I teach home food preservation, using science as the basis.
Sugar is not a necessary (from a science perspective) ingredient for any preserve, whether pressure canned or water bath processed. Sugar/syrups help fruits to retain colour and texture, and so make the final result more appealing. Physically/chemically speaking, one could chop fruit, cook it down to evaporate the water, and then have “jam”. Most fruits are naturally high acid enough to prevent mold, fungus and bacterial growth AFTER appropriate waterbath processing. Otherwise, keep your preserve in the fridge.
Some types of pectin react to high heat, and so are less suitable for waterbath processing; think “freezer jam” or “no-cook jams”. The flavour of the preserve is much fresher, as there is no cooking involved.
Small batch (i.e. 1 kg of fruit or less) jams, using little or no sugar can still be processed for shelf-stable storage, using a water-bath. See “Small Batch Preserves” or “Food In Jars” books as reference.
Many recipes for fruit preserves and pickles are scaled for larger quantities, due to the perceived effort of water-bath processing several jars at a time. FWIW, I use an asparagus cooker (deep narrow pot) to process one or two small jars of specialty items, or for test batches.
We are a household of two, and I make many recipes that yield two or four 250ml jars.
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