Preservation is about keeping food fresh for as long as possible by delaying or stopping natural degradation processes. What happens in the food when you preserve it, and what methods are available? This article will give you the answers.
Almost all food contains biological material which will decompose if the conditions are right – such as temperature, humidity or whether any air is present. Preservation in a food context means delaying the degradation processes in the food.
What happens when the food goes bad?
The decomposition process is based on chemical or enzymatic reactions in the food, but the main culprit responsible for breaking down food is active micro-organisms such as fungi or bacteria. In sufficient numbers, they can both ruin the quality and make the food harmful to eat. In some cases, food can become harmful to eat due to bacteria, fungi or viruses long before we are able to detect this from a bad taste.
Today we know how the different degradation processes happen and how micro-organisms grow. We can therefore stop the process, or inhibit it so that it slows down. That way we can maintain the longevity and food safety in a controlled manner.
Many of today’s preservation methods came into use thousands of years ago, such as drying, salting, boiling and fermentation. These methods are still in use, but we also find entirely new approaches being adopted.
Some preservation methods affect the taste, appearance and nutrient contents, and so food producers need to carefully consider what kind of product they want to make, and its desired shelf life. Once you have clarified that part, you can work out which method is best suited.
Heating food to kill or inhibit microorganisms is a method with many different approaches.
Sterilization uses the most extreme heat treatment – a method used for canned food. Sterilized food is heated to a moderately high temperature, for example 112 °C for an hour, or a slightly higher temperature, for example 121 °C for 3 minutes. This treatment eliminates all pathogenic bacteria in the food.
Canned food can be stored at room temperature for many years without going bad. Hermetically sealed cans are heat-treated in a pressure cooker (autoclave), but today we also have new heat-treatment methods such as microwave heating and Shaka autoclaves. These methods will heat the food even faster, so the food retains more of its beneficial properties and the manufacturer can save energy on the heating process.
Hermetically sealed packaging from materials other than metal – such as plastic and cardboard – have also been developed, providing us with brand-new, modern products which have long shelf lives.
Pasteurisation uses a milder form of heat treatment, which means some bacteria can survive. The shelf life of a pasteurised product in a chilled state is often 3-5 weeks, depending on the raw material and treatment method used. For pasteurisation, the temperatures often go up to 90 °C for 10 minutes. With lower pasteurisation temperatures, such as for milk which is pasteurised at 72 °C for 2 minutes, the shelf life of the product is around 10-14 days if it is kept chilled continuously. This treatment method is used extensively in the catering industry and in cafes and restaurants.
Pasteurised and mildly heat-treated products are available in a number of varieties for grocery store products, in canteens and hospitals and in nursing homes. Examples of these techniques include sous vide, cook-and-chill, cook-and-serve, HTST (high temperature, short time) and LTLT (low temperature, long time).
At the other end of the temperature scale, we have cold storage(temperatures of 4°C or lower) and deep freezing (–20°C or lower). A relatively new cooling method called “supercooling”, with a temperature of –1°C, has been shown to provide a three-week shelf life for fresh salmon when combined with a modified atmosphere (see below).
Food in modified atmosphere packaging
Microorganisms thrive in oxygen-rich air, but are inhibited by higher concentrations of carbon dioxide (CO2) than what we find in regular air. That is why fresh products are often packed in a protective or modified atmosphere (MAP) before they are refrigerated and distributed to grocery stores.
Before the packaging is sealed, all air is extracted from it and replaced with a mixture of CO2 and nitrogen (N2) – gases that are also found naturally in the air. Specific gas mixtures are used for different foods; some foods cannot tolerate high levels of CO2, while others require oxygen to maintain a long shelf life.
Part of the gas mixture will naturally seep in to the food, which is why the gas volume should be quite large relative to the product with this packaging method. To introduce more gas into the packing with a smaller volume, the food can be saturated with the gas mixture before packing – a method known as SGS ( soluble gas stabilisation).
High pressure technology
Exposing food to high pressure is a gentle and healthy method that significantly increases shelf life. High pressure has proved very suitable for meat products and juices among, other things. The food is exposed to very high pressures, up to 6500 atmospheres, for a short time. This kills bacteria and alters the properties of the food.
This technology is healthy as it allows for reduced use of additives such as salt. Factors such as taste, texture, quality and nutrient content all suggest that high pressure processing is a preferred technique for extending the shelf-life of foods. Food items preserved using high pressure processing appear fresh, but have a much longer shelf life. For example, raspberries are still fresh and good 90 days after high pressure processing. In recent years, many high pressure-processed products (e.g. juice) have entered the Norwegian market.
The most commonly used means of chemical preservation is to place the food in salt, acid and/or sugar, which many of us are familiar with from making pickled food in our own kitchen. This is a safe and well-known preservation method, but on the other hand the use of salt and sugar should be limited from a health perspective.
Another method is to use acids such as vinegar and citric acid, which lowers the pH level. The greater the acidity, the less friendly the environment is for pathogenic bacteria.
This principle is used in fermentation, where beneficial lactic acid bacteria are applied to partly break down the food into useful or tasty products. Both carbohydrates, sugars and starch can be broken down and used in products such as sourdough bread, cheese, yogurt, cultured milk, salami, rakefisk (fermented trout) and of course wine and beer which use yeast cultures.
The use of artificial preservatives has been a widespread practice for a long time, but is strictly regulated in Norway and many other countries. Regulations are in place for which preservatives can be used and in what concentrations or quantities, based on the average annual human consumption. In recent years, there has been increased focus on using fewer artificial preservatives and instead using natural agents that have a preservative or antibacterial effect, but do not need to be listed as preservatives on the product label.
Combination technology – using several inhibitory factors
In order for foods to retain their freshness as long as possible, a combination of several mild preservation methods can be used instead of a single high-impact method. A typical example is brined shrimp, which keeps for up to 9 weeks by combining cold storage, lowering the pH slightly, and adding salt and preservatives (benzoic acid and sorbic acid) in small quantities. Another example is smoked salmon, where a combination of phenols in the smoke, salt, drying and vacuum packing provides durability and food safety.
Future preservation methods
Nofima scientists provide food manufacturers with guidance and advice on how methods for preservation, packaging and durability can be combined to make the product just the way they want while still being safe for consumers.
In recent years, we have conducted research into how we can best preserve the quality of food through proper freezing and thawing techniques, and we are continuing to explore the full potential of traditional preservation methods.
But we are also researching a number of innovative technologies that can increase food safety and improve the quality of the food, while helping to meet the demand for sustainable, healthy and affordable food products for a growing population.
Here are examples of some of the preservation technologies we are currently researching:
- Pulsed electric fields (PEF)
The food is exposed to pulsating electric fields that break down the cellular walls and make the product easier to process while extending its shelf life. Over the past ten years, PEF has been used by potato and vegetable producers as it makes the raw materials easier to cut up and deep-fry. Scientists are working to optimize the method and document its effect on quality and durability.
- Ultraviolet Light (UVC)
UVC inactivates bacteria on the surface of the food. The method is currently applied to bread products, but for packaged goods, the technology still needs further development. We expect to be able to increase the shelf life by 5-6 days for packaged fresh food products.
Ozone is an oxidizer and can be used in water treatment, washing and disinfection of equipment, odour removal and processing of fruits, vegetables, meat and seafood. Excess ozone quickly breaks down into oxygen, leaving no residual ozone in the food.
- Cold plasma
Cold plasma is a new disinfection technology that is environmentally friendly and eliminates the need to use chemicals. Plasma is an ionized gas that can destroy cell membranes and DNA in bacteria. Berries and vegetables can get up to five days longer shelf life when treated with cold plasma.
- Plasma-activated water (PAW)
By using cold plasma in water, and then rinsing the foods in the water, fresh goods can have a longer shelf life. PAW creates an acidic environment that produces a number of changes in the chemical properties of the water and can serve as an alternative method of microbial disinfection.
- Essential oils
Oils from plants and seeds contain chemical components such as alcohols, esters, aldehydes, ethers, ketones, phenols and oxides. Plant-based oils in certain proportions have anti-bacterial and anti-fungal properties, making them useful in several areas for both the food and pharmaceutical industries.