What are the types of preservation

Food preservation

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Before food from all parts of the world was transported fresh to Europe every day, there was an oversupply of food during harvest time; and in the coming winter no fresh → fruit, → vegetables or cereals. To prevent the shortage, food had to be preserved. Usual processes around 1900 were smoking and curing (dehydration), boiling down and the production of canned food (heating), pickling z. B. Sauerkraut and beans as well as the cool storage of food. A book from 1908 recommended the addition of salicylic acid as a preservative to prevent mold formation. These methods have been optimized over time and supplemented by newer processes.

Causes of spoilage

Once fruits and vegetables have been harvested, the metabolic processes in the food by no means stop. Some of the plant parts continue to grow after the harvest and gas exchange takes place (→ metabolism of plants). These processes use up reserve materials, water is lost and heat is developed. As a result, the properties of the food such as smell, texture, ingredients and taste change. The heat generated promotes microbial growth and chemical processes in the food.
Food can be spoiled by microorganisms such as bacteria, yeasts and molds (further information on microorganisms can be found in the → chapter "Hazards for food"). Microorganisms need moisture, usually a neutral pH value and warmth to grow. The optimal temperatures are between 15 ° C and 50 ° C. Food stored at room temperature in a damp kitchen therefore offers good living conditions for molds, yeasts and the like. In order to reduce the growth of microorganisms to a minimum, they have to be deprived of their essential livelihoods. Bacteria can be killed by heating food for a long time (70 ° C and higher). Some bacteria such as → Clostridium botulinum form highly toxic substances (toxins), which can often also be deactivated by heat. Heat-resistant permanent stages such as spores represent a particular difficulty, some of which are only killed at very high temperatures of 120 ° C. These spores survive simple boiling, then germinate and then form toxins that are harmful to humans. Molds are easier to spot. Moldy food should always be thrown away whole and not attempted to save parts by cutting out.

The reaction with oxygen or light can also negatively affect food. → Fat spoils (becomes rancid), the taste of → milk changes under the influence of light and the color of food can change. For example, light and oxygen-impermeable packaging or the addition of → additives such as antioxidants help to reduce the chemical processes.

Conservation method

Withdrawal of moisture

A classic way is drying food such as dried fruit or stockfish / clipfish. Spray drying is a more modern process. For this purpose, the liquid is finely atomized and then dried in a drying tower (e.g. milk powder).
Sugar can also remove liquid from food, such as candying or cooking jam and marmalade. The same effect can be achieved with salting (→ osmosis). As with salted herring, fish have been preserved by salt, just like eggs pickled in salt solution, the so-called sole eggs. In contrast to simple salting, nitrates such as potassium or sodium nitrate or potassium or sodium nitrite (= nitrite curing salt) are added to the salt in curing salt. The curing leads to a red coloration of the meat and the typical aroma. The type of curing salt must be indicated on the packaging (Section 9 of the Additive Admissions Ordinance, Paragraph 1).

When smoking, the water content of food is reduced by around 40%, and the smoke also gives it a special aroma. Fish, but also meat and meat products, are smoked. These are usually salted or cured before smoking. A more modern process is freeze-drying, in which food is first deep-frozen at normal pressure and then the products are dried in a vacuum and heat. A well-known example of this process is freeze-dried soluble coffee.
When soaked in alcohol, the water in the food is displaced by the high percentage alcohol such as rum or brandy. The method is used, for example, with fruits. One variant is soaking in oil. The oil is absorbed in the food, which greatly increases the energy content (e.g. tuna in oil or pickled tomatoes).

Reduction of the pH value

The growth of microorganisms is inhibited in an acidic environment. The lowering of the pH value is achieved e.g. B. by adding citric acid or vinegar. Since the taste of the food must not be as acidic as would be necessary for preservation, the → vegetables or → fruit used are cooked, blanched or pre-cooked beforehand. The combination of the preservation methods achieves a longer shelf life. As with sauerkraut, acid can also arise from fermentation processes. For this purpose, the white cabbage is sliced ​​and alternately placed in a stone pot with → salt. The salt removes water from the cabbage, creating a brine that covers the cabbage. Over the next 4-6 weeks, the herb's carbohydrates are converted to lactic acid. Important nutrients are retained during this preservation process.


Food can also be protected against premature spoilage by adding chemical preservatives. Preservatives work in low concentrations and ensure that the reproduction of microorganisms is strongly inhibited or stopped, or microorganisms are destroyed without changing the taste of the food. For many preservatives are so-called ADI values ​​established.

Sorbic acid (E 200), potassium sorbate (E 202), benzoic acid (E 210), PHB ester (E 214-219), sulfur dioxide (E 220), orthophenylphenol (E 231), sodium orthophenylphenolate (E 234) can be used as preservatives , as well as potassium nitrite (E 249), acetic acid (E 260), lactic acid (E 270), boric acid (E 284), sodium lactate (E 325) and calcium phosphate (E 341). The addition of preservatives must be indicated on food (Section 9 of the Additive Admissions Ordinance, Paragraph 1).


When food is irradiated, it is exposed to various sources of radiation such as gamma, x-ray or electron beams. Irradiation not only kills microorganisms, it also eliminates pests such as insects and maggots. In addition, it prevents the germination of z. B. from → potatoes and onions. The subsequent ripening of → fruit can also be delayed. In Germany are allowed to be loud Food Irradiation Ordinance of December 14, 2000 as well as the EU Directives 1999/2 / EG and 1999/3 / EG only dried → herbs and → spices are irradiated. These foods must be marked with the words "irradiated" or the words "treated with ionizing radiation".
In other European countries such as Belgium, France, Italy, the Netherlands and the United Kingdom, other foods may also be irradiated. For example, in the UK, irradiation of fruits, potatoes, fish, poultry and grains is allowed. In France z. B. the irradiation of cereal flakes and germs for dairy products and poultry is permitted and in the Netherlands it is possible to preserve pulses, chicken, dried vegetables and fruits or even shrimp by irradiation. These foods may only be offered in Germany with a special permit.

Health concerns

According to the Federal Office for Consumer Protection and Food Safety, irradiation does not make the food radioactive. The European Commission's Scientific Committee on Food has not recognized a health risk. Whereas the environmental institute in Munich is critical of the irradiation of food. Although they also confirm that an accumulation of radioactive radiation in the body through these foods is not possible, they see a danger in the fact that the radiation changes the products chemically. Longer storage reduces the vitamin and nutrient content.

Deprivation of oxygen

Oxygen-impermeable packaging can reduce the flow of oxygen. One step up is vacuum packaging, in which the existing air, including oxygen, is removed as much as possible. Another possibility is to exchange the oxygen in a packaging with a so-called protective atmosphere. Depending on the food, this is a gas with different proportions of carbon dioxide (CO2), Oxygen (O2) and nitrogen (N2). According to Additive Approval Ordinance - ZZulV § 9 Paragraph 7, the note "packed in a protective atmosphere" must appear on the packaging.


The heating of food is used in many areas. For example, when the food is cooked, it can be kept at temperatures between 75 ° C and 100 ° C. An invention for easier warfare was the Appert method of conservation. The French Nicolas François Appert, born in 1749, managed to develop the first canned food to ensure the nutrition of Napoleon's troops. Food was heated and stored in airtight bottles. Later the glass bottles were replaced by tinplate cans. The first canned food had to struggle with sensory impairments. The canned food was later heated with overpressure in a rotary autoclave, which significantly improved the quality.

Movie: Show with the mouse: Why is the tin can round?


Pasteurization is a process developed by the chemist Louis Pasteur (1822-1895) to reduce germs in → milk. The milk is heated and this kills the microorganisms. Since the milk is not sterilized, only the number of germs is reduced, the milk can only be stored for a limited time.
There are three types of pasteurization:

Type of pasteurizationtemperaturetime
Continuous heatingat least 63 ° C30 minutes
Short-term heatingat least 72 ° C15 seconds
High temperature85 ° C to 127 ° Cz. B. 4 seconds at 85 ° C

Ultra high temperature

Another possibility of preservation by killing germs is ultra high temperature (UHT heating or ultra high temperature process). In this method, a liquid such as milk is briefly brought to a temperature between 135 ° C and 150 ° C. In addition to milk, soups and flavor-sensitive fruit juices are also preserved in this way. There are two types of ultra-high-temperature heating: In indirect ultra-high-temperature heating, the milk is heated to 135-140 ° C in a heat exchanger for about 6-10 seconds and then quickly cooled.
With direct ultra-high-temperature heating, hot steam is introduced into the milk under pressure, whereby the temperature rises to approx. 150 ° C and is held for about three seconds. The milk is then quickly cooled by releasing it in a vacuum. Brief heating can reduce the vitamin content of → vitamins B1, → B6 and → pantothenic acid. Long-life milk can be stored for several months at room temperature. However, there is a clear difference in taste between pasteurized milk and long-life milk → milk and dairy products.

Cooling and freezing

The recommended storage temperatures for various foods can be found in the → Chapter "Storing foods". The storage time can be increased significantly by deep freezing. At a storage temperature of -18 ° C, the water is frozen out and is therefore not available to the microorganisms. In addition, microorganisms hardly multiply at these temperatures → refrigerator.

Modern procedures

New methods of preserving food are constantly being worked on. The aim is to find preservation methods that change the structure and taste of the food as little as possible. A common practice in the USA, Japan and some European countries is high pressure treatment. The technique comes from Japan and was used there as early as 1990 to preserve strawberry and kiwi icecream fruit. Products are exposed to high hydrostatic pressure (4,000 - 8,000 bar), which compresses the cell wall of bacteria and inactivates the microorganisms [1,3,4,6].

A process that also works at high pressure is disinfection by high pressure homogenization (Ultra High Pressure Homogenization, UHPH). It is used, for example, in the dairy and beverage industry. The liquid is compressed in a high pressure pump and then injected through a nozzle. When the drops are relaxed, shear forces act on them, which crush the drops. The high pressures of up to 1,000 MPa (= 10,000 bar) destroy viruses, bacteria and, in some cases, the heat-resistant spores of bacteria (e.g. clostidia) [2].

E-beam is another technique for rendering microorganisms harmless. E-beam is called an electron beam sterilization process. Here, the shelf life of food is extended with the help of electrons. These damage the bacteria by destroying their genetic make-up (DNA, RNA). With e-beam, electrons are emitted and accelerated by an electron source. An electromagnetic field deflects the accelerated electrons and creates a vertical curtain of electrons through which the products are guided. The advantage of this process is that e-beam packages can penetrate more than 70 cm in height. Tetra packs, for example, are treated with e-beam [2,5].

Non-thermal atmospheric pressure plasma is suitable for removing microorganisms from surfaces. A plasma is a neutral gas that is made to glow through the action of energy. The cold plasma has a microbicidal effect and may also be used in food in the future [2,4].

A promising approach is the use of bacteriophages (Greek "bacteria eater") ( BfR: Questions and answers on bacteriophages). Bacteriophages are viruses that infect certain bacteria. There is hope that phages will be used to fight bacteria in food in the future [2].

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Interesting links

German Additive Museum: https://www.zusatzstoffmuseum.de/
How was food preserved in the Middle Ages? The video shows how food lasts through winter. Video: How to keep food fresh without a refrigerator | Quarks


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