How Does Sugar Preserve Fruits and Berries When Making Jams and Preserves?

There’s nothing unique about the use of sugar to preserve foods.

In principle, you could make your strawberry jam with salt instead of sugar and it would keep just as long. Much longer, in fact, because nobody would go near it after the first taste. Salt has been used for thousands of years, however, to preserve fish and meats. The wonderful cured salmon called gravlax is usually made with a mixture of salt and sugar.

Although sugar and salt work quite well in killing or deactivating microorganisms to keep foods from spoiling, they function only when they are in very concentrated form. You can’t sterilize foods just by sprinkling these familiar kitchen chemicals on them. But if you use enough sugar or salt, so that when it dissolves in the foods’ juices it makes a solution of at least 20 or 25 percent, then most bacteria, yeasts, and molds simply can’t survive. And no, it’s not because they die from diabetes or high blood pressure.

What happens is that the sugar or salt solution sucks most of the water out of the little buggers, dehydrates them, so they just shrivel up and either die or become inactive. Practically nothing can live indefinitely without water, and these microscopic, one-celled organisms are no exception.

How can a solution of salt or sugar pull water out of an object? The answer is by osmosis, that seemingly all-purpose word that people throw around to refer to any mysterious kind of seepage. (“I never really studied Urdu, but my parents spoke it and I guess I got it by osmosis.”)

Osmosis is actually only one particular kind of seepage. It’s the seepage of water through a thin membrane, and it occurs whenever there happen to be two solutions of different concentrations (strengths) on opposite sides of the membrane. The membrane must be semipermeable. It must allow water molecules to seep through, but not other molecules. Most of the thin, sheet-like membranes that separate organs from one another in plants and animals are semi permeable. In our own bodies, that includes the walls of our red blood cells and the walls of our capillaries.

In osmosis, there is a net transfer of water molecules through the membrane from one of the solutions to the other, but not in the reverse direction. In a sense, the membrane functions as a one-way street for water molecules. The direction of the traffic depends on the relative concentrations, or strengths, of the two solutions. The water will flow from the less concentrated solution to the more concentrated one. Let’s see how that happens in the case of those villainous bacteria on your strawberries.

A bacterium is essentially a tiny blob of jelly-like protoplasm encased within a cell wall that functions as a semipermeable membrane. The bacterium’s protoplasm is water with various kinds of stuff dissolved in it, proteins and many other chemicals that are terribly important to the bacteria but of little concern to us at the moment.

Now, let’s deluge this blob-within-a-membrane with a flood of very salty or sugary water. Suddenly the concentration of dissolved matter outside the cell is higher than it is inside. That means that there are relatively fewer freely-moving water molecules in the outside solution, because they are hindered by the dissolved substances.

We thus have an unbalanced situation in which there are different concentrations of free water molecules on the two sides of a thin, water-penetrable membrane. Now, Mother Nature just hates imbalances, and she always tries to even things up if she can. In this case, the balance can be restored if some of the free water molecules on the inside migrate through the membrane to the outside. And that’s just what happens.

Osmosis behaves almost as if there were some kind of pressure forcing water through a membrane from the low-concentration side to the high-concentration side. Scientists actually do talk in terms of an osmotic pressure, and they deal with it pretty much in the same way as they deal with gas pressures.

For our hapless bacterium, the net result is that water will have been sucked out of it, whereupon it promptly bites the dust. At the very least, it is so weakened that it is incapable of reproducing. (“Not tonight, dear, I’m dehydrated.”) In either case, the threat to our health has been eradicated.

For the same reason, shipwrecked people, when stranded in a lifeboat or on a raft at sea, can’t drink any of the “water, water everywhere.” Perversely, drinking this water could dehydrate them fatally.

The same fate is likely to befall a fresh-water fish when put into salt water. Osmosis will draw water out of the fish’s cells and into the saltier sea, and the fish can die of dehydration, a rather ironic mode of demise for a fish.