Winemaking involves some simple steps. In general these amount to:
extracting the juice of certain fruits (for REAL wine this has to be grape)
adding water and sugar (the more sugar - the more alcohol - up to a point!)
(to begin the fermentation)
Allowing it to settle, then racking off (winemakers talk for siphoning the clear liquid off the settled yeast)
Racking again, then bottling.
Initially, the fermentation is begun in an open bucket. The yeast
begins to grow if oxygen is present. This is the aerobic phase of yeast reproduction. During this aerobic phase, the yeast will grow, carbon dioxide gas will be generated, but not much alcohol will be produced. After a week or so of "fermentation on the pulp"
the pleasant fruity flavours are extracted into the must. The wine is then transferred to a demijohn - and the oxygen supply is shut off with an airlock. This is crucial: it causes the yeast to switch to a less efficient anaerobic form of respiration. Now,
alcohol is produced. Another good reason to employ an airlock is that it gives a pretty good indication when the fermentation is over since the number of bubbles produced falls to an imperceptible minimum. It is worth stating that fermentation only allows
up to 18% alcohol production. The reason is that as alcohol builds up in the vessel, it begins to poison the yeast. In effect, it renders it useless. Table wine is generally about 12 - 13% alcohol. Some yeasts can tolerate the higher levels of 18% but you
need to get these from a specialist winemaking supplier.
In fact, you should only use proper wine yeast for all fermentations - unless you want off flavours in your wine!
In order to get wine of quite high alcohol level, it is important not to
add all the sugar at once. Rather the wine is "fed" the sugar at intervals as the fermentation proceeds. This allows the yeast to become aclimatised to the conditions - it stops the organisms being overwhelmed by (to them) toxic ingredients.
Now a word
about cleanliness: since the process involves microorganisms (yeast) we want to ensure we only have these present - and no bacteria or wild yeasts! For this reason it is essential that you sterilise all equipment by washing it in a weak solution of
sodium metabisulfite. You can buy this at all good winemaking stores. Generally, about a level teapoonful in a pint of water will be OK. The solution releases sulfur dioxide which kills bacteria and unwanted - so called - wild yeasts. Sometimes, the fruit
itelf is sterilised with this material. It doesn't matter if a little is left on the fruit - your wine yeast should be able to cope, provided it's only a small quantity. Sometimes a recipe calls for boiling water to be added. This, of course, also sterilises
the fruit. A word about the consequences of poor hygiene and sloppy practice: your wine could easily develop off-taints. Have a look at this link to see what can happen:
A word about Campden tablets. These are just a compressed form of sodium metabisulfite. Some have a little starch added as a binder. All Campden tablets are useful if
you want to add a small measure of sodium metabisulfite to your wine. Sometimes this is done deliberately to ensure fermentation has well and truly stopped prior to bottling. It helps prevent burst bottles if the bottling is done too enthusiastically! Also,
since sulfur dioxide is a reducing agent, it can help prevent oxidised flavours occuring in your wine. Be careful not to overdo the Campden tablets though - it is possible to get quite nasty sulfur compounds generated - which smell truly awful!