Characterization of Pseudomonas spp. contamination and in situ spoilage potential in pasteurized milk production process

To investigate the prevalence of Pseudomonas in the pasteurized milk production process and its effect on milk quality, 106 strains of Pseudomonas were isolated from the pasteurized milk production process of a milk production plant in Shaanxi Province, China. The protease, lipase and biofilm-producing capacities of the 106 Pseudomonas strains were evaluated, and the spoilage enzyme activities of their metabolites were assessed by simulating temperature incubation in the refrigerated (7 °C) and transport environment (25 °C) segments and thermal treatments of pasteurization (75 °C, 5 min) and ultra-high temperature sterilization (121 °C, 15 s). A phylogenetic tree was drawn based on 16S rDNA gene sequencing and the top 5 strains were selected as representative strains to identify their in situ spoilage potential by examining their growth potential and ability to hydrolyze proteins and lipids in milk using growth curves, pH, whiteness, Zeta-potential, lipid oxidation, SDS-PAGE and volatile flavor compounds. The results showed that half and more of the isolated Pseudomonas had spoilage enzyme production and biofilm capacity, and the spoilage enzyme activity of metabolites was affected by the culture temperature and sterilization method, but ultra-high temperature sterilization could not completely eliminate the enzyme activity. The growth of Pseudomonas lundensis and Pseudomonas qingdaonensis was less affected by temperature and time, and the hydrolytic capacity of extracellular protease and lipase secreted by Pseudomonas lurida was the strongest, which had the greatest effect on milk quality. Therefore, it is crucial to identify the key contamination links of Pseudomonas, the main bacteria responsible for milk spoilage, and the influence of environmental factors on its deterioration.

The use of biomimetic surfaces to reduce single- and dual-species biofilms of Escherichia coli and Pseudomonas putida

The ability of bacteria to adhere to and form biofilms on food contact surfaces poses serious challenges, as these may lead to the cross-contamination of food products. Biomimetic topographic surface modifications have been explored to enhance the antifouling performance of materials. In this study, the topography of two plant leaves, Brassica oleracea var. botrytis (cauliflower, CF) and Brassica oleracea capitate (white cabbage, WC), was replicated through wax moulding, and their antibiofilm potential was tested against single- and dual-species biofilms of Escherichia coli and Pseudomonas putida. Biomimetic surfaces exhibited higher roughness values (SaWC = 4.0 ± 1.0 μm and SaCF = 3.3 ± 1.0 μm) than the flat control (SaF = 0.6 ± 0.2 μm), whilst the CF surface demonstrated a lower interfacial free energy (ΔGiwi) than the WC surface (-100.08 mJ m-2 and -71.98 mJ m-2, respectively). The CF and WC surfaces had similar antibiofilm effects against single-species biofilms, achieving cell reductions of approximately 50% and 60% for E. coli and P. putida, respectively, compared to the control. Additionally, the biomimetic surfaces led to reductions of up to 60% in biovolume, 45% in thickness, and 60% in the surface coverage of single-species biofilms. For dual-species biofilms, only the E. coli strain growing on the WC surface exhibited a significant decrease in the cell count. However, confocal microscopy analysis revealed a 60% reduction in the total biovolume and surface coverage of mixed biofilms developed on both biomimetic surfaces. Furthermore, dual-species biofilms were mainly composed of P. putida, which reduced E. coli growth. Altogether, these results demonstrate that the surface properties of CF and WC biomimetic surfaces have the potential for reducing biofilm formation.

The sensory evaluation and antimicrobial efficacy of Lactobacillus acidophilus supernatant on Salmonella enteritidis in milk

Postbiotics are metabolites derived from living probiotic bacteria like Lactobacillus strains, during the fermentation process and/or produced in pure form on laboratory scales. These compounds, depending on the type of probiotic from which they are prepared, have specific antibacterial agents such as: organic acids, bacteriocins, short-chain fatty acids, and peptides. The objective of this study was to investigate the effect of Lactobacillus acidophilus supernatant (LAS) on the growth pattern of Salmonella enteritidis at fluctuating temperatures and the sensory evaluation of milk that contains this probiotic. Baranyi and Roberts's model determined the best-fit curve for the microbial growth. According to mathematical equations, the highest and lowest specific growth (μ max) rates of S. enteritidis were obtained at 0.055 h-1 and 0.0059 h-1 and also highest and lowest maximum generation time (MGT) values were obtained at 20.06 h and 8.85 h, respectively. Sensory evaluation by the Triangel test reveals that LAS could not establish a significant (p > .05) adverse effect on milk perceptible. Regarding the results obtained in the present study, LAS, without causing adverse sensory change, could act as a safe food additive for the control of bacterial pathogens and reducing food waste, particularly in milk and milk-containing food products.

Combined effect of nitrogen and phosphorus on growth and biochemical composition of Tetradesmus obliquus (Turpin) M.J. Wynne

Microalgae have many biotechnological applications in various industries including food and feed, fertilizer, biofuel, cosmetics, pharmaceutics, and wastewater treatment. Since hey produce secondary metabolites under stress conditions such as pigments, carotenoids, hydrocarbons, and vitamins, investigating the effects of stress factors on growth parameters and biochemical composition of microalgal biomass is needed in producing bioproducts. In this paper, the combined effects of nitrogen and phosphorus on growth and the protein/amino acid and Lipid-FAMEs profiles of microalgae Tetradesmus obliquus (MAKUMACC-037) were investigated. Nitrogen and phosphorus deficiency reduced the algal growth. Biochemical composition was changed in a nitrogen and phosphorus dependent manner. High concentration of protein and lipid were associated with increased nitrogen and phosphorus concentration However, the FAMEs profiles were changed depending on only the nitrogen concentration.

Development of a Monte Carlo simulation model to predict pasteurized fluid milk spoilage due to post-pasteurization contamination with gram-negative bacteria

Psychrotolerant gram-negative bacteria introduced as post-pasteurization contamination (PPC) are a major cause of spoilage and reduced shelf life of high-temperature, short-time pasteurized fluid milk. To provide improved tools to (1) predict pasteurized fluid milk shelf life as influenced by PPC and (2) assess the effectiveness of different potential interventions that could reduce spoilage due to PPC, we developed a Monte Carlo simulation model that predicts fluid milk spoilage due to psychrotolerant gram-negative bacteria introduced as PPC. As a first step, 17 gram-negative bacterial isolates frequently associated with fluid milk spoilage were selected and used to generate growth data in skim milk broth at 6°C. The resulting growth parameters, frequency of isolation for the 17 different isolates, and initial concentration of bacteria in milk with PPC, were used to develop a Monte Carlo model to predict bacterial number at different days of shelf life based on storage temperature of milk. This model was then validated with data from d 7 and 10 of shelf life, collected from commercial operations. The validated model predicted that the parameters (1) maximum growth rate and (2) storage temperature had the greatest influence on the percentage of containers exceeding 20,000 cfu/mL standard plate count on d 7 and 10 (i.e., spoiling due to PPC), which indicates that accurate data on maximum growth rate and storage temperature are important for accurate predictions. In addition to allowing for prediction of fluid milk shelf life, the model allows for simulation of "what-if" scenarios, which allowed us to predict the effectiveness of different interventions to reduce overall fluid milk spoilage due to PPC through a set of proof-of-concept scenario (e.g., frequency of PPC in containers reduced from 100% to 10%; limiting distribution temperature to a maximum of 6°C). Combined with other models, such as previous models on fluid milk spoilage due to psychrotolerant spore-forming bacteria, the data and tools developed here will allow for rational, digitally enabled, fluid milk shelf life prediction and quality enhancement.

Evaluation of the Effect of Essential Oil Addition on the Quality Parameters and Predicted Shelf Life of Potato Yogurt

ABSTRACT

Potato, the third most important food crop worldwide, is rich in nutrients but low in protein. In contrast, milk is rich in protein. Yogurt produced through the cofermentation of potatoes and milk is a highly nutritious food. The quality and shelf life of yogurt are important topics in the dairy industry. The objective of this study was to explore the effect of the addition of essential oil (EO) on the shelf life and quality of potato yogurt. The antimicrobial effects of several EOs, the effect of perilla leaf EO (PLEO) concentration on potato yogurt, and the volatile flavor components of PLEO and PLEO potato yogurt were evaluated. The effects of storage time and temperature on the pH, microbial counts, and sensory characteristics of PLEO potato yogurt also were analyzed to establish a shelf-life model. PLEO had an antimicrobial effect and was the appropriate EO for use in the potato yogurt. A total of 69 compounds were detected in PLEO, and limonene was the main compound. PLEO had an effect on the pH, sensory characteristics, and viable bacterial counts of potato yogurt during storage. The optimal concentration of added PLEO was 0.04%. PLEO had considerable influence on volatile flavor components, and the consumer acceptance of 0.04% PLEO potato yogurt was higher than that of potato yogurt without PLEO in the later stage of storage. The shelf life of potato yogurt with PLEO was 6 days longer than that of the control yogurt. PLEO also improved the concentrations of active terpene substances in potato yogurt. The prediction models based on pH and sensory scores at 5°C were established as A = A0e0.00323t and A = A0e0.00355t, respectively. Comparison of the accuracy factor and the deviation factor of the models revealed that the sensory prediction model was more accurate than the pH prediction model. The results of this study provide theoretical and data support for the industrial development of yogurt with EOs, including extension and prediction of its shelf life.

HIGHLIGHTS

Influence of Nitrogen and Phosphorus on Microalgal Growth, Biomass, Lipid, and Fatty Acid Production: An Overview

Microalgae can be used as a source of alternative food, animal feed, biofuel, fertilizer, cosmetics, nutraceuticals and for pharmaceutical purposes. The extraction of organic constituents from microalgae cultivated in the different nutrient compositions is influenced by microalgal growth rates, biomass yield and nutritional content in terms of lipid and fatty acid production. In this context, nutrient composition plays an important role in microalgae cultivation, and depletion and excessive sources of this nutrient might affect the quality of biomass. Investigation on the role of nitrogen and phosphorus, which are crucial for the growth of algae, has been addressed. However, there are challenges for enhancing nutrient utilization efficiently for large scale microalgae cultivation. Hence, this study aims to highlight the level of nitrogen and phosphorus required for microalgae cultivation and focuses on the benefits of nitrogen and phosphorus for increasing biomass productivity of microalgae for improved lipid and fatty acid quantities. Furthermore, the suitable extraction methods that can be used to utilize lipid and fatty acids from microalgae for biofuel have also been reviewed.

A protein-free chemically defined medium for the cultivation of various micro-organisms with food safety significance

AIMS

To develop a broadly applicable medium free of proteins with well-defined and reproducible chemical composition for the cultivation of various micro-organisms with food safety significance.

METHODS AND RESULTS

The defined medium was designed as a buffered minimal salt medium supplemented with amino acids, vitamins, trace metals and other nutrients. Various strains commonly used for food safety research were selected to test the new defined medium. We investigated single growth factors needed by different strains and the growth performance of each strain cultivated in the defined medium. Results showed that the tested strains initially grew slower in the defined medium compared to tryptic soy broth, but after an overnight incubation cultures from the defined medium reached adequately high cell densities.

CONCLUSIONS

The newly designed defined medium can be widely applied in food safety studies that require media with well-defined chemical constituents.

SIGNIFICANCE AND IMPACT OF THE STUDY

Defined media are important in studies of microbial metabolites and physiological properties. A defined medium capable of cultivating different strains simultaneously is needed in the food safety area. The new defined medium has broader applications in comparing different strains directly and provides more reproducible results.

Summary of the COVID-19 epidemic and estimating the effects of emergency responses in China

Coronavirus disease-2019 (COVID-19) pandemic has affected millions of people since December 2019. Summarizing the development of COVID-19 and assessing the effects of control measures are very critical to China and other countries. A logistic growth curve model was employed to compare the development of COVID-19 before and after the emergency response took effect. We found that the number of confirmed cases peaked 9–14 days after the first detection of an imported case, but there was a peak lag in the province where the outbreak was concentrated. Results of the growth curves indicated that the fitted cumulative confirmed cases were close to the actual observed cases, and the R² of all models was above 0.95. The average growth rate decreased by 44.42% nationally and by 32.5% outside Hubei Province. The average growth rate in the 12 high-risk areas decreased by 29.9%. The average growth rate of cumulative confirmed cases decreased by approximately 50% after the emergency response. Areas with frequent population migration have a high risk of outbreak. The emergency response taken by the Chinese government was able to effectively control the COVID-19 outbreak. Our study provides references for other countries and regions to control the COVID-19 outbreak.

Effect of Cold Chain Logistic Interruptions on Lipid Oxidation and Volatile Organic Compounds of Salmon (Salmo salar) and Their Correlations With Water Dynamics

Lipid oxidation and water migration are important factors in the quality changes of aquatic products. This study investigated the relationship between water migration and lipid oxidation in salmon filets under four different storage conditions (control: 0°C; T1: 4°C; T2 and T3: two temperature fluctuation groups) by detecting thiobarbituric acid–reactive substances, changes of fatty acids, and volatile organic compounds (VOCs), and other quality indicators including redness, microorganism, total volatile basic nitrogen (TVB-N), and water-holding capacity (WHC) were also measured. The results of low-field nuclear magnetic resonance (LF-NMR) showed that more trapped water (T₂₂) turned to form free water (T₂₃) in groups suffering temperature fluctuations. A more significant decrease in fatty acids was found in T2 and T1 groups, especially oleic acid (C18:1n9c), linoleic acid (C18:2n6c), and palmitic acid (C16:0). The VOCs with off-flavors (1-penten-3-ol, 2-penten-1-ol, (Z)-, 1-octen-3-ol, hexanal) in the groups suffered from simulated cold chain interruptions increased faster than the other two groups during storage. T₂₂ was negatively correlated (p < 0.05) with stearic acid (C18:0), 1-penten-3-ol, hexanal, and nonanal, whereas T₂₃ was positively correlated with 1-penten-3-ol, hexanal, and heptanal. Therefore, the temperature fluctuation accelerated the loss of polyunsaturated fatty acids and the increase of unpleasant odors related to water migration.

Characterization of biofilm production by Pseudomonas fluorescens isolated from refrigerated raw buffalo milk

Pseudomonas fluorescens can often be isolated from refrigerated raw milk. Two strains of P. fluorescens PL5.4 and PL7.1, isolated from raw buffalo milk, were evaluated for their proteolytic capacity, exopolysaccharide production and biofilm production. Proteolytic activity was observed in both strains. The P. fluorescens PL5.4 strain presented fluorescence in the presence of calcofluor, indicating exopolysaccharide production. Both strains were able to produce biofilm at 7 °C for 72 h. For the biofilm production test on stainless steel, adherent cell counts of up to 7.1, 7.3 and 8.8 log CFU/cm² at 7, 23 and 30 °C were obtained. Through scanning electron microscopy, it was possible to observe the biofilm produced by the P. fluorescens PL5.4 strain. Proper cleaning and disinfection practices in order are important to reduce bacterial contamination and extend the useful life of raw material and its derivatives.

Proteolytic and lipolytic potential of Pseudomonas spp. from goat and bovine raw milk

ABSTRACT: Pseudomonas, the main genus of gram-negative microorganisms isolated from milk, is psychrotrophic, biofilm-forming, and thermo-resistant deteriorating enzyme producers. The aim of this study was to quantify Pseudomonas spp. in goat’s and cow’s milk produced in the Paraná state, Brazil, to evaluate the deteriorating activity of the isolates at mesophilic and psychrotrophic conditions and to identify, at the species level, the isolates with alkaline metalloprotease (aprX gene) production potential. Microbiological, biochemical and molecular methods were used for isolating, confirming and identifying of isolates. The mean counts were 1.6 (±6.3)x104 and 0.89(±3)x102 CFU/mL for goat and bovine milk samples, respectively, immediately after milking. Of the Pseudomonas colonies isolated from goat milk (n=60), 91.7% showed proteolytic potential when incubated at 35°C/48 h and 80% at 7°C/10 days, and lipolytic potential was observed in 95% of the isolates incubated in mesophilic and 78.3% at refrigeration conditions. From the isolates of bovine milk (n=20), 35% showed proteolytic activity only when incubated at 35°C/48 h, and lipolytic potential was observed in 25% of the isolates incubated at 7°C/10d and 35°C/48h. It was observed that 83.3% and 25% of the isolates genetically confirmed as Pseudomonas spp. of goat and bovine milk showed the potential for alkaline metalloprotease production, with the species P. azotoformans, P. koreensis, P. gessardii, P. monteilii and P. lurida being the most frequent in goat milk and P. aeruginosa the only species identified in cow milk.