Changes during heating air as the heat transfer medium

4 Changes during heating air as the heat transfer medium 

Heat is transferred more slowly by hot air than by hot water because of differences in heat conductivity. Therefore, in most circumstances higher temperatures and longer processing times should be used than is the case with boiling. Hot air reaches the surface of food, which is then changed more intensively than the inner layers where the temperature does not exceed about 100°C. Losses of antioxidants are, of course, also higher on the surface than in the interior of heated food. 

In inner layers, changes are much the same as during boiling. Tocopherols and tocotrienols present in wheat and rye are partially destroyed during baking. In ordinary wheat bread, losses of a-tocopherol amount to about 25%, but in the case of rye bread, prepared by traditional technology, a loss of about 50% was reported (Piironen et al, 1987). Losses of natural antioxidants in coffee brews and tomato puree were also observed (Nicoli et al, 1997). 

---