Sugar-tolerant yeasts occur naturally in honey. If they are not killed by heat, they can cause fermentation when honey moisture levels exceed 17 percent. High moisture levels can result from extracting honey that is not fully ripened by the bees, allowing honey to absorb moisture during processing and storage, or from granulation. Fermentation usually occurs after granulation. Since there is more moisture in the top layers of crystallized honey than in the bottom layers, fermentation begins at the top of the containers and works downward.

The yeasts develop on the dextrose and levulose of the honey, producing alcohol and carbon dioxide. The alcohol in the presence of oxygen is further broken down into acetic acid and water, giving the honey a sour taste. The first signs of fermentation are whitish streaks appearing in granulated honey and honey leaking from the container. As the honey is heated, considerable foaming will occur. The degree of spoilage or effect upon flavor and quality depends upon the length of time fermentation is allowed to proceed before being stopped by heating.

Storage of honey below 50° F or above 80° F will prevent fermentation indefinitely. Lower temperatures are preferred since higher temperatures can cause deterioration in honey color and flavor. Honey heated at 145° F for thirty minutes will be safe from fermentation if protected from further yeast contamination. Higher temperatures (155° to 160° F) are sometimes recommended, but only for a few minutes. Failure to lower the high temperatures quickly will result in a damaged product. Honey should always be heated in a double-jacketed heating vat to avoid contact with direct heat. Have at least 1 inch of water around the bottom and sides of the container. This ensures circulation of the hot water around the bottom and prevents scorching the lower honey layer by direct heat. Keep the water level near the top of the container and loosen the container's lid to allow for expansion.