Water distribution, physicochemical and microstructural properties of scallop adductors as affected by different drying methods

Effect of three drying methods, viz. hot air drying (HAD), heat pump drying (HPD) and vacuum freeze drying (VFD) on the drying kinetics, water composition and distribution, physicochemical and microstructural properties of scallop adductors were investigated. Drying methods significantly affected drying rate following by the order of HAD>VFD>HPD. Low field nuclear magnetic resonance (LF NMR) data demonstrated the transverse relaxation time (T2) of immobilized water was the main component governing the drying process. As drying proceeded, T2 of tightly and loosely bound water maintained a relatively stable state, whereas T2 of free water and immobilized one shifted leftwards. Drying led to an increase in hardness and chewiness, and a decrease in total color difference, adhesiveness and crude protein content. Rehydration ratio was increased with time accompanied by a prompt in the initial period and relatively stable thereafter. Compared to drying process, opposite tendency of LF NMR relaxation properties was observed in scallop adductors during rehydration procedure. HAD resulted in compact structure, severe denaturation and degradation of protein secondary structure, while HPD products had a looser microstructure than that of HAD, and VFD products had the most porous structure.