In order to achieve the desired levels of DOM, duration of whitening was altered and DOM was measured using a digital whiteness meter (KM, Model C-100, Japan). After obtaining the desired DOM level, the samples for each level of DOM having the initially existing broken grains were poured into a laboratory rice grader and sieved for 60 s to completely separate the broken grains from sound kernels. Size of the grader cylinder groove was 3 mm and the separated grains were collected in the rice grader container at a set angle of 30° from the vertical.
The laboratory analyses were performed in the Rice Research Institute of Iran (RRII). The evaluated rice variety, Hashemi, is one of the common varieties of rice in north of Iran (Zareiforoush et al. 2010b). This variety is categorized as “long kernel” rice according to the standard provided by the Institute of Standards and Industrial Research of Iran (ISIRI) (ISIRI 2012). Moisture content of the evaluated samples was determined by means of a digital moisture meter (GMK model 303RS, Korea) to be 11–13.5 % (w.b.). Before image acquisition, milled rice grains were classified by milling experts into four classes based on the standard provided by ISIRI (ISIRI 2012). The classes included Low-processed sound grains (LPS), Low-processed broken grains (LPB), High-processed sound grains (HPS), and High-processed broken grains (HPB). The mentioned grades were defined based on two quality indices, namely degree of milling and length of rice grains.
This paper reveals design of a rice destoner for separating stones from rice based on their different properties. It has a sieve with opening based on the size of rice grain to separate the rice from the stones larger than it and also a second sieve which allows sands and stones smaller than the rice only. Both sieves are vibrated, powered by a 0.50kW DC and rotational speed of 700rpm electric motor. The destoner has the efficiency of 80% and mass flow rate of 2.10 kg/s which is equivalent to 7.50t/hr capacity. The design of the destoner was carried out economically. A machine of this nature can be manufactured for small entrepreneurs and rural level applications in the developing countries, due to its low cost of and easy production and maintenance, where rice is locally produced for better quality and quantity.