I remember as a new brewer it took me quite a while before I understood exactly what was going on inside the fermentor while I anxiously waited for my latest batch of beer to be ready for bottling. Eventually I did learn that a lot of the flavors that develop in a batch of beer are contributed by the yeast, those flavors taste great when the yeast is healthy, not so much when the yeast is stressed out. Today I still see posts on beer forums, from both new and conditioned brewers alike, questioning why they ended up with off flavors in their beer and how.
Looking back now, if I've learned anything at all about yeast and beer fermentation, it's that the fermentation process shouldn't be looked at as a single process at all. When you look closer it's actually the combination of four separate processes, or unique phases, that take place from the second you pitch your yeast until you've racked the last ounce of beer from the fermentor. Each of the four phases play a key role in producing a great tasting beer and knowing how to provide the optimal conditions needed for each phase of the fermentation will make off flavors a thing of the past.
Phase 1 - The Lag Phase
The lag phase begins the moment you pitch your yeast and it's one thing we as brewers need to keep as short in duration as possible. During this phase the yeast cells are using the sugars and oxygen in the wort solely as food to boost up their energy reserves, they won't be converting anything to alcohol until they've been very well fed. As long as you don't stress the yeast out during this phase, with temperatures that are too high or with too few healthy cells to do the job, the lag phase will be short. The length of the lag phase is an indicator on how the remaining fermentation phases will go. Generally speaking increasing the time it takes the fermentation to complete also increases the amounts and types of off flavors, like diacetyl, that are likely to develop that may or may not ever condition out of a beer.
Phase 2 - The Growth Phase
The growth phase starts as soon as the lag phase ends now that the yeast have stored up enough energy they're ready to start multiplying and creating more cells. During the growth phase you'll see foam beginning to form on the surface the wort caused by the production of carbon dioxide. As the yeast cells begin consuming the oxygen in the wort and producing Co2 the pH and oxygen levels of the wort will both begin to get lower. At this point if you've pitched enough healthy yeast cells, at the right temperature, into well aerated or oxygenated wort the lag time since first pitching the yeast should be 6-12 hours. If everything has gone right up to the end of this phase there will be enough viable yeast cells full of energy and ready to kick off a very healthy fermentation.
Phase 3 - The Fermentation Phase
The fermentation phase begins as soon as the growth phase has completed and it's triggered by the lack of oxygen in the wort. It's during this phase that the production of Co2, alcohol and beer flavor is at its peak, the cells are active and using so much energy that the temperature of the wort can rise 3F to 5F higher than the ambient air temperature outside the fermentor. The yeast cells stay in suspension so that they can come into contact with as many fermentable sugars in the wort as possible. At this rate the wort will run out of sugars for the cells to eat within 3-7 days and at that point the yeast cells will begin to flocculate out to the bottom of the fermentor. Higher temperatures during the fermentation phase will produce more esters, or fruity flavors and aromas, like the banana flavors in a Hefeweizen for example.
Phase 4 - The Sedimentation Phase
The sedimentation phase is the final phase of the fermentation process where the yeast cells have eaten all the fermentable sugars in the wort, now the yeast cells start consuming let over flavor precursors like diacetyl that will produce off flavors in your beer. It's during this phase that the yeast cells prepare to go dormant by storing up energy reserves for their deep sleep and then dropping out of suspension and settling at the bottom of the fermentor. All but the most advanced home brewers typically flush the dormant yeast down the drain as they clean out their fermentor but if handled properly the left over cells can easily be washed, stored and reused many times over.
The amount of diacetyl and other off flavor causing precursors left for the yeast to consume is completely dependent on how well the cells were treated, or mistreated, during the first three phases of fermentation. Raising the temperature of the wort in the fermentor 3F - 5F during this phase and holding it there for at least three days before packaging your beer also reduces off flavors. The yeast will only convert the flavor precursors it created earlier if the fermenting beer is warmer than it was when the yeast cells originally created them. There are limits though since the yeast can only cleanup so much before they go dormant and once they are dormant any flavor precursors will be left in your beer to produce off flavors.
Vince "ScrewyBrewer" Feminella
www.thescrewybrewer.com
***
Vince is back teaching us all about the greatness of yeast. A beloved contributor, and friend of the community, please be sure to subscribe to Vince's blog to get the latest and greatest from this Screwy Brewer!
Looking back now, if I've learned anything at all about yeast and beer fermentation, it's that the fermentation process shouldn't be looked at as a single process at all. When you look closer it's actually the combination of four separate processes, or unique phases, that take place from the second you pitch your yeast until you've racked the last ounce of beer from the fermentor. Each of the four phases play a key role in producing a great tasting beer and knowing how to provide the optimal conditions needed for each phase of the fermentation will make off flavors a thing of the past.
Phase 1 - The Lag Phase
The lag phase begins the moment you pitch your yeast and it's one thing we as brewers need to keep as short in duration as possible. During this phase the yeast cells are using the sugars and oxygen in the wort solely as food to boost up their energy reserves, they won't be converting anything to alcohol until they've been very well fed. As long as you don't stress the yeast out during this phase, with temperatures that are too high or with too few healthy cells to do the job, the lag phase will be short. The length of the lag phase is an indicator on how the remaining fermentation phases will go. Generally speaking increasing the time it takes the fermentation to complete also increases the amounts and types of off flavors, like diacetyl, that are likely to develop that may or may not ever condition out of a beer.
Phase 2 - The Growth Phase
The growth phase starts as soon as the lag phase ends now that the yeast have stored up enough energy they're ready to start multiplying and creating more cells. During the growth phase you'll see foam beginning to form on the surface the wort caused by the production of carbon dioxide. As the yeast cells begin consuming the oxygen in the wort and producing Co2 the pH and oxygen levels of the wort will both begin to get lower. At this point if you've pitched enough healthy yeast cells, at the right temperature, into well aerated or oxygenated wort the lag time since first pitching the yeast should be 6-12 hours. If everything has gone right up to the end of this phase there will be enough viable yeast cells full of energy and ready to kick off a very healthy fermentation.
Phase 3 - The Fermentation Phase
The fermentation phase begins as soon as the growth phase has completed and it's triggered by the lack of oxygen in the wort. It's during this phase that the production of Co2, alcohol and beer flavor is at its peak, the cells are active and using so much energy that the temperature of the wort can rise 3F to 5F higher than the ambient air temperature outside the fermentor. The yeast cells stay in suspension so that they can come into contact with as many fermentable sugars in the wort as possible. At this rate the wort will run out of sugars for the cells to eat within 3-7 days and at that point the yeast cells will begin to flocculate out to the bottom of the fermentor. Higher temperatures during the fermentation phase will produce more esters, or fruity flavors and aromas, like the banana flavors in a Hefeweizen for example.
Phase 4 - The Sedimentation Phase
The sedimentation phase is the final phase of the fermentation process where the yeast cells have eaten all the fermentable sugars in the wort, now the yeast cells start consuming let over flavor precursors like diacetyl that will produce off flavors in your beer. It's during this phase that the yeast cells prepare to go dormant by storing up energy reserves for their deep sleep and then dropping out of suspension and settling at the bottom of the fermentor. All but the most advanced home brewers typically flush the dormant yeast down the drain as they clean out their fermentor but if handled properly the left over cells can easily be washed, stored and reused many times over.
The amount of diacetyl and other off flavor causing precursors left for the yeast to consume is completely dependent on how well the cells were treated, or mistreated, during the first three phases of fermentation. Raising the temperature of the wort in the fermentor 3F - 5F during this phase and holding it there for at least three days before packaging your beer also reduces off flavors. The yeast will only convert the flavor precursors it created earlier if the fermenting beer is warmer than it was when the yeast cells originally created them. There are limits though since the yeast can only cleanup so much before they go dormant and once they are dormant any flavor precursors will be left in your beer to produce off flavors.
Vince "ScrewyBrewer" Feminella
www.thescrewybrewer.com
***
Vince is back teaching us all about the greatness of yeast. A beloved contributor, and friend of the community, please be sure to subscribe to Vince's blog to get the latest and greatest from this Screwy Brewer!
