Stop shivering in fear: it is not Halloween witches stirring up cauldrons of warm, seething and frothing grape juice - it's yeast. Yeast cells are so small it took thousands of years before humans could figure out how fermentation works. Until Louis Pasteur's experiments in 1857, fermentation was considered a mysterious act by spooky spirits or a gift from ancient Gods, which is why wine has long figured prominently in religion, most especially in Judeo-Christian creeds. Fermentation is a simple process: hungry yeasts gorge on grape-juice sugars converting them to alcohol, heat and bubbles. Fermentation bubbles are carbon-dioxide gas, the same tongue-tingler found in carbonated soft drinks. Although some winemakers, such as those in France's Champagne region, trap these bubble in the bottle to create sparkling wine, most winemakers let the gases escape. Carbon-dioxide gas is heavier than oxygen, so after rising to the top of the tank, it quietly lies on top of the juice, like a scarecrow reclined on a blanket, to ward off harmful oxygen. The energy, or heat, produced by the yeasts directly affects the final character of the wine, so winemakers carefully watch and often control the fermentation temperature. It is especially important when it comes to white wine because if fermentation temperatures creep up, a white wine's delicate floral and fruit nuances are destroyed. With red wine, excessively warm fermentations result in jammy, almost cooked flavours. On the other hand, when fermentation temperatures are as chilly as a witch's toe, wines can develop a confectionery character and banana-like aromas. Temperature control, a relatively recent development in the wine industry, has greatly improved wine quality around the world. Previously, natural cellar temperatures or climate dictated fermentation temperatures, especially in regions where tanks were stored outdoors. Alcohol levels are determined by the amount of natural sweetness in the grape juice. The higher the sugar levels, the higher the resulting alcohol levels. Alcohol helps to form the skeleton of wine, the other bone support being provided by tart acids and, in the case of red wines, tannins. Most winemakers rely on the grape juice's natural sweetness levels, but some cooler regions like to spill a few spoonfuls of sugar into the juice - not to make the wine sweeter, but to raise the final alcohol level. Higher alcohol levels create the illusion of a fuller body or weightier wine, but they also have a role to play during fermentation. Alcohol, combined with fermentation heat, has a vampire affect, sucking red colour and flavour from the grape skins, but benignly infusing them into the wine. Too much alcohol, however, and the wine balance topples over like a tipsy witch on a broom. But let's face it, without any alcohol, wine would be ghastly.