Sterilise all tools, equipment and hands with food safe disinfectant or at least soap and hot water.
Slice and toast the bread in a dry pan or toaster until nice and dark. Don't just dry it in the oven. It's pointless.
Dissolve the sugar and honey in a small portion(1 L) of hot water before adding the rest of the water along with the molasses.
Place into a large bucket and add the yeast and toasted bread.
Let the kvass ferment at room temperature, away from direct sunlight. The fermentation process will be visible after 24 hours.
Let the kvass ferment for up to 4 days, or even longer if you like a dryer drink. You judge the readiness by tasting it daily. It's that simple.
Once you are happy with the flavour, you can optionally and recommended strain out the bread. Bottle it to carbonate the kvass. Preferably plastic bottles. This is more of a safety reason, and it's impossible to judge carbonation level in glass. You can also pop a few raisins and a strip or two of fresh ginger into the bottles before closing.
Let it ferment for another day or two at room temperature before chilling down in the fridge. Only consume when fully chilled.
Video
Notes
About ingredients
The spices in Borodinsky bread are a very important component to the overall flavour. If you can not find it or don't want to make it, then simply add a bit of toasted coriander seeds and caraway seeds to the kvass when you would have added the bread.
Molasses come in two different varieties normally. Blackstrap and regular. Blackstrap molasses is the one you want, although you could also use regular or even malt syrup to good effect.
Honey is used only to kickstart the fermentation process. If you are allergic or don't have it, then just skip it.
You can use fresh, dried or sourdough starter. If you want t0 use fresh, use 20 grams instead of 10 grams dried. If you want to use sourdough starter, then use 100 grams of starter. Although sourdough starter is the traditional way of making it, it also gives a slightly sour taste to the final product. Keep that in mind when choosing.
Only use natural sugar. It does not matter much which type, but "fake" sugars will not ferment, and it will also taste bad.
Storage instructionsAlways keep chilled in the fridge. Kvass is a live ferment and the cooler the temperature, the slower the fermentation and the longer you can store it. At room temp it will keep fermenting and eventually just be completely dry or the bottles will explode because of the pressure. It will keep well for up to 8 weeks in your fridge.SafetyBecause of the overpressure risk it's best to carbonate and store kvass or any other fermented drink for that matter in plastic bottles. This way you can judge the pressure build up and move it to the fridge on time or let out some gas.