As with any living organism, Yeast have to be fed to survive. Unfortunately, honey does not have much in the way of nutrients, particularly if it has been boiled. It is therefore up to the brewer to ensure the right combination of nutrients is added to the Must to ensure a healthy fermentation. If this is not done, then the Mead could experience a number of negative results, such as Stuck Fermentation and Hydrogen Sulfide Contamination (Rotten Egg Smell). Fortunately, it is fairly easy to meet the nutritional needs of the yeast.
Once yeast have been placed into a solution of oxygen, nutrients, and food, they begin to prepare themselves for growing. This does not mean the individual yeast will grow, rather that they will begin to multiply (i.e. grow the colony). To do this, they will immediately begin to suck up as many nutrients as possible (the Lag phase), will begin to reproduce (Accelerating Growth phase), will reproduce rapidly (Exponential Growth phase), will slow down as the nutrients are depleted (Decelerating Growth phase), and finally will maintain their numbers (Stationary phase). Each stage correlates directly to the amount of nutrient provided.
Lag Phase – This occurs during the first few hours after addition of the yeast, during which there are no apparent signs of fermentation or growth. The yeast are becoming acclimated to their new environment. The yeast may be “shocked” by their new environment and it may take some time to adjust. They absorb all of the available oxygen, using it to synthesize all the enzymes and other building blocks needed for growth and fermentation, and storing oxygen up in the form of sterols for later use. This stage is critical to fermentation and should occur as rapidly as possible, preferably within a few hours, so lots of oxygen is needed at the start.
Accelerating Growth Phase – The yeast cells start to grow and divide, and some fermentation will begin. The yeast store the sugars they absorb for later use.
Exponential Growth Phase – The yeast’s rate of reproduction and metabolism is at its highest, dividing every 90 – 180 minutes, resulting in an increase in numbers of up to 1000 times in 24 hours. Fermentation begins in earnest and a Krausen may be beginning to form.
Decelerating Growth Phase – This should occur between 12 to 24 hours after pitching. At this time the oxygen is fully depleted and fermentation and CO2 production is taking over. Lots of bubbling should be occurring at this stage.
Stationary Phase – During this time the fermentables and nutrients are completely consumed. All yeast growth has stopped and they are beginning to fall out of suspension or flocculate. The stored oxygen and sugars inside the yeast will begin to be used to continue growth. Prolonged exposure in this phase (weeks) can lead to autolysis or total breakdown of the cell.
The goal of the brewer is to make sure that the correct amount of nutrients are added at every stage during the yeast’s life-cycle to ensure they have what they need to be healthy. Healthy yeast leads to fewer esters and sulfur compounds being produced. Beware though that too much nutrients, or oxygen added after around day 3, will create additional problems. Learn what to look out for to know what stage your yeast are in, and dose them appropriately.
Yeast is a peculiar organism in that it does not necessarily need straight oxygen to survive. Instead, it can obtain the O2 required to reproduce by fermenting any available sugars. But, since O2 is used to build up strong cell walls, the yeast is able to reproduce far quicker, and with fewer unwanted byproducts, if it can get the O2 without having to break it off from the sugar molecules. It is therefore important to dissolve O2 into the Must before the yeast is pitched, and to add more for the first 2 to 3 days until the yeast population has grown sufficiently. This is a fairly simple process and can be achieved in a number of ways:
- When pouring the water (or Must if you have heated or boiled the mixture) into the fermenter, make sure it splashes a lot so that you can see a lot of air bubbles. Pouring it through a strainer or a funnel achieves this.
- If you are siphoning the Must, hold the end of the tube near the top of the fermenter to allow the liquid to fall freely through the air.
- If you are using a 1gallon Carboy as the primary, pick it up and shake it for at least 5 minutes (if you feel strong enough to do this to a 5 gallon Carboy, go ahead but be aware that they can be slippery when wet).
- Stir the Must vigorously with a long handled spoon for at least 5 minutes.
- Use a Lees Stirrer or Wine Degasser attached to an electric drill.
Use a 0.5 micron stainless steel aeration stone and either an O2 tank for at least 2 minutes or an aquarium pump fitted with an in-line filter for at least 5 minutes.
Whichever method you choose, make sure that you completely sanitize the tool before each use. Also, boiling the Must drives out any dissolved oxygen so it is very important to add it back in some way.
Yeast have two types of nitrogen they are looking for when metabolizing the honey into Mead: YANC (Yeast Assimilable Nitrogen Content) and FAN (Free Amino Nitrogen). YANC includes all of the assimilable nitrogen from ammonium ions and the assimilable FAN present in the juice/must. Nitrogen is essential for the yeast to be able to reproduce. Without a growing yeast population, the fermentation will be extremely slow and may stop altogether if the yeast you do have conk out on you. Nitrogen also helps the yeast build up a tolerance for alcohol, which will not only help prevent a stuck fermentation, but will allow the brewer to create Meads with a higher ABV. Yeast use Nitrogen to create natural enzymes that clarify and age the Mead quicker. Finally, low levels of YANC are associated with the production of undesirable sulfide compounds (H2S).
Nitrogen sources are available from most Homebrew stores and are called “Yeast Nutrient”. Generally, yeast need a range of 250 ppm-350 ppm or higher depending on the initial Gravity/Brix level of the must. This is why it is often necessary to supplement the available nitrogen.
Basically, these are anything else needed by the yeast to build strong cell walls, reproduce, build up tolerance to alcohol, and prevent the production of unwanted compounds that can change the flavor of the Mead. Vitamins and minerals needed include amino acids, urea, phosphorous, citric and pantothenic acids. The micronutrients are often called “Yeast Energizer” by most Homebrew stores.
The following are some of the commercial sources for the required nutrients:
- DAP (Diammonium Phosphate) – Contains fermentable Nitrogen and phosphorus.
- Fermax™ – contains diammonium phosphate, dipotassium phosphate, magnesium sulfate, autolyzed yeast.
- Fermaid K™ – A more complete nutrient than DAP, it contains a variety of compounds such as amino acids, sterols, yeast hulls, and vitamins; also contains a limited concentration of fermentable nitrogen. It supplements the thiamine levels in the Must, which if too low will cause excess H2S production. It also supplements the Pantothenate levels which if too low will cause excess volatile acid production.
- Yeast Hulls – Commercial brand is Ghostex™. The remains of dead yeast in powder form. The hulls provide lipids for your yeast to build strong cell walls, and absorb autotoxic yeast byproducts that could inhibit alcoholic and malolactic fermentations. They can be used if you encounter a stuck or sluggish fermentation.
Note that many Homebrew stores have their own mixture that they provide, often with no name other than “Nutrient” and “Energizer”, but each will have a similar combination of compounds that give the yeast what they need.
DAP – 1/4 tsp per Gallon. For a 5 gallon batch, dose the Must at a rate of 1/2 tsp a day for the first three days while aerating. That way the DAP is mixed in and the Must aerated at the same time. 2 teaspoons per five gallon batch is plenty if a single dose is being made before pitching the yeast.
Fermaid K – 1 gram (1/4 tsp) per gallon of Fermaid K at 1/3 sugar break.
Use both Fermaid K and DAP before adding the yeast, especially in lower color honeys. Initially, the nitrogen in DAP is adsorbed very readily. As the fermentation progresses, use Fermaid K and supplement with yeast hulls. The nitrogen from the amino acids (in the Fermaid K) is absorbed more easily further along in the fermentation than from ammonia salts (DAP).
Note that some manufacturers make products that require slightly different dosage amounts, so it is important to read the instructions carefully. Wyeast, for example, have a yeast nutrient that instructs the brewer to use only 1 tsp. for 5 gallons.
Warning – Do not use too much of either or you risk changing the flavor of the Mead and creating compounds that may spoil your batch.
If there is no Homebrew store nearby, it is possible to use alternatives for the nutrients. For example, certain fruit have sufficient nitrogen and minerals in them for a healthy fermentation. Raisins are an excellent choice, although it must be noted that the flavor profile of the Mead will be changed if they are used. Use between 5 to 10 raisins per gallon of Must, crushing them or cutting them in half to allow the yeast to reach the inner pulp. In the case of a Braggot, the addition of the malt extract also gives the yeast sufficient nutrients.
Using nutrients is not an absolute necessity. Many brewers prefer the straight Honey/water/yeast method and they achieve very good results. If you attempt to brew without any additions and encounter a stuck fermentation, add a little energizer or yeast hulls to get the process working again.
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