Food-grade lactic acid is an organic acid widely used in food, beverages, pharmaceuticals, and daily chemicals, requiring stringent purity and safety standards (e.g., heavy metals ≤0.001%, arsenic salts ≤0.0001%) . The mainstream production method is microbial fermentation, which uses renewable resources and combines environmental sustainability with economic efficiency. The core process is analyzed below by stage:
I. Raw Material Pretreatment and Saccharification
- Raw Material Selection and Treatment
- Primary Raw Materials: Corn starch (highest proportion), sucrose, whey, cellulose .
- Pretreatment Process:
- Crush raw materials (corn/cassava), blend with water at 4:1 ratio.
- Add amylase (10 units/g dry material) for gelatinization, hydrolyzing starch into sugars .
- Sterilization Requirements: Heat treatment at 90–95°C for 5–10 minutes to inactivate contaminants and enhance fermentation efficiency.
- Homogenization
- Gelatinized liquid is homogenized at 65–70°C under 15–20 MPa pressure to optimize fluidity and fermentation uniformity.
II. Microbial Fermentation Process
- Strain Selection and Inoculation
- Dominant Strains:
- Homolactic Fermentation Bacteria: Lactobacillus delbrueckii—sugar conversion rate >90%, producing DL-lactic acid.
- Genetically Engineered Strains: Recombinant yeast (with pyruvate decarboxylase gene knockout + lactate dehydrogenase gene insertion), directly producing L(+) or D(−)-optical pure lactic acid, tolerant to low pH (pH=3) .
- Inoculation Volume: 2.5–3% of fermentation broth volume, with strain-to-gelatinized material ratio at 9:1.
- Fermentation Control
- Temperature and pH Regulation:
- Traditional process: Fermentation at 50±1°C, requires continuous CaCO₃ addition to neutralize lactic acid (maintain pH 5–6).
- Recombinant yeast process: Fermentation at 40–45°C and pH=3, eliminating neutralization and simplifying purification .
- Cycle and Output: 40–45 hours, lactic acid concentration reaches 100–130 g/L.
III. Separation and Purification Technology
- Pretreatment: Impurity Removal
- Defects of Traditional Process:
Requires multi-step procedures (heating → precipitation → evaporation → crystallization → washing), taking 7–10 days with low yield and high pollution. - Modern Improvement—Flocculation-Sedimentation Method:
- Maillard Reaction: Add magnesium salt catalyst to convert residual sugars/proteins into colloidal brown complexes.
- Sensitized Flocculation: Adjust pH to alkaline, add flocculant to form colloids adsorbing impurities (e.g., fats, pigments).
- Sedimentation Separation: Polymer settling agents (e.g., polyacrylamide) neutralize charges for solid-liquid separation .
- Advantages: Completed within 6–8 hours in a single reactor, increasing yield by >30%.
- Refined Purification
- Ion Exchange: Removal of metal ions (e.g., Ca²⁺, Mg²⁺).
- Concentration and Filtration: Vacuum evaporation → double decomposition → secondary concentration → suction filtration.
- Product Standards: Food-grade lactic acid concentration ≥80%, must pass chloride, sulfate, and carbonizable substance tests .
Conclusion
Food-grade lactic acid production is transitioning from high-energy, multi-step traditional processes (e.g., crystallization) to efficient, green integrated technologies (flocculation-sedimentation, recombinant yeast fermentation). Future breakthroughs lie in strain performance optimization (e.g., acid resistance, optical purity) and purification cost control to meet dual market demands for food additives and biodegradable plastics.
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