In the UK, beer fermentation and maturation (secondary fermentation) tends to be a batch process in stainless steel vessels which can be cooled. Figure 2.1 (a) illustrates a schematic of a typical dual purpose fermenter with the recommended filling level (working volume) and the total
Fouling process example Induced by temperature?
Protein deposition in heat exchangers Yes
Mineral deposition in heat exchangers Yes
Ice build up in freezers Yes
Scale build up in cooling water systems Yes
Fat burn on in ovens Yes
Product solidification Yes
Growth of biofilm No
Accumulation of material in low flow areas of equipment No
31 volume including CO2 atmosphere (gross volume). Figure 2.1 (b) illustrates a dual purpose
fermenter with the convection pattern of beer, identified by the arrows, under different cooling regimes: 1 – high level cooling and 2 – low level cooling. The * indicates that the cooling jacket is on. Secondary fermentation is slower and at lower temperature with lesser amounts of yeast (Lewis and Young, 2002). During both processes, a foam forms above the beer. This is called kräusen by microbrewers meaning “frizzy” in German. This foaming leads to the deposition of material on the wall of the vessel within the head space. This foam has been commented on by microbrewers and seen by other authors including Cluett (2001). Photographic documentation of the material is presented in Figure 2.2.
The amount and severity of the foam is dependent on carbon dioxide evolution during fermentation, which is dependent on
(i) the metabolic activity of the yeast. (ii) the size and shape of the vessel.
Rapid production of carbon dioxide bubbles is believed to enhance convection currents in fermenters which results in a large volume of foam above the beer (Briggs et al., 2004). Cylindroconical fermentation vessels are normally 3 – 4 times taller than their diameter, which could be up to 4 m in large scale breweries. Larger height to diameter ratio tends to produce carbon dioxide bubbles more quickly generating a larger volume of foam.
32
(a) (b)
Figure 2.1: (a) Schematic of a dual purpose cylindroconical fermenter (Briggs et al., 2004). SB – spray ball, TPA – top plate assembly, TP – temperature probe, PT – pressure transmitter. (b) Schematic of beer movement in tall cylindroconical fermenters (Lewis and Young, 2002), 1 – high level cooling when the beer is above the temperature of maximum density, 2 – low level cooling when the beer is below the temperature of maximum density. The cooling panels are labelled in (a).
(a) (b)
Figure 2.2: Kräusen remaining of the walls of (a) the Caledonian Brewery open square fermenter and (b) the top interior of a 500 l working capacity cylindroconical fermenter (from Cluett, 2001).
HIGH
LOW
CONE
33 Figure 2.3 (a) indicates the rate of CO2 evolution during batch fermentation. An initial lag in CO2
production is followed by accelerated evolution reaching a maximum. There is a linear deceleration phase after this. Fermentation is exothermic due to yeast metabolism, thus the rate of CO2 evolution can be related to the increase and decrease in temperature seen during typical ale
fermentation. An example of ale fermentation is given in Figure 2.3 (b). The temperature can in turn be related to the foaming action during fermentation. Figure 2.3 (c) illustrates typical lager fermentation. The time frame to attain the desired gravity is longer for lagers than ales. This is because lager fermentations are done at lower temperatures than ale fermentations. Lower temperatures will reduce yeast metabolism and the rate of CO2 production which will in turn
reduce the foam produced. This also suggests that foaming produced during secondary fermentation will also be less due to lower temperatures used and yeast contained in the beer.
34 (a)
(b)
(c)
Figure 2.3: (a) CO2 evolution (from Boswell et al., 2003), (b) the fermentation profile typically of ale and (c) the
fermentation profile typically of lager (from Briggs et al., 2004). SG – specific gravity, T – temperature, fa – fusel alcohols (mg l-1), e – esters (mg l-1). The pH tends to fall as amino acids and ammonium ions are taken up by the
35 The heat given out during fermentation will peak at the maximum fermentation rate, typically within the first 40 - 60 h for ale fermentation (Lewis and Young, 2002). At maximum fermentation rate foaming is most vigorous and material from the foam attaches to the vessel wall in the head space. The foam collapses when the temperature and thus yeast activity decreases. During the remaining fermentation time may be changing the adhesion of the material on the surface. The duration of lager fermentation and maturation is longer than for ale. This may mean that even though less foam and less deposit are produced, the deposit formed may be harder to remove due to the longer aging time. The head space will also be at a higher temperature than the beer due because it is located above the level of the high cooling jacket. The heated head space could be further baking the material onto the wall of the vessel, especially in summer.
When fermentation vessels are emptied residual material that looks like yeast slurry sticks to the vessel wall. This creamy deposit is illustrated in Figure 2.4. Salo et al., (2008) added riboflavin which is fluorescent under a UV lamp to the beer before emptying the vessel. Upon emptying the vessel the walls were not fluorescent; as yeast is not naturally fluorescent the wall material is most likely yeast. Yeast cells were cultured from contact agars of the vessel in the study. In a large scale vessel the emptying time is minimised but can be anywhere from 3 – 12 h (Briggs et al., 2004). If the duration of emptying is increased the yeast can age on the surface for longer.
Salo et al., (2006) used fermentation cone deposit to create fouling on stainless steel plates. The cone deposit was aged on surfaces for 2 weeks; much longer than in normal beer fermentation operation. The plates were held at different cone angles during rinsing: 15, 35 and 55° (from the horizontal), which gave a flow velocity of 0.23 – 1.13 m s-1, 0.34 – 1.68 m s-1 and 0.4 – 2.01 m s-1
36 from the top to the bottom of the plate. The authors found that this deposit could not be wholly removed using ambient water at 648 l h-1 (Re 1760) in any case. In fact only 20 - 30% of the soil was removed in all cases. No significant effect of cone angle was found. The cone angle of fermentation vessels tends to be 70° (see Figure 2.1) to enable efficient separation of yeast from beer (Briggs et al., 2004). This angle would enable higher flow velocities at the cone.
Figure 2.4: 80 l stainless steel tank (0.8 m by 0.4 mm) with residual yeast fouling attached to the wall and the cone. The wall was also sampled by contact agar (from Salo et al., 2008).