Keep the jam

Sugar: a natural preservative

Jam is a food product obtained by cooking fruit pulp added with sugar;

the subsequent cooling gives rise to a homogeneous mass similar to a gelatin, which assumes a variable consistency according to the production methods adopted (possible addition of gelling agents). If the fruit, deprived of the peel, is reduced to a pulpy puree, it is commonly referred to as jam, while when whole pieces are kept it is referred to as jam. From the legislative point of view (Decree 20 February 2004, n.50) the question is not so simple:

Products prepared with the pulp and / or puree of one or more fruits can be considered jams. More precisely, the jams must have no less than 35% of pulp and fruit; for extra jams the legal minimum of fruit pulp increases to 45%. For jams containing particular fruits such as rowan, sea buckthorn, cashew apple, etc. other percentages are indicated.
The definition of jams, on the other hand, belongs to products prepared with pulp, puree, juice, aqueous extracts and citrus peel with a minimum of 20% fruit, of which at least 7.5% must come from the endocarp.

Sugar, the main ingredient together with fruit, is added with the dual purpose of improving the organoleptic characteristics and increasing storage times. At this point a question might arise ... but how is it possible that sugar, the nutritional element par excellence of cells, including microbial ones, can inhibit their growth? To answer this question we need to brush up on the concept of osmotic pressure, that force which regulates the passage of water from the solution with a lower concentration towards that with a higher concentration. If we apply this law to jam, since the higher concentration is in the middle, it is the cell that releases water, depriving itself of an essential element for life, and not vice versa . The use of hypertonic solutions, therefore, increases the conservation times of food because it reduces the availability of water, an essential element for the degenerative processes of food.

Classic pasteurization

In order to preserve the jam in the best possible way, it is however important to carry out a second operation called pasteurization which allows, among other things, to reduce the use of sugar; this technique involves immersing jars filled with jam in water, which will then be brought to a boil for about thirty minutes (the duration depends on the type of preserves and the size of the jar). This method of conservation destroys most of the germs and creates a vacuum inside the jar ensuring its hermetic closure (the heated air increases its volume which is then reduced by cooling, creating the suction effect). Pasteurization also prevents the formation of droplets under the lid, where part of the sugar would solubilize, favoring the growth of some microbial spores. This technique allows to eliminate most of the microbes except for the Botox which can survive even in the absence of air.

Once opened, the jam must be kept in the refrigerator.

How to pasteurize the jam

Close the jars well (previously left open to cool the freshly cooked jam), then place them in a high and large pot in which the water level must exceed the capsule of the jars by 2-5 cm.

On the bottom of the pot it is good to place some cloths to avoid exposure of the jar to direct heat; in the same way it is good to put some rags between one jar and the other.

Heat, bring to a boil and let it boil for about 20-40 minutes, taking care to check the water level in order to top it up if it drops.

Turn off and remove the jars only when the water has become cold, then dry them and place them in a cool, ventilated and dark place for storage.

Alternatives to classic pasteurization

There is a particular method that allows you to avoid boiling jars filled with jam, while still ensuring optimal food preservation.
First of all, the empty glass containers, including caps, must undergo a preliminary and long boiling (sterilization), and remain totally immersed in water while continuing to boil for the entire period necessary for the jam to gelatinize. In other words, the jars must not undergo drastic temperature changes starting from the moment of boiling, up to the filling with jam. To avoid the above, the jars - as well as the caps - should be removed from the boiling water only shortly before being filled with boiling jam. This means that the jars do not have to cool before potting the fruit to be preserved: for the same reason, those who usually sterilize the jars in the dishwasher or in the microwave, will still have to proceed with the classic post-potting pasteurization precisely because the jars tend to cool. quickly. It should also be remembered that before pouring the boiling jam, the jars must be carefully poured from the water, taking care to also remove any droplets to avoid creating an environment suitable for the proliferation of bacteria.

On the other hand, classical pasteurization is indispensable in some circumstances:

The jars are filled when the jam is cold;
The glass containers do not undergo preliminary sterilization practices;
The jars are allowed to cool to room temperature before being filled with jam;
Preparation of fruit in syrup: unlike jam, fruit in syrup - which must not be boiled - is placed in jars raw, cold and possibly cut into pieces. For this reason, after having in any case sterilized the empty containers in boiling water, post-potting pasteurization is essential for the correct preservation of fruit in syrup.

To conclude, if the glass jars (with the relative caps) are boiled for a long time in water and removed only shortly before filling them with the still steaming jam, the classic post-potting pasteurization would be a superfluous procedure for the correct preservation of the jam.