BREWING
undertaking these steps in the brewhouse on their own will be insuf cient to produce an AFB worth marketing. They need to be combined with the following adjustments in fermentation and beyond. Arrested fermentation In arrested fermentation, the wort is pitched with yeast and allowed to pro- ceed until the required level of attenuation is achieved. It has been found that the lower the OG at the start of fermentation the lower the perceived level of worty notes of the beer produced. Fermentation can be stopped by applying rapid cooling, removal of the yeast through a centrifugal separator or by applying a high pressure to the fermentation vessel. Where pressure is applied, CO 2 is added through the bottom of the FV af- fording a degree of cold stripping at the same time. The temperature of fermen- tation has been found to be relatively unimportant with regard to reduction of worty avour compounds – so arrested fermentations tend to be lower than standard fermentations due to the potential for fermentation to overrun if a higher temperature is used.
Of the three methods to arrest fermentation applying crash cooling is most practical and hence common. A cold maturation (0-1 o C) of at least seven days is used for beers produced using arrested fermentation to reduce the level of sulphur containing off notes. Restricted fermentation The cold contact process (CCP) – not to be confused with a critical control point – is fermentation carried out at 0-4 ° C often at a high pitching rate (circa 30x10 6 cells per ml) with a wort which has not been oxygenated. The high pitching rate means that compounds important to beer which originate from yeast cells are added to the wort without signi cant yeast growth or fermentation taking place. Yeast slurries can contain around 6% ABV so are often washed before pitching to ensure no alcohol pick up. The lower temperature of CCP means that ethanol production is slow so the contact time with the yeast can be extended. Yeast contact time is thought to be important not only for the production of aroma compounds typical of fermenta-
One of the most signi cant avour defects in biologically produced AFBs is a worty avour. Strecker aldehydes (3-Methylbutanal, 2-methylbutanal and 3-methylthiopropionaldehyde) formed in kilning and heating processes in the brewhouse are implicated as the culprits, or at least as an index to them. In the kettle, hot stripping can be per- formed where either nitrogen or CO 2 is blown through the wort to help remove these aldehydes. The same approach in fermenta- tion is called cold stripping (a lot less comfortable) which in some breweries is undertaken using the exhaust from a standard fermentation. Cold strip- ping using this method has the added bene t of increasing the level of typical fermentation volatiles in the AFB, al- though some brewers feel the risk of the entrainment of unwanted volatiles like sulphur-containing off notes and vicinal diketones is too great. Another obvious adjustment to stand- ard brewhouse practice is to produce a wort with a lower OG and hence potential alcohol level, 1020-1030 SG (5.0-7.6 Plato) is typical. It should be noted that
“Over the past few years we looked at lots of differ- ent ways of making low alcohol beers, the reverse osmosis method gave by far the best tasting beer. It allows us to brew and ferment as normal and then at cold temperatures remove the alcohol whilst leaving the other avours of the fermentation in the beer. It’s all very high-tech magic allowing our Ghost Ship to sail away free from alcohol but remaining full of its original avours and aromas.” Fergus Fitzgerald, Head Brewer at Adnams.
7 ● BREWER AND DISTILLER INTERNATIONAL
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