Chalkiness in Rice

Why in News?

Recently, scientists from Yangzhou University's Agricultural College in China discovered a gene that helps prevent rice grains from breaking during milling.

• Rice – It is a stable food for over half of the world’s population.
• The cultivated rice plant is an annual grass and grows to about 1.2 metres (4 feet) in height.
• The leaves are long and flattened and are borne on hollow stems.
• The fibrous root system is often broad and spreading.
• Varieties differ greatly in the length, shape, and weight of the panicle and the overall productivity of a given plant.
• Layers – A rice grain can be made up of several layers.
  • Husk – The outermost layer of the rice has to be removed.
  • Bran - Often, the next layer is also removed to whiten the rice.

• Traditionally, these layers have been removed by pounding the paddy rice with pestles in a mortar, then winnowing the grains from the chaff.
• Present day, mechanical rollers graze the layers in a process called milling.
• Chalkiness – It is a trait found in rice grains that significantly increases breakage during milling.
• It reduces the recovery of commercially acceptable grain and downgrades the quality.
• It disappears during cooking and does not affect the taste & aroma.
• Chalky grain rate – It is measured by the proportion of chalky grains among all rice grains and the degree of chalkiness, which reflects the extent of chalkiness in them.
• Properties – Chalky rice grains have an opaque appearance after milling and a brittle structure, which leads to higher breakage during milling.
• However, it has no impact on the taste, aroma, or cooking quality of the rice.
• Genetic determinants – Chalkiness in rice varieties is influenced by many genes and by environmental factors like high temperature and nutrient availability.
• The researchers identified a small stretch of DNA on chromosome 9 whose presence was strongly associated with low chalkiness in rice varieties and absence with high chalkiness.
• Low-chalkiness varieties contained the segment and showed higher expression of Chalk9 in endosperms compared to high-chalkiness varieties.
•  The endosperm is the part of the paddy grain that makes up the bulk of the milled rice.
• Impact – Higher chalkiness leads to more breakage during milling, and it reduces the yield of whole, marketable grains and affects rice quality.
• The modern breeding has unintentionally selected for the low-chalkiness version (Chalk9-L), improving rice quality over time.
• Breeders can directly introduce the Chalk9-L variant into high-chalkiness varieties to reduce breakage and enhance grain quality.

Reference

The Hindu| Chalkiness in Rice