Influence of Soaking on Crushing Strength of Raw and Parboiled Rice

Abstract

Hardness (crushing strength) of rice decreased with increase in soaking time. A remarkable reduction was observed after complete hydration in parboiled rice (∼85%) than raw rice (27%), which was attributed to greater hydration capacity of parboiled rice (63% d.b.) compared to raw rice (38% d.b.). Crushing strength of unsoaked parboiled rice (∼150 N) was greater than raw rice (∼76 N). On the contrary, crushing strength of fully hydrated parboiled rice (∼23 N) was lower than raw rice (∼55 N). The significant decrease in the hardness of parboiled rice grains upon hydration, however, could not explain the greater energy requirement for wet grinding parboiled rice studied earlier, which, perhaps, could be attributed to the higher slurry viscosity of parboiled rice compared to raw rice.

Keywords:

• Crushing strength
• Hardness
• Moisture
• Raw and parboiled rice
• Slurry viscosity
• Soaking
• Wet grinding energy

ACKNOWLEDGMENTS

This research was funded by the Council of Scientific and Industrial Research, New Delhi, India. S. Krishna Murthy helped in hardness measurements and Pankaj Sharma helped in viscosity measurements.