The surface of fresh grapes is always covered with a thin layer of white powder. Many people mistakenly believe this to be pesticide residue, but in fact, this white powder is a good thing.
Generally, any white powder that is evenly distributed and does not cover the grape's natural skin color can be considered a natural substance secreted by the grape itself, known as bloom or pruina. It is completely harmless to the human body and acts as a "protector" for the grape. Because sugar alcohols are not water-soluble, they prevent the grape's surface from absorbing moisture and creating a humid environment, which could otherwise lead to the growth and infection of pathogens. They also prevent the grapes from dehydrating too quickly and shriveling after being picked.
Additionally, the wine yeast abundant in the white powder can also prevent microbial contamination during the winemaking process. Therefore, when selecting grapes, do not intentionally avoid those with a white powder. Sometimes, the amount of this powder can be an important indicator of the grape's quality.