Monitoring the microbiological and chemical hazards of dairy products in China

Abstract

China is one of the largest producers and consumers of dairy products in the world, but there has also been an increase in dairy safety issues in recent years. This research topic aims to collect recent studies on the following themes: (1) research on testing dairy products for residues and microbiological safety; (2) new dairy product development in China; (3) the future development of the dairy industry in China; and (4) control of foodborne pathogens in dairy products. This research topic comprises seven original research articles and one review article that covers the areas discussed above.

Dairy products are easily contaminated by microorganisms from different sources, and subject to rapid deterioration or even cause safety issues. Some strains are extensively distributed in diary processing environments and possessed unusual abilities to survive under adverse environmental conditions, including desiccation, osmotic and thermal stress. For example, a study assessed the prevalence and risk-related phenotypes of Bacillus cereus in fluid milk products, and B. cereus (19.27%) was the most abundant species showing a high genetic diversity with seven random amplified polymorphic DNA patterns in 41 milk samples. Most B. cereus strains were classified as good biofilm formers with strong motility, EPS-producing ability, high proteolytic and lipolytic activities. Antibiotic resistance analysis suggested the majority B. cereus strains were resistant to tetracycline (95.24%), oxacillin (90.48%) and penicillin G sodium (47.62%). Proteomic and phenotypic evaluation of LuxS-mediated hyperosmotic tolerance in Cronobacter malonaticus was also analysed, and LuxS served as a quorum-sensing transcription factor in C. malonaticus to tolerate hyperosmotic stress, illustrating that LuxS contributed to survival, biofilm formation and reduced cellular injury when grown with 5.5% NaCl stress.

Effective analytical methods play a major role in dairy safety. In this view, a propidium monoazide-loop-mediated isothermal amplification (PMA-LAMP) assay was developed for fast and cost-effective visual detection of viable Cronobacter sakazakii in dairy products. The detection could be finished in 75 min, and the results could be identified by the naked eye, with detection limits of the assay as low as 1.2 × 10 cfu/mL and 1.2 × 10² cfu/mL for gel electrophoresis and naked eyes, respectively. In another study, smartphone-integrated device-assisted ratiometric fluorescent sensors for on-site rapid detection of Listeria monocytogenes in dairy products were developed. This smartphone-based biosensor presents significant advantages in terms of detection limit and speed. More importantly, the ratiometric mode employed in our approach enhances the sensitivity of detection by circumventing interference caused by the complex dairy components.

In order to control the key dairy-derived strains in dairy processing environment and dairy products, a phage-based method was developed to control Pseudomonas fluorescens contamination in the dairy industry. a lytic bacteriophage YZU_PF006 against P. fluorescens was isolated from sewage samples in Yangzhou, China. Morphology and one-step growth results indicated that this short-tailed phage (a tail length of 5 nm and a head diameter of 58 nm) had a high burst size (180 PFU/infected cells) but a relatively short latent period (60 min). This phage can be inactive after pasteurisation and Clean-In-Place (CIP) procedures. The genomic sequence showed that phage YZU_PF006 had 47 open reading frames (ORFs) but did not have genes for virulence, lysogeny and antibiotic resistance. Phage YZU_PF006 effectively controlled P. fluorescens growth at 7°C and 28°C in milk, which can be used to control P. fluorescens contamination in the dairy industry.

In addition, dairy products are also potential carriers of veterinary drugs, chemical pollutants and microbial toxins. The government reformed the management of dairy products and associated laws to strengthen food safety regulations and raise technical standards to improve the safety of dairy products. Heavy metals have been categorised as chemical pollutants, and Hg²⁺ ions ranked third among hazardous heavy metals in the US Agency list of Toxic Substances and Disease Registry, which may interrupt the digestive, nervous and immune systems. Methods for monitoring Hg²⁺ levels in dairy products include inductively coupled plasma–mass spectroscopy (ICP–MS), high-performance liquid chromatography (HPLC) together with UV–visible or fluorescence detectors; however, these methods have several limitations such as lengthy sample preparation, expensive, labour-intensive and high cost equipment. In a study, a simple, sensitive and selective colorimetric probe for the detection of Hg²⁺ in milk samples employing redox interaction of Hg²⁺ ions with citrate-stabilised silver nanoparticles was developed. This method provided a detection limit (LOD) of 6.17 ppb using UV–visible spectrophotometer with linear correlation at 10–2000-ppb solution of Hg²⁺ ions.

On the contrary, fermented dairy products have been indicated as having functional potential, that is beneficial effects on the health of those who consume them. These functional effects may be related to the probiotics present in these dairy products, as well as the metabolites produced by them. In one study, the positive effect of Limosilactobacillus plantarum grx16 on nonalcoholic fatty liver disease (NAFLD) in rats was evaluated. This work compared the level of NAFLD in rats before and after L. plantarum grx16 intervention, as well as changes in gut digestive enzymes and inflammatory factors. L. plantarum grx16 inhibited the absorption of starch and triglycerides by inhibiting the activity of gut amylase and lipase to alleviate insulin resistance and reduce lipid accumulation in the body, thereby alleviating the inflammatory response, and thus alleviating NAFLD in rats. Still, it is interesting to select starter cultures, or novel technologies in dairy processing that could be used for new dairy products development.

At last, a review article on the applications of metabolomics approaches in dairy science was summarised. This review summarised the most recent developments and applications of metabolomics as an efficient tool for comprehensively characterising the composition and dynamic changes of metabolites in the area of food science and nutrition research in the process of getting dairy products from factory to human from 2018 to 2022, focusing on potential marker metabolites and metabolic mechanisms related to dairy product quality, authenticity/traceability and dairy intake monitoring. The future direction of metabolomics in the field of dairy science was also discussed. The review will provide key knowledge for the further application of metabolomics technology to develop their quality, safety and nutritional value of dairy products.