Ensiling (silage making)

Ensiling is the controlled preservation of moist, chopped biomass through lactic acid fermentation in an oxygen-free environment. It works through a sequence of four stages. In the first 4–6 hours after sealing, residual oxygen is consumed by plant cell respiration and aerobic bacteria, producing CO2 and dropping the silo atmosphere to anaerobic. In the next 2–3 days, facultative anaerobes (enterobacteria, yeasts) ferment the most accessible sugars, producing acetic acid and ethanol. From day 3 onward, lactic acid bacteria (LAB) — Lactobacillus, Pediococcus, Enterococcus species — outcompete other organisms, fermenting soluble sugars into lactic acid that drives pH down to 3.8–4.2. Below this pH, all spoilage organisms are inhibited and the biomass is stable for 12–24 months.

For ensiling to succeed, four conditions must be met. Moisture content at 60–70% — drier material cannot compact tightly enough to exclude air, wetter material loses nutrients through effluent runoff. Sufficient soluble sugars (8–12% of dry matter) to feed the LAB; low-sugar crops like alfalfa often need molasses addition. Adequate compaction to a bulk density above 220 kg DM/m3 to eliminate air pockets. Airtight sealing with polythene sheeting (200–250 micron, weighted with tyres or sandbags) within 24–48 hours of cutting. Modern Indian operations increasingly use commercial silage inoculants — Lactobacillus plantarum and L. buchneri cultures at Rs 200–400 per tonne of biomass — to ensure rapid acidification and stable storage.

Ensiling format varies with scale and capital availability. Bunker silos (concrete walls, 50–500 tonne capacity) are the standard for industrial-scale CBG plants at Rs 200–400 per tonne capacity. Bag silos (250–500 kg flexible bags) suit smaller plants and offer plug-and-play handling. Pit silos (excavated, lined) are the cheapest but require good drainage. Tower silos are uncommon in India due to high capex. Properly made silage retains 90–95% of the original dry matter and adds value: methane yield is typically 5–15% higher than fresh biomass of the same crop because partial pre-fermentation breaks down cell walls and exposes carbohydrates. For Indian CBG plants running on seasonal energy crops, ensiling is what converts a 4-month harvest into 12 months of continuous digester feed.