The Truth About Yeast
Open that homebrew book and find the perfect recipe, perhaps an India pale ale. Lots of malt, a ton of that popular Yakima Valley hop that you love so much, and the brewer even tells you how to adjust the water to achieve the perfect result. But wait, what about the yeast?
Yeast is the single most important contributor to beer flavor and the cause of more funky, scary, wild homebrew flavors than anything else.
Add a little too much crystal malt to your IPA and you will have a darker beer with more caramel flavors, but add too little yeast (or worse yet, contaminated yeast) and you may end up with anything from a pineapple and banana beer to a cardboard and vinegar ale. Ferment this concoction at the wrong temperature and your problems multiply. With a few easy steps you can take the Russian Roulette out of yeast pitching and control the flavor of your precious homebrew.
Fresh Air
After it’s boiled, the wort is totally devoid of oxygen. In this type of medium, yeast must depend on its existing supply of glycogen (stored carbohydrates) for energy required for growth. How would you like to be asked to reproduce several times over without so much as a snack in between? The yeast might be able to do it once, but after that, it’s through.
Lucky for us, it doesn’t take much oxygen for the yeast to do their dirty deed. About 20 percent oxygen saturation is perfect. Since air is composed of approximately 20 percent oxygen, 100 percent saturation with air will create the same effect (the 79 percent nitrogen in air is extremely insoluble in wort). The trick is to get the oxygen to the yeast without introducing bacteria, which could cause off-flavors.
There are a few things that can be done to increase the rate of oxygen transfer into the wort. The first is to increase the gas/liquid interface area. Splashing the wort into the fermenter or shaking the fermenter creates bubbles that increase the surface area and therefore increase oxygen transfer. A carbonation stone (also called an aerating stone) is a device that forces a gas into tiny bubbles, thereby increasing the gas-liquid surface area.
The temperature of the wort greatly affects the equilibrium concentration of the wort. Equilibrium concentration is simply the point at which the same amount of gas is being dissolved into a liquid as is being released. Lowering the temperature of the wort allows oxygen to travel much more quickly to the hungry yeast.
Pressure is another factor in the equilibrium concentration of oxygen in wort. If you’re fermenting in Cornelius kegs or other vessels that can withstand it, try applying pressurized air to aid aeration.
Finally, the rate of aeration depends on the type of gas being absorbed into the liquid. Oxygen is absorbed much easier than nitrogen. Bubbling pure oxygen into the wort will transfer oxygen five times faster than bubbling with air (which is only one-fifth oxygen).
Heavy Breathing
Finding a way to introduce oxygen into the wort is the easy part. Getting it into solution without introducing beer spoilage organisms is a challenge for which there are at least a hundred practical solutions.
One method is to use an aquarium-type aerator with a filter to remove microorganisms. A typical kit includes the air pump, a carbonating stone, air hose, and a filter. You may find, however, that the filter blinds easily.
Instead of using the filter provided, you can make your own with a two-inch piece of pipe and two quarter-inch hose barbs. Fill the two-inch pipe with cotton that has been sprayed with 70 percent ethanol (diluted grain alcohol) to filter and kill any microorganisms. The carbonating stone and air hose are soaked in a 50-parts-per-million iodophor solution before use (concentration dilutions are listed on the back of the iodophor bottle). The residual iodophor is blown out of the tube by the air pump and drips off the outside of the hose.
This system causes a lot of foaming. You may have to turn the air on and off five or six times to get 45 to 60 minutes of aeration. The only aeration supplies you will need to replace each brew are a cotton ball and a squirt of alcohol.
Pitching Is the Key
Once the wort is properly oxygenated, it must be pitched with the proper amount of yeast. The standard pitching rate used by the big brewers is one million cells per milliliter per degree Plato (or approximately 12 million cells per milliliter).
That’s an easy value for the big guys to determine with their microscopes, hemocytomers, spectrophotometers, and williwonkaumpalumpometers, but what about the homebrewer and his hydrometer?
Several sources estimate that this pitching rate is equal to approximately 50 milliliters of yeast slurry per gallon of wort. This means that if you save 250 milliliters (approximately eight ounces) of thick yeast from the bottom of a starter or previous fermentation, you will be somewhere in the ballpark of 12 million cells per milliliter. This pitching rate will allow yeast to grow enough to replenish its supply of healthy young cells but not so much as to produce off-flavors in the finished product.
Single-Cell Scourges
Wort oxygen and pitching rates are important factors in yeast flavor, but what are the consequences of underpitching or underaerating? When yeast is introduced to beer at the proper rate of 10 to 12 million cells per milliliter, it wants to grow to achieve a concentration of around 60 million cells per milliliter. This is equal to a sixfold growth, or about 2.5 doublings of the population.
If the same yeast is introduced at a much lower rate (1 million to 2 million cells per milliliter), it will still try to grow to 60 million cells per milliliter. However, instead of |doubling 2.5 times, it must double six times to grow to this proportion. This growth takes time, which leads to a lag in fermentation.
During this lag time the wort is susceptible to contamination from other microorganisms. Additionally, this yeast growth increases the level of esters such as isoamyl acetate (the banana ester) and the level of fusel alcohols. Lower pitching rates also increase the level of diacetyl (a buttery or rancid flavor).
The effects of a lack of wort oxygen are most pronounced when reusing under-oxygenated yeast. This yeast, which may have reproduced three or four times with very little oxygen to replenish its membranes, is weak and will not be able to proceed along the proper fermentation pathway. A gross lack of oxygen will immediately decrease the health of the yeast, causing increased ester formation and possibly a stuck fermentation.
One very important factor in any fermentation is the temperature. Often homebrewers ferment at a much warmer temperature than optimal for the particular strain. Most ale yeasts ferment very cleanly (and quickly) at 58° to 60° F, while lager yeasts show their best character down around 46° to 48° F.
Increasing the fermentation temperature above these values will cause fermentation to occur more rapidly, but it will also increase the level of esters, possibly producing a winey and fruity product. Higher levels of fusel alcohols may also be a result, increasing the aroma and fragrance of the beer as well as the headaches of the beer drinker.
Conversely, fermentation temperatures that are too low may cause a premature flocculation of the yeast and a corresponding stuckfermentation. In any event a cooler fermentation will be slower and cleaner.
If you keep records concerning your homebrewing efforts, remember to record what you did with the yeast. That award-winning IPA will taste great if well aerated, pitched with the right amount of healthy yeast, and fermented at the correct temperature. But if you change any of these variables, you will change the final product.
