8 Thawing and tempering Thawing has received much less attention in the literature than either chill- ing or freezing. In commercial practice there are relatively few controlled thawing systems. Frozen meat, as supplied to the industry, ranges in size and shape from complete hindquarters of beef to small breasts of lamb, although the major- ity of the material is ‘boned-out’ and packed in boxes ca. 15 cm thick weigh- ing between 20 and 40kg. Thawing is usually regarded as complete when the centre of the block or joint has reached 0 °C, the minimum temperature at which the meat can be boned or cut by hand. Lower temperatures (e.g. -5 to -2 °C) are acceptable for meat that is destined for mechanical chop- ping, but such meat is ‘tempered’ rather than thawed. The two processes should not be confused because tempering only constitutes the initial phase of a complete thawing process. Thawing is often considered as simply the reversal of the freezing process. However, inherent in thawing is a major problem that does not occur in the freezing operation. The majority of the bacteria that cause spoilage or food poisoning are found on the surfaces of meat. During the freezing operation, surface temperatures are reduced rapidly and bacterial multiplication is severely limited, with bacteria becoming completely dormant below -10 °C. In the thawing operation these same surface areas are the first to rise in temperature and bacterial multiplication can recom- mence. On large objects subjected to long uncontrolled thawing cycles, surface spoilage can occur before the centre regions have fully thawed. Most systems supply heat to the surface and then rely on conduction to transfer that heat into the centre of the meat. A few systems use electro- magnetic radiation to generate heat within the meat. In selecting a thawing system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of product, processing problems such as effluent disposal and the capital and operating costs of the respec- tive systems. Of these factors, thawing time is the principal criterion that governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed condi- tion but are less so if the meat is for processing. 8.1 Considerations The design of any thawing system requires knowledge of the particular environmental or process conditions necessary to achieve a given thawing time, and the effect of these conditions on factors such as drip, evaporative losses, appearance and bacteriological quality. The process of freezing a high water content material such as meat takes place over a range of temperatures rather than at an exact point, because as freezing proceeds the concentration of solutes in the meat fluid steadily increases and progressively lowers the freezing temperature. Thawing simply reverses this process. Thawing time depends on factors relating to the product and the envi- ronmental conditions, that include: ? dimensions and shape of the product, particularly the thickness ? change in enthalpy ? thermal conductivity of the product ? initial and final temperatures ? surface heat transfer coefficient ? temperature of the thawing medium. The total amount of energy that must be introduced into the product is equal to the enthalpy change between the initial temperature and the average temperature required within the material after thawing. For the thawing process to be complete, no ice should remain and the minimum temperature has to be above -1 °C. To thaw 1kg of meat from a starting temperature of -40 °C would require the addition of 300 kJ of energy if the meat was very lean, falling to about 180 kJ if very fat. Frozen meat that requires boning has to be completely thawed. However, an increasing pro- portion of meat is boned before freezing and if it is subsequently used in products such as pies, sausages, and so on, it can be cut by machine in a semi-frozen (tempered) state. To temper meat from -40 °C to an average temperature of -4 °C requires a heat input of approximately 100 kJkg -1 , only one third of that required for complete thawing. Thermal conductivity has an important effect on thawing. The conduc- tivity of frozen lean meat is three times that of the thawed material. When thawing commences, the surface rises above the initial freezing point. Sub- 160 Meat refrigeration sequently, an increasing thickness of poorly conducting material extends from the surface into the foodstuff, reducing the rate of heat flow into the centre of the material. This substantially increases the time required for thawing. The main environmental factors are the temperature of the thawing medium and the surface heat transfer coefficient (h) which is a function of the shape and surface condition of the product, the thawing medium used and its velocity. Except for very simple configurations, h cannot be derived theoretically and must be measured experimentally. Few such measure- ments have been made for the thawing of foodstuffs (Arce and Sweat, 1980; Vanichseni, 1971), but typical ranges of h for the main thawing systems are given in Table 8.1. In air thawing, h is not constant and is a function of relative humidity (James and Bailey, 1982). In the initial stages, water vapour condenses onto the frozen surface, immediately changing to ice. This is followed by a stage where vapour condenses in the form of water until the surface temperature is above the dew point of the air and all condensation ceases. The varying rate of condensation produces substantial changes in the value of h during the thawing process. 8.2 Quality and microbiological considerations There are few published data relating thawing processes to the palatability of meat and eating quality is generally independent of the thawing method. However, two reports indicated that cooking directly from the frozen state produced less juicy lamb rib loins (Woodhams and Smith, 1965) and less tender beef rolled rib joints (James and Rhodes, 1978) when compared with meat that had been thawed before cooking. The main detrimental effects of freezing and thawing meat is the large increase in the amount of proteinaceous fluid (drip) released on final cutting, yet the influence of thawing rate on drip production is not clear. There was no significant effect of thawing rate on the volume of drip in beef (Empey, 1933; Ciobanu, 1972) or pork (Ciobanu, 1972). Several authors Thawing and tempering 161 Table 8.1 Typical surface heat transfer coefficients (h) for different thawing systems System Surface heat transfer coefficients (W m -2 K -1 ) Air-free convection 5–15 Air-forced convection 10–70 Water 100–400 Vacuum steam heat 150–1000 Plate 100–300 (Cutting, 1974; Love, 1966) concluded that fast thawing rates would produce increased drip, while others showed (Finn, 1932; Singh and Essary, 1971) the opposite. Thawing times from -7 to 0°C of less than 1min or greater than 2000 min led to increased drip loss (James et al., 1983). The results are therefore conflicting and provide no useful design data for optimising a thawing system. The principle criteria governing quality of thawed meat are the appear- ance and bacteriological condition. These are major factors if the product is to be sold thawed but are less important if the food is destined for pro- cessing and heat treatment. Microbiological problems can arise during thawing of food in bulk.While centre temperatures may not exceed 0 °C, the exterior surface may be held at 10–15 °C for many hours, or even days. During this time extensive growth of spoilage organisms can occur on the surface.The time required for micro- biological numbers to reach ‘spoilage’ levels will largely be dependent upon the number of microbes initially present and the temperature. Since freez- ing and frozen storage have little effect on the number of viable microbes present, material of poor microbiological quality before freezing is likely to spoil more quickly during thawing (Roberts, 1974). The use of high thawing (>10 °C) temperatures for carcass meats tends to lead to large increases in microbial numbers (James and Creed, 1980; Bailey et al., 1974). Little published data exist on microbiological effects of thawing meat. Buttiaux (1972) reported that water thawing was more successful for beef than for pork if the meat was to be stored. Consequently, care must be exer- cised in extrapolating from one meat species to another. Results with pork suggested that air thawing gives final counts about ten times higher than thawing in 3% brine, whereas with beef Heinz (1970) reported counts lower by a factor of about 10 for air (4–5ms -1 ; 10 °C) as opposed to flowing water (10 °C). Kassai (1969) also found no significant increase in bacteriological numbers when thawing beef carcasses in air (0.2–0.3m s -1 , 15–20°C, 96% relative humidity, (RH)). Shoulders of lamb (Vanichseni et al., 1972) thawed in air (0.2 m s -1 ; 18 °C) or water (45°C) had bacterial counts that increased respectively by factors of 1.74 and 1.12; humidity and air velocity also influ- enced the results of air thawing. It is often asserted that thawed food is more perishable than fresh or chilled produce, but experiments (Kitchell and Ingram, 1956; Kitchell and Ingram, 1959) have failed to demonstrate any difference of practical significance between the growth of meat spoilage organisms on fresh or thawed slices of meat. Greer and Murray (1991) found that the lag phase of bacterial growth was shorter in frozen/thawed pork than in fresh pork, while the generation time was unaffected. Under commercial conditions, microbiological sampling of frozen meat may be of limited relevance. Small frozen samples will be thawed in a la- boratory, probably under conditions unlike those used later to thaw whole blocks. On the laboratory samples, extensive microbial growth during 162 Meat refrigeration thawing is unlikely, while on commercial blocks it is probable. Hence, the laboratory count reflects the number of microbes on the frozen meat but not necessarily on meat after commercial thawing. Microbial counts incubated at 1 °C and 20–25 °C assess the storage life of meat at chill and intermediate temperatures. Counts at 37 °C give an indi- cation of contamination from human and animal sources. Thawing under conditions that permit growth of bacteria counted at 1°C and 25 °C will result in meat of poorer quality in terms of storage life. Thawing conditions allowing heavy growth of bacteria counted at 37 °C are undesirable since food-poisoning bacteria (such as Salmonella spp.) may be capable of growth. The appearance of the surface of thawed meat is similarly related to the time spent in a given environment. Since this time will be a function of the material thickness, it is not possible to define one overall set of conditions for optimal appearance. For example, the air temperature, velocity and rela- tive humidity required to thaw small joints satisfactorily in a reasonably short time would almost certainly cause problems if used to thaw whole quarters of beef. In general both the appearance and final bacterial condi- tion in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unaccept- able for other reasons related to operating requirements. A compromise must therefore be reached which for a given material could well differ from one factory to the next. 8.3 Thawing systems There is no simple guide to the choice of an optimum thawing system (Table 8.2). A thawing system should be considered as one operation in the pro- duction chain. It receives frozen material which should be within a known temperature range and of specified microbiological condition. It is expected to deliver that same material in a given time in a totally thawed state. The weight loss and increase in bacterial numbers during thawing should be within acceptable limits, which will vary from process to process. In some circumstances, for example a direct sale to the consumer, the appearance of the thawed product is crucial, in others it may be irrelevant. Apart from these factors the economics and overall practicality of the thawing opera- tion, including the capital and running costs of the plant, the labour require- ments, ease of cleaning and the flexibility of the plant to handle different products, must be considered. 8.3.1 Conduction The main conduction-based thawing methods rely on air, water or steam condensation under vacuum. Thawing and tempering 163 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. 8.3.1.1.1 Still air Thin blocks (<10 cm) 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 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 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 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 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 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, 5ms -1 , 85–88% RH, to a 0.51 weight gain at 25°C, 1ms -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, low- ering the surface temperature to that of the ambient air. Since this tem- perature 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 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. 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 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 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 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). 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 homoge- neous, 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 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 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 tempera- ture 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 placing the bellies in oil, water and saline baths and wrapping the meat in cheesecloth 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 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 electro- magnetic 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, 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- perature 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 wide- spread 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 shoul- ders 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 increasing the water velocity had very little additional effect. VHT was not appreciably faster than water thawing at any temperature, demonstrating that for materials of this thickness conductivity is the rate controlling factor. The pork legs increased in weight by 1 ±0. 3% under any of the condi- tions of thawing in air or water, with the exception of high velocity air where losses of l% were recorded (Table 8.3). A similar increase was recorded in VHT at 10 °C but there were small losses of weight at 20 and 30 °C. Thawing and tempering 169 5000 4000 3000 2000 1000 500 400 300 200 100 40 30 20 10 50 A B C D E 0 10 20 30 40 50 Thawing time (h) A = Vacuum steam heat B = Water, 0.023 m s –1 C = Water, 0.006 m s –1 D = Air, 5 m s –1 E = Air, 0.25 m s –1 3 kg leg. –30 ° to 0 ° C 6 kg leg. –30 ° to 0 ° C Thawing from –10 ° to 0 ° C (Shaded area shows reduction in thawing time for initial temperature of –10 ° C under the fastest and slowest thawing conditions) 40 ° C30 ° C20 ° C10 ° C5 ° C Temperature of thawing medium Surface film heat transfer coef ficient (W m –2 ° C –1 ) Fig. 8.2 Prediction of thawing times of frozen pork legs from -30–0 °C (source: Bailey and James, 1974a). Table 8.3 Mean percentage weight losses (fresh to thawed states) for pork legs thawed in air, water or vacuum Thawing Velocity of Thawing temperature (°C) medium medium (m s -1 ) 10 20 30 Air 0.25 +0.9 (18) +1.0 (27) +1.1 (20) 5.5 +0.9 (18) -1.0 (14) -1.0 (15) Water 0.006 +1.0 (8) +1.1 (10) +1.2 (8) 0.023 +1.2 (8) +1.1 (13) +0.7 (6) VHT +1.3 (14) -0.2 (14) -0.6 (13) () Number of samples. Source: Bailey and James, 1974a. The surface of legs thawed in air at 10°C, 85% RH at low velocity was moist but not wet and the colour of both skin and cut surface was good. At higher temperatures, the appearance was less attractive. The skin of legs thawed at high air velocity was light brown in colour and rather dry and parchment like. This condition did not improve with storage. Legs from the water and VHT systems were very wet and the colour of the cut surface was extremely pale. However, considerable improvements in the condition of the surface and in the colour of legs thawed at 10 °C were noted after holding for some time in a chill room. Changes in bacterial numbers during thawing in air or water could not be related solely to the temperature of the thawing medium or its velocity; in both cases, the interactions of thawing medium and its velocity were significant (Table 8.4). Air at 10°C/0.25 m s -1 gave the best result and both 10°C/5.5ms -1 and 20 °C/0.25 m s -1 were satisfactory. In water, decreases in bacterial counts were obtained at 10°C/0.023 m s -1 and small increases at 10°C/0.006ms -1 . In VHT, the difference between initial and final bacterial numbers increased with increased thawing temperature. 170 Meat refrigeration Table 8.4 Bacterial changes in pork legs thawed in air, water or VHT Thawing Temp. Velocity Number Mean bacterial count (log 10 cm -2 ) medium of of of at incubation temperature medium medium samples Initial (before Final (after(°C) (m s -1 ) freezing) thawing) 37 °C 25 °C 1 °C 37 °C 25 °C 1 °C Air 10 0.25 6 4.4 5.0 4.7 4.3 4.6 5.2 5.5 6 4.1 4.9 5.0 4.6 5.5 5.6 20 0.25 6 4.2 4.9 4.9 5.1 5.4 5.1 5.5 6 2.7 3.3 1.6 5.2 6.3 6.1 30 0.25 6 3.4 4.2 1.1 4.6 5.0 3.9 5.5 6 3.5 4.1 2.5 4.2 4.4 4.6 Water 10 0.006 10 4.8 7.0 – 5.1 7.5 – (4.5) (5.6) 0.023 10 5.7 7.7 – 5.0 7.0 – (4.4) (5.6) 20 0.006 10 4.8 7.4 – 6.0 6.9 – (6.3) (7.1) 0.023 10 5.1 6.6 – 6.1 6.7 – (6.0) (6.2) 30 0.006 10 5.2 6.9 – 6.6 7.2 – (6.6) (7.7) 0.023 10 4.8 6.9 – 7.0 7.4 – (6.1) (6.6) VHT 10 – 6 3.7 4.8 4.8 3.3 4.1 5.0 20 – 6 4.3 5.1 3.6 5.8 6.2 6.0 30 – 6 3.5 4.8 3.1 4.8 5.2 3.4 Source: Bailey et al., 1974. Two-stage air thawing of hams was the subject of a patent issued to the Danish Meat Research Institute. It was claimed that thawing times could be reduced by thawing in near saturated air at 45°C until the surface ap- proaches 30 °C followed by subsequent thawing in air at 12 °C and 90% RH. Little information was given on appearance and bacteriological condition. 8.3.3.2 Thawing of lamb shoulders and carcasses Vanichseni et al. (1972) found that the thawing times of lamb shoulders (3.9–4.6 kg) ranged from 60 h in air at 2 °C, 70% RH and 0.1 ms -1 to 2h in water at 50 °C and 0.05 m s -1 .The appearance of water-thawed shoulders was judged to be quite attractive after drying and equilibration for 30 min in air at 10 °C, whilst that of air-thawed shoulders deteriorated with increasing air velocity and decreasing relative humidity. A high relative humidity was maintained to minimise weight loss and no significant changes in bacterial numbers were recorded on the small number of samples tested. From their data, Creed et al. (1979) produced a time–temperature rela- tionship plot for determining thawing times of wrapped and unwrapped lamb carcasses at air velocities of 0.75 and 2.25 m s -1 over the temperature range from 5 to 20 °C (Fig. 8.3). Applying this relationship to commercial requirements they concluded that the optimal condition for thawing wrapped lambs in 24 h was to use air at 10 °C and 0.75 m s -1 and for unwrapped lambs 7.5°C and 0.75ms -1 . For overnight schedules (ca. 15h) wrapped lambs would require air at 20 °C and 0.75m s -1 and unwrapped lambs 15 °C and 0.75 m s -1 . Changes in bacterial numbers for these condi- tions would be insignificant and produce meat of sufficiently good appear- ance to be sold in the thawed state. Thawing and tempering 171 20 15 10 5 10 20 30 HU LU HW LW Thawing time from –30 to 0 °C (h) Thawing temperature ( ° C) 40 Fig. 8.3 Relationship between thawing time and thawing temperature for lambs of 20kg average weight, thawed unwrapped (U), wrapped (W) at 0.75 m s -1 (L) or 2.25 m s -1 (H) (source: Creed et al., 1979). 8.3.3.3 Thawing of beef quarters Heinz (1970) investigated the thawing of beef hindquarters in water at 10 °C and concluded that water thawing had no advantage over air thawing in terms of thawing time. Subsequent drying in air was thought to be a nec- essary process, after which the quarters showed weight gains of 0.5–1.7% over the weight in the frozen state. Surface bacterial counts were a factor of 10 greater than those thawed in air. The EEC policy of intervention purchase, freezing and storage of beef quarters created a demand for data on thawing such cuts, and consequently an experimental investigation was carried out by James et al. in 1977. This work was consequently extended to provide comprehensive charts for the determination of thawing times of frozen beef forequarters (Fig. 8.4) and beef hindquarters (Fig. 8.5) over a range of air, water and VHT thawing conditions (James and Creed, 1980). Thawing quarters at 5 and 10°C had little effect on appearance. The use of higher temperatures led to a darkening of the lean, most marked on the cut surfaces, and the drying out of thin sections, especially on the fore- quarters, giving a parchment like appearance. Owing to this deterioration all quarters that were thawed at 30 °C, and those without very good fat cov- ering that were thawed at 20°C, were only suitable for further processing. 172 Meat refrigeration 10 50 100 500 1000 5000 20 30 40 50 60 70 80 90 Thawing time (h) Air, 0.25 m s –1 Air, 5 m s –1 Water, 0.006 m s –1 Water, 0.023 m s –1 Vacuum steam heat 30 ° C20 ° C10 ° C5 ° C 70 kg quarter 60 kg quarter 50 kg quarter Surface film heat transfer coef ficient (W m – 2 ° C – 1 ) Fig. 8.4 Predicted relationship between thawing time from -30–0 °C and surface film heat transfer coefficient for frozen beef forequarters (source: James and Creed, 1980). Quarters with a good fat covering thawed at 20°C were of retail quality after some trimming.A small number of quarters were thawed using a high humid- ity (95% RH) at 30°C. This procedure stopped drying but produced a pale damp slimy surface that was deemed to be commercially unacceptable. Changes in bacterial numbers showed there to be a trend for the final count to increase with increased thawing temperature.This was most clearly seen with hindquarters thawed at 30°C, in which counts incubated at 35, 25 or 1 °C were significantly higher than those from any other thawing tem- perature (Table 8.5). Temperature curves showed that during such thawing the surface spent times in excess of 40 h at temperatures above 5 °C. The equivalent figure for forequarters was only 20h, but in a commercial situa- tion both hindquarters and forequarters would be left to thaw for the same time resulting in similarly large increases in the bacterial counts on the forequarters. 8.3.3.4 Thawing of boned-out meat blocks The majority of frozen boned-out meat, ranging in size from large primal joints to small pieces and trimmings, is packaged in 25 kg lots within solid or corrugated fibreboard cartons, usually containing a polyethylene sheet inner liner. An average carton size is 61 ￥ 40 ￥ 15 cm (Creed and James, 1981). Minimum thawing times are attained by defrosting the blocks in the unwrapped state, but it is not always possible to remove the polyethylene liner prior to thawing. Thawing and tempering 173 10 50 100 500 1000 5000 20 30 40 50 60 70 80 90 100 110 Thawing time (h) Air, 0.25 m s –1 Air, 5 m s –1 Water, 0.006 m s –1 Water, 0.023 m s –1 Vacuum steam heat 30 ° C20 ° C 10 °C5 °C 70 kg quarter 60 kg quarter 50 kg quarter Surface film heat transfer coef ficient (W m – 2 ° C – 1 ) Fig. 8.5 Predicted relationship between thawing time from -30–0 °C and surface film heat transfer coefficient for frozen beef hindquarters (source: James and Creed, 1980). 174 Meat refrigeration Table 8.5 Bacterial changes on beef quarters thawed in air at 5, 10, 20 and 30 °C Temp. Number Mean bacterial count (log 10 cm -2 ) at incubation temperature of of Initial (before freezing) Final (after thawing) (°C) 37 °C 25 °C 1 °C 37 °C 25 °C 1 °C Fat Lean Fat Lean Fat Lean Fat Lean Fat Lean Fat Lean Fores 5 13 3.47 2.89 4.30 3.49 3.84 3.12 3.64 3.04 4.43 4.20 4.69 4.27 10 13 3.58 2.77 4.13 3.37 3.53 2.89 3.71 4.07 4.55 6.11 4.31 5.92 20 18 3.40 3.02 3.91 3.63 3.52 3.10 4.74 4.43 4.91 5.32 4.54 5.09 30 12 4.16 3.55 4.73 4.05 4.13 3.46 6.09 5.12 5.92 5.70 5.63 5.02 Hinds 5 10 3.44 3.58 3.82 4.03 2.89 3.00 3.82 3.78 4.75 4.64 4.30 4.24 10 13 3.28 3.68 4.31 4.30 3.72 3.59 4.55 4.48 5.39 5.49 5.04 5.23 20 14 3.64 3.86 3.86 4.37 3.41 3.46 5.45 5.26 5.93 5.81 5.20 5.52 30 12 3.88 3.54 4.73 4.52 4.24 3.83 6.45 6.70 7.39 7.37 6.98 6.96 Source: James et al., 1977. medium samples James and Bailey (1980) studied the thawing of such blocks in air and by VHT. This work was extended by Creed and James (1981) to provide a comprehensive chart (Fig. 8.6) relating the thawing of unwrapped boneless beef blocks to various environmental conditions from a combination of pre- dicted and experimental data. The commercial advantage of VHT in terms of reduced thawing time is small because conductivity is the rate control- ling factor for such thick materials. The real advantage of VHT is attained with thin (2.5 cm) blocks where thawing times of less than 1h are possible, but such blocks do not normally exist in commercial operations. Bailey and James (1974b) also examined two-stage air thawing of com- mercial meat blocks. For commercial convenience it is desirable that two- stage thawing operates with 7 day’s supply in the first (conditioning) phase and 1 day’s supply in the second (thawing) phase. The thawing phase envi- ronmental conditions in this experiment were defined by an industrial user as 10°C and 1ms -1 . The investigation was therefore concerned with estab- lishing the temperature and air velocity in the conditioning phase necessary to satisfy the requirements that: ? the final thawing period be as short as possible and no greater than 24h, ? the block remains in good physical condition at the end of the condi- tioning phase to allow movement to the second phase, i.e. no break-up or excessive drip, ? acceptable bacteriological levels are achieved at the end of both condi- tioning and thawing phases. Thawing and tempering 175 0 50 100 150 Thawing time (h) 1000 A B 500 C D 100 E 50 F G H 10 5 Surface film heat transfer coef ficient (W m – 2 ° C – 1 ) Thawing medium temperature 30 ° C20 ° C10 ° C5 ° C Code Thawing method Surface heat transfer coefficient, W m –2 ° C –1 Vacuum steam heat Water 0.023 m s –1 Water 0.006 m s –1 Water Spray 30 ml cm –2 min –1 Air 5 m s –1 Air 3 m s –1 Air 1 m s –1 Air 0.25 m s –1 A B C D E F G H 1000 600 400 200 60 40 30 15 13 cm thick block 16 cm thick block Fig. 8.6 Predicted relationship between thawing time from -30–0 °C and surface film heat transfer coefficient for unwrapped boneless meat block (source: Creed and James, 1981). It was shown that these requirements could be met if the polyethylene wrapped blocks were conditioned in air at +0.5°C and 0.25ms -1 . However, the method was only successful if the mean air temperature was exactly 0.5 °C. Increasing this temperature by 0.5 °C caused ‘overconditioning’ and consequent handling problems; decreasing the temperature by 0.5°C caused ‘underconditioning’ and final thawing times in excess of 24h. The process was therefore considered impracticable for commercial operations. James (1984) demonstrated that thawing times of wrapped meat blocks could be significantly reduced using a hybrid microwave/vacuum system. Using a prototype microwave/vacuum system (2.5kW, 915 Mhz), single 15 cm thick meat blocks inside solid fibreboard cartons were thawed in 1 or 2h cycles. Weight losses averaged 7.6%. Unpublished values from indus- trial thawing systems that handle similar types of blocks range from 3–10%. 8.3.4 Commercial practice The previous sections have described experimental data on meat thawing systems. The following few examples illustrate the wide range of thawing systems used by industry (James and Crow, 1986). Despite considerable interest in the use of different thawing systems, for example vacuum/steam, plate and microwave, all the commercial processes investigated used either air or water. 8.3.4.1 Air systems 1 30 tonnes per day of intervention beef quarters were thawed for boning in a modified chill room from -18 °C to a minimum deep temperature above 0 °C. The process was carried out in two stages. In a first stage, lasting 38 h, live steam was injected into the room to maintain it at 10–12 °C and 95% RH. The air temperature was then reduced and maintained at 0–1 °C for a further 6 h. This system was very effective, achieving a maximum surface temperature of 10°C and a small tem- perature difference, 0–2 °C, throughout most of the quarters at the end of thawing. Average weight loss was 0.2%. The only real problem was caused by the wide weight range, 60–110 kg, of quarters being thawed. To avoid the overthawing of light and underthawing of heavy quarters the meat was sorted into weight groups and the heavy quarters placed in the area with the highest air velocity. 2 100 tonnes per week of manufacturing beef in 15cm thick cartons were thawed by a canned meat company in a large (46 ￥ 15 cm) uninsulated shed with heater units. Pallets of cartoned meat were placed in an adja- cent cold store at 0 °C for 16h before the packaging was removed and the blocks restacked in a single layer on racks. The racks were then placed in front of the heater units for 24–48 h. During this time some were moved away from the heaters to a cooler part of the room and any fully thawed material not immediately required was moved back into 176 Meat refrigeration the cold store. Meat entering the process varied in temperature from - 15 to -3 °C and the aim was to have the meat fully thawed but below 7 °C on exit. In general this aim was achieved, however, some surfaces rose to ambient temperature while ice crystals were found in deep tissues. The method was very weather dependent and required double handling, almost constant supervision, and its operation relied heavily on the subjective skills of the operator. Drip was a problem. 3 A large company that deboned beef quarters and lamb legs and pre- pared them for cooking or refreezing, and cut, diced or minced beef primals, used three slightly different thawing methods. All the frozen meat entered the thawing systems at -20 °C. Cartons of beef primal and lamb legs, and wrapped quarters were placed in factory air heated to 16 °C by propane heaters. Lamb legs were thawed for 16–18h and beef quarters and primals for 60 h. Other quarters were thawed for 7 days in a chill room at 4–5°C. Cartons of beef primals and lamb legs on pallets were also thawed in a combined system for 3–4 days at 4–5°C then 12–16 h at 16 °C. The first method tended to produce very variable thawing, with ice still present in the centre, discolouration at the surface and high drip losses. The others produced good results but were very slow. 8.3.4.2 Water systems 1 One hundred 27 kg cartons of pork legs, beef topsides or silversides were thawed per day by one company in tubs of mains water. The tubs 45 ￥ 75 ￥ 150 cm were loaded with either 70 legs of pork or joints from 20 cartons of beef. Water was supplied to the tubs by hose and overflowed onto the floor, with effective circulation only occurring in the top 5 cm. Thawing times were typically 36–42h in winter when the water tem- perature was 4 °C reducing to 16–24 h in summer. The system was con- sidered to be satisfactory but trimming and deboning of pork was difficult if thawing was not complete. Up to 3% in weight was lost from the silversides. 2 A large canning company thawed 10 tonnes a day of 10 cm thick frozen blocks of kidney in tanks of water. The blocks on pallets were removed from a -l8 °C cold store and left for a few hours in the ambient air before the carton and wrappings were removed. The blocks then thawed for 12 hours in static water before the taps were turned on and thawing con- tinued for 5–12 h. At the end of the cycle the kidneys at the top of the tank were at the water temperature while some at the bottom would still be frozen. Complete removal of the polyethylene film was also a real problem. Introducing compressed air into the bottom of the tank reduced temperature stratification but caused frothing. 3 One company thawing 1–1.5 tonnes of beef topsides per day found that final temperature affected weight gain. At -1–0 °C the gain was 2.1%, 0–4 °C, 1.3% and at 4–6 °C, 0.2%. The joints were placed in warm water Thawing and tempering 177 at 35 °C and within an hour the water cooled to 10 °C. Ten hours later the temperature was 6 °C at the top and 2 °C in the middle of the tank. Warm water was added again after 18h. The addition of some form of water circulation and temperature control would have produced a more consistent final product. 4 In another application a purpose-built thawing system steam heated water in a reservoir and then passed it through 90 ￥ 120 ￥ 90 cm tanks at a constant temperature of 10 °C. Blocks of tongue thawed from -20 °C to between 0 and 2°C in an overnight 14–18 h cycle. This system showed the advantage of designing a system to meet the requirements of the product. 8.4 Tempering Hamburgers, sausages, canned meats, pet foods, frozen prepared foods, portion controlled steaks and specialities rely heavily on frozen ingredients. Much of this frozen meat is tempered rather than thawed before process- ing. Tempering as an alternative to thawing eliminates the accompanying problems of drip loss, bacterial growth and other adverse changes. 8.4.1 Requirements for cutting and processing equipment Tempering of frozen block meat allows much better control over product quality and texture and enables the use of a greater percentage of frozen ingredients. Frozen meat blocks are processed from both a frozen and a tempered condition in the manufacture of sausages, canned meats and hamburger patties. The processing of frozen (untempered) blocks that are prebroken is somewhat limited. Grinding employs torque, a force that produces a twisting effect. When grinding through a perforated plate with small openings, 3 mm, this force on frozen meat lowers the freezing point allowing it to pass through the plate and the meat refreezes as it exits. This phenomenon is called regelation and most processing equipment down- stream is unable to accommodate this frozen putty type material (Koberna, 1986). Chopping in a rotating bowl will shatter frozen meat with little control over particle size. However, if warm water and additives are introduced some products can be produced, particularly pet food. There are many examples of mechanical requirements for uniform product temperature (Koberna, 1986). If we again consider slicing and then dicing, the consequences of processing below the zone of optimum temper (undertempering) may be blade breakage and yield loss from shattered meat, excessive fines and slices of non-uniform thickness.The consequences of overtempering may also be in the form of yield loss from ragged edges and incomplete shearing of connective tissue resulting in pearling. Temper 178 Meat refrigeration for particle size and definition in comminuting is very important to the appearance and texture of coarse-ground sausages, meat sauces and yield in protein extraction. Tempering is important for heat balance considera- tions, such as controlling the temperature of ground meat in the patty forming machine or controlling the temperature of a sausage blend. Table 8.6 summarises some of the results associated with undertempering and overtempering for some meat processing applications. 8.4.2 Requirements for prebreaking The first unit operation in many meat product lines is termed prebreak- ing, and is the process of taking large meat pieces (especially in the form of frozen blocks, of up to 30kg each) and producing smaller pieces which can be handled by the primary comminution procedure. A variety of pre- breakers are available commercially, including chippers, guillotines, flakers and grinders, some of which can handle a 30kg block in its entirety, whilst others require the block to be divided initially, usually by band sawing. Thawing and tempering 179 Table 8.6 Effects of undertempering and overtempering on some meat processing applications Process Effects of undertemper Effects of overtemper Grinding Equipment failure, Crushes meat tissue, blood shattered or regelated loss meat, excessive fines Bowl chopping Shattering, blade damage, Connective tissue or gristle excessive fines oversized, soft meat overchopped Slicing Equipment failure, curled Incomplete or ragged slices and shattered slices Dicing Equipment failure, Incomplete dices, pearling excessive fines, broken dices Log pressing Equipment failure, plain Squeeze out (blood loss), slippage, shattering ragged ragged surface surface Individual portion Fracturing, incomplete die Squeeze out, forming not pressing filling possible Temperature control Equipment failure, lack of Lack of weight control, for patty forming weight control, incomplete ragged edges, sticky patties, forming, lack of patty bind wasted refrigeration Temperature control Unsatisfactory protein Protein loss, increased yield for sausage blending extraction and stuffing, loss in cooking, wasted smeared appearance refrigeration Before the meat enters the process, it is usually tempered, although some prebreakers are able to operate on hard frozen meat. Equipment failure through the use of meat that is too cold is a very clear effect of using undertempered meat, but, if we assume the meat is within the range the prebreaker can handle, the effects of different temperatures at prebreaking may be more subtle. Similarly, although it seems to be well known that prebreaking hard frozen meat is detrimental to product cohe- sion or ‘bind’, information on other quality aspects is hard to come by. The influence of prebreaking on product quality has received less attention than the effects of later stages of manufacture, and what literature there is has usually been restricted to the use of meat grinders for prebreaking beef. Some characteristics of frozen meat are worth mentioning before product quality is considered. Information on the tensile properties of frozen and thawed lean beef has shown that anisotropy of tensile strength according to fibre orientation is far less marked in frozen beef than in thawed beef (Munro, 1983). From changes in the ultimate tensile strength as a function of strain rate, the shape of stress/strain curves, and the observed mode of fracture and appearance of fracture surfaces, it has been suggested that frozen beef exhibits viscoelastic fracture at high tempera- tures and low strain rates, and brittle fracture at low temperatures and high strain rates (Munro, 1983). This tendency towards brittle fracture with lower temperature is the reason frozen meat shatters under various practical conditions, such as bowl chopping. Tensile strength and sample modulus (the maximum slope of the stress/strain curve prior to fracture) increased with decreasing temperature, the most marked increase occurring between 0 °C and -10 °C. Dramatic changes in the physical properties of meat also occur over this range (Miles, 1974). 8.4.2.1 Effect on cooking loss and ‘bind’ Gumpen (1978) showed that prebreaking at low temperatures produces poor quality emulsion type sausages. Meat which had been prebroken at -20 to -30 °C, then thawed, gave products with reduced fat binding, showed considerable oiliness, and had a loose and unsatisfactory texture. In a second study (Harbitz and Egelandsdal, 1983) similar sausages from meat ground at -20 or -7 °C were judged less hard, more coarse, more adhesive, more oily, more juicy, and lost more fat on microwave reheating than those from meat ground at -2, -0.5 (at which temperature the meat was presumably thawed) or +4 °C. Cooking losses from beefburgers made from the meat prebroken at either -20 or -7 °C were greater than the other treatments. At Langford, flaked and formed patties were made from beef tempered to either -8 C or -3 °C then prebroken by grinding. Prebroken meat from each temperature was then ‘retempered’ before flaking in order to check for any interactive effect between the two operations. Patties prebroken 180 Meat refrigeration and flaked at -8 °C had significantly greater cooking loss than the other treatments (Table 8.7). Instrumentally assessed ‘texture’ (Jones et al., 1985) was also influenced by prebreak temperature; with meat flaked at -3°C, resistance to deformation, compressive strength and residual strength were all significantly reduced in products from meat prebroken at -8 °C than that from meat prebroken at -3°C. The fact that grinding causes more shattering (i.e. brittle fracture) at -8 °C than at -3°C can be simply but dramatically demonstrated by com- paring the number of particles present in a constant mass sample. Using video image analysis, differences in particle size distribution have been observed between meat prebroken at -5°C and at -3.5°C. The effect on particle size is potentially of direct relevance because the perception of particle size is a key aspect of quality in this type of product (Jones et al., 1985), but its effect on bind is puzzling. In the presence of salt, myofibrillar proteins should be easier to extract from smaller pieces, and the accepted wisdom is that this will lead to better bind (although an increased proportion of smaller particles produced at lower temperatures can be expected to produce a ‘mushy’ texture (Dransfield et al., 1984) in the absence of salt). However, there seems to be good agreement that lower temperature at prebreak leads to poorer bind. Particles of the same size but produced at different temperatures could conceivably have different struc- tures that might influence bind; an earlier suggestion that the high pressures and shear forces occurring during grinding colder meats leads to denatura- tion of proteins (Gumpen, 1978) is probably not correct (Jolley et al., 1986). Evans and Ranken (1975) established that increased cooking losses from meat ground whilst frozen are attributable to the release of free lipid from broken fat cells. Thawing and tempering 181 Table 8.7 Effect of prebreaking by grinding at either -3 or -8 °C on cooking loss and ‘texture’ of flaked and formed beef patties Temperature at prebreaking Significant (°C) -3 -8 difference Temperature at flaking (°C)* -3 -8 -3 -8 Cooking loss (% uncooked 25.6 b 27.3 b 25.6 b 32.1 a <0.05 weight) Resistance to deformation 5.3 a 4.6 b 4.4 b 4.7 a,b <0.001 (N cm -1 ) Compressive strength (N cm -1 ) 352.5 a 299.7 a,b 274.8 b 287.8 b <0.001 Residual strength (N cm -1 ) 826.3 a 511.0 d 683.0 b 575.2 c <0.001 Means in the same row with the same superscript are not significantly different (P > 0.5). * Prebroken meat was retempered by holding at indicated temperature for at least 72 h before flaking. Source: Ellery, 1985. 8.4.2.2 Influence on other aspects of quality Poor control over temperature at prebreaking has been blamed for vari- ability in the amount of fluid released when a cooked, flaked and formed patty is cut (Jolley and Rangeley, 1986); the phenomenon is known by names such as ‘welling’ or ‘bursting’. The effect seemed to depend on the degree of comminution (Table 8.8). With comparatively coarse comminu- tion (240 flake head) more fluid was released from patties made from meat prebroken at -3 °C than those from meat prebroken at -6°C. The reverse was true for comparatively fine comminution (further grind through 10mm plate, then flaked through 120 head). The juice expelled from finely com- minuted products made from meat prebroken at -3 °C had proportionally more fat than that from meat prebroken at -6°C. These results on ‘welling’ come from a factory-based experiment aimed at assessing the importance of interactions between three processing factors (temperature at prebreaking, degree of comminution and blend time) and two raw material factors (level of added fat and level of added salt). Products (excluding those finely comminuted) from the same study were assessed for eating quality by consumer panels at two locations. Results obtained using simple analysis of variance are shown in Table 8.9 which shows that the effect of temperature at prebreaking again depended on another factor, this time the level of salt in the final product. With 1% salt, patties produced from meat prebroken at -6 °C tended to be favoured; with 0.5% salt the effect of prebreak temperature was clearer, with prod- ucts from meat at -3 °C being favoured, and considered more meaty. 8.4.3 Microwave tempering Despite the widespread industrial use of tempered meat there is little pub- lished process design data for meat tempering operations, with the excep- tion of commercial claims for microwave processing units. 182 Meat refrigeration Table 8.8 Effect of prebreaking by grinding at either -3 or -6 °C on cooked yield and ‘welling’ of flaked and formed beef patties Temperature at prebreaking (°C) -3 -6 Significant Degree of comminution Coarse Fine Coarse Fine difference Weight of fluid released on 5.6 a 3.5 c 4.0 b,c 5.4 a,b P < 0.001 puncturing (g) (1) Weight of fat expressed as weight of 44 a,b 54 a 47 a,b 37 b P < 0.05 fluid released (%) (2) Cooking loss before puncturing (%) 24 b 30 a 26 b 26 b P < 0.01 (1) Cooking loss after puncturing (%) 28 b 33 a 29 b 30 b P < 0.05 (2) Means in the same row with the same superscript are not significantly different (P > .05). Sources: (1) Jolley and Rangeley, 1986; (2) unpublished data from the same study. James and Crow (1986) provide some data on the use of microwaves for tempering meat blocks. The batch unit investigated would accept 5 meat cartons in a single layer on a pallet that was pushed over rollers man- ually into the microwave chamber. Microwave power was provided by a 30 kW magnetron (variable down to 20 kW) operating at 896 MHz, from which the microwaves entered the chamber via waveguides situated at the top and bottom. Rotating metal discs were positioned above and below the product to provide a more uniform microwave field and the loaded pallet was subjected to cyclic lateral movements of 90 mm during irradiation. It is clear from Table 8.10 that blocks processed directly from frozen storage can be acceptably tempered in a batch microwave unit to a mean temperature of ca. -3°C (range -5–0 °C) with no hot spots.Tempering times varied from 3.5 to 5.0 min with block types (1) and (2) (Table 8.10) and at least two combinations of microwave power and processing time produced acceptable results. The frozen cartons of flank that contained a higher percentage of fat caused more runaway heating problems, and low micro- wave power (20 kW) applied for 6.5 min was required to achieve the desired results. In general, microwave tempering of blocks which had been allowed to warm up in factory ambient temperatures for 8 h was unsatisfactory. Surface temperatures, especially at the corners of the meat blocks, rose to unacceptable levels and there was substantial drip loss from thawed surfaces. These results indicate that it would be difficult if not impossible to produce a uniform power/time combination for all types of ‘standard 27 kg blocks’. For optimal tempering, trials have to be carried out to determine the correct power and time setting for each type of block. Blocks sorted into batches of similar type should be processed directly from frozen storage under the predetermined conditions. Thawing and tempering 183 Table 8.9 Effect of prebreaking by grinding at either -3 or -6 °C on eating quality of flaked and formed beef patties as assessed by consumer panel Temperature at prebreaking (°C) -3 -6 Significant Salt in product (%) 0.5 1.0 0.5 1.0 difference Texture 53 46 48 52 <0.05 Taste 54 b 46 a 45 a 50 a,b <0.05 Meatiness 64 b 53 a 52 a 55 a <0.05 Overall 50 44 45 48 0.053 Meatiness: not very meaty (0), very meaty (100) (no mid-point). Means in the same row with the same superscript are not significantly different (P > 0.05). Source: Jolley et al., 1986. Successful tempering can be achieved in minutes using a microwave system, compared with 1–14 days that is required in industrial air temper- ing systems. Continuous conveyerised microwave tempering systems using either a single 60 kW magnetron or two 40 kW magnetrons can temper 2–2.5 tonnes per hour depending upon fat content. In large throughput operations the continuous microwave tempering plant provides considerable flexibility, in that changes in raw material requirements, for the post-tempering processes, can be accommodated in minutes. Using air tempering systems, at least 1 day and up to 8 days are required to accommodate equivalent changes. Many advertisements for microwave systems claim higher product yield because of reduction in evaporative and drip loss during tempering. Since the majority, if not all, of conventional plants temper material in a wrapped form, evaporative losses are insignificant, whilst substantial periods at air temperatures above 0 °C would be required before thawing of surface tissues occurred and drip became apparent. 184 Meat refrigeration Table 8.10 Effects of block types, weights of frozen meat and initial meat temperature on final meat temperature and condition of meat blocks tempered in a batch microwave unit Block Initial Microwave Process Final temperature (°C) type temperature power time (meat condition) (weight) (°C) (kW) (min) 1 -15 28 3.5 -7 to -2 25–30 kg 4.0 -4 to -2 (soft corners) 4.5 -4 to -1 (soft surfaces) 4.5 -3 to -2 (locally +2) 4.75 -4 to -2 (surfaces +2) 25 5.0 -3 to -2 (surfaces 0) 5.0 -4 to 0 -8 deep, 30 3.0 -5 to -2 surface -2 25 5.5 -3 to -2 (corners +3) 2 -15 30 5.0 -5 to +2 27–37 kg 4.0 -5 to -4 (soft spots) 4.5 -4 to -3 (soft spots) 28 5.0 -4 to -2 (surface <2) -8 deep, 30 6.0 -4 to +27 (v. variable) surface -1 6.0 -6 to +25 (v. variable) 5.0 -5 to +6 (spot at +54) 3 -15 30 4.0 -6 deep, -1 surface 25–27 kg 5.0 -5 deep, 0 surface 6.0 -5 deep, some cooking 6.0 -5 deep, some cooking 20 6.5 -4 to -2, uniform -8 deep, 20 7.0 -5 deep, hot spots surface -1 5.0 -5 uniform 30 5.0 -5 deep, hot spots Source: James and Crow, 1986. 8.4.4 Commercial practice The following data from a limited survey of tempering systems (James and Crow, 1986) indicate that most meat tempering was carried out in air- based conduction systems. At the time there were ca. 17 fully operational microwave tempering systems for meat in the UK, 12 of which were small batch units and the remainder continuous tempering tunnels. A number of the batch systems were used as either one stage of a hybrid microwave/con- duction system or to augment large conduction systems by fast tempering of small batches of urgently required material. The survey revealed substantial variations in all aspects of conduction- based tempering systems, with the exception of the raw material which was consistently 27 kg frozen blocks of meat in 15 cm thick cartons.Throughputs of frozen material ranged from less than a 1 tonne pallet of ca. 40 cartons to over 20 tonnes per day, and the tempered material was used in a variety of products, including beefburgers, re-formed meat, minced products, pies and canned goods. Typical data from a selection of tempering operations (Table 8.11) show initial temperatures ranging from -10 to -27°C, final tem- peratures from -2.5 to -15 °C, and tempering times from less than 1 day to 12 days. In one operation, times of up to 14 days were commonly used. Tempering operations within individual companies were very uncon- trolled and variable, as demonstrated by factory A where initial meat tem- peratures ranged from -10 to -18 °C and tempering times from 7–12 days. In the majority of operations, the blocks, on the pallets used during frozen storage, were transferred by fork lift or stacker trucks directly into the tempering rooms. In a few cases attempts were made to improve the Thawing and tempering 185 Table 8.11 Tempering configurations, initial and final product and environment temperatures, and tempering times for cartons of frozen meat (ca. 27 kg in weight, 15 cm thick) in industrial systems Factory Tempering Temperatures (°C) Tempering configuration Initial Final Ambient time (days) A Palletised -14/-12 -3 -8 then -3/-1 5 then 7 Palletised -10 -3 -8 then -3/-1 1 then 6 Palletised -15 -2.5 -8 then -3/-1 3.5 then 6 Palletised -18 -3 -8 then -3/-1 6 then 5 B Palletised -18 -15/-6 0 0.9 Racked -18 -4.5 14 then 0 0.2 then 0.7 C Palletised -20/-18 -10/-5 0 1.0 D Palletised -20 -6 -5/-3 6 to 7 E Palletised -27/-23 -8/-5.5 -3/-1 0.9 Racked -27/-23 -5/-4 -3/-1 0.9 F Palletised -18 -8/-3.5 -4/-3 1.0 On trolleys -18 -5/-3.5 -4/-3 0.8 Source: James and Crow, 1986. rate of heat transfer into the cartons and reduce the effective unit thick- ness by placing the cartons in single layers on racks or trolleys. This pro- duced a more even temperature distribution in the tempered blocks of meat. In most cases, however, the gap between blocks was too small and/or the air velocity in the tempering room too low to optimise tempering times. 8.5 Conclusions 1 Available thawing systems are based either on heat conduction into the product from the surface or on internal generation of heat using electromagnetic radiation. The latter systems appear to be unsuitable if complete thawing is required because of problems of thermal insta- bility and runaway heating. 2 There is no evidence to suggest that the method of thawing significantly affects the palatability of the meat when subsequently cooked. Quality is therefore assessed by appearance and bacteriological condition. 3 The efficiency of a given thawing process is a function of the thick- ness of the material to be thawed. The advantage of systems utilising high surface film heat transfer coefficients lessens markedly as mater- ial thickness increases; consequently VHT and water thawing systems offer little advantage over air thawing systems in terms of reduced thawing time for most commercial operations. 4 Appearance and bacteriological condition of the thawed meat are generally better in air thawing than in either water thawing or VHT. The optimal temperature for most operations appears to be 10 °C, 0.25ms -1 and 85% RH. However, thawing times are long, e.g. 2–3 days for beef quarters. 5 The choice of a practical thawing system for meat is limited. Standard 25kg, 15cm thick cartoned blocks can only be thawed in air blast sys- tems. Large quantities of beef quarters or lamb or pig carcasses are again restricted to air thawing systems. Water thawing is possible for vacuum packaged primal meat cuts and pork joints that are to be sub- sequently cured. 6 Industrial thawing systems use either air or water as the thawing medium. Few of these systems are well designed and consequently thawing tends to be very variable with, in some cases, high surface tem- peratures combined with ice in deep tissues of the same material. Thawing times in air are long, 2–3 days being typical for quarters or meat blocks, and drip losses are high. Water thawing can be very effec- tive if circulation is maintained and temperatures are controlled but effluent disposal can cause problems. 7 Quite small differences in temperature during processing operations such as prebreaking or cutting can have quite large effects on product quality. 186 Meat refrigeration 8 The amount of ice to a certain extent determines the physical proper- ties of meat; this changes rapidly within the range -2 to -1 °C. It is very difficult to measure temperature accurately under commercial condi- tions, and an error of 0.5 °C within the range represents 25% ice. 9 Unless tempering is well controlled, product variability is liable to be high. 10 Prebreaking at lower temperatures produces smaller particles which do not bind together well; this is not necessarily a bad thing, as a soft texture may be desirable in some products. 11 Industrial microwave tempering systems can provide a practical alter- native to conventional air-based systems in many situations especially if flexibility and/or large throughputs are important. However, capital costs are high and running costs substantial. 12 The majority of commercial tempering is carried out using air con- duction systems in which tempering times can vary from 0.8–14 days. Many of these systems have not been particularly well designed and are often operated in a haphazard manner, consequently there is considerable variation in temperatures, -10 to -2.5 °C, in the tempered product. 8.6 References arce j and sweat v e (1980), Survey of published heat transfer coefficients encoun- tered in food refrigeration processes, ASHRAE Trans, 86(2) 235–260. bailey c and james s j (1974a), Predicting thawing time of frozen pork legs, ASHRAE J, 16 (3) 68–69. bailey c and james s j (1974b), Air-, water-, and vacuum-thawing of frozen pork legs and meat blocks, in Cutting C L, Meat Freezing: Why and How? Meat Research Institute Symposium No. 3 42.1. bailey c, james s j, kitchell a g and hudson w r (1974), Air-, water- and vacuum- thawing of frozen pork legs, J Sci Food Agriculture, 25 81–97. 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