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Reused yeast: repitching and successive generations

Few people drinking beer realise that the yeast used to brew it probably did not work for the first time. Breweries rarely use yeast only once - instead they harvest it after the finished fermentation and use it again, in the next batch, and then once more. This practice is called repitching, and it is the foundation of both the economy and the quality of beer. Reused yeast is not a saving at the cost of quality - it is the key to the repeatability and consistency of successive batches. But reuse has its limits: after a certain number of generations the yeast has to be replaced. In this post you will learn how repitching works, how many generations you can get, why yeast mutates and why it is such an important practice. It is a journey into the everyday life of a brewery. Let us start with what repitching is.

What repitching is

Repitching is a practice involving the harvesting of yeast biomass after the finished fermentation and using it in subsequent fermentations. Instead of buying or growing fresh yeast each time, the brewery recovers the yeast that has just finished working in one batch and pitches it into the next. Yeast multiplies during fermentation, so after it finishes there is far more of it than at the start - enough to serve subsequent batches. This serial practice, called serial repitching, is a standard in almost every brewery in the world, from home to industrial. Repitching is not only a way to save, but also to maintain the consistency of beer. Reusing yeast is one of the best ways to improve the repeatability of beers, and at the same time save on the cost of raw material. Understanding what repitching is is the key to this post. So let us get to know how exactly yeast is recovered after fermentation.

How yeast is harvested

Harvesting yeast is the first step of repitching. Yeast is harvested after the finished fermentation, when it settles - bottom-fermenting (lager) yeast gathers at the bottom of the vessel, and top-fermenting (ale) yeast gathers on the surface, from where it is collected (top cropping). Key is the moment and way of harvesting. It is best to harvest yeast when the temperature has dropped and the trub has been removed, which ensures a large amount of clean, homogeneous slurry. The harvested yeast slurry is stored cold until the next batch. An important rule: always select the youngest available generation of yeast for harvest, because fewer generations means less risk of mutation and contamination. Time also matters - after about four weeks the viability of stored slurry usually drops to 50 percent or lower. Harvesting yeast is an art requiring hygiene and a sense of timing. Well-harvested yeast is the basis of successful repitching. It is the stage at which the quality of subsequent batches is decided.

How many generations you can get

The key question is: how many times can the same yeast be reused? The answer depends on practices and conditions. The general rule says that after about 3-5 generations it is usually best to start with a fresh portion of yeast. However, with good harvesting practices you can get from seven to ten generations from one initial purchase. And if you brew mainly light, lightly hopped beers of low alcohol content, you can get even more than ten generations. The limit thus depends on the style of beer and the care of the brewery: strong, heavily hopped beers stress the yeast more, shortening its viability. The milder the conditions of fermentation, the more generations. The number of generations is a compromise between saving and the risk of a drop in quality. Breweries track the generations of yeast to know when to replace it. This planning lets the yeast be used to the maximum, without risking the quality of the beer. The generation limit is the practical knowledge of every good brewer. Let us now get to know what happens to yeast with successive uses.

Mutations and drop in viability

Repitching has its limits, because yeast is not eternal - with each generation it changes and weakens. Yeast can mutate if it is reused too many times, especially in stressful conditions of fermentation, like high alcohol content or high bitterness. Mutations can change the behaviour of the yeast and the flavour profile of the beer, moving it away from the desired effect. The second problem is the drop in viability: along with the generation number the viability of yeast decreases for all strains. The target is a minimum viability of 85 percent for harvested yeast - below this threshold fermentation can be problematic. Therefore it is recommended to always select the youngest available generation, because fewer generations means fewer opportunities for mutation or contamination. Stress, time and the number of uses are the three enemies of reused yeast. Understanding these limits is key: repitching is not an infinite process, but a cycle with a natural end. That is why breweries monitor viability and replace yeast at the right moment. The limit is real and must be respected. We write more about the role of yeast in our post on brewing yeast.

Why breweries do it

Since repitching has its risks, why is it so common? There are several reasons, and each is important. First, economy: fresh yeast, especially high-quality, is costly, and its recovery significantly reduces production costs. Second, and equally important, consistency: reusing yeast is one of the best ways to improve the repeatability of beer - the same yeast gives the same, predictable profile batch after batch. Third, yeast in successive generations is already adapted to the conditions of a given brewery, which can improve fermentation. Fourth, it is a sustainable practice, reducing waste and the use of raw materials. For the brewery repitching is a combination of saving, quality and predictability. That is why practically every brewery uses it, treating yeast recovery as routine. Contrary to appearances, reused yeast is not worse - often it is even more reliable than fresh. Repitching is a wise, proven solution. It is proof that saving and quality can go hand in hand.

Quality control of yeast

For repitching to succeed, breweries have to control the quality of the recovered yeast. Several practices are key. First, monitoring viability: checking what percentage of yeast cells is alive, to ensure efficient fermentation (the target is a minimum of 85 percent). Second, tracking generations: keeping a record of how many times a given portion of yeast has already been used, to replace it at the right moment. Third, hygiene: contamination with bacteria or wild yeast can spoil the whole batch, so harvesting and storage must be sterile. Fourth, selecting the youngest generation for harvest, to minimise the risk of mutation. Fifth, proper storage in the cold and use at the right time, before viability drops. Quality control of yeast is both a science and a craft, requiring discipline and knowledge. Good breweries treat their yeast almost like a living treasure, caring for it carefully. It is this care that lets you reap the benefits of repitching without losing quality. Quality control is the foundation of successful yeast recovery. Without it, repitching would be risky.

Repitching in a table

Let us set the key facts about yeast recovery side by side:

The table shows that repitching is a practice of clear rules and limits. Yeast is recovered after fermentation and reused, usually for 3-5 generations, and with good practices even 7-10 or more. The limit is set by mutations and a drop in viability below 85 percent. The benefit is twofold: saving costs and improving the repeatability of beer. It is a practice that combines economy with quality, used in almost every brewery. Repitching is the everyday reality that shapes the flavour and consistency of beer.

Why it is worth knowing this

Understanding repitching enriches the knowledge of beer and its production. First, it shows how huge a role yeast plays - they are living organisms that the brewery has to manage like a herd. Second, it explains where the consistency of a favourite beer comes from: the same, recovered yeast gives the same profile batch after batch. Third, it makes you realise that behind the scenes of a brewery there is constant work on the quality and viability of yeast. Fourth, it is an interesting example of how economy and quality can go hand in hand, and sustainable practices reduce waste. A conscious beer lover knows that the yeast in their favourite beer is perhaps already the tenth generation, carefully cultivated by the brewery. Next time, drinking a consistent, good beer, it is worth appreciating the invisible work on yeast that stands behind it. It is knowledge that deepens respect for the craft of brewing. Repitching is the quiet hero of the repeatability and quality of beer.

The key points in a nutshell

Breweries rarely use yeast only once - the practice called repitching involves harvesting yeast after fermentation and reusing it, generation after generation. Yeast is harvested from the bottom of the vessel (lager) or from the surface (ale), best the youngest generation, with care for hygiene and viability. Usually 3-5 generations are obtained, and with good practices 7-10 or more, depending on the style of beer. The limit is set by mutations and a drop in viability below the target of 85 percent, especially with stressful, strong batches. Breweries do it for saving and, crucially, for the repeatability of beer. It requires strict quality control of yeast. Want to explore the secrets of beer and record your impressions? Keep tasting notes in the GustoNote app. See also our posts on brewing yeast and on Norwegian kveik yeast.