Microbial inactivation

Journal of Applied Microbiology 2005, 98, 1387–1399

doi:10.1111/j.1365-2672.2005.02561.x

A REVIEW Microbial inactivation by new technologies of food preservation
´ ˜ P. Manas and R. Pagan
Tecnologı´a de los Alimentos, Facultad de Veterinaria, Universidad de Zaragoza, Zaragoza, Spain
2004/0771: received 5 July 2004, revised and accepted 26 November 2004

1. Summary, 1387 2.Introduction, 1387 3. Mechanisms of inactivation, 1388 3.1 Inactivation targets and mode of action, 1388 3.2 Sublethal injury, 1390 3.3 Stress adaptation and resistance, 1391 4. Factors affecting microbial resistance, 1392 4.1 Process parameters, 1392

4.2 Microbial characteristics, 1393 4.3 Product parameters, 1394 5. Kinetics of inactivation, 1395 6. Concluding remarks, 1396 7. References, 1397

1.SUMMARY The increasing consumer demand for Ôfresh-likeÕ 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 themicrobiological 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. 2. INTRODUCTION Micro-organisms are the mainagents 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 environmentalconditions such as, for
´ Correspondence to: Pilar Manas, Tecnologıa de los Alimentos, Facultad de ˜ Veterinaria, Universidad de Zaragoza, C/Miguel Servet, 177, 50013 Zaragoza, Spain (e-mail: manas@unizar.es).

instance, the maintenance of the chill chain. Thermal treatment is the most widely used procedure for microbial inactivation in foods. However, heat causes unwanted sideeffects in thesensory, 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 involvesthe 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 ¼ 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)] increasessubstantially 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

ª 2005 The Society for Applied Microbiology

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were conducted at the end of...