Psychrotrophic bacterial populations in Chinese raw dairy milk

Comprehensive review of enzymes (protease, lipase) in milk: Impact on storage quality, detection methods, and control strategies

Enzymes play a crucial role in determining the storage quality of milk by influencing various biochemical processes. Among these enzymes, proteases and lipases are of particular significance due to their impact on flavor, texture, and shelf-life stability. This study offers a thorough examination of proteases and lipases in milk, focusing on their enzymatic activities and mechanisms of action during storage. The present review addresses the techniques for monitoring enzyme activity, including fluorescence-based assays, spectrophotometry, fluorometry, mass spectrometry, biosensors, ELISA, polymerase chain reaction, and next-generation sequencing, emphasizing their sensitivity and applicability in quality control. Furthermore, various strategies for controlling enzyme activity in milk are examined, encompassing both thermal and non-thermal treatments, pH modulation, and the use of enzyme inhibitors. Additionally, the review explores the regulatory frameworks governing enzyme activity in dairy products to ensure compliance with safety and quality standards. A thorough understanding of the dynamics of proteases and lipases in dairy products is crucial for optimizing storage conditions, ensuring product quality, and meeting consumer demands for purity and nutritional integrity.

Improving Foaming Properties and Quality of Pasteurized Milk Using Antimicrobial Agents from Wild Pediococcus acidilactici

Pasteurized milk foam has become a quality issue in some applications, such as cappuccino-style drinks, as it should be stable and high-capacity. The extended shelf life of pasteurized milk is also a challenge. Some factors affect the foam capacity and stability; among them, the increasing amount of free fatty acids in raw milk is critical. The psychrotrophic bacteria can produce a lipase-like enzyme, which is responsible for increasing the level of free fatty acids in raw milk. Therefore, this work aims to utilize the cell-free supernatant of a bacteriocin-producing culture as a natural preservative against psychrotrophic and spore-forming bacteria to enhance the foaming capacity and stability and improve the final product's quality and shelf life. Milk samples from 15 dairy farms were assessed for free fatty acids, microbiological quality, and foaming capacity. Raw milk was divided into four portions: a control without any additive and cell-free supernatant (CFS) treatments, with CFS added at concentrations of 5, 10, and 15 mL/L in each portion. Raw milk was stored for 5 days before heat treatment at 75 °C/30 s, then cooled at 5 °C. All samples were examined for microbiological, free fatty acid, and foaming properties immediately after heat treatment and during storage up to 14 days. The results of this study reveal that there is a negative impact of free fatty acids on the capacity and stability of foaming. The cell-free supernatant (15 mL/L) of the traditional dairy isolate Pediococcus acidilactici inhibits the psychrotrophic bacteria in raw milk during storage for 5 days, a phenomenon which has a direct impact on reducing the free fatty acids, improving the foaming capacity and stability, as well as reducing the bitterness at the end of the shelf life of pasteurized milk up to 14 days compared to the detection of bitterness after 8 days in the control pasteurized milk. It is concluded that, to produce pasteurized milk with a high foaming capacity and extended shelf life, raw milk with low amounts of free fatty acids should be used and fast pasteurized or treated with a bacteriocin of lactic acid bacteria.

Effectiveness of a bacteriophage YZU_PF006 in controlling dairy spoilage caused by Pseudomonas fluorescens

Pseudomonas fluorescens is a psychrophilic bacterium that can cause dairy spoilage by producing heat-stable enzymes. Bacteriophages are proved as one of the alternatives to control spoilage bacteria in today's dairy industry. This study aimed to investigate how a previously identified phage YZU_PF006 prevents dairy spoilage caused by P. fluorescens. Results demonstrated that phage YZU_PF006 effectively controlled P. fluorescens growth and production of protease at 7°C and 28°C in milk in a phage concentration-dependent way. Phage YZU_PF006 at a multiplicity of infection (MOI) of 100 increased the pH values of milk by 1.43 at 28°C and 0.81 at 7°C, increased the particle size of milk by 2.74 μm at 28°C and 1.74 μm at 7°C. Phage YZU_PF006 reduced the free AA content by 15.36% at 28°C and 32.03% at 7°C, and decreased the contents of Glu (206.678 mmol/L at 28°C and 40.481 mmol/L at 7°C), Phe (94.137 mmol/L at 28°C and 144.137 mmol/L at 7°C) and other amino acids in milk. In contrast, high-throughput sequencing analysis revealed that phage YZU_PF006 specifically prevented the growth of Pseudomonas in raw milk at low temperatures. Results demonstrated that phage YZU_PF006 can be used alone or in combination with other control strategies to serve as one of the good antimicrobial candidates to control P. fluorescens contamination in dairy processing environments, and to promote the safety and sensory quality of raw milk and milk products.

Relationship between microorganisms and milk metabolites during quality changes in refrigerated raw milk: A metagenomic and metabolomic exploration

Although cold storage at 4 °C can effectively prolong the shelf life of raw milk, it cannot prevent its eventual spoilage. In this study, we analyzed the main physicochemical and microbial indexes of raw milk stored at 4 °C for 6 days. The changes in microbial profiles and milk metabolites and their relationship during refrigeration were also explored. Metagenomic analysis performed using the Illumina Hiseq Xten sequencing platform revealed that the dominant genera in raw milk evolved from Acinetobacter, Streptococcus, Staphylococcus, and Anaplasma to Flavobacterium, Pseudomonas, and Lactococcus during cold storage. Using the UHPLC-Q-TOF MS method, 77 significantly different metabolites (p < 0.05) were identified, among which lipids were the most abundant (37). The most significant metabolic changes largely occurred at 3-4 days of refrigeration, coinciding with the rapid increase in dominant psychrotrophic bacteria. Subsequently, correlation analysis demonstrated that these lipid-related metabolites were significantly associated with Acinetobacter, Flavobacterium, and Pseudomonas. Both macro indicators and microanalysis indicated that the key stage of quality changes in raw milk was 3-4 days. Thus, this stage can be targeted for the quality control of raw milk.

Characterization of psychrotrophic and thermoduric bacteria in raw milk using a multi-omics approach

Psychrotrophic and thermoduric bacteria are the main reasons for the spoilage of dairy products. This study aims to address the composition and function of psychrotrophic and thermoduric bacteria in eight groups of raw milk samples obtained from Heilongjiang Province and Inner Mongolia (China). Microbial enumeration showed an average total bacterial count of 4.63 log c.f.u. ml-1 and psychrotrophic bacterial counts of 4.82 log c.f.u. ml-1. The mean counts of mesophilic and thermophilic thermoduric bacteria were 3.68 log and 1.81 log c.f.u. ml-1, respectively. Isolated psychrotrophic bacteria (26 genera and 50 species) and mesophilic thermoduric bacteria (20 genera and 32 species) showed high microbial diversity. Through metagenomic and proteomic analyses, significant disparities in the concentration and community structure of psychrotrophic and thermoduric bacteria were observed among different locations. A large number of peptidases were annotated by metagenomics, which may result in milk spoilage. They mainly come from some typical psychrotrophic and thermoduric bacteria, such as Chryseobacterium, Epilithonimonas, Pseudomonas, Psychrobacter, Acinetobacter, Lactococcus, Escherichia and Bacillus. However, the main proteins detected in fresh raw milk were associated with bacterial growth, reproduction and adaptation to cold environments. This investigation provides valuable insights into the microbial communities and protein profiles of raw milk, shedding light on the microbial factors contributing to milk deterioration.

Recent Development in Detection and Control of Psychrotrophic Bacteria in Dairy Production: Ensuring Milk Quality

Milk is an ideal environment for the growth of microorganisms, especially psychrotrophic bacteria, which can survive under cold conditions and produce heat-resistant enzymes. Psychrotrophic bacteria create the great problem of spoiling milk quality and safety. Several ways that milk might get contaminated by psychrotrophic bacteria include animal health, cowshed hygiene, water quality, feeding strategy, as well as milk collection, processing, etc. Maintaining the quality of raw milk is critically essential in dairy processing, and the dairy sector is still affected by the premature milk deterioration of market-processed products. This review focused on the recent detection and control strategies of psychrotrophic bacteria and emphasizes the significance of advanced sensing methods for early detection. It highlights the ongoing challenges in the dairy industry caused by these microorganisms and discusses future perspectives in enhancing milk quality through innovative rapid detection methods and stringent processing controls. This review advocates for a shift towards more sophisticated on-farm detection technologies and improved control practices to prevent spoilage and economic losses in the dairy sector.

Psychrotrophic Bacteria Threatening the Safety of Animal-Derived Foods: Characteristics, Contamination, and Control Strategies

Animal-derived foods, such as meat and dairy products, are prone to spoilage by psychrotrophic bacteria due to their high-water activity and nutritional value. These bacteria can grow at refrigerated temperatures, posing significant concerns for food safety and quality. Psychrotrophic bacteria, including Pseudomonas, Listeria, and Yersinia, not only spoil food but can also produce heat-resistant enzymes and toxins, posing health risks. This review examines the characteristics and species composition of psychrotrophic bacteria in animal-derived foods, their impact on food spoilage and safety, and contamination patterns in various products. It explores several nonthermal techniques to combat bacterial contamination as alternatives to conventional thermal methods, which can affect food quality. This review highlights the importance of developing nonthermal technologies to control psychrotrophic bacteria that threaten the cold storage of animal-derived foods. By adopting these technologies, the food industry can better ensure the safety and quality of animal-derived foods for consumers.

Footprint analysis of CO2 in microbial community succession of raw milk and assessment of its quality

With the growing production of raw milk, interest has been increasing in its quality control. CO2, as a cold processing additive, has been studied to extend the cold storage period and improve the quality of raw milk. However, it is yet uncertain how representative microbial species and biomarkers can succeed one another at distinct critical periods during refrigeration. Therefore, the effects of CO2 treatment on the succession footprint of the microbial community and changes in quality during the period of raw milk chilling were examined by 16S rRNA analysis combined with electronic nose, and electronic tongue techniques. The results indicated that, the refrigeration time was shown to be prolonged by CO2 in a concentration-dependent way. And CO2 treatment was linked to substantial variations in beta and alpha diversity as well as the relative abundances of various microbial taxa (p < 0.01). The dominant bacterial phylum Proteobacteria was replaced with Firmicutes, while the major bacterial genera Acinetobacter and Pseudomonas were replaced with lactic acid bacteria (LAB), including Leuconostoc, Lactococcus, and Lactobacillus. From the perspective of biomarkers enriched in CO2-treated sample, almost all of them belong to LAB, no introduction of harmful toxins has been found. The assessment of the quality of raw milk revealed that CO2 improved the quality of raw milk by lowering the acidity and the rate of protein and fat breakdown, and improved the flavor by reducing the generation of volatiles, and increasing umami, richness, milk flavor and sweetness, but reducing sourness. These findings offer a new theoretical foundation for the industrial use of CO2 in raw milk.

Dynamic Changes in the Microbial Composition and Spoilage Characteristics of Refrigerated Large Yellow Croaker (Larimichthys crocea) during Storage

The quality changes, dynamic changes in microbial composition, and diversity changes in large yellow croaker (Larimichthys crocea) during 4 °C refrigeration were studied using 16S rDNA high-throughput sequencing technology, and the total viable count (TVC), total volatile basic nitrogen (TVB-N), and thiobarbituric acid-reactive substances (TBARS) were determined. The results revealed a consistent increase in TVC, TVB-N, and TBARS levels over time. On the 9th day, TVC reached 7.43 lg/(CFU/g), while on the 15th day, TVB-N exceeded the upper limit for acceptable quality, reaching 42.56 mg/100 g. Based on the 16S rDNA sequencing results, we categorized the storage period into three phases: early storage (0th and 3rd days), middle storage (6th day), and late storage (9th, 12th, and 15th days). As the storage time increased, both the species richness and diversity exhibited a declining trend. The dominant genus identified among the spoilage bacteria in refrigerated large yellow croaker was Pseudomonas, accounting for a high relative abundance of 82.33%. A comparison was carried out of the spoilage-causing ability of three strains of Pseudomonas screened and isolated from the fish at the end of storage, and they were ranked as follows, from strongest to weakest: P. fluorescen, P. lundensis, and P. psychrophila. This study will provide a theoretical basis for extending the shelf life of large yellow croaker.

Identification, proteolytic activity quantification and biofilm-forming characterization of Gram-positive, proteolytic, psychrotrophic bacteria isolated from cold raw milk

Psychrotrophic bacteria of raw milk face the dairy industry with significant spoilage and technological problems due to their ability to produce heat-resistant enzymes and biofilms. Despite extensive information about Gram-negative psychrotrophic bacteria in milk, little is known about Gram-positive psychrotrophic bacteria in milk, and their proteolytic activity and biofilm-forming characteristics. In the present study, Gram-positive, proteolytic, psychrotrophic bacteria of cold raw milk were identified, and their proteolytic activity and biofilm-forming capacity were quantified. In total, 12 genera and 22 species were represented among the bacterial isolates, however 50% belonged to three genera, namely Staphylococcus (19.4%), Bacillus (16.7%), and Enterococcus (13.9%). Different levels of proteolytic activity were detected in the identified isolates, even among the strains belonging to the same species. In addition, proteolytic activity was significantly higher at 25°C than at 7°C for all isolates. The crystal violet staining assay in polystyrene microtitre plates revealed a high level of variation in the biofilm-forming capacity at 7°C. After 72 hours of incubation, 11.1% of the strains did not produce a biofilm, while 27.8%, 52.8%, and 8.3% produced low, moderate, and high amounts of biofilm on polystyrene, respectively. The psychrotrophic bacteria were also able to produce biofilms on the surface of stainless steel coupons in ultra-high temperature milk after 72 h of incubation at 7°C; the number of attached cells ranged from 1.34 to 5.11 log cfu/cm2. These results expand the knowledge related to the proteolytic activity and biofilm-forming capacity of Gram-positive psychrotrophic milk bacteria.

The Impact of Low-Temperature Inactivation of Protease AprX from Pseudomonas on Its Proteolytic Capacity and Specificity: A Peptidomic Study

The destabilization of UHT milk during its shelf life can be promoted by the residual proteolytic activity attributed to the protease AprX from Pseudomonas. To better understand the hydrolysis patterns of AprX, and to evaluate the feasibility of using low-temperature inactivation (LTI) for AprX, the release of peptides through AprX activity on milk proteins was examined using an LC-MS/MS-based peptidomic analysis. Milk samples were either directly incubated to be hydrolyzed by AprX, or preheated under LTI conditions (60 °C for 15 min) and then incubated. Peptides and parent proteins (the proteins from which the peptides originated) were identified and quantified. The peptides were mapped and the cleavage frequency of amino acids in the P1/P1′ positions was analyzed, after which the influence of LTI and the potential bitterness of the formed peptides were determined. Our results showed that a total of 2488 peptides were identified from 48 parent proteins, with the most abundant peptides originating from κ-casein and β-casein. AprX may also non-specifically hydrolyze other proteins in milk. Except for decreasing the bitterness potential in skim UHT milk, LTI did not significantly reduce the AprX-induced hydrolysis of milk proteins. Therefore, the inactivation of AprX by LTI may not be feasible in UHT milk production.

Microbial Properties of Raw Milk throughout the Year and Their Relationships to Quality Parameters

Raw milk microbiota is complex and influenced by many factors that facilitate the introduction of undesirable microorganisms. Milk microbiota is closely related to the safety and quality of dairy products, and it is therefore critical to characterize the variation in the microbial composition of raw milk. In this cross-sectional study, the variation in raw milk microbiota throughout the year (n = 142) from three farms in China was analyzed using 16S rRNA amplicon sequencing, including α and β diversity, microbial composition, and the relationship between microbiota and milk quality parameters. This aimed to characterize the contamination risk of raw milk throughout the year and the changes in quality parameters caused by contamination. Collection month had a significant effect on microbial composition; microbial diversity was higher in raw milk collected in May and June, while milk collected in October and December had the lowest microbial diversity. Microbiota composition differed significantly between milk collected in January−June, July−August, and September−December (p < 0.05). Bacterial communities represented in raw milk at the phylum level mainly included Proteobacteria, Firmicutes and Bacteroidota; Pseudomonas, Acinetobacter, Streptococcus and Lactobacillus were the most common genera. Redundancy analysis (RDA) found strong correlations between microbial distribution and titratable acidity (TA), fat, and protein. Many genera were significantly correlated with TA, for example Acinetobacter (R = 0.426), Enhydrobacter (R = 0.309), Chryseobacterium (R = 0.352), Lactobacillus (R = −0.326), norank_o__DTU014 (R = −0.697), norank_f__SC-I-84 (R = −0.678), and Subgroup_10 (R = −0.721). Additionally, norank_f__ Muribaculaceae was moderately negatively correlated with fat (R = −0.476) and protein (R = −0.513). These findings provide new information on the ecology of raw milk microbiota at the farm level and contribute to the understanding of the variation in raw milk microbiota in China.

Diversity and proteolytic activity of Pseudomonas species isolated from raw cow milk samples across China

Pseudomonas spp. are common microorganisms from cold-storage raw milk, and protease secreted by Pseudomonas spp. can cause the deterioration of stored milk. However, analyses of Pseudomonas spp. diversity and proteolytic activity in raw milk from different regions of China have not been extensively examined. With this aim, the diversity and proteolytic activity of Pseudomonas isolated from 25 raw cow milk samples from Inner Mongolia, Heilongjiang, Gansu, Henan, Anhui, Jiangsu, Chongqing and Hunan of China in different seasons were evaluated by PCR targeting 16S rDNA and rpoD, as well as TNBS method, respectively. A total of 116 Pseudomonas isolates from 25 raw cow milk samples were identified at the species level, including P. fluorescens, P. veronii, P. psychrophila, P. lundensis, P. lactis, P. azotoformans, P. granadensis, P. lurida, P. rhizosphaerae, P. rhodesiae and P. extremorientalis. P. fluorescens accounted for 75.8% of the total. Of all 116 Pseudomonas isolates, 68.9% of them displayed proteolytic activity at 4 °C, 81.9% at 10 °C and 85.3% at 25 °C, respectively. The aprX gene encoded a secreted and heat-resistant metalloprotease that was present in 60.3% of the Pseudomonas isolates tested. The proteases showed residual activity ranged from 73 ± 4% to 84 ± 7% residual activity after the heat treatment at 72 °C for 15 s and 62 ± 3% to 74 ± 2% after the heat treatment at 132 °C for 4 s. This is the first report to compare Pseudomonas spp. diversity and proteolytic activity at species levels in raw milk from different regions of China. The results of this study provide valuable data about the diversity and spoilage potential of Pseudomonas species in raw milk and the thermal resistance of the proteases. Therefore, these findings provide a reference for the importance to prevent Pseudomonas spp. contamination of raw cow milk to ensure the quality and safety of milk and dairy products.

Identification of Pseudomonas jessenii and Pseudomonas gessardii as the most proteolytic Pseudomonas isolates in Iranian raw milk and their impact on stability of sterilized milk during storage

Identification of the most proteolytic Pseudomonas strains that can produce heat-resistant proteases and contribute to the Ultra High Temperature (UHT) milk destabilization is of great interest. In the present study, among the 146 Pseudomonas isolates that encoded the aprX gene, five isolates with the highest proteolytic activity were selected and identified based on 16S rRNA, rpoD and gyrB gene sequences data. The identification results were confirmed by phylogenetic analysis based on multilocus sequence analysis and identified the representative isolates as P. jessenii (two isolates) and P. gessardii (three isolates). Casein zymography demonstrated the ability of these species to produce heat-resistant enzymes, AprX, with molecular mass of about 48 kDa during storage at 7° C for 72 h. In sterilized milk samples, the residual activity of AprX caused a considerable enhancement in the degree of protein hydrolysis, non-protein nitrogen and non-casein nitrogen contents of the samples during a two-month storage. This enhancement was slightly higher in samples containing enzyme produced by P. jessenii compared to P. gessardii ones, resulting in earlier onset of sterilized milk destabilization. Hence, this study revealed that P. jessenii and P. gessardii can play a considerable role in deterioration of Iranian commercial long-life milk.

Interspecies Interactions in Dual-Species Biofilms Formed by Psychrotrophic Bacteria and the Tolerance of Sessile Communities to Disinfectants

ABSTRACT

Biofilms on the surface of food processing equipment act as potential reservoirs of microbial contamination. Bacterial interactions are believed to play key roles in both biofilm formation and antimicrobial tolerance. In this study, Aeromonas hydrophila, Chryseobacterium oncorhynchi, and Pseudomonas libanensis, which were previously isolated from Chinese raw milk samples, were selected to establish two dual-species biofilm models (P. libanensis plus A. hydrophila and P. libanensis plus C. oncorhynchi) on stainless steel at 7°C. Subsequently, three disinfectants, hydrogen peroxide (100 ppm), peracetic acid (100 ppm), and sodium hypochlorite (100 ppm), were used to treat the developed sessile communities for 10 min. Structural changes after exposure to disinfectants were analyzed with confocal laser scanning microscopy. The cell numbers of both A. hydrophila and C. oncorhynchi recovered from surfaces increased when grown as dual species biofilms with P. libanensis. Dual-species biofilms were more tolerant of disinfectants than were each single-species biofilm. Peracetic acid was the most effective disinfectant for removing biofilms, followed by hydrogen peroxide and sodium hypochlorite. The results expand the knowledge of mixed-species biofilms formed by psychrotrophic bacteria and will be helpful for developing effective strategies to eliminate bacterial mixed-species biofilms.

HIGHLIGHTS

Microbial Populations of Fresh and Cold Stored Donkey Milk by High-Throughput Sequencing Provide Indication for A Correct Management of This High-Value Product

Donkey milk is receiving increasing interest due to its attractive nutrient and functional properties (but also cosmetic), which make it a suitable food for sensitive consumers, such as infants with allergies, the immunocompromised, and elderly people. Our study aims to provide further information on the microbial variability of donkey milk under cold storage conditions. Therefore, we analysed by high-throughput sequencing the bacterial communities in unpasteurized donkey milk just milked, and after three days of conservation at 4 °C, respectively. Results showed that fresh donkey milk was characterized by a high incidence of spoilage Gram-negative bacteria mainly belonging to Pseudomonas spp. A composition lower than 5% of lactic acid bacteria was found in fresh milk samples, with Lactococcus spp. being the most abundant. The occurrence of microbial species belonging to risk group 2 was found in fresh milk. After three days of cold storage, the bacterial biodiversity of donkey milk was strongly reduced, since about 93% of the bacterial communities were identified as different species of psychrotrophic Pseudomonas. In conclusion, we report a preliminary description of the microbial diversity of donkey milk by using a metagenomic approach and encouraging a correct exploitation of this high-value niche product.

Understanding spoilage microbial community and spoilage mechanisms in foods of animal origin

The increasing global population has resulted in increased demand for food. Goods quality and safe food is required for healthy living. However, food spoilage has resulted in food insecurity in different regions of the world. Spoilage of food occurs when the quality of food deteriorates from its original organoleptic properties observed at the time of processing. Food spoilage results in huge economic losses to both producers (farmers) and consumers. Factors such as storage temperature, pH, water availability, presence of spoilage microorganisms including bacteria and fungi, initial microbial load (total viable count-TVC), and processing influence the rate of food spoilage. This article reviews the spoilage microbiota and spoilage mechanisms in meat and dairy products and seafood. Understanding food spoilage mechanisms will assist in the development of robust technologies for the prevention of food spoilage and waste.

Insights into Psychrotrophic Bacteria in Raw Milk: A Review

HIGHLIGHTS

Levels of psychrotrophic bacteria in raw milk are affected by to habitats and farm hygiene. Biofilms formed by psychrotrophic bacteria are persistent sources of contamination. Heat-stable enzymes produced by psychrotrophic bacteria compromise product quality. Various strategies are available for controlling dairy spoilage caused by psychrotrophic bacteria.

Spoilage potential of psychrotrophic bacteria isolated from raw milk and the thermo-stability of their enzymes

The storage and transportation of raw milk at low temperatures promote the growth of psychrotrophic bacteria and the production of thermo-stable enzymes, which pose great threats to the quality and shelf-life of dairy products. Though many studies have been carried out on the spoilage potential of psychrotrophic bacteria and the thermo-stabilities of the enzymes they produce, further detailed studies are needed to devise an effective strategy to avoid dairy spoilage. The purpose of this study was to explore the spoilage potential of psychrotrophic bacteria from Chinese raw milk samples at both room temperature (28 °C) and refrigerated temperature (7 °C). Species of Yersinia, Pseudomonas, Serratia, and Chryseobacterium showed high proteolytic activity. The highest proteolytic activity was shown by Yersinia intermedia followed by Pseudomonas fluorescens (d). Lipolytic activity was high in isolates of Acinetobacter, and the highest in Acinetobacter guillouiae. Certain isolates showed positive β-galactosidase and phospholipase activity. Strains belonging to the same species sometimes showed markedly different phenotypic characteristics. Proteases and lipases produced by psychrotrophic bacteria retained activity after heat treatment at 70, 80, or 90 °C, and proteases appeared to be more heat-stable than lipases. For these reasons, thermo-stable spoilage enzymes produced by a high number of psychrotrophic bacterial isolates from raw milk are of major concern to the dairy industry. The results of this study provide valuable data about the spoilage potential of bacterial strains in raw milk and the thermal resistance of the enzymes they produce.

Insights into Bacterial Milk Spoilage with Particular Emphasis on the Roles of Heat-Stable Enzymes, Biofilms, and Quorum Sensing

Milk spoilage caused by psychrotrophic bacteria and their heat-stable enzymes is a serious challenge for the dairy industry. In many studies, spoilage has been explored based on the simplistic view of undesirable enzymes produced by planktonic cells. Recently, biofilms and quorum sensing (QS) have been suggested as important factors in the deterioration of milk, which opens new avenues for investigation of the processes and challenges. Production and heat stability of enzymes are enhanced in biofilms, mainly because of inherent differences in physiological states and protective shielding by extracellular polymeric substances. QS plays a key role in modulating expression of hydrolytic enzymes and biofilm formation. To date, few studies have been conducted to investigate the complex interplays of enzyme production, biofilm formation, and QS. This review provides novel insights into milk spoilage with particular emphasis on the roles of biofilms and QS and summarizes potential effective strategies for controlling the spoilage of milk.