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Food preservation (2): chemical and biological methods

Preserving a food means slowing down the processes that can alter its safety, quality and characteristics over time. In food production, everything rarely depends on a single method. Temperature, water availability, acidity, the presence of oxygen and packaging can act together, creating conditions that are less favourable to the growth of microorganisms and to changes in the product.

In the first article in this series (read it here), we looked at how physical methods act through heat, cold, the reduction of available water or the control of the atmosphere surrounding the food. But there are other possibilities. Salt, sugar and acids have been used for centuries; modern preservatives make it possible to act on specific phenomena; in other cases, microorganisms themselves, through fermentation or protective cultures, contribute to preservation.

The distinction between physical, chemical and biological methods is useful for understanding them, but the boundaries are not always clear-cut. Salt, for example, is a substance added to food, but an important part of its effectiveness comes from its ability to reduce the water available to microorganisms. More than any single technique, therefore, what matters is how the different barriers are combined.

Salt, sugar and acidity: changing the food environment

Long before industrial refrigeration and microbiology, people had already learned that certain foods lasted longer when salted, immersed in brine, sweetened, acidified or fermented. The mechanisms were not yet understood, but the principle was correct: changing the environment in which microorganisms have to live can make their growth more difficult. Salt and sugar act largely on the water available. What matters to microorganisms is not only how much water a product contains, but how much of it they can actually use. Dissolving high amounts of salt or sugar binds part of the water present in the food and reduces its activity. In this way, conditions become less favourable to the growth of many bacteria, yeasts and moulds.

This is the principle behind very different applications, from salting and brining to jams, syrups and products preserved with high concentrations of sugar. This does not mean that salt and sugar have the same effect on all microorganisms or that simply adding a certain amount is enough to make a food safe. Some species tolerate conditions in which others can no longer grow, while the concentration required depends on the product and the process.

Another approach is to modify acidity. pH measures how acidic or alkaline an environment is and is one of the factors that influence the ability of microorganisms to grow. Many bacteria responsible for spoilage or food safety risks struggle to develop when pH decreases, although there are yeasts, moulds and bacteria capable of tolerating acidic environments. Acidity can be achieved by directly adding food acids, as happens in many acidified products, or it can result from a biological process. In lactic fermentations, for example, certain microorganisms transform sugars and produce lactic acid. The environment gradually becomes more acidic and less favourable to many unwanted organisms. This is one of the reasons why fermentation has also been used for centuries as a preservation method.

Alcohol can also contribute to the same purpose. At adequate concentrations, it interferes with the growth of many microorganisms and is one of the factors that help stabilise products such as wine, spirits and foods preserved in alcohol. But here too, the result depends on the concentration, the type of food and the simultaneous action of other factors.

How preservatives and antioxidants work

In everyday language, the word preservative is often used to describe almost any substance added to a food to make it last longer. From a technical point of view, however, the functions are different. European legislation defines preservatives as substances that extend the shelf life of foods by protecting them from deterioration caused by microorganisms or from the growth of pathogenic organisms. Antioxidants act on a different phenomenon: oxidation. They can, for example, limit the rancidity of fats or certain changes in colour. In both cases, the shelf life of the product can increase, but through different mechanisms.

Among preservatives, there are families of substances with specific applications. Sorbates and benzoates, for example, are used in certain food categories mainly to counteract yeasts and moulds. Nitrates and nitrites are authorised for use in some products, including various meat preparations. Sulphites can be used, within the permitted conditions, in products such as wine, dried fruit and other food categories. However, there is no single preservative suitable for every food. Effectiveness can depend on pH, the amount used, temperature, product composition and the microorganisms to be controlled. This is also why the use of additives is governed by a precise framework of authorisations and conditions of use.

In the European Union, an additive can be used only if it is authorised for that food category and under the conditions established by legislation. The assessment considers safety at the permitted doses, technological necessity and the requirement that its use must not mislead the consumer. The general reference is European Regulation 1333/2008 and its subsequent updates. The letter E followed by a number is also often misunderstood. It does not mean that a substance is necessarily artificial. It is the code used by the European Union to identify an authorised additive. Some substances used as additives also occur naturally: ascorbic acid, or vitamin C, for example, is identified as E 300 when used as an additive.

When microorganisms are the ones doing the preserving

It may seem paradoxical: to limit the growth of unwanted microorganisms, we can use other microorganisms. This is what humans have been doing for a very long time through fermentation, even though for centuries the process took place without its causes being understood. During fermentation, certain bacteria, yeasts or moulds transform some of the substances present in food. They can produce acids, alcohol, carbon dioxide and many other compounds. The result is not only about preservation: flavour, aroma, texture and the very identity of the food change. Yoghurt, cheese, bread, wine, beer and fermented vegetables exist precisely thanks to the activity of microorganisms.

In many cases, however, these transformations also have a protective effect. The production of acids can lower the pH, the production of alcohol can make the environment less favourable to certain organisms, and the presence of a dominant microbial population can hinder the growth of unwanted species. Not all fermentations guarantee food safety on their own, but they can provide one of the barriers on which preservation is based.

In modern production, the process can be guided through starter cultures: selected microorganisms added to initiate and control fermentation with more predictable characteristics. In this way, the aim is to achieve the desired result more consistently, both from a sensory and a technological point of view. Alongside starter cultures, the concept of protective cultures has also developed. In this case, the role of microorganisms is specifically linked to their ability to counteract organisms that could spoil the food or pose a risk. This is one of the tools of biopreservation, a field that uses microorganisms or substances produced by them to contribute to food safety and shelf life.

Among these substances are bacteriocins, molecules produced by certain bacteria that are capable of inhibiting other bacteria. One of the best known is nisin, produced by some lactic acid bacteria and used as a food preservative in specific applications. Protective cultures and bacteriocins have been studied for years and represent an important area of research in biopreservation, also in relation to the possibility of combining several barriers without resorting to excessively intensive treatments.

Physical, chemical and biological remain useful categories for explaining the different tools available. Real food products, however, do not recognise these divisions. Every product is a system in which water, temperature, acidity, oxygen, ingredients, microorganisms and packaging interact. For this reason, preservation does not mean applying the strongest possible treatment or adding one more substance. It means understanding the product, identifying the changes that may occur over time and building a combination of barriers capable of controlling them while maintaining safety and quality.

Read the first part of the article here

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