Secondary Fermentation(book excerpts)
Secondary fermentation refers to the period post-primary fermentation where beer is transferred from the primary fermentation vessel to a “secondary” vessel (e.g., keg or bottle) during which most of the carbohydrates in the wort are assimilated. If no secondary fermentation is done, then all carbohydrates are assimilated during primary fermentation. The yeast remaining in the beer at the end of primary fermentation can utilize fermentable carbohydrates remaining in the beer or small quantities of fermentable carbohydrates are added in the form of “priming sugar.” In some systems wort is added to provide the fermentable material or actively fermenting wort a process referred to as krausening. Yeast activity achieves carbonation, purges undesirable volatiles, removes of all residual oxygen, and chemically reduces many compounds such as reduction of vicinal diketones and aldehydes, thus leading to improved flavor and aroma. Beer produced in this way will have flavor attributes that cannot be achieved with finely filtered beer. Secondary fermentation processes include lagering, kräusening, and cask- and bottle-conditioned beers. If a secondary fermentation is desired, the transfer from the fermenter will be done with some fermentable extract still remaining (1–4°Plato).
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