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 carcasseswarm 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, 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, 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