88 Meat refrigeration 5.2.1 Chilling Immediately after slaughter the surface of the carcass is hot(ca 30C)and wet so the rate of evaporation is high. Pork carcasses lose 0. 4% moisture between 0.5 and 1.0h post-mortem when held at approximately 15C (Cooper, 1970). Spray-washed lamb carcasses show an even greater rate of weight change, ca. 1.0%, during this time(James, unpublished work). Con sequently, the time at which initial hot weight is obtained is crucial in all weight loss measurements. The majority of carcasses in the UK are chilled n a single stage system, pork at a nominal temperature of 4 C, air velocity of 0.4ms-,85-90% relative humidity (RH), lamb and beef at 0C, 0.5ms, 85-90%RH. In practice the majority of chill rooms have under powered refrigeration plants and are overloaded, so the rooms take several hours to reach their designed operating conditions. Typical weight losses in these single stage systems for beef are 2-3.5%, for lamb 2-2.8%, and for pork18-3.5% In a single stage chilling process, the factors in equation [ 5.1] that can be controlled by the refrigeration designer are Pa and m, since both are a func tion of air humidity and temperature Humidity is controlled by the tem- perature difference(AT) across the evaporator coil. There are two ways of designing a coil to extract the same amount of heat: it can either have a very large surface area and a small AT, or a small area and a large AT The former is expensive but produces air at a high humidity, whilst the latter is cheap but dries the air. If we assume that in the initial stages of chilling the surface of a carcass is saturated and is above 30 C, then in air at 0C, 90% RH, Pm-Pa=0.054 bar, and at 70% rh, Pm -Pa= 0.055 bar. The initial effect of RH on weight loss is therefore small, but as cooling proceeds, Pn reduces and RH becomes increasingly important. Hodgson(1970)in South Africa showed that beef sides cooled for 20h in air at a temperature of 1.7C, and velocity of 0. 75ms lost 2.75% in weight at 90%RH, and 3.4% t 70% RH, i.e. a 0.65% difference. Hodgson also stated that the maximum return on investment was achieved using a large coil with a ATof 5'C. Since that time the price of beef has risen faster than the capital and the runnin costs of refrigeration equipment, and it is probable that the AT for a maximum return is now even smaller The lower the air temperature the faster the rate of fall of the surface temperature, which controls the maximum value of Pm. Lower air temper atures should therefore reduce weight loss during chilling. Beef sides of average UK weight(140kg) lost 1. 2% in air at 4C, 0.5ms, 90%Rh and 0.2% less at 0.C0.5ms-l 90% RH when cooled to a maximum centre tem perature of 10C (Bailey and Cox, 1976). The initial weight was recorded ca. 2h after slaughter Since air velocity is directly related(via h), to the mass transfer coeffi- cient it would seem from equation [5.1 that increasing the air velocity during chilling would produce a greater weight loss. However, higher air velocities also increase the rate of fall of surface temperature and hence5.2.1 Chilling Immediately after slaughter the surface of the carcass is hot (ca. 30 °C) and wet so the rate of evaporation is high. Pork carcasses lose 0.4% moisture between 0.5 and 1.0 h post-mortem when held at approximately 15 °C (Cooper, 1970). Spray-washed lamb carcasses show an even greater rate of weight change, ca. 1.0%, during this time (James, unpublished work). Con￾sequently, the time at which initial hot weight is obtained is crucial in all weight loss measurements. The majority of carcasses in the UK are chilled in a single stage system, pork at a nominal temperature of 4 °C, air velocity of 0.4 ms-1 , 85–90% relative humidity (RH), lamb and beef at 0 °C, 0.5 m s-1 , 85–90% RH. In practice the majority of chill rooms have under￾powered refrigeration plants and are overloaded, so the rooms take several hours to reach their designed operating conditions. Typical weight losses in these single stage systems for beef are 2–3.5%, for lamb 2–2.8%, and for pork 1.8–3.5%. In a single stage chilling process, the factors in equation [5.1] that can be controlled by the refrigeration designer are Pa and m, since both are a func￾tion of air humidity and temperature. Humidity is controlled by the tem￾perature difference (DT) across the evaporator coil. There are two ways of designing a coil to extract the same amount of heat: it can either have a very large surface area and a small DT, or a small area and a large DT. The former is expensive but produces air at a high humidity, whilst the latter is cheap but dries the air. If we assume that in the initial stages of chilling the surface of a carcass is saturated and is above 30°C, then in air at 0°C, 90% RH, Pm - Pa = 0.054 bar, and at 70% RH, Pm - Pa = 0.055 bar. The initial effect of RH on weight loss is therefore small, but as cooling proceeds, Pm reduces and RH becomes increasingly important. Hodgson (1970) in South Africa showed that beef sides cooled for 20 h in air at a temperature of 1.7 °C, and velocity of 0.75m s-1 lost 2.75% in weight at 90% RH, and 3.4% at 70% RH, i.e. a 0.65% difference. Hodgson also stated that the maximum return on investment was achieved using a large coil with a DT of 5 °C. Since that time the price of beef has risen faster than the capital and the running costs of refrigeration equipment, and it is probable that the DT for a maximum return is now even smaller. The lower the air temperature the faster the rate of fall of the surface temperature, which controls the maximum value of Pm. Lower air temper￾atures should therefore reduce weight loss during chilling. Beef sides of average UK weight (140 kg) lost 1.2% in air at 4 °C, 0.5 m s-1 , 90% RH and 0.2% less at 0 °C, 0.5 m s-1 , 90% RH when cooled to a maximum centre tem￾perature of 10 °C (Bailey and Cox, 1976). The initial weight was recorded ca. 2 h after slaughter. Since air velocity is directly related (via h), to the mass transfer coeffi- cient it would seem from equation [5.1] that increasing the air velocity during chilling would produce a greater weight loss. However, higher air velocities also increase the rate of fall of surface temperature and hence 88 Meat refrigeration