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Yeast Management during Fermentation - Top Cropping and Skimming |
You have heard of these terms mentioned in reference to yeast management during fermentation and wonder what they are. You will also wonder if you should or can do this. Well you can and its very easy. So firstly what do the terms means and why do you use them?
Everyone would have heard of ale yeast being top fermenting and lager yeast being bottom fermenting. Yet when you look in your fermenters it all seems to settle at the bottom. Well this is true at the end of fermentation, but at the beginning ale and lager yeast will behave slightly differently. Lager yeast will almost immediately head to the bottom of the fermenter, but for the very first part of fermentation, ale yeasts will actually concentrate near the top, then settle out.
Certain ale yeasts will concentrate at the top better than others. English ale yeasts in particular are particularly good at that. In fact the foam at the top of a fermenting English ale is very concentrated with yeast. Harvesting this foam will give you very active healthy yeast for your next beer. This is top cropping. For a typical English ale you do this about the second day.
Now any brewer has seen their fermenter at high krausen, (lot of foam) or at the end of fermentation and noticed all that brown stuff all over the place. Simply put, there are bitter hop resins and other awful compounds that the gas of fermentation takes out of your beer. If you taste this stuff, its extremely bitter and not very nice at all. Fortunately it gets deposited on the sides of the fermenter, so it doesn't get in your beer to affect the flavour.
But some will fall back through your beer to the bottom. Many brewers believe that if you skim this stuff off, you reduce the chance of it falling back through you beer and affecting its flavour. It can also produce a smoother flavour. This is especially important if you open ferment, that is don't cover the fermenter. If oxygen can react with these compounds, it will produce terrible flavours.
So skimming is just removing these compounds early on in fermentation. Normally day one to day three .
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The photos show its so easy (click on the photo for a bigger image). The procedure is the same if you skim or top crop. With a sanitised spoon, just scoop the foam off. If you are skimming, you are only concerned with getting all the crap off the top of the foam. |
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Top croppers skim automatically. They have to skim the crap off the first day, so that the foam (which is highly concentrated with yeast) is fairly clean for the second day. You put the foam in a sanitised jar to use again. So top croppers should skim on day one and harvest (crop) on day two. |
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Graham Sanders