86 Meat refrigeration comparison have been obtained from the available literature, and from an unpublished survey and experimental information gathered by the MR (Meat Research Institute at the Institute of Food Research, Bristol Labo- ratory(IFR-BL) In the concluding section, areas and systems that require further investigations are discussed 5.1 Theoretical considerations The rate at which a piece of meat loses weight through its surface depends upon two related processes: evaporation and diffusion. Evaporation is the process that transfers moisture from the surface of the meat to the sur- ounding air Diffusion transfers water from within the meat to its surface The rate of evaporation(Me) from the surface of a food is given by Daltons law Me=mA(P-P) where m is the mass transfer coefficient. a the effective area and p and Pa the vapour pressure at the surface of the meat and in the surrounding r,respectively If each term in the right-hand side of the equation is examined in turn, the difficulty of predicting the rate of mass transfer from a meat carcass or joint becomes apparent. In most systems a value for the mass transfer coe ficient is not obtained directly, but by analogy with the overall surface heat transfer coefficient (h). Some work has been carried out to measure m and h simultaneously(Kondjoyan et al., 1993). The surface heat transfer coeffi- cient itself is a function of the shape of the body and the properties of the medium flowing over it. It can be calculated for simple shapes, but must be obtained experimentally for irregular bodies such as meat joints and car casses. Arce and Sweat(1980)carried out one of the most comprehensive reviews of publishing values of h for foodstuffs. However, only 4 references relate to meat and these cover a very limited range of refrigeration condi tions. It is well established for forced air conduction systems that h becomes larger as air velocity increases. Therefore, all other factors being equal, weight loss will increase as air velocity increases. The effective area A can be difficult to measure, for example, the surface area of an irregular shape such as a meat carcass. In many commercial situations joints and/or carcasses are packed tightly together making an estimate of the'effective area even more problematic. Even meat blocks contain a number of irregularly shaped pieces of meat and do not normally present flat continuous surfaces to the air stream. Only in limited applica tions such as plate freezing or thawing can an accurate estimate be made of the effective surface area Pa is a function of both air humidity and temperature and values are readily available in standard text books. Pm is dependent upon the rate of diffusion and thus difficult to determine. After slaughter and flaying, freecomparison have been obtained from the available literature, and from an unpublished survey and experimental information gathered by the MRI (Meat Research Institute at the Institute of Food Research, Bristol Labo￾ratory (IFR-BL)). In the concluding section, areas and systems that require further investigations are discussed. 5.1 Theoretical considerations The rate at which a piece of meat loses weight through its surface depends upon two related processes: evaporation and diffusion. Evaporation is the process that transfers moisture from the surface of the meat to the sur￾rounding air. Diffusion transfers water from within the meat to its surface. The rate of evaporation (Me) from the surface of a food is given by Dalton’s law: [5.1] where m is the mass transfer coefficient, A the effective area and Pm and Pa the vapour pressure at the surface of the meat and in the surrounding air, respectively. If each term in the right-hand side of the equation is examined in turn, the difficulty of predicting the rate of mass transfer from a meat carcass or joint becomes apparent. In most systems a value for the mass transfer coef- ficient is not obtained directly, but by analogy with the overall surface heat transfer coefficient (h). Some work has been carried out to measure m and h simultaneously (Kondjoyan et al., 1993). The surface heat transfer coeffi- cient itself is a function of the shape of the body and the properties of the medium flowing over it. It can be calculated for simple shapes, but must be obtained experimentally for irregular bodies such as meat joints and car￾casses. Arce and Sweat (1980) carried out one of the most comprehensive reviews of publishing values of h for foodstuffs. However, only 4 references relate to meat and these cover a very limited range of refrigeration condi￾tions. It is well established for forced air conduction systems that h becomes larger as air velocity increases. Therefore, all other factors being equal, weight loss will increase as air velocity increases. The effective area A can be difficult to measure, for example, the surface area of an irregular shape such as a meat carcass. In many commercial situations joints and/or carcasses are packed tightly together making an estimate of the ‘effective’ area even more problematic. Even meat blocks contain a number of irregularly shaped pieces of meat and do not normally present flat continuous surfaces to the air stream. Only in limited applica￾tions such as plate freezing or thawing can an accurate estimate be made of the effective surface area. Pa is a function of both air humidity and temperature and values are readily available in standard text books. Pm is dependent upon the rate of diffusion and thus difficult to determine. After slaughter and flaying, free M mA P P e ma = - ( ) 86 Meat refrigeration