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Food preservation (1): physical methods

Preserving a food means slowing down the processes that can alter its safety, quality and characteristics over time. In food production, however, 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. The available techniques are numerous and can, with some simplification, be divided into physical, chemical and biological methods. The first group includes some of the best-known preservation systems, from cold treatment to heat treatment, from drying to vacuum packaging. These are very different methods, but they share the ability to act on the environment in which microorganisms, enzymes and chemical reactions are able to operate.

Heat and cold: preserving food by controlling temperature

Putting food in the refrigerator is probably the most common preservation method. The principle behind it is simple: lowering the temperature slows down many chemical and enzymatic reactions and reduces the growth rate of numerous microorganisms. Slowing them down, however, does not mean eliminating them. Refrigeration extends the shelf life of many foods, but it does not make them stable indefinitely and does not prevent the growth of all microorganisms. This is why maintaining the cold chain is an integral part of the method: if the temperature rises, even temporarily, conditions that are more favourable to spoilage can develop again.

With freezing, the effect is more pronounced. Part of the water contained in the food turns into ice and becomes less available for microbial activity and for many reactions. Even in this case, however, it would be wrong to consider cold a sterilisation system: many microorganisms can survive freezing and begin multiplying again during or after thawing. Freezing and quick-freezing are not synonyms either. In quick-freezing, the product must pass rapidly through the zone of maximum ice crystal formation; European legislation also requires that, after thermal stabilisation, the product be maintained at a temperature of -18°C or below at all points. The speed of the process also affects the formation of ice crystals and, consequently, the characteristics of the product after thawing.

Heat follows the opposite logic. In this case, the objective is not to slow down microbial activity, but to inactivate microorganisms and enzymes through a specific combination of temperature and time. Pasteurisation uses relatively moderate treatments, calibrated according to the product and the result to be achieved. It significantly reduces the microbial population and inactivates heat-sensitive pathogenic microorganisms, but it does not necessarily eliminate all microbial forms, including the most resistant spores. For this reason, a pasteurised food may also require refrigeration or other storage conditions.

Industrial sterilisation, on the other hand, uses more intensive treatments and aims to achieve so-called commercial sterility: a condition in which the product, if properly packaged and stored, does not contain microorganisms capable of developing under the normal conditions expected for its storage. Time and temperature are not universal values. They vary according to the food, its acidity, consistency, the presence of particles and also the characteristics of the container, because the heat must adequately reach even the most difficult point to treat.

This is precisely where it becomes clear how closely process and packaging are connected. Heat-treating a product and then exposing it again to possible contamination would undermine much of the work carried out. Hot filling, treatment of the already packaged product and aseptic packaging are different solutions, but they arise from the same need: to control not only the treatment itself, but also what happens before and after it.

Less water, less oxygen

Temperature is not the only lever available. To grow and multiply, microorganisms also need water. More precisely, what matters is the amount of water actually available, expressed through the concept of water activity. Two foods can contain similar amounts of water and behave very differently. Part of the water may in fact be bound to other components and therefore not be available in the same way to microorganisms and chemical reactions. This is why, in preservation, the total amount of water tells only part of the story.

Drying acts precisely on this balance. By removing a significant part of the water, conditions are created in which microbial growth becomes more difficult or, below certain thresholds, impossible. It is one of the oldest preservation methods, but the principle remains central in modern industrial production as well. What mainly changes are the techniques used. Water can be removed by circulating hot air, through vacuum systems or with more complex processes. In freeze-drying, for example, the product is first frozen and then placed under conditions that allow the ice to pass directly into vapour. The result is a food with a very low amount of available water, capable in many cases of keeping for a long time and of recovering some of its characteristics through rehydration. Concentration also reduces the amount of water relative to the other components. Juices, purées, extracts and other products can be concentrated to obtain specific technological characteristics and greater stability. Once again, however, water reduction may be only one of the barriers used: depending on the product, a heat treatment, a certain level of acidity or specific packaging conditions may also be required.

Another approach is to act on the air surrounding the food. With vacuum packaging, most of the air inside the package is removed. By reducing oxygen, oxidative phenomena can be slowed down and the growth of some aerobic microorganisms can be limited. This does not mean making the product immune to spoilage: some microorganisms do not need oxygen, and vacuum packaging must be managed together with the other safety parameters required for that specific food.

Modified atmosphere packaging takes this one step further. The air inside the package is replaced or modified using gas mixtures, generally based on carbon dioxide, nitrogen and, when necessary, oxygen. Composition and proportions vary depending on the product: what works for fresh meat is not necessarily suitable for vegetables, cheese or baked goods.

The aim may be to slow the growth of certain microorganisms, limit oxidation or control physiological processes such as respiration and ripening in vegetables. Modified atmosphere packaging does not, however, automatically replace temperature control: for many products, the two techniques work together.

One barrier is rarely enough

Cold, heat, water and oxygen already help explain why apparently similar foods can have very different storage times and conditions. Above all, however, they allow us to move beyond an overly simple idea of industrial preservation: choosing one technique and applying it to the product. In reality, several barriers are often used together. A lower temperature can be combined with a modified atmosphere. A pasteurised food can be refrigerated. A partially dehydrated product can also be protected through packaging. In other cases, acidity, salt, sugar, preservatives or fermentation come into play.

The combination can also make it possible, where appropriate, to avoid treatments that are more aggressive than necessary. Heat is extremely effective, but it can alter colour, texture, aroma and some nutrients. The aim of a well-designed process is not to apply the most intense treatment possible, but to achieve the required level of safety and stability while limiting unwanted effects on the product.

This search has also led to physical technologies that differ from traditional ones. High-pressure processing, for example, subjects foods to very high pressures and can inactivate various microorganisms while reducing the need for intense heat treatments. It is not a universal solution and its effectiveness depends on the product, the microorganisms present and the process parameters.

Another method is the use of ionising radiation, applied under controlled conditions to reduce microorganisms and parasites, slow certain ripening processes or prevent sprouting in specific vegetables. In the European Union, the treatment is regulated and its use is limited to authorised conditions and categories; it does not make food radioactive and cannot be used as a substitute for correct hygiene and production practices.

Different technologies, then, but a common logic: modifying one or more of the conditions necessary for food spoilage. Temperature, water and oxygen are some of the main levers, but they are not the only ones. Salt, sugar, acidity and even the activity of selected microorganisms can contribute to the same result. In food preservation, it is almost always the balance between several factors that makes the difference.

Read the second part of the article here

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