Saturday, 21 April 2012

Rinsing Yeast

The following article is a copy from AussieHomeBrewer forum and with permission we've been allowed to reproduce it here as I think it is a great resource to have when considering your yeast management.

Rinsing yeast is a technique used to 'wash' the yeast/trub slurry found on the bottom of the fermentor after the beer has been removed. The 'cleaned' yeast can be pitched directly into a new batch of beer, or saved for later re-use. I've used the term 'rinsing' (as used in the recently published 'Yeast' book) to differentiate between 'washing' the yeast in water to remove trub and debris from 'acid-washing', a process used to to eliminate biological contaminants.

While the process of yeast rinsing is dependent on the yeast-strain, wort composition and what (if any) fininings were used, the basic process relies on the fact that the yeasty-trub will form three distinct layers when left to settle:

Here you can see the fermentor-dregs of a dark UK-style ale. The top clear layer of left-over beer, the creamy-yeasty middle layer, and then the trub (protiens, break material and other debris) on the bottom. The process of rinsing the yeast aims to retain only the middle layer of yeast and discard the other two layers, especially the trub and debris.

I wash my yeast in (~400ml) glass pickle jars, they can be easily heat-sterilized (in a pressure cooker or boiled in a large pot) and boiling water can be tipped directly into them to enhance the sanitation procedures. Ideally sterile distilled water should be used for yeast washing, however I use plain boiled and cooled tap-water (here in Melbourne the water is very soft and contains few minerals, but if you live in an area with hard water it may be a good idea to use treated or even bottled water). When working with yeast try to keep the equipment and procedures as clean and sterile as possible, and if possible keep the temperature constant, don't shock the yeast by tipping in water too cold or hot. I try to pre-boil the water and seal the jars a day or two in advance so that the yeast and the water will be the same temperature.

Here is the trub in the bottom of the fermetor after a recently brewed Australian Ale (using Coopers re-cultured yeast):

Add a jar full of cooled-boiled water (approx 400ml) and shake/swirl and mix it around and it should look like this:

Let the yeasty-liquid-trub settle for 10 to 15mins, and then the first step of the rinsing process is to decant the majority of the liquid into the rinsing-jars, but leaving behind any large bits of gunk, break material, hop or other debris:

Depending how much beer was left behind, how much yeast and how much trub you have, you should easily be able to fill 1 or 2 jars. Generally yeast rinsed from one jar is enough to pitch into a new batch of beer, but I also wanted to save some for later re-use, so I ended up with 2 jars full of yeasty-trub and the left overs in the fermentor shown above:

Now we need to be patient and watch what happens, after about 10 to 30mins you should start to see three distinct layers in the jar, clear water/beer ontop, a creamy layer of suspended yeast in the middle and with the trub and break material falling to the bottom of the jar.
The time it takes, and exactly what it looks like depends on the yeast, the wort, the grain, finnings and many other factors, so if you are not familiar with the process or the yeast it can be a good idea to let the jars sit for a number of hours (even a day) and watch as the trub and yeast settle out over time. However, the longer you let it sit the clearer the layers will become, but once the yeast starts to settle out of suspension it is harder to separate (so if you let it settle out to observe, simply shake it up so you can start the process again).

We want to keep the middle layer since that contains our yeast in suspension. If you do not leave it long enough you will still have trub mixed in, but if you leave it too long the more floculant yeast will have settled out. The process is a bit of trial-and-error, but by careful observation it's not too hard.

I've found that by the time the layers start to form about 1/2 to 2/3 of the jar's contents is the milky-homogenous-yeasty-suspension that we want to keep, the trub and debris on the bottom is usually a darker, grainy layer, with the jars having a thin-clear layer on the top.
By carefully decanting the liquids we should be able to discard the clear top-layer, keep the middle layer which is our yeast in suspension, and discard the bottom layer of trub.
The middle jar is the original one, with the trub remaining on the bottom, the jar on the right contains our top clear layer and the jar on the left is our yeast-in-suspension that we will keep:

The suspended-yeast-layer from the second jar was decanted into a flask ready for re-pitching, the top clear layer discarded and the trub (with some suspended yeast), that will also be discarded, remaining in the original jar:

If we have done it right and/or were lucky, by letting the saved yeast-in-suspension layer settle out, we should have only single layer of nice clean creamy yeast:

But if you find you still have a distinct layer of trub settling more quickly, simply repeat the process again.

The yeast in the flask was left to settle (refrigeration would have sped-up the process, however since I was going to directly re-pitch the yeast I did not want to risk thermal-shocking it, so instead just waited a day for it to settle out at fermentation temps).
Once the yeast has been rinsed and settled to form a thick compact layer of yeast-slurry (if it is viable and healthy) 50 to 100ml should be adequate for pitching into the next batch of beer.

The yeast in the jar, which will be stored for later use was washed several times more, until the liquid remained clear, each time decanting the liquid from the top and giving the yeast time to settle out:

When the liquid above the yeast is clear, it was split into 3 storage containers (ie: sterilized beer bottles):

Once they have been capped and labeled the washed yeast should remain viable if stored in the fridge for 6 to 9 months.

The thick layer of yeast on the bottom of the bottles should be adequate to pitch directly into a starter (I step it from 300ml to 1.5L before pitching), however yeast stored this way does not usually remain viable more than about 1 year.   

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