Table of content A-Z


Conserves or Preserves


englisch: conserves


Conservation includes all processes for preserving foods. Conserves are thus products with a longer shelf life, some of them filled in airtight containers. These may be metal tins (e.g. for maize), glass jars (e.g. for dill pickles), or composite packaging (e.g. for puréed tomatoes). According to this definition, deep-frozen products also count as conserves, but for better searchability, they are discussed in a separate chapter.

Without conservation, the metabolic processes of fruits and vegetables continue after they are harvested, whereby ingredients are very quickly degraded and thus no longer available to the consumer. The produce spoils. Micro-organisms or harmful insects that adhere to or have penetrated the produce can also be dangerous for human beings.

The technology of conservation makes possible longer storage of fruits and vegetables that are not processed directly following the harvest but are to be consumed later. Thus it gained great importance in the past century. Today, deep-freezing of fruits and vegetables has become the more important method for giving them a longer shelf life because it is more effective in preserving the ingredients. Conserves still play a large role for sour products, preserves, marmalades and fruit sauces.

Production of tinned vegetables

Some types of vegetables are more suitable than others for conserving. These are then processed directly following the harvest in order to keep the loss of nutrients to a minimum. First they are washed to free them of coarse impurities and micro-organisms from the soil. This is followed by trimming and peeling to remove all inedible parts. If so desired, the vegetables can also be chopped during this step. The sorting out that follows is not only for visual purposes. Technologically, this is necessary in order to coordinate the temperature and timing exactly in the later heating steps: When the heat is adapted to the larger pieces during blanching or sterilisation, smaller pieces are damaged by the exposure to high heat, while larger pieces aren’t cooked correctly or their enzymes are not completely inactivated and the adherent micro-organisms are not completely killed if the heat is adjusted to the smaller pieces.
The next step is inspection, during which pieces that differ in colour, have defects or are spoiled are eliminated.

With most vegetable conserves, the goods used to be blanched after they were sorted. Blanching serves to inactivate the enzymes they contain, which cause changes in the colour, texture and aroma. Moreover, the germ content should also be reduced at this point, a certain degree of cooking should be reached (e.g. change in texture), air should be removed from the intracellular membranes, and shrinkage of the vegetables should be attained. Today, blanching has largely been replaced by pre-heating, by which not all of the plants’ own enzymes are deactivated, as they are with blanching, but only those are targeted that cause a reduction in quality. This new method therefore protects the ingredients better. Two procedures are possible:
a) Conventional sterilisation: The hot (blanched) vegetables are filled immediately into hot containers (glass jars, tins, soft packaging). Earlier, an additional hot infusion served to eliminate the oxygen from the cooked produce as far as possible and to maintain the necessary temperature for sterilisation later, thereby minimizing the additional exposure to heat. However, this entailed a rather great loss of vitamins and minerals; they should have been saved but were leached into the water and thus not available as nutrients. Today, the amount of additional liquid is very small and the loss is accordingly less or even avoided entirely. Usually for reasons of taste, the added hot water frequently contains salt. For this reason, conserved vegetables generally have a higher sodium content than fresh vegetables do. After they have been filled, the containers are closed (not yet airtight). The actual preservation takes place via heat treatment. This kills micro-organisms and reduces or inhibits the activity of their enzymes as well as those of the food, and thus arrests spoilage.

Most vegetable conserves are sterilised. They are heated at temperatures between 115°C and 123°C for 18–60 minutes in autoclaves (gas-tight sealable vessels in which substances can be sterilised with superheated steam under high pressure; this is equivalent to the pressure cooker used in households). Any oxygen still present in the container is forced out, creating a vacuum, and the container becomes airtight.

b) Aseptic filling: The vegetables are first sterilised and then filled in tins under aseptic, i.e. highly sterile, conditions. The advantage of this procedure is that, thanks to direct contact with the source of heat, the vegetables can reach the necessary temperatures, and a shorter period of heating for sterilisation overall is possible. This helps to preserve the ingredients: vitamins, aromas and flavours are appreciably better than with the conventional procedure.

Production of pickled vegetable conserves

Pickling was one of the first possibilities for increasing the shelf life of vegetables. A low, i.e. acidic, pH prevents microbial growth. But the micro-organisms that are responsible for the fermentation process and are therefore needed at the beginning also cause foods to spoil if the fermentation is not stopped. Therefore, vegetables fermented with lactic acid keep longer only after they have also undergone heat treatment (pasteurisation), which kills the lactic-acid bacteria.

Production of fruit conserves
Tinned fruit

Fruit conserves include whole or chopped fruit in juice, marmalades, jams, purées, jellies and other spreads, as well as juices, fruit juice drinks and sauces. The production of juice and fruit juice drinks is discussed under this heading later in this chapter; therefore, we concentrate here only on the firm and semi-firm fruit products.
As a basic principle, all fruit must be washed before it is processed. Before or after washing, the fruits are separated according to size. As with vegetables, this step serves the appearance, but also ensures quality in processing. Only if all pieces are of the same size can the subsequent heat treatment be correctly set as to temperature and length of time, so that the fruit neither is heated too much or too long and therefore loses structure and ingredients, nor is insufficiently heated, making the preservation incomplete. Depending on the type of fruit, all parts are then removed that are either not suitable for consumption or conservation or are inedible (cores, seeds, possibly skins). If necessary, this is followed by chopping. The fruit is then filled in glass jars or tins together with an infusion usually containing sugar. In high concentrations, the sugar also acts as a preservative and at the same time supports the thickening effect of pectin, a natural gelling agent contained in fruit. However, the sugar goes into the fruit and thus increases its energy value. It is recommended to make sure that you buy only unsweetened tinned fruit. Unfortunately, the sugar content is not explicitly given on the label. ‘Very lightly sugared’ means a sugar content of 9–14%, ‘lightly sugared’ equals 14–17%, ‘sugared’ stands for 17–20% sugar and ‘highly sugared’ means over 20%. If no sugar, only water, was added in the processing, we speak of steamed fruit. Prior heating of the infusion results in air displacement and at the same time reduces the necessary time for heating when the goods are pasteurised.

In contrast to vegetables, most fruits have a low, acidic pH; therefore, it is sufficient to pasteurise the fruit, and sterilisation is not necessary. Normally the products are heated to over 80°C. Although this kills only the vegetative germs, an acidic environment (below pH 4.5) plays its part in preventing the growth of micro-organisms. Following pasteurisation, the products are cooled to 40°C.

Semi-firm fruit products (marmalades, jams, jellies, purées)
Marmalades are made from citrus fruits, jams from the pulp of fruits other than citrus fruits, and jellies from fruit juice or juice extract.
The terms ‘extra jam’ and ‘extra jelly’ mean that for 1 kg of the end product 450 g fruit or juice was used; just ‘jam’ and ‘jelly’ contain 350 g fruit or juice, respectively, per kilogram. For jams from certain fruits there are special regulations, according to which less prepared fruit per kilogram may be used. The fruits used to make jams must also be cleaned and freed from inedible parts, just as for the production of classical fruit preserves.

Together with sugar, they are heated to about 80°C and then steamed at about 65°C until the desired dry weight is obtained. Only then are a hot pectin solution and citric acid added. Pectin serves as a natural gelling agent, and citric acid supports the gelling process. To remove germs the mixture is heated again to 80°C; after this it is pre-cooled and filled in glasses that are tightly closed, pasteurised and again cooled to 40°C.

Shelf life

Products have varying shelf life depending on the temperature and duration of treatment. The conserves that are sold (also marmalades, jams and jellies) are mostly fully preserved and will keep at a storage temperature of below 25°C for at least a year, often for up to 4 years. During production, temperatures between 110° and 140°C are maintained for varying periods of time depending on the type of vegetable or fruit. This treatment kills vegetative micro-organisms,* bacilli of mesophilic germs* and spores* of the genus Clostridium.

There are also semi-preserved products which will keep for 6 months at below 5°C. With these only the vegetative micro-organisms have been killed at relatively low temperatures of 65°–99°C. Boiled conserves, which have been heated to just below 100°C so that the vegetative micro-organisms are also killed, can be kept for up to 1 year at up to 10°C.

Three-quarter conserves keep for 6–12 months at below 15°C. They have been heated to above 100°C so that, in addition to the vegetative micro-organisms, the bacilli of mesophilic germs have also disappeared.

For the special conditions in the tropics so-called tropical conserves are produced at a temperature of ca. 121°C for 16–20 minutes. They will keep for up to 1 year at up to 40°C, as the thermophilic spores* of Bacillus and Clostridium have also been killed.

So-called shelf-stable products are conserves that have been heated to below 100°C to kill vegetative germs. As with semi-preserved products, the spores are still alive. However, due to the combination of heat treatment with regulation of the pH value to an acidic level, keeping the amount of water available for micro-organisms to a minimum (water activity / aw-value), and the addition of other ingredients, they cannot germinate or give rise to the formation of further micro-organisms.

In rare cases it may happen that the heat treatment is not sufficient to ensure the desired effect of conservation. Then micro-organisms remain in the food that can under certain circumstances proliferate and cause the product to spoil. Some species form gases (hydrogen and carbon dioxide) that lead to buckling of the tin or the lid. The tins look puffed out or the lids bulge upward. Other types of packaging can also buckle. For instance, if fermentation takes place in a milk carton the sides also puff out. Buckling is almost always a sign of spoilage, and these products should be discarded immediately. With some sour products (e.g. sauerkraut), however, a deliberate secondary fermentation takes place in the packaging and causes buckling; in this case the food is even better to eat.

Effects on quality and ingredients

In addition to the reasons for conserving foods that have already been mentioned, heat treatment has further desirable effects (+) but unfortunately also some that are undesirable where quality is concerned (-). The advantages and disadvantages of conservation by heating are compared in the following table.

Effect on



Sugar (carbohydrates)

(The amount of carbohydrates does not change with heating. They also remain digestible.)

- Carbohydrates, i.e. sugar, react with amino acids and other substances at high temperatures. Browning occurs and aromas develop that usually are not wanted (e.g. caramel and roasted flavours).

Proteins and thus also amino acids and enzymes

+ Loosening of the protein structure improves digestibility.


+ Release of amino acids from the proteins makes it possible for the human organism to resorb and make use of them.

- Unfortunately, however, amino acids are also degraded and therefore no longer available for uptake by the human body.

+ Denaturation of proteins leads to the inactivation of certain enzymes that are responsible for the degradation of colours and flavours as well as for changes in texture. Moreover, certain enzymes (e.g. in legumes/pulses) are inactivated that inhibit the digestion of protein in the human body.

- At the same time, such proteins are also degraded by the heating process that would have been important for texture and water-binding capacity.

Fats (lipids)

+ Inactivation of enzymes that degrade fats and can thus change texture, flavour and mouthfeel (lipases).




- Heat is fundamentally bad for water-soluble vitamins. Fat-soluble vitamins are somewhat less damaged by sterilisation processes. The average losses through heating for all vitamins are between 10% and 50%. However, this can be higher depending on the duration of heating and the temperature. Also to be noted is the loss through leaching into the water that is added, by which vitamins are transported from the food into the liquid. Since the liquid is usually poured away once the container is opened, these vitamins are lost for the human diet.



- Added salt increases the sodium content in vegetables. This makes it more difficult for individuals to monitor how much salt or sodium they are ingesting, which is a disadvantage particularly for persons with certain diseases.


+ Through reactions that take place during heating, aromas develop that are desirable for a few products (the majority of them animal-based).

- These aromas are not desired for most plant-based foods.


Even chopping, which precedes conservation, entails changes to the ingredients.
Whenever the cells of fresh fruits and vegetables are destroyed, i.e. by peeling, coring or deseeding, chopping, etc., reactions take place that change and ultimately degrade substances. Owing to the destruction of the cell walls, substances that have been separated can mix and come into contact with oxygen from the air, such that uncontrolled reactions can take place. This contact with oxygen in particular causes visible and tastable changes: the foods turn brown; their flavour changes, e.g. apples taste overripe.

Types of packaging

Preserved foods can be packaged in various kinds of containers with correspondingly different characteristics. It is regulated by law that no substances that are harmful to health, or only stipulated maximum quantities thereof, may pass into the food from the packaging.

Tins made of tin-plated steel are the most common option. Foods that are conserved in these usually have a long shelf life. Very sour contents can react with the metal, however, meaning that metal ions are transferred to the foodstuff, leading to changes in aroma and flavour, and the simultaneous formation of hydrogen causes buckling (see above). When the tin is opened and the food comes into contact with oxygen, the process is accelerated. Other foods contain certain amino acids that may react with the tinplate. In this case, black or grey discolourations appear on the inside walls of the tin.
Coating the insides of the tins with food-safe materials is therefore standard procedure. It prevents reactions of the metal with the contents. When tins are opened and the conserved products are removed the coating can be invisibly damaged, however, and any remainders should therefore not be kept in the tin, but rather always transferred to other, airtight containers.

Glass bottles or jars are an equally popular type of packaging. They have the advantage that the consumer can see the contents. There are no substances that may be transferred from the glass to the food or react with it. On the other hand, normal clear glass does not protect the contents from ultraviolet rays, and thus certain ingredients, such as aroma and flavour components, can be damaged by the light. The green or brown glass bottles used for beer, juice or milk are helpful here, but they minimize the advantage of transparency. Basically, brown glass bottles absorb more damaging UV rays than green, and these absorb more than clear glass bottles. The beer industry has used clear glass bottles with UV filters for several years now. They protect most of the ingredients. However, these bottles or jars are quite expensive; consequently, the normal clear glass is still used for fruit and vegetable preserves, for which the consumer is not prepared to pay too much, and the loss of ingredients is accepted.
Therefore, if you buy foods conserved in glass containers for storage at home, it is best to keep them in a cool, dark room or cupboard.

Composite packaging (e.g. Tetra Pak, Combibloc) consists of several layers of the synthetic material polypropylene (PP), cardboard and aluminium. These are bonded together with adhesives that are not harmful to health. Composite packages protect the food they contain from light; they are shatterproof, lightweight, easy to stack, and when they are folded up in the consumer’s trash bin they take up little room.


* Water-soluble vitamins are transferred to the liquid in the conservation process and are therefore no longer available as ingredients of the vegetables. If you use vegetable or fruit conserves to prepare dishes such as soups, use the infusion as well instead of discarding it.

* Store conserves in glass containers in a dark room, so that the ingredients are not exposed to damaging UV rays.

* To avoid changes in flavour do not keep opened tins. Instead, transfer leftover contents to an airtight container and put this in the refrigerator.


* Autoclave: an apparatus for sterilizing foodstuffs and objects while at the same time building up heat and pressure. Overpressure makes it possible to heat water to more than 100°C.

* Mesophilic germs are those whose optimal growth temperature lies between 20° and 42°C. This comprises most of the known soil and water bacteria, e.g. Escherichia coli, Pseudomonas and Staphylococcus. The word mesophilic consists of meso for middle and -phile, having an affinity for or a strong attraction to.

* Pasteurisation: This process of partial sterilisation kills only the vegetative micro-organisms. It uses temperatures that are lower than those for sterilisation. Overpressure is not necessary, and thus pasteurisation can be carried out without autoclaves.

* Spores are resistant, enduring forms of bacteria. Their metabolism is extremely low, enabling them to survive for a long time under unfavourable conditions (heat, desiccation, chemicals, radiation – not ultraviolet).

* Sterilisation: This procedure kills all micro-organisms and their resting stages (spores). Depending on which germs are present, temperatures of up to 140°C are required. Since water does not get hotter than 100°C in normal cooking processes, overpressure must be built up. For this reason an autoclave is needed to sterilise food conserves. The greater the contamination with micro-organisms, the longer the food must be heated. Although we usually speak of sterilisation in the production of conserves, even fully preserved foods are not totally free of germs. It would make more sense to use the term ‘virtual sterilisation’.

* Thermophilic germs grow or proliferate optimally at more than 40°C. At 70°C, however, even they can no longer multiply, but their dormant forms are more stable under heat.
Extremely thermophilic germs grow best at temperatures above 65°C. Among these are some species of Bacillus and Clostridium. The word thermophilic consists of thermo for temperature and -phile, having an affinity for or a strong attraction to.

* Vegetative micro-organisms are germs that proliferate asexually by means of division. They are not very heat-stable and can normally be killed by heating at 80°C for 10 minutes.




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