Adjuncts are unmalted grains, such as corn, rice, rye, oats, barley, and wheat, used in brewing beer which supplement the main mash ingredients (such as malted barley), often with the intention of cutting costs, but sometimes to create an additional feature, such as better foam retention.
Ingredients which are standard for certain beers, such as wheat in a wheat beer, may be termed adjuncts when used in beers which could be made without them — such as adding wheat to a pale ale for the purpose of creating a lasting head. The sense here is that the ingredient is additional and strictly unnecessary, though it may be beneficial and attractive. Under the Bavarian Reinheitsgebot purity law it would be considered that an adjunct is any beer ingredient other than water, barley and hops; this, however, is an extreme view and is not standard.
The term adjunct is often used to refer to corn and rice, the two adjuncts commonly used by pale lager brewing companies as substitutes for barley malt. This use of ingredients as substitutes for the main starch source, usually to lower the cost of production, is where the term adjunct is most often used.
Adjuncts can be broadly separated into solids and liquid syrups. Solid adjuncts are ingredients such as cereals, flakes, grits and flours which must be added to the mash tun in order to convert the starch into simple sugars which the yeast can utilise during fermentation. Some cereals have a higher gelatinisation temperature than the standard mashing temperatures and must be cooked in a cereal cooker to gelatinise the starch before adding to the mash. Liquid syrups, on the other hand, are designed to be added directly to the kettle and therefore can be used to reduce loading on the mash and lauter tun and effectively increase the brewhouse capacity.
Other benefits of using adjuncts include reducing cost, improving consistency, diluting wort nitrogen (thereby improving shelf life) and reducing colour (or increasing colour with roasted cereals and caramels.) (Full article...)