Introduction Trends in food spoilage and safety Foods deteriorate as a result of physical and chemical changes, the activities of enzymes and of micro- organisms (Table I). In addition, post-harvest losses occur due to insect pests. The activities of micro- organisms are by far the most important quantitatively, leading to enormous levels of spoilage (Table 11). Losses of commodity foods, particularly in the less-well- developed countries of the world, are estimated to exceed 50% for fruits and vegetables and 10% for cereal grains and legumes (Anon, 1993). Deterioration in colour, taste and texture of foods is catalyzed' by endogenous enzymes, and undesirable physiological changes, such as ripening and sprouting, degrade food quality. The presence of certain micro-organisms in foods may lead to food poisoning by infection or, if the micro- organisms have multiplied in a food, to intoxication, in some instances (Table 11). Unfortunately, in many developed countries, despite public awareness of food poisoning risks, the numbers of food poisoning cases are rising, rather than falling, year by year (e.g. reported cases of disease caused by Salmonella and Campylo- bader approximately doubled between 1983 and 1993, in the UK, with substantial economic consequences (Roberts and Sockett, 1994)). In developing countries, food poisoning remains one of the major causes of morbidity and mortality. Control measures are evidently failing or, at least, not making the progress that we should expect in the final decade of this millennium. New approaches to the effective elimination of the most important of the food poisoning micro-organisms from the relatively small number of most frequently contami- nated foods are urgently needed. Food preservation technologies The major food preservation technologies, which are employed to counteract the deleterious effects of micro- organisms in foods, mostly act by inhibiting or delaying their growth rather than by inactivating them (Table 111). For example, the use of cold, low pH, salts, sugars, preservatives, etc., all act essentially by inhibition. Many Table I Quality loss reactions of foods (adapted from Gould, 1989). Physical Chemical Enzymic Microbiological Mass transfer, movement of low Oxidative rancidity Lipolytic rancidity Multiplication of spoilage MW components micro-organisms Drying, loss of succulence, caking Loss of colour Proteolysis and other enzyme activities organisms Presence of infectious micro- Hydration, loss of crisp textures Non-enzymic browning Enzymic browning Multiplication of toxinogenic Loss of flavours Freeze-induced damage micro-organisms Loss of nutrients 1 2 Introduction Table I1 Microbial food spoilage and food poisoning problems (adapted from Gould, 1989). Problems Examples Food spoilage Excretion of major metabolic products Excretion of minor metabolic products Secretion of enzymes Biomass Food poisoning Presence of infectious micro-organisms Multiplication of toxinogenic micro- organisms Lactic and acetic acids causing souring; gases (carbon dioxide, hydrogen) causing blowing. Low odour threshold compounds (amines, esters, thiols) causing off- odours, discolouration. Lipases, proteases, cellulases, etc., causing flavour and texture changes. Visible presence of micro-organisms (slime, haze, mould colonies, etc.) Salmonella, Campylobacter; Listeria. Staphylococcus aureus, Clostridium botulinum. of the new developments, which have come into use or have been proposed in recent years in reaction to consumers’ requirements for less severely processed, more natural, additive-free foods, also act by inhibition (e.g. ‘modified atmosphere packaging’, use of naturally- occurring antimicrobials; Dillon and Board, 1994). Since the major underlying cause of microbial food spoilage and food poisoning is ultimately the presence of the micro-organisms in the foods in the first place, it follows that inactivation techniques are ideally prefera- ble to inhibitory ones. Heat is the only food preservation technique, which is used on a large scale, that acts primarily by inactivation. A problem with inactivation techniques, such as high- temperature processing, has been that they often tend to produce unacceptable damage in the quality of food products. For this reason, procedures that minimize heat- induced damage are being pursued, e.g. rotary retorting, microwave heating, ohmic heating, etc., for pasteuriza- tion and sterilization. Also, essentially non-thermal techniques are being explored and some are already being exploited on a small scale, e.g. enzymic tech- niques such as the addition of lysozyme, other enzymes and naturally-occurring antimicrobials to foods; physical techniques such as the application of ultra-high pressure, high-voltage electric discharges (‘electroporation’), ultrasonics combined with mild heat and slightly raised pressure (‘manothermosonication’) (Table 111, Gould, 1995). These ‘emerging’ techniques are novel and scientif- ically challenging but few of them are widely employed. As yet, one of the most effective alternatives to heat for the inactivation of micro-organisms is ionizing radiation. Table 111 Food preservation techniques (updated from Gould, 1989). Mode of action Preservation technique Inhibition or slowing of growth freezing. Lowered temperature by chilling, Reduced water activity achieved by drying, curing with added salts, conserving with added sugars. Restricted availability of nutrients in water-in-oil emulsions. Removal of oxygen from vacuum packs. Increased carbon dioxide, in ‘modified atmosphere’ packs. Addition of acids, directly or by fermentation. Increased ethanol levels by fermentation, fortification, release in packs from sachets. Addition of preservatives including naturally-occurring antimicrobials. Inactivation Heat, to blanch, pasteurize or sterilize, by hot air, water or high- pressure steam; by newer methods including microwaves and electrical (ohmic) methods. Ionizing radiation to inactivate pathogenic or spoilage micro- organisms in foods. Ultraviolet radiation to inactivate micro-organisms in water or on the surfaces of foods and packaging materials. High-intensity visible laser and non- coherent light to inactivate micro- organisms in water and on surfaces. Application of ultra-high pressure Application of high-voltage electric discharges. Application of ultrasound with mild heat and pressure (manothermosonication). Addition of bacteriolytic (e.g. lysozyme) and other enzymes and natural antimicrobials. Acid dips and sprays for carcase decontamination. Ionizing radiation Food irradiation is the use of ionizing radiation to increase food storage life, reduce post-harvest food losses and eliminate food poisoning micro-organisms. The effectiveness of ionizing radiation, its penetrating Introduction 3 the extensive losses that now occur. Food irradiation could fulfil these requirements for some foods if wider understanding and acceptance of the treatment could be achieved. power and its straightforward kinetics make it much simpler, in practice, to use than heat. It does bring about serious organoleptic changes in some foods, but very little change in others. In this respect, it is analogous to most of the other means of food preservation that alter the quality attributes of different foods to some extent. The toxicological aspects of food irradiation have been studied more extensively than for any other food preservation technique. As a result of these studies, the toxicological safety and ‘wholesomeness’ of foods, irradiated up to specified doses, have been judged to be satisfactory and to introduce no special or nutritional problems (WHO, 1981). This has led to acceptance by 130 governments of a Codex General Standard for Irradiated Foods (Codex Alimentarius Commission, 1984) and to approval by 37 countries of over 40 foods or groups of foods for consumption. Currently, full-scale implementation is inhibited by issues concerning eco- nomic viability and the levels of consumer acceptance of the process (Lagunas-Solar, 1995). Conclusions Substantial advances have been made in understanding the basis of efficacy of food irradiation for the reduction of food spoilage and for the improvement in food safety. However, although a surge in application was expected, the expansion in the use of food irradiation has been slow. Without doubt, a major reason for this has been the reluctance by consumers in many countries to accept that the process is satisfactorily safe, in spite of the extensive scientific evidence that now exists. New inactivation techniques are urgently needed to safely supplement the use of heat, and other more severe preservation procedures, for the improvement of food quality and safety. New techniques to extend the storage life of commodity foods are necessary in order to reduce References Anon (1993) Report and recommendations of a working group, in Cost-benefit Aspects of Food Irradiation Processing, Proceedings of an IAEA/FAO/WHO International Symposium, Aix-en-Provence, p. 48 1. Codex Alimentarius Commission (1984) Codex general standard for irradiated foods and recommended inter- national code of practice for the operation of radiation facilities used for the treatment of foods, Codex Alimentarius Volume Xv 1st edition, Food and Agriculture Organization of the United Nations/Wor�d Health Organization, Rome. Dillon, V.M. and Board, R.G. (eds) (1994) Natural Antimicrobial Systems and Food Preservation, CAB International, Wallingford, Oxon. Gould, G.W. 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