Effects of Harvest Location and Moisture Content on Kernel-To- Kernel Breaking Force Distributions

One medium-grain rice variety, six long-grain varieties, and two long-grain hybrids, harvested from 13.4 to 26.0% moisture content (MC) from Keiser, Ark., Stuttgart, Ark., and Alvin, Texas, were used to determine the influence of harvest location and MC on kernel-to-kernel breaking force distributions (BFDs). Harvest MC affected BFDs, however, a greater influence was imposed by the growing location. Head rice yield (HRY) was plotted against the percentage of kernels in a sample having breaking forces greater than 20 N, which was used to define “strong” kernels. The strongest correlations between HRY and the percentage of strong kernels were found for variety/growing location lots having low to mid-level HRY levels; little to no correlation was observed for samples having high HRYs. INTRODUCTION Rice MC at harvest is one of the most important factors influencing milling quality and overall economic value of rice. Calderwood et al. (1980) and Lu et al. (1992) found that HRY reached a maximum at an intermediate harvest date, and then declined. Lu et al. (1995) demonstrated that HMC affected the field yield, drying charges, and milling quality; HMC thus has a dramatic impact on the final gross income to a producer. Since rice kernels on a panicle mature unevenly, the MC and resultant mechanical properties of individual kernels at harvest varies (Chau and Kunze, 1982). Freshly harvested rice kernels exhibit a wide range of MCs, particularly during the early AAES Research Series 517 398 stages of the harvest season (Kocher et al., 1990; Siebenmorgen et al., 1992). The production of fissures due to rapid moisture adsorption by low MC kernels (Kunze and Prasad, 1978; Siebenmorgen and Jindal, 1986) is the primary reason for HRY reductions at low HMCs. Siebenmorgen et al. (1998) showed that the percentage of kernels with MCs less than critical levels before moisture adsorption was related to the HRY reduction incurred. While it is known that harvest MC influences kernel MC variability, information relating kernel-to-kernel mechanical strength to harvest MC is not available. This information would lend a more direct understanding as to why harvest MC affects HRY. Further, differences in kernel strength distributions across growing locations could possibly explain HRY variability due to growing location. The objectives of this research were to determine: (1) the influence of HMC and growing location on the kernel-to-kernel BFDs of rice lots; and (2) the relationships between BFDs and milling quality. PROCEDURE Rough rice lots of ‘Bengal’ (8 harvest MCs), ‘Cypress’ (8 harvest MCs), and ‘Drew’ (8 harvest MCs) were plot combine-harvested from the Northeast Research and Extension Center at Keiser, Ark., from 1 October to 31 October 2002. Lots of ‘1093’ (9 HMCs), ‘Francis’ (9 HMCs), and ‘Wells’ (8 HMCs) were harvested from the Rice Research and Extension Center at Stuttgart, Ark., from 21 August to 12 September 2002. RiceTec Inc. at Alvin, Texas, similarly harvested ‘Cocodrie’ (14 HMCs), ‘XL7’ (17 HMCs), and ‘XL8’ (15 HMCs) samples from 30 July to 30 August 2002. After harvest, samples were cleaned using a dockage tester (Carter-Day Co., Minneapolis, Minn.). Two kg samples of each lot were dried slowly in a chamber maintained at approximately 21°C and 65% RH (rice equilibrium MC of 12.5%). The samples were stored in plastic bags at 2°C until testing. Two hundred kernels were randomly selected from each location/variety/HMC lot and hulled by hand. Three-point bending tests were conducted on each kernel using a texture analyzer (TA.XT2i, Texture Technologies Corp., Scarsdale, N.Y.) with a flat-faced loading head (Fig. 1). The deformation rate was set at 0.5 mm/s. After placing a kernel across the supporting span, the bending test was initiated and the maximum force attained before the kernel failed was recorded as the breaking force. Two 150 g subsamples from each location/variety/HMC lot were milled with a McGill No. 2 mill. Head rice yield was determined from the percentage head rice in each milled sample, which was determined using an image analyzer (Graincheck 2312, Foss North America, Minneapolis, Minn.).