Module 7

Fermentation & Wine Biochemistry

From sugar-laden grape must to bottled wine, fermentation is a biochemistry performance by Saccharomyces cerevisiae and (often) Oenococcus oeni. This module covers primary alcoholic fermentation, malolactic conversion, yeast-derived esters, SO₂ preservation, and the biochemistry of wine aging.

1. Alcoholic Fermentation

S. cerevisiae converts glucose and fructose to ethanol + CO₂via the Embden-Meyerhof-Parnas glycolytic pathway, with pyruvate decarboxylated by PDC1 and reduced by ADH1:

\[ \text{C}_6\text{H}_{12}\text{O}_6 \;\longrightarrow\; 2\,\text{C}_2\text{H}_5\text{OH} + 2\,\text{CO}_2 + 2\,\text{ATP} \]

Gay-Lussac stoichiometry: theoretical yield ~51% ethanol by mass. Actual conversion 46–48% because yeast diverts some carbon to biomass + glycerol. Typical wine fermentation takes 7–21 days at 15–28 °C; temperature affects aroma-compound retention. Commercial inoculation uses selected strains (EC-1118, D254, BDX) with defined fermentation kinetics and sulfur-compound profiles.

2. Aromatic Esters & Higher Alcohols

Yeast secondary metabolism produces fruity esters (ethyl acetate, isoamyl acetate, ethyl hexanoate) from acetyl-CoA + fusel alcohols. Wines from cooler ferments retain more volatile esters (Saerens 2010). Higher alcohols (isoamyl, isobutanol, phenylethanol) give floral character; thiols (3-sulfanylhexan-1-ol in Sauvignon Blanc) contribute to varietal aroma.

Simulation: Primary & Malolactic Kinetics

Python

script.py44 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Primary fermentation kinetics
days = np.linspace(0, 21, 400)
sugar = 250 * np.exp(-days/3)
ethanol = 14 * (1 - np.exp(-days/3))
temp = 18 + 10*np.exp(-((days-3)/1.5)**2)

# Malolactic fermentation (Oenococcus oeni) after primary
MLF_days = np.linspace(21, 60, 200)
malate = 3.0 * np.exp(-(MLF_days-21)/8)
lactate = 2.0 * (1 - np.exp(-(MLF_days-21)/8))

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(14, 5), facecolor='#0a0a1a')
for ax in (ax1, ax2):
    ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
    for s in ax.spines.values(): s.set_color('#334155')
    ax.grid(True, color='#334155', alpha=0.3)

ax1.plot(days, sugar, color='#fbbf24', lw=2.5, label='Sugar (g/L)')
ax1.plot(days, ethanol*10, color='#f472b6', lw=2.5, label='Ethanol x10 (% v/v)')
ax1b = ax1.twinx()
ax1b.plot(days, temp, color='#dc2626', lw=2, ls='--', label='Must T (C)')
ax1.set_xlabel('Days from inoculation', color='#cbd5e1')
ax1.set_ylabel('Concentration (g/L or x10 %)', color='#cbd5e1')
ax1.set_title('Primary fermentation (S. cerevisiae)',
              color='#e9d5ff', fontweight='bold')
ax1.legend(loc='upper right', facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
ax1b.legend(loc='center right', facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

ax2.plot(MLF_days, malate, color='#fbbf24', lw=2.5, label='Malate (g/L)')
ax2.plot(MLF_days, lactate, color='#a78bfa', lw=2.5, label='Lactate (g/L)')
ax2.set_xlabel('Day', color='#cbd5e1')
ax2.set_ylabel('Concentration (g/L)', color='#cbd5e1')
ax2.set_title('Malolactic fermentation (Oenococcus oeni)',
              color='#e9d5ff', fontweight='bold')
ax2.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Ethanol yield ~51% of sugar mass (theoretical max by Gay-Lussac stoichiometry)')
print('Malolactic fermentation: malate -> lactate + CO2, softens acidity')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Malolactic Fermentation

Oenococcus oeni (lactic acid bacteria) converts malic acid (diacid) to lactic acid (monoacid) + CO₂, reducing total acidity and softening mouthfeel. MLF is almost universal in red wines and optional in whites. Induction is controlled by temperature, pH, and SO₂ levels. Diacetyl production during MLF contributes a buttery aroma, especially in Chardonnay.

4. SO₂ Preservation & Aging

Sulfur dioxide binds acetaldehyde, anthocyanins, and keto sugars. Free SO₂ (molecular + bisulfite) is antimicrobial; bound SO₂is inert. Target free SO₂ 20–40 mg/L in red wines, higher in whites. Aging reactions include polymerisation of phenolics (anthocyanin → polymeric pigments), ester hydrolysis, and oxygen-mediated tannin modification. Bottle aging over decades produces the characteristic “tertiary” aromas (leather, truffle, dried fruit) of aged Bordeaux and Burgundy.

Key References

• Ribereau-Gayon, P. et al. (2006). Handbook of Enology, 2nd ed. Wiley.

• Saerens, S. M. G. et al. (2010). “Production and biological function of volatile esters in Saccharomyces cerevisiae.” Microb. Biotechnol., 3, 165–177.

• Bartowsky, E. J. (2005). “Oenococcus oeni and malolactic fermentation.” Aust. J. Grape Wine Res., 11, 174–187.

• Waterhouse, A. L. et al. (2016). Understanding Wine Chemistry. Wiley.

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