Effect of Drying on Grain Quality -- Moisture Readsorption Causes Fissured Rice Grains
Abstract
Among others, drying can affect the quality of rice or other cereal grains (with a hard vitreous endosperm) in three prevalent and distinctive ways. These are:
1. Through Moisture Readsorption by Dry Rice Grains: Rice, as well as other cereal grains, is hygroscopic. The low-moisture (dried) grain readsorbs moisture from any source to which it is exposed. Moisture adsorbed through the grain surface causes the starch cells to expand and produce compressive stresses. Since the grain is a "free body", compressive stresses are countered by equal but opposite tensile stresses at the grain centre. When the compressive stresses at the surface exceed the tensile strength of the grain at its centre, a fissure develops. Fissured grains usually break during milling. Sources for grain moisture readsorption are discussed.
2. Through Moisture Readsorption by Field Rice Grains: When rice grains in the field reach harvest moisture (22%), the field sample may contain grains with moisture contents (MC) between 15 and 45%. Many individual grains may dry to below 15% MC during the day. Such grains can fissure on the stalk when they readsorb moisture at night.
3. Through Rapid Drying (To near storage moisture): Rapid drying produces a steep moisture gradient in the grain. As this gradient reclines after drying, the grain surface receives moisture from the interior and expands while the grain interior loses moisture and contracts. As this combination of stresses (compressive at the surface and tensile at the centre) develops with time, the grain fails in tension by pulling itself apart at its centre. The rheological aspects (stress, strain and time) that cause the grain failure are discussed.
Most breakage in rice processing can be attributed to grains which were fissured before milling. Moisture readsorption by low-moisture grains in the field before harvest is the most prevalent cause of fissured grains. Mechanical harvesting at a higher moisture content can minimize this type of grain failure. -- Fissuring of the rice grain after drying can be controlled by the drying procedure. Rapid drying which leaves a steep moisture gradient in the grain at the end of drying will cause the grain to fissure after drying. With time the grain fails in tension. Rice drying should not be terminated with a steep moisture gradient in the grain.
