Yeastie Beasties!

The saying goes “brewers make wort, yeast make beer”. While strictly true, the wort we make and the conditions that we then present for the yeast will help consistently produce great beers.

Yeast Fermenting

Yeast is a single cell organism, classed in the fungi family. Over 1500 species of yeast have been identified but there are still many that are unknown. They exist in our natural environment but particular species have selected and islolated over many years of re cropping by breweries.

Brewers are usually interested in Saccharomyces cerevisiae (ale yeast), Saccharomyces pastorianus (lager yeast) and Brettanomyces (wild/brett yeast).

When mixed with sugary solutions, like wort, yeast will reproduce and consume the sugars. During this process they will create co2, ethanol (alcohol) and lots of other flavour compounds. Esters, fusel alcohols and diacetyl are the most common types referred to by brewers and these can give beer unique characters but can also cause problems if not appropriate for the beer style.

Yeast require suitable conditions to get the best out of them.  They will work in most conditions but this will have a huge effect on the flavour and quality of the beer. Quality/quantity of yeast pitched, oxygen levels, available nutrients and temperature are the most important factors. Having control over these and ensuring that these are favourable to produce the desired effect will help ensure consistent results.

The quantity of yeast you require will depend on the fermentation temperature and volume/gravity of the wort. If you are using liquid yeasts you will also have to make sure the there is enough viable cells available as the age of the pack will have a great impact. Dried yeasts are not as susceptible as they generally store better but make sure you rehydrate them before pitching. There are lots of calculators online and in brewing software packages which will help ensure you pitch the correct amount.

If you want really accurate pitching rates then a sample of the yeast should be put under a microscope so the number of viable cells / ml can be calculated and the pitch adjusted accordingly. Perhaps not so important for home brewers but this is essential for commercial brewers when reusing yeast.

Oxygen is another important factor for healthy fermentation and one thing that’s often overlooked by home brewers. Yeast need oxygen in the initial stages of fermentation so they can start reproducing. They ideally need around 8 – 10 ppm which is actually quite a lot and reasonably difficult to achieve without pumping pure oxygen directly into the wort. That said, vigorous shaking or stirring for 5 mins should get you in the realms of 4 -5 ppm which will suffice in home brewing environments.

Wort produced from predominately barley contains most of the nutrients that the yeast need although they can benefit from additional zinc so the specialist brewers nutrients like Wyeast nutrient or Whitelabs Servomyces can be useful.

Temperature will have a huge effect on the yeast. High temps will favour the production of the yeast and speed up the fermentation process but this will also lead to many more bi products and flavour compounds which may not be appropriate for the beer style. Most of the flavour compounds are produced at the beginning of fermentation so this is where temperature is most critical.

There are many different strains available and they are usually measured by in terms of flocculation and attenuation.

Flocculation is how easily the yeast will fall of suspension. This will effect how quickly and easily the beer clears as well as how much sugar it will ferment. Highly flocculating yeasts may require rousing during fermentation to get them back into suspension to avoid the fermentation not completing. It’s usually measured as low to high, the higher being the quicker clearing.

Attenuation is the amount the yeast will ferment the available sugars. It is usually measured as a percentage, 100% meaning that the yeast will ferment all the possible sugars to alcohol. Generally they range from 60 - 85%. Yeasts that have high attenuation figures generally have low flocculation and vice versa.

Main brands

Yeast available to home brewers usually come in two types of packaging: dried vacuum packed or fresh liquid. The dried yeasts are more stable which means they have a much better shelf life and are generally easier to use. The downside is that there is a much more limited range due to lots of strains not being suitable for the freeze drying process. They are also selected for their ability to be stable once packed and fast reliable fermentation rather than flavour. The main brands of dried yeast are Fermentis and Danstar, both producing a range of yeast strains. They come in ready to pitch 11g sachets which is enough yeast for a medium gravity 23 litre batch.

Fresh Liquid yeasts usually come in foil smack packs or vials. They have a huge range of strains available but they have a short shelf life and deteriorate quickly so require a yeast starter before pitching. This makes them more complicated to use but opens up a much wider range of beers styles which can be created using them. The two main brands of liquid yeast are Wyeast and whitelabs both made in the US. If you’ve not a tried liquid yeast then they are well worth the extra effort as they open a whole world of flavour opportunities.

For further in depth information on yeast check out the great “yeast – the practical guide to beer fermentation ” by Chris White, founder of Whitelabs.