Why is sugar considered a preservative in jams?

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Offline cryonicfry

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Why is sugar considered a preservative in jams?
« on: 30/06/2011 02:32:07 »
Hi, [:)]
I don't really know if this fits under this category but since shelf-life is related to the growth of microorganisms it seemed okay.
I was wondering why and how sugar preserves jam (if it does). The reason I'm asking this is because I was wondering if one could make sugarless ( without added sugar to be more precise) jams and the information I've found online states that sugar prolongs the shelf life of the product. If sugar is a nutrient for many microorganisms how can it help preserve such products?
I was thinking that it might raise the temperature of the cooked mixtures and thus allow a more sterile product but technically, cooking on lower temperature for longer periods should yield similar results. Another possibility that crossed my mind is that it would limit the motility of microorganisms... [???]
Any detailed information would be much appreciated.
« Last Edit: 30/06/2011 05:59:47 by Geezer »


Offline CliffordK

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Why is sugar considered a preservative in jams?
« Reply #1 on: 30/06/2011 08:33:08 »
The need for a preservative would be dependent on a few things.

1 - Canning temperature (are  you using a pressure cooker?)
2 - How long between opening and consumption.  Jelly might be in your fridge for months.
3 - Color and Texture Stability.  There may be some chemical breakdown independent of biologic breakdown.  Apparently NASA has problems with ready-to-eat meals for hypothetical multi-year trips.

Sugar/syrup may be an osmotic barrier for many micro-organisms.  I.E.  not enough water, and it would tend to dry out the organisms by sucking the moisture out of them.

If your canning is intended to be consumed in a single setting, you are probably ok without any sugar or "preservatives" added.  Just make sure you get it hot with the pressure cooker.  Watch for signs of contamination such as smell or broken seals.

If you are planning on eating the jelly over an extended period of time, consider adding something that would create an osmotic barrier.

It may not be as important now as it was 100 years ago as we now have refrigeration for a secondary preservative.


Offline Mazurka

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Why is sugar considered a preservative in jams?
« Reply #2 on: 01/07/2011 16:58:31 »
The principal way the jam making process preserves fruit is by boiling a fruit / sugar mix to the right consistency will sterilise the mixture.  It is the reaction between pectin and sugar at 104C that makes jam set.  Most jam making failures are due to too little pectin.  If the jam does not reach this temperature it will not set, so slow cooking is not an option.  Fruit syrup (unset fruit/ sugar mix) will last in the fridge for quite a while (but to my mind, except in small quantities, keeping "preserves" in the fridge defeats the point...

Putting jam (whilst fairly warm) into a sterilised jam jar and covering (either with a sterilised airtight lid or waxed disk) rapidly, prevents microbes/ mould spores from getting in, thus preserving the food.  If this is done well enough, and the jar remains airtight a good batch of jam will last for months if not years stored in a cool dark place.   

It is possible to make sugar free preserves.  These are often called fruit leathers and there are basically an (oven) dried fruit puree.  Taste lovely (often much better than they look) and can be eaten in a variety of ways, although they do not spread on toast [:-'(] 

The other way to preserve whole fruits is canning or bottling and their are ways to do this without sugar, but I have only tried this using a hot syrup

Depending on the fruit/ veg also consider chutney, pickle or salting... 


Offline Mr. Data

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Why is sugar considered a preservative in jams?
« Reply #3 on: 02/07/2011 16:08:42 »
Or simply sugar is an excellent preserve for foods in much the same way salt is.

See, food is considered inedible if the food contains certain micro-organisms that begin to reproduce by eating the surface of the food. Foods like this become this way because of water contained within it. Water is the fastest way to spread germs, if you didn't know! High suger content and high salt content in foods can literally dry the system out from the micro-organisms which can develop on its surface, which may include a host of deadly organisms, including very spurious types of fungi.