Overview of food preservation technologies

The control of microbial access and growth in foods from ‘farm to fork’ is important to ensure consumer health and well-being and minimise losses of foods through spoilage. Whilst it seems almost impossible to achieve a good and consistently hygienic production of raw materials, there are many different ways of controlling both access and growth of important microorganisms. Good Manufacturing Practices (GMP), i.e. hygienic handling of the raw materials, should start on the farm to minimise pathogenic species that are naturally present in farm environments and can then be transferred to raw materials for food production. The whole environment of a manufacturing plant needs to be subjected to the HACCP principles to control ‘persistent pathogens’ which can be transferred to food ‘in-process’ and avoid post-process contamination.

The basic characteristics of food preservation technologies addressing chemical, biological, thermal and non-thermal processes is presented below.

Thermal food preservation

Heat processing of foods is designed to result in a specific reduction in numbers of foodborne pathogens or elimination of food spoilage organisms in the target product, thus ensuring microbiological safety and increased shelf life. Microbial heat resistance is described via D- and z-values; D-value equates to the heating time required at a specific temperature to destroy 90 per cent of the viable cells or spores of a specified organism. The z-value represents the change in temperature needed to change the D-value by 1 log cycle, thereby giving an indication of the relative heat resistance of an organism. Increasing heating temperatures will reduce the time required at that temperature to achieve the required lethal effect. Typical D-values for Salmonella spp are one to 10 minutes at 60°C, and 0.1-0.2 minutes for C. botulinum spores at 121°C.

Depending on the level of microbial destruction required, a pasteurisation or a sterilisation process may be applied to the product. Pasteurisation is a relatively mild heat treatment, the purpose of which is to kill non-spore forming pathogenic bacteria. It also kills most spoilage organisms and deactivates enzymes and is normally conducted between 60 and 80°C. Sterilisation at temperatures exceeding 100°C, implies killing all micro organisms including pathogenic sporeforming organisms, as well as spoilage organisms. In reality, commercial sterilisation results in the death of almost all organisms. Viable spores may persist in the product, but they are prevented from growing by other factors in the product, e.g. low pH (pH <4.5), low water activity and preservatives such as nitrite and salt. P

asteurisation of milk has proved a highly effective microbial control measure, drastically reducing tuberculosis in the population and controlling several other pathogens that can enter the milk chain. Long term preservation of safe foods by sterilisation in cans, for distribution worldwide, is a very large component of food businesses. However, setting up a canning line is an expensive investment in equipment and the energy costs are high and likely to increase, at least in the shorter term. The very long heating process for commercial or full sterilisation of large cans reduces the quality of the contents and therefore ultra high temperature (UHT) treatments requiring only short exposure times in a continuous process and followed by aseptic filling into pre-sterilised cartons, pouches or bags, is now an increasing proportion of heattreated foods production. Another important benefit of UHT is that much of the heat used to sterilise the product can be recovered in heat exchangers, although this is not true for all UHT treatments, as steam-into-product processes do not allow heat recovery. Other methods of heating foods such as steam-into-product, high heat infusion (product-into-steam), infra red, radio frequency, microwave and ohmic heating, have been developed but are not commonly used, as they all have important drawbacks and / or are applicable only in certain foods and package configurations.

Preservation using acids and natural antimicrobials

Organic acids are natural constituents of many foods and they are widely used in food preservation, whether by direct addition e.g. acetic acid (vinegar) for pickles and mayonnaise or by microbial fermentation processes, e.g. lactic acid in yoghurt and cheese. Their antimicrobial efficacy is based mainly on their ability to lower the pH in the water phase of foods, restricting microbial growth. However, the weak organic acids (acetic, sorbic, benzoic and propionic) also penetrate the cell membrane into the cytoplasm in the undissociated form, i.e. at low pH, acidifying the cytoplasm and causing cell death. The antimicrobial efficacy of organic acids depends on the type of acid used, its concentration, product storage temperature, water activity, salt, oxygen and particularly pH value. Substituting citric or lactic acid, either partially or completely, for acetic acid in pickles and mayonnaises, markedly reduces the antimicrobial effect of the preservation system (which also includes salt, sugar and low pH), and should only be undertaken with caution and after experimental proof of efficacy. However, lactic acid, or its salts, may be a more effective preservative and more acceptable organoleptically, in other products such as meat products.

Sorbic acid, that occurs naturally in some fruits, and benzoic acid are both effective antimicrobials against yeasts and moulds, however many of these strains have now gained resistance to them. Certain types of bacteria can also degrade them to organoleptically unacceptable compounds, e.g. benzene, a carcinogen, and are consequently being phased out as food preservatives.

Other organic acids are being used in a restricted range of products as preservatives, such as propionic acid and its salts (in bread to inhibit bacterial spore germination, growth and production of ‘ropy bread’), fumaric, malic, succinic and tartaric acids. These are ‘Generally Regarded as Safe’ (GRAS).

In the search for alternative food preservatives that are ‘natural’ and can therefore subscribe to the ‘clean label’ concept, plant extracts are being actively researched by many groups. Antimicrobial compounds derived from plants are generally present in the oil fraction of leaves (e.g. rosemary and sage), flowers and flower buds (e.g. clove), bulbs (e.g. garlic and onion), rhizomes (e.g. asafoetida), fruit (e.g. pepper, cardamom) or other parts of the plant. The major antimicrobial activity of these herbs and spices belong to phenolic compounds, terpenes, aliphatic alcohols, aldehydes, ketones, acid and isoflavonoids and are highly flavoured. These antimicrobial compounds may act by inhibiting the production of a metabolite (e.g. mycotoxins) normally produced by bacteria, yeasts and moulds or by inhibiting the growth of the microorganism itself and have been linked with the natural defence systems used by plants against pathogens. Moreover, representatives of these groups of compounds can be bactericidal, fungicidal and (with respect to bacteria) sporostatic and sporicidal. More than 1,000 plants are known to be potential sources of antimicrobial compounds but many more still need to be investigated.

Despite many of the antimicrobial components of herbs having been successfully tested in model laboratory systems, when evaluated in real foods they may lack efficacy. Factors such as temperature, pH and the presence of fats and proteins, surfactants and minerals and other food components can greatly influence the outcome of antimicrobial testing. Furthermore, for the plant extracts to be effective antimicrobials in foods, concentrations need to be considerably higher than when used for flavouring, which may result in a modification of the sensory characteristics of the product, making it unacceptable. However, many published reports have demonstrated that combining a processing treatment (such as pH and temperature) with a ‘natural’ antimicrobial compound at a lower level can result in reduced spoilage within an overall sensory acceptability.

There are several known antimicrobials produced by bacteria, e.g. the bacteriocins, but also bacteriophages. Among the bacteriocins, nisin produced by lactic acid bacteria (LAB), is the most well-known and permitted in certain foods. This compound is the most potent of the bacteriocins known; common pasteurisation or sterilisation processes do not affect its activity, and therefore has been successfully used in high pH / heat-treated food products such as canned vegetables and pasteurised liquid egg. Nisin is active against Gram-positive but not against Gram-negative bacteria, yeasts or moulds. However, in combination with a chelating agent, nisin can be active against Gram-negative bacteria as the chelating agent destabilises the outer cell membrane, permitting nisin to interact with the cytoplasmic cell membrane. Nisin acts on the cell cytoplasmic surface, where it forms pores, causing leakage of small molecules such as K+ ions, cellular solutes and metabolites such as ATP and amino acids. In a food matrix, nisin activity depends on several factors, the most important of which is the interaction with the food components. Nisin works better in a homogeneous liquid than in a solid and heterogeneous system, probably due to better diffusibility. Other factors that can affect diffusion and activity include pH, salt concentration, nitrite and nitrate, aqueous phase availability, fat content, proteolytic activity, etc. There is a growing interest in the development of bacteriocins as natural food preservatives and antimicrobial agents, especially against Listeria monocytogenes. Several studies have demonstrated effective control of this pathogen in lightly processed or preserved foods where it can be a problem, e.g. lightly fermented meat sausages, coldsmoked fish, by stimulation of specific bacteriocin-producing organisms of the autochthonous microflora.

A specific anti-mould and yeast compound, Natamycin, has proved markedly effective in this role, but it has no effect on bacteria, due to its specific attachment to a cell wall structure unique to moulds and yeasts. This antimicrobial is produced specifically by a strain of Streptomyces natalensis, and its widespread use has not yet resulted in any natamycin-resistant yeasts and moulds.

Non-thermal food preservation

Non-thermal processes for food preservation include freezing, modified atmosphere packaging (MAP), ozone treatment, high pressure treatments, irradiation, pulsed electric fields and drying. MAP has been extensively researched and is now used for many food products to limit microbial growth, e.g. in fresh and processed meats and fish, to minimise oxidation of essential oils in foods, etc. Whilst ozone has proved a very successful microbicide in water treatment without the residual effects of chlorine, and experimentally has been proved effective against certain pathogens and spoilage flora, it has an oxidative effect on unsaturated fatty acids that may result in rancidity. It is also difficult to apply in practice in an open system. High pressure processing has also been extensively researched and is now successfully applied on some foods of high quality / price, such as fruit juices, and speciality cured meats where L. monocytogenes is still a problem. It is an expensive process in terms of the equipment required and power needed. Although ionising irradiation (electron-beam or from a radioactive source) has been very extensively researched and proven highly effective against a very wide range of microorganisms, it is still not acceptable to consumers. Pulsed electric fields and high intensity pulsed light have also been subjects of research but have not as yet been applied in practice. Pulsed electric fields have problems of achieving uniform application to foods and pulsed light is essentially a surface effect treatment. The latter may achieve some application in reducing surface contamination of fresh produce, e.g. of Campylobacter on chicken.

Finally, drying is a most satisfactory method of food preservation, whether by solar, freezedrying or spray-drying, producing a range of very valuable preserved products, from sundried fruits (raisins, sultanas etc.) to ready meals and dried milk powders. Each methodology has its own advantages and disadvantages, in terms of the product and energy consumption. Freezedrying is by far the most expensive in energy terms, but can produce excellent results. Bulk drying of liquids by spray-drying is a very large part of the dairy industry, producing a shelfstable food ingredient relatively cheaply. Although the liquid milk is subjected to high temperatures during the drying process, since the drying is very rapid, the process achieves very little in reducing the microbial load.

Discussion and conclusions

Even though there is a significant trend towards the use of ‘natural’ preservatives to replace chemical synthetics used in foods, research on identifying the sources of the natural alternatives has largely focused on plant extracts and products of microbial fermentation. Many of them result in organoleptically unacceptable products since the levels necessary for antimicrobial activities are usually significantly high. Organic acids, although commonly used in certain foods such as pickles, sauces and conserves, are most often used in combination with other preservation methods, such as heating and may not necessarily be applicable to other food types.

Thermal processing used in the production of pasteurised, short shelf-life chilled foods or commercially sterile foods has been practised for a long time with significant benefits to the health of the population. Some traditional methods of pasteurisation and retorting have been developed into automated processes over the years; however, the cost of energy consumed still remains an important consideration. More modern heating methods may be more efficient and rapid; however, a number of developmental hurdles are yet to be addressed. Similarly, non-thermal processes for food preservation include technologies that, although effective, may be either unacceptable to consumers or require a significant capital investment.