The question of whether sugar can grow mold often arises. While many organic substances support microbial life, sugar possesses unique properties that deter mold growth under specific conditions. Understanding these characteristics reveals why pure, dry sugar behaves differently from other foods concerning spoilage. This distinction is rooted in fundamental scientific principles governing water availability for microorganisms.
The Science Behind Sugar and Mold Growth
The primary reason granulated white sugar resists mold growth lies in water activity (aw). Unlike water content, water activity quantifies the available water microorganisms need for growth. Mold, like all living organisms, requires available water to thrive. Granulated white sugar is highly hygroscopic, attracting and binding water molecules.
This strong binding reduces water activity to extremely low levels, typically around 0.06. Most molds cannot grow in environments with water activity below 0.65. Sugar acts as a preservative by dehydrating microbial cells, preventing their proliferation. This principle explains its historical use in items like jams and jellies.
When Sugar Appears to Spoil
While pure granulated sugar resists mold, certain conditions or sugar products can spoil. Brown sugar, for instance, contains molasses, providing higher moisture than white granulated sugar. This raises its water activity, making it more susceptible to mold or yeast growth if exposed to humidity or contaminants. Hardening or caking in brown sugar is due to moisture loss, not mold.
Sugar syrups like honey and maple syrup also contain more available water than crystalline sugar. While most molds struggle, specialized molds called xerophiles can grow in highly concentrated sugar solutions if not stored correctly. Yeasts can also cause fermentation, leading to bubbling or off-flavors, especially if syrups are diluted or contaminated. If granulated sugar contacts external moisture, such as from a wet spoon, this localized water creates pockets of higher water activity. Mold can then grow on these wet spots or impurities, rather than on the sugar itself.
Preventing Sugar Spoilage
Proper storage prevents sugar spoilage by minimizing moisture exposure. Storing sugar in airtight containers prevents it from absorbing humidity and keeps out contaminants. This maintains the sugar’s low water activity for long-term stability.
Keeping sugar in a cool, dry place minimizes exposure to fluctuating temperatures and high humidity. Using clean, dry utensils prevents introducing moisture or foreign particles. For brown sugar or syrups with visible mold or fermentation, discarding the product is recommended for safety.