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Jam and marmelade
Jams and marmelades are fruit jellies. The fruit has a shelf life of several months to more than a year due to addition of sugar, gelation and vacuum packaging. Once opened (release of the vacuum) the fruit jellies have a durability of a few weeks, at least when it is stored at cool temperatures (under 7°C). Besides jam and marmelade another fruitjelly is known, named confiture. In confiture hole fruits are used. For processing of jam the fruits are cut fine or even mashed. Marmelade consists of citric fruit most times the skins are used too. This gives a bitter sweet taste to the marmelade.
|Processing of traditional jam|
|Cutting and mashing|
|Processing of diet jam|
The fruit contains a small amount of pectin. This gives gelation. But the amount of natural gelatine is too small to gelate all the mass. Therefore extra pectin is needed. The sugar has, besides the function of giving taste, a function in gelating the mass.
The pectin molecules can not make a gel network without sugar and acid. Due to the high water content of the fruit mass a hydratation layer (water molecules binded to the hydrogen bindings) is formed around the pectin molecules. Therefore the pectin molecules can not move freely and can not come close enough to each other to bind. Besides this, the free carboxyl groups are dissociated. H+ is loosened, a negative end is the result. This results in a a negative charge of the pectin molecules. A negative charge makes the molecules depart from each other. Binding is impossible.
Sugar binds water and takes water from the hydratation layer. The hydratation layer gets thinner and the pectin molecules can move more freely. But the negative charge is actually still very high, so the molecules keep pushing off each other.
When the acid is added the pH drops under 3.5. The positive hydrogen molecules bind the dissociated groups. The negative charge gets more neutral and the pectin molecules can approach each other so they can bind at several places.
A vacuum in the pot can be reached easily by closing the pots in a normal way and turn the pots upside down for several minutes. The air can, while the jam is still warm, escape trough the pot and the lid. Because the jam is right behind the lid, air can not go into the pot.
After vacuum packaging the jam can be turned to its normal position and cooled to 20°C. Cooling of the jam pots can be done effective in streaming cold water. In this way gel forming will be obtained easily and the fruit has no possibility to float and is kept well divided.
The network of LM-pectin can be formed at a higher pH (higher then 3,5) and a low sugar content. But it requires cations like calcium ions. The calcium ions form a binding with the dissociated carboxyl groups and a calcium-pectinate-gel is formed.