London Amateur Brewers

London Amateur Brewers

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How to Get the Best Out of Your Yeast by Gillian Grafton

Reproduced with kind permission from CBA (UK) Brewers Contact – Volume 6, Issue 2, June 2006.

We all know about the friendly little fungus that does the business of converting our best efforts at mashing into fine beer, but how many of us treat them with the care and affection they deserve? It may seem somewhat anthropomorphic, but it really is true that most of us abuse our yeast and they reward this misuse with beer that isn’t as good as it could be. So this short article (based on a talk given to the Midlands Craft Brewers) is all about the care and nurture of your most important brewing friend – your yeast.

Most homebrewers encounter yeast either as dried preparations in packets or liquid yeasts in foil packs or tubes. We’ll leave slants for the serious yeast rancher, but even so, most of what’s written here applies in that case too. Dried yeast used to have a terrible reputation. More often than not they contained a high level of spores, were slow to start, and produced beer that tasted suspiciously like bread. However big improvements in the processing of dried yeasts mean that some top quality yeasts are now available in this form and some commercial breweries rely entirely on them for their beer production. The main drawback these days is the relatively limited range of strains available, which is where liquid yeasts come into their own. But regardless of whether the yeasts are dried or liquid, you need to treat them with a little care and attention to make the very best use of your yeast.

Numbers Count

It’s my opinion that the single biggest mistake that most homebrewers make is that they use far too few yeast. You’ll probably think me quite mad when I tell you that I use the equivalent of a one gallon starter for a five gallon batch of beer, and you won’t be the first. Let me see if I can convince you. The reality of homebrewing is quite different to brewing in a large commercial brewery. The latter can spend money and time on maintaining a sterile environment whereas we’re competing for space in a house with kids, pets, and the irate spouse who just wants to use the kitchen for cooking. Add to that the fact that we’re creating a rich medium to encourage microbiological growth (the wort) and the end result is that sterility is a distant dream. In this context the best way of avoiding an infected brew is to crowd out the bacteria with overwhelming numbers of yeast. The longer the lag time between pitching the yeast and the start of fermentation, the greater the chance that the beer will become infected. A lag time of over 24 hours is indicative of serious problems with your yeast, and you should be aiming for four to six hours at the most. The other consideration is the health of your yeast. Straight from the manufacturer, dried or liquid yeast are in top condition, but the delay between distribution and sale, and a period of sitting around in the homebrewer’s cupboard or even fridge takes the edge off their health. This directly contributes to long lag times and unhealthy yeast can produce off- flavours.

These considerations dictate that we should pay close attention to the numbers and health of our yeast. It’s worth it since they pay us back in top quality beer.

Rehydration

Dried yeast is generally sold in packets of about 10g and one packet is recommended for use for five gallons of beer. I strongly advise using two packets of yeast. Check the sell-by date on the yeast – how long has it been since it left the manufacturers? Remember, recommendations are for yeast straight from the factory. If you do nothing else, please rehydrate your yeast. They’ve been carefully dried to stop all cellular activity. They need water to come back to life. It can be argued that they get that water from the wort, but if it is a high gravity wort the rehydration may not occur. In any case there is a significant lag while the yeast revive themselves with the obvious implications for infection.

To rehydrate your yeast simply boil and cool some water (about five times the weight of the yeast, so for 10 g, use 50 ml of water). Add the yeast when the water is at room temperature and leave it for about an hour. You can use boiled and cooled wort (so long as it is a relatively low gravity) as an alternative. If you use wort the added advantage is that you should see some fermentation starting after an hour, which is a useful check that your yeast are in fact alive. You can do this check with water by then simply adding a teaspoon of sugar.

Starters

If you use two packets of dried yeast properly rehydrated then there ought to be no need for a starter. The way to check is to measure the lag time between pitching the yeast and the onset of fermentation. If it is in the order of a couple of hours and you get a big crop of yeast on top of the fermenting wort, then you’re doing just fine. Otherwise consider using more packets, or alternatively use a starter.

For liquid yeasts you almost always need a starter. They are sold in 50-ml packets or 35-ml tubes which provide roughly one quarter of the amount of yeast in a 10-g packet of dried yeast. Storage and delivery delays further reduce this amount. Some manufacturers now supply “ready-to-pitch” cultures of 175 ml, which if nothing else is an admission that the 50-ml packets are not enough. These cultures are not easy for UK home brewers to get hold of, so if you want to use liquid yeasts, you are committed to making starters. So to get to the equivalent of two packets of 10-g of dried yeast you need to step up the amount of yeast (50 ml) from the packet 16 times; that’s 800 ml of yeast. However if we allow for the fact that the yeast will reproduce in numbers during the growth of the starters, then 250 ml of yeast is about right. Please note that it’s 250 ml of yeast, not 250 ml of starter. A much greater volume of starter is required. The actual amount will depend on the starter’s OG and how quickly the yeast grow in your home brewery. For me, the total amount of starter comes to about one gallon. Don’t be alarmed, I’m not advocating throwing one gallon of pretty awful tasting starter into your prize-winning beer.

Here’s how to do it: Dissolve some malt extract in boiling water. Cover and cool it. You should be aiming for about 2 pints of starter with an OG in the range of 1030 to 1040 – 100 g of dried extract in a litre gives 1038. Put it into a sterilised container (a demijohn is perfect) shake it up well to aerate it and fit an airlock. Bring the liquid yeast up to room temperature (this is important) and then add to the starter. Mix well and put the starter in a dark place at room temperature – even 24°C is fine. Keep a careful eye on it. It will take a while to get going but after a day or two you should see some activity in the airlock. Once the fermentation has died down, let the yeast settle to the bottom of the demijohn. Pour off the liquid carefully so as not to disturb the yeast. A little liquid will be left. Swirl the container so that the yeast is resuspended into the liquid. Use this to pitch into another starter. The next starter will be much bigger in volume, say about 6 pints (6+2 = 8 pints = 1 gallon, that’s where my 1 gallon starter comes from). Repeat as before. Allow the yeast to settle, pour off the liquid, and what’s left is sufficient yeast to ferment one 5-gallon batch of wort. The second starter should have an OG as close as possible to the OG of the beer you’re brewing to avoid “osmotic shock”.

Many authorities recommend pitching yeast at “high kräusen” which is taken to mean “at their peak activity”, in other words when the fermentation is going at full blast. If you do this, the yeast are distributed throughout the starter and it effectively means that you have to pitch the whole volume of the starter. That doesn’t do your beer any favours at all. The yeast will remain on top form for about 2 days after high kräusen, so it is perfectly fine to use them up to two days after they have settled out. One day afterwards is probably best. After two days the yeast will start to drop off in activity and die, and it’s best not to use them at this point. So although I’ve used one gallon of starter, I’ll probably end up with about half a pint (roughly 250 ml) of yeast slurry and the fermentation will certainly get going quickly.

Fermentation Temperature

Having got all that right it’s tempting to sit back and think that all is well in the yeast world, but the final consideration is temperature in the fermentation. I’m sure that sooner or later all of us come across beer that’s been fermented in an airing cupboard (I’ll admit to having done it when I was a student) and I’d lay money on you having had a horrible hangover if you drank it. Temperature directly affects the types of alcohol produced and a whole host of other flavours too, so it pays dividends to keep an eye on what’s going on. Traditional ale yeasts like a temperature in the range of 13° to 24°C but even in this range there is a great deal of variation. In practice ale yeasts produce the best results if the temperature is kept below 18°C. Speciality beers such as Trappist ales or wheat beers may profit from temperatures in the range 20° to 24°C, but most British ales would start to produce odd flavours and headache inducing properties at the higher end of the temperature scale.

In contrast, lager yeasts tolerate a wider range of temperatures in the order of 0° to 24°C. It is usual to ferment lagers at a higher temperature, say 12° to 18°C to start off with and to progressively reduce the temperature towards lagering temperatures of about 4°C. As always there’s an exception: the Californian speciality, steam beer, uses lager yeasts but fermented at the higher end of the ale yeast spectrum: 20°C. In all of this discussion, the temperature referred to is the temperature inside the fermenting wort. If you have a lot of yeast and a good vigorous fermentation, that temperature can be a few degrees higher than the external temperature in your room. Homebrewers vary in the degree to which they monitor and control the fermentation temperature, but it is worth paying attention to. A simple method of cooling the fermentation is to wrap the vessel in towels soaked in cold water, but you can go all the way to thermocoupled cooling devices if you wish. Whatever your sticking point is in terms of diminishing returns for effort invested, please do bear in mind that fermentation temperature can have large effects on the resulting beer. There is always much more to say about yeast, but these practical tips ought to help you get the most out of your yeast.

Happy brewing.

Additional comments…

Advances in dried yeast production since the article was written mean that the note about rehydration of dried yeast is a little out of date and you should follow the manufacturer’s recommendations.


Posted: Sunday, March 1st, 2015

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