Thawing and tempering 161 Table 8.1 Typical surface heat transfer coefficients (h) for different thawing System Surface heat transfer coefficients(WmK) free convection Air-forced convection Vacuum steam heat Plate equently, 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 thaw 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 nd its velocity. Except for very simple configurations, h cannot be derived heoretically 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 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 pr rocess 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 authorssequently, 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-2K-1 ) Air-free convection 5–15 Air-forced convection 10–70 Water 100–400 Vacuum steam heat 150–1000 Plate 100–300