Inactivation and recovery of Staphylococcus aureus in milk, apple juice and broth treated with ohmic heating

Inactivation of Bacillus cereus spores by ohmic heating: Efficiency and changes of spore biological properties

Bacillus cereus spores pose a significant concern during food processing due to their high resistance to environmental stress. Ohmic heating (OH) is an emerging and alternative heating technology with potential for inactivating such spores. This study evaluated the inactivation effects and the biological property changes of Bacillus cereus spores during OH treatments. OH effectively inactivated spores in milk, orange juice, broth, rice soup, and buffer solution in less time than oil bath heating (OB). A decrease in NaCl content improved spore inactivation at the same temperature. Spores were more sensitive to acid at 80-85 °C with OH treatment. Furthermore, OH at 10 V/cm and 50 Hz could reduce the spore resistance and inhibit an increase in spore hydrophobicity and spore aggregation. Both heating methods resulted in significant dipicolinic acid (DPA) leakage and damage to the cortex and inner membranes of the spores. However, OH at 10 V/cm and 50 Hz had the lowest DPA leakage and inflicted the least damage to the inner membrane. The damage to the spore's inner membrane was considered the primary reason for inactivation by OB and OH treatments. Still, OH at 10 V/cm and 50 Hz might also block the germination or outgrowth of treated spores or cause damage to the spore core.

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.

Viability, Sublethal Injury, and Release of Cellular Components From Alicyclobacillus acidoterrestris Spores and Cells After the Application of Physical Treatments, Natural Extracts, or Their Components

Alicyclobacillus acidoterrestris is a spoiling microorganism regarded as one of the most important causes of spoilage of fruit juices and acidic products. In this paper, four strains of A. acidoterrestris (type strain-DSM 3922; two wild strains isolated from soil-C8 and C24; wild strain isolated from a spoiled pear juice CB1) were treated through natural extracts/active compounds from essential oils (EOs), and physical treatments were used to assess their susceptibility and the presence of sublethal injury. The characterization of damage was also performed. The results suggest that it is possible to control A. acidoterrestris through alternative approaches, although the effect relied upon the age of spores. In addition to the mere antimicrobial effect, some treatments could cause a sublethal injury on spores. Lemon extract was the most effective treatment for both the antimicrobial effect and the sublethal injury, as evidenced by the release of proteins, and calcium dipicolinate [dipicolinic acid (DPA)] by fresh spores and only DPA (with an exception for C8) by old spores. A sublethal injury with protein release was also found for physical treatments [US (ultrasound) or heating]. For the first time, this paper reports on the existence of a sublethal injury for A. acidoterrestris, and this evidence could also be a challenge, because injured microorganisms could restore their metabolism, or an opportunity to design new preserving treatments.