How to Make a Yeast Starter
Making a Liquid Yeast Starter (from Palmer's How To Brew)
Liquid yeast packets should be stored in the refrigerator to keep the yeast dormant and healthy until they are ready to be used. There are two types of liquid yeast package - Those with inner nutrient packets and those without. The packages that contain an inner bubble of yeast nutrient (i.e. a "smack pack") are intended to function as a mini-starter, but are really not adequate. They still need to be pitched to a starter wort after activation. The package must be squeezed and warmed to 80°F at least two days before brewing. The packet will begin to swell as the yeast wake up and start consuming the nutrients. When the packet has fully swelled, it is time to pitch it to a starter to increase the total cell count to ensure a good fermentation. I prefer to prepare all my liquid yeast packages yeast four days before brewday.
1. If you are going to brew on Saturday, take the yeast packet out of the refrigerator on Tuesday . Let it warm up to room temperature. If it is a smack pack, place the packet on the countertop and feel for the inner bubble of yeast nutrient. Burst this inner bubble by pressing on it with the heel of your hand. Shake it well. If you are not using a smack pack, proceed directly to step 3. You will be making two successive starters to take the place of the mini-starter smack pack.
2. Put the packet in a warm place overnight to let it swell. On top of the refrigerator is good. Some brewers, who shall remain nameless, have been known to sleep with their yeast packets to keep them at the right temperature. However, their spouse assured them in no uncertain terms that the presence of the yeast packet did not entitle them to any more of the covers. So, just put the packet somewhere that's about 80°F, like next to the water heater.
3. On Wednesday (or Tuesday for slants) you will make up a starter wort. Boil a pint (1/2 quart) of water and stir in 1/2 cup of DME. This will produce a starter of about 1.040 OG. Boil this for 10 minutes, adding a little bit of hops (a few pellets) if you want to. Put the lid on the pan for the last couple minutes, turn off the stove and let it sit while you prepare for the next step. Adding a quarter teaspoon of yeast nutrient (vitamins, biotin, and dead yeast cells) to the starter wort is always advisable to ensure good growth.
4. Fill the kitchen sink with a couple inches of cold water. Take the covered pot and set it in the water, moving it around to speed the cooling. When the pot feels cool, about 80°F or less, pour the wort into a sanitized glass mason jar or something similar. Pour all of the wort in, even the sediment. This sediment consists of proteins and lipids which are actually beneficial for yeast growth at this stage.
Ideally, the starter's temperature should be the same as what you plan the fermentation temperature to be. This allows the yeast to get acclimated to working at that temperature. If the yeast is started warmer and then pitched to a cooler fermentation environment, it may be shocked or stunned by the change in temperature and may take a couple days to regain normal activity.
5. Sanitize the outside of the yeast packet before opening it by swabbing it with isopropyl alcohol. Using sanitized scissors, cut open a corner of the packet and pour the yeast into the jar. Two quart juice or cider bottles work well, and the opening is often the right size to accept an airlock and rubber stopper. Cover the top of the jar or bottle with plastic wrap and the lid.
Shake the starter vigorously to aerate it. Remove and discard the plastic wrap, insert an airlock and put it somewhere out of direct sunlight. (So it doesn't get too hot in the sun.) If you don't have an airlock that will fit, don't worry. Instead, put a clean piece of plastic wrap over the jar or bottle and secure it loosely with a rubber band. This way the escaping carbon dioxide will be able to vent without exposing the starter to the air.
6. On Thursday (or Wednesday for slants) some foaming or an increase in the white yeast layer on the bottom should be evident. These small wort starters can ferment quickly so don't be surprised if you missed the activity. When the starter has cleared and the yeast have settled to the bottom it is ready to pitch to the fermenter, although it will keep for 2-3 days without any problems. However, I recommend that you add another pint or quart of wort to the Starter to build up the yeast population even more.
The starter process may be repeated several times to provide more yeast to ensure an even stronger fermentation. In fact, a general rule is that the stronger the beer (more fermentable/higher gravity), the more yeast you should pitch. For strong beers and barleywines, at least 1 cup of yeast slurry or 1 gallon of yeast starter should be pitched to ensure that there will be enough active yeast to finish the fermentation before they are overwhelmed by the rising alcohol level. For more moderate strength beers (1.050 gravity) a 1-1.5 quart starter is sufficient. One consideration when pitching a large starter is to pour off some of the starter liquid and only pitch the yeast slurry. One recommendation when pitching a large starter is to chill the starter overnight in the refrigerator to flocculate all of the yeast. Then the unpleasant tasting starter beer can be poured off, so only the yeast slurry will be pitched.
When is My Starter Ready to Pitch?
A yeast starter is ready to pitch anytime after it has attained high krausen (full activity), and for about a day or two after it has settled out, depending on the temperature. Colder conditions allow the yeast to be stored longer before pitching to a new wort. Yeast starters that have settled out and sat at room temperature for more than a couple days should be fed fresh wort and allowed to attain high krausen before pitching.
A key condition to this recommendation is that the composition of the starter wort and the main wort must be very similar if the starter is pitched at or near peak activity. Why? Because the yeast in the starter wort have produced a specific set of enzymes for that wort's sugar profile. If those yeast are then pitched to a different wort, with a different relative percentage of sugars, the yeast will be impaired and the fermentation may be affected. Kind of like trying to change boats in mid-stream. This is especially true for starter worts made from extract that includes refined sugars. Yeast that has been eating sucrose, glucose/dextrose, or fructose will quit making the enzyme that allows it to eat maltose - the main sugar of brewer's wort.
If you make your starter using a malt extract that includes refined sugar, it is better to wait until the yeast have finished fermenting and settled out before pitching to the main wort. Why? Because towards the end of fermentation, yeast build up their glycogen and trehalose reserves; kind of like a bear storing fat for the winter. Glycogen and trehalose are two carbohydrates that act as food reserves for the yeast cell. Yeast slowly feed off these reserves when other food is not present, and use this food extensively to fuel the synthesis of essential lipids, sterols, and unsaturated fatty acids when pitched to an oxygenated wort. (Yeast will rapidly deplete their glycogen reserves when exposed to oxygen.) While glycogen can be likened to the fat that a bear stores for winter, the other component, trehalose, acts more like the bear's heavy fur coat. Trehalose seems to get built up on both the inside and outside of the cell membrane, and is generally believed to make the membrane structure more robust and more resistant to environmental stresses. By allowing the yeast starter fermentation to go to completion, these reserves are built up, and upon pitching, the yeast starts out with a ready fuel supply and a clean slate to better adapt it to the new wort. As noted earlier, though, these same reserves are used by the yeast while in hibernation, so if the yeast are left too long before pitching, the reserves may be depleted and should be replenished with a fresh starter wort fermentation before use.