You can’t get very far in a brewery without yeast. One way to keep a yeast culture going is to harvest it. Harvesting yeast implies that the cell culture has already been used to make beer. The basic principle is that you are collecting it to use again, then again and probably 10 more times.
In between going from one beer to the next it is important to give the yeast a break. However, letting yeast sit at the bottom of a cone for too long for a break will begin to impart autolysis flavors in your beer. These flavors are never yummy and the conditions at the bottom of a cone are not recommended for the health of the yeast. No one likes to sit in their own waste.
Harvesting the Harbingers of Beer
The size of a brewery usually dictates how yeast is harvested and stored. The most common method is to harvest the yeast slurry into a brink. A brink can be a propagator, a keg or a fancy agitation tank. The brink allows the yeast to be separated from the low pH, high alcohol and high pressure environment within the cone of a fermenter.
All of these factors can cause deterioration of yeast viability and vitality. Pitching cone to cone is never recommended as creates irregular practices and does not promote quality control. Historically, yeast could have been harvested in a few different ways depending on if the brewer was using a bottom or top fermenting yeast. However, in today’s world, things have been standardized to using cylindro-conical fermentation systems (a snooty way of saying a cone shaped fermenter). The course of events is generally as follows:
- Wort and yeast go into the fermenter
- Yeast produces alcoholic goodness
- Tank is chilled to induce yeast to fall to the bottom of the tank
- Yeast is collected or sent to waste
This process is called yeast cropping and the main issue with it is that it is non-selective. When the yeast is collected from the bottom you have to take what you can get and it often contains trub. The point of this is that once the yeast is moved to a brink it is imperative treat the yeast right. This means keeping the culture cold and minimizing oxygen content.
Yeast cells store glycogen as energy for that boost of getting from lag phase to log phase growth. Oxygen contact induces the conversion of glycogen into energy. When oxygen is present this signals the cells to begin the arduous task of producing sterols and lipids. This in turn produces lots of ATP which then begins the biochemical process of phosphorylation to perform the hydrolysis of glycogen to glucose. So basically the yeast is blowing all of its energy for nothing as it sits in a brink!
The glycogen levels can drop approximately 20% from where it originally started leaving the yeast nothing to work with when it gets re-pitched into a new tank of beer. This results in sluggish and even stalled fermentations. To slow down this process the most effective way is through temperature. The sooner the yeast is cooled the less metabolically active it becomes. These glycogen stores are replenished during fermentation. By keeping the culture cool the brewer is able to prevent the yeast from using its energy stores so they are there when they are really needed.
Another carbohydrate store that yeasts produce is trehalose. Trehalose is a disaccharide that is often used as a cryoprotectant in freezing cells and tissues to help them survive the freezing. It serves a similar function in yeast. Although trehalose is in relatively low quantities in brewer’s yeast, it provides protection to cell membranes from stressful conditions. The basic concept is that trehalose will take the place of water in the membrane of a cell, reducing damage. Keeping this sugar in check is important to helping a harvest yeast source survive its containment. Again reducing exposure to oxygen, short storage times and low temperatures helps keep trehalose from degrading.
What Can Go Wrong
One thing that most people fail to see is that a yeast slurry can be a fabulous place for bacteria to hide. It only takes one bacteria to ruin everything, so keep it clean, enclosed, and monitor the health frequently. There are not too many bacterium out there that can spoil beer but there are several that will spoil wort. Most of these wort spoiling bacterium will only operate when sufficient oxygen is present, but they will often produce sulfur, diacetlyl and acidic off-flavors that will be carried all the way to the final beer.
The best thing to do is to check for microbial cleanliness through microbiological techniques, but other ways to monitor a yeast culture are through understanding and being familiar with the yeast. This means knowing its morphology, monitoring viability, checking for budding, pH and understanding its growth curve. If an infection is bad enough, bacteria will be visible under the microscope with a simple wet mount slide.
At the end of the day yeast should not be taken for granted. It is relied on heavily to produce our favorite beverage. Proper handling for harvesting and storing of yeast can be one of the most important functions in any brewery.
|Post written by Jessica Davis, our Quality Specialist who makes sure our beer & yeast are healthy. Jess has worked in many a lab, including the one at Stone Brewing Co.|