Effect of drying and milling modes on the quality of white rice of an Indonesian long grain rice cultivar

Abstract

Background. Many studies have revealed the susceptibility of long grain rice to breaking during milling, while others have demonstrated the variation in the yield of head rice due to different rough rice drying methods. Thus, this study aims to determine appropriate drying and milling methods to improve the head rice yield and nutritional quality of long grain rice. Materials and methods. A series of drying experiments were performed on rough paddy rice employing shallow bed, oven and sun drying methods. Then, the dried rough rice grains were milled with various dehusking (H), separation (S) and polishing (P) configurations to obtain white rice. The resulting batches of white rice were analyzed and compared in terms of head rice yield, broken rice yield, brewer yield, whiteness and nutritional quality. Results. Milling configurations strongly affected the total, head rice, broken rice and brewer yields. The configuration of one dehusking, one separation and one polishing (H -S -P) resulted in the highest milling and head rice yields. Although the whiteness of the rice samples was significantly affected by the milling configuration, their values fell within an acceptable range preferred by consumers in Southeast Asia (39-47). The milling of dried rough rice obtained from shallow bed and sun drying using a concrete floor and white tarpaulin resulted in a comparable total (65%) and head rice yield (51%). However, the milling of rough rice dried using an oven and sun drying on black tarpaulin resulted in a slightly lower total yield (64.50%) and head rice yield (50.50%). The moisture, ash, protein and lipid contents of the white rice were significantly lower than those of manually dehulled rice, whereas the carbohydrate and amylose contents of the white rice were significantly higher. Conclusion. Application of shallow bed or sun drying on a concrete floor followed by milling with the H -S -P configuration produced the highest head rice yield with an acceptable whiteness and nutritional composition. These combined postharvest technologies are simple, efficient and economical for both small-and large-scale applications. Further research on consumer acceptance and on the nutritional and cooking qualities of the white rice obtained from these combined postharvest technologies is essential.

Effect of Pads and Thickness of Paddy on Moisture Removal under Sun Drying

“…The required moisture content of 14% (dry basis) was obtained in 5 to 9 hours from 28% initial moisture content depending on the type of drying pads used. Kumoro et al (2019) conducted another similar experiment from 7.00 am to 12.00 noon by evenly spreading rough paddy onto a concrete floor and tarpaulin (white and black) with the rice layer thickness 2 to 5 cm. Hellevang ( 2004) reported that plastic sheets could result in condensed water and tend to hold them in low places; thus, not suitable for drying pads.…”

Physiochemical and Cooking Properties of Rice Treated With Different Drying Methods

“…During drying, the heat flow induces the development of moisture and temperature gradients and thermal stress inside the rice kernel, leading to the formation of fissures on the kernel. Usually, fissured kernels break easily during milling causing a serious reduction in the head rice yield, cooking qualities and market value (Kumoro et al, 2019).…”

Cracking and breaking response in four rice varieties as influenced by fertilization regime and storage duration

“…At moisture contents less than 14%, the grain becomes brittle and cracks or break during milling. Both instances lead to reduction in head rice yield (Schluterman and Siebenmorgen, 2007;Akowuah et al, 2012;Kumoro et al, 2019). Generally, a good quality paddy constitutes about 55% head rice, 15% broken, 20% husk, and 10% bran.…”

“…Rice cracking was known as sun-cracking by earlier researchers, latter research led to the observation that rice cracking was due to moisture absorption of dried rice (Schluterman and Siebenmorgen, 2007;Akowuah et al, 2012). Some studies in different varieties found that rice cracking begins at moisture content below 14.2 and 18.3% for crack resistant and crack susceptible varieties, respectively (Kumoro et al, 2019). Another factor that may cause paddy to crack before harvest is nonsynchronous blooming which leads to uneven grain maturity (Schluterman and Siebenmorgen, 2007;Akowuah et al, 2012;Kumoro et al, 2019).…”

“…Some studies in different varieties found that rice cracking begins at moisture content below 14.2 and 18.3% for crack resistant and crack susceptible varieties, respectively (Kumoro et al, 2019). Another factor that may cause paddy to crack before harvest is nonsynchronous blooming which leads to uneven grain maturity (Schluterman and Siebenmorgen, 2007;Akowuah et al, 2012;Kumoro et al, 2019). In bulk storage of paddy, moisture migration causes lowmoisture grains to absorb moisture from the highmoisture grains and crack when the high-moisture grains had only 4 to 5% more moisture than the low-moisture grains (Akowuah et al, 2012).…”