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