Postcollection additions

Although wort composition is largely determined by the time it is cooled and pumped into the fermenting vessel, some modifications may be made by way of a miscellany of post-collection additions. Some of these, principally those that have a direct influence on wort composition, are made immediately after collection is complete. Others may be dosed in during the course of fermentation. These may be viewed as process aids or additions made to correct abnormal fermentation behaviour. A third class may be added at the end of primary fermentation and may be viewed as being the first part of post-fermentation processing.

The most common nutrient addition is zinc, usually added as a solution of the hydrated sulphate and at a concentration of 0.05 to 0.15 ppm Zn2 + . This metal ion, which may be limiting in malt worts, is an essential component of several yeast enzymes, notably alcohol dehydrogenase (see Section 2.4.6). Occasionally other inorganic salts, such as chlorides of sodium or potassium, may be added.

Additions may be directly to fermenter, or in-line, via a small dissolving tank located after the wort cooler. Inorganic chloride supplements are made because they are flavour active, contributing to beer fullness. Salts are added to fermenter rather than during wort preparation because their presence during wort boiling may con tribute to corrosion of stainless steel vessels. Flavour considerations apart, they will change the mineral composition of the wort. They will contribute to yeast nutrition of yeast and may influence the expression of yeast flocculence.

Enzymes may be added to fermenter to modify the sugar spectrum of the wort. For example, wort with a high dextrin content may be treated with amyloglucosidases to increase the proportion of fermentable sugars. This increases the potential yield of ethanol. This approach has been used to produce so-called 'low carbohydrate' beers. Thus, the reduction in dextrin concentrations and concomitant increased wort fer-mentability allows the production of more fully attenuated beers. Commercial preparations of these enzymes tend to be of relatively low activity and are suited to addition to fermenter because of the opportunity for providing a long exposure time.

Antifoam may be added to wort at the end of collection in a single dose to control fobbing. Alternatively, it may be added throughout fermentation as required. To avoid the possibility of microbial contamination the antifoam should be sterilised prior to application. The total dosage rate should be limited to avoid the possibility of carry-over into beer. Failure to do this may result in beers with poor foaming properties.

Post-collection additions may be made where fermentation performance is abnormal. This may take the form of a very slow attenuation rate or failure to achieve the desired final gravity - a 'sticking or hung fermentation'. The causes and treatments of abnormal fermentation performance are discussed in Section 6.5.4. Typically, more oxygen may be added, preferably via a stainless steel candle or some other means of improving gas transfer rates, located at the bottom of the fermenter. In deep fermenting vessels, such treatments should be approached with caution since the possibility of sudden and catastrophic breakout of carbon dioxide from the lower super-saturated regions is a very real risk. Treatment with oxygen or air has the additional benefit of agitating the contents of the fermenter thereby re-suspending yeast, which may have settled out. Should application of oxygen fail to re-start the fermentation it may be necessary to add further yeast. The yeast must be accompanied by more oxygen since conditions within the fermenter will be anaerobic and addition of yeast alone will have little effect.

If addition of further oxygen and/or yeast fails to restart the fermentation, a supplement of vitamins and metals may be made. Such preparations are available commercially and sold under the generic name of 'yeast foods'. They usually consist of mixtures of vitamins, trace metals, lipids and phosphate. Cruder preparations are simply dried powdered extracts of yeast. Dosage rates are given by the manufacturer. However, caution should be exercised since over-dosage, particularly in a stuck fermentation where the yeast may be very stressed, can alter the nutritional balance of the wort such that adventitious microbial contamination becomes a risk.

In traditional ale fermentations the process of clarifying the green beer is sometimes started in fermenter when the attenuation gravity has been reached and the yeast top crop removed. Thus, silica-based auxiliary finings may be added directly to fermenter after cooling and cropping. These contain approximately 2% silica in the form of silicic acid. In beer, which has a pH of approximately 4.0, the silicic acid forms an insoluble precipitate and co-flocculates with suspended proteinaceous matter

(Vickers & Ballard, 1974). This pre-treatment improves the fining behaviour of the beer down-stream of the fermenter.

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