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1
Slice the rye bread and toast it in a dry oven or under a grill until it reaches the colour of deep mahogany, approaching but not crossing the line of burning. The Maillard compounds released at this stage are the structural flavour of the kvass; pale toast produces pale, flat results. The cue is colour: the darkest brown the bread will take without carbonising.
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2
Place the toasted bread in a clean container. Pour the hot water over it and add the sugar. Stir to dissolve the sugar. Allow the steep to stand until the liquid is a deep reddish-brown and carries a roasted, bread-like aroma — this is the bread tea that fermentation will transform.
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3
Once the liquid has cooled to room temperature, leave the container loosely covered at room temperature. Wild yeast present on the bread and in the environment will inoculate the steep without any added starter. Fermentation is under way when small bubbles form at the surface and a faintly sour tang is detectable.
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4
Ferment for 1–3 days, tasting daily. The kvass is ready when it tastes pleasantly sour and carries a light effervescence on the tongue. Over-fermentation produces an aggressively sour, alcoholic result unsuited to drinking; do not allow it to proceed beyond this point.
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5
Strain the liquid through a fine sieve or cloth, discarding the spent bread. Transfer to a sealed bottle or jar and refrigerate immediately. The cold halts active fermentation and holds the kvass at the desired flavour stage.