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Yeast Propagation in a Whisky Distillery

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  • Harry
    Cultivation of distillery yeast Most of the yeast used in distilleries is purchased from manufacturers of baking and distilling yeast. The final production
    Message 1 of 2 , May 1, 2008
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      Cultivation of distillery yeast

      Most of the yeast used in distilleries is purchased from manufacturers of

      baking and distilling yeast. The final production stage culture is in fermentors

      of up to 5x105 litres capacity, but the inoculum is propagated from the

      laboratory stock culture through a succession of fermentation vessels of

      twenty-fold increases in size. The culture medium of molasses, from either

      the beet or cane sugar industries, is supplemented with ammonium salts and

      any trace nutrients shown to be necessary by analysis of the current batch of

      bulk molasses (Burrows, 1970). All cultures are grown with accurate temperature

      control at 30°C and vigorous aeration, and often with mechanical agitation,

      although for unicellular yeasts the aeration alone should suffice for

      efficient mixing.

       

      For efficient cultivation of both baking and distilling strains of yeast, it is

      essential to maintain a low concentration of fermentable sugar in the culture

      medium. To avoid the Crabtree effect** and maintain energy-efficient aerobic

      conversion of sugars to carbon dioxide and water, the fermentor is filled with

      a medium of NH4 salts and other trace nutrients and the sugar is added as

      sterilized concentrated molasses – slowly at first, but increasingly rapidly as

      the yeast biomass becomes greater throughout the fermentation. The aim is to

      provide a consistent level of about 0.5 per cent sugar. Ultimately the yeast

      concentration exceeds the ability of the aeration system of the culture vessel to

      maintain the necessary aerobic conditions for efficient growth, so as the dissolved

      oxygen concentration approaches zero; after about 30 hours' growth

      the propagation is stopped.

       

      [Source:  Whisky

      Technology, Production and Marketing

      Elsevier, 2003

      Edited by: Inge Russell, Graham Stewart, Charlie Bamforth and Inge Russell

      ISBN: 978-0-12-669202-0]

       

       

      ** Crabtree effect: 

      From Wikipedia, the free encyclopedia

      Named after the English biochemist Herbert Grace Crabtree, the Crabtree effect describes the phenomenon whereby the yeast, Saccharomyces cerevisiae, produces ethanol (alcohol) aerobically in the presence of high external glucose concentrations, rather than producing biomass via the tricarboxylic acid cycle, the usual process occurring aerobically in most yeasts.

       

      HTH

      Slainte!
      regards Harry

    • Harry
      Further Info: Saccharomyces spp. and other fermentative yeasts cannot grow indefinitely under anaerobic conditions without certain nutrient supplements that
      Message 2 of 2 , May 1, 2008
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        Further Info:

        Saccharomyces spp. and other fermentative yeasts cannot grow

        indefinitely under anaerobic conditions without certain nutrient supplements

        that are unnecessary aerobically. Sterols and unsaturated fatty acids, the

        essential cell membrane components of yeasts (and indeed of all eukaryotic

        organisms), cannot be synthesized anaerobically, thus limiting anaerobic

        yeast growth to two or three generations unless these compounds are provided

        in the culture medium. Secondly, fermentative yeasts are not true

        facultative anaerobes, since the Crabtree effect (Fiechter

        et al., 1981; Young,

        1996) takes precedence. Above about 1 per cent fermentable sugar in the

        culture medium (the exact percentage varying between strains) the yeast

        ferments the sugar by anaerobic metabolism, no matter how well the culture

        medium may be aerated. Instead, the dissolved oxygen is used for the

        biosynthesis of membrane fatty acids and sterols, thereby permitting more

        extensive growth when aerobic conditions are no longer available. Therefore,

        although deliberate addition of unsaturated fatty acids and sterols to wort

        to encourage yeast growth is certainly not permitted for Scotch whisky

        fermentations, the same effect can be achieved legally by aeration of the

        wort.

        In a paper partly reporting their own experience but also extensively

        reviewing the literature on the Pasteur and Crabtree effects, O'Connor-

        Cox

        et al. (1996) stated that of the dissolved oxygen present in the wort

        at the start of the brewery fermentations, only about 30 per cent was used

        directly for synthesis of unsaturated fatty acids and sterols. It has been

        well established that none is used for the oxidative metabolism of sugars

        (see, for example, Lagunas, 1986).


         [Source: Whisky

        Technology, Production and Marketing
         
        Elsevier, 2003

        Edited by: Inge Russell, Graham Stewart, Charlie Bamforth and Inge
        Russell
         
        ISBN: 978-0-12-669202-0]

        HTH

        Slainte!
        regards Harry

        >

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