Aeration
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Wort is usually aerated in line on transfer between the wort clarification vessel (whirlpool tank) through the wort cooler to the fermenting vessel prior to yeast addition. Most breweries oxygenate the wort on the cold side after the wort cooler (see table below). It is surprisingly difficult to get oxygen to dissolve in water (or wort). There are several systems available which include:[1]
- Aeration in the mid section of two stage wort cooler to benefit from the turbulent flow conditions of a plate heat exchanger.
- Use of stainless steel and ceramic candles in the cold wort line to produce micro bubbles.
- The use of in line static mixers to promote turbulent flow
- The use of venturi systems which produce pressure increase to forcing gas into solution.
In any system only part of the gas supplied is dissolved. A good aeration system should also include a measuring device appropriately located sufficiently far from the injection point so that it accurately measures the dissolved oxygen and can feed back to control system. The amount of dissolved oxygen required depends on the yeast strain and the original gravity of the worts. Traditional ale and lager worts were usually not collected higher than 1045 (12% Plato) and required 6 to 8 ppm dissolved oxygen. With high gravity brewing original gravities have increased up to 1080 (20% Plato) and require dissolved wort oxygen levels of 16 ppm or higher. From the table below it can be seen it is impossible to provide this level of dissolved oxygen from air alone and pure oxygen injection is used.[1]
Air or oxygen is added just prior to fermentation to stimulate yeast growth. Live active yeast has a huge capacity to adsorb oxygen and it is rapidly assimilated before any chemical oxidation can occur. At the end of fermentation the green beer is totally anaerobic and free from oxygen.[1]
Wort aeration or, for higher level of dissolved O2 , oxygenation, is the only example of deliberate addition of oxygen in brewing processes and is strictly controlled to the required level. Oxygen aids the yeast to form unsaturated fatty acids that are necessary for construction of cell membranes; this is assisted by unsaturated fatty acids and sterols derived in wort from malt and utilized by yeast, with the expenditure of energy, to form acyl-CoAs. This added oxygen is exhausted early in fermentation and the process is thereafter anaerobic. Yeast is an efficient scavenger for entrained air and so oxygen pick-up in beer that contains yeast is much less damaging than in bright beer, though both are bad practices.[2]
Oxygen is added to the wort as a nutrient for the yeast. There is some reaction of oxygen with wort constituents and some believe that this is not only wasteful but also detrimental to the flavor stability of beer. Accordingly, it has been argued that it makes more sense to oxygenate the yeast imme- diately before pitching per se, rather than the wort. Provided the physical challenges inherent in trying to get oxygen to all of the cells in a thick slurry are overcome, it is certainly the case that improved fermentation control can be achieved by pitching defined quantities of oxygenated yeast.[2]
Oxygen is required by the yeast to produce the lipid molecules (sterols and unsaturated fatty acids) that are significant components of its membranes. If sufficient quantities of these materials are directly provided to the yeast then the addition of oxygen is superfluous. No wort contains enough lipid; however, it is possible to supplement with lipid, and spent grain pressings have been suggested as one potential source. Such an addition will also act as an antifoam. As yet, no brewer adopts such a practice.[2]
The brewing trials were performed with pitching wort containing DO levels of 8, 16, and 24 ppm. The increase of wort DO from 8 to 16 ppm caused an acceleration of the fermentation rate, but an increase from 16 to 24 ppm caused no further acceleration of the fermentation rate. However, the EA value and flavor stability of the beer was significantly reduced with increasing DO. It was found that the consumption of excess oxygen by yeast resulted in the deterioration of flavor stability without a proportional acceleration of the fermentation rate. However, too little DO in wort caused very poor fermentation performance, resulting in an undesirable taste in the finished beer. Thus, the optimum aeration conditions must be determined by considering not only the desired fermentation performance but also the desired beer quality, including flavor stability. The sulfite content in beer decreased with the DO content in wort. The effect of DO in wort on the EA value might be mainly caused by the differences in the sulfite content of beer.[3]
Although not directly stimulatory to fermentation, oxygen is required by yeasts for synthesis of cell membrane precursors including steroids (primarily ergosterol) and lipids (principally oleanoloic acid). Yeast propagated aerobically contain a higher proportion of unsaturated fatty acids and up to three times the steroid level of anaerobic yeast. Without initial oxygen, replication is usually restricted to 4-5 generations as each yeast budding cycle reduces the sterol content of the membrane by approximately half. When the level reaches a critical point, replication stops and fermentation must continue with the population present at that point.[4]
The focus of studies by Moonjai et al.148,149 has been the potential of lipid supplements to decrease the oxygen demand of the system. A smaller input of oxygen to wort will increase the flavour stability of the final beer and will limit potential oxidative stress in the yeast. An exogenous supply of UFA, which is normally synthesised via oxygen-consuming reactions, reduces the cell’s demand for oxygen. The potential for UFA-supplementation to replace wort oxygenation at an industrial scale has been shown by Hull et al.90 In this case, exposure of pitching yeast to olive oil (a source of oleic acid) prior to fermentation was used rather than wort aeration, without major effects on the acceptability of the beer produced. UFA supplementation therefore acts as a form of oxygen credit.[5]
Despite its potential to cause oxidative stress, there has been no direct evidence of oxidative damage to yeast cell components under brewing conditions and oxygen’s role in relation to yeast stress may rather be a positive one.[5] ??? This doesn't sound right.
See also[edit]
References[edit]
- ↑ a b c O'Rourke T. The role of oxygen in brewing. Brewer International. 2002;2(3):45–47.
- ↑ a b c Lewis MJ, Bamforth CW. Chapter 12: Oxygen. In: Lewis MJ, Bamforth CW, eds. Essays in Brewing Science. Springer; 2006:131–142.
- ↑ Uchida M, Ono M. Technological approach to improve beer flavor stability: analysis of the effect of brewing processes on beer flavor stability by the electron spin resonance method. J Am Soc Brew Chem. 2000;58(1):8–13.
- ↑ Gump BH, Zoecklein BW, Fugelsang KC. Prediction of prefermentation nutritional status of grape juice: The formol method. Food microbiology protocols. 2001:283-96.
- ↑ a b Gibson BR. 125th anniversary review: improvement of higher gravity brewery fermentation via wort enrichment and supplementation. J Inst Brew. 2011;117(3):268–284.