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616.936 (497.2) : 633.18

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Rice (Oryza sativa L.) is one of the most important food crops in the world and the staple food for more than half of the world’s population. It is harvested as a covered grain, which is called rough rice. Before consumption, the cover or hull is removed, yielding brown rice. It is common to remove the bran and germ from the brown rice kernel in a process referred to as milling. During processing, some rice grains break. It is of economic and industrial importance to reduce the breakage susceptibility of rice grains. Parboiling, a three-step hydrothermal treatment consisting of soaking, steaming and drying of rice, can substantially reduce the breakage susceptibility of the grains. The parameters impacting the milling breakage susceptibility of parboiled brown rice are, however, rather unclear at present. The aim of this doctoral dissertation was to study the impact of brown rice parboiling on milling breakage susceptibility. Hereto, we focused on the impact of raw rice characteristics on the one hand, and parboiling conditions and the resulting structural changes in starch and proteins on the other. This way, raw material specifications and parboiling conditions can be optimized so that rice kernel breakage can be decreased without compromising on the end product quality. In a first part of this work, the raw rice characteristics impacting the milling breakage susceptibility of raw and parboiled rice were investigated. Hereto, the breakage susceptibility of bulk samples and fractions thereof was related to the properties of the raw material, including the composition, dimensions and microstructure of the grains, and to the changes occurring in the starch and protein fraction during parboiling. Both thin and chalky (im)mature rice grains had a higher tendency to break during milling. The increased breakage susceptibility of the first fraction was explained by kernel dimensions, whereas that of the latter was due to the kernel internal structure. The chemical composition of the grains had little if any effect on their bending strength. Parboiling increased the resistance of rice grains to breakage during milling due to the homogenous and compact internal structure obtained as a result of full starch gelatinization and/or protein aggregation. It was concluded that, when executed properly, parboiling decreases the milling breakage susceptibility of rice, irrespective of the characteristics of the unprocessed starting material. The impact of parboiling conditions on milling breakage susceptibility and changes in starch and proteins was investigated in a second part. First, brown rice was parboiled using combinations of soaking and steaming conditions and one standard drying step. Next, the parameters impacting the milling breakage susceptibility of the parboiled end products were investigated. More in particular, the different types of parboiled rice grains that lack strength were determined and it was examined whether their strength is related to changes in starch and proteins as a result of parboiling. The soaking and steaming conditions, and thus the heat-moisture conditions applied during parboiling, impacted the structuring and the milling breakage susceptibility of the parboiled rice grains. Both white bellies, i.e. parboiled rice grains with translucent outer layers and an opaque center, and fissured parboiled rice grains were more susceptible to breakage than intact parboiled rice grains. In contrast to the extent of protein aggregation, the degree of starch gelatinization impacted the abundance of such grains. Incomplete starch gelatinization upon steaming caused white bellies to be present and increased fissuring incidence in the parboiled end product. This led to a higher milling breakage susceptibility. In contrast, when starch had been completely gelatinized, mainly intact parboiled rice grains were present, and thus the milling breakage was reduced. Next, the impact of soaking conditions on the level of breakage susceptible rice kernels in the parboiled end product was determined. Fissures developed inside the rice grains as a result of moisture absorption during soaking. As the rice grains absorbed more moisture, these fissures, however, disappeared due to starch granule swelling. In addition, with increasing soaked rice grain moisture content, the degree of starch gelatinization during subsequent steaming increased, and thus less white bellies were present in the parboiled end product. The fissuring incidence in the soaked rice samples was related to that of the parboiled rice samples. It was hypothesized that, when the moisture content in the center of the rice grains is too low for the starch granules to swell during soaking, and thus to make the fissures disappear, it is also too low for the starch to gelatinize during subsequent steaming. As a consequence, fissured rice grains and white bellies are present in the parboiled rice samples. In addition, the impact of parboiling on the properties of rice proteins, and, more in particular, the relative contribution of non-covalent interactions and covalent bonds within and between the different proteins was examined. During brown rice parboiling, proteins aggregated. In general, more severe steaming conditions resulted in a denser protein network. In this protein network, glutelins were the main building blocks, which polymerized by disulfide bonds and interacted with one another by hydrogen bonds and hydrophobic interactions. Albumins, globulins and prolamins were either incorporated in the glutelin network or formed a separate protein network. In conclusion, this doctoral work contributed to a better understanding of the parameters impacting the milling breakage susceptibility of parboiled brown rice. It is clear that the rice grains need to absorb sufficient moisture during the soaking step to heal the induced fissures and allow complete starch gelatinization during the subsequent steaming step. In that way, the milling breakage susceptibility of the parboiled rice is independent of kernel defects inside the raw material and can be kept to a minimum. However, one should avoid rice grain fissuring during the final drying step of the parboiling process, as it lowers the rice grain mechanical strength. Because of their dimensions, some rice grains or cultivars are more susceptible to fissuring during drying than others even when exposed to the same environmental conditions. As such, careful selection of the raw material and optimal drying conditions are of utmost importance to obtain a parboiled end product with minimal breakage susceptibility

academic collection --- 633.18 --- Rices. Oryza --- Theses --- 633.18 Rices. Oryza

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633.18 --- 633.18 Rices. Oryza --- Rices. Oryza --- Rice --- Rice. --- Planting.

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633.18 --- 632.07 --- 632.07 Analysis and diagnosis --- Analysis and diagnosis --- 633.18 Rices. Oryza --- Rices. Oryza

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633.18 --- 632.9 --- Rices. Oryza --- Control of plant diseases and pests --- 632.9 Control of plant diseases and pests --- 633.18 Rices. Oryza