Preserves, jams and jellies are usually high-acid from the fruits used (or from the acidic juices or the vinegars added), which makes them safe from bacterial growth if the jars are properly processed. [In the same way that you may can high acid fruits sugar free in just water as the canning liquid.]
In a very few instances, however, there’s a distinct lack of acid in recipes that are to be water-bathed. This is usually a huge no-no — if it’s low-acid it should be pressure canned instead to kill off the nasties, so what’s going on here?
What’s going on here is that there is one and only one rare exception, and that is if a recipe (of which there are only a handful) provides for a product which is low in water activity.
The scientific abbreviation for water activity is “aw”, standing for “activity of water.”
A small handful of recipes depend on pure sugar for safety
Consider the classic recipe for corn cob jelly:
The ingredients are:
- 3 cups corncob juice
- 1 (1¾ ounce) package powdered pectin
- 3 cups sugar
Corn is low-acid, with a pH of 6.0 – 7.5 . So, it’s probably roughly around the pH of the water that was used to make the juice from it. So in theory, this recipe ought to be pressure canned, because the pH of the only real ingredient is so far above the safe cut-off for water-bathing of 4.6.
What makes this recipe safe for water-bathing is that the sugar and the pectin bind up a great deal of the water in the food product. In doing so, they don’t leave enough water free for any bacteria to use to do anything. It is with this rare type of recipe, where you don’t see an acid at all, that it is not safe to even consider reducing the sugar or using a sugar substitute. This is a rare example in home canning where sugar is a critical safety preservative, rather than just a flavour, colour or texture enhancer or conserver, as it is most of the time.
Another good example of a water-bathed recipe which relies entirely on low-water activity for safety is Ball’s Maple Walnut Syrup: the ingredients are walnuts, water, corn syrup, maple syrup and sugar. The overwhelming amount of sugars in the recipe would lower the water activity immensely.
In contrast, the classic USDA simple recipe for water-bath canning blueberries in plain water, with a very high water activity yet no added refined sugar, is safe because the acidity of the blueberries (pH of 3.1 – 3.4) is providing the safety (combined of course with proper processing of the jar to inactivate spoilage mechanisms.)
Pectin and water activity
With the new no-sugar needed pectins such as Pomona Pectin, you can set almost anything even with high water activity and low-acidity, which could lead into unsafe territory. If water activity is not being controlled, then the acidity needs to be high (as we have seen above.) Such recipes therefore also usually call for some added acid as well in the form of apple, grape, lemon, etc juice if the main ingredient is of low or unreliable acidity (such as melon, Asian pear, banana, or fig.)
It’s the same as the USDA’s advice for home canning figs and Asian Pears: those are low-acid fruits, and you may can them in plain water if you wish, but regardless of whether you add sugar or not to the canning liquid, it’s the mandatory added acidity (see their guidelines) that will provide the safety for those.
If you wanted to go off the beaten track of the already hundreds of recipes that Pomona has tested and approved, do first ask the advice of the people there to ensure that what you are thinking of doing will be safe acidity-wise — their customer service is very responsive and helpful.
Measuring water activity
You can buy meters to measure the water activity in a product. But unlike pH meters, these water activity meters really are still very expensive. The cheapest portable one is $1,500.00 US (as of 2015) and most are many thousands of dollars more.
Here is a video of the portable $1,500.00 meter in use:
Here is how a table-top meter works:
Don’t muck with low-water activity recipes
So, if you see a recipe from an approved source, which is a water-bathed recipe with no sign of acid (either added or from within the food ingredients themselves) but copious amounts of sugar, it’s likely that that recipe critically depends on actual, real sugar for its safety. Again, there are only a handful of recipes like that, but they are out there. You have to either make the recipe as it is, or walk on by and let it go, or, hunt instead for a tested, replacement recipe from a reputable source.
Miscellaneous information about water activity
“Primary growth-limiting factors for C. botulinum include environmental temperature above 122°F (50°C) or below 50°F (10°C); high acidity (pH <4.6); low water activity ”  Keith R. Schneider, Rachael Silverberg, Alexandra Chang, and Renée M. Goodrich Schneider. . Preventing Foodborne Illness: Clostridium botulinum. Document FSHN04-06 . Revised December 2014. Accessed March 2015 at https://edis.ifas.ufl.edu/fs104.
“Canning is the method of food preservation where food is treated by the application of heat alone, or in combination with pH and water activity …”  Parto, Naghmeh. Home canning: literature review. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Toronto, ON: Queen’s Printer for Ontario; 2014. ISBN 978-1-4606-4166-8 [PDF] page 9. https://www.publichealthontario.ca/en/eRepository/Home_Canning_2014.pdf
“The water content of food is another major determinant of vulnerability to growth of C. botulinum. Available water is ‘free’ water that is not chemically bound to other molecules like salt or sugar. This is why salt and sugar were traditional food preservatives. Available water is referred to as ‘water activity’. The water activity of pure water is 1.0. C. botulinum is inhibited below a water activity of .93. Generally, pathogenic bacteria cannot grow in foods with a water activity level of less than or equal to .85. The water activity of sugar is 0.85; other canned foods have a water activity greater than 0.90.”  Rayner, Lisa. The Natural Canning Resource Book. The Natural Canning Resource Book. Flagstaff, Arizona: Lifeweaver LLC. 2010. Page 35.
“The second factor is water activity (aw), which is defined on a scale of 0 (bone dry) to 1 (pure water). The good news is that C. botulinum generally cannot exist in an aw lower than 0.93. However, other pathogens such as staphylococcus aureus can exist in environments with an aw of 0.86. ”  Timberlake, Sean. Can I Can Jams Made with Chia? Accessed March 2015 at https://foodpreservation.about.com/od/Canning/fl/Can-I-Can-Jams-Made-with-Chia.htm
Low-water activity alone is not a guarantee of safety. The National Center says, “More recent research has shown that some species of Salmonella are tolerant of … low water activity conditions.” National Center for Home Food Preservation. https://nchfp.uga.edu/questions/FAQ_canning.html#34 Accessed August 2017. Heat processing would be required to deal with nasties such as salmonella, etc.
|↑1||Keith R. Schneider, Rachael Silverberg, Alexandra Chang, and Renée M. Goodrich Schneider. . Preventing Foodborne Illness: Clostridium botulinum. Document FSHN04-06 . Revised December 2014. Accessed March 2015 at https://edis.ifas.ufl.edu/fs104.|
|↑2||Parto, Naghmeh. Home canning: literature review. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Toronto, ON: Queen’s Printer for Ontario; 2014. ISBN 978-1-4606-4166-8 [PDF] page 9. https://www.publichealthontario.ca/en/eRepository/Home_Canning_2014.pdf|
|↑3||Rayner, Lisa. The Natural Canning Resource Book. The Natural Canning Resource Book. Flagstaff, Arizona: Lifeweaver LLC. 2010. Page 35.|
|↑4||Timberlake, Sean. Can I Can Jams Made with Chia? Accessed March 2015 at https://foodpreservation.about.com/od/Canning/fl/Can-I-Can-Jams-Made-with-Chia.htm|
|↑5||National Center for Home Food Preservation. https://nchfp.uga.edu/questions/FAQ_canning.html#34 Accessed August 2017|