Friday, April 8, 2011

Jam Making, Mould and the Boiling Water Bath Method

There is nothing that tastes as good as home made preserves. Unfortunately, there are still many people out there who are making their own jams and other preserves that are not using "world’s best practice" when it comes to food safety. I am constantly running into people who make jam, then pore the hot jam into hot, sterilized jars, put on a sterilised lid and think that they have guaranteed against contamination. This is simply not so. The jars then need to be processed using the boiling water bath method.

As an experiment, I made five jars of grape jam/jelly. I then processed three of the jars in the boiling water bath and left two of the jars, with lids secured, on the kitchen counter to see what happened. Here is the result.

As you can see, the two jars that I did not process in the boiling water bath were both contaminated with mould. This is because even with putting hot jam into hot, sterilized jars, you still have some air trapper in the jar with your jam. The air around us in contaminated with bacteria, natural yeasts and moulds. Once these contaminates are trapped in a jar with the remaining air, the moisture and food they have access to can explode their population significantly. Just one bacterium can multiply to over 2,000,000 in as little as 7 hours.

What you need to do is use the boiling water bath method to create a full vacuum, by expelling all the air out of the jar. When you put hot jam into a hot jar, as the jar cools, the contents contract and it appears to create a vacuum; however, this is only a partial vacuum and is not enough to prevent the above contamination.

Micro-toxins and Your Health

There is an excellent web site for further information here:

I have quotes just a few key points below.

“Micro-toxins are poisonous substances produced by certain moulds found primarily in grain and nut crops, but are also known to be on celery, grape juice, apples, and other produce. There are many of them and scientists are continually discovering new ones. The Food and Agriculture Organization (FAO) of the United Nations estimates that 25% of the world's food crops are affected by micro-toxins, of which the most notorious are aflatoxins.

Some moulds cause allergic reactions and respiratory problems. And a few moulds, in the right conditions, produce "micro-toxins," poisonous substances that can make people sick. When you see mould on food, is it safe to cut off the mouldy part and use the rest? To find the answer to that question, delve beneath the surface of food to where moulds take root.”

Moulds can thrive in high-acid foods like jams, jellies, pickles, fruit, and tomatoes. But these microscopic fungi are easily destroyed by heat processing high-acid foods at a temperature of 212 °F in a boiling water canner for the recommended length of time.

Looking back at the photographs of my mouldy jam, you can see a definite green tinge to the mould.  Here is some back and green mould under a microscope.

You can see the long roots that would penetrate into he food on the green mould, so if your jam, or any other food for that matter, gets mouldy, throw it away.

So boiling water bath please and enjoy wonderful food and good health.


  1. Just wondering for how long you recommend to put into the Boiling Water bath for? Does it depend on what you're preserving??

  2. The time you process in a boiling water bath does depend on what you are preserving and also on the size of the jar. For jam, I use small jars and process for 5 minutes. All the books we have on the Green Living Australia web site have processing time for each recipe.

  3. This is really helpful thanks:)

  4. No problem. It is a stark reminder that we need to be ever vigilant in regards to food hygiene, but that it can be done safely with a little effort.

  5. what is a water bath?

  6. Hi just wondering if anyone could tell me if its to late to process my chutney in water bath? I made them yesterday afternoon & added the chutney to sterilised jars & lids & thought this is all I had to do.

  7. have a look here

    for full information on the boiling water bath method for safely preserving high acid foods.

    In regards to processing preserves you made yesterday, I believe I answered this in another post, but yes that would be OK.


  8. I bought a couple of really delicious blackcurrant jams 2 weeks. When I used some of them but did not refrigerate them afterwards. I later placed them in the fridge - this morning I'm horrified to find small white mould growing on the jam. I'm reluctant to throw them out. Would scooping out the jam and boiling it again help?

  9. I would not eat this jam and scraping off the visible mould and eating the rest would not be recommended. It is just not worth a case of food poisoning. The safe thing to do is throw this jam out