2 Introduction Table II Microbial food spoilage and food poisoning Table III Food preservation techniques(updated from Gould, 1989 Problems Examples Mode of action Preservation technique Excretion of majot Lactic and acids of growth etabolic products rbon dioxide Reduced water activity conserving with added lability of nutrients in Secretion of enzyme ipass, proteases, cellulases, etc. ausing flavour and texture changes Removal of oxygen from vacuum Biomass slime, haze, mould colonies, etc. Presence sf infectious salmonella, Campy Addition of acids, directly or by multiplication of Staphylococcus aureus, Clostridium Increased ethanol levels by botulinum fermentation fortification release in Addition of preservatives including have been proposed in recent years in reactio Inactivation consumers'requirements for less severely proce sterilize, by hot air, water or high m: by newer met more natural, additive-free foods, also act by inhibition luding microwaves and electrical (e.g. 'modified atmosphere packaging, use of naturally- (ohmic) methods occurring antimicrobials; Dillon and Board, 1994) lonizing radiation to in Since the major underlying cause of microbial food spoilage and food poisoning is ultimately the presence of rgan 以 the micro-organisms in the foods in the first place, it Ultraviolet radiation to inactivate follows that inactivation techniques are ideally prefera 0-organisms in water or on the ble to inhibitory ones. Heat is the only food preservation surfaces of foods and packaging chnique which is used on a large scale, that acts primarily by inactivation High-intensity visible laser and a problem with inactivation techniques, such as high erature processing, has been that they often tend to sms in water and on surfaces e unacceptable damage in the quality of food Application of ultra-high roducts. For this reason, procedures that minimize heat duced damage are being pursued, e.g. rotary retorting, crowave heating, ohmic heating, etc, for pasteuriza- of ultrasound with mild on and sterilization. Also, essentially non-thermal echniques are being explored and some are already sonication) ing exploited on a small scale, e.g. enzymic tech Addition of bad ques such as the addition of lysozyme, other enzymes nd naturally-occurring antimicrobials to foods; physical natural antimic techniques such as the application of ultra-high pressure, elec ed with mild heat and slightly rmosonication')(Table Ill These 'emerging'techniques are novel and scientif- Ionizing radiation ically challenging but few of them are widely employ Food irradiation is the use of ionizing radiation to As yet, one of the most effective altematives to heat for increase food storage life, reduce post-harvest food he inactivation of micro-organisms Is zing losses and eliminate food poisoning micro-organisms The effectiveness of ionizing radiation, its penetrating2 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