Abstract: Vacuum precooling technology can play an important role in the cooling of low-temperature cooked meat by virtue of its fast cooling rate. In this study, the effect of vacuum precooling on the water mobility and pore structure of cooked pork ham injected with different levels (10%, 20%, 30% and 40%) of brine solution was evaluated, and in turn the effects of the water mobility and pore structure on the cooling rate were discussed using partial least squares regression (PLSR). The results showed that samples injected with 10% brine solution had a higher average cooling rate (0.94 ℃/min) compared to those injected with 20% (0.76 ℃/min), 30% (0.56 ℃/min) and 40% (0.68 ℃/min) brine solution during vacuum precooling (P < 0.05). The nuclear magnetic resonance (NMR) results showed that the relaxation peak areas of bound water (A21 and A22), immobile water (A23) and free water (A24 and A25) in all samples exhibited a decrease during vacuum cooling while the relaxation time of immobile water (T23) remained unchanged. In addition, vacuum precooling produced a wider range of pore size distribution, higher porosity and larger apertures than did the conventional precooling method. The loading plot showed that the average cooling rate exhibited a significant correlation with the transverse relaxation time and peak area of immobile water, cumulative pore volume and area, average pore diameter, tortuosity and permeability. The variable importance in projection (VIP) results revealed that immobile water exhibited a more significant effect on the average cooling rate compared to bound water and free water. In addition, the pore structure parameters average pore diameter, permeability and tortuosity more significantly affected the average cooling rate compared to the porosity determined by mercury porosimetry.

Key words: vacuum precooling; cooked pork ham; pore structure; water mobility; cooling mechanism