Brewing yeast is supplied in both dry and liquid form but only certain strains of yeast can be dried. Dry yeast has several advantages as it is more tolerant to warm storage temperatures and is packaged with nutrient reserves meaning it can be directly pitched without the need for starters. Liquid yeast on the other hand is a live culture which is more sensitive to storage temperatures and as it is a perishable product is usually much more expensive than dry yeast. The amount of viable yeast cells in a fresh vial of yeast is usually enough to pitch directly into a standard gravity beer. Over time and depending on the storage conditions, the viability of the yeast will decrease where eventually a starter will be required to replenish the yeast count. The advantage of liquid yeast is its vastly superior variety of strains for brewing a broad range of beer styles.
Making yeast slants has several advantages, the primary advantage being cost as you are getting multiple batches from a single vial of yeast as opposed to buying a fresh vial for every brew. Slants are also very portable which makes them easy to distribute to fellow brewers and also very easy to store so that you can easily retrieve them for brews later in the year. This guide is made to be a simple introduction to the slanting process, if you want more in depth information, have a look at these guides:
Inoculation loop (This can also be made with a guitar string twisted at the end)
Pot or flask for heating the mixture
Tinfoil (for covering items in the pressure cooker)
Cloth/Towel for lifting hot items and cleaning spills
Jewellers scales (If you don’t have these scales then 1tsp agar ≈ 2g)
Electricians tape (For sealing vial lids)
Small Funnel (for pouring solution into vials)
Nothing needs to be sterile/sanitised at this stage
The first step is to make a wort starter which is pretty much a mini batch of beer except without the hops. The target specific gravity is in the range of 1.030 and 1.040 which is a comfortable concentration level for yeast colonisation. The easiest way to do this is by using a 10 to 1 ratio of water and dry malt extract. For every 1g of dry malt extract (DME) there would be 10g of water, so if you are aiming to fill 10 vials, with 25ml each then you will need 250ml of water and 25g of DME. Slants are generally made with a ~2% concentration of agar in the solution which corresponds to approximately 2g of agar for every 100ml of wort. Continuing with our sample size of 10 vials, this would require 5g of agar. Heat the wort, add agar gradually to the solution and boil until the agar is fully dissolved. In a similar fashion to boiling beer, be careful of boil-overs for the initial stage of boiling the mixture.This may require you to lift the mixture off the heat for a few seconds for the foam to subside. Once a rolling boil has been reached, the chances of boil-overs drop dramatically as long as the heat and mixture remain unchanged.
The ratio of wort to agar is worth getting right as if its too thin it will not set correctly. At the other end of the scale, if the mixture is too thick the jelly will be too hard to properly inoculate. Gelatin may also be used as an alternative to agar but it is less stable at room temperature compared to agar which remains solid until close to boiling temperatures. Gelatin, being available as a cooking ingredient in most stores, is much easier to obtain than agar which is normally only available in Asian food markets. As recommended by Richie, who also has a guide on slanting vials, agar comes in two forms. It can be purchased in a ‘stringy’ form (with the constancy of paper for packaging) or powder form. He recommends powder form as it dissolves much easier and is much more straightforward to work with.
While the wort and agar solution is boiling on the stove, prepare the vials so that they are ready to receive the solution. As stated in the equipment section these vials should be capable of withstanding autoclave temperatures which is in the region of 120 degrees C. These vials are scientifically known as “centrifuge tubes” and are available on ebay, homebrew stores, scientific supply websites and group buys organised by the NHC.
When the agar is completely dissolved, take the wort/agar solution off the heat and pour 20ml (this is for a 50ml vial) into each vial using the funnel to make the process easier. When all the vials are half full, screw the lids loosely onto each vial and put them into the pressure cooker for 15 minutes. Use the storage trays or tinfoil inside the pressure cooker to hold the vials upright. If you have any left over agar solution at this stage, pour it into an old container, allow it to set and throw it out into the rubbish. If you pour it down the drain it will set and either block the U-bend in your sink or your pipes.
After 15 minutes take the pressure cooker off the heat and allow it to cool. It will cool quicker if you put the pressure cooker into a sink of cold water. If your pressure cooker has a manual pressure release valve in addition to the main valve then it is possible to release all the pressure inside without having to wait. Make sure to read the instructions on your pressure cooker to verify that this is or is not the case.
Take the vials out and leave them at an angle on their side to cool either on a plate or on top of a piece of wood. The angle should be such that the solution almost reaches the lid of the vial, this maximizes the inoculation area. Do not screw the caps on tightly until they have cooled down fully, doing so immediately results in condensation build up within the vial. This buildup can reduce the life of yeast colonies, drip contaminants from the condensed wall and make it difficult to identify the status/health of the slant.
Everything must be sterile/sanitized after this stage
Once the vials have cooled they are ready to be inoculated with yeast, this doesn’t necessarily have to be done on the same day as the sterilization process. Your slants can be kept as ‘blanks’ which can be used as you acquire new strains of yeast. At this point ensure the work area is sanitized and free of drafts. Turn the ‘flame’, this serves several purposes, killing bacteria near it, a sterilization source for the inoculation loop and it also causes a hot updraft which prevents dust/nasties falling down and settling in your sterile vial. At this stage loosen the lids on both the source yeast vial and the blank slant vial.
Insert the inoculation loop in the flame until it glows and allow to cool for a few seconds, this can be verified by dipping the loop into the agar in the slant. Open a slant and open the source yeast vial. Once cool, dip the loop in the source vial and drag the loop though the blank slant in a zigzag fashion. Close the slant and repeat for subsequent vials. Store the vials at room temperature for 3 or 4 days until the yeast becomes established on the agar. During this time you will need to loosen the lid occasionally to “burp” out the small amount of CO2 produced by the growing yeast. Once it has become established you can put some electric tape on the lid of the vial to keep it secure. The tape isn’t completely necessary but it ensures that there is no chance of the lid accidentally coming off the inoculated vial.
Store the inoculated slant’s in the coldest part of the fridge, ideally storing them in a polystyrene box which will insulate them from the temperature fluctuations which occur when the fridge door is opened. Yeast slants will generally remain viable for up to a year, when the yeast starts to turn brown it’s time to re-slant. The table below gives a general life expectancy for yeast for different storage techniques.
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