Grain Cleaning

Foreign matter left in a grain bulk causes several problems. Fine dust and broken kernels pack together, impede airflow, and hold moisture, making the grain harder to dry uniformly and more prone to hot spots. Chaff, weed seeds, and broken grain are also preferred food and shelter for storage insects and moulds, so cleaner grain stores more safely. Finally, buyers pay by grade, and excessive foreign matter, weed seeds, or damaged grain reduces the grade and price.

Cleaning also supports later processing: mills and seed plants need grain free of stones and contaminants, and clean grain gives better milling and germination results.

Winnowing

Grain is dropped or tossed in a current of air or wind so that lighter chaff and dust are carried away while heavier grain falls; a simple, widely used traditional method.

Screening and sieving

Grain passes over screens with openings sized to retain grain while letting smaller material through, or to hold back larger straw and stones.

Aspiration

A controlled airstream lifts light material away from the grain, often combined with screens in a single cleaning machine.

These principles are combined in machines ranging from hand-operated cleaners to large air-screen units and, for seed, more precise graders. Larger combines and cleaning plants integrate screens and aspiration to clean grain as it is handled.

• Clean grain before drying and storage to improve airflow and reduce insect and mould harbourage.
• Match screen sizes to the grain so sound kernels are retained and foreign matter removed.
• Manage dust safely, since grain dust is a health and explosion hazard.
• Remove weed seeds and contaminants that lower grade or affect food safety.