Microbial inactivation by new technologies of food preservation

Abstract

The increasing consumer demand for 'fresh-like' foods has led to much research effort in the last 20 years to develop new mild methods for food preservation. Nonthermal methods allow micro-organisms to be inactivated at sublethal temperatures thus better preserving the sensory, nutritional and functional properties of foods. The aim of this review is to provide an overview of the microbiological aspects of the most relevant nonthermal technologies for microbial inactivation currently under study, including irradiation, high hydrostatic pressure, pulsed electric field and ultrasound under pressure. Topics covered are the mechanisms of inactivation, sensitivity of different microbial groups and factors affecting it and kinetics of inactivation. Micro-organisms are the main agents responsible for food spoilage and food poisoning and therefore food preservation procedures are targeted towards them. Food preservation methods currently used by the industry rely either on the inhibition of microbial growth or on microbial inactivation. Methods which prevent or slow down microbial growth cannot completely assure food safety, as their efficacy depends on the environmental conditions such as, for instance, the maintenance of the chill chain. Thermal treatment is the most widely used procedure for microbial inactivation in foods. However, heat causes unwanted side-effects in the sensory, nutritional and functional properties of food. This limitation together with increasing consumer demand for fresh-like foods has promoted the development of alternative methods for microbial inactivation, among which ionizing irradiation, ultrasound under pressure, high hydrostatic pressure (HHP) and pulsed electric field (PEF) are attracting much interest. The irradiation process involves the application of electromagnetic waves or electrons to foods. Radiation sources are either gamma rays from cobalt-60, electron beams or X-rays, and the amount of irradiation absorbed by a food is measured in kGy (1 Gy = 1 J kg−1). Commercial application of ionizing radiation (IR) treatment on foods was started at the beginning of the 1980s, but its success has been prevented by consumer concerns. Nowadays, social perception of IR is changing and this technology is being re-examined. Ultrasound is defined as sound waves with frequencies above the threshold for human hearing (>16 kHz). Although ultrasound was initially discarded for food preservation because of its weak lethal action, the application of an external hydrostatic pressure of up to 600 kPa [manosonication (MS)] increases substantially the lethality of the treatment. In addition, a combination of MS with temperature [manothermosonication (MTS)] has been proposed (Raso et al. 1998a). The HHP involved the application of pressures from 100 to 1000 MPa. The first studies on the lethal effect of HHP were conducted at the end of the 19th century, but it has been in recent years when commercial applications of this procedure have started. Finally, PEF technology consists in the application of short duration (1–100 μs) high electric field pulses (10–50 kV cm−1) to a food placed between two electrodes. Like ultrasound under pressure, PEF technology is not yet being used to preserve food commercially. This review discusses the state of the art of the research of the microbial aspects of the four technologies described above. The successful implementation of a novel technology for food preservation relies on the progress in the field of mechanisms of inactivation. An adequate knowledge of the physiological behaviour of micro-organisms towards inactivation agents is essential for the development of safe foods. It is necessary for understanding the effect of environmental factors on resistance and also for identifying critical factors. It would help to interpret kinetics of inactivation and to develop mathematical models based on parameters with a biological meaning and therefore more useful and able to predict microbial inactivation in a wider range of conditions. A better knowledge of the effect of the preservation agents on the micro-organisms would lead to a more rational design of processes. Cell death has been associated with either structural damage or physiological dysfunctions. Among structural damage, disruption of the envelopes, DNA conformational changes, ribosome alterations or protein aggregation are the most frequently described. Also physiological disorders, such as membrane selective permeability alterations or loss of function of key enzymes have been proposed as events leading to cell death (Gould 1989). Perhaps the most important difficulties that researchers encounter in this area is that several of these lesions may occur simultaneously when the cells are subjected to an agent and it is therefore difficult to attribute the loss of viability of the cell to a single event. Nevertheless, it must be kept in mind that also multitarget inactivation is feasible, this being the addition