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.