|Saccharomyces Cerevisiae Top Fermenting Ale Yeast|
|Saccharomyces Uvarum Bottom Fermenting Lager Yeast|
Both ale and lager yeast cells reproduce by a process known as budding, where each yeast cell grows as many as four buds that eventually separate into daughter cells of the original increasing the the overall population. Yeast cells are able to live in both aerobic as well as anaerobic conditions, when living in well oxygenated wort the cells begin taking in sugars and building up their food reserves, multiplying and preparing their cells walls for the task of transferring large amounts of sugars and alcohol during primary fermentation.
|Oxygen Tank And 2 Micron Stainless Steel Diffuser|
|2 Liter Starter Made With 1.040 Gravity Wort|
|My 420 Special Wheat Beer Aerated Using Pure Oxygen|
The Four Phases Of Fermentation
Phase 1 begins as soon as you pitch your yeast and is referred to as the lag phase, which we brewers want to keep as short as possible. The yeast are using up the sugars and oxygen in the wort to load up their food reserves, they won't ferment anything until they've been well fed. Stressing the yeast out with too high temperatures or too low numbers of viable cells will prolong the lag phase and the fermentation will take longer to complete while increasing the amounts and types of off flavors like diacetyl that may or may not ever condition out.
Phase 2 starts as soon as the lag phase ends because now the yeast have enough energy stored up to start multiplying, this is referred to as the growth phase. This is where you begin to see a bit of foam floating at the surface the wort from the production of Co2 and the pH and oxygen levels of the wort will start dropping. If you've pitched enough healthy yeast at the right temperatures into well aerated wort the lag time should have been 6-12 hours and the yeast are now full of energy and off to a very healthy start.
Phase 3 begins as soon as the growth phase is done and is triggered by a lack of oxygen in the wort, this is known as the fermentation phase. This is the phase where the production of Co2, alcohol and your beer's flavor is at it's peak and the wort temperature rises 3-5F higher than the ambient air outside the fermentor. The yeast will stay in suspension, so they come in contact with as many sugars as possible, over the next 3-7 days before they run out of sugars to eat and flocculate out to the bottom of the fermentor. Higher temperatures during this phase will produce more esters or fruity flavors and aromas, like the banana flavors in a hefeweizen. It's interesting to note that another cause of ester production is wort that hasn't been aerated enough.
Phase 4 is the final phase of the fermentation process and it's referred to as the sedimentation phase where the yeast begin consuming and converting any remaining flavor precursors in the wort like diacetyl that will produce off flavors in your beer. During this phase the yeast cells are preparing themselves to go dormant and storing up energy reserves for their deep sleep, even though this is where most of us flush them down the drain. I'd like to point out that the amount of cleanup work left for the yeast to do is dependent on how well we treated, or mistreated, our yeast during the first 3 fermentation phases.
During the sedimentation phase I raise the temperature of my fermentors 3-5F and hold it there for at least 3 days before bottling or kegging my beer. I do this because the yeast will only convert the flavor precursors it created earlier if it's warmer than it was when they created them. There is a limit to this rule though since the yeast can only do so much cleanup before they go dormant. Once the yeast go dormant any remaining flavor precursors will be left in your beer to produce off flavors.
The availability of high grade beer brewing ingredients has led many celebrated home brewing authors to rewrite sections of their published works regarding yeasts and fermentation times. For today's brewers moving beer to a secondary fermentor is no longer the absolute necessity it once was. There are still some conditions, like adding fruit or when conditioning the beer for extended lengths of time, when you may want to move your beer off of the primary fermentation yeast.
Home brew and yeast experts Jamil Zainasheff and Chris White have said that homebrewers using the proper amount of healthy yeast to achieve a good fermentation don't really have to worry about autolysis like they used to. Both authors agree that fermenters having a large flat surface area at their bottom will accumulate a thin even layer of yeast cells at the bottom as they flocculate out, preventing the yeast from dying off and introducing off flavors due to autolysis.
Pitching enough healthy yeast at the right temperature in a carboy or plastic bucket should allow homebrewers to keep their fermenting beer in the primary fermentor for up to several months without worry. Of course if you are adding hops, fruits or other adjuncts to your recipe you may have other compelling reasons to rack to a secondary or if you're lagering your beer for extended periods. Overall if you've been racking to a secondary fermentor to prevent yeast cells from introducing autolysis' burnt rubber flavors into your beer, you can take it from the experts and rethink your current process.
Yeast Makes A Difference
The type of yeast used to ferment your beer makes a huge difference in the way your finished beer tastes. There are recommended types of yeast that have been cultured for use in different styles of beer, some strains designed to ferment Ales and other strains designed for use in fermenting Lagers. Several companies offer their yeast products in dry form while others provide a wider array of yeast packaged in liquid form. Either way you choose to go, liquid or dry, the yeast you use and how well your fermentation process treats them will have a big impact on your finished beer.
|Rehydrating Dry Yeast In Sanitized Water To Prepare For Fermentation|
Dry yeast should always be rehydrated in tap water, not deionized or distilled water, that has been boiled and allowed to cool to between 95 to 105F before sprinkling on top of the water. Once the yeast begins to absorb the warm water they will sink after about five minutes and you will be able to stir the solution into a cream. This creamy yeast solution should ideally be pitched after 30 minutes into wort that has cooled to within 15F of the yeast solution. Rehydrating dry yeast within this optimal temperature range will help you to get nearly 100% viability of the recovered yeast.
The interesting thing understand about rehydrating dry yeast is that the cells are very fragile and they lack the ability to block sugars or toxic material that will damage their cells from passing through the cell. Only until the cell walls have been properly rehydrated with water can they can regulate what passes through to the inner cell. This is one of the main reasons for rehydrating dry yeast with water or very diluted wort. Rehydrating dry yeast in water that is below 105F will cause the cells begin to leach their insides damaging the cells and reducing the recovery rate of viable cells.
Each gram of dry yeast contains about 20 billion live yeast cells and improper rehydration steps can reduce the number of viable cells by more than half. A lower cell count translates into longer fermentation times and increases the possibility for off flavors to be introduced into the finished beer. When stored in a refrigerator dry yeast will lose about 4% of viable cells per year, when stored at room temperature the loss of viable cells can be as high as 20% per year or more depending on how high the storage temperature goes.
|Liquid Provides The Largest Selection Of Yeast To Choose From|
Making a 1.040 gravity wort is easy just add a cup of extra light DME to 2 liters of filtered water, boil for 15 minutes and let cool to 70F. Pour the wort into an Erlenmeyer flask with a package of liquid yeast a stirbar and set it spinning on a stirplate for 24 hours. Put a loosely folded piece of sanitized tin foil over the flask opening to keep any dust like particles out of the flask and allow room air to replace the Co2 that escapes.I like to add a pinch of yeast nutrient or dry yeast to starter wort when boiling it so the liquid yeast will have plenty of nutrition available to them.
|1.040 Starter Wort With Yeast Nutrient And Hops|
|Erlenmeyer Flask, Washed Yeast, Funnel And Stir Bar|
1) Find a good clean gallon or so sized glass jar, I use large pickle jars after getting rid of the pickle odors.
2) Fill jar with water and boil for 10-15 minutes to sanitize it then keep it covered with a tight fitting lid
3) Once the water is the same temperature as the beer you bottled, pour the sanitized water into the fermentor
4) Use a sanitized spoon to swirl the water to get the yeast into suspension
5) Pour the washed yeast back into the same sanitized glass jar
6) Spray the top of the jar lightly with StarSan, screw on the sanitized lid, spray the top of the jar again and cover with plastic wrap and an elastic band.
7) Cold crash the yeast slurry for 5-7 days in the refrigerator with it still tightly sealed and covered
8) Decant off most of the clear beer and use a sanitized turkey baster to fill the sanitized vials
9) Cap the vials tightly soak in StarSan then refrigerate until ready to use in your next beer
10) Make a 1.040 starter wort from one of the vials, decant as needed and pitch propagated the yeast
Your nose and sense of smell is the best tool you own for testing the viability of the yeast and detecting any bacteria infections. Believe me contaminated yeast is a smell you wouldn't soon forget, if the yeast smells like beer it's ready to use, if it smells at all funky toss it out and get a new supply.
|Vials Of Washed Yeast Refrigerated For A Week|
I estimate vials stored in my dedicated brewing mini-refrigerator will have a 10% per month cell die off rate and take that into account when estimating my pitching rates and starter sizes. I've successfully used up to 5 generations of yeast to brew perfectly good beer, the yeast provided attenuation just as good as the original generation one and a single purchased vial of yeast allowed me to brew for an entire year.