Dextrin malt, also known as carafoam or carapils, is a type of specialty malt that is often used in the brewing process to improve the body and head retention of the beer. It is made by kilning malted barley at a slightly higher temperature than other malts, which converts the starches into dextrins.
Dextrins are complex sugars that are not easily fermentable by yeast. They are the result of partial starch hydrolysis by enzymes, such as alpha-amylase and beta-amylase, during the mashing process. These enzymes break down the starches in the malt into simpler sugars that can be consumed by the yeast during fermentation. However, some of the starches are only partially broken down, resulting in the formation of dextrins.
Unlike fermentable sugars, dextrins do not get fully converted into alcohol and carbon dioxide during fermentation. They remain in the beer, contributing to its body, mouthfeel, and sweetness. This is why dextrin malt is often used in beers where a fuller body and improved head retention are desired, such as in English ales, stouts, and porters.
However, it is important to note that dextrins do have a small degree of fermentability. While they are not as easily fermented as other sugars, some yeast strains are capable of metabolizing them to a certain extent. The degree of fermentability of dextrin malt will depend on various factors, including the yeast strain used, fermentation temperature, and the overall composition of the wort.
In my personal experience as a brewer, I have found that dextrin malt can contribute a slight increase in the final gravity of the beer, indicating that some of the dextrins have been fermented. However, the impact on the overall fermentability of the beer is relatively low compared to other fermentable sugars present in the wort.
To summarize, dextrin malt does have some degree of fermentability, although it is lower compared to other sugars present in the wort. It is primarily used to enhance the body and mouthfeel of the beer, rather than to increase its alcohol content. The exact fermentability of dextrin malt will depend on various factors and can be influenced by the brewing process and yeast strain used.