Primary Fermentation
During primary (alcoholic) fermentation, yeast — either wild or cultured — metabolises sugars over 1 to 4 weeks. Temperature control is critical: whites typically ferment at 12-18°C to preserve delicate aromas, while reds ferment warmer at 25-30°C to aid extraction.
Wild vs. Cultured Yeast
- Cultured yeast — commercial strains selected for reliability, predictable flavour profiles, and alcohol tolerance
- Wild (indigenous) yeast — naturally present on grape skins and in the cellar; prized by natural winemakers for complexity but riskier
Fermentation Vessels
Wine can ferment in stainless steel tanks, oak barrels, concrete eggs, clay amphorae, or open-top vats. Each vessel imparts different characteristics — oak adds vanilla and spice, concrete maintains purity, and amphora enhances texture.
Stuck Fermentation
When yeast dies prematurely (from heat, nutrient deficiency, or high alcohol), fermentation stops before all sugar is consumed. This "stuck" fermentation can leave unwanted residual sugar and is a winemaker's nightmare.