Leuconostoc citreum, a Gram-positive lactic acid bacterium isolated from fermented grains and vegetables, can express various enzymes involved in the biosynthesis of different extracellular polysaccharide (EPS). In this study, Leuconostoc citreum BD1707 was cultured in tomato juice. After 24 and 72 h of culture, samples were collected for ammonium sulfate precipitation followed by in situ activity analysis using native-polyacrylamide gel electrophoresis (native-PAGE). The results showed that Leuconostoc citreum BD1707 could secrete three EPS synthases with molecular masses of about 150, 90 and 80 kDa, respectively, which were higher than those of the known levansucrases.

In this study, the complete antigens of fipronil and its analogue were synthesized using glutaraldehyde (GA) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) as a cross-linker, respectively, and were identified by ultraviolet absorption spectroscopy and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). BALB/c mice were immunized with fipronil-bovine serum albumen to obtain hybridoma cell 3F6. Monoclonal antibodies were prepared by inducing ascites in mice. An enzyme-linked immunosorbent assay (ELISA) was developed and evaluated to determine fipronil in milk. The results showed that the concentration of the prepared monoclonal antibody, belonging to the IgG1 subclass, was 8.5 mg/mL, and the titer was 2.0 × 106. The cross-reaction rates with fipronil-desulfinyl, fipronil sulfone and fipronil sulfoxide were 9.21%, 14.02% and 21.46%, respectively. The recoveries of fipronil in spiked milk were 87%–118%, and the coefficient of variation for precision was less than 15%. In conclusion, this method is highly accurate and precise.

The effects of ripening temperature and time on the type and concentration of biogenic amines in Monascus-fermented cheese were examined in this paper. The results showed that tyramine, β-phenylethylamine and histamine were the typical biogenic amines of Monascus-fermented cheese during ripening. Their concentrations increased with increasing ripening time and temperature. β-phenylethylamine appeared at the late stage of maturity, and histamine appeared at high temperature. The level and growth rate of tyramine were higher than those of the other two biogenic amines, and it appeared the earliest of all. The optimal ripening conditions were found to be ripening at 14 ℃ for 12–14 days. The results of exposure assessment to biogenic amines in Monascus-fermented cheese showed that the average exposure of Chinese population was 8.55 μg/(kg·d), 94.72 μg/(kg·d) in P99. It is recommended that the per capita intake of Monascus-fermented cheese should not exceed 58 g/d in order to ensure the safety of 99.9% of populations with dietary exposure to biogenic amines through the consumption of Monascus-fermented cheese.

The optimization of medium components and culture conditions for improved spore production of Monascus purpureus M-4 in submerge culture was performed using a combination of one-factor-at-a-time (OFAT) method and response surface methodology (RSM). The OFAT results showed that potato extract powder and glucose were the optimal nitrogen and carbon source, respectively and that their optimal concentrations were 0.4 and 2.0 g/100 mL, respectively. The optimal culture conditions that provided the maximum number of spores were as follows: initial medium pH 6.72 (natural pH), medium volume in a 500-mL triangular flask 120 mL, concentration of added CaCO3 2.0 g/100 mL, number of passages 2, and culture time 72 h. Potato extract powder and glucose concentration as well as culture time were identified as main variables that influence spore production and their optimal levels were determined to be respectively 0.38 g/100 mL, 2.2 g/100 mL and 6.5 d using RSM. The highest spore number of (7.47 ± 0.15) × 106 CFU/mL was experimentally obtained under the optimized conditions, which was close to the predicted value (7.53 ± 0.31) × 106 CFU/mL. The average error between the predicted and experimental values was 0.79%. A 31.13-fold increase in spore number was achieved by the optimization. Besides, the spore-producing ability per unit volume of strain M-4 was greatly improved.

The histamine content in commercial fermented dairy products was determined by high performance liquid chromatography (HPLC) with precolumn derivatization. Four derivatization conditions, dosage of derivatizing agent (dansyl chloride, Dns-Cl), pH value, time and temperature were optimized using one-factor-at-a-time method and response surface methodology (RSM). The results showed that the optimized derivatization conditions were easy to manipulate and allowed for efficient, accurate and precise determination of histamine. The method met the requirements for accurate determination of histamine dairy products.

The major free middle-chain fatty acids (hexanoic acid, octanoic acid, decanoic acid) in raw milk and dairy products are regarded as important compounds that affect liquid milk quality and safety due to their strong flavor characteristics. This article provides an analytical method using headspace solid phase microextraction and gas chromatography-mass spectrometry (HS-SPME-GC-MS) with an internal standard for the quantitative determination of free middle-chain fatty acids in liquid milk. The method was verified as an approach with high accuracy, simple operation and high extraction efficiency.

The safety of dairy products has increasingly become the focus of both social and scientific attention. This article provides an overview of the concepts of dairy product safety and microbial risk assessment. From the point of view of the hazard factors affecting the safety of dairy products, a comprehensive review is executed with respect to the sources of microbial hazards and the current status of microbial risk assessment at home and abroad. And differences between microbial risk assessment and chemical hazard risk assessment are pointed out. Finally, a perspective outlook is proposed to expect the future direction of microbial risk assessment of dairy products in China.