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| Beauty and the Yeast |
Sep, 2003 |
| by Donald Million |
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| You make the wort. but it's the yeast that turns it into beer. What you can do to ensure that your fungal friends get off to a fast start and finish at the proper specific gravity. |
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 Many brewers believe yeast is the single biggest factor affecting the final taste of their beer. The effort we put into mashing, boiling and the rest simply prepares an environment where the yeast can do their work.
Some of the flavor and aroma compounds that are affected by yeast are esters (ăfruity,ä often banana, aroma), diacetyl (buttery or butterscotch flavor), fusel alcohols (solvent-like flavor), and aldehydes (green apple aroma and flavor). Most lager yeasts also produce noticeable amounts of sulfur. These characteristics are acceptable, sometimes even desirable, in low concentrations, but are considered flaws in higher concentrations. And for beer styles such as German hefeweizen or some Belgian beers, key flavor and aroma characteristics of the beer are a product of the special yeast strains used in their brewing. Thatâs why choosing the right yeast strain and handling the yeast correctly is so important.
Selecting the Right Yeast
Should you use dry yeast or liquid? In the past, dry yeast got a bad reputation for a number of reasons. One of the main ones was the generic package stuck under the lid of all-in-one can kits of an earlier era. This yeast was sometimes old and could have overheated during shipping.
Name brand dry yeast suppliers, like Lallemand and Safale, have made dramatic improvements in the last few years. If you havenât tried dry yeast recently, you havenât tried dry yeast. The only drawback these days is that it is not available in as many varieties as liquid. If you are making a basic pale, brown or amber ale, there are dry yeasts available that are as good as the best liquid yeast you can buy. If you are making lagers, many Belgian ales or a German-style wheat beer, you will need one of the appropriate liquid yeast strains.
An important step in selecting a yeast strain is checking the suppliersâ information. Most yeast suppliers provide information about attenuation, flocculation and appropriate temperatures for each of their yeast strains. In addition, they will describe the style of beer the strain is best suited for and whether or not it can tolerate high gravity wort.
Attenuation: Attenuation refers to how completely the yeast can ferment wort. If your wortâs starting gravity is 1.048 and its final gravity is 1.012, then the attenuation is calculated as (48-12)/48 = 0.75, or 75%. This value is about average. Some yeast attenuate less ÷ in the 70-73% range ÷ resulting in a sweeter, fuller-bodied beer. Wyeast 1968, White Labs WLP002 and Danstar Windsor are examples of low to moderate attenuating yeast. Other yeasts attenuate more ÷ as much as 80-85% ÷ resulting in a drier, lighter-bodied beer. Wyeast 1010, Danstar Nottingham and White Labs WLP007 are examples of highly-attenuating yeast strains.
Flocculation: Flocculation refers to the tendency of the yeast to clump together and settle out when fermentation is complete. All yeast does this to some extent, but there is considerable variation between strains. For example, White Labs WLP300 and Wyeast 1007 are both known as low flocculating yeasts while WLP002 and Wyeast 2112 are both highly flocculent. The important difference to homebrewers is that with a highly flocculent yeast, the beer will clear more quickly and completely than with a low flocculating strain.
Temperature: Most ale yeasts work best between 65ö75 ˇF (18ö24 ˇC) and will slow down or stop working completely if the temperature drops much below 60 ˇF (15.6 ˇC). Lager yeast work at much lower temperatures, in the range of 45ö60 ˇF (7ö16 ˇC). As always, there are exceptions; ale yeasts are available that work as cold as 55ˇ F (13ˇ C) and lager yeasts that work as warm as 68ˇ F (20ˇ C) without producing off flavors. Both ale and lager yeast will ferment at temperatures above their recommended range, but then produce excessive amounts of esters and other flavor compounds, resulting in off tastes.
Once you have chosen a strain and gotten it home, it should be refrigerated until you are ready to use it.
Pitching Enough Yeast
Yeast want lots of company when they are making beer. Once in the wort, yeast will reproduce until there are 6-10 million cells per milliliter, per degree Plato ÷ for a total of as many as two trillion cells in a typical 5-gallon (19 L) batch!
Commercial breweries have established the ideal amount of yeast to pitch at about 1 million cells per milliliter per degree Plato. The yeast then accomplish a six to ten-fold increase in population between pitching and the time when the majority of alcohol production occurs. If the yeast must reproduce more than that, there will be a correspondingly longer lag time.
A long lag time leaves the wort susceptible to contamination by bacteria and wild yeast. The increase in yeast growth that goes along with low pitching rates is also associated with increases in diacetyl, esters and fusel alcohols. Finally, under-pitching can result in under-attenuation ÷ causing high final gravities ÷ or an extremely slow fermentation.
If you do the math, you would find that you should be pitching 228 billion yeast cells for an average strength ale. A larger batch or higher gravity calls for more yeast.
How do you get that many yeast cells? First of all, you donât need exactly that many yeast cells. This is an ăidealä amount, but good beer
has been made with far less. If you
get into the neighborhood, your beer will be fine.
On the other hand, if youâre experiencing long lag times, or any of the problems associated with them, your pitching rate should be increased. It is also a good idea to err on the side of too much yeast rather than too little for lagers and high gravity ales. This is because the fermenting conditions for these kinds of beer put more demands on the yeast.
According to their respective manufacturers, large smack packs and ăpitchableä tubes of liquid yeast contain up to 60 billion cells while an 11-gram packet of dry yeast may have as much as 160 billion cells. A packet of dry yeast gets you fairly close to your target pitching rate and two is plenty even for a high gravity beer. The liquid yeast should really be stepped up at least once.
Making a Starter
To step up the quantity of yeast, we make a yeast starter. This is a simple process, but you need to be careful about sanitation. You want to grow brewers yeast, not bacteria or wild yeast. Remember, too, that you are growing yeast. While the process is similar to making a mini batch of beer, the objective is different.
The correct specific gravity for starter wort is a matter of some disagreement. Wyeast says it should be between 1.020 and 1.030. White Labs recommends about 1.040. Many commercial breweries make starters with gravities of 1.048. What everyone seems to agree on is that yeast grow more rapidly in lower gravity wort. The disagreement revolves around whether lower gravity wort properly prepares the yeast for pitching into the actual batch.
My recommendation is to begin with relatively low gravity wort; 60% of the projected gravity of the actual batch is a good rule of thumb. For instance, if your batch is going to have a gravity of 1.050, you would make a starter with a gravity of (50*0.60 = 30) 1.030. For a high gravity beer, I would step the yeast up a second time into a starter that was closer in gravity to the batch, perhaps 75ö80%. How big of a starter do you need? Since you
probably donât have the equipment to count yeast cells, you have to base it on the volume of the starter. A standard rule of thumb is to step up the volume of yeast to 6ö10 times the amount of wort that you start with. That means, if you are going to make a 5-gallon (19 L) batch, you would ideally pitch from a 2-quart (~ 2 L) starter. Any of the ăpitchableä yeast tubes or large smack packs can be pitched directly into a 2-quart (~2 L) starter.
A Starter, Step by Step
Here is how to make a 2-qt. (~2L) starter with a gravity of about 1.030.
1. Heat 2 qts. (~2 L) of water to boiling.
2. Add 6 ozs. (170 g), or about
11/2 cups, of dry malt extract.
3. Add 1/4 tsp. of yeast nutrient. (This is optional, but recommended).
4. Boil for 15-20 minutes. (Watch out for boilovers!)
5. Remove from heat and cool to below 90ˇ F (32ˇ C).
6. Transfer starter wort to a sanitized container that provides at least a couple of inches of headspace. A one-gallon (3.8 L) apple cider bottle works well, as does a brewpub growler or 3-liter soda bottle.
7. Aerate well.
8. Add yeast.
9. Close the container with a stopper and airlock or simply cover with aluminum foil.
Keep the starter warm, at the upper end of ÷ or even slightly above ÷ the supplierâs recommended temperature range. Donât be surprised if you donât see a lot of foaming or airlock activity. Starters often ferment quickly, but quietly. If, after a day or two, you see yeast sediment in the container, your starter has been active.
Using the Starter
Once the yeast goes through the growth phase, the starter is ready. At this point, a layer of gray, putty-like yeast will have settled to the bottom of the container. You can decant the liquid and only pitch this yeast sediment, or you can pitch the entire starter. If you decant the liquid, you will lose some yeast still in suspension, but will not dilute your batch as much. If your starter was similar in gravity and color to your batch, however, diluting is not an issue and I recommend pitching the entire starter. In either case, you need to swirl the container to loosen the sediment on the bottom.
Other Sources of Yeast
An alternative to a starter is using the yeast sediment from a previous batch. Time your batches so that you are ready to pitch a new one just as you are bottling the previous one, or moving the previous one from primary to secondary, and you have a ready-made starter.
Some homebrewers pour the new batch in on top of the yeast sediment from the previous batch after moving the previous batch out of the primary fermenter. An alternative, which separates the new batch from the cold-break and hop residue of the previous batch, is to measure out about a cup (about 250 mL) of the previous batchâs yeast sediment from either the primary or secondary fermenter and pitch that into the new batch. As usual, pitch more for a lager or high gravity ale.
Be aware that over several generations the yeast can mutate. For that reason, any changes for the worse in your beer should signal that it is time to stop reusing that yeast. Also, if a batch exhibits any signs of contamination, itâs time for fresh yeast.
Should you use the yeast from the primary or secondary fermenter? Yeast from the secondary has gone through a longer fermentation period and spent more time in an alcohol-rich environment. For that reason, some brewers believe they are ătiredä and less suitable than yeast from the primary. On the other hand, the yeast from the primary will have more trub mixed in with them, so some brewers feel the yeast from the secondary are ăcleaner.ä Iâd prefer fresh, ădirtyä to ătired,ä clean yeast, but good beer has been made using both methods.
A final option with some strains is to skim the floating yeast from the top of the primary. This is my favorite way to harvest yeast, because you get the freshness of yeast from the primary, but avoid the trub. White Labs WLP022 and Wyeast 1007 are examples of strains that usually leave enough yeast floating on top for this to work. Use a carefully sanitized spoon to skim the yeast and save it in a sanitized jar under distilled water.
Pitching Temperature
Wherever you get your yeast from, avoid shocking it with a large temperature change when you pitch. Most brewers yeast strains can survive temperatures well over optimal fermentation temperatures. However, yeast do not like rapid temperature changes, for instance from pitching cold yeast into a batch of beer that has not been cooled adequately. Try to ensure that the temperature difference between your yeast and beer is 10 ˇF (5 ˇC) or less. Remove the yeast from the fridge, if thatâs where youâre storing it, at least three hours before you use it to let it warm up.
Nutrients and Oxygen
Pitching enough yeast isnât all there is to it, though. You also have to provide the right environment for them. This means the right nutrients and enough oxygen for them to go through a healthy reproductive phase and move vigorously into fermenting your wort.
Yeast nutrients are available at most homebrew stores. The contents of these mixtures vary, but most contain zinc and diammonium phosphate (DAP). Some also contain common minerals like thiamin, niacin and folic acid, and many contain yeast hulls (the empty cells of dead yeast).
With all-grain recipes, the wort should contain all the nutrients the yeast will need. If the recipe includes a lot of adjuncts, there may be deficiencies. Extract or partial mash recipes may also benefit from the addition of yeast nutrients. Extremely high gravity wort, where the yeast is going to be stressed just converting all of the sugar to alcohol, can also benefit from additional nutrients. Usually a teaspoon or two, added to the boil, is all that is needed.
You need to get as much oxygen as possible into the wort immediately after it cools. This can be done by shaking the fermenter, splashing the wort, allowing air to be sucked in and mixed with the wort while siphoning, or by direct injection of air or oxygen using an aquarium pump or oxygen cylinder. Yeast require oxygen during their growth phase, when they are rapidly reproducing. Without it, they canât reproduce adequately and will come out of the growth phase weakened and unable to fully ferment the wort. The affect on the beer is the same as not pitching enough yeast.
Oxygen is more soluble in cool wort and aerating hot wort causes it to darken; this is why we add oxygen only after the wort has cooled. Likewise, oxygen introduced after the yeast have passed the growth phase results in oxidation, causing early staling of the beer. It will also lead to diacetyl production in the wort. This is why we aerate only immediately after cooling the wort, not throughout fermentation.
Fermentation
Fermentation is where it all comes together, as far as homebrewing is concerned. Each strain of yeast has different characteristics it will impart to the beer and different conditions in which it will ferment most effectively. If youâre making an imperial stout, for instance, you want yeast that can tolerate high gravity and the high alcohol content towards the end of fermentation. Examples would be White Labs WLP007 and Wyeast 1728. If your fermentation area is on the warm or cool side, pick a strain that will do well at that temperature. White Labs WLP001, for instance, ferments well at warmer temperatures while Wyeast 1007 does well at cooler temperatures.
Some extremely flocculent yeast strains may need to be roused during fermentation. You will need to shake the fermenter or stir the wort to get the yeast off the bottom and back into suspension. Wyeast 1968 is an example of this type of yeast.
The attenuation your yeast achieves is dependent on all the things weâve discussed up to now, as well as on the fermentability of the wort. If you under-pitch, fail to aerate, keep the fermenter too cool or use a yeast intended for low gravity beers in a high gravity wort, then under-attenuation is a likely result.
During fermentation, about all you have to do as a homebrewer is ensure that the temperature remains in the recommended range and (at least roughly) constant. Also, keep your carboy away from bright lights, especially bright sunlight. A dark t-shirt over the carboy is all youâll need for light protection if your fermenting area is brightly lit. If you have taken care of yeast nutrition and aeration, sit back and let the yeast do their work.
Yeast for Bottle-Conditioning
After the wort ferments, the yeast will eventually settle out and the beer will appear clear. Even after beer has fallen clear, it still has yeast in it ÷ enough yeast for bottling in almost all cases. Some homebrewers, however, still prefer to add some bottling yeast for quicker conditioning. If you add fresh yeast, it is not necessary to pitch as much as for a completely new batch. The Sierra Nevada brewery, for instance, adds only enough yeast to their bottle conditioned beers to get about 250,000 cells/mL, or the equivalent of 1 teaspoon (5 mL) of yeast slurry in a 5-gallon (19 L) batch.
Don Million wrote ăKeg Your Beerä in the March-April 2003 issue of BYO.
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