Post-Harvest Engineering: From Paddy to Polish

Harvesting timing is a calculation of GVM (Grain Visual Maturity). If you harvest too early, you get "immature green grains" that are low in starch. If you harvest too late, the grains become overly dry and develop fissures (micro-cracks) due to the daily cycle of re-wetting from dew and drying from the sun. These fissures are invisible to the eye but cause the grain to shatter into "brokens" during milling, which significantly reduces the market value of the crop.

Once harvested, the immediate priority is Moisture Content (MC). Paddy comes off the field at roughly 24–26% MC. In this tropical state, the grain is respiring rapidly, generating heat. If not dried within 24 hours, the grain undergoes "Yellowing"—a microbial fermentation that permanently stains the white interior. The goal is to reach a stable 14% MC, which is the equilibrium moisture content for safe long-term storage in most climates.

The Milling Process is a two-step mechanical separation. First, the De-husker (usually using rubber rollers) applies shear force to remove the silica-rich outer husk. This produces "Brown Rice." While brown rice is more nutritious, the oils in the bran layer are prone to oxidation (rancidity), which is why it has a shelf life of only a few months. To produce white rice, the grain enters the Polisher or Whitener, where an abrasive surface rubs off the bran and the germ.

The final stage is Grading and Color Sorting. Modern mills use optical sensors to "fire" a puff of air at any grain that isn't perfectly translucent. This removes discolored grains, weed seeds, and small stones. The result is "Head Rice"—the whole, unbroken grains that command the highest price. The by-products (hulls and bran) are not wasted; hulls are often used as fuel for the drying furnaces, and bran is processed for high-vitamin rice bran oil, completing a zero-waste industrial cycle.