How To Choose Your Yeast by Dr. Keith Thomas, of Brewlab

Sadly, the most readily available source of yeasts is from packets containing dried powder or granules. Although providing some consistency these are often baking yeasts or brewing strains with limited character.

Today, many homebrewers are looking for more.

For a wider range of options many homebrewers look towards the sediment in bottle conditioned samples. Here, at least, are distinctive varieties. The beers come from different breweries, they can be chosen according to the beer style and matched for the intended homebrew. Above all, they are available off the shelf, either in the off licence round the corner or in your nearest supermarket. However, take care in your selection. Not all bottle conditioned yeasts guarantee a quality beer. Just because a bottle comes from a well known brewery doesn't mean your beer will taste the same. Just because the beer tastes clean in your glass doesn't certify purity in your own brew.

Bottle conditioned beers are a valuable and exciting resource of yeasts for the homebrewer but beware the following:-

Not all bottles with sediment contain live yeasts. Some breweries allow a bottle conditioned beer to mature in the brewery but pasteurise the bottles in an effort to extend shelf life.

Not all bottles contain a primary fermenting yeast. Some breweries filter out their brewing yeast and re-inoculate with a different yeast, often a lager strain, more suited to sedimenting in a bottle. Such yeasts may ferment slowly, are likely to settle rapidly during the brew and form little head. They are also likely to produce atypical flavours.

Many yeast cells may have died in the bottle. Yeasts are mortal after all and easily die if mistreated. Even age takes it's toll, resulting in low viabilities after time, particularly in high gravity beers. Too low a viability and your culture may take so long to grow that you loose interest. Wild yeasts and bacteria may grow instead, giving a totally undesirable result. Chose a fresh bottle where possible.

Many bottles may be contaminated. No brewery is perfect and bacteria creeps in at all stages. In some cases these may be part of the brewery's character but held in check by careful handling. In your homebrew these may grow without restraint particularly if the bottle yeast takes an extended time to recover and multiply.

Of course, you may be lucky and select a bottle which avoids all of these potential problems and has pure brewing yeast of hips viability. However, a recent technical survey of 31 bottle conditioned beers showed that 14 were contaminated with bacteria, one was also contaminated with wild yeast, and 4 beers had no viable yeast cells so were not bottle conditioned at all!

Choosing a bottle off the shelf needs more than a degree of luck to succeed!

This isn't to say that homebrewers can't maximise their options. Using a microscope to view the yeast is the obvious way to check yeast cell numbers, viability and for contamination but is an option not open to most homebrewers. Using a starter bottle to culture up the yeast to pitching volume is essential. Smelling, and even tasting, the culture after growth will often indicate problems. A good, healthy growth of yeast is essential before pitching.

Bottle yeasts can yield a rich harvest for the homebrewer and are worth using if precautions are taken and expectations kept under control. When successful, results can be excellent but select your samples with care.

Summary:	Use bottles with a stated long shelf life, indicating freshness. Avoid bottles where the beer is cloudy. Look for a good, consistent sediment. Avoid bottles with a scum on the beer or a ring of dried matter inside the neck. Taste the beer and reject the yeast if any off-flavours are present.
Postscript:	You should all be aware of the Light-struck' effect when beer bottled in clear or green bottles is exposed to light. Some bottle conditioned beers are so bottled and there is a suggestion that the resultant off-flavour can be carried forward in the yeast. Ask for a bottle straight from the box in the storeroom rather than one off the shelf.