How do salt and sugar prevent microbial spoilage?
—K. RAJYAGURU, LIBERTYVILLE, ILL.
Mickey Parish, chair of the nutrition and food science department at the University of Maryland, explains:
Salt (usually sodium chloride) and sugar (generally sucrose) interfere with microbial growth in several ways to block decay in food.
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The most notable way is through simple osmosis, resulting in dehydration. The salt or sugar, whether in solid or dissolved form, attempts to reach equilibrium with the salt or sugar content of the food product with which it is in contact. This action has the effect of drawing available water from within the food to the outside and inserting salt or sugar molecules into the food interior. The result is a reduction of the so-called water activity (aw), a measure of unbound, free water molecules in the food that are necessary for microbes to survive and reproduce. The aw of most fresh foods is 0.99, where-as the aw required to halt growth of most bacteria is roughly 0.91. Yeasts and molds usually survive with even lower amounts of water.
Microorganisms differ widely in their ability to resist salt- or sugar-induced reductions of water content. Most disease-causing bacteria do not grow below 0.94 aw (roughly 10 percent sodium chloride concentrations), whereas most molds that spoil foods grow at levels as low as 0.80, corresponding to highly concentrated salt or sugar solutions.
In addition to dehydrating food, salt and sugar interfere with a microbe's enzyme activity and weaken its DNA molecular structure. Sugar may also provide an indirect form of preservation, by serving to accelerate the accumulation of antimicrobial compounds from the increase of certain other organisms. Examples include the conversion of sugar to ethanol in wine by fermentative yeasts and the transformation of sugar to organic acids in sauerkraut by lactic acid bacteria.
The practice of adding salt or sugar, or both, to food has ancient roots and has many names. Among them are salting, salt or sugar curing, and corning (pieces of rock salt are sometimes called corns—hence the name “corned beef”).
Curing may utilize solid crystals of salt or sugar or solutions in which salt or sugar is mixed with water. For instance, brine is the term for salt solutions used in pickling. Examples of foods preserved with salt or sugar are bacon, salt pork, sugar-cured ham, fruit preserves, and jams and jellies. Curing has numerous permutations and may include additional preservation techniques, such as smoking, or ingredients such as spices. These processes not only prevent spoilage of foods but also serve to inhibit or prevent growth of food-borne pathogens such as Salmonella or Clostridium botulinum.
Why do bubbles form if a glass of water is left alone for a while?
—D. TEYP, BUENOS AIRES, ARGENTINA
Rick Watling, a meteorologist at the National Oceanic and Atmospheric Administration, offers this answer:
The bubbles come from gases in the water. Atmospheric gases such as nitrogen and oxygen can dissolve in water. The amount present depends on the temperature of the water and the atmospheric pressure at the air-water interface. Colder water and higher pressure allow more gas to dissolve; conversely, warmer water and lower pressure permit less gas.
When you draw a glass of cold water from your faucet and let it to come to room temperature, nitrogen and oxygen slowly exit the solution, with tiny bubbles forming and coalescing at sites of microscopic imperfections on the glass. If the atmospheric pressure happens to be falling as the water warms, the equilibrium between gas molecules leaving and joining the air-water interface becomes unbalanced and tips in favor of them leaving the water, which in turn causes even more gas to come out of solution.
For a complete text of these and other answers from scientists in diverse fields, visit www.scientificamerican.com/askexpert