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The cropping process

There is not an agreed universal fermenter design; an observation that is frequently reinforced when visiting breweries. Ideally, the CCV can be independently cooled to minimize the development of hostile conditions that damage the cropped yeast. Typically to aid stratification and rundown, the included cone angle is 60 or 70° but flatter or steeper cones are not uncommon. Practically, there are a limited number of processing options yeast cropping from the cone. Firstly and foremost is whether to recover the entire crop or to focus recovery on a particular fraction or ‘cut' so as to select the best yeast. Perhaps the most universal approach (Noble, 1997; O'Connor-Cox, 1997) is to recover the ‘middle cut' whilst discarding the first runnings, which are rich in trub and dead yeast and (if they are there) enriched with flocculent yeast variants whose selection can cause processing problems (see ‘Population heterogeneity and genetic instability'). The first cut or cold break is either removed within a day or so of fermentation commencing or more usually as part of the cropping process itself. The uppermost ‘fuzzy' fraction is often specifically not recovered during cropping, as it is lower in solids and enriched in small chronologically young cells together with potentially less flocculent yeast.

The process of yeast recovery can be controlled in a variety of ways ranging from a standard volumetric cut-off, visual assessment, use of turbidity sensors or, better still, application of an in-line viable biomass probe as used to manage pitching yeast (see above). Cropping from the vessel cone is managed via an appropriate pump. Less common is the recovery of powdery, less floccu- lent yeast by centrifugation. With such yeast this process is unavoidable, but the approach requires capital (for the centrifuge and in-line cooler), adds complexity and is comparatively energy intensive. Cropping from the top of a cylindroconical vessel is unusual and is a demanding process hygienically. Whatever the approach taken to recover cropped yeast, it is preferably stored as slurry in dedicated chilled and mixed yeast storage tanks (see ‘Storage', below).

As elsewhere in brewing, time and temperature are key variables of yeast cropping. Typically, where diacetyl reduction is managed in fermenter, yeast is cropped as the final process activity after VDK specification is achieved and the vessel contents have been cooled. Such ‘cold' cropping prolongs yeast residence time in fermenter and in doing so inevitably results in more physiologically damaged yeast. An alternative approach with suitably flocculent yeast is to practice ‘warm' cropping (Noble, 1997; O'Connor-Cox, 1997; Loveridge et al., 1999). Here, yeast cropping is independent of diacetyl reduction and occurs sooner (rather than later) after fermentation has achieved racking gravity (usually 24 hours) but before cooling is applied. The advantage of this approach is that the yeast is cropped two or more days earlier - with the attendant benefit to yeast quality - but with the additional process complexity of a second crop (that goes to waste) prior to beer transfer. The timelines for cold and warm cropping are compared in Fig. 3.5.

 
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