High pressure homogenization inactivation of Escherichia coli and Staphylococcus aureus in phosphate buffered saline, milk and apple juice

Effect of High-Pressure Micro-Fluidization on the Inactivation of Staphylococcus aureus in Liquid Food

High-pressure homogenization has been extensively studied for its excellent homogenization effect and the prospect of continuous liquid food production, but its sterilization ability still needs to be improved. In this study, we replaced the homogenization valve with two opposing diamond nozzles (0.05 mm inner diameter) so that the fluid collided at high velocity, corresponding to high-pressure micro-fluidization (HPM). Moreover, HPM treatment significantly inactivated Staphylococcus aureus ~7 log in the liquid with no detectable sub-lethal state at a pressure of 400 MPa and a discharge temperature of 50 °C. The sterilization effect of HPM on S. aureus subsp. aureus was attributed to a significantly disrupted cell structure and increased membrane permeability, which led to the leakage of intracellular proteins, resulting in bacterial death. At the same time, HPM treatment was able to significantly reduce the ability of S. aureus subsp. aureus to form biofilms, which, in turn, reduced its virulence. Finally, compared to the simulated system, more effective sterilization was observed in apple juice, with its color and pH remaining unchanged, which suggested that HPM can be used to process other liquid foods.

Validation of High-Pressure Homogenization Process to Pasteurize Brazil Nut (Bertholletia excelsa) Beverages: Sensorial and Quality Characteristics during Cold Storage

The effect of high-pressure homogenization (HPH) on the inactivation of Escherichia coli and the stability of the quality properties of Brazil nut beverages were studied. E. coli was used as target microorganism to validate the HPH process (pressures from 50 to 180 MPa and inlet temperatures (Ti) from 25 to 75 °C). Cold storage (5 °C) for 21 days was conducted to establish the shelf-life of BN beverages, in terms of their microbiological, physical, physicochemical, and sensorial stability. HPH-treated samples were compared to pasteurized BN beverages (63 °C for 20 min). The combination of Ti and the pressure of the HPH process (50 to 150 MPa/75 °C and 180 MPa/25 °C) had a significant effect on E. coli inactivation (8.2 log CFU/mL). During storage at 5 °C, the growth of mesophilic aerobes in processed BN beverages was controlled by the HPH process. Oxidative stability (TBAR assay) and physicochemical properties (pH, acidity, and °Brix) were evaluated during cold storage, showing good stability. Additionally, HPH-treated beverages showed a reduction in their particle size and the formation of more stable protein aggregates, which favored the beverages’ whiteness (color). The HPH process could be an alternative to pasteurization to obtain Brazil nut beverages with an acceptable microbiological shelf life (≥21 days at 5 °C) and high-quality characteristics without the use of any additives.

A novel gdmH-related gene, ghl, involved in environmental stress tolerance and vancomycin susceptibility in milk-derived Staphylococcus aureus

Staphylococcus aureus is one of the main microorganisms that contaminate dairy products and pickled foods, and has a great impact on economy and human health. GdmH-related proteins, as important functional units widely present in Staphylococcus species, have not been reported in S. aureus so far. In this study, we identified a gdmH-related gene, named ghl. We found that mutation of ghl gene could decrease the tolerance of environmental stresses (heat, desiccation, salt and hydrogen peroxide) of S. aureus and enhanced the capacities of biofilm formation. In addition, the ghl mutant was more sensitive to vancomycin on CAMHB solid plates but more resistant to vancomycin in CAMHB liquid medium compared to wild type RMSA24. These results indicated that ghl is an important factor to respond to environmental stress in foodborne S. aureus. This paper for the first time reported that a GdmH-related protein plays an important role in environmental tolerance, providing a new direction for the follow-up study of GdmH-related proteins, as well as a potential target gene for further research on the tolerance mechanism of Staphylococcus aureus in food processing and the control of biofilm formation.

Effect of ultra-high pressure homogenization on microorganism and quality of composite pear juice

In this study, composite pear juice was processed by ultra-high pressure homogenization (UHPH) at four different pressures (50, 100, 150, and 200 MPa) with six different temperatures (4, 20, 30, 40, 60, and 80°C), then microorganism and physicochemical and nutritional properties of the samples were investigated. The counts of total aerobic bacteria (TAB) and yeasts and molds (Y&M) were reduced by 0.89-4.72 log10 CFU/ml and 0.40-3.03 log10 CFU/ml after processing, respectively. There was no significant change on total soluble solid and color, but significant decreases of pH and particle size value were observed, and the antioxidant activity, total phenolic content, viscosity, and suspension stability significantly increased in treated samples. Compared to the untreated samples, polyphenol oxidase (PPO) and peroxidase (POD) activity of UHPH-treated samples varied between 97%-126% and 81%-165%, respectively, indicating that the PPO and POD activities could be inactivated or activated by UHPH. This study introduced proper temperature combined with UHPH could improve the microbial inactivation and the quality of the compound juice.