When I spend the extra few quid on liquid yeast I like to think I get a little more for my money and hope that I coax the best from the yeast during the propagation stage. There is much debate about the merits of various forms of yeast and what they bring to a beer, but one thing is certain; you're wasting your money if you don't prepare a viable starter from a liquid yeast pack. Dry yeast is ready and eager to ferment once it has been re-hydrated because the producers of it have gone to much trouble to ensure that the yeast is dessicated in prime condition. Making a starter with dried yeast is a waste of time because the primary reason for making a starter is to ensure the yeast you pitch is fighting fit and ready to ferment, and dried yeast is already in this condition. Liquid yeast require more nurturing because they are not in stasis like dried yeast and deplete all their cellular reserves while hanging around in the packet.
A few simple pieces of equipment can aid in yeast propagation. The first of which is a Pyrex conical flask, and the second a magnetic plate stirrer.
To see why these pieces of equipment are useful in propagating yeast we must venture into a little yeast biochemistry and metabolism for an explanation. Yeast are wonderful survival machines with the ability to thrive in both aerobic and anaerobic environments. In an anaerobic environment such as brewer's wort yeast use the sugars to produce energy with ethanol produced as a glorious waste product. If however yeast find themselves in an aerobic environment they respire much like us producing carbon dioxide and water. With this is mind we can see that it is very important that the environment that yeast finds itself in during beer fermentation must be anaerobic otherwise we would have the horrifying situation of no ethanol production, vast amounts of yeast cells and a strange liquid that couldn't really be described as beer.
Thankfully, there is no way that brewers can accidentally aerate to such an extent that ethanol production is completely inhibited. However at the beginning of fermentation a certain degree of oxygenation is essential to set the yeast up for the work they have ahead. Before pitching, yeast is usually stored for a period during which it has to rely upon its own reserves to survive. These reserves are rapidly depleted during storage and yeast is in no position to effectively ferment beer when it is pitched. A ready supply of oxygen at the beginning of fermentation is necessary for the yeast to replenish itself and and produce sterols to make the cell wall permeable to the wort constituents. Also during this period there is an increase in the number of cells, as cell proliferation occurs at the expense of ethanol production in the aerobic environment. It is after this stage that things kick off and a vigorous fermentation ensues with lots of gas production accompanied by copious foam and ethanol.
With this little bit of information in mind the objective of yeast propagation can be laid out. Liquid yeast for home brewing is in a depleted state, ready and waiting for a chance to grow. The number of cells in the average yeast pouch is insufficient to pitch, so a propagation step is needed. This step is essentially the same as the first aerobic stage of fermentation and this is where the equipment comes into use. Sterile aerated wort is placed in a flask and the depleted yeast pouch is added to the wort where it is continually mixed on the magnetic stirrer. The aim of this exercise is to produce as many yeast cells as possible to meet the pitching rate required for the gravity of the beer, which is easily done in an aerobic environment where the yeast will proliferate rather than produce ethanol. The mixing plate keeps the cells in contact with the wort ensuring maximum use of the sugar available, and also mixes the oxygen making it available to the yeast. Studies suggest that greater cell counts can be achieved with this method and it is also a hell of a lot handier than having to shake the flask every time you walk by it.
Once you have managed to propagate some yeast the question of how much should be pitched arises. For most home brewers this is not an issue at all; they merely pitch whatever has grown and invariably enjoy a rapid and intense primary fermentation with satisfactory results. However if we were in a position to easily count the yeast in our starter we would like to achieve around 1 million cells per mililitre of wort per degree Plato. This translates as 1 billion cells per litre of wort per degree Plato. So if you have 20 litres of 10 Plato (1.040) wort you will need 200 billion cells to achieve generally recommended pitching rate. This figure is based on re-pitched yeast which is less viable than freshly propagated yeast, so a reduction of 25% in the pitching rate will still do the job.
The type of beer your plan to produce should be reflected in the amount of yeast pitched by favouring the upper or lower end of the generally accepted pitching rate. This alteration to the rate is necessary because the concentration of yeast in the wort influences the flavour profile of the beer through the production of metabolic by-products. High pitching rates produce fewer by products while the opposite is true for low pitching rates. It is these by-products that give beer a great deal of its flavour and why breweries carefully maintain their yeast culture and ensure uniform pitching rates for each fermentation. An alteration to the pitching rate will cause a different spectrum of yeast by-products and inconsistent flavour profiles from brew to brew.
This would suggest that English ales should be pitched at the lower end at between 0.25 - 0.75 billion cells per litre to produce fruity and yeasty characteristics, while the figure can be doubled for lagers to ensure a clean fermentation with little contribution from yeast by products. The fermentation temperature is a further factor to consider because yeast metabolism is accelerated at higher temperatures resulting in more flavour compounds, many of which are undesirable. Lagers are fermented cold for this reason, ensuring a clean profile from the high pitching rate and low fermentation temperature.
Pro brewers are all about consistency, their reputation often depends upon it, but home brewers rarely produce the same beer twice and so are spared the torment of worrying about exact pitching rates and fixed fermentation temperatures. Yeast counts are not really necessary in our nano breweries, but our success or otherwise at producing viable yeast from a starter is very important to the quality of the beer we brew.