《肉制品冷冻技术》（英文版） Part 10 Chilled and frozen storage.pdf_大学文库

208 Meat refrigeration 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 table for consumption or the intended process. Bogh-Sprensen(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 wa 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"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 the ere 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 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 imum 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

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 consumption 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’. Bøgh-Sørensen (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 selecting 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 minimise quality changes. Although well suited to research work, some drawbacks have been noted. The actual definition of storage life and the way it is measured has therefore 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, prefreezing 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

210 Meat refrigeration Table 10.2 General levels of microbiological contamination reported on meat carcasses throughout the world Type of ry APC*K Reference (logo org 19-3.7 Ingram and Roberts(1976) Ingram and roberts (1976) New zealand Ingram and Roberts (1976) New Zealand Norway 1.3-3.9 chanson et aL. (1983) Roberts et al. (1984) 4-3.8 Hudson et al. (198 New zealand 04-3.3 Bell et al.(1993) australia Anon(1997) Canad 1.5-3.2 Gill et al.(1998b) Hinton et al.(1998) Lamb/ New Zealand Newton et al.(1978 Prieto et al. (1991) New Zealand 2.3-4.1 Bell et al. (1993) New Zealand Biss and Hathaway(1995) australia 2.5-3.3 Ingram and Roberts(1976) Hanson et al.(1983) 16-3.8 Christensen and Sorensen(1991) 4.3-5.0 Barbuti et aL. (1992) 4 Values are not directly comparable since different sampling techniques and incubation temperatures have been used 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 4.0-5.0logio cfucm- and an anaerobic population of between 3.7 and 4.7" would be reasonable for wholesale cuts of meat Surveys from the mid-1970s have shown that in general levels of between 1 and logo cfug- 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 10.0logo cfucm" after 11 days in a moist environment at 5C (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 orated 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-

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 4.0–5.0 log10 cfucm-2 and an anaerobic population of between 3.7 and 4.7 log10 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 4 log10 cfu g-1 can be expected on red meat carcasses (Table 10.2). Specific surfaces of the carcass can have very high levels of initial contamination. 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 log10 cfu cm-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 incorporated 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 (log10 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 Sørensen (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. Investigations 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 produced 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 (BøghSørensen 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 (4.1 log10 cfu g-1 ) than those from cold-boned meat (2.7 log10 cfu g-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 log10 cfug-1 , in both cases. In vacuum-packaged primals, Egan et al. (1986) have shown that the temperature 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 0 2 4 7 9 11 14 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)

212 Meat refrigeration Table 10.3 Storage life and nature of spoilage of vacuum-packaged pork 5°C Storage life Spoilage Storage life (weeks) characteristics characteristics 5.4-5.8 -4 Flavour changes. souring 6.2-6.5 Variable Greening, odour of His, putrefaction Source: Egan et al. 1986. 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)or vacuum-packaged pork at-15C to 9 weeks. Based on appear ance, CO2-packaged and vacuum-packed pork loin had storage lifes of ove 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 -01C from hot-boned bull beef that had been cooled and stored in vacuum or CO2 packs. On opening, the appearance of the strip loins was also acceptable. However, overageing as 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-4C ( Lee et aL., 1985), whilst green discolouration was significant after 14 days at 3 and 7C and 28 days at 0C. 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 5C 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 3C compared with 5 weeks at-15C. Cuts

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 (CO2) or vacuum-packaged pork at -1.5 °C to 9 weeks. Based on appearance, CO2-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 CO2 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 microbial 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 substantially 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 longissimus dorsi muscle (skin on) were grossly spoiled by Brochothrix thermosphacta 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 H2S, putrefaction Source: Egan et al., 1986

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 CO2 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 CO2 back-flushed packs. Total plate counts increased from 2.7 to 7.2 log10 cfucm-2 after 7 days and to 8.6 log10 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: Bøgh-Sørensen et al., 1986

Chilled and frozen storage 215 chilled and stored at 4C. 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 Increase Addition of 0. 25 or 0.5% polyphosphate to restructured, battered and breaded, cooked beef and pork nugget products protected them from off favours 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 5C, 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 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 50-55C in a microwave oven before tasting. In general the untrained taste panel found sandwiches acceptable after maximum microbial levels ere a Substantial differences in counts were found between replicates and between samples from the same replicates(Table 10.5). Coliform count never exceeded 2cfug. Further studies investigated laboratory packing in 30, 50 and 70%CO2 with either air or nitrogen. Excluding oxygen from the beefburger packs extended the shelf-life from 14 to 35 day Table 10.5 Growth of lactic acid bacteria in beefburgers stored in a modified gas 4°C Lactic acid bacteria(cfu g) ample 1 sample 2 <1×102 <1×102 <1×102 <1×102 <1×102 <1×102 54×103 1.4×102 2.7×10 1.0×102 9.2×105 4.5×107 1.0×1032 28×104 4.1×105 4.7×10° 1.4×10 12×107 14×105 2.0×10 3.0×10 <1×102 14×109 Source: McMullen Stiles. 1989

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 (JurdiHaldeman 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% CO2 : 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% CO2 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 ¥ 102 <1 ¥ 102 <1 ¥ 102 <1 ¥ 102 2 <1 ¥ 102 <1 ¥ 102 6.0 ¥ 102 5.4 ¥ 103 3 1.4 ¥ 102 2.7 ¥ 105 1.0 ¥ 102 9.2 ¥ 105 4 4.5 ¥ 107 1.0 ¥ 102 2.8 ¥ 104 4.1 ¥ 106 5 4.7 ¥ 106 1.4 ¥ 104 1.2 ¥ 107 1.4 ¥ 105 6 2.0 ¥ 106 3.0 ¥ 103 <1 ¥ 102 1.4 ¥ 106 Source: McMullen & Stiles, 1989

216 Meat refrigeration 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) 10.3.1.7 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 gen 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 cids 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 favour. 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

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 quotation 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 palatability – of the meat’.This view has been reiterated many times, and as freezing 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 oxidation 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 breakdown products of the peroxides that produce the characteristic objectionable 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 necessary 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 contains, 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 accelerated by heat, light, organic iron catalysts and traces of metal ions, especially copper and iron. The breakdown products of the peroxides cause the oxygen to react more rapidly with the fatty acids, thus producing the autocatalytic effect. 216 Meat refrigeration