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 individuals 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 tempered 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 freezing 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). Petrovic 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 waterbinding 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 cryogenics was >2% less than in those using air freezing. The most recent comparison (Sundsten et al., 2001) revealed some commercial 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. CastellPerez 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 economical, 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 purposebuilt 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 individual 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 reduced 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 transfer 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 carcasses 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 overcomes 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 mechanical 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