Rye bread flour is a product of rye grain milling, an essential ingredient for traditional rye and rye-wheat bread. Russia is the world's largest producer of rye (over 2 million tons/year). Flour grades: Wholemeal (ash content ≤1.45%) — the main industrial grade for Borodinsky, Darnitsky, Rizhsky, Minsky bread; greyish-white with a creamy tint. Sifted (ash content ≤0.75%) — fine grind, white, for light rye bread, flatbreads, gingerbread, bagels. Wholegrain (ash content ≤2.0%) — coarse grind from whole grain with maximum fiber. Rye flour does not form elastic gluten (pentosans and water-soluble proteins form a viscous structure), so dough is prepared with sourdough starters — lactic acid fermentation acidifies the medium, stabilizes starch, and gives the bread its characteristic sour taste and aroma. GOST 7045-2017. Moisture ≤15%. Falling number ≥150 s for wholemeal, ≥160 s for sifted. Packaging: bags 25–50 kg, packages 1–2 kg. Sourdough for rye bread is a culture of lactic acid bacteria and yeast, maintained continuously (refresh every 4–8 hours). Dough acidity (7–12° Neumann) stabilizes starch and prevents crumb collapse. Rye bread stays fresh longer (3–5 days vs 1–2 days for wheat) due to increased acidity and pentosan content.