State of the art of nonthermal and thermal processing for inactivation of micro-organisms.

Despite the constant development of novel thermal and nonthermal technologies, knowledge on the mechanisms of microbial inactivation is still very limited. Technologies such as high pressure, ultraviolet light, pulsed light, ozone, power ultrasound and cold plasma (advanced oxidation processes) have shown promising results for inactivation of micro-organisms. The efficacy of inactivation is greatly enhanced by combination of conventional (thermal) with nonthermal, or nonthermal with another nonthermal technique. The key advantages offered by nonthermal processes in combination with sublethal mild temperature (<60°C) can inactivate micro-organisms synergistically. Microbial cells, when subjected to environmental stress, can be either injured or killed. In some cases, cells are believed to be inactivated, but may only be sublethally injured leading to their recovery or, if the injury is lethal, to cell death. It is of major concern when micro-organisms adapt to stress during processing. If the cells adapt to a certain stress, it is associated with enhanced protection against other subsequent stresses. One of the most striking problems during inactivation of micro-organisms is spores. They are the most resistant form of microbial cells and relatively difficult to inactivate by common inactivation techniques, including heat sterilization, radiation, oxidizing agents and various chemicals. Various novel nonthermal processing technologies, alone or in combination, have shown potential for vegetative cells and spores inactivation. Predictive microbiology can be used to focus on the quantitative description of the microbial behaviour in food products, for a given set of environmental conditions.

The effect of high-power ultrasound and gas phase plasma treatment on Aspergillus spp. and Penicillium spp. count in pure culture.

AIMS: The aim of this study was to investigate and compare two nonthermal techniques in the inactivation of moulds. METHODS AND RESULTS: High power ultrasound (20 kHz) and nonthermal gas phase plasma treatments were studied in the inactivation of selected moulds. Aspergillus spp. and Penicillium spp. were chosen as the most common mould present in or on food. Experimental design was introduced to establish and optimize working variables. For high power ultrasound, the greatest reduction of moulds (indicated by the total removal of viable cells) was obtained after ultrasound treatments at 60°C (thermosonication) for 6 and 9 min (power applied, 20-39 W). For plasma treatment, the greatest inactivation of moulds was observed for the longest treatment time (5 min) and lowest sample volume (2 ml), (AP12, AP13, PP12 and PP13). CONCLUSIONS: The great amount of applied energy required for achieving a partial log reduction in viable cells is the limiting factor for using high-power ultrasound. However, both treatment methods could be combined in the future to produce beneficial outcomes. SIGNIFICANCE AND IMPACT OF THE STUDY: This study deals with nonthermal food processing techniques and the results and findings present in this study are the root for further prospective studies. The food industry is looking for nonthermal methods that will enable food preservation, reduce deterioration of food compounds and structure and prolong food shelf life.