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Yeast are our friends

Published: 18/06/2016 08:47 am

Yeast are our friends, so we really need to be looking after them. After all, theyíll make you a beer to drink when you get home after a long day at work.

During anaerobic metabolism in brewing applications, yeast need trace amounts of oxygen so they can produce lipids in the cell wall. If yeast are deprived of sufficient oxygen, they canít  metabolise squalene (Squalene is a naturally occurring oily organic compound found in just about all life forms on the planet). Yeast require the metabolism of the squalene into lipids that are stored in the cell wall to make it elastic and to protect it from environmental factors within the beer, as well as needing elastic cell walls to produce lots of healthy buds which are the next generation of yeast cells.

Without these lipids, the cell walls will ultimately become inelastic, leading to poor replication rates and also leading to susceptibility of the toxic effects of alcohols.

With each generation of yeast cells, the original colony will use its reserves of lipids and ultimately, by the time a fresh batch of dry yeast has replicated 3-4 times, these stores are pretty much exhausted.

When the yeast reach a stage where their lipid stores are exhausted, it will stop regenerating and the colony will start to decline.

Your yeast will also need a wide range of vitamins, minerals and nitrogen. While the wort we produce along with some of the break material from the brewing process also contains most of what yeast need, a touch of other vitamin and mineral compounds such as we see in the Whitelabs or Wyeast Yeast nutrients may also be of benefit in high gravity brewing. However, overloading the wort, either starter wort or the beer you are making with nutrients can have adverse effects on the overall flavour of the beer and the yeast colony, so follow the instructions.

So, when it comes to brewing low to mid gravity beers, how much oxygen do we really need? Basically, if you are pitching sufficient healthy yeast, you should need no more than what you can adequately get into the wort by vigorous whipping with a paddle or spoon or shaking the carboy. Since Iím essentially a lazy person, I canít be bothered shaking something that weighs 24 odd kg, so I stir up a frenzy with a sanitised stainless paddle. Also note: It is pointless doing this if the wort is too warm as the oxygen you put into the wort will simply not stay dissolved. Make sure you are cooling the wort down to below 20-22 degrees C. This will assist to keep a little more oxygen dissolved until the yeast need it post rehydration.

For wort up to about 1.060 ish in gravity, with sufficient healthy fresh yeast pitched, you should never run into problems with stuck fermentation.

Once we start to get up to the higher gravity ranges for beers like AIPAís IIPAís and Russian Imperial Stouts etc, there is a lot to be said for oxygen injection a couple / few times over the first 24 hrs. This doesnít negate the need to pitch a LOT of healthy yeast cells at the beginning though. Thereís nothing more frustrating than a stuck fermentation in a really big beer. The wait for them to condition to drinking stage is enough of a tease & we donít need to prolong it any further.

For transfer to the fermentation vessel, allowing the cooled wort to run widely over a large surface area exposes the wort to a lot more oxygen, improving oxygen uptake. Simply streaming it into the bottom as an unbroken stream will not get the dissolved oxygen levels where we want them.

On pitch rates: Use an online pitch rate calculator and if you are going to brew high gravity imperial beers, youíll be wanting to invest in one of the Oxygen Injection systems you will see discussed on various forums.  It will help protect the yeast from the ill effects of alcohol toxicity.