Specific heat of apple at different moisture contents and temperatures

• Specific heats C of apples versus moisture content W and temperature T were measured.
• The important role of bound water in determination of C(W,T) behaviour was revealed.
• The strongly nonlinear C(W) dependencies were observed at small humidity (W ⩽ 0.1).
• The new additive model was used for estimation of the specific heat of bound water.
• The amount of bound water Wb was comparable with monolayer moisture content.

This work discusses results of experimental investigations of the specific heat, C, of apple in a wide interval of moisture contents (W = 0–0.9) and temperatures (T = 283–363 K). The obtained data reveal the important role of the bound water in determination of C(W,T) behaviour. The additive model was applied for description of C(W) dependence in the moisture range of 0.1 ⩽ W < 1, where the apple was considered as a mixture of water and hydrated apple material (water plasticised apple) with specific heat Ch. The difference between Ch and specific heat of dry apple, ΔCb = Ch–Cd, was proposed as a measure of the excess contribution of bound water to the specific heat. The estimated amounts of bound water Wb were comparable with the monolayer moisture content in the apple.