Harvesting and reusing yeast is a fairly simple and cost effective technique homebrewers can use to brew like a pro. Professional brewers routinely harvest and reuse (repitch) yeast and take advantage of the fresh, healthy, bountiful supply of yeast from a previous batch. Each time we brew beer we strive to use healthy yeast at an adequate pitching rate. By harvesting yeast from a freshly fermented batch of beer we have the perfect combination of both quality and quantity of yeast ready to inhabit a new fermenter of wort.
Collecting, storing, and repitching yeast from batch to batch is not complicated; it just requires a little planning and special attention to sanitation. Each and every vessel, spoon, funnel, tube, or other implement that touches the yeast you plan to repitch must be absolutely clean and sanitized before use. Clean means no crud or other unwanted residue attached to your equipment. Sanitizing means killing any unwanted microorganisms in or on your equipment.
There are several cleaners available for use on brewing equipment, such as powdered brewery wash, and several sanitizers, such as StarSan, Iodophor, household bleach and alcohol. A solution of one fluid ounce of bleach per gallon of water is great for glass or plastic. Using a non-bleach sanitizer is your best bet for stainless steel. (With long contact times, or in a lower pH solution, some pitting of the metal can result.)
At least a ten-minute soak in bleach solution is necessary to do the job, and should be followed by rinsing to remove residual chlorine. If your tap water is sanitary for use in rinsing all should be well. If the sanitation of your water is questionable, either boil the water for a half hour before use or employ a sanitizer, other than bleach, that does not necessitate rinsing. Alcohol (like cheap vodka) sanitizes on contact and dries quickly so it is handy for sanitizing small items or vessels. As with any cleaning or sanitizing product, proceed with caution and follow the directions for use and storage.
How you collect yeast from your fermenter will depend on the type of yeast to be collected and your fermenting equipment. If you have a fermenter that is easily accessed from the top (such as a plastic bucket) and are using ale yeast, simply skim some yeast off of the surface of the beer during active primary fermentation. An alternative method would be to wait for the primary fermentation to subside and rinse some of the trub layer away with sterile water before collecting a sample of yeast. Then deposit the harvested yeast into a sanitized container, attach a loose fitting lid or airlock and place it in the refrigerator.
If you are fermenting your ale in a carboy or other vessel that is not readily accessible from the top, you will need to wait until primary fermentation is complete to harvest your yeast. In this case, rack the ale into a secondary fermenter soon after primary fermentation begins to subside and harvest the sedimented yeast from your primary fermenter. Although yeast harvested after a secondary fermentation will have less trub associated with it, the yeast from a primary fermentation will be healthier. To transfer the yeast to bottles or a keg, carefully swirl the yeast in the bottom of the fermenter and decant between a cup and a pint (8 fluid ounces or 237 mL to 16 fluid ounces or 473 mL) into a sanitized container. Attach a loose fitting lid or airlock to the container and place it in the refrigerator. If you wish to collect yeast from a lager fermentation, follow this same procedure of collecting yeast from the bottom of the secondary fermenter.
If you are fortunate enough to have a cylindrical-conical fermenter equipped with an outlet on the bottom and a removable lid on the top, you can either harvest ale yeast from the top during active fermentation or draw off ale or lager yeast from the bottom after primary fermentation subsides. When accessing yeast from the bottom of a fermenter do your best to collect the middle yeast. The sediment at the very bottom often contains dead yeast and trub, and the middle layer contains the best yeast for repitching. The bottom layer of dead yeast and trub can be distinguished by its darker color, while the preferred layer of yeast will appear more yellow and putty-like. Harvest the yeast as soon as fermentation is complete and the yeast has settled.
Now that you have a sample of yeast collected, it must be stored properly to assure continued viability. Store the yeast in the refrigerator as close to 32 °F (0 °C) as practical to keep it dormant. This will reduce both the risk of spoilage and of autolysis. Autolysis is when a (yeast) cell goes into self-destruct mode and essentially digests itself. This process often produces a rubbery stench that, needless to say, is undesirable in beer. As mentioned earlier, the yeast should be stored in a container with a loose fitting lid or an airlock. After you collect the yeast and put it in the refrigerator it will not immediately go dormant as most refrigerators are not very cold and the temperature fluctuates considerably over time. Because of this, yeast stored in a refrigerator may produce some carbon dioxide that could cause problems. To store the yeast at a lower, more stable temperature, place the yeast container in a small cooler (or a box with baggies of ice in it). It is important to vent the container daily, for the first three days, because excessive CO2 will damage yeast quickly. Likewise, some containers may rupture under excess CO2 pressure. Yeast should be used in a batch of beer within two weeks of being collected from a previous batch. Yeast that is not actively fermenting can lose its viability rather quickly when stored using the simple methods and conditions outlined here. This is where scheduling to brew successive batches of beer is important if you wish to collect and repitch yeast.
When you are ready to retrieve your stored yeast from the fridge and pitch it into a new batch of wort it is important to give the yeast a wake up call before putting them to work. Remove the container with the stored yeast from the refrigerator and allow it to slowly warm up to the temperature of the wort it will be pitched into. You may want to give the yeast a shot of oxygen to help them become active. This can be accomplished by simply stirring the yeast up with a sanitized spoon or whisk to develop a bit of froth. If you have a magnetic stir plate you can sanitize the stir bar, slide it into the yeast vessel and place it on the stir plate and fire it up. If you have an aquarium pump with an inline sanitary air filter, you could also pump air into your yeast in addition to giving it a stir to provide aeration. If your wort is adequately aerated, however, you don’t need to worry too much about aerating the yeast sample. Once the yeast has been stirred and had a chance to warm, it can be pitched directly into the waiting wort.
A few final considerations for harvesting and reusing yeast relate to beer style. It is best to brew successive batches of beer that are within a fairly narrow style range of original gravity and bitterness if you plan to reuse yeast. Therefore plan your batches accordingly so the yeast will be appropriate for each style without any undesirable flavor carryover. If you would like to use harvested yeast for a particularly dark or bitter style of beer, make that beer the last batch in the series so you can dead-end the yeast and not have to be concerned with any carryover of flavors.
With a little planning and preparation you can stretch your homebrewing dollar and achieve a quick start to fermentation by harvesting and repitching yeast from batch to batch. Pay special attention to both sanitation and beer style, and you too can brew like a pro!