Why do bacteria dislike jam?
Sweet jam could also be tempting for bacteria - but it isn't: home-made or bought jam can be kept even at room temperature. The high sugar content preserves the cooked fruit very reliably. The reason for this is the reduced water activity, i.e. the amount of water available (not the water content!). Like all living organisms, bacteria need water to live. As sugar binds a lot of water, there is little of it freely available in the jam. Worse still: bacteria are actually deprived of water when they come into contact with jam, they "die of thirst".
Another preservation trick is to boil the jam for several minutes: the high temperatures are lethal for the vast majority of bacteria. Hot jam poured into cleanly rinsed jars will therefore keep for a very long time. However, opened jam can still go mouldy on the surface - because both fungal spores and nutrients get onto the surface of the jam through cutlery or air.
Other preservation methods are also based on the reduction of water activity: Candying food uses sugar, just like jam production. Cooking salt has the same effect: when salting (pickling) it reduces water activity. Smoking meat, fish and sausages, sometimes at high temperatures, also dries these foods and reduces their water activity.
Some microorganisms have developed strategies to survive even when water activity is low. They produce "water-binding" dissolved substances inside their cells. This enables them to keep water inside and better tolerate environments with low water availability. The species Halobacterium salinarum, for example, is adapted to extremely salty conditions and even survives in salt pans. Only a few fungi of the genus Penicillium or Saccharomyces can colonise jams and sweet fruit cakes.
The water activity aw is defined as the ratio of the vapour pressure of the food in relation to that of pure water (aw = p / p0). Jam has an aw value of 0.7-0.8 and, like salami (0.82), is lower than cheese (0.85) and bread (0.95). Nuts (0.7), dried fruit (0.6) and pasta (0.5) have a lower aw value (and therefore a longer shelf life).
If you want to know what influence these values have on the spoilage of food at the breakfast table, you don't have to test it yourself. The Quarks & Co. team prepared a substantial meal and then left it to stand for three weeks: The result: please don't replicate it! And: jam simply lasts the longest!
Read more:
US Food and Drug Administration (1984, 2014): Water activity aW in Foods
© Text and figure: Adrian Hochkeppel, UFZ/ VAAM, adrian.hochkeppel[at]ufz.de, use according to CC 4.0
