Preparing a yeast starter

The reason for using a yeast starter is quite simple. We simply need to outnumber the wild yeast cells and make sure we have a relative short fermentation period.

Preparing Dry Yeast
Dry yeast should be re-hydrated in water before pitching. Often the concentration of sugars in wort is high and the yeast can not draw enough water across the cell membranes to restart their metabolism. For best results, re-hydrate 2 packets of dry yeast in warm water (95-105°F) and then proof the yeast by adding some sugar to see if they are still alive after de-hydration and storage.

If it's not showing signs of life (churning, foaming) after a half hour to one hour, your yeast may be too old or dead. Unfortunately, this can be a common problem with dry yeast packets, especially if they are the non-name brand packets taped to the top of malt extract beer kits. Using name brand brewers yeasts like those mentioned previously usually prevents this problem.

Re-hydrating Dry Yeast
1. Put 1 cup of warm (95-105F/ 35-40C) boiled water into a sanitized jar and stir in the yeast. Cover with Saran Wrap and wait 15 minutes.
2. "Proof" the yeast by adding one teaspoon of malt extract or sugar that has been boiled in a small amount of water. Allow the sugar solution to cool before adding it to the jar.
3. Cover and place in a warm area out of direct sunlight.
4. After 30 minutes or so the yeast should be visibly churning and/or foaming, and is ready to pitch in the starter.

Note: Lallemand/Danstar does not recommend proofing after rehydration of their yeast because they have optimized their yeast's nutrional reserves for quick starting in the main wort. Proofing expends some of those reserves.

Preparing Liquid Yeast
Liquid yeast is generally perceived as being superior to dry yeast because of the greater variety of yeast strains available. Liquid yeast allows for greater tailoring of the beer to a particular style. However, the amount of yeast in a liquid packet is much less than the amount in the dry. Liquid yeast usually must be pitched to a starter wort before pitching to the main wort in the fermenter. Using a starter gives yeast a head start and increases the population preventing weak fermentations due to under-pitching.

Making a Liquid Yeast Starter
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. Take the yeast packet out of the refrigerator and 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. So, just put the packet somewhere that's about 80°F, like next to the water heater.

3. On day two we can 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. 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 or 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 days three 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 wash (more fermentable/higher gravity), the more yeast you should pitch.

Ratio starter/wash
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. 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 the 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. This is 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 the grain based wort.

Yeast nutritional needs
From a yeast cells point of view, its purpose in life is to grow, eat, and reproduce. Yeast can do all this with or without oxygen, but using oxygen makes the processes easier for the cell. Yeast use oxygen in the biosynthesis of the compounds that make up their cell membranes that allow them to process sugars for food and grow. Being able to process food and grow more efficiently allows them to reproduce more effectively also. Without oxygen, yeast cannot reproduce as fast. Therefore, to ensure a good fermentation, we need to provide the yeast with sufficient oxygen to allow them to grow quickly and reproduce when they are first pitched to the fermenter. Once they have reproduced to sufficient numbers, we can let them get on with turning our sweet wort into alcohol.

Nutrients
Besides sugar, yeast needs nitrogen, and amino and fatty acids to enable them to live and grow. The primary source for these building blocks is the free amino nitrogen (FAN) and lipids from the malted barley. Refined sugars like table sugar, corn sugar or candy sugar do not contain any of these nutrients. And, it is common for extracts (especially kit extracts targeted toward a particular style) to be thinned with refined sugars to lighten the color or reduce the cost of production. An all-malt beer has all the nutrition that the yeast will need for a good fermentation, but all-extract beers may not have sufficient FAN to promote adequate growth. Since malt extract is commonly used for yeast starters, it is always a good idea to add some yeast nutrients to ensure good yeast growth.

If you use ion-exchanged softened water for brewing, the water may not have adequate calcium, magnesium, and zinc for some of the yeast’s metabolic paths. Magnesium plays a vital role in cellular metabolism and its function can be inhibited by a preponderance of calcium in the wort. Brewers adding calcium salts for water chemistry adjustment may want to include magnesium salts as part of the addition if they experience fermentation problems. Usually the wort supplies all the necessary mineral requirements of the yeast, except for zinc which is often deficient or in a non-assimilable form. Additions of zinc can greatly improve the cell count and vigor of the starter, but adding too much will cause the yeast to produce excessive by-products and cause off-flavors. Zinc acts as a catalyst and tends to carry over into the succeeding generation—therefore it is probably better to add it to either the starter or the main wort but not both. The nutrient pouches in the Wyeast smack-packs already contain zinc in addition to other nutrients. For best performance, zinc levels should be between 0.1-0.3 mg/l, with 0.5 mg/l being maximum. If you experience stuck fermentations or low attenuation, and you have eliminated other variables such as: temperature, low pitching rate, poor aeration, poor FAN, age, etc., then lack of necessary minerals may be a significant factor.

There are three types of yeast nutrients on the market that can supplement a wort that is high in refined sugars or adjuncts.

- Di-ammonium Phosphate: This is strictly a nitrogen supplement that can take the place of a lack of FAN.
- Yeast Hulls: This is essentially dead yeast, the carcasses of which act as agglomeration sites and contain some useful residual lipids.
- Yeast Nutrient or Energizer:- The name can vary, but the intent is a mixture of di-ammonium phosphate, yeast hulls, biotin and vitamins. These mixtures are a more complete dietary supplement for the yeast and what I recommend.

Oxygen
Yeast need oxygen to synthesize sterols and unsaturated fatty acids for cell membrane biosynthesis. Without aeration, fermentations tend to be underattenuated because oxygen availability is a limiting factor for yeast growth—the yeast stop budding when sterol levels become depleted. Higher gravity worts need more yeast for proper fermentation, and thus need more oxygen, but the higher gravity makes it more difficult to dissolve oxygen in the first place. Boiling the wort drives out the dissolved oxygen normally present, so aeration of some sort is needed prior to fermentation. Proper aeration of the wort can be accomplished several ways:

- Shaking the container, e.g. the starter jar
- Ppouring the cooled wort into the fermenter so it splashes,
- Using a bronze or stainless steel airstone with an aquarium air pump and using it to bubble air into the fermenter for an hour.

For the beginning distiller, I recommend the simplest methods of shaking the starter and pouring/shaking the wort. This method is especially effective if you are doing a partial boil and adding water to the fermenter to make up the total volume. Instead of shaking the wort, you can shake the water.
Pour the water into the fermenter and cover it tightly. The fermenter should be about half full. Shake it vigorously for several minutes to aerate it well.
Now you can pour your cooled wort to the fermenter and not worry about trying to shake the entire five gallons.

The last method mentioned method uses an airpump and airstone to bubble air into the fermenter. The only precaution you need to take, other than sanitizing the airstone and hose, is to be sure that the air going into the fermenter is not carrying any mold spores or dust-borne bacteria. To guard against contamination, a filter is used in-line to prevent airborne contamination from reaching the wort. One type is a sterile medical syringe filter and these can be purchased at hospital pharmacies or a your local brewshop. An alternative, build-it-yourself bacterial filter is a tube filled with moist cotton balls. The cotton should be changed after each use.

Aeration is Good, Oxidation is Bad
The yeast is the most significant factor in determining the quality of a fermentation. Oxygen can be the most significant factor in determining the quality of the yeast. Oxygen is both your friend and your enemy. It is important to understand when which is which.

You should not aerate when the wort is hot, or even warm. Aeration of hot wort will cause the oxygen to chemically bind to various wort compounds. Over time, these compounds will break down, freeing atomic oxygen back into the beer where it can oxidize the alcohols and hop compounds producing off-flavors and aromas like wet cardboard or sherry-like flavors. The generally accepted temperature cutoff for preventing hot wort oxidation is 80°F.

Oxidation of your wort can happen in several ways. The first is by splashing or aerating the wort while it is hot. Other beginning-brewing books advocate pouring the hot wort after the boil into cold water in the fermenter to cool it and add oxygen for the yeast. Unfortunately the wort may still be hot enough to oxidize when it picks up oxygen from the splashing. Pouring it down the side of the bucket to minimize splashing doesn't really help either since this increases the surface area of the wort exposed to the air. Thus it is important to cool the wort rapidly to below 80°F to prevent oxidation, and then aerate it to provide the dissolved oxygen that the yeast need. Cooling rapidly between 90 and 140°F is important because this temperature region is ideal for bacterial growth to establish itself in the wort.

In addition, if oxygen is introduced after primary fermentation has started, it may cause the yeast to produce more of the early fermentation byproducts, like diacetyl. However, some strains of yeast respond very well to "open" fermentations (where the fermenter is open to the air) without producing off-flavors. But even for those yeast strains, aeration or even exposure to oxygen after fermentation is complete can lead to staling of the beer.

To summarize, you want to pitch a sufficient amount of healthy yeast, preferably grown in a starter that matches your intended fermentation conditions. You want to cool the wort to fermentation temperature and then aerate the wort to provide the oxygen that the yeast need to grow and reproduce. Then you want to protect the wash from oxygen once the fermentation is complete to prevent oxidation and staling.

 

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