When you strain it, do you save the water you've strained? Do you save the solids? Is this the "starter" you later refer to?
You save the liquid portion and yes it’s called a starter. Just remember when adding it to boiling water make sure you are constantly stirring it and adding it slowly otherwise you will end up with lumps instead of nice consistency.
Is it the equivalent of yeast?
No it’s not. Apart from its use as a beverage, kwas is often used as a base or additive for soups and stews in East European cuisine especially in Russian and Ukrainian dishes. Although non-alcoholic It contains anywhere from 0.7% - 2.2% of alcohol. This drink is very popular in the Eastern Europe, in countries like Russia, Ukrainian, Belarus, Lithuania. Also available in eastern part of Poland. Go to any local ethnic deli like Russian, Ukrainian and ask for: Russian, Belarusian, Serbian and Ukrainian: “квас” (kvas) in Polish it’s “kwas chlebowy” (lit. "bread leaven"), “kwas” on its own means acid. In Lithuanian “gira” and Estonian “kali”. I’m sure they will be more than happy to assist you and recommend the product. Lithuanian and Ukrainian brands are good but I’m sure the other ones are just as good.
Here’s a little tip on what to look for.
Kwas chlebowy is made by the natural fermentation of bread made from whole grain bread like wheat, rye and flavored with fruit, berries, raisins. Homemade “kwas chlebowy” most often uses dark or rye bread, dried and baked into croutons or fried with an addition of sugar, fruit and with yeast culture. Unlike mass-produced varieties, the home made one has no preservatives and spoils within relatively short period of time usually up to a week or so. Commercial one the cheap brands are made just like any other soft drink product, using sugar, carbonated water, malt extract, and flavoring. Better brands are those made by beer rather than soft drink manufacturers, usually they use a variation of the traditional process to brew their products. For cooking purpose look for a product made by beer manufacturer with as little flavoring ingredients added in as possible. Kwas chlebowy is high in vitamin B content and commonly served unfiltered, with the yeast still in it.
If you’re really hard pressed and there’s nothing available where you live you can make your own here’s a basic recipe for you.
To make your own home made Kwas Chlebowy
25 dag of whole grain bread (preferably dark variety)
2.5 L of water
20 dag of sugar
1 dag of yeast (the one used in wine and beer making process not the one for baking so be careful here) Cooper's Ale Yeast is fine.
Raisins, lemon peels (optional if you are making drink version for extra flavor)
Crumble the bread, dry it in the oven till it's slightly toasted and dry. When the bread is dried out lay it at the bottom of a pot and pour boiling water over it, place it aside for 24h. After 24 hours has passed strain it through a fine sieve (keeping the liquid portion) add sugar, and yeast that had been dissolved in small amount of warm water, blend it thoroughly and set it aside in a worm environment. When reaction takes place carefully remove the froth. When the reaction finally subsides remove the remnants of the froth and it’s ready for bottling. For cooking purpose use it as it is with the setlement on the bottom of the container.
If you decide to bottle some you can reuse the bottles from Grolsch for this purpose, (the ones with the cork attached to them not the bottle cap type). If you’re making it to be consumed as a drink place the lemon peel and couple of raisins in the bottle to give it extra flavor, before pouring in the liquid. Close the bottles tightly and keep it in a warm environment for one more day. Then store it in a dark cool place for later consumption. If properly bottled and stored in a suitable environment it will keep for approximately 4 weeks, if not the bottles can explode. If you decide to experiment with making your own kwas keep it away from other food products that could be contaminated with glass just in case the accidents happen. Anything that’s stored longer than 5 days should be checked if it’s not spoiled before consumption. If you decide to drink it, watch out it’s an acquired taste but according to some folks it’s good for stomach ailments and used as a home medicine for this purpose.
Recent scientific studies have proven that people who regularly consume kwas chlebowy are 3 time less likely to catch a flu. It’s very beneficial for proper digestion and speeds up the metabolism. It contains enzymes which regulate the bacterial flora in human digestive system.
YOU SHOULD NOT DRINK THIS PRODUCT IF YOUR BODY IS UNABLE TO TOLERATE YEAST.