Effect of Water Content on the Glass Transition and Caking of Fish Protein Hydrolyzates

The glass transition temperature (T_g) and structure collapse (volumetric shrinkage) of a freeze‐dried fish protein hydrolyzate (HFP) were studied. An increase in the water activity from 0 to 0.64 reduced the T_g of HFP from 79.1 to −42.8 °C. The Gordon—Taylor equation was a good predictor for the plasticizing effect of water on T_g. At room temperature (19 °C), collapse was initiated at a_w = 0.44 corresponding to a T — T_g value of 35.8 °C, and browning was evident above a_w = 0.55. Above these critical values several structural changes occurred: shrinkage, collapse, browning, and setting into a sticky, high‐viscosity brown liquid. The viscosity of the matrix at the onset of collapse was 10⁵–10⁷ Pa·s, as estimated using the Williams—Landel—Ferry equation.