By Harold McGee
Many spirits are chill-filtered: chilled to below the freezing point of water, and then filtered to remove the cloudy material that forms. The substances that form the cloud are poorly soluble fusel oils and volatile fatty acids from the original spirits, and a variety of similar substances extracted from the barrel. Their removal prevents the spirits from clouding when the drinker chills them or dilutes them with water, but it also removes some flavor and body, so some producers choose not to chill-filter. Clouding does not occur in spirits with more than about 46% alcohol, so such undiluted “cask-strength” spirits are often not chill-filtered. (Some spirits cloud spectacularly;).