These simple sugars are generally all either a single sugar molecule (monosaccharide) or two sugar molecules (disaccharide), which makes them readily available to be consumed by the yeast as-is. This ranges from refined sugars (think table sugar or corn sugar) to Belgian candi sugars and raw sugars to molasses. It would also include sugars added by the introduction of fruit.
These adjuncts need to be mashed along with the base malt in order for the sugars to become available to the yeast. The base malt supplies the necessary enzymes to convert the starches or longer chain sugars from the adjuncts into simpler sugars that the yeast can use. Complex adjuncts would include any un-malted cereal grains (often flaked, could be corn, rye, wheat, barley, rice, oats, etc), as well as any additional source of starch.
So what are adjuncts good for in addition to adding fermentable sugars? Brewing with adjuncts gives the brewer and additional tool to use to dial in the beer they want to make. Most often, adjuncts are used to increase the fermentables and thus the alcohol content, while decreasing the body and mouthfeel for a drier finished beer. Good examples of this would be the use of sugars in Belgian golden strong ales or in some imperial IPAs that still want a dry finish. Other adjuncts can be used to add increased mouthfeel and body, such as oats in an oatmeal stout. Still other adjuncts bring flavors or aromas in addition to their sugar content, such as sugars from fruit additions.
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