Pasteurized milk is highly regarded for its ability to effectively retain nutrients and sensory quality, but its shelf life is limited by the heat-resistant microorganisms and their metabolic products remaining during the mild sterilization process. To solve this problem, the key is to identify the core residual flora and its metabolic network that lead to quality degradation. In this study, pasteurized milk stored at 4 ℃ for 0, 6, 9, and 12 days was examined to explore the changes in physicochemical properties, microbial communities, and functional metabolism during storage. Physicochemical analysis showed a slight decrease in fat, protein, and lactose content and a significant increase in acidity, particle size, and yellowness during storage. Among three pasteurization treatments (65 ℃/30 min, 75 ℃/15 s, and 85 ℃/15 s), the 75 ℃/15 s protocol optimally preserved nutrients and sensory quality, providing critical parameters for process optimization. Metagenomic analysis by Illumina HiSeq sequencing platform showed that Acinetobacter, Pseudomonas and Lactococcus might be the main core microflora leading to the deterioration of pasteurized products during storage. Pasteurized milk processed for 75 ℃/15 s can maintain good quality when stored at 4 ℃ for 0-9 days. Storing at 4℃ for 9 days may be the key storage point for microbial dynamics. Functionally, genes related to carbohydrate metabolism, amino acid metabolism, and energy metabolism increased with storage time. These metabolic pathways are crucial for microbial growth and may affect the nutritional composition and quality of pasteurized milk. Core microbiota primarily impacted milk quality through pyruvate metabolism and valine, leucine, and isoleucine biosynthesis pathways, leading to changes in acidity, bitterness, and texture. This study provides a basis for the monitoring and quality safety management of the main microorganisms and active metabolites during the production, processing and storage of pasteurized milk, and lays a foundation for obtaining high-quality pasteurized milk with long shelf life.

To investigate the correlation between the beany odor of raw cow’s milk and feed types, this study selected two samples of raw cow’s milk with beany odor and five common feeds: soybean meal, alfalfa, flaked corn, oat grass, and corn silage, and measured their volatile flavor compounds by headspace solid phase microextraction-gas chromatography-mass spectrometry (GC-MS). Key flavor compounds were identified using relative odor activity value (ROAV). The results indicated that acids and aldehydes contributed primarily to the beany odor of raw milk. A total of 49 volatile substances were identified in the two milk samples, among which hexanoic acid, octanoic acid, decanoic acid, decanal, nonanal, (E)-2-octenal, and (E)-2-decenal were the main beany odor substances. A total of 257 volatile substances were detected in all feed samples, including decanal, nonanal, and (E)-2-octenal. According to ROAV analysis, the beany odor compounds were more abundant in the feed samples than in the milk samples. The relative contents of beany odor compounds in the feed samples followed the following decreasing order of alfalfa > oat grass > soybean meal > flaked corn > corn silage, and flaked corn exclusively contained vanillin, with a strong milky aroma.

Pasteurization and ultra-high temperature treatment can kill most microorganisms in milk. However, since some bacteria such as Bacillus subtilis can resist pasteurization and ultra-high temperature treatments, even the most stringent heat treatments used to eliminate pathogenic microorganisms in the dairy industry cannot completely inactivate all microorganisms. In addition, highly heat-resistant spores can survive ultra-high temperature processing, so sterilized milk may be contaminated, causing bacterial spoilage in milk and dairy products during storage. In this review, the harms of Bacillus subtilis and its spores, spoilage-causing enzymes and biofilm to sterilized milk and the control measures for them are summarized so as to provide strategies for the prevention and control of Bacillus subtilis in sterilized milk and the quality assurance of milk and dairy products.