A food-chemistry and agronomy lab. Watch a season of heat build up on a terraced vineyard: sugar (degrees Brix) climbs while acid falls. Choose when to harvest, press the grapes, then follow the yeast in a cutaway tank as sugar turns into ethanol and carbon dioxide and releases heat. Drag to orbit, pinch or wheel to zoom. Educational only: no recipe, and nothing here encourages drinking.
Each check compares the simulation with a second method: closed-form integration of the temperature curve; the atomic-mass stoichiometry of C6H12O6; carbon balance; the adiabatic heat equation; and a 4x finer time step.
Growing degree days. Vines grow only when it is warm enough. The heat accumulated since budbreak is GDD = ∫ max(0, T − 10) dt, with 10 °C as the base temperature, in practice summed daily from the daily mean temperature. For the smooth yearly temperature used here, T(t) = M + A cos(ω(t − 200)) with ω = 2π/365, the integral has a closed form: with θ = arccos((10 − M)/A) the warm window is |ω(t−200)| < θ and GDD = (M−10) t + (A/ω) sin(ω(t−200)) evaluated over that window. Weather in the game adds a random day-to-day wobble and a rain calendar; the closed form is the noise-free benchmark (check 1).
Sugar. Sugar is measured in degrees Brix (grams of dissolved solids per 100 g juice). It stays low (about 5) while berries are green, then after veraison (colour change, at a ripening-group-specific GDD) rises along an S-curve Brix(G) = 5 + (Bmax − 5) / (1 + e−(G − Gm)/70). The ceiling falls as crop load rises, Bmax = 28.5 − 0.65 (load − 6), because a heavy crop shares the same leaf area. These thresholds and the logistic shape are illustrative, not measured for a particular variety.
Acid. Titratable acidity (as tartaric acid, g/L) starts near 28 and falls after veraison because berries burn malic acid, faster when warm: TA = 4 + 24 exp(−0.035 Σ 2(T−20)/10) (a Q10 of 2). A warm year therefore gives sweet but flat fruit; a cool year gives sharp fruit that may never get sweet. pH is approximated by 4.0 − 0.042 TA. The “balance score” is only the distance to the Brix and acid of the target you pick, reduced by the rot percent; it is a teaching device, not a quality standard. Rot grows on rainy days once berries are sweet, more in autumn.
Sugar to ethanol. Yeast turn glucose into ethanol and CO2: C6H12O6 → 2 C2H5OH + 2 CO2. From atomic masses, 2 × 46.07 / 180.16 = 0.5114 g ethanol per g sugar and 0.4886 g CO2. In real tanks about 5 % of the sugar goes into yeast cells, glycerol and by-products, so the simulation uses an efficiency η = 0.95 (practical yield about 0.486). Must sugar is taken as 93 % of the Brix solids, so sugar (g/L) = 10 · Brix · SG · 0.93. Predicted alcohol by volume: ABV = 100 · (0.5114 η S0) / 789.4, close to the rule of thumb ABV ≈ 0.55–0.6 × Brix.
Kinetics. Yeast biomass X grows at μ = μmax f(T) S/(Ks+S) (1 − (E/Emax)2) after a lag, only while assimilable nitrogen remains (10 g cells per g nitrogen), and takes up sugar at q = qmax f(T) S/(Ks+S) (1 − (E/Emax)2) X V. The temperature factor is Arrhenius (Ea = 50 kJ/mol, so rate about doubles per 10 °C near 25 °C) with a sharp thermal cut-off near 37 °C; viability V falls with heat and with ethanol. Ethanol tolerance drops with temperature (about 140 g/L at 20 °C). So: cold means slow, hot means fast but risky, low nitrogen means little yeast and a sluggish tail, very sweet must may stop before it is dry.
Heat. Fermenting sugar releases about 586 kJ per kg (about 1.3 °C per degree Brix with no cooling), ΔT = 586 ΔS / (ρcp) with ρcp = 4104 kJ m−3 K−1. The jacket removes min(0.30, 0.3 (T − Tset)) kW/m³, and a small loss goes to the 16 °C cellar. Switch cooling off and start warm to watch a runaway.
Simplifications. Smooth sinusoidal climate plus random weather; no frost, drought, pests or canopy management; one logistic ripening curve; no water uptake or dehydration of berries; the press is a visual step (no juice-yield model, no skin contact or tannin); fermentation is one well-mixed 1 m³ batch with a single yeast population and no malolactic step, no volatile aromas, no oxygen. All constants are round numbers chosen to be realistic in order of magnitude, not fitted to a variety or a cellar. The sim is a teaching aid, not a production guide, and the product is for adults only. Not affiliated with any estate or brand.