You might have already heard your fellow brewers talking about starters and how important they are to making great beer. Proper fermentation is what sets apart great beers from just-OK beers, and starters can help ensure it. Proper fermentation results in a beer with the correct appearance, flavor, body, and aroma: clean, complete, consistent, and reproducible.
To have proper fermentation, you must have the right amount of clean, healthy yeast. If you're a professional brewer with plenty of yeast every few days, this isn't hard. As an amateur, you're often starting with a single fresh pitch (a White Labs PurePitch pouch or a Wyeast Activator pack), and a starter can improve its performance.
The answers below are based on my experience and lab work, plus conversations with David Logsdon (Founder/Owner, Wyeast), Greg Doss (Wyeast microbiologist), and Neva Parker (White Labs laboratory manager). Thanks to Graham Sanders for his extensive feedback.
A note on this update: the answers below are the original article, brought up to date where the products have changed. The biggest change since I first wrote this is packaging. White Labs has retired the vial in favor of PurePitch, and the current PurePitch Next Generation pouch is built around a higher cell count, about 145 billion cells, sized to pitch five gallons of ale at 7.5 million cells per milliliter, while a Wyeast Activator pack still carries a minimum of 100 billion. I have updated the math that leans on those numbers and left the principles, which have not changed, alone.
Do I always need to make a starter?
No, but in many cases a starter provides better fermentation. Always make one if you suspect viability might be low: an old vial or pack, or yeast left warm for an extended period (for example, several days in shipping). You can also make a starter to grow a smaller amount of yeast into an appropriately sized pitch. Larger batches, higher-gravity worts, and lager fermentations require more yeast. You can pitch more containers or repitch from a previous batch, but a starter from a fresh pouch or pack is an excellent solution.
When shouldn't I make a starter?
Never make one if you can't handle the steps in a sanitary way or provide proper nutrition. Yeast health is the whole point. For some small or low-gravity beers (an ordinary bitter), there's a slight chance of overpitching if you get carried away, which can give an undesirable profile (low esters, autolysis notes, poor head retention). This matters in styles where yeast character is foremost, such as Bavarian wheat beers. And with dry yeast, it's usually cheaper and easier to buy more than to build a starter large enough; putting dry yeast in a starter just depletes the reserves the manufacturer built in. For a standard-gravity wort you can skip both the starter and the rehydration step and sprinkle the yeast straight onto the surface. Rehydrating becomes worthwhile in bigger beers. See the full guide to rehydrating dry yeast for when it matters and how to do it properly.
How do I make a starter?
It's easy, like a mini-batch of beer where the focus is yeast growth and health, not drinkability. You'll need a clean, sanitized container with headroom, aluminum foil, dried malt extract (DME), yeast nutrient, and water.
Keep the starting gravity between 1.030 and 1.040 (7–10°P). You do not want a high-gravity starter. As a ballpark, use about 6 ounces by weight of DME to 2 quarts of water. In metric it's easy: a 10-to-1 ratio, 1 gram of DME for every 10 ml of final volume (for a 2-liter starter, add water to 200 g of DME until you reach 2 liters total). Add ¼ teaspoon of yeast nutrient, boil 15 minutes, cool, and add yeast. With a borosilicate Erlenmeyer flask it's even easier: boil gently right on the burner under a foil cap, then cool.
If you have oxygen, add it; otherwise shake every few hours. A stir plate is better still (good gas exchange, yeast kept in suspension, CO₂ driven off) for roughly 2–3× the growth of a still, unshaken starter. Keep the starter around room temperature (72°F, 22°C). Remember the four main factors for yeast growth and health: nutrients, temperature, sugars, and pH. Key nutrients are oxygen, zinc, amino acids, and nitrogen; oxygen tends to be the most limiting. Use an all-malt wort so the sugar is maltose, not simple sugar. Yeast grown on simple sugars adapt to them and can be slower to take up maltose when they hit your wort, so keeping the starter all-malt keeps them tuned for the job ahead. Target a pH around 5; a high-quality DME will land in the right range. When pitching into the starter, work in a draft-free area and keep containers open as briefly as possible. Sanitize the outside of pouches and packs before opening.
Should I add hops to my starter wort?
No. It doesn't hurt, but it doesn't help much either. The antibiotic effect is minimal. The final product of a good starter is healthy yeast, not beer, so it's better to have less material, less expense, and fewer steps.
Should I add oxygen to my starter?
Yes. You'll get far healthier yeast and far more growth with oxygen throughout the process. Yeast use oxygen to synthesize unsaturated fatty acids and sterols, which are critical to a healthy cell membrane. With oxygen, yeast convert sugar to CO₂ and water and grow rapidly; without it they make alcohol, grow slowly, and reach a lower mass. Options include intermittent shaking, a stir plate, an air pump with a sterile filter, or pure oxygen. A stir plate is perhaps the most effective. Don't seal the vessel with an airlock; foil or a breathable foam stopper allows better gas exchange. Shaking every hour or two makes a large difference and can be nearly as effective. Continuous pure oxygen from a tank is both expensive and unnecessary.
How much yeast (or how big a starter) do I need?
You should always know your target, and the first thing to know is that the two packages are no longer the same size. A Wyeast Activator pack still carries a minimum of 100 billion cells. White Labs has moved on from the vial to PurePitch Next Generation, built around roughly 145 billion cells, enough on its own to pitch 5 US gallons (19 L) of an average-gravity ale at about 7.5 million cells/ml, which is the rate White Labs now recommends for standard ales. That is a good deal more yeast than the old one-vial-per-batch math assumed, so it is worth knowing which target you are aiming at, because the figures people quote vary widely. A lighter fresh pitch of around 5 million cells/ml will make good beer with fresh, healthy yeast; the more conservative rates below ask for noticeably more. Either way, higher-gravity worts (especially above 1.060 / 15°P), larger volumes, and lager fermentations all need higher rates or a starter.
A common rule for repitched yeast is 6–10 million cells/ml for ales and 10–15 for lagers, but that ignores gravity. Better: pitch about 1 million viable cells per ml per degree Plato, a little less for ale, a little more for lager. George Fix gives 0.75 million for ale and 1.5 million for lager, the conservative end of the range, and the rate I'll use for the worked example below. The simple math, for an ale:
(0.75 million) × (ml of wort) × (°Plato)
5.25 gallons ≈ 20,000 ml. One degree Plato ≈ 1.004 SG (divide the decimal part of the SG by 4). So 5.25 gal of 1.060 wort needs about 225 billion cells: 750,000 × 20,000 × 15 = 225,000,000,000.
Put another way, you need about 3¾ billion cells per point of OG into a little over 5 gallons. Double that for a lager. A single current White Labs PurePitch pouch (~145 billion) gets you roughly two-thirds of the way for this beer, and a 100-billion Wyeast pack closer to half; a 2-liter starter, which roughly doubles a single pack, clears the target from either one, as does simply pitching two packs. Don't overdo it: Logsdon stays within 20% of his ideal rate and prefers to slightly under-pitch; changes in flavor are noticeable even at 20% over.
If you would rather not run this arithmetic by hand, the Yeast Pitching Rate Calculator does all of it for you: viable cells by age, the number of packs or grams you need, and the size of the starter required to grow whatever is missing. For more on the math behind these rates, see Yeast Pitching Rates & How to Make Proper Starters.
Does the starter need to be at fermentation temperature?
No, but there are practical limits. Warmer starters (up to 98°F, 37°C) grow faster but produce weaker, less stable yeast; lager strains are especially sensitive. Cooler starters grow slower but can be healthier. Keep starters between 65°F (18°C) and 75°F (24°C); the low 70s strikes the best balance. If you'll pitch at high krausen, keep the starter within 5°F (3°C) of the main wort. Pitching a very warm, active starter into cold wort can stun the yeast and, with lagers, raise petite-mutant rates, hurting attenuation and flocculation and increasing hydrogen sulfide.
At what point do I pitch the starter?
Much discussion rages here. Most experts say high krausen is optimal, which can range from a few hours to a day or more depending on inoculation, health, and temperature. Doss says an XL pack into 2 liters reaches maximum cell density within 12–18 hours; very small inoculations into large starters can take 24+ hours. I like to pitch while the starter is still very active, as soon as the bulk of reproduction is done (usually 12–18 hours), which conveniently lets me make a starter the night before or the morning of brew day.
Should I decant the starter before pitching?
This is the other half of the timing question, and it comes down to what you put into the batch along with the yeast. When I pitch at high krausen, as above, the starter is small and active and I pitch the whole thing, wort and all, because the volume is minor and the yeast are at their best. That is my usual approach.
The alternative is to let the starter ferment all the way out, cold-crash it in the refrigerator for a day so the yeast settle into a firm cake, pour off the spent beer on top, and pitch just the slurry. The reason to do this is simple: a finished starter is not good beer. It is low-gravity, fully attenuated, essentially hop-free, and often a little oxidized, and none of that is something you want to add to a carefully built wort. With a small starter the splash you would add is trivial. But once you are making the 2-liter and larger starters this article calls for (especially for lagers and big beers), that spent starter beer becomes a real fraction of a 5-gallon batch, and you can taste it. In those cases I decant.
So: pitch at high krausen and pour it all in when the starter is small relative to the batch; crash and decant when the starter is large, when the beer is delicate, or any time you would rather not dilute your wort with a liter or two of flat, tired starter beer. The one tradeoff to know is that decanting means pitching settled, less-active yeast, so give them a little oxygen and a bit of patience at the start.
Can a starter be too small or too large?
Yes. Parker notes that a fresh vial into just 500 ml, taken to completion, can leave the yeast less ready. They don't rebuild reserves or gain much mass. The minimum for significant growth is about 1 liter; one vial or pack into 1 liter yields roughly a 50% increase in cell mass.
For a high-gravity beer, should I make a high-gravity starter?
No. Starter wort should be 1.030–1.040 (7–10°P). To revive stressed yeast (from a bottle, an old slant), use a lower gravity around 1.020 (5°P). Lower gravity is easier on the yeast (less growth); higher gravity grows more but is more stressful. Yeast don't "acclimate" to high gravity; the osmotic pressure just stresses them.
When stepping up a starter, what size should each step be?
There's leeway, but step size matters. A very large step can shift yeast metabolism so the last-fermented sugars fall out of favor, making later generations lower-attenuating. Many small steps increase contamination risk. Five to ten times the prior step is considered correct, balancing handling, sanitation, and growth. You don't have to plan the steps by hand: the Yeast Pitching Rate Calculator works out the starter size you need, and when a single starter can't grow enough from the yeast you have, it shows you how to build it up in steps with the numbers carried over.
Do I need a stir plate?
No, but a stir plate produces more cells from the same starter and, more importantly, healthier cells. Logsdon: it drives off CO₂, exchanges air in (raising oxygen), and eliminates dead spots so yeast easily reach the sugars. Two cautions: some plates add heat (one I tested added 5°F / 3°C; a thin piece of Styrofoam helps), and the constant air draw makes the starter track room-temperature swings, which hurts results. No plate? Shake as often as you can. In my tests, vigorous hourly shaking roughly doubled cell count versus no shaking, and a stir plate added about 40% over shaking.
Can I pitch part of a pack into the same-size starter and get the same cells?
No. You'll get more replication from fewer starting cells, but not the same total. One full pack of about 100 billion cells into 2 liters of 1.040 yields about 220 billion; one-tenth of a pack into the same volume yields only about 600 million. To grow a lot from a little, step it up while keeping the starter aerated and well-nourished. Pushing growth without enough oxygen-produced sterols hurts vitality.
In summary
The products from White Labs and Wyeast are excellent. When making a bigger beer, or when the yeast is old or abused, a clean, properly prepared starter delivers consistent, well-fermented beer. Keep your process simple rather than complex, and always ask whether the beer is the way you like it. Keep notes on both process and results. Above all, keep the big picture in mind: healthy yeast first, proper quantities second.