Modern Industrial Microbiology and Biotechnology

Production of Beer 249separator which is now more common. In this equipment the wort which is fed into a flatcentrifuge, is thrown at the side of the equipment and finds its way out through an outleton the periphery. The heavier particles (the trub) are thrown to the center and withdrawnthrough a centrally located outlet. The separated wort is cooled in a heat exchanger.When the temperature has fallen to about 50°C further sludge known as ‘cold break’begins to settle, but it cannot be separated in a centrifuge because it is too fine. In manybreweries the wort is filtered at this stage with kieselghur, a white distomaceous earth.The cooled wort is now ready for fermentation. It contains no enzymes but it is a richmedium for fermentation. It has therefore to be protected from contamination. During thetransfer to the fermentor the wort is oxygenated at about 8 mg/liter of wort in order toprovide the yeasts with the necessary oxygen for initial growth.12.1.3.4 FermentationThe cooled wort is pumped or allowed to flow by gravity into fermentation tanks andyeast is inoculated or ‘pitched in’ at a rate of 7-15 x 10 6 yeast cells/ml, usually collectedfrom a previous brew.12.1.3.4.1 Top fermentationThis is used in the UK for the production of stout and ale, using strains of Saccharomycescerevisiae. Traditionally an open fermentor is used. Wort is introduced by a fish tail sprayso that it becomes aerated to the tune of 5-10 ml/liter of oxygen for the initial growth of theyeasts. Yeast is pitched in at the rate of 0.15 to 0.30 kg/hl at a temperature of 15-16°C. Thetemperature is allowed to rise gradually to 20°C over a period of about three days. At thispoint it is cooled to a constant temperature. The entire primary fermentation takes aboutsix days. Yeasts float to the top during this period, they are scooped off and used forfuture pitching. In the last three days the yeasts turn to a hard leathery layer, which is alsoskimmed off. Sometimes the wort is transferred to another vessel in the so-calleddropping system after the first 24-36 hours. The transfer helps aerate the system and alsoenables the discarding of the cold-break sediments. Sometimes the aeration is alsoachieved by circulation with paddles and by the means of pumps. Nowadayscyclindrical vertical closed tanks are replacing the traditional open tanks. A typical topfermentation cycle is shown in Fig. 12.3.12.1.3.4.2 Bottom fermentationWort is inoculated to the tune of 7-15 x 10 6 yeast cells per ml of wort. The yeasts thenincrease four to five times in number over three to four days. Yeast is pitched in at 6-10°Cand is allowed to rise to 10-12°C, which takes some three to four days; it is cooled to about5°C at the end of the fermentation. CO 2 is released and this creates a head called Krausen,which begins to collapse after four to five days as the yeasts begin to settle. The totalfermentation period may last from 7-12 days (Fig. 12.4).12.1.3.4.3 Formation of some beer componentsDuring wort fermentation in both top and bottom fermentation anaerobic conditionspredominate; the initial oxygen is only required for cell growth. Fermentable sugars areconverted to alcohol, CO 2 and heat which must be removed by cooling. Dextrins and