Two foods are left out on the counter – fresh tomatoes and a bowl of sugar. Within a week or so, one will develop black spots and the other remains pristine, albeit perhaps a little clumpy depending on the humidity of the air. The reason? Osmosis.

While microorganisms love sugar, they also need a certain amount of water to thrive. This level of freely available water, called “water activity (a_w),” for bacteria is about 0.91, for molds it is 0.8 and for fungi (yeasts), it must be at least 0.6. The a_w of fresh foods is generally about 0.99, while crystalline sucrose (table sugar) is a paltry .06.

In its crystal form bone dry, sucrose (C12H22O11) loves to bind with water (H20). When present in sufficient concentrations, table sugar will suck all of the water around it. This is why sugar is an excellent food preservative. Via osmosis, the sugar pulls the available water from within the foodstuff, reducing the food’s a_w, thus making it unsuitable for microbes to grow, or even survive.

More specifically, at the outer edge of a cell is its membrane, a semi-permeable barrier that allows some substances, including nutrients and wastes, to move in and out.  With a higher concentration of sugar outside the cell, the solution is hypertonic, meaning it will draw water from the cell, causing the bacteria (or whatever cell) to shrivel and die. (The reverse could potentially happen as well if the sugar concentration was higher inside the cell, hypotonic, with it drawing water in, perhaps to the point of bursting the cell.)

On a chemical level, it’s pretty interesting as well. Notice all the hydrogen and oxygen involved; between the two molecules, there are 24 hydrogen atoms and 12 oxygen. Each oxygen atom has a slight negative charge and each hydrogen atom has a slight positive charge, and in chemistry, opposites attract. Together, all of these hydrogen and oxygen atoms pull at each other – initially to form their respective molecules (table sugar or water), and then in the process that kills the microbe.

You can also observe this absorption effect simply by taking some cotton candy, which is made of pure spun sugar, and placing it in a humid environment. With just 33% relative humidity, cotton candy left out in the air will completely collapse and crystallize in just 3 days as it absorbs the moisture in the air. At 45% relative humidity, it will completely collapse in just one day. At 75% humidity, it takes just 1 hour. This is why it has only been since 1972 that non-“made on demand” cotton candy has been available. (1972 was when the first fully automated cotton candy machine was invented that could make the fluffy treat and quickly package it in water tight containers).

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Bonus Facts:

• While it may seem like cotton candy, which is made of pure sugar (sometimes with food coloring or other flavoring added), would be pretty much the worst thing in the world for you to eat,  it should be noted that it only takes about 30 grams of sugar to make a typical serving size of cotton candy, which is about 9 grams less than a 12 ounce can of Coke.  Further, cotton candy has no fat, no preservatives and is only about 115 calories per serving.  While certainly not a health food, nor filling in any way, there are numerous things people consume every day that are much worse for them health-wise.
• Even when dissolved in small amounts of water, table sugar remains toxic to most microbes – think of jams and jellies, which have an a_w of about 0.8 and so don’t (usually) spoil very easily. Of course, there are several microorganisms, called osmophilic, which can thrive in relatively low water activity environments. Two of these, Pediococcus halophilus, a bacteria, and Saccharomyces rouxii, a yeast, work together with a mold, Aspergillus sojae (or oryzae), to create shoyu, a fermented soy sauce.
• We use other microbes in food production as well. Bacterial cultures form the basis of cheeses (such as Lactococcus lactis) and yogurt (e.g. Lactobacillus bulgaricus), as well as fermented sausages, like chorizo and pepperoni (e.g. Lactobacillus plantarum). Lactic acid bacteria are also used to help stabilize the malic acid in wines.
• To make blue cheeses (think: Roquefort, Gorgonzola and Stilton, as well as Bleu), the molds Penicillum Roqueforti and P. Glaucum,are added. Note that although some molds can be toxic (when they produce aflatoxins and mycotoxins), the composition of cheese prevents this – thus rendering cheese mold generally safe to eat.
• Yeasts (e.g. Saccharomyces cerevisiae and S. pastorianus) are used for fermentation, integral for making breads, spirits, wine and beer. These processes require both a fair bit of sugar and water to make, but because there is sufficient water, this doesn’t have the effect of killing off the microbes via dehydration as happens with pure, dry table sugar. More specifically, relying on the same process that dehydrated the microbe, the water and sucrose molecules seek each other out, but this time, in the presence of sufficient water, the bonds between individual sucrose molecules are broken – and thus each molecule is separated and surrounded by water molecules, making a sugary solution. At a mixture of 50% water to 50% sucrose, the solution has an a_w of .927 – high enough for yeast, mold and bacteria to thrive off the abundant sugar source.
• The cell membrane of a microbe like bacteria has small pores that are large enough to let small water molecules (with a molecular weight (MW) of 18) to pass through, but are too small for large sugar molecules (342 MW) to normally traverse. Thus, to get sugars like sucrose and glucose to pass through a cell membrane, rather than osmosis, sugars can enter a cell through a special channel. In this process, called facilitated diffusion, proteins on the membrane bind with the sugar, which opens a portal that allows the molecules to enter and exit; with facilitated diffusion, no energy is expended and the substance moves from the area of high concentration to that of low concentration. A similar process, although one that requires the expending of energy, active transport, moves substances from areas of low concentration to those of high concentration.