Q: Does a starter need to be kept at the same temperature as it is going to ferment the batch of beer later?
No, but there are practical limits to how high or low you can go.
Warmer starters (up to 98°F, 37°C) equal more rapid yeast growth, but using these very high propagation temperatures negatively affects the viability and stability of the resulting yeast. Very rapid growth or excessive growth can result in weaker cell membranes due to lower unsaturated fatty acid concentrations. Lager yeasts tend to be especially sensitive to high temperatures.
The cooler you ferment the starter (down to the planned fermentation temperature for the main batch) the slower the yeast growth, but the yeast can be healthier than yeast coming from a high temperature starter.
Keep starters between 65°F (18°C) and 75°F (24°C). A temperature around the low 70s (72°F, 22°C) strikes the best balance for the propagation of yeasts. Lager yeast starters can be kept a few degrees cooler and ale yeasts can be kept a few degrees warmer, but this temperature strikes a good balance of yeast health and efficient propagation for both types of yeast.
If you are going to pitch the starter at high krauesen, it is best to keep the starter within 5 degrees Fahrenheit (3°C) of the wort temperature of the main batch. Pitching a very warm, active starter into cold wort can stun the yeasts and with lager yeasts this can cause a higher incidence of petite mutants, which can negatively affect attenuation, flocculation, and increase hydrogen sulfide production.
You can add small amounts of cool wort to the starter over time, to bring the temperature down gradually, but it is really better to keep everything closer to fermentation temperatures from the beginning. Any time yeast sense a big drop in temperature, they slow down and drop out.
Q: At what point do I pitch the starter into the wort?
A great deal of discussion rages over this topic. Should the starter be fermented completely, the spent liquid decanted, and the yeast pitched or should the entire starter be pitched when at the height of activity?
Most yeast experts say that when propagating yeast, moving at high krausen is optimal. The time of high krauesen can range anywhere from a few hours to twenty-four or more. It depends on the amount of yeast added to the starter wort, yeast health, temperature, and several other factors.
Doss says a starter made from an XL pack of yeast into 2 liters of wort will reach its maximum cell density within 12-18 hours. If youre starting with a very small amount of yeast in a large starter, it can take 24 hours or more to reach maximum cell densities. For the average starter, let's just say that the bulk of the yeast growth is done by 12-18 hours.
I like to pitch starters while they're still very active and as soon as the bulk of reproduction is finished, usually within 12 to 18 hours. This is really convenient, because I can make a starter the morning of the brew day or the night before and it is ready to go by the time the batch of wort is ready.
Of course, if you have a large starter volume in relation to your batch of beer or a starter that was continuously aerated, then you probably dont want to pitch the entire starter into your wort. Adding a large starter or a heavily oxidized starter to your wort can alter the flavor of the finished beer.
If youre going to pitch only the yeast from the starter, make sure the starter attenuates fully before decanting the spent wort. The yeast rebuild their glycogen reserve at the end of fermentation and it is this glycogen that they use when preparing to ferment a new batch of beer. Separating the spent wort from the yeast too early also selectively discards the less flocculent, higher attenuating individuals in your yeast population. You may end up with a pitch of yeast that wont attenuate the beer fully. Allow the fermentation to go complete cycle, chill, decant the beer and pitch just the yeast.