Antimicrobial compounds derived from nature as an alternative to synthetic preservatives in food
Preservatives have been on the receiving end of some very negative publicity in recent years to the extent that the phrase “contains no preservatives” has become a positive selling point for some foods. However unfair that may be, consumers and hence retailers are becoming increasingly unwilling to accept products with a shelf life maintained by use of the synthetic preservatives currently permitted for use in food. The pressure for ‘clean-labelling’ of food products is driving a trend towards so-called ‘natural’ additives, and preservatives are no exception. Researchers have been studying antimicrobials derived from nature for many years, but interest is now stronger than ever and is driving the development of a number of potentially marketable natural alternatives.
On the face of it, the current trend for clean-labelling of foods is driven not by science, but by ignorance and lack of understanding. Many perfectly serviceable food additives are now seen as undesirable simply because of unfounded negative publicity, or on the basis of an unattractive ‘chemical’ name. For example, it is ironic that the ‘E-numbers’ assigned to permitted additives are now widely seen as unfavourable on an ingredient list, despite that fact that the number means that the additive has been thoroughly assessed for safety and is strictly controlled in how it can be used. The same cannot be said of some natural alternatives. Taken to its extreme, this clean label trend can become ridiculous. For example, in 2008 one UK product was promoted on the basis that it was “100% chemical free”. This prompted the Royal Society of Chemistry to offer a prize of £1 million for anyone who could prove that they had such a material. So far, the prize remains unclaimed.
Food manufacturers may be faced with demands to reformulate products without synthetic additives that are at best unscientific and at worst unreasonable, but in many markets such action is now necessary to maintain sales. This is particularly true of synthetic preservatives such as sodium benzoate, which has been used safely and successfully for many years, primarily to prevent fungal spoilage in acid foods. Recent UK research on the effect of synthetic colours on child behaviour found that a cocktail of colours together with sodium benzoate could be linked to hyperactivity in some children. Despite the fact that there was no evidence for any causal effect, there is now considerable pressure for benzoic acid and benzoates to be removed from foods and beverages. It therefore comes as no surprise that many manufacturers are looking for alternatives from natural sources, which can maintain product safety and quality, but which look less unfriendly on a label.
A wide spectrum of natural antimicrobials
Natural substances with antimicrobial action have been identified from a very wide range of sources, including herbs and other edible and medicinal plants, microorganisms and animals. Many of these have been investigated, but few have yet been exploited as food preservatives on a commercial basis. Some examples are given below.
Some examples of natural antimicrobials with potential as food preservatives
Herbs and other plants with antimicrobial activity
Antimicrobials from microorganisms
Antimicrobials from animals
Antimicrobials from plants
Many researchers have focused on extracts from herbs, such as oregano, rosemary and thyme. It is the essential oils derived from these plants that contain most of their antimicrobial activity and they contain a variety of individual components that seem to be able to kill, or inhibit the growth of, microorganisms. Antimicrobial components include phenolic compounds, terpenes, alcohols, aldehydes, ketones and isoflavonoids, but there are four individual compounds that seem to occur most frequently and at the highest levels. These are carvacrol, citral, eugenol and thymol, but other compounds too, such as borneol, cinnamaldehyde and thejone have been identified as important. Essential oils from different plants show activity against different types of microorganisms. In general, most are more active against Gram-positive bacteria, such as Bacillus species, than against Gram-negatives like E. coli and Salmonella, but a few, notably oregano, clove and cinnamon, have been found to be effective against both, and these therefore have more potential for food use.Plants need to protect themselves from microbial infection just as animals do and the defences of many species are boosted by the presence of antimicrobial compounds in their leaves, fruits, buds, bulbs and seeds. Many of these compounds have already been identified and investigated by researchers, though it is certain that a great many more remain undiscovered. Not surprisingly, most attention has been given to herbs and other edible plants and to plants known for their medicinal qualities – in other words, plants known not to be highly toxic to humans – but it is mainly herbs and spices that are of interest as sources of antimicrobials for use in food preservation.
In other plants, different chemical compounds provide protection against attack. For example, in garlic and other members of the Allium family, such as onions, precursors and enzymes are present, which together generate an antimicrobial called allicin, but only when the plant is physically damaged or stressed. The antimicrobial and medicinal properties of allicin have been widely studied. Other plants, including mustard, horseradish and wasabi, use a similar mechanism to produce various isothiocyanates, one of which, allyl isothiocyanate, is a powerful antimicrobial and has antifungal as well as antibacterial activity. Green tea extracts containing catechins have also been found to show activity against a wide range of pathogenic bacteria.
Plant extracts do have a number of disadvantages as potential food preservatives, not least their cost in relation to synthetic compounds. Since most are derived from herbs and other plants used to provide flavour, they may have an adverse effect on the organoleptic properties of foods in which they are not normally used. Other components in the food product may also reduce the antimicrobial effect of extracts considerably. They may work well in a liquid culture in the laboratory, but be much less effective in a real food product. This means that higher concentrations need to be used, exacerbating any problems caused by the strong taste. Nevertheless, essential oils from a number of plants, including clove, mustard, oregano and thyme have shown sufficient potential to be seriously investigated as possible preservatives with real applications.
Antimicrobials from microorganisms
Many microbes produce chemicals that inhibit the growth and development of other microbial species and this has been known for many years. Some therapeutic antibiotics, including penicillin, were originally derived from microbial cultures. It is thought that producing antimicrobial compounds may give microbes in complex and diverse environments like soil a competitive advantage over their neighbours. Many natural antibiotics are completely unsuitable for use in food preservation, but there is one group of compounds that has great potential. Bacteriocins are antimicrobial proteins produced by both Gram-positive and Gram-negative bacteria, but it is the compounds produced by lactic acid bacteria – sometimes known as lantibiotics – that are of most interest for food manufacturers. The reason for this is that lactic acid bacteria are generally harmless and are often present in foods, especially in products like cheese and fermented meats. Furthermore, lactic acid bacteria tend not to cause detectable spoilage of foods unless they are present in very large numbers. This means that food manufacturers can safely exploit the ability of some strains to produce bacteriocins by using them in starter cultures, or even adding them to certain foods. Furthermore, lantibiotics are not generally used as therapeutic agents and so the development of antibiotic resistance is not an issue.
A wide range of bacteriocins has been discovered, but the most important from a food preservation point of view is nisin, which is currently used commercially. Nisin has a 40-year history of use and is approved for use in food in the EU (E234), the USA and many other countries. It is a stable polypeptide compound produced by some strains of Lactobacillus lactis and is quite effective against many Gram-positive bacteria. It works mainly by damaging the cell membrane. It is used in a number of applications, but is particularly useful for preventing the spoilage of cheese by the bacteria Clostridium tyrobutyricum and Cl. butyricum, which cause ‘late blowing’ during ripening. Nisin has other applications in canned foods, dressings, bakery products and cooked sausages, but is somewhat limited by its comparatively narrow spectrum of activity – it is not effective against Gram-negative bacteria or fungi – and by the fact that it is most effective at low pH. However, the effectiveness of nisin against Gram-negative bacteria can be improved if chelating agents, such as EDTA, are present. These work by increasing the permeability of the bacterial cell wall to nisin. Nisin is marketed under the trade name Nisaplin(r) by Danisco.
Other bacteriocins of special interest include pediocin, a stable protein produced by strains of Pediococcus acidilactici, which has generally recognised as safe (GRAS) status and is active against many Gram-positive bacteria over a wide pH range. Pediocin-producing bacteria are used in starter cultures for fermented sausage products, where their presence helps to inhibit both spoilage bacteria and pathogens, including Listeria. Other bacteriocins produced by lactobacilli, but not yet exploited commercially, are attracting interest. These include acidophilin, bulgaricin, lactacin and plantaricin. Reuterin, a non-protein bacteriocin produced by certain strains of Lactobacillus reuteri, has also been widely investigated. It has a wide spectrum of activity and is effective against both Gram-positive and Gram-negative bacteria, yeasts and moulds. It is water-soluble, works over a wide pH range and is quite stable, making it potentially a very useful food preservative.
Antimicrobials from animals
Antimicrobials derived from animals are the least studied, but potentially the most interesting and useful from a food preservation point of view. Some, such as lysozyme found in egg white, the lactoperoxidase system in milk and chitosan from the shells of crustaceans and arthropods, are well known, but researchers are finding that antimicrobial peptides (AMPs) are widespread in the animal kingdom and can be found in insects, fish, amphibians, birds and mammals. These compounds are currently attracting a lot of attention. Most of them seem to act by rapid general destruction of the microbial cell membrane so that even fast-growing bacteria are unlikely to develop resistance to them.
Lysozyme is a bacteriolytic enzyme naturally present in the albumen of birds’ eggs, which helps to protect the developing egg from microbial attack. Like nisin, lysozyme has been found to be effective against the clostridia that cause late blowing in cheese and has been shown to help prevent spoilage of wine by lactobacilli. It can also inhibit growth of Gram-positive spoilage organisms and pathogens, including Listeria and Bacillus cereus. Lysozyme has been commercialised and is available in purified preparations like inovapure(tm) marketed by Neova Technologies. The lactoperoxidase system relies on reactivating the enzyme lactoperoxidase, naturally present in raw milk, by adding thiocyanate and a source of peroxide. The effect is to block bacterial metabolism and inhibit growth, so extending the shelf life of raw milk, and it is approved by Codex for this purpose. The lactoperoxidase system is effective against both bacteria and fungi, but Gram-negative bacteria seem to be the most susceptible. Chitosan is a polymer, but in low molecular weight form it has been shown to be effective in controlling growth of both bacteria and fungi.
Researchers have only scratched the surface when it comes to investigating the huge numbers of AMPs that are thought to be present in animals, but some of them have already been shown to be promising as food preservatives. Lactoferrin for example, found in cows’ milk, is a glycoprotein that binds iron and has antimicrobial activity against bacteria and fungi. It has an application as a preservative in meat, and has recently received USDA approval for use in beef products. Pleurocidin is an AMP found in the skin of a fish called the winter flounder and has antimicrobial activity against both Gram-positive and Gram-negative bacteria. It is also heat stable and salt tolerant and has been shown to be effective at quite low concentrations against some important foodborne pathogens and spoilage bacteria, including Listeria and E. coli O157:H7. It appears not to be toxic to human cells and so its potential as a preservative is clear. Another promising group of AMPs are the defensins found in the epithelial cells of mammals and birds. They are present as part of the animals protection against infection and are reported to be effective against a wide range of microorganisms. Other AMPs currently under investigation include protamine, salmine and clupeine from fish and magainin from frogs, all of which are active against important foodborne bacteria and could be evaluated further for possible use as natural preservatives.
Turning potential into reality
There is no shortage of candidates to become the food preservatives of the future, but there is still a long way to go before synthetics can be phased out entirely and many obstacles on the road to all-natural preservation.
One obvious problem is that there are very few natural antimicrobials that can be used as direct replacements for existing preservatives. Either they are not as effective, are too costly, or cause product quality problems. Replacing proven preservatives like sodium benzoate with a natural alternative needs to be very carefully researched and tested before it can be implemented, especially where there are safety concerns. One approach that might make this easier is to use natural antimicrobials in combination with one another or with other technologies in a multi-hurdle preservation system. For example, nisin in combination with carvacrol has been shown to be more effective than nisin alone. Much research has focused on using blends of essential oils from different herbs to preserve foods and in some cases definite synergies can be obtained. Bacteriocins work better in combination with chelating agents, and non-thermal processing technologies like ultrasound, high pressure processing and ozonation have all been found to enhance the performance of bacteriocins.
Even if a natural antimicrobial system with potential as a food preservative can be shown to be sufficiently effective in foods it will still need to be approved before it can be used as a food additive. Ironically, this is likely to mean acquiring one of those E numbers that consumers are so wary of. This has already happened to nisin, which is proven to be both effective and safe, but must be declared as an additive on the label if it is used as a preservative. The problem is that consumers may not recognise which additives are natural and which are not and may reject declared additives wholesale.
A possible answer to this is for natural preservatives to be included as ingredients. For instance, it may be possible to formulate products flavoured using herbs that also have significant antimicrobial action, such as oregano and thyme, or to source herbs bred specifically for high levels of antimicrobials. Using bacterial starter culture strains known to produce bacteriocins is also an option. Another possibility would be to look at classifying some natural antimicrobials as processing aids if they are not detectable in the product at point of retail. These options may provide a way round the stumbling block of the approval process and labelling, but they are not a long term answer. If natural antimicrobials are to be successfully exploited as food preservatives it will probably require changes in legislation and better consumer education. We have only just begun to tap the huge potential of natural preservation and in the future it could even replace conventional processing technologies like pasteurisation in some applications, as well as replacing synthetic preservatives. It is an exciting field, but one needing a lot more research input before it can make a real contribution to safer, fresher food.