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An answer on another SE site mentions that sugar "at a certain level acts as a preservative". I've always been taught that microorganisms eat sugar and expel acids, that is why sugary food are damaging to teeth. How is it that sugar acts as a preservative, then?

Googling the question I've found conflicting answers though I don't credit the sources as being especially reputable. What is the real reason that sugar can act as a preservative?

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up vote 9 down vote accepted

Sugar in high concentrations acts osmotic. This means that the water available in the cells is drawn towards the high concentration of a solutant (sugar), like in the image below (this is demonstrated with a plant cell, but the principle is the same for bacteria and other microorganisms):

enter image description here

Since microorganisms can not survive without water, they are not able to grow or reproduce. This effectively safes food. The same mechanism works when salt is used to conserve meat or fish.

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This is also why quite large amounts of sugar (or salt) is needed in order for it to work as a preservative. With too little sugar there will be no osmotic effect (or too little) killing of the microorganisms spoiling what you're trying to preserve. Which in turn is why e.g. jam always is so sugary. –  RickardSjogren Jul 18 at 5:53

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