| Term | Definition |
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| Couch | In olden times, "couching" was a stage of barley germination that allowed for taxing authorities to assess the grain production. In agricultural application, barley in the stage of beginning to sprout; to allow the removed (from immersion) sprouts to breathe. This often produces some slight fermentation resulting in the production of acids.
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| Culture | As a noun, cultivation of living organisms in prepared medium; as a verb, to grow in prepared medium.
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| Culture medium | Any nutrient system for the artificial cultivation of bacteria or other cells; usually a complex mixture of organic and inorganic materials.
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| Cytase | An enzyme which has the power of dissolving the cellulose of the cell walls surrounding the starch granules.
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| Decoction | Exhaustive system of mashing in which portions of the wort are removed, heated, then returned to the original vessel. It does not occur without the presence of water.
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| Dextrin | A soluble, gummy unfermentable carbohydrate formed by the decomposition of starch by heat, acids or enzymes in barley. It gives the beer flavor, body, and mouthfeel. Lower temperatures produce more dextrin and less sugar. While higher temperatures produce more sugars and less dextrin.
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| Diacetyl | A volatile compound in beer that contributes to a butterscotch flavor, measured in parts per million.
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| Diastase | An enzyme mixture capable of gelatinizing and converting starch to dextrins and sugars.
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| Diffusion | The flow of molecules, usually, but not necessarily, through a membrane.
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| Dimethyl sulfide (DMS) | A sulfur compound offing, giving the taste and aroma of sweet corn as a result of the short or weak boil of the wort, slow wort chilling, or bacterial infection.
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| Disaccharide | The sugar resulting from the condensation of two molecules of a monosaccharide, with the loss of water. Examples: sucrose, maltose, and lactose.
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| Dispersion | Any mixture where one substance is very intimately intermingled with another. Most frequently a dispersion refers to a colloidal suspension.
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| Dissociation | The splitting of a compound into two or more simpler molecules, atoms, or ions.
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| Distillation | The process of heating a liquid to its boiling point, removing the vapors through a cooling and condensing apparatus, and finally collecting the condensed vapors, as a liquid, in a separate receiver. Method of separating liquids from solids or liquids having different boiling points from each other by evaporation and condensation of the more volatile component.
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| Dosage | The addition of yeast and/or sugar to the cask or bottle to aid secondary fermentation.
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