The purpose of this study was to investigate the growth kinetics of Bacillus cereus in different milk products and its impact on milk quality. Fifty dairy products (including 25 raw milk from factory pre-treatment tanks and 25 commercial milk packaged in boxes) were collected, and suspected strains were isolated using Mannitol-Egg-Yolk-Polymyxin Agar (MYP). Morphological observation, protein toxin crystallization experiment, Vitek-2 biochemical identification, and 16S rRNA sequencing were used to isolate and identify B. cereus. Further, one strain with the fastest growth rate was selected as the experimental object, and the strain was inoculated into three different milk products (pure milk, lactose-decomposed milk, and grain milk) at initial concentrations of 101, 102, and 103 CFU/mL. The growth kinetics of B. cereus in three different products were studied at two temperatures (25 ℃ and 35 ℃), and a Logistic equation was used to fit the growth model. The pH and sensory changes of the three milk products were simultaneously monitored. The results showed that the three strains (B1, B2, and B3) of B. cereus originated from raw milk were isolated and identified and they were not detected in commercial milk products. By measuring the growth curve, B1 strain with a faster growth rate was selected as the experimental object to study its effects on three different milk products. The results showed that temperature and initial inoculation concentration had a more significant impact on the growth of B. cereus. At 35 ℃ and high initial inoculation concentration, B. cereus in dairy products entered the logarithmic and stable phases earlier than at 25 ℃, with a shorter lag period. Among the three types of products, lactose-decomposed milk was more conducive to the growth of B. cereus. The fitting degree of the Logistic model was R2>0.9857. It was found that the pH of milk changed significantly as the bacterial concentration increased. Different dairy products had their own characteristics in terms of sensory degradation. Among them, the odor deterioration of lactose-decomposed milk was the most significant. This study provides a scientific basis for the early warning and safety control of B. cereus contamination in dairy products.
| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |