Free Bread Matters by Andrew Whitley Page B

page 126), a relatively small amount of initial yeast will reproduce itself until there are enough cells to provide the fermentative activity that aerates the dough during the final proof stage.
    Apart from food in the form of sugars, yeast needs water and warmth. The optimal temperature for yeast fermentation is 25-28°C. At this temperature, water feels slightly warm to the hand. Above about 46°C and below about 10°C yeast works very slowly. It is killed at around 60°C. It follows that dough temperature and fermentation speed are closely connected – a fact that can be used both to produce better bread and to fit the whole process into the life of the home baker. It also follows that you may need to adjust yeast quantity and dough temperature to reflect the changing seasons.
    Fresh yeast
    This should be stored at around 5°C – the temperature of a domestic refrigerator on a fairly high setting – and used within a couple of weeks. Its gassing power progressively reduces as it gets older. If you store fresh yeast at 15°C for two weeks, it will take twice as long to prove your bread. As it ages, fresh yeast becomes crumbly and may form dark patches on its surface. It will still work in this condition, but it may lack gassing power and may also introduce by-products into the dough that have a negative effect on the strength of the gluten network 5 . As a last resort, fresh yeast can be frozen, but it turns to a runny mess on defrosting and loses a significant amount of its efficacy. Yeast is best stored in its dried form.
    Dried yeast
    There are two kinds of dried yeast – the traditional ‘active dried’ yeast, in the form of small granules, and ‘instant’ or ‘fast-action’ yeast, which looks more like fine iron filings. The latter may come already mixed with additives such as vitamin C and ‘rehydration agent’. Quite why this form of yeast needs something to help it get wet (other than water) is doomed to remain a mystery for as long as the manufacturers follow that irritating food industry habit of adding ingredients with opaque or meaningless descriptions and no indication of any valid reason for their inclusion. Presumably it is a belt-and-braces measure to ensure that this kind of yeast lives up to its ‘instant’ name, enabling it to be chucked into the dough mix without prior dispersal in water. In fact, both fresh and traditional granular dried yeast could also be added straight to the dough mix.
    Fast-action yeast is more concentrated than traditional dried yeast and both are, weight for weight, more concentrated than fresh yeast. The recipes in this book use fresh yeast but will work equally well with either of the dried versions. The rule of thumb when converting a recipe for the kind of yeast you have available is:
    10g fresh yeast = 5g traditional ‘active dried’ yeast = 3g ‘fast-action’ yeast
    I prefer to dissolve all yeast, including the ‘fast-action’ variety, in water prior to mixing, simply to ensure its complete dispersal throughout the dough. If granular dried yeast is added without being dissolved, some of it may fail to disperse if you don’t knead thoroughly, and can show up as little brown specks in a dough that may not rise as well as it should. A further reason for dissolving dried yeast in water before adding it to the flour is as a check on its efficacy. If it has been hanging around in a not-very-airtight tub or opened sachet for a while, it may not be in good condition. Sprinkled into some warm water (30°C), or mixed with a little flour and water into a sponge or ‘ferment’, it should start to work in a few minutes. If it doesn’t show any signs of life after half an hour, you should consider it moribund and start again with a new source; at least you won’t have wasted a whole dough and all the effort expended in kneading it.
    Organic yeast
It may come as something of a surprise to discover that the yeast used in organic bread is not necessarily organic.