The joy of sex (a voyeuristic approach to fermentation)
Sorry, but I thought I’d try the attention grabbing headline, so beloved of the British tabloid media! I’m talking about yeast sex of course … apart from the fact that yeast obviously don’t “do” sex. Our good friend Saccharomyces cervisae (“sugar fungus of beer”) is a unicellular budding fungi … the mother cell buds off a daughter cell, the mother is left with a scar but the daughter is not (a yeast caesarian?) Both those yeast cells will bud off daughter cells and hence the original mother cell will now have 2 scars and the original daughter cell one scar and so on. Each yeast cell “ages” as measured by the accumulation of birth scars and at some point the yeast cell membrane will become so damaged by the process of scarring that it can no longer participate in fermentation. However the overall yeast population does not age as such as half of the yeast cells at any given time will have no birth scar. Our house strain of yeast is the (infamous) Ringwood Ale yeast, a typical top fermenting ale yeast that forms a thick frothy head on the surface of the fermenting beer, supported by the generation of carbon dioxide bubbles by the vigorous fermentation of the yeast still suspended in the beer. Because we ferment in traditional British open vessels we, as brewers, can “connect” with our yeast in a way not possible in a closed system … we can see and smell what’s happening in the fermentor as well as being able to monitor the progress of fermentation by the measurement of specific gravity, temperature and pH. Indeed, by looking and smelling the fermentation at the start of the day we have an insight into the fermentation before any technical measurements are made! We are also able to strip yeast from the yeast head of an active fermentation to pitch into a new brew and, with a hardy yeast such as Ringwood, we are able to use the yeast for some 30 generations before we start with a fresh pitch grown from the housed strain. The Ringwood yeast is highly flocculent and well suited to separation from the beer with the use of isinglass finings.
The video below shows a fermentation using a Belgian yeast strain (courtesy of The Brewer’s Art). Unlike our English Ale yeast there is no thick yeast head, allowing us to see an active “rolling” fermentation, the fermenting wort looking like it is being heated and approaching boiling. Pretty damn cool!
The photograph below shows wort transfer to an open fermenting vessel.
The photo below shows an active fermentation with Ringwood Ale yeast with it’s characteristic frothy yeast head!
The photo below shows the yeast “skin” left on the surface of the beer post fermentation (excess yeast has been skimmed off and pitched into a new batch of wort). You can clearly see the height that the yeast head reached above the surface of the fermenting wort.
The photo below shows yeast that has accumulated at the bottom of the fermenting vessel. As fermentation progresses yeast becomes less active as the conditions (lowering of pH and rising alcohol level) become less favorable. Yeast flocculates and much forms a sediment at the bottom of the vessel, accelerated by chilling the beer at an appropriate specific gravity (i.e. when it has achieved a desirable attenuation).
This past Summer my good friend and fellow Brit Paul Pendyck (owner of UK Brewing Supplies) was fortunate to tour The Black Sheep Brewery in Masham, Yorkshire and was kind enough to share these wonderful photographs of their open fermentation system. There is a”shelf” above the vessel on which yeast collects and fermenting wort is sprayed upon it and this then drains back into the vessel below.
One of the original slate square open fermenting vessels which were still in use as recently as ten years ago before production was moved into the new stainless steel round vessels.