Biosystems Engineering (2004) 89 (3), 321–330 doi:10.1016/j.biosystemseng.2004.08.010 PH—Postharvest Technology Effects of Processing Conditions and Environmental Exposure on the Tensile Properties of Parboiled Rice Saifullah M.H. Saif 1 ; Dwayne A. Suter 2 ; Yubin Lan 1 1 Agricultural Engineering Technology Department, Fort Valley State University, 1005 State University Drive, Fort Valley, GA 31030, USA; e-mail of corresponding author: saifs@fvsu.edu 2 Department of Agricultural and Biological Engineering, Texas A&M University, University Avenue, College Station, TX 77843, USA; e-mail: dsuter@tamu.edu (Received 2 December 2003; accepted in revised form 11 August 2004; published online 5 November 2004) Ultimate tensile strength (UTS) and tensile modulus of elasticity (MOE) of parboiled rice were investigated. UTS and tensile modulus were measured for two rice varieties, Lemont and Rico-1, with respect to 5 and 10 min steaming durations, 21 and 40 1C drying temperatures, four storage environments of no exposure, 65, 86 and 100% relative humidity (RH) at room temperature, and six exposure durations of 0, 1, 2, 3, 9 and 24 h. Long grain, high amylose Lemont was found superior in UTS and MOE than medium grain, low amylose Rico-1, both before and after parboiling. Increasing steaming duration from 5 to 10 min increased the degree of parboiling, and consequently increased the UTS and MOE in both the varieties. Increasing drying temperature from 21 to 40 1C decreased the strength properties. Effect of storage environment and RH on the tensile strength properties revealed that in no exposure condition as well as 65% RH environment the tensile strength properties increased slightly or remained stable. When exposed to higher than 65% RH environment at room temperature the strength properties progressively decreased over storage period. The decrement rate was fast with high humidity of 100% and prolonged storage of 9–24 h, and the decrease was as high as 666% of the initial strength. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd 1. Introduction Rice is the staple food of about one-half of the world’s population. It is estimated that a quarter of the total production is consumed as parboiled rice. Parboiled rice is mostly consumed in south Asian nations. However, it has gained popularity as a part of cereal diet in recent years in the western countries including the Americas due to its non-sticky plum cooking as well as nutritional qualities. The parboiling process is applied to rice with a preliminary objective of hardening the kernel in order to maximise head rice yield in milling. Besides milling yield, it was also the realisation of the nutritional and health benefits of parboiled rice (Larsen, 2000) compared to raw milled rice that created the awareness and importance of parboiling among consumers and processors. Despite the known phenomena of hardening of the kernels due to parboiling, the processors also experience that the breakage of kernels in milling could not be eliminated altogether, as desired, due to parboiling alone unless the process conditions are controlled carefully. Even after controlled drying, the parboiled rice kernels are prone to fissure when exposed to high- humidity environment (Chattopadhyay & Kunze, 1986). They found that the hypothesis of fissuring of raw rice due to moisture adsorption (Kunze & Hall, 1965) that was later attributed to tensile failure of the raw rice kernel (Kunze & Choudhury, 1972) holds equally well for parboiled rice kernels, however, with reduced effect. Marshall et al. (1993) reported that extensive parboiling is not necessary to obtain maximum head rice yields, indicating the influence of various processing conditions on the strength characteristics of the kernel. ARTICLE IN PRESS 1537-5110/$30.00 321 r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd