Using metabolomics, this study explored the relationship between the quality deterioration and the metabolic changes in refrigerated large yellow croaker (Larimichthys crocea) during refrigeration. Key differential metabolites associated with quality deterioration across fish samples after 0, 9, and 18 days of refrigeration were identified by gas chromatography-quadrupole-time-of-flight mass spectrometry, and the metabolic pathways in which the differential metabolites may be involved were analyzed. The results showed that as refrigeration time increased, the types and contents of metabolites such as amino acids, carbohydrates, and nucleotides significantly changed in large yellow croaker muscle. The differential metabolites were primarily concentrated in amino acid metabolism pathways, such as arginine and proline metabolism; glycine, serine, and threonine metabolism; and cysteine and methionine metabolism. Proline, glutamate, isoleucine, glycine, methionine, tyrosine, valine, guanosine, uracil, ribose, thymidine, xanthine, and 5’-inosine monophosphate were identified as key differential metabolites. The degradation of amino acids and the generation of biogenic amines were closely related to the decline in the freshness and shelf life of large yellow croaker, while the degradation of nucleotide substances was related to the decrease in the taste.

In order to improve the quality of fermented skim milk, the effects of adding aloe extract on the physicochemical properties and sensory quality of fermented skim milk were investigated. Three levels of aloe extract (0.1%, 0.2% and 0.3%) were used. The pH value, titratable acidity, water-holding capacity, dehydration shrinkage, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity, Lactobacillus count, color, viscosity and sensory evaluation of fermented milk were measured. The experimental results showed that the addition of aloe extract improved the problem of the low acidity of fermented skim milk during refrigeration, increased the viscosity, imparted better texture to it, improved the water-holding capacity, decreased the dehydration shrinkage, and increased the cold storage stability, the Lactobacillus count and the antioxidant capacity. However, the flavor and appearance of the fermented skim milk with aloe extract decreased.