In the days before yeast was 'discovered' and produced, bakers had to depend on wild yeasts - floating in the air - to leaven their bread. What is it? Living organisms that, when exposed to moisture, food and warmth, produce carbon dioxide. What's it used for? Making bread doughs rise, giving alcohol to wine and bubbles and alcohol to beer and champagne. It's also an ingredient in Marmite and Vegemite. What are the differences? There are hundreds of types of yeast cultures, but for home use, we'll concern ourselves only with the types used for leavening dough. Professionals prefer cake yeast, also called fresh or compressed yeast, which is moist and needs to be refrigerated (or frozen for longer storage). It's said to give better flavour to breads but it goes mouldy quickly. Dry yeast keeps longer and is easier to find. Active dry yeast needs to be dissolved in liquid, while instant yeast can be mixed with the flour and other dry ingredients before being activated with liquid. Wild yeasts are unreliable but certain types (such as Lactobacillus sanfranciscensis, which is indigenous to San Francisco, California) give a unique flavour to breads. What else? Breads can also be leavened with 'sour' or 'starter' (a batter-like mixture of yeast, flour and water) or 'chef' (a piece of dough taken from the previous day's bread and incorporated into the new batch), which are often made initially with wild yeast. The sour, which is fed with flour and water on a regular basis, and the chef, which is reserved from a batch of dough each time a new one is made, gain flavour over time. Their leavening power is sometimes supplemented with commercial yeast. Yeast is killed if the liquid used to dissolve it is too hot. Use liquid from 35 to 45 degrees Celsius. How to use: dry yeast has approximately double the power of cake yeast, so if your recipe calls for one type and you have only the other, make the necessary adjustments. If you want to attempt to capture wild yeasts, combine equal quantities of flour, water and rinsed, crushed, organic grapes. Leave for several days in a clean bowl that's been covered with cheesecloth (which keeps out insects but allows the mixture to breathe). If it starts to ferment and give off a good, yeasty smell, you've been successful; if it gives off a sickly odour and turns green, throw it away. Store the sour in an air-tight jar in the fridge and refresh several times a week by adding more flour and warm water. The sour will die if it isn't fed frequently enough.