Existing methods
Heat treatment is one of the most widely used physical methods for food preservation because it can effectively inactivate enzymes and microorganisms. Sterilisation, which requires high temperatures, is in fact currently one of the few technologies that can ensure the destruction of bacterial spores in food (64).
While the effectiveness of heat on inactivation of enzymes depends mainly on their thermostability, the inactivation of microorganisms can be more complex. Thus, the mechanisms of microbial inactivation by heat have been extensively studied and many different cellular alterations have been reported. It is generally assumed that heat disrupts multiple structures and functions of the microorganisms, with the sum of all changes leading to cell death. In general, when studying the effect of heat, the cellular structures or elements that have often been studied as susceptible are the outer and inner membrane, the peptidoglycan cell wall, nucleic acids (nucleoid and RNA), ribosomes and various enzymes (65).
Some of the limitations of conventional thermal treatments are uneven heating and low heat transfer efficiency in some foods, so more rigorous treatments must be applied to ensure that all parts of the whole food receive the minimum treatment required. In this regard, electromagnetic technologies have gained increased interest and demonstrated their potential for faster and more uniform heat treatments of food. Technologies such as infrared heating, microwave heating, radiofrequency heating and instant pressure drop control technology have shown to reduce these drawbacks and provide, at least in part, an alternative to conventional heat treatment in food preservation (66).
Despite advances in the use of thermal treatments, undesirable side effects of heat on the sensory, nutritional and functional properties of food could still occur (67). These changes in foods and the growing consumer demand for minimally processed foods have prompted the development of alternative, non-thermal preservation methods that offer safe foods with minimal processing treatments due to their antimicrobial efficacy (68). Technologies such as high hydrostatic pressure (HHP), pulsed electric field (PEF), ionising radiation (IR) and non-thermal atmospheric plasma (NTAP) have shown to be promising alternatives for food preservation.
Emerging technologies
High hydrostatic pressure. Industrial HHP processing is based on treatments with increased pressure (100–600 MPa) transmitted throughout the product using a pressure-transmitting medium with a duration of 1 to 20 min. These treatments at refrigeration or room temperature (4-25 °C) allow the extension of shelf-life and reduce microbial loads to levels comparable to those achieved by thermal pasteurisation, without affecting nutritional and sensory attributes (69). HHP treatments eliminate or inactivate vegetative forms of spoilage and pathogenic microorganisms, but not their spore form. However, HHP treatments have been shown to induce the germination of bacterial spores of several bacterial genera, such as Bacillus spp., and thus facilitate their inactivation by subsequent treatments (70).
The mechanisms of microbial inactivation by HHP are very complex and diverse. Microbial death is thought to be caused by the accumulation of all the damage to different cell structures and functions: disruption of the outer membrane, damage to the cytoplasmic membrane, alteration of pH homeostasis and osmoregulation, damage to cytoplasmic components (such as disintegration of ribosomes into their subunits, protein and enzyme unfolding) and damage to DNA and RNA by reactive oxidative stress (ROS) (71, 72).
Nowadays, HHP is the most-widely used non-thermal technology for the commercial pasteurisation of food. According to Khouryieh (73), 35.6 % of food processing companies in the USA have implemented HHP technology in their production lines, followed by PEF technology at 20 %. HHP is mainly used for the preservation of fruit- and vegetable-derived products, egg products, dairy products, seafood, meat products and alcoholic beverages (74).
Pulsed electric field. PEF technology is a non-thermal method of food preservation that uses short pulses of electricity for microbial inactivation. Basically, PEF is based on the application of high-voltage pulses (20–80 kV/cm) for short periods of time (in the order of ms or μs) to a product enclosed by or flowing between two electrodes (75).
The mechanism of action of PEF on microorganisms is based on the disruption of the membranes by the induction of electromechanical compression that leads to the pore formation. This phenomenon is called electroporation and it can be reversible, i.e. the cell is only sublethally damaged and can recover, or irreversible, i.e. the damage cannot be repaired and the cell is inactivated (76, 77). The inactivation of pathogenic and spoilage vegetative cells of bacteria and yeasts by PEF has been widely demonstrated. However, the few studies conducted on PEF inactivation of bacterial spores describe these structures as PEF-resistant and show that their inactivation is only achieved in combination with heat (78). For this reason, this technology is used as an alternative to pasteurisation rather than sterilisation. Therefore, PEF treatments can ensure food safety and extend the shelf life of certain foods, avoiding detrimental changes in food quality and maintaining physical, sensory and nutritional attributes compared to pasteurisation (79).
PEF is generally not suitable for preservation of solid food (79). Pasteurisation by PEF has been extensively demonstrated for various liquid food products such as fruit juices, milk and dairy products, soup, liquid egg and beverages, among others (80, 81). However, this technology has several limitations in the food industry: food must be free from air bubbles and have low electrical conductivity. Moreover, PEF has proven to be useful in some processing steps such as for the extraction of compounds of interest, or in dehydration or freezing/thawing processes of solid food (82). Most consumers recognise and appreciate the benefits of PEF as an alternative pasteurisation method to heat, for example in apple juice. Nevertheless, there are still some consumers who have doubts about the safety of using PEF as a food preservation method (83), especially for traditional food such as wine.
Ionising radiation (IR). Food irradiation is a process in which food is exposed to a carefully measured amount of intense radiant energy called ionising radiation (IR), which is capable of ionising atoms or molecules by detaching electrons from them. This non-thermal technology is a preservation method with minimum effect on the quality, taste, appearance and texture of food. It assures the inactivation of pathogenic and spoilage bacteria, yeasts and moulds at the legal doses permitted in the European Union, except in their spore form (84, 85).
The mechanism of bacterial inactivation in IR treatments is based on the damage to microbial DNA, either directly or indirectly (86). In the direct action, the IR directly hits the DNA molecules and disrupts the molecular structure. Such structural change leads to cell damage or even cell death. In the indirect action, the IR hits the water molecules, which are the major constituent of the microbial cell and other organic molecules inside the cell, and produces free radicals such as hydroxyl ion (OH˙) and hydrogen atoms (H˙). These free radicals have an unpaired electron in the structure that is very reactive and therefore reacts with DNA molecules and causes a molecular structural damage. In addition, these reactive species, also called reactive oxygen species (ROS), can damage other cellular structures, such as membranes, where alterations have been observed after IR treatment. Direct and indirect IR treatments lead to cell death of the microorganisms present in the food.
As per the General Standard for Irradiated Foods (87), food irradiation can be achieved using three different types of ionising radiation: (i) gamma rays emitted by radionuclides like cesium-137 (137Cs) or cobalt 60 (60Co), (ii) X-rays generated by machine sources operating at or below 5 MeV energy level and (iii) electron beams produced by electron accelerators at or below 10 MeV energy level (also known as e-beam).
For food irradiation, it is essential to ensure that the average absorbed radiation dose in processed food remains below 10 kGy and that none of this radiation has sufficient energy to produce radioactivity. Regarding the safety of IR, according to FAO, IAEA, WHO and the Scientific Committee on Food of the European Commission, foods irradiated with appropriate technologies and doses are safe and nutritionally adequate (88).
However, the use of this technology to treat food is very restricted in some countries by legal regulations. In the EU (88), the following foods may be treated (note that this depends on each member country): fruits and vegetables, including root vegetables; cereals, cereal flakes and rice flour; spices and condiments; fish and shellfish; fresh meat, poultry and frogs’ legs; raw milk camembert; gum arabic, casein/caseinates, egg white and blood products. It should be noted that in many cases the permitted irradiation is not aimed at microbial inactivation but at preventing or slowing down the germination of some vegetables, e.g. potatoes. All imported, exported, advertised, distributed, stored, manufactured and sold food items that have been irradiated, contain irradiated ingredients or have been processed from irradiated materials must bear the food irradiation logo on their labels (87).
IR offers several advantages over traditional heat-based or chemical food preservation methods including: treatment of packaged food, chemical and temperature independence, liquid and solid food and minimal organoleptic changes (89). However, IR still has some limitations for its application in some sectors of the agri-food industry, such as the legal restrictions on its use, the cost of facilities for its application, the high and costly safety measures for its safe and efficient use and the insufficient acceptance by consumers/manufacturers (73, 84).
Non-thermal atmospheric plasma. NTAP is generated at room temperature and normal pressure by electrical discharge of a gas, resulting in ionisation, dissociation and excitation of its atoms and molecules. NTAP consists of numerous reactive units, such as electrons, positive and negative ions, free radicals, excited or unexcited atoms and molecules, and electromagnetic radiation (UV rays) (90). However, depending on the gas and the method used to generate NTAP, its constitution differs, including both ROS and reactive nitrogen species (RNS). These include ozone (O3), superoxide (O2˙̄), hydrogen peroxide (H2O2), hydroxyl (OH˙), peroxyl (ROO˙) radicals, singlet oxygen (1O2), atomic oxygen (O), nitric oxide (NO) or nitrogen dioxide (NO2) (91).
NATP has demonstrated strong antimicrobial properties against a broad group of food pathogens, such as Listeria monocytogenes, Escherichia coli and Salmonella spp. (92). However, the mechanism of action of the NATP components that cause microbial inactivation is not yet fully understood. Many studies have observed the oxidative damage to various structural and functional cellular components by the reactive species produced in NATP. So far, the hypothesis supporting its good antimicrobial properties against bacteria is that these reactive species act on multiple cellular targets, and the sum of all damage would lead to cell death. Many authors have observed that the components of NATP (reactive species) can cause DNA damage, cell leakage, protein modification, lipid peroxidation and morphological changes, highlighting the membrane as one of the main structures altered by this technology (93).
This technology has several advantages that make it one of the most studied techniques in recent years for its use in the food industry: (i) low application cost, (ii) short treatment times, (iii) versatility in treating food and contact surfaces, (iv) treatment within the packaging to avoid subsequent contamination and (v) environmentally friendly technique as it can use ambient air (91). However, there are still some limitations to this technology that hinder its application for food preservation. In this sense, it is necessary to evaluate the nutritional and sensory effects depending on the intensity of the treatment and the food, the role of individual plasma components responsible for its antimicrobial properties, the lack of short- and long-term toxicity for consumers and the development of equipment that is easy to use (93).
, Diego García-Gonzalo1
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