In situ investigation of ice fractions and water states during partial freezing of pork loins and shrimps

Ice fractions and water states in partially frozen muscle foods greatly affect their quality. In the study, a variable temperature nuclear magnetic resonance (VT-NMR) with a liquid nitrogen temperature control system was employed to in situ investigate the relationship between ice fractions and temperatures and changes in water states during partial freezing and thawing of pork and shrimp. Results indicated that changes in ice fractions ranging from −2 ∼ −20 °C could be divided into 3 stages including slow increase, random leap and remarkable leap. More serious damages to the structures related to immobile water occurred in shrimp than in pork, and partial freezing also caused deterioration in muscle fibres related to free water. Additionally, −2 ∼ −3 °C and − 3.5 °C were the appropriate partial freezing temperatures for pork and shrimp, respectively. Therefore, the VT-NMR method possessed great potential for fundamental studies and applications of partial freezing of muscle foods. •An in situ method by variable temperature nuclear magnetic resonance was developed.•Ice fractions and water states during partial freezing (PF) and thawing were studied.•Ice fraction change shows slow increase, random leap and remarkable leap stages.•Suitable PF temperatures for pork of −2 ∼ −3 °C and shrimp of 3.5 °C were obtained.