《肉制品冷冻技术》（英文版） Part 8 Thawing and tempering

8 hawing and tempering T Thawing has received much less attention in the literature than either chill- ing or freezing. In commercial practice there are relatively few controlled thawing systems. Frozen meat, as supplied to the industry, ranges in size and shape from complete hindquarters of beef to small breasts of lamb, although the major ity of the material is'boned-out and packed in boxes ca. 15 cm thick weigh ng between 20 and 40kg. Thawing is usually regarded as complete when he centre of the block or joint has reached0C, the minimum temperature at which the meat can be boned or cut by hand. Lower temperatures(e 5 to C)are acceptable for meat that is destined for mechanical chop- ping, but such meat is 'tempered rather than thawed. The two processes should not be confused because tempering only constitutes the initial phase of a complete thawing process. Thawing is often considered as simply the reversal of the freezing rocess. However, inherent in thawing is a major problem that does not occur in the freezing operation. The majority of the bacteria that cause spoilage or food poisoning are found on the surfaces of meat. During the freezing operation, surface temperatures are reduced rapidly and bacterial multiplication is severely limited, with bacteria becoming completely dormant below -10C In the thawing operation these same surface areas are the first to rise in temperature and bacterial multiplication can recom nence On large objects subjected to long uncontrolled thawing cycles, surface spoilage can occur before the centre regions have fully thawed Most systems supply heat to the surface and then rely on conduction to transfer that heat into the centre of the meat. A few systems use electro magnetic radiation to generate heat within the meat. In selecting a thawing

8 Thawing and tempering Thawing has received much less attention in the literature than either chill￾ing or freezing. In commercial practice there are relatively few controlled thawing systems. Frozen meat, as supplied to the industry, ranges in size and shape from complete hindquarters of beef to small breasts of lamb, although the major￾ity of the material is ‘boned-out’ and packed in boxes ca. 15 cm thick weigh￾ing between 20 and 40kg. Thawing is usually regarded as complete when the centre of the block or joint has reached 0 °C, the minimum temperature at which the meat can be boned or cut by hand. Lower temperatures (e.g. -5 to -2 °C) are acceptable for meat that is destined for mechanical chop￾ping, but such meat is ‘tempered’ rather than thawed. The two processes should not be confused because tempering only constitutes the initial phase of a complete thawing process. Thawing is often considered as simply the reversal of the freezing process. However, inherent in thawing is a major problem that does not occur in the freezing operation. The majority of the bacteria that cause spoilage or food poisoning are found on the surfaces of meat. During the freezing operation, surface temperatures are reduced rapidly and bacterial multiplication is severely limited, with bacteria becoming completely dormant below -10 °C. In the thawing operation these same surface areas are the first to rise in temperature and bacterial multiplication can recom￾mence. On large objects subjected to long uncontrolled thawing cycles, surface spoilage can occur before the centre regions have fully thawed. Most systems supply heat to the surface and then rely on conduction to transfer that heat into the centre of the meat. A few systems use electro￾magnetic radiation to generate heat within the meat. In selecting a thawing

160 Meat refrigeration system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of product, processing problems such as effluent disposal and the capital and operating costs of the respec- tive systems. Of these factors, thawing time is the principal criterion that governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed cond tion but are less so if the meat is for processing 8.1 Considerations The design of any thawing system requires knowledge of the particular environmental or process conditions necessary to achieve a given thawing time, and the effect of these conditions on factors such as drip, evaporative losses, appearance and bacteriological quality The process of freezing a high water content material such as meat takes place over a range of temperatures rather than at an exact point, because as freezing proceeds the concentration of solutes in the meat fluid steadily increases and progressively lowers the freezing temperature. Thawing simply reverses this process. Thawing time depends on factors relating to the product and the envi onmental conditions that include dimensions and shape of the product, particularly the thickness change in enthalpy thermal conductivity of the product initial and final temperatures surface heat transfer coefficient temperature of the thawing medium. equal to the enthalpy change between the initial temperature and ns The total amount of energy that must be introduced into the product average temperature required within the material after thawing. For the thawing process to be complete, no ice should remain and the minimum emperature has to be above -1C To thaw lkg of meat from a starting temperature of -40C would require the addition of 300kJ of energy if the meat was very lean, falling to about 180kJ if very fat. Frozen meat that requires boning has to be completely thawed. However, an increasing pro- portion of meat is boned before freezing and if it is subsequently used in products such as pies, sausages, and so on, it can be cut by machine in a semi-frozen( tempered) state To temper meat from -40C to an average temperature of-4C requires a heat input of approximately 100kJkg-, only one third of that required for complete thawing Thermal conductivity has an important effect on thawing The conduc t ty of frozen lean meat is three times that of the thawed material. When wing commences, the surface rises above the initial freezing point. Sub-

system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of product, processing problems such as effluent disposal and the capital and operating costs of the respec￾tive systems. Of these factors, thawing time is the principal criterion that governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed condi￾tion but are less so if the meat is for processing. 8.1 Considerations The design of any thawing system requires knowledge of the particular environmental or process conditions necessary to achieve a given thawing time, and the effect of these conditions on factors such as drip, evaporative losses, appearance and bacteriological quality. The process of freezing a high water content material such as meat takes place over a range of temperatures rather than at an exact point, because as freezing proceeds the concentration of solutes in the meat fluid steadily increases and progressively lowers the freezing temperature. Thawing simply reverses this process. Thawing time depends on factors relating to the product and the envi￾ronmental conditions, that include: • dimensions and shape of the product, particularly the thickness • change in enthalpy • thermal conductivity of the product • initial and final temperatures • surface heat transfer coefficient • temperature of the thawing medium. The total amount of energy that must be introduced into the product is equal to the enthalpy change between the initial temperature and the average temperature required within the material after thawing. For the thawing process to be complete, no ice should remain and the minimum temperature has to be above -1 °C. To thaw 1kg of meat from a starting temperature of -40 °C would require the addition of 300 kJ of energy if the meat was very lean, falling to about 180 kJ if very fat. Frozen meat that requires boning has to be completely thawed. However, an increasing pro￾portion of meat is boned before freezing and if it is subsequently used in products such as pies, sausages, and so on, it can be cut by machine in a semi-frozen (tempered) state. To temper meat from -40 °C to an average temperature of -4 °C requires a heat input of approximately 100 kJkg-1 , only one third of that required for complete thawing. Thermal conductivity has an important effect on thawing. The conduc￾tivity of frozen lean meat is three times that of the thawed material. When thawing commences, the surface rises above the initial freezing point. Sub- 160 Meat refrigeration

Thawing and tempering 161 Table 8.1 Typical surface heat transfer coefficients (h) for different thawing System Surface heat transfer coefficients(WmK) free convection Air-forced convection Vacuum steam heat Plate equently, an increasing thickness of poorly conducting material extends from the surface into the foodstuff, reducing the rate of heat flow into the centre of the material. This substantially increases the time required for thaw The main environmental factors are the temperature of the thawing medium and the surface heat transfer coefficient(h) which is a function of the shape and surface condition of the product, the thawing medium used nd its velocity. Except for very simple configurations, h cannot be derived heoretically and must be measured experimentally. Few such measure- ments have been made for the thawing of foodstuffs(Arce and Sweat, 1980; Vanichseni, 1971), but typical ranges of h for the main thawing systems are given in Table 8.1. In air thawing, h is not constant and is a function of relative humidity where vapour condenses in the form of water until the surface temperature is above the dew point of the air and all condensation ceases. The varying rate of condensation produces substantial changes in the value of h during the thawing pr rocess 8.2 Quality and microbiological considerations There are few published data relating thawing processes to the palatability of meat and eating quality is generally independent of the thawing method However, two reports indicated that cooking directly from the frozen state produced less juicy lamb rib loins(Woodhams and Smith, 1965)and less tender beef rolled rib joints (James and Rhodes, 1978 )when compared with meat that had been thawed before cooking The main detrimental effects of freezing and thawing meat is the large increase in the amount of proteinaceous fluid ( drip) released on final cutting, yet the influence of thawing rate on drip production is not clear There was no significant effect of thawing rate on the volume of drip in beef (Empey, 1933: Ciobanu, 1972) or pork(Ciobanu, 1972). Several authors

sequently, an increasing thickness of poorly conducting material extends from the surface into the foodstuff, reducing the rate of heat flow into the centre of the material. This substantially increases the time required for thawing. The main environmental factors are the temperature of the thawing medium and the surface heat transfer coefficient (h) which is a function of the shape and surface condition of the product, the thawing medium used and its velocity. Except for very simple configurations, h cannot be derived theoretically and must be measured experimentally. Few such measure￾ments have been made for the thawing of foodstuffs (Arce and Sweat, 1980; Vanichseni, 1971), but typical ranges of h for the main thawing systems are given in Table 8.1. In air thawing, h is not constant and is a function of relative humidity (James and Bailey, 1982). In the initial stages, water vapour condenses onto the frozen surface, immediately changing to ice. This is followed by a stage where vapour condenses in the form of water until the surface temperature is above the dew point of the air and all condensation ceases. The varying rate of condensation produces substantial changes in the value of h during the thawing process. 8.2 Quality and microbiological considerations There are few published data relating thawing processes to the palatability of meat and eating quality is generally independent of the thawing method. However, two reports indicated that cooking directly from the frozen state produced less juicy lamb rib loins (Woodhams and Smith, 1965) and less tender beef rolled rib joints (James and Rhodes, 1978) when compared with meat that had been thawed before cooking. The main detrimental effects of freezing and thawing meat is the large increase in the amount of proteinaceous fluid (drip) released on final cutting, yet the influence of thawing rate on drip production is not clear. There was no significant effect of thawing rate on the volume of drip in beef (Empey, 1933; Ciobanu, 1972) or pork (Ciobanu, 1972). Several authors Thawing and tempering 161 Table 8.1 Typical surface heat transfer coefficients (h) for different thawing systems System Surface heat transfer coefficients (W m-2K-1 ) Air-free convection 5–15 Air-forced convection 10–70 Water 100–400 Vacuum steam heat 150–1000 Plate 100–300

162 Meat refrigeration (Cutting, 1974: Love, 1966) concluded that fast thawing rates would produce increased drip, while others showed(Finn, 1932; Singh and Essary, 1971) the opposite. Thawing times from -7 to 0C of less than 1 min or greater han 2000 min led to increased drip loss (James et al., 1983). The results are therefore conflicting and provide no useful design data for optimising a thawing system. The principle criteria governing quality of thawed meat are the appear- ance and bacteriological condition. These are major factors if the product is to be sold thawed but are less important if the food is destined for pro- cessing and heat treatment Microbiological problems can arise during thawing of food in bulk. while centre temperatures may not exceed0oC, the exterior surface may be held at 10-15C for many hours, or even days. During this time extensive growth of spoilage organisms can occur on the surface. The time required for micro- biological numbers to reach'spoilage' levels will largely be dependent upon he number of microbes initially present and the temperature. Since freez ing and frozen storage have little effect on the number of viable microbes present, material of poor microbiological quality before freezing is likely to spoil more quickly during thawing(Roberts, 1974). The use of high thawing (10C) temperatures for carcass meats tends to lead to large increases in microbial numbers(James and Creed, 1980; Bailey et al., 1974) Little published data exist on microbiological effects of thawing meat. Buttiaux(1972)reported that water thawing was more successful for beef than for pork if the meat was to be stored. Consequently, care must be exer cised in extrapolating from one meat species to another. Results with po suggested that air thawing gives final counts about ten times higher tha thawing in 3% brine, whereas with beef Heinz(1970) reported counts lower by a factor of about 10 for air(4-5ms": 10C)as opposed to flowing water (10%C). Kassai (1969)also found no significant increase in bacteriological numbers when thawing beef carcasses in air(0. 2-0.3ms", 15-20C,96% elative humidity, (RH)). Shoulders of lamb( Vanichseni et al., 1972)thawed in air(0.2ms: 18C)or water(45C) had bacterial counts that increased respectively by factors of 1.74 and 1. 12: humidity and air velocity also influ enced the results of air thawing It is often asserted that thawed food is more perishable than fresh or chilled produce, but experiments(Kitchell and Ingram, 1956; Kitchell and Ingram, 1959) have failed to demonstrate any difference of practical significance between the growth of meat spoilage organisms on fresh or thawed slices of meat. Greer and Murray(1991) found that the lag phase of bacterial growth was shorter in frozen/thawed pork than in fresh pork while the generation time was unaffected. Under commercial conditions, microbiological sampling of frozen meat may be of limited relevance. Small frozen samples will be thawed in a la boratory, probably under conditions unlike those used later to thaw whole blocks On the laboratory samples, extensive microbial growth during

(Cutting, 1974; Love, 1966) concluded that fast thawing rates would produce increased drip, while others showed (Finn, 1932; Singh and Essary, 1971) the opposite. Thawing times from -7 to 0°C of less than 1min or greater than 2000 min led to increased drip loss (James et al., 1983). The results are therefore conflicting and provide no useful design data for optimising a thawing system. The principle criteria governing quality of thawed meat are the appear￾ance and bacteriological condition. These are major factors if the product is to be sold thawed but are less important if the food is destined for pro￾cessing and heat treatment. Microbiological problems can arise during thawing of food in bulk.While centre temperatures may not exceed 0 °C, the exterior surface may be held at 10–15 °C for many hours, or even days. During this time extensive growth of spoilage organisms can occur on the surface.The time required for micro￾biological numbers to reach ‘spoilage’ levels will largely be dependent upon the number of microbes initially present and the temperature. Since freez￾ing and frozen storage have little effect on the number of viable microbes present, material of poor microbiological quality before freezing is likely to spoil more quickly during thawing (Roberts, 1974). The use of high thawing (>10 °C) temperatures for carcass meats tends to lead to large increases in microbial numbers (James and Creed, 1980; Bailey et al., 1974). Little published data exist on microbiological effects of thawing meat. Buttiaux (1972) reported that water thawing was more successful for beef than for pork if the meat was to be stored. Consequently, care must be exer￾cised in extrapolating from one meat species to another. Results with pork suggested that air thawing gives final counts about ten times higher than thawing in 3% brine, whereas with beef Heinz (1970) reported counts lower by a factor of about 10 for air (4–5ms-1 ; 10 °C) as opposed to flowing water (10 °C). Kassai (1969) also found no significant increase in bacteriological numbers when thawing beef carcasses in air (0.2–0.3m s-1 , 15–20 °C, 96% relative humidity, (RH)). Shoulders of lamb (Vanichseni et al., 1972) thawed in air (0.2 m s-1 ; 18 °C) or water (45°C) had bacterial counts that increased respectively by factors of 1.74 and 1.12; humidity and air velocity also influ￾enced the results of air thawing. It is often asserted that thawed food is more perishable than fresh or chilled produce, but experiments (Kitchell and Ingram, 1956; Kitchell and Ingram, 1959) have failed to demonstrate any difference of practical significance between the growth of meat spoilage organisms on fresh or thawed slices of meat. Greer and Murray (1991) found that the lag phase of bacterial growth was shorter in frozen/thawed pork than in fresh pork, while the generation time was unaffected. Under commercial conditions, microbiological sampling of frozen meat may be of limited relevance. Small frozen samples will be thawed in a la￾boratory, probably under conditions unlike those used later to thaw whole blocks. On the laboratory samples, extensive microbial growth during 162 Meat refrigeration

Thawing and tempering 163 thawing is unlikely, while on commercial blocks it is probable. Hence, the laboratory count reflects the number of microbes on the frozen meat but not necessarily on meat after commercial thawing Microbial counts incubated at 1C and 20-25C assess the storage lif of meat at chill and intermediate temperatures. Counts at 37C give an indi- cation of contamination from human and animal sources. Thawing under conditions that permit growth of bacteria counted at 1C and 25C will result in meat of poorer quality in terms of storage life. Thawing conditions food-poisoning bacteria(such as Salmonella spp. may be capable ce allowing heavy growth of bacteria counted at 37C are undesirable since growth e. The appearance of the surface of thawed meat is similarly related to the he spent in a given environment. Since this time will be a function of the material thickness, it is not possible to define one overall set of conditions for optimal appearance. For example, the air temperature, velocity and rela- tive humidity required to thaw small joints satisfactorily in a reasonably short time would almost certainly cause problems if used to thaw whole quarters of beef. In general both the appearance and final bacterial condi tion in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unaccept- able for other reasons related to operating requirements. A compromise must therefore be reached which for a given material could well differ from one factory to the next 8.3 Thawing systems There is no simple guide to the choice of an optimum thawing system(Table 8.2). A thawing system should be considered as one operation in the pro- duction chain. It receives frozen material which should be within a known temperature range and of specified microbiological condition. It is expected to deliver that same material in a given time in a totally thawed state. The reight loss and increase in bacterial numbers during thawing should be within acceptable limits, which will vary from process to process. In some circumstances, for example a direct sale to the consumer, the appearance of the thawed product is crucial, in others it may be irrelevant. Apart fre hese factors the economics and overall practicality of the thawing opera tion, including the capital and running costs of the plant, the labour require- nents, ease of cleaning and the flexibility of the plant to handle different products, must be considered 8.3.1 Conduction The main conduction-based thawing methods rely on air, water or steam condensation under vacuum

thawing is unlikely, while on commercial blocks it is probable. Hence, the laboratory count reflects the number of microbes on the frozen meat but not necessarily on meat after commercial thawing. Microbial counts incubated at 1 °C and 20–25 °C assess the storage life of meat at chill and intermediate temperatures. Counts at 37 °C give an indi￾cation of contamination from human and animal sources. Thawing under conditions that permit growth of bacteria counted at 1°C and 25 °C will result in meat of poorer quality in terms of storage life. Thawing conditions allowing heavy growth of bacteria counted at 37 °C are undesirable since food-poisoning bacteria (such as Salmonella spp.) may be capable of growth. The appearance of the surface of thawed meat is similarly related to the time spent in a given environment. Since this time will be a function of the material thickness, it is not possible to define one overall set of conditions for optimal appearance. For example, the air temperature, velocity and rela￾tive humidity required to thaw small joints satisfactorily in a reasonably short time would almost certainly cause problems if used to thaw whole quarters of beef. In general both the appearance and final bacterial condi￾tion in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unaccept￾able for other reasons related to operating requirements. A compromise must therefore be reached which for a given material could well differ from one factory to the next. 8.3 Thawing systems There is no simple guide to the choice of an optimum thawing system (Table 8.2). A thawing system should be considered as one operation in the pro￾duction chain. It receives frozen material which should be within a known temperature range and of specified microbiological condition. It is expected to deliver that same material in a given time in a totally thawed state. The weight loss and increase in bacterial numbers during thawing should be within acceptable limits, which will vary from process to process. In some circumstances, for example a direct sale to the consumer, the appearance of the thawed product is crucial, in others it may be irrelevant. Apart from these factors the economics and overall practicality of the thawing opera￾tion, including the capital and running costs of the plant, the labour require￾ments, ease of cleaning and the flexibility of the plant to handle different products, must be considered. 8.3.1 Conduction The main conduction-based thawing methods rely on air, water or steam condensation under vacuum. Thawing and tempering 163