Beer Yeast Byproducts
Acetaldehyde creates a sour (green) apple aroma in beer. It is present when primary fermentation does not complete, or when too much oxygen is present during conditioning or aging. Saccharomyces yeast create Acetaldehyde at an intermediate stage when converting simple sugars into ethanol.
Esters generally provide aromas associated with fruit and sweetness. Esters are present in beer when ethanol reacts with acids.
Ethanol + Acid => Ester + Water + Heat
This reaction is known as esterification. More esters are created depending on how the Saccharomyces yeast are pitched at the beginning of fermentation in the cellaring stage of brewing. When yeast are pitched at low populations and must reproduce heavily before primary fermentation, more esters are created. When yeast are pitched at slightly warmer temperatures (5 F or 3 C above) than their average fermentation temp, more esters are created. When yeast are pitched without adequate oxygen in the wort, more esters are created.
Ales tend to have more esters than lagers, because they ferment at higher temperatures to begin with. This is one of the reasons why ales are considered complex, while lagers are considered smooth.
Ethyl Acetate creates a subtle fruit or floral aroma at low concentrations. At very high concentrations in beer, it creates an aroma of solvents, such as nail polish remover or acetone. It is present when ethanol reacts with acetic acid.
Ethyl Butyrate creates a pineapple aroma in beer.
Ethyl Hexanoate creates a sweet (red) apple aroma in beer.
Isoamyl Acetate creates a banana aroma in beer.
4-Vinyl Guaiacol creates a clove aroma in beer. It is present when the Saccharomyces yeast has the enzyme ferulate decarboxylase (not all strains do), and is able to process ferulic acid that is naturally present in malted barley and wheat.
Simple alcohols have only one or two carbon atoms.
Also known as ethyl alcohol or drinking alcohol. At high concentrations it can create a warming sensation on the tongue. It is created during the fermentation step of cellaring, if the yeast have enough simple sugar (from the mashing step during brewhouse) and a sufficient fermentation temperature.
Also known as fusel alcohols. Higher alcohols generally provide aromas associated with alcohol and solvents. Higher alcohols have three or more carbon atoms. They are heavier alcohols, with an oily consistency. Higher alcohols may create legs in beer. When fermentation occurs at warmer temperatures (above 75 F or 24 C), more higher alcohols are created. When the wort pH is more acidic, more higher alcohols are created during fermentation. When yeast are pitched without adequate nitrogen in the wort, more higher alcohols are created.
Diacetyl creates a butter or butterscotch aroma in beer. It is present when beer is not conditioned long enough after primary fermentation.
It is created during primary fermentation by the Saccharomyces yeast, as well as the Pediococcus and Lactobacillus wild bacteria. After primary fermentation during conditioning, the Saccharomyces yeast processes diacetyl and produces other compounds not perceivable in beer.