Could you please tell me if it is possible to use a Bain Marie to heat food so that it can be preserved. I seem to remember my grandmother and my mother using the technique many years ago in Zambia to make preserves, etc.  I need to know how long one should leave the bottled substances in the pan above the pan with water, so that afterwards one can seal the bottles and they will last for a season. I have been leaving my bottles in the oven at 100 degrees Centigrade for up to 3 hours.  I just don't know if this is too much or not enough.

-- Lucy

Although I can't find any specific reference to using a Bain Marie for canning, there are a variety of methods that have been used historically but are not recommended nowadays.  These unsafe methods include steam canning, oven canning and open-kettle canning.  The last, for those unfamiliar with the term involves preparing a large kettle full of whatever is to be canned, and then filling and sealing the jars without any further processing.

In oven canning, which is the closest to what you are describing, it is not possible to assure that the contents of the jars have been heated to, and maintained at, an adequately high temperature for long enough to kill any dangerous organisms.  This is because the temperature regulators in ovens are neither particularly accurate, nor do they maintain a constant temperature.  Even quality ovens may experience temperature swings of as much as 50°C (90°F).  This means that your oven may be running as cool as 75°C (167°F) over part of the time you are canning the food.  With oven canning, there is also a greater risk of injury from glass canning jars exploding.

While using a water bath will act to buffer these temperature variations somewhat for the food that is below the water line, anything above will still be subject to oven temperature variations.

Currently, the only two recommended methods of canning are in boiling water, where the filled canning jars are submerged in boiling water, and steam pressure canning, which allows temperatures to be raised well above the boiling temperature of water.  The boiling water method is adequate for high-acid and some other foods.  Steam pressure canning is the only reliable method for canning low-acid foods.

Further information on canning can be found in the KitchenSavvy posting In a Jam.  Also, the following additional information resources are listed at the end of that article:

National Center for Home Food Preservation at the University of Georgia.

Complete Guide to Home Canning and Preserving (Second Revised Edition) US Department of Agriculture

Canning & Preserving for Dummies

Joy of Cooking: All About Canning & Preserving

One final note:  the fundamentals of canning were rewritten in about 1994, based on new knowledge and up-to-date research.  Heirloom recipes may not benefit from this new information and should be used with caution.