《肉制品冷冻技术》（英文版） Part 7 Freezing of meat.pdf_大学文库

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

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 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 temperature, 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 automatic 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 substantially shorter freezing times (Fleming et al., 1996). Twice as many freezing 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

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 refrigerant, 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 transfer 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 refrigerants 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 insulated 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)