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 chilling 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 majority of the material is ‘boned-out’ and packed in boxes ca. 15 cm thick weighing 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 chopping, 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 recommence. 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 electromagnetic 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 respective 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 condition 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 environmental 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 proportion 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 conductivity 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 measurements 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 appearance 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 processing 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 microbiological numbers to reach ‘spoilage’ levels will largely be dependent upon the number of microbes initially present and the temperature. Since freezing 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 exercised 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 influenced 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 laboratory, 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 indication 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 relative 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 condition in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unacceptable 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 production 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 operation, including the capital and running costs of the plant, the labour requirements, 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
164 Meat refrigeration Table 8.2 Advantages and disadvantages of different thawing system Advantages Disadvantages Easy to install: can be Very slow, unless high tems adapted from chill rooms. velocities and high ow velocity systems temperatures are used retain good appearance then there can b weight loss, spoilage and appearance problems Faster than air systems Effluent disposal. microbiological ndition Unsuitable for composite blocks Vacuum-heat Fast Deterioration in Very controllable ane Batch size limited Electrical Microwav Problems of limited systems Infra red absorption Can cause High Resistive Problems of contact on irregu 8.3.1.1 Air thawing Air thawing systems transfer heat to the frozen material by conduction through the static air boundary layer at the product surface and the rate of heat transfer is a function of the difference in temperature between the product and the air and the air velocity. Air systems are very flexible and may be used to thaw any size of meat cut from whole carcasses to individ- ual steaks 8.3. 11.1 Still air Thin blocks(<10cm) of meat can be thawed overnight at room tempera ture and, provided the surface of the product does not become too dry, the thawed product can be perfectly acceptable. Air temperatures should not be greater than15°C For thicker materials still air thawing is not recommended, since thawing times extend to days, rather than hours, and the surface layers may become
8.3.1.1 Air thawing Air thawing systems transfer heat to the frozen material by conduction through the static air boundary layer at the product surface and the rate of heat transfer is a function of the difference in temperature between the product and the air and the air velocity. Air systems are very flexible and may be used to thaw any size of meat cut from whole carcasses to individual steaks. 8.3.1.1.1 Still air Thin blocks (<10 cm) of meat can be thawed overnight at room temperature and, provided the surface of the product does not become too dry, the thawed product can be perfectly acceptable. Air temperatures should not be greater than 15 °C. For thicker materials still air thawing is not recommended, since thawing times extend to days, rather than hours, and the surface layers may become 164 Meat refrigeration Table 8.2 Advantages and disadvantages of different thawing systems Advantages Disadvantages Conduction Air Easy to install: can be Very slow, unless high systems adapted from chill rooms. velocities and high Low velocity systems temperatures are used, retain good appearance when there can be weight loss, spoilage and appearance problems Water Faster than air systems Effluent disposal. Deterioration in appearance and microbiological condition. Unsuitable for composite blocks Vacuum-heat Fast. Deterioration in (VHT) Low surface temperatures. appearance. Very controllable. High cost. Easily cleaned Batch size limited Electrical Microwave/ Very fast Problems of limited systems Infra red penetration and uneven energy absorption. Can cause localised ‘cooking’. High cost Resistive Fast Problems of contact on irregular surfaces
Thawing and tempering 165 warm and spoil long before the centre is thawed. Still air thawing is prac- ticable only on a small scale, because considerable space is required, the process is uncontrolled and the time taken is often too long to fit in with processing cycles. The sole advantage is that little or no equipment is required 8.3.1.1.2 Moving The majority of commercial thawing systems use moving air as the thawing medium Not only does the increased h value produced by moving air result in faster thawing but it also produces much better control than using still air Control of relative humidity is important with unwrapped products to reduce surface desiccation and increase the rate of heat transfer to the food stuff, 85-95% RH being recommended for meat(Bailey et aL., 1974) With 250g slabs of meat(Zagradzki et al, 1977) weight loss was a func- tion of temperature, velocity and relative humidity. In all cases, increasing the air temperature, or decre 85-88% RH or an increase in weight gain at 95-98% RH. Changes ranged from a 2.5% weight gain at 5C, 5ms 85-88% RH, to a 0.51 weight gain at 25C,1ms, 95-98%RH 831.1.3 Two-stage air Two-stage air thawing has often been proposed as a means of shortening the thawing process. In the first stage, a high air temperature is maintained until the surface reaches a predetermined set temperature, thus ensuring a apid initial input of energy. The air temperature is then reduced rapidly and maintained below 10C until the end of the thawing process Heat flows from the hotter surface regions to the centre of the frozen foodstuff, low ering the surface temperature to that of the ambient air. Since this tem- erature is below 10C, and the overall thawing time is short, total bacteria growth is small. A patent(1974)has been taken out on a two-stage thawing system using almost saturated air between 35 and 60C, followed by ai between 5 and 10C after the surface temperature of the product has reached 30-35C. The first stage normally takes 1-1.5h, the second 15-20 h and it is claimed that weight loss is low and drip loss minimal 8.3.1.2 Water thawing The mechanism of heat transfer in water is similar to that in air but because the heat transfer coefficients obtained are considerably larger, the thawing times of thinner cuts are effectively reduced. However, there are practical problems that limit the use of water thawing systems: boxed or packaged goods(unless shrink-wrapped or vacuum-packed) must be removed from their containers before they can be water thawed, composite blocks of boned-out pieces break up and disperse in the thawing tank, and handling difficulties arise which preclude the use of large cuts such as carcasses
warm and spoil long before the centre is thawed. Still air thawing is practicable only on a small scale, because considerable space is required, the process is uncontrolled and the time taken is often too long to fit in with processing cycles. The sole advantage is that little or no equipment is required. 8.3.1.1.2 Moving air The majority of commercial thawing systems use moving air as the thawing medium. Not only does the increased h value produced by moving air result in faster thawing but it also produces much better control than using still air. Control of relative humidity is important with unwrapped products to reduce surface desiccation and increase the rate of heat transfer to the foodstuff, 85–95% RH being recommended for meat (Bailey et al., 1974). With 250g slabs of meat (Zagradzki et al., 1977) weight loss was a function of temperature, velocity and relative humidity. In all cases, increasing the air temperature, or decreasing the air velocity produced a decrease in percentage weight loss at 85–88% RH or an increase in weight gain at 95–98% RH. Changes ranged from a 2.5% weight gain at 5°C, 5 m s-1 , 85–88% RH, to a 0.51 weight gain at 25 °C, 1 ms-1 , 95–98% RH. 8.3.1.1.3 Two-stage air Two-stage air thawing has often been proposed as a means of shortening the thawing process. In the first stage, a high air temperature is maintained until the surface reaches a predetermined set temperature, thus ensuring a rapid initial input of energy. The air temperature is then reduced rapidly and maintained below 10 °C until the end of the thawing process. Heat flows from the hotter surface regions to the centre of the frozen foodstuff, lowering the surface temperature to that of the ambient air. Since this temperature is below 10 °C, and the overall thawing time is short, total bacteria growth is small. A patent (1974) has been taken out on a two-stage thawing system using almost saturated air between 35 and 60 °C, followed by air between 5 and 10 °C after the surface temperature of the product has reached 30–35 °C. The first stage normally takes 1–1.5 h, the second 15–20 h and it is claimed that weight loss is low and drip loss minimal. 8.3.1.2 Water thawing The mechanism of heat transfer in water is similar to that in air, but because the heat transfer coefficients obtained are considerably larger, the thawing times of thinner cuts are effectively reduced. However, there are practical problems that limit the use of water thawing systems: boxed or packaged goods (unless shrink-wrapped or vacuum-packed) must be removed from their containers before they can be water thawed, composite blocks of boned-out pieces break up and disperse in the thawing tank, and handling difficulties arise which preclude the use of large cuts such as carcasses. Thawing and tempering 165
166 Meat refrigeration Meat racks in working section Water Water sump Vacuum pump Fig.8.1 APV-Torry vacuum thawing plant(source: Bailey and James, 1974b) 8.3.1.3 Vacuum-heat thawing A vacuum-heat thawing(VHT) system(Fig 8.1)operates by transferring he heat of condensing steam under vacuum to the frozen product. Theo- retically, a condensing vapour in the presence of a minimum amount of a non-condensable gas can achieve a surface film heat transfer coefficient far higher than that achieved in water thawing. The principle of operation is hat when steam is generated under vacuum, the vapour temperature will correspond to its equivalent vapour pressure. For example, if the vapour pressure is maintained at 1106Nm-2, steam will be generated at 15.C.The steam will condense onto any cooler surface such as a frozen product. The benefits of latent heat transfer can be obtained without the problems of cooking which would occur at atmospheric pressure With thin materials, thawing cycles are very rapid, enabling high daily throughputs to be achieved. The advantage of a high h value becomes less marked as material thickness increases and beef quarters or 25 kg meat blocks require thawing times permitting no more than one cycle per day Under these conditions, the economics of the system and the largest capac ity unit available(10-12 tonnes)severely restrict its application 8.3.2 Electrical methods In all of the methods described above, the rate of thawing is a function of he transfer of heat from the thawing medium to the surface of the meat and the conduction of this heat into the centre of the carcass or cut. In heory, electrical systems should overcome these problems because heat is generated within the material and the limitations of thermal conductivity are circumvented. In such systems the kinetic energy imparted to molecules by the action of an oscillating electromagnetic field is dissipated by inelas
8.3.1.3 Vacuum-heat thawing A vacuum-heat thawing (VHT) system (Fig. 8.1) operates by transferring the heat of condensing steam under vacuum to the frozen product. Theoretically, a condensing vapour in the presence of a minimum amount of a non-condensable gas can achieve a surface film heat transfer coefficient far higher than that achieved in water thawing. The principle of operation is that when steam is generated under vacuum, the vapour temperature will correspond to its equivalent vapour pressure. For example, if the vapour pressure is maintained at 1106 N m-2 , steam will be generated at 15 °C. The steam will condense onto any cooler surface such as a frozen product. The benefits of latent heat transfer can be obtained without the problems of cooking which would occur at atmospheric pressure. With thin materials, thawing cycles are very rapid, enabling high daily throughputs to be achieved. The advantage of a high h value becomes less marked as material thickness increases and beef quarters or 25 kg meat blocks require thawing times permitting no more than one cycle per day. Under these conditions, the economics of the system and the largest capacity unit available (10–12 tonnes) severely restrict its application. 8.3.2 Electrical methods In all of the methods described above, the rate of thawing is a function of the transfer of heat from the thawing medium to the surface of the meat and the conduction of this heat into the centre of the carcass or cut. In theory, electrical systems should overcome these problems because heat is generated within the material and the limitations of thermal conductivity are circumvented. In such systems the kinetic energy imparted to molecules by the action of an oscillating electromagnetic field is dissipated by inelas- 166 Meat refrigeration ~~ ~ ~ ~~ ~ ~~~ ~~~~ ~~~~~~ ~~ ~~~ ~~~~~~~~ Meat racks in working section Water sump Vacuum pump Steam Water Air In-place cleaning Fig. 8.1 APV-Torry vacuum thawing plant (source: Bailey and James, 1974b)
Thawing and tempering 167 tic collisions with surrounding molecules and this energy appears as heat Thus electromagnetic radiation may be used to heat foodstuffs. Three regions of the electromagnetic spectrum have been used for such heating: resistive 50Hz; radio frequency 3-300 GHz and microwave 900-3000GHz 8.3.2.1 Resistive thawing A frozen foodstuff can be heated by placing it between two electrodes and applying a low voltage at normal mains frequency. As the electric current flows through the material, it becomes warm(ohmic heating). Electrical contacts are required and product structure must be uniform and homoge eous, otherwise the path of least resistance will be taken by the current resulting in uneven temperatures and runaway heating Frozen meat at a low temperature does not readily conduct electricity, but as it becomes warmer, its electrical resistance falls, a larger current can flow and more heat is generated within the product. In practice, the system is only suitable for thin (5cm) homogeneous blocks such as catering blocks of liver since current flow is very small through thick blocks and inhomogeneities lead o runaway heating problems. 8.3.2.2 Radio frequency During radio frequency thawing, heat is produced in the frozen foodstuff because of dielectric losses when a product is subjected to an alternating electric field. In an idealised case of radio frequency heating the foodstuff, a regular slab of homogeneous material at a uniform temperature is placed between parallel electrodes and no heat is exchanged with its surroundings. When an alternating electro magnetic force is applied through the elec trodes the resulting field in the slab is uniform, so the energy and the resul tant temperature rise is identical in all parts of the food (Sanders, 1966) In practice this situation rarely applies. Foodstuffs are not generally the shape of perfect parallelepipeds, frozen meat consists of at least two components, fat and lean. During loading frozen meats pick up heat from the surroundings, the surface temperature rises and the dielectric system not presented with the uniform temperature distribution required for even heating. By using a conveyorised system to keep the product moving past the electrodes and/or surrounding the material by water, commercial systems ave been produced for blocks of oily fish and white fish(Jason and Sanders, 1962). Successful thawing of 13 cm thick meat blocks and 14cm thick offal blocks have also been reported (Sanders, 1961)but the tempera- ture range at the end of thawing(44 min) was stated to be -2-19C and -24"C, respectively, and the product may not have been fully thawed To overcome runaway heating with slabs of frozen pork bellies, workers Satchell and Doty, 1951) have tried coating the electrodes with lard
tic collisions with surrounding molecules and this energy appears as heat. Thus electromagnetic radiation may be used to heat foodstuffs. Three regions of the electromagnetic spectrum have been used for such heating: resistive 50 Hz; radio frequency 3–300 GHz and microwave 900–3000 GHz. 8.3.2.1 Resistive thawing A frozen foodstuff can be heated by placing it between two electrodes and applying a low voltage at normal mains frequency. As the electric current flows through the material, it becomes warm (ohmic heating). Electrical contacts are required and product structure must be uniform and homogeneous, otherwise the path of least resistance will be taken by the current, resulting in uneven temperatures and runaway heating. Frozen meat at a low temperature does not readily conduct electricity, but as it becomes warmer, its electrical resistance falls, a larger current can flow and more heat is generated within the product. In practice, the system is only suitable for thin (5 cm) homogeneous blocks such as catering blocks of liver since current flow is very small through thick blocks and inhomogeneities lead to runaway heating problems. 8.3.2.2 Radio frequency During radio frequency thawing, heat is produced in the frozen foodstuff because of dielectric losses when a product is subjected to an alternating electric field. In an idealised case of radio frequency heating the foodstuff, a regular slab of homogeneous material at a uniform temperature is placed between parallel electrodes and no heat is exchanged with its surroundings. When an alternating electro magnetic force is applied through the electrodes the resulting field in the slab is uniform, so the energy and the resultant temperature rise is identical in all parts of the food (Sanders, 1966). In practice this situation rarely applies. Foodstuffs are not generally in the shape of perfect parallelepipeds, frozen meat consists of at least two components, fat and lean. During loading frozen meats pick up heat from the surroundings, the surface temperature rises and the dielectric system is not presented with the uniform temperature distribution required for even heating. By using a conveyorised system to keep the product moving past the electrodes and/or surrounding the material by water, commercial systems have been produced for blocks of oily fish and white fish (Jason and Sanders, 1962). Successful thawing of 13cm thick meat blocks and 14cm thick offal blocks have also been reported (Sanders, 1961) but the temperature range at the end of thawing (44min) was stated to be -2–19 °C and -2–4 °C, respectively, and the product may not have been fully thawed. To overcome runaway heating with slabs of frozen pork bellies, workers (Satchell and Doty, 1951) have tried coating the electrodes with lard, Thawing and tempering 167
168 Meat refrigeration placing the bellies in oil, water and saline baths and wrapping the meat in heesecloth soaked in saline solution. Only the last treatment was success- ful but even that was not deemed practical. 8.3.2.3 Microwave thawing Microwave thawing utilises electromagnetic waves directed at the product through waveguides without the use of conductors or electrodes. whilst he heating of frozen meat by microwave energy is potentially a very fast method of thawing, its application is constrained by thermal instability. At its worst, parts of the food may be cooked whilst the rest is substantially frozen. This arises because the absorption by frozen food of electro- magnetic radiation in this frequency range increases as the temperature rises, this dependence being especially large at about -5C, increasing as the initial freezing point is approached. If for any reason during irradiation a region of the material is slightly hotter than its surroundings, propor tionately more energy will be absorbed within that region and the original difference in enthalpy will be increased. As the enthalpy increases so the absorption increases and the unevenness of heating worsens at an ever- increasing rate. Below the initial freezing point the temperature increase is held in check by thermal inertia since for a given energy input the tem is continued after the hot spot has reached its initial freezing poin, th? perature rise is inversely proportional to the thermal capacity. If irradiation temperature rises at a catastrophic rate. A hybrid microwave/vacuum system, in which boiling surface water at a low temperature was used to cool the surface, thawed 15 cm thick cartoned meat in 1-2h without runaway heating, but problems of control and cost would appear to limit the commercial use (James, 1984). Despite a wide- pread belief to the contrary, microwave thawing systems have not been commercially successful. However, microwave tempering systems(see later) have found successful niche applications in the meat industry 8.3.3 Published thawing data for different meat cuts ders and carcasses, beef quarters and boned-out meat blode s, lamb shoul Process design data is available on thawing of frozen pork legs, lamb shoul 8.3.3.7 Thawing of pork legs/hams Bailey et al.(1974)made a comparative experimental study of thawing of frozen pork legs of different weights in air, water and vacuum heat thawing VHT) systems with respect to thawing time, weight loss and appearance A comprehensive chart(Fig. 8.2) was produced for the determination of thawing times over a range of process operating conditions(Bailey and James, 1974a) Thawing time increased almost linearly with leg weight for all systems hawing in water was faster than in air at any given temperature, but
placing the bellies in oil, water and saline baths and wrapping the meat in cheesecloth soaked in saline solution. Only the last treatment was successful but even that was not deemed practical. 8.3.2.3 Microwave thawing Microwave thawing utilises electromagnetic waves directed at the product through waveguides without the use of conductors or electrodes. Whilst the heating of frozen meat by microwave energy is potentially a very fast method of thawing, its application is constrained by thermal instability. At its worst, parts of the food may be cooked whilst the rest is substantially frozen. This arises because the absorption by frozen food of electromagnetic radiation in this frequency range increases as the temperature rises, this dependence being especially large at about -5 °C, increasing as the initial freezing point is approached. If for any reason during irradiation a region of the material is slightly hotter than its surroundings, proportionately more energy will be absorbed within that region and the original difference in enthalpy will be increased. As the enthalpy increases so the absorption increases and the unevenness of heating worsens at an everincreasing rate. Below the initial freezing point the temperature increase is held in check by thermal inertia since for a given energy input the temperature rise is inversely proportional to the thermal capacity. If irradiation is continued after the hot spot has reached its initial freezing point, the temperature rises at a catastrophic rate. A hybrid microwave/vacuum system, in which boiling surface water at a low temperature was used to cool the surface, thawed 15cm thick cartoned meat in 1–2 h without runaway heating, but problems of control and cost would appear to limit the commercial use (James, 1984). Despite a widespread belief to the contrary, microwave thawing systems have not been commercially successful. However, microwave tempering systems (see later) have found successful niche applications in the meat industry. 8.3.3 Published thawing data for different meat cuts Process design data is available on thawing of frozen pork legs, lamb shoulders and carcasses, beef quarters and boned-out meat blocks. 8.3.3.1 Thawing of pork legs/hams Bailey et al. (1974) made a comparative experimental study of thawing of frozen pork legs of different weights in air, water and vacuum heat thawing (VHT) systems with respect to thawing time, weight loss and appearance. A comprehensive chart (Fig. 8.2) was produced for the determination of thawing times over a range of process operating conditions (Bailey and James, 1974a). Thawing time increased almost linearly with leg weight for all systems. Thawing in water was faster than in air at any given temperature, but 168 Meat refrigeration
