Role of temperature and humidity fluctuations in shrimp quality deterioration during frozen storage

Household refrigerators commonly use -18 °C for frozen storage of fresh food, but temperature and humidity control during storage are often overlooked. This study used shrimp for the research subject, with -18 °C set as the target temperature. The experiment was designed to investigate the effects of different temperature fluctuation amplitude (±1.0 °C and ±3.0 °C), different freezing rates (quick freezing and slow freezing) and different humidity levels (high humidity and low humidity) on shrimp quality over frozen storage periods (0, 15, 30, 45, and 60 d). The results showed that after 60 d of storage, the melanosis of shrimp heads and the relative activity of polyphenol oxidase (PPO) in the small-fluctuation quick-freezing high-humidity group (±1 QFHH) were significantly better than those in the large-fluctuation slow-freezing low-humidity group (±3 SFLH). Meanwhile, the drip loss, total viable bacterial count (TVC), total volatile basic nitrogen (TVB-N), purine and biogenic amine contents in the ±1 QFHH group were also significantly lower (P < 0.05) than those in the ±3 SFLH group. In addition, with prolonged storage time, the number of dehydration channels in muscle tissue increased and their width expanded, leading to an increase in protein surface hydrophobicity (bromophenol blue (BPB) bound) and myofibrillar fragmentation index (MFI), as well as a decrease in Ca2+-ATPase activity. In summary, larger temperature fluctuations, slower freezing rates and lower humidity environments exacerbate the destruction of muscle structural integrity in shrimp, promote water loss and widen ice crystal channels, thereby significantly accelerating the deterioration process.