《肉制品冷冻技术》（英文版） Part 7 Freezing of meat

Freezing of meat leat for industrial processing is usually frozen in the form of carcasses, quarters or boned out primals in 25 kg cartons. Most bulk meat, consumer portions and meat products are frozen in air blast freezers. Some small ind viduals items, for example beefburgers, may be frozen in cryogenic tunnels and a small amount of offal and other meat is frozen in plate freezers. It is not unusual for meat to be frozen twice before it reaches the consumer During industrial processing frozen raw material is often thawed or tem- pered before being turned into meat-based products, i.e. pies, convenience meals, burgers, etc or consumer portions, fillets, steaks, and so on. These consumer-sized portions are often refrozen before storage, distribution and 7.1 Freezing rate There are little data in the literature to suggest that, in general, the method f freezing or the rate of freezing has any substantial influence on a meats subsequent storage life, its quality characteristics or final eating quality There is some disagreement in the literature about whether fast or slow freezing is advantageous. Slightly superior chemical and sensory attributes ave been found in food cryogenically frozen in a few trials(Sebranek et al., 1978: Dobryschi et al., 1977; Sebranek, 1980) but other trials did not show any appreciable advantage(Lampitt and Moran, 1933) especially during short term storage(Hill and Glew, 1973). Jackobsson and Bengtson ( 1973) indicated that there is an interaction between freezing rate and cooking method. Meat that had been cooked from frozen was found to

7 Freezing of meat Meat for industrial processing is usually frozen in the form of carcasses, quarters or boned out primals in 25kg cartons. Most bulk meat, consumer portions and meat products are frozen in air blast freezers. Some small indi￾viduals items, for example beefburgers, may be frozen in cryogenic tunnels and a small amount of offal and other meat is frozen in plate freezers. It is not unusual for meat to be frozen twice before it reaches the consumer. During industrial processing frozen raw material is often thawed or tem￾pered before being turned into meat-based products, i.e. pies, convenience meals, burgers, etc or consumer portions, fillets, steaks, and so on. These consumer-sized portions are often refrozen before storage, distribution and sale. 7.1 Freezing rate There are little data in the literature to suggest that, in general, the method of freezing or the rate of freezing has any substantial influence on a meat’s subsequent storage life, its quality characteristics or final eating quality. There is some disagreement in the literature about whether fast or slow freezing is advantageous. Slightly superior chemical and sensory attributes have been found in food cryogenically frozen in a few trials (Sebranek et al., 1978; Dobryschi et al., 1977; Sebranek, 1980) but other trials did not show any appreciable advantage (Lampitt and Moran, 1933) especially during short term storage (Hill and Glew, 1973). Jackobsson and Bengtson (1973) indicated that there is an interaction between freezing rate and cooking method. Meat that had been cooked from frozen was found to

138 Meat refrigeration show a favourable effect from faster freezing rates. Mittal and Barbut (1991) showed that freezing rate affected the modulus of rigidity of meat after cooking. Similar values to fresh meat were produced in meat frozen in liquid nitrogen. The value of the modulus increased as the rate of freez ing decreased. In 1980 Anon and Calvelo reported a relationship between the rate of freezing and drip loss, drip loss reaching a maximum when the freezing time from-1 to-7C was ca 17 min Mascheroni (1985)used this relationship to produce a method for determining the rate at which frozen meat had been frozen. However, attempts to replicate the work at Langford(James et al., 1983)were unsuccessful because of the variability in drip loss from meat before freezing. Studies using differential scanning calorimetry (dSC) on fresh and frozen bovine muscle at different freezing rates show a decrease of denaturation enthalpies; the slower the freezing rate the greater the loss(Wagner and Anon, 1985). Investigations covered freezing times from 5 to 60min Experiments with pork M. longissimus dorsi found no difference in drip loss between samples frozen at -20oC or -80oC(Sakata et al., 1995). At -20 and"C samples took 6 and 3h, respectively to pass from -lC to ca.-6'C In the -20C samples inter- and intracellular ice were seen but only intracellular ice was seen at-80C. Methods of freezing clearly affect the ultrastructure of muscle. Slow freezing(1-2mmh- for example( Buchmuller, 1986)tends to produce large ice crystals extracellularly, whilst quick freezing (e.g. 50mmh-)gives smaller crystals in and outside cells(Buchmuller, 1986: Bevilacqua et al 1979). Obviously a temperature gradient will occur in large pieces of meat and result in a non-uniform ice morphology(Bevilacqua et al., 1979) Petrovic et al. (1993)found that slowly frozen meat, 0.22 and 0.39cmh- lost more weight during freezing, thawing and cooking than that frozen at 3.95-5.66cmh-(Table 7.1). However, higher weight losses during thawing were measured at an intermediate freezing rate of 3. 33 cm-. Meat frozen at rates of 3.33 cmh and faster was rated as more tender and juicier after cooking than unfrozen controls and slow frozen samples (Table 7.2). Petro- vic et al. stated that the optimal conditions for freezing portioned meat are those that achieve freezing rates between 2 and 5cmh-to-7oC Grujic et al.(1993)suggest even tighter limits, 3.33-3.95cmh-. Slow freezing at up to 0.39cmh- resulted in decreased solubility of myofibrillar proteins, increase in weight loss during freezing, thawing and cooking, lower water- binding capacity and tougher cooked meat. Very quickly frozen meat (4.9cmh-)had a somewhat lower solubility of myofibrillar proteins, lower yater-binding capacity and somewhat tougher and drier meat. The samples were thawed after storage times of 2-3 days at -20oC so the relationship etween freezing rates and storage life was not investigated Storage times of 48h and 2.5 months were used during investigations of the effect of different freezing systems and rates on drip production from

show a favourable effect from faster freezing rates. Mittal and Barbut (1991) showed that freezing rate affected the modulus of rigidity of meat after cooking. Similar values to fresh meat were produced in meat frozen in liquid nitrogen. The value of the modulus increased as the rate of freez￾ing decreased. In 1980 Añón and Calvelo reported a relationship between the rate of freezing and drip loss, drip loss reaching a maximum when the freezing time from -1 to -7 °C was ca. 17 min. Mascheroni (1985) used this relationship to produce a method for determining the rate at which frozen meat had been frozen. However, attempts to replicate the work at Langford (James et al., 1983) were unsuccessful because of the variability in drip loss from meat before freezing. Studies using differential scanning calorimetry (DSC) on fresh and frozen bovine muscle at different freezing rates show a decrease of denaturation enthalpies; the slower the freezing rate the greater the loss (Wagner and Añón, 1985). Investigations covered freezing times from 5 to 60 min. Experiments with pork M. longissimus dorsi found no difference in drip loss between samples frozen at -20 °C or -80 °C (Sakata et al., 1995). At -20 and -80 °C samples took 6 and 3h, respectively to pass from -1 °C to ca. -6 °C. In the -20 °C samples inter- and intracellular ice were seen but only intracellular ice was seen at -80 °C. Methods of freezing clearly affect the ultrastructure of muscle. Slow freezing (1–2 mm h-1 for example (Buchmuller, 1986) tends to produce large ice crystals extracellularly, whilst quick freezing (e.g. 50 mm h-1 ) gives smaller crystals in and outside cells (Buchmuller, 1986; Bevilacqua et al., 1979). Obviously a temperature gradient will occur in large pieces of meat and result in a non-uniform ice morphology (Bevilacqua et al., 1979). Petrovic et al. (1993) found that slowly frozen meat, 0.22 and 0.39 cmh-1 , lost more weight during freezing, thawing and cooking than that frozen at 3.95–5.66 cmh-1 (Table 7.1). However, higher weight losses during thawing were measured at an intermediate freezing rate of 3.33 cm h-1 . Meat frozen at rates of 3.33 cm h-1 and faster was rated as more tender and juicier after cooking than unfrozen controls and slow frozen samples (Table 7.2). Petro￾vic et al. stated that the optimal conditions for freezing portioned meat are those that achieve freezing rates between 2 and 5 cmh-1 to -7 °C. Grujic et al. (1993) suggest even tighter limits, 3.33–3.95 cm h-1 . Slow freezing at up to 0.39 cmh-1 resulted in decreased solubility of myofibrillar proteins, increase in weight loss during freezing, thawing and cooking, lower water￾binding capacity and tougher cooked meat. Very quickly frozen meat (>4.9 cmh-1 ) had a somewhat lower solubility of myofibrillar proteins, lower water-binding capacity and somewhat tougher and drier meat. The samples were thawed after storage times of 2–3 days at -20 °C so the relationship between freezing rates and storage life was not investigated. Storage times of 48 h and 2.5 months were used during investigations of the effect of different freezing systems and rates on drip production from 138 Meat refrigeration

Freezing of meat 139 Table 7.1 relationship between freezing rate of beef M rsi and weight loss during freezing. thawing and cooking Freezing rate(cmh Weight loss during Freezing Tha Cookin Control 0.22 1.15 3747 0.63 003 ource: Petrovic et al. 1993. Table 7.2 Relationship between freezing rate of beef M. zing rate Texture Tenderness juicines Control 7.0 0.22 7.0 6.0 0.39 777 000 5 7.0 .0 8.5 5.66 7.0 7.3 Source: Petrovic et al.. 1993 Texture: 1=extremely tough, 7= extremely fine. Tenderness: 1 =extremely hard, 9= extremely tender Juiciness: 1 =extremely dry, 9= extremely juicy. small samples of mutton muscle ( Sacks et al, 1993). In all cases, drip loss after 2.5 months was at least double the percentage measured after 48h (Table 7.3). After 2.5 months, drip loss from samples frozen using cryogen ics was >2%less than in those using air freezing. The most recent comparison(Sundsten et aL., 2001)revealed some com- mercial advantages of fast freezing, but no quality advantages. The studies ompared three different freezing methods, spiral freezing(SF), cryogenic freezing(liquid nitrogen, LN) and impingement freezing(IF). The times equired to freeze a 10mm thick 80g hamburger from +4 C to -18C in the sf,in and if were 22 min, 5min 30s and 2min 40s, respectively. The uthors state that dehydration was significantly higher for hamburgers frozen in SF (1.2%)compared to LN (0.4%)and IF(0.4%). No significant

small samples of mutton muscle (Sacks et al., 1993). In all cases, drip loss after 2.5 months was at least double the percentage measured after 48 h (Table 7.3). After 2.5 months, drip loss from samples frozen using cryogen￾ics was >2% less than in those using air freezing. The most recent comparison (Sundsten et al., 2001) revealed some com￾mercial advantages of fast freezing, but no quality advantages. The studies compared three different freezing methods, spiral freezing (SF), cryogenic freezing (liquid nitrogen, LN) and impingement freezing (IF). The times required to freeze a 10mm thick 80g hamburger from +4 °C to -18 °C in the SF, LN and IF were 22 min, 5 min 30 s and 2 min 40 s, respectively. The authors state that dehydration was significantly higher for hamburgers frozen in SF (1.2%) compared to LN (0.4%) and IF (0.4%). No significant Freezing of meat 139 Table 7.1 Relationship between freezing rate of beef M. longissimus dorsi and weight loss during freezing, thawing and cooking Freezing rate (cm h-1 ) % Weight loss during Freezing Thawing Cooking Control – – 36.32 0.22 2.83 0.78 38.41 0.39 2.58 0.72 38.00 3.33 1.15 1.21 37.47 3.95 1.05 0.18 37.24 4.92 0.87 0.10 37.15 5.66 0.63 0.03 37.14 Source: Petrovic et al., 1993. Table 7.2 Relationship between freezing rate of beef M. longissimus dorsi and texture Freezing rate Texture Tenderness Juiciness (cm h-1 ) Control 7.0 6.8 7.0 0.22 7.0 6.0 6.7 0.39 7.0 6.5 7.0 3.33 7.0 7.5 7.5 3.95 7.0 8.0 8.0 4.92 7.0 8.5 8.5 5.66 6.0 7.0 7.3 Source: Petrovic et al., 1993. Texture: 1 = extremely tough, 7 = extremely fine. Tenderness: 1 = extremely hard, 9 = extremely tender. Juiciness: 1 = extremely dry, 9 = extremely juicy

140 Meat refrigeration Table 7.3 Drip loss(%)from 776g samples of longissimus lumborum et thoracis frozen under different methods and thawed at 4 c eez Freezing time Freezing rate Storage tim (cmh-) at-20°C 48h 2.5 months Cryogenic,-90°C 3.342 Cryogenic,-65°C 4.70Pb 972 Blast freezer.-21°C 1274 Walk-in-freezer. -21C 1.88h Domestic freezer. -25C 0.5 1172 Means in the same column with different superscripts are different at P>0.05h difference could be seen in cooking losses, even after storage for 2 months. Ice crystals were significantly larger in hamburgers frozen in SF compared to LN and IF. Sensory analysis revealed no difference in eating quality between the three freezing methods, even after storage for 2 months. Slow freezing from a high initial temperature can provide conditions for microbial growth compared with a very rapid freezing process. Castell- Perez et al.(1989) predicted that slow freezing from an initial product temperature of 30C could result in an 83% increase in bacterial numbers compared with a 4% increase from 10%C. 7.2 Freezing systems 7.2.1 Air Air is by far the most widely used method of freezing food as it is eco- nomical, hygienic and relatively non-corrosive to equipment Systems range from the most basic, in which a fan draws air through a refrigerated coil and blows the cooled air around an insulated room(Fig. 7.1), to purpose- built conveyerised blast freezing tunnels or spirals. Relatively low rates of heat transfer are attained from product surfaces in air systems. The big advantage of air systems is their versatility, especially when there is a requirement to freeze a variety of irregularly shaped products or individ ual products. In practice, air distribution is a major problem, often overlooked by the stem designer and the operator. The freezing time of the product is re- duced as the air speed is increased. An optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds. This optimum value can be as low as 1.0ms" air speed when freezing beef quarters rising to 15ms or more for thin products

difference could be seen in cooking losses, even after storage for 2 months. Ice crystals were significantly larger in hamburgers frozen in SF compared to LN and IF. Sensory analysis revealed no difference in eating quality between the three freezing methods, even after storage for 2 months. Slow freezing from a high initial temperature can provide conditions for microbial growth compared with a very rapid freezing process. Castell￾Perez et al. (1989) predicted that slow freezing from an initial product temperature of 30°C could result in an 83% increase in bacterial numbers compared with a 4% increase from 10 °C. 7.2 Freezing systems 7.2.1 Air Air is by far the most widely used method of freezing food as it is eco￾nomical, hygienic and relatively non-corrosive to equipment. Systems range from the most basic, in which a fan draws air through a refrigerated coil and blows the cooled air around an insulated room (Fig. 7.1), to purpose￾built conveyerised blast freezing tunnels or spirals. Relatively low rates of heat transfer are attained from product surfaces in air systems. The big advantage of air systems is their versatility, especially when there is a requirement to freeze a variety of irregularly shaped products or individ￾ual products. In practice, air distribution is a major problem, often overlooked by the system designer and the operator. The freezing time of the product is re￾duced as the air speed is increased. An optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds. This optimum value can be as low as 1.0 m s-1 air speed when freezing beef quarters rising to 15 m s-1 or more for thin products. 140 Meat refrigeration Table 7.3 Drip loss (%) from 77.6 g samples of longissimus lumborum et thoracis frozen under different methods and thawed at 4 °C Freezing conditions Freezing time Freezing rate Storage time to -2.2 °C (cm h-1 ) at -20 °C 48 h 2.5 months Cryogenic, -90 °C 15 min 6.4 3.34a 9.49a Cryogenic, -65 °C 22 min 4.4 4.70ab 9.72a Blast freezer, -21 °C 1.83 h 0.55 5.53b 12.74b Walk-in-freezer, -21 °C 1.88 h 0.53 4.71ab 13.18b Domestic freezer, -25 °C 1.96 h 0.51 5.26b 11.72b Means in the same column with different superscripts are different at P > 0.05 h Source: Sacks et al., 1993

Freezing of meat 141 Fig. 7.1. Example of a freezing tunnel with longitudinal air circulation. The use of impingement technology to increase the surface heat trans fer in air and other freezing systems has received attention recently (Newman, 2001; Sundsten et al., 2001; Everington, 2001). Impingement is the process of directing a jet or jets of fluid at a solid surface to effect a change. When the jets of fluid are very cold gas, the change is a dramatic ncrease in convective surface heat transfer coefficients. The very high velocity(20-30ms-)impingement gas jets, "breakup' the static surface boundary layer of gas that surrounds a food product. The resulting medium around the product is more turbulent and the heat exchange through this zone becomes much more effective 7.2.1.1 Batch systems Placing food items in large refrigerated rooms is the most common method of freezing. Fans circulate air through refrigerated coils and around the products in an insulated room. Large individual items such as meat car casses are hung from overhead rails, smaller products are placed either unwrapped or in cartons on racks, pallets, or large bins. 7.2.1.2 Continuous systems In a continuous system, meat is conveyed through a freezing tunnel or refrigerated room usually by an overhead conveyor or on a belt. This over omes the problem of uneven air distribution since each item is subjected to the same velocity/time profile. Some meat products are frozen on racks of trays(2m high), pulled or pushed through a freezing tunnel by me- chanical means. For larger operations, it is more satisfactory to use feed meat on a continuous belt through linear tunnels or spiral freezers. Linear

The use of impingement technology to increase the surface heat trans￾fer in air and other freezing systems has received attention recently (Newman, 2001; Sundsten et al., 2001; Everington, 2001). Impingement is the process of directing a jet or jets of fluid at a solid surface to effect a change. When the jets of fluid are very cold gas, the change is a dramatic increase in convective surface heat transfer coefficients. The very high velocity (20–30 m s-1 ) impingement gas jets, ‘breakup’ the static surface boundary layer of gas that surrounds a food product. The resulting medium around the product is more turbulent and the heat exchange through this zone becomes much more effective. 7.2.1.1 Batch systems Placing food items in large refrigerated rooms is the most common method of freezing. Fans circulate air through refrigerated coils and around the products in an insulated room. Large individual items such as meat car￾casses are hung from overhead rails, smaller products are placed either unwrapped or in cartons on racks, pallets, or large bins. 7.2.1.2 Continuous systems In a continuous system, meat is conveyed through a freezing tunnel or refrigerated room usually by an overhead conveyor or on a belt. This over￾comes the problem of uneven air distribution since each item is subjected to the same velocity/time profile. Some meat products are frozen on racks of trays (2 m high), pulled or pushed through a freezing tunnel by me￾chanical means. For larger operations, it is more satisfactory to use feed meat on a continuous belt through linear tunnels or spiral freezers. Linear Freezing of meat 141 Evaporator Reversible fan False ceiling Product on trolleys Fig. 7.1. Example of a freezing tunnel with longitudinal air circulation

142 Meat refrigeration tunnels are of simpler construction but are restricted by the length of belt necessary to achieve the cooling time required and on the space available in most factories. Spiral freezers are therefore a more viable alter native 7.3 Contact freezers Contact freezing methods are based on heat transfer by contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact freezing offers several advantages over air cooling, i.e. there is much better heat transfer and significant energy savings However, the need for regularly shaped products with large flat surfaces is a major hindrance Modern plate cooling systems differ little in principle from the first contact freezer patented in 1929 by Clarence Birdseye. Essentially the refrigerant(Fig. 7. 2). A hydraulic cylinder is used to bring the freezing plates into pressure contact with the product. These plates can be either horizontal or vertical Good heat transfer is dependent on product thickness, good contact and the conductivity of the product. Plate freezers are often limited to a maximum thickness of 50-70mm Good contact is a prime requirement. Air paces in packaging and fouling of the plates can have a significant effect on cooling time, for example a water droplet frozen on the plate can lengthen the freezing time in the concerned tray by as much as 30-60% General advantages of plate freezers over air-blast carton fre Hydraulic ram Product Fig. 7.2. Example of a horizontal plate freezer

tunnels are of simpler construction but are restricted by the length of belt necessary to achieve the cooling time required and on the space available in most factories. Spiral freezers are therefore a more viable alternative. 7.3 Contact freezers Contact freezing methods are based on heat transfer by contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact freezing offers several advantages over air cooling, i.e. there is much better heat transfer and significant energy savings. However, the need for regularly shaped products with large flat surfaces is a major hindrance. Modern plate cooling systems differ little in principle from the first contact freezer patented in 1929 by Clarence Birdseye. Essentially the product is pressed between hollow metal plates containing a circulating refrigerant (Fig. 7.2). A hydraulic cylinder is used to bring the freezing plates into pressure contact with the product. These plates can be either horizontal or vertical. Good heat transfer is dependent on product thickness, good contact and the conductivity of the product. Plate freezers are often limited to a maximum thickness of 50–70 mm. Good contact is a prime requirement.Air spaces in packaging and fouling of the plates can have a significant effect on cooling time, for example a water droplet frozen on the plate can lengthen the freezing time in the concerned tray by as much as 30–60%. General advantages of plate freezers over air-blast carton freezers include: 142 Meat refrigeration Hydraulic ram Freezing plate Product Separated plates Closed plates Fig. 7.2. Example of a horizontal plate freezer

Freezing of meat 143 Freezing is either faster for the same refrigerant evaporating tempera- ture, or can take place at a higher evaporating temperature for a given freezing time. Product temperatures are easier to control, especially for smaller cuts Power consumption is significantly reduced -savings of at least 30% nd possibly 50% or more, may be expected because air-circulating fans are not required and because higher evaporating temperatures can be used for the same effective cooling medium temperature. In many cases, less building space is required The product remains uniform and flat after freezing, unlike air-blast frozen cartons which often bulge. The flat cartons result in stable loads, giving up to 30% higher space utilisation in cold stores. For transport, the stable pallets facilitate unitised loading, and some 8-10% more product can be loaded into a container Disadvantages of plate freezers relate mainly to cost aspects Capital costs are significantly higher than for equivalent air-blast freezers. Manually loaded plate freezers are comparable in cost to auto- matic air-blast tunnel freezers. Fully automatic plate freezers are more High circulation rates of liquid refrigerant are required; this results in additional costs for larger accumulators and higher capacity pumps For manual plate freezers, simultaneous loading and unloading may require higher labour input than for a batch air freezer Each plate must be loaded with product of the same thickness Damp cartons can stick to plates or cause jams when ice forms. Air infiltration must be minimised to prevent frost build up on plates. Freezing unpacked meat has significant advantages because of the sub- stantially shorter freezing times( Fleming et al., 1996). Twice as many freez ng cycles per day can be achieved with the bare product Table 7. 4) Overall costs for plate freezing can be comparable to those for air-blast freezing. De Jong(1994)carried out a cost analysis(Table 7.5)for a beef plant using either plate or air blast freezers in New Zealand which assumed Table 7. 4 Predicted freezing time of meat blocks in a plate freezer operating at -30C Thickness(mm) Freezing time(h) Cycles per day Cartoned Bare Cartoned Bare ource: Fleming et al., 1996

• Freezing is either faster for the same refrigerant evaporating tempera￾ture, or can take place at a higher evaporating temperature for a given freezing time. • Product temperatures are easier to control, especially for smaller cuts. • Power consumption is significantly reduced – savings of at least 30%, and possibly 50% or more, may be expected because air-circulating fans are not required and because higher evaporating temperatures can be used for the same effective cooling medium temperature. • In many cases, less building space is required. • The product remains uniform and flat after freezing, unlike air-blast frozen cartons which often bulge. The flat cartons result in stable loads, giving up to 30% higher space utilisation in cold stores. For transport, the stable pallets facilitate unitised loading, and some 8–10% more product can be loaded into a container. Disadvantages of plate freezers relate mainly to cost aspects: • Capital costs are significantly higher than for equivalent air-blast freezers. Manually loaded plate freezers are comparable in cost to auto￾matic air-blast tunnel freezers. Fully automatic plate freezers are more expensive. • High circulation rates of liquid refrigerant are required; this results in additional costs for larger accumulators and higher capacity pumps. • For manual plate freezers, simultaneous loading and unloading may require higher labour input than for a batch air freezer. • Each plate must be loaded with product of the same thickness. • Damp cartons can stick to plates or cause jams when ice forms. • Air infiltration must be minimised to prevent frost build up on plates. Freezing unpacked meat has significant advantages because of the sub￾stantially shorter freezing times (Fleming et al., 1996). Twice as many freez￾ing cycles per day can be achieved with the bare product (Table 7.4). Overall costs for plate freezing can be comparable to those for air-blast freezing. De Jong (1994) carried out a cost analysis (Table 7.5) for a beef plant using either plate or air blast freezers in New Zealand which assumed Freezing of meat 143 Table 7.4 Predicted freezing time of meat blocks in a plate freezer operating at -30 °C Thickness (mm) Freezing time (h) Cycles per day Cartoned Bare Cartoned Bare 80 6.3 2.5 3 6 160 16.5 8.5 1 2 Source: Fleming et al., 1996

144 Meat refrigeration Table 7.5 Energy and cost requirements for beef plant freezing 3000 cartons per freezers blast ezer Freezing time(h Freezer load (kw 53 Power consumption (kW) Capital cost Annual capital charges Annual energy costs Annual labour cost Total annual cost Source: De Jong. 1994 a net electricity cost of NZso10 per kilowatt hour, a capital recovery over 10 years and an interest rate of 12% 7.4 Cryogenic freezing Cryogenic freezing uses refrigerants, such as liquid nitrogen or solid carbon dioxide, directly. The method of cooling is essentially similar to water-based evaporative cooling, cooling being brought about by boiling off the refrig erant, the essential difference being the temperature required for boiling As well as using the latent heat absorbed by the boiling liquid, sensible heat is absorbed by the resulting cold gas. Owing to very low operating temperatures and high surface heat fer coefficients between product and medium, cooling rates of cryogenic systems are often substantially higher than other refrigeration systems. Most systems use total loss refrigerants, i.e. the refrigerant is released to the atmosphere and not recovered. Alternatively dichlorodifluoromethane (CCl, F2)(otherwise known as Freon 12, R 12 or F12) may be used in a recovery and recycle system, however, R 12 is not generally accepted in all countries. Because of environmental and economic factors total loss refrig erants must be both readily available and harmless, which limits the choice to atmospheric air and its components, liquid nitrogen(LN) and liquid or solid carbon dioxide(cO2) The particular ch cs of total loss refrigerants that may be regarded as advantages dvantages are listed in Table 7.6. eezing is used for small products such as burgers, ready meals, and so on. The most common method is by direct spraying of liquid nitrogen onto a food product while it is conveyed through an insu lated tunnel

a net electricity cost of NZ$0.10 per kilowatt hour, a capital recovery over 10 years and an interest rate of 12%. 7.4 Cryogenic freezing Cryogenic freezing uses refrigerants, such as liquid nitrogen or solid carbon dioxide, directly. The method of cooling is essentially similar to water-based evaporative cooling, cooling being brought about by boiling off the refrig￾erant, the essential difference being the temperature required for boiling. As well as using the latent heat absorbed by the boiling liquid, sensible heat is absorbed by the resulting cold gas. Owing to very low operating temperatures and high surface heat trans￾fer coefficients between product and medium, cooling rates of cryogenic systems are often substantially higher than other refrigeration systems. Most systems use total loss refrigerants, i.e. the refrigerant is released to the atmosphere and not recovered. Alternatively dichlorodifluoromethane (CCl2F2) (otherwise known as Freon 12, R.12 or F12) may be used in a recovery and recycle system, however, R.12 is not generally accepted in all countries. Because of environmental and economic factors total loss refrig￾erants must be both readily available and harmless, which limits the choice to atmospheric air and its components, liquid nitrogen (LN) and liquid or solid carbon dioxide (CO2). The particular characteristics of total loss refrigerants that may be regarded as advantages or disadvantages are listed in Table 7.6. Cryogenic freezing is mainly used for small products such as burgers, ready meals, and so on. The most common method is by direct spraying of liquid nitrogen onto a food product while it is conveyed through an insu￾lated tunnel. 144 Meat refrigeration Table 7.5 Energy and cost requirements for beef plant freezing 3000 cartons per day Four-batch air Automatic air Manual plate freezers blast freezers Energy analysis Freezing time (h) 38 38 17 Freezer load (kW) 573 477 400 Power consumption (kW) 689 570 462 Economic analysis (NZ$000) Capital cost 1500 2200 2000 Annual capital charges 266 389 354 Annual energy costs 463 383 310 Annual labour cost 60 0 90 Total annual cost 789 772 754 Source: De Jong, 1994

Freezing of meat 145 Table 7.6 Advantages and disadvantages of total loss refrigerants in comparison with mechanical refrigeration Advantages Disadvantages Low capital investment High operating cost Igh refrigerating High refrigerating capaci Low weight when out of use High weight at start of use No residual weight(dry ice) Limited duration without fillin No noise or temperature control Advantageous storage atmosphere(N2) Reduced humidi Bacteriostatic affect(CO2) Suffocation hazard Low maintenance requirements Limited availability Foolproof once installed (dry ice) Impingement technologies are being used to increase heat transfer further(Newman, 2001). Newman states that when comparing the overall heat transfer coefficients of cryogenic freezing tunnels, impingement heat transfer is typically 3-5 times that of a conventional tunnel utilizing axial flow fans With the increased overall heat transfer coefficient. one can either increase the freezing temperature to increase overall cryogen efficiency of continue to run at very cold temperatures and dramatically increase the overall production rate. Impingement freezing is best suited for products with high surface area to weight ratios, for example hamburger patties or products with one small dimension. Testing has shown that products with a thickness of less than 20mm freeze most effectively in an impingement heat transfer environment. When freezing products thicker than 20mm, the benefits of impingement freezing can still be achieved, however, the surface heat transfer coefficients later in the freezing process should be reduced to balance the overall process efficiency. The process is also very attractive for products that require very rapid surface freezing and chilling 7.5 Freezing of specific products 7.5.1 Meat block James et al.(1979) showed that air temperatures below -30C and air velocities exceeding 5ms are required to freeze 15cm thick meat blocks in corrugated cardboard cartons in less than 24h(Fig. 7.3).Creed and James (1981)carried out a survey which indicated that only 58% of industrial throughput is frozen in times within +% of the actual freezing time 7.5.2 Beef quarters James and Bailey(1987a)reported that brine spray and liquid nitrogen immersion systems had been used to freeze beef quarters. However, most

Impingement technologies are being used to increase heat transfer further (Newman, 2001). Newman states that when comparing the overall heat transfer coefficients of cryogenic freezing tunnels, impingement heat transfer is typically 3–5 times that of a conventional tunnel utilizing axial flow fans.With the increased overall heat transfer coefficient, one can either increase the freezing temperature to increase overall cryogen efficiency or continue to run at very cold temperatures and dramatically increase the overall production rate. Impingement freezing is best suited for products with high surface area to weight ratios, for example hamburger patties or products with one small dimension. Testing has shown that products with a thickness of less than 20 mm freeze most effectively in an impingement heat transfer environment. When freezing products thicker than 20mm, the benefits of impingement freezing can still be achieved, however, the surface heat transfer coefficients later in the freezing process should be reduced to balance the overall process efficiency. The process is also very attractive for products that require very rapid surface freezing and chilling. 7.5 Freezing of specific products 7.5.1 Meat blocks James et al. (1979) showed that air temperatures below -30 °C and air velocities exceeding 5 m s-1 are required to freeze 15 cm thick meat blocks in corrugated cardboard cartons in less than 24 h (Fig. 7.3). Creed and James (1981) carried out a survey which indicated that only 58% of industrial throughput is frozen in times within ±20% of the actual freezing time required. 7.5.2 Beef quarters James and Bailey (1987a) reported that brine spray and liquid nitrogen immersion systems had been used to freeze beef quarters. However, most Freezing of meat 145 Table 7.6 Advantages and disadvantages of total loss refrigerants in comparison with mechanical refrigeration Advantages Disadvantages Low capital investment High operating cost High refrigerating capacity High refrigerating capacity Low weight when out of use High weight at start of use No residual weight (dry ice) Limited duration without filling No noise Poor temperature control Advantageous storage atmosphere (N2) Reduced humidity Bacteriostatic affect (CO2) Suffocation hazard Low maintenance requirements Limited availability Foolproof once installed (dry ice)

146 Meat refrigeration EO°Ni9寸E° Fig. 7.3. Freezing time of 15 cm thick boxed blocks(source: James et aL, 1979). Table 7. Ay zing time from4to-7° thermal centre of 50, 75 and 100kg beef quarters 50k 100k Hindquarter Forequarter Source: James et al.. 1987 investigations had used air Temperature in air systems ranged from -ll to 40C and weight loss from 0.3 to 1. 19%. In their own investigations beef quarters ranging in weight from 40 to 140kg were frozen in air at -32C Im s On average, hindquarters below 50kg and forequarters below 75kg could be frozen in a 24 h operation (Table 7.7). There was no statistical dif ference in bacterial counts before and after freezing. 7.5.3 Mutton carcasses Mutton production is seasonal and continuity of supply for processing can be achieved by frozen storage and subsequent thawing and boning(Creed and James, 1984) Data from the investigations of Creed and James were used to verify a predictive program for freezing of mutton carcasses. The predictions indi- cated that any condition more severe than -20C, 0.5ms, would achieve a 24 h freezing operation for unwrapped carcasses(Table 7. 8). To guaran tee an overnight(15-16h) freezing cycle for wrapped carcasses, conditions more severe than -30C, 4ms, would be required. 7.54 ofal Although edible offal comprises 3-4% of the cold weight of a carcass there little published data on its refrigeration(Creed and James, 1983 ). The

investigations had used air. Temperature in air systems ranged from -11 to -40 °C and weight loss from 0.3 to 1.19%. In their own investigations beef quarters ranging in weight from 40 to 140kg were frozen in air at -32 °C, 1.5 m s-1 . On average, hindquarters below 50 kg and forequarters below 75 kg could be frozen in a 24 h operation (Table 7.7). There was no statistical dif￾ference in bacterial counts before and after freezing. 7.5.3 Mutton carcasses Mutton production is seasonal and continuity of supply for processing can be achieved by frozen storage and subsequent thawing and boning (Creed and James, 1984). Data from the investigations of Creed and James were used to verify a predictive program for freezing of mutton carcasses. The predictions indi￾cated that any condition more severe than -20 °C, 0.5 m s-1 , would achieve a 24 h freezing operation for unwrapped carcasses (Table 7.8). To guaran￾tee an overnight (15–16h) freezing cycle for wrapped carcasses, conditions more severe than -30 °C, 4 m s-1 , would be required. 7.5.4 Offal Although edible offal comprises 3–4% of the cold weight of a carcass there is little published data on its refrigeration (Creed and James, 1983). The 146 Meat refrigeration 0 20 40 60 80 100 120 38 25 55 38 112 75 –30 –20 –10 Time from 4 to –7 °C (h) Air temperature (°C) 0.5 m/s 5.0 m/s Fig. 7.3. Freezing time of 15 cm thick boxed blocks (source: James et al., 1979). Table 7.7 Average freezing time from 4 to -7 °C at thermal centre of 50, 75 and 100 kg beef quarters Weight 50 kg 75 kg 100 kg Hindquarter 22 29 33 Forequarter 13 20 25 Source: James et al., 1987