 0 °C. The amount of drip loss increased substan- tially with both length and temperature of storage (Fig. 10.4). Drip loss from pork liver tends to be higher than that from muscle and increases more rapidly during storage. At a storage temperature of 5°C losses increased from 1.9% after 1 day to ca. 6% after 6 days (Strange et al., 1985). Gill and Harrison (1989) found that vacuum-packed cuts of pork longis- simus dorsi muscle (skin on) were grossly spoiled by Brochothrix thermo- sphacta after 2 weeks storage at 3 °C compared with 5 weeks at -1.5°C. Cuts 212 Meat refrigeration Table 10.3 Storage life and nature of spoilage of vacuum-packaged pork Meat pH 0 °C 5 °C Storage life Spoilage Storage life Spoilage (weeks) characteristics (weeks) characteristics 5.4–5.8 6 Flavour changes, 3–4 Flavour changes, souring souring 6.2–6.5 4–5 Variable 2–3 Greening, odour of H 2 S, putrefaction Source: Egan et al., 1986. packed under CO 2 spoiled after 5.5 weeks storage at 3 °C. Growth of Bro- chothrix thermosphacta was suppressed when the pork was stored under CO 2 at -1.5 °C. Growth of Enterobacteriaceae caused gross spoilage of an increasing proportion of cuts between 18 and 26 weeks. Until spoilage occurred, the eating quality of the pork was little affected by the length of storage. In audits carried out in New Zealand to improve the shelf-life of vacuum- packed chilled lamb, changing the chilling practice was found to have the largest effect (Gill, 1987). It was found that the significance of the relatively small numbers of organisms added to carcasses during dressing was greatly magnified by their growth during carcass cooling. Small changes to the chilling practices alone extended the storage life by up to 50%. The length and conditions used during ageing can also affect the storage life of meat (Nortjé and Shaw, 1989). Beef loin steaks from primals that had been aged Chilled and frozen storage 213 10 8 6 4 2 0 0 7 14 21 28 Storage time (days) –4 °C 0 °C 3 °C 7 °C Log cfu g – 1 Fig. 10.3 Growth of psychrotroph counts (log 10 cfu g -1 ) on vacuum-packed cubed pork at -4, 0, 3 and 7 °C (source: Lee et al., 1985). 5 4 3 2 1 0 0 7 14 21 28 Storage time (days) –4 °C 0 °C 3 °C 7 °CDrip loss (%) Fig. 10.4 Drip loss from vacuum-packed cubes of pork stored at -4. 0. 3 and 7 °C (source: Lee et al., 1985). for 3 weeks in vacuum packs discoloured more rapidly and off-odours developed sooner than those from meat that had been hung in air for one week or vacuum packed for one week. The poorer storage stability was explained by higher initial levels of bacteria because of growth during ageing. Rancidity development was only detected in the 3-week-aged steaks that were stored at 6 °C. Studies have also shown that there is an interaction between storage and retail display. The retail display life of pork from CO 2 packaged primals depends on the length of time the primals have been stored (Greer et al., 1993). On appearance criteria, the display life in days is 4.60–0.15 (weeks in storage), whilst on odour criteria the display life in days is 5.03–0.17 (weeks in storage). The tenderness, juiciness and flavour of beef patties has been found to deteriorate during chilled storage with the flavour deteriorating less at 0 °C than at 4 or 8 °C (Bentley et al., 1989). Drip loss increased with storage temperature from 3.2% at 0 °C to 3.3% at 4 °C and 4.6% at 8 °C. Drip loss was also affected by the type of packaging with greater drip in vacuum packs than in 100% nitrogen or CO 2 back-flushed packs. Total plate counts increased from 2.7 to 7.2 log 10 cfucm -2 after 7 days and to 8.6 log 10 cfu after 21 days of storage but no effect of storage temperature or packaging type was detected. 10.2.3 Cooked products The influence of temperature on the storage life of vacuum packed sliced cured meat products is shown clearly in Table 10.4. Precooked beef roasts can be stored for 28 days at 4 °C (Stites et al., 1989).The roasts of beef chuck were prepared in vacuum cooking bags with phosphate salt and cooked to a centre temperature of 70 °C.They were then 214 Meat refrigeration Table 10.4 Storage life for vacuum-packed sliced cured meat products Temperature Storage life (days) (°C) Bologna Smoked Cooked Cooked sausage fillet pork loin pork loin 12 7 – 21 – 8 11.5 9 33 16.5 5 21.5 10 66 31.5 2 33 11.5 78 52.5 0 42 22.5 141 64 -3 11 33 165 >64 Source: B?gh-S?rensen et al., 1986. chilled and stored at 4 °C. Sensory attributes were acceptable after 28 days and total plate counts less than 100 per gram. Studies by Tanchotikul et al. (1989) have shown that the susceptibility to oxidation in precooked roasts during chilled storage increased as the end-point cooking temperature increased. Addition of 0.25 or 0.5% polyphosphate to restructured, battered and breaded, cooked beef and pork nugget products protected them from off- flavours and lipid oxidation during chilled storage (Huffman et al., 1987). Similar effects have been shown for garlic and onion juices (Jurdi- Haldeman et al., 1987). Garlic and onion juices were added to minced lamb which was then made into patties and cooked. After 0, 3 and 7 days of storage at 5 °C, TBA values were lower in patties from the two treatments than the control. Although the market for preprepared sandwiches is expanding rapidly there are little published data on their chilled storage life. One study in the USA has looked at the storage life of commercially processed sandwiches including processed meats, roast beef and hamburgers packed in 50% CO 2 : 50% air mixture and stored at 4 °C (McMullen and Stiles, 1989). Storage life ranges from 35 days for processed meat sandwiches, 28 to 35 days for roast beef and only 14 days for hamburgers. Both microbiological and taste panel tests were used to determine shelf-life. The sandwiches were heated to 50–55 °C in a microwave oven before tasting. In general the untrained taste panel found sandwiches acceptable after maximum microbial levels were achieved. Substantial differences in counts were found between replicates and between samples from the same replicates (Table 10.5). Coliform counts never exceeded 2 cfu g -1 . Further studies investigated laboratory packing in 30, 50 and 70% CO 2 with either air or nitrogen. Excluding oxygen from the beefburger packs extended the shelf-life from 14 to 35 days. Chilled and frozen storage 215 Table 10.5 Growth of lactic acid bacteria in beefburgers stored in a modified gas atmosphere at 4 °C Week Lactic acid bacteria (cfu g -1 ) Replicate 1 Replicate 1 Replicate 2 Replicate 2 sample 1 sample 2 sample 1 sample 2 1 <1 ￥ 10 2 <1 ￥ 10 2 <1 ￥ 10 2 <1 ￥ 10 2 2 <1 ￥ 10 2 <1 ￥ 10 2 6.0 ￥ 10 2 5.4 ￥ 10 3 3 1.4 ￥ 10 2 2.7 ￥ 10 5 1.0 ￥ 10 2 9.2 ￥ 10 5 4 4.5 ￥ 10 7 1.0 ￥ 10 2 2.8 ￥ 10 4 4.1 ￥ 10 6 5 4.7 ￥ 10 6 1.4 ￥ 10 4 1.2 ￥ 10 7 1.4 ￥ 10 5 6 2.0 ￥ 10 6 3.0 ￥ 10 3 <1 ￥ 10 2 1.4 ￥ 10 6 Source: McMullen & Stiles, 1989. 10.3 Frozen storage This chapter is a brief summary of a full review by James and Evans (1997). The factors that influence the storage life of frozen meat may act in any one of three stages: prior to freezing, during the actual freezing process and postfreezing in the storage period itself. 10.3.1 Oxidative rancidity The importance of fat oxidation in frozen meat is illustrated by a short quo- tation from a paper published by Lea (1931);‘it is often the deterioration of the fat which limits the storage life – from the point of view at least of palata- bility – of the meat’.This view has been reiterated many times, and as freez- ing technology has improved it is true to say that fat oxidation remains the obstacle to very long-term storage of frozen meat. Early studies on fat oxi- dation and freezing were reviewed by Lea (1938) and Watts (1954). 10.3.1.1 Mechanism of oxidation The reaction of oxygen with fatty acids produces peroxides. It is the break- down products of the peroxides that produce the characteristic objection- able odour and flavour of rancid meat. The development of oxidative rancidity in meat is affected by two groups of factors, one group consisting of the built-in characteristics of the meat and the other group consisting of those factors involved in the treatment of the meat. The former are mainly under the control of the farmer or are innate characteristics of the living animal, whereas the latter can be controlled by the abattoir, the meat packer or the cold store operator. Although the first group cannot be changed by the meat processor it is necessary to consider their effect so that procedures may be modified to limit them. Before discussing either group it is neces- sary to look at the process of fat oxidation in the hope that knowledge of the process will indicate the ways in which it may be controlled. The reaction of oxygen with fat is an autocatalytic process. Once the reaction starts, the products of the reaction stimulate it to go faster. The initial reaction is between a molecule of oxygen and a fatty acid to form a peroxide.This is a slow reaction but like any other chemical reaction its rate is increased by raising the temperature. The rate is also influenced by the type of fatty acid. Saturated fatty acids react slowly, but unsaturated fatty acids react more rapidly, and the more double bonds that a fatty acid con- tains, the more reactive it is. The presence of peroxides in fat does not change the flavour, it is the breakdown products of the peroxides which produce the rancid odour and flavour. The breakdown of peroxide is accel- erated by heat, light, organic iron catalysts and traces of metal ions, espe- cially copper and iron. The breakdown products of the peroxides cause the oxygen to react more rapidly with the fatty acids, thus producing the auto- catalytic effect. 216 Meat refrigeration The type of fatty acid present in the meat is therefore of major impor- tance in determining its oxidative stability. Beef and mutton tallow, both very hard fats, which contain few unsaturated fatty acids are much more stable than lard, a softer fat, which contains a large quantity of unsaturated fatty acids.The effect of fatty acid composition can, however, be much more subtle. Lea (1936) observed that feeding an ounce of cod liver oil per day to pigs increased the susceptibility of the fat to oxidation by a factor of 5, although the change in the iodine value of the fat was negligible. This result may be compared with the studies of Dahl (1957) where an increase in the linoleic acid content of pig fat from 7% to 15.6%, which increased the iodine number by 10 units, only reduced the oxidative stability by one quarter. The reason for the difference is that the quantity of polyunsatu- rated fatty acids from the cod liver oil that was stored in the pig fat was small. However, since these fatty acids contained 4, 5 and 6 double bonds, the initiation of oxidation was much easier than in the fat where the high iodine number was produced by an increase in a fatty acid with only 2 double bonds. The relative susceptibly of oleic, linoleic and linolenic acids, with 1, 2 and 3 double bonds respectively, to oxidation is 1 : 12 : 100 (Kuhn and Meyer, 1929). 10.3.1.2 Natural antioxidants In view of the ease with which fat oxidation takes place one might enquire whether it occurs in the living animal, and if not, why not? The answer is that it does but only to a small extent. Nor does it occur in meat imme- diately after slaughter. The reason is the presence in the animal tissues of antioxidants which prevent the peroxide breakdown products from catalysing the oxidation. The major antioxidant in meat is alpha-tocopherol or vitamin E. Deficiency of vitamin E in many animals leads to the oxida- tion of the adipose tissue, which turns yellow (Dam, 1957). Such material from pigs would of course not pass inspection, but even meat having a lowered alpha-tocopherol content would be less stable in frozen storage. The situation is well documented in the case of the turkey, which has low levels of alpha-tocopherol. The injection of alpha-tocopherol into turkeys decreased the thiobarbituric acid value, a measure of oxidation of the frozen carcass, and improved the flavour (Webb et al., 1972). Fatty acid composition and the antioxidant status of the tissue are the main factors affecting oxidation, which are fixed in the animal before slaughter. The fatty acid composition of the diet readily changes the fatty acid composition of pig fat, but has little effect in ruminants. The antioxi- dant levels of the tissues are not greatly affected by changes in the quan- tity in the diet, since little of the added alpha-tocopherol finds its way to the fat. However, prolonged low levels of alpha-tocopherol in the diet can reduce the quantity in the animal. Feeding high levels of polyunsaturated fatty acids can also reduce the body’s stores of alpha-tocopherol because of the extra quantity needed to prevent oxidation. The alpha-tocopherol Chilled and frozen storage 217 content of cereals may be reduced if the grain is stale and the lipids have started to oxidise and destroy it. Not all the fat depots on the carcass are equally susceptible to oxidation. Subcutaneous fat in most species is much softer (more unsaturated) than the internal fat. 10.3.1.3 Phospholipids Although the bulk of the fat in a carcass is in the visible fat depots, all cells in the animal’s body contain phospholipids as part of their structure. Furthermore, the phospholipids contain large quantities of polyunsaturated fatty acids that are not readily influenced by the nature of the dietary fatty acids. The greater susceptibility of phospholipids to oxidation com- pared with the neutral lipids of ground pork was observed by Younathan and Watts (1960). Phospholipids have been used as antioxidants under certain conditions, but the mechanism by which they function is not under- stood and it is not known if they exhibit any antioxidant activity in fresh meat. 10.3.2 Prefreezing treatment Some prefreezing factors, that is, species differences, animal to animal vari- ation or differences between cuts of meat, are inherent in the animal. There are also other factors including feeding and transport that may have an effect on frozen storage. Species is the main prefreezing factor that is commonly believed to influ- ence the frozen storage life. Table 10.6 provides data from three sources on the storage life of meat from different species and the average and range from all the publications located. There is up to a two-fold difference between species in recommended storage times, but more important is the relative ranking, in terms of which can be stored for the longest time, which varies between the sources. When all the available data found in the literature are considered the picture becomes even more confusing. Average values for the storage lives of the different species at -18 °C (Table 10.6) have a different ranking to that gen- erally accepted and the range of storage lives is very large. 218 Meat refrigeration Table 10.6 Frozen storage life (months) for different species Source 1 2 3 Average at Range at temperature (°C) -20 -18 -18 -18 -18 Species Beef 12 12 18 10.2 2.8–19.4 Pork 6 8 12 17.4 2.8–23.3 Lamb 10 12 24 7.8 2.8–24.3 Chicken 12 10 18 13.6 6.0–23.3 Source: James and Evans, 1997. Few publications compare the meat of more than two species under directly comparable conditions. Under similar ageing and packaging regimes beef was found to store for 69 weeks, pork for 53 weeks and lamb for 44 weeks at -18°C. At -18 °C pork remained palatable for longer than lamb but at higher temperatures the lamb was more stable. It seems fair to conclude that most work points towards a difference, but not necessarily a consistent difference, in frozen storage life between species. To look at animal to animal variation, two trials were carried out in New Zealand where lamb was stored at -5 °C. In the first trial the lamb was judged rancid after 20 weeks and in a duplicate trial the lamb was found to store for 40 weeks. The only variation that could be determined was that different animals were used in the two trials. There appear to be large vari- ations between animals which cause changes in the storage life of meat, but why these differences exist is not completely understood. Feeding influences frozen storage life. Pork from pigs that had been fed materials containing offal or household refuse had half the practical storage life than that from pigs which had been fed conventional diets (Bailey et al., 1973). Rations with large amounts of highly unsaturated fatty acids tend to produce more unstable meat and fat. The feeding of fish oils or highly unsaturated vegetable oils to poultry is known to produce fishy flavours in the meat but there is some debate as to whether this diet directly affects frozen storage times. The linoleic acid content of meat probably plays a major role in storage. There has been a general trend in the UK for pigs to be leaner and therefore to have greater proportions of linoleic acid in their tissues.There is a possibility that pork may store less well than might be expected from results dating from 10 or 20 years ago. Reports of variations in the storage life of different cuts of meat are scarce and primarily show that light meat stores for a longer time than dark meat.This is thought to be due to either higher quantities of haem pigments in the dark muscle, or to higher quantities of phospholipids which are major contributors to oxidised flavour in cooked meat. Increased stress or exhaustion can produce PSE or DFD meat, which is not recommended for storage mainly due to its unattractive nature and appearance. Meat is generally not frozen until rigor is complete and a degree of con- ditioning has taken place, otherwise toughening and increased drip can occur. In red meat, there is little evidence of any relationship between chill- ing rates and frozen storage life. However, there is evidence that increas- ing the time in chilled storage before freezing reduces frozen storage life. Carcasses which have been electrically stimulated have prolonged storage lives and this could be attributed to the shorter interval between slaughter and freezing. In poultry, the chilling method does have an effect on storage life. Air chilled broilers had significant flavour changes after 3 months at -12°C and -20 °C, whereas immersion chilled birds only exhibited changes at -12 °C after 6 months and were stable at -20 °C. Water chilling of Chilled and frozen storage 219 broilers produced a more favourable taste in the leg and breast meat than air chilling. Processing of meat prior to freezing generally results in a lengthened storage time. Heating prior to freezing can result in a 50% longer PSL for sausages. However, the heating process could be critical since muscles cooked to higher temperatures are most susceptible to oxidative changes during storage. Heat treatments such as frying tend to produce short storage lives, probably because of the high fat content of the product. Breaded products are often fried and although breading alone may have a pro- tective effect on a product, the addition of oil may have a counteractive effect. A process such as mincing has been found to affect storage of commin- uted products and this is probably due to the induced heating and the increased surface area that results. Addition of fat to mince can lower storage life unless a high grade wrapping material, which has the ability to exclude air, is used to wrap the product. Smoking is generally advantageous owing to the antioxidant properties of the smoke. Smoked broilers and ham store well for over a year without serious quality change. Additives, such as spices, seasoning, antioxidants and protein concen- trates can influence storage life. The use of vegetable extracts such as onion juice, yellow onion peel, hot water extracts of aubergine (egg plant), pota- toes and sweet potatoes have been shown to help control rancidity in beef and turkey meat. However, an addition of salt may also reduce the storage life because of increased rancidity. Mechanically recovered meat is used in a range of meat products, but can cause storage problems owing to its high fat content and increased rancidity. 10.3.3 Freezing process There are few data to suggest that in general the method of freezing or the rate of freezing has any substantial influence on the subsequent storage life of a food. There is some disagreement in the literature about whether fast (cryogenic) or slow (blast) freezing is advantageous. Slightly superior chemical and sensory attributes have been found in food cryogenically frozen in a few trials, but other trials did not show any appreciable advan- tage, especially during short-term storage. The method of freezing clearly affects the ultrastructure of the meat. Slow freezing (1–2 mmh -1 for example) tends to produce large ice crystals extracellularly, whilst quick freezing (e.g. 50 mmh -1 ) gives smaller crystals in and outside cells. Obviously, a temperature gradient will occur in large pieces of meat and result in a non-uniform ice morphology. Fast freezing tends to produce a lighter coloured product as the small ice crystals scatter the light more than larger crystals and this enhances the surface appear- ance of poultry skin. However, there are no data to suggest that the ultra- structure influences storage life. 220 Meat refrigeration 10.3.4 During frozen storage Three factors concerning storage; the storage temperature, the degree of fluctuation in the storage temperature and the type of wrapping/packaging in which the meat is stored, are commonly believed to have the main influ- ence on frozen storage life. 10.3.4.1 Storage temperature To quote from the IIR Red book ‘storage life of nearly all frozen foods is dependent on the temperature of storage. . .’ and in the book a table is pro- vided of practical storage lives of different foods at three storage tem- peratures. An extract is given in Table 10.7. However, few papers have been located where data are presented from experiments on the PSL of meat at different storage temperatures. Many of those that have been located are on products that do not meet the lower temperature longer storage rule (normal stability). Experimental data from many different publications have been plotted against the temperature of storage for beef (Fig. 10.5), pork (Fig. 10.6) and lamb (Fig. 10.7). There is a clear effect of temperature on storage life, with lower temperatures resulting in extended storage, but considerable scatter between results at any one temperature. It has been shown that rancidity in bacon is increased by higher salt content and that the rates of chemical reactions are accelerated as the tem- perature is lowered when packed in permeable wrap. Cured pork products are known to have an abnormal temperature profile between -5°C and -60 °C and store less well between -30°C and -40 °C. Chilled and frozen storage 221 Table 10.7 Practical storage life (months) at different storage temperatures Product -12 °C -18 °C -24 °C Beef carcasses 8 15 24 Beef steaks/cuts 8 18 24 Ground beef 6 10 15 Veal carcass 6 12 15 Veal steaks/cuts 6 12 15 Lamb carcasses 18 24 >24 Lamb steaks 12 18 24 Pork carcasses 6 10 15 Pork steaks/cuts 6 10 15 Sliced bacon (vac.) 12 12 12 Chicken, whole 9 18 >24 Chicken parts/cuts 9 18 >24 Turkey, whole 8 15 >24 Ducks, Geese, whole 6 12 18 Liver 4 12 18 Source: IIR, 1986. Improved aroma scores have been found to be moderately related to lower freezing temperatures, but not related to flavour. Aroma scores for minced beef improved during a 6–12 month storage period at -12.2, -17.8 or -23.3 °C, although a slight increase in rancidity also occurred. Work in New Zealand (Winger, 1984) has found that consumer panels are often not very sensitive to quality changes and could not tell the dif- 222 Meat refrigeration 1200 1000 800 600 400 200 0 –40 –30 –20 –10 0 Temperature (°C) Storage life of beef (days) Fig. 10.5 Experimental data on storage life of beef at different temperatures (source: James and Evans, 1997). 1200 1000 800 600 400 200 0 –40 –30 –20 –10 0 Temperature (°C) Storage life of pork (days) Fig. 10.6 Experimental data on storage life of pork at different temperatures (source: James and Evans, 1997). ference between samples of lamb stored at -5 and 35 °C. A trained taste panel could differentiate between the two temperatures and scored the samples stored at -5 °C as rancid. 10.3.4.2 Temperature fluctuation Fluctuating temperatures in storage are considered to be detrimental to the product. However, it has been reported that repeated freeze–thaw cycles do not cause any essential change in the muscle ultrastructure (Carrol et al., 1981) and that several freeze–thaw cycles during the life of a product cause only small quality damage (Wirth, 1979) or possibly no damage at all. In fact, a slight but significant improvement in samples that had been frozen and unfrozen several times was found by one taste panel (Jul, 1982). Minor temperature fluctuations in a stored product are generally con- sidered unimportant, especially if they are below -18 °C and are only of the magnitude of 1–2 °C. Well-packed products and those that are tightly packed in palletised cartons are also less likely to show quality loss. How- ever, poorly packed samples are severely affected by the temperature swings. There is disagreement about how much effect larger temperature fluctuations have on a product. Some authors consider that tempera- ture fluctuations have the same effect on quality of the product as storage at an average constant temperature (Dawson, 1969); others consider fluc- tuations may have an additive effect (Van Arsdel, 1969; Bech-Jacobsen and B?gh-S?rensen, 1984). There is evidence that exposure to temperatures above -18°C rather than temperature fluctuations may be the major factor influencing quality deterioration (Gortner et al., 1948). Chilled and frozen storage 223 1200 1000 800 600 400 200 0 –40 –30 –20 –10 0 Temperature (°C) Storage life of lamb (days) Fig. 10.7 Experimental data on storage life of lamb at different temperatures (source: James and Evans, 1997). 10.3.4.3 Packaging Packaging has a large direct effect on storage life, especially in fatty foods and in extreme cases it has an indirect effect owing to substantial increases in the freezing time. A number of examples have occurred where large pallet loads of warm boxed meat have been frozen in storage rooms. In these cases, freezing times can be so great that bacterial and enzymic activity results in a reduction of storage life. In most cases, it is the ma- terial and type of packaging that influences frozen storage life. Wrapping in a tightly fitting pack having a low water and oxygen permeability (such as a vacuum pack) can more than double the storage life of a product. Waterproof packing also helps to prevent freezer burn and tight packing helps to prevent an ice build up in the pack. When a product is breaded, packaging appears to have little effect and in a trial where breaded pork chops and breaded ground pork were packed in poor and very good packs an effect of packing could not be found. Rancidity occurs in unwrapped meat because its surface dries, allowing oxygen to reach subcutaneous fat. Without wrapping, freezer burn may occur causing extreme toughening and the development of rancidity in the affected area. Packaging can be effective in some cases in reducing dis- colouration by lessening oxygen penetration into the meat. Lighting, espe- cially ultraviolet, can also increase fat oxidation (Volz et al., 1949; Lentz, 1971). Exposure to the levels of light found in many retail frozen food display areas can cause appreciable colour change within 1–3 days. Devel- opment of off-flavour can be accelerated and may be noticeable within 1–2 months on display. Products kept in dark or opaque packages may there- fore be expected to retain colour longer than those exposed to the light. 10.4 Types of storage room 10.4.1 Bulk storage rooms Most unwrapped meat and poultry and all types of wrapped foods are stored in large rooms where air is circulated. To minimise weight loss and appearance changes associated with desiccation, air movement around the unwrapped product should be the minimum required to maintain a con- stant temperature. With wrapped products low air velocities are also desir- able to minimise energy consumption. However, many storage rooms are designed and constructed with little regard to air distribution and localised air velocities over products. Horizontal throw refrigeration coils are often mounted in the free space above the racks or rails of product and no attempt is made to distribute the air around the products. Using a false ceiling or other form of ducting to distribute the air throughout the storage room can substantially reduce variations in velocity and temperature. It is claimed that an even air distribution can be maintained using air socks, with localised velocities not exceeding 0.2ms -1 . 224 Meat refrigeration 10.4.2 Controlled atmosphere storage rooms Controlled atmosphere storage rooms were developed for specialised fruit stores, especially those for apples. Interest is growing in the application of this technique to other commodities including meat. In addition to the normal temperature control plant these stores also include special gas-tight seals to maintain an atmosphere which is normally lower in oxygen and higher in nitrogen and carbon dioxide than air. An additional plant is required to control the CO 2 concentration, generate nitrogen and consume oxygen. There is growing interest in the use of controlled atmosphere retail packs to extend the chilled storage and display life of red meats, poultry and meat products. Since the packs tend to be large and insulate the products, effi- cient precooling before packaging is especially important if product quality is to be maintained. 10.4.3 Jacketed cold stores Cooling the walls, floor and ceiling of a store produces very good tem- perature control in the enclosed space with the minimum of air movement. It is especially suitable for controlled atmosphere (CA) storage and for unwrapped produce that is very sensitive to air movement or temperature fluctuations. The refrigerated jacket can be provided by embedding pipe coils in the structure or utilising a double skin construction through which refrigerated air is circulated. Although the refrigerated jacket is efficient in absorbing any heat input from the surroundings, the lack of air circulation within the enclosed space means that heat removal from the product is very limited. Care must there- fore be taken to (1) attain the desired storage temperature throughout the product before storing, (2) minimise any heat loads produced during loading and unloading, and (3) provide the supplementary refrigeration required for any products which respire. 10.5 Conclusions The rate of spoilage of meat depends upon the numbers and types of organ- isms initially present, the conditions of storage (temperature and gaseous atmosphere), and characteristics (pH, water activity a w ) of the meat. Tem- perature is by far the most important factor. Spoilage is characterised by off-odours, slime formation and discoloura- tion. The type of micro-organisms present defines the pattern of spoilage. The dominance, and thus type of spoilage, is dependent on the storage con- ditions (temperature and gaseous atmosphere). In general spoilage occurs when the microbial population reaches ca. 7–8logcfucm -2 . Chilled and frozen storage 225 Those bacteria responsible for the spoilage of carcass meat grow most rapidly above 20 °C. Any reduction below this temperature will extend the storage life. Broadly speaking bacterial growth will be half as fast at 5 °C as at 10 °C and half as fast again at 0°C, i.e. meat should keep roughly four times longer at 0 °C than at 10 °C. Although a great deal has been written on the frozen storage life of different meats, the underlying data are backed up by a relatively small number of controlled scientific experiments. Most of the scientific data date back to the time when meat was either stored unwrapped or in wrapping materials that are no longer used. It is not surprising when we consider the changes in packaging and handling methods over the last century that there is a considerable scatter in data on storage lives for similar products. In recent years energy conservation requirements have caused an increased interest in the possibility of using more efficient storage tempera- tures than have been used to date. Researchers such as Jul have questioned the wisdom of storage below -20 °C and have asked whether there is any real economic advantage in very low temperature preservation. There is a growing realisation that storage lives of several foods can be less depen- dent on temperature than previously thought. Since research has shown that meat and poultry often produce non-linear time–temperature curves there is probably an optimum storage temperature for a particular product. Improved packing and preservation of products can also increase storage life and may allow higher storage temperatures to be used. 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