how to cultivate the beer yeast?

Since you can buy beer illegally, buy a beer you like. It will still have some active yeast in the bottle. Then get a stir plate, add corn sugar, and cover the top with aluminum foil - but leave it open enough to allow air in around the edges. You can also heat up the bread yeast you have to inactivate it and add it to this solution as a nutrient booster. Every 18 hours the yeast will have fully metabolized the sugar - you can repeat this a couple of times to build up a population. Temperature here can be higher - up to 27 degrees C. You will need to decant the solution after you are finished. This can be done by sticking it in the fridge, letting the yeast settle then pouring off the liquid.

See joe92's reply to my answer here for an easy stir plate build guide - Alternative to stir plate for yeast starter (and erlenmeyer flask)

When you're done with fermentation, a bunch of yeast you can reuse will be left at the bottom. You can pitch fresh fermentables right on top of that cake. Just know that every part of this process - the grains/malts/sugars/temperatures the yeast is exposed to will eventually change the qualities of the yeast. So, if you want to keep a strain with consistent properties you will have to restart this process with the same beer from the beginning.

A hugely significant component of beer flavour is from the yeast. It's important. But ... are you sure the undesirable flavours you encountered were from the yeast you used?

You really should use brewing yeast. Obviously you can buy yeast, or possibly grow up a small amount from a bottle of beer that has not been pasteurised.

For a long time bread yeast and brewing yeast were the same thing。It was not until 1883 when Emil Hansen isolated a single strain of yeast that the concept of "pure yeast brewing" came into beer production.

So if you cannot get beer yeast at all, using a bread making yeast would be the next best alternative. Most wild-captured yeasts perform much like a saison-type yeast, and have relatively low alcohol tolerance. Also it's difficult to remove other things from your microbial culture, like bacteria - which may add a sour note to your beer flavour.

Yeast produces the bulk of the flavour during the initial growth phase. So you can increase the flavour by using less yeast, or decrease the flavour by pitching more.

Thus If you cannot get brewing yeast at all, I recommend you get a good quality bread yeast. Make a few batches with differing amounts, maybe starting around 0.6 grams per litre (i.e. 12g for 20 litres).

There is a great book called "Yeast" by Chris White & Jamil Zainasheff. It can tell you all you need to know about growing yeast.

In addition to this, it's very important to ensure all aspects of your brewing process are sanitary and your fermentation process is best-practice. It may be worth reading of a few brewing sites.

You do not need beer yeast to make beer. If you are using bread yeast try to keep the temperature down.

But. If you are trying to recreate a given style then having the correct yeast is very important. A large amount of the flavour of the beer comes from the yeast you use. Also, a large amount of how well the yeast makes the flavours is from how warm the yeast gets. If you can keep the temperature of your brew down at 18-20C you should get a cleaner flavoured brew.

Also, have you considered trying a Lambic style brew and just using your local micro organisms, leave some wort to cool on a window sill by a open window, with a thin muslin cloth on top to stop flies getting in.

Saied, experimenting with yeasts and microorganisms is fun, as long as you do prior research. The other three answers have great points and I would just like to elaborate more on what they have suggested. Bread yeast is a starting point as it is readily available and you get well cultivated strains. I think the next step is to use microorganisms that occur naturally. For example chickpeas have Lactobacillus brevis, Lactococcus lactis and Lactobacillus plantarum. These bacteria will turn milk to yoghurt, corn to silaged animal feed and corn syrup to alcohol. Likewise many fruits have microorganisms on their skin which will ferment the sugars of the fruit, like grapes, fresh date plums (not the ones that are dried to eat, but the ones that resemble tomato), figs, etc. Under ordinary circumstances (if you don't have a lab suited for this purpose) it may not be easy to cultivate these organisms from fresh fruits or vegetables. For example developing yoghurt culture from chickpeas requires 6 generations (you need to cultivate it 6 times, filtering & feeding each time) before being eligible to be used. Most likely you would be able to utilize microorganisms from the skin of these fruits to further develop your bread yeast. A common approach used in Turkey is to use morning dew of plants/fruits and develop the strain in milk or filtered fruit juice. But be careful to use sanitized equipment as you wouldn't want to contaminate it with household bacteria. And definitely do research, because some organisms will inhibit each other, they would require certain pH values of the medium, temperature, etc. Good luck!

You can make decent beer using bread yeast, and certainly stuff that you'll enjoy drinking. You might have a problem trying to match a particular style of beer with bread yeast, but you can easily make beer, wine and mead with bread yeast because bread, ale, and lager yeast are all Saccharomyces cerevisiae. They all eat the same kind of sugars, including maltose. Their differences are are more subtle in how they react to their environment (e.g., temperature, pH), what byproducts they produce, and how well they flocculate. You'll have a hard time making a classic German hefeweizen with bread yeast because you won't be able to produce its distinctive banana and clove taste, but you can make a tasty wheat beer, or you'd have trouble making a crisp and clear lager because bread yeast (in my experience) doesn't flocculate as well.

As part of a science fair project with my son, we went the other way and made bread using different kinds of yeasts. We used bread, ale, lager, two different wine yeasts, and a strain of Brettanomyces bruxellensis. We made a basic white loaf out of all of them and gave out samples for taste tests. They all made decent bread, but (I think) the lager and the brett did just as well as the bread yeast. Personally, my least preferred was the wine/champagne yeast EC-1118; there wasn't much yeasty flavor and it was pretty bland (though others picked it as their preferred).

Well, first of all check your local legislation to make sure you're not in trouble with the law for making alcohol at home :)

Second, yes, to make e.g. beer of the taste you might be familiar with (e.g. english ale or european lager) you need a cultured yeast. While liquid yeast should be most likely too difficult to find, you could order dry yeast via internet (again, subject to legal restrictions). Google for something like "fermentis dry yeast" or "mangrove jacks dry yeast".

If you definitely can't get your hands on real yeast.. well, if I had such an issue, I'd try to cultivate something. After all, yeast are ubiquitous and exist everywhere, you just need to capture and tame the right kind. One option might be from wild yeast from grapes/fruits. To avoid conditioning of yeast on fructose (brewing yeast should be able to eat maltose), I'd just wash freshly picked grapes/apples with boiled lukewarm water and then add that wash into sanitary malt extract. In a rural environment, you might try and put out plates with malt extract solution overnight, collect them next morning, then incubate and check what taste you get within a couple of weeks. 1 from 5 chance you get something suitable, so you need to keep trying. Google "spontanous fermentation" for more details. Good luck!