Effect of processing on thiamin and riboflavin contents of some high-yielding rice varieties of punjab

Six high-yielding varieties of rice (Oryza sativa L) were taken and subjected to 6 and 8% degree of milling. The raw rice and the parboiled rice were analysed microbiologically for thiamin and riboflavin contents. Milled parboiled rice contained more thiamin and riboflavin that milled raw rice at both the levels of milling because parboiling of paddy results in inward diffusion of water-soluble vitamins to the endosperm. The thiamin content of brown rice is reduced after parboiling. The loss may be due to the partial decomposition of thiamin during the stages of parboiling, but the riboflavin content of brown rice is found to be increased after parboiling.     

Rice degree of milling effects on hydration,texture, sensory and energy characteristics. Part 2. Cooking using fixed,water-to-rice ratios

The purpose was to assess the effects of degree of milling on hydration and textural characteristics of rice cooked using a range of water-to-rice ratios, and to compare the energy requirements when using these fixed water-to-rice ratios to the energy required when cooking with excess-water. Surface lipid contents (SLCs) ranged from 0.15% to 0.55% for non-parboiled rice and from 0.40% to 0.95% for parboiled rice. Cooking degree was assessed by measuring cooked-rice peak force, moisture content and percentage gelatinized kernels. Milling degree had little to no effect on cooking characteristics of all milled samples. Differences in cooking characteristics between milled and brown rice were less pronounced for parboiled than non-parboiled rice. Non-parboiled milled rice required the least energy to be ‘well-cooked’, followed by parboiled milled rice, non-parboiled brown rice, and parboiled brown rice (there were no significant differences between non-parboiled and parboiled brown rice). In general, excess-water cooking required more energy than fixed water-to-rice ratio cooking.     

Some physical properties of paddy and rice and their interrelations

Grain dimensions, density, bulk density, porosity and angle of repose of paddy and rice were studied with respect to (a) varietal difference, (b) effect of moisture content, and (c) effect of degree of milling.

• (a) Grain breadth and thickness were mutually correlated; all dimensions of milled rice were closely related to those of the corresponding paddy. Density was nearly constant at 1.452 g/ml in all rice varieties; in paddy, the value was around 1.182 g/ml for round varieties and around 1.224 g/ml for others. Bulk density varied appreciably in both rice (0.777–0.847) and paddy (0.563–0.642 g/ml), and so did porosity (41–46% in rice, 46–54% in paddy). These were related to the grain shape (l/b ratio); the more round the grain, the greater was the bulk density and the lower the porosity and vice versa. Angle of repose was relatively constant in different varieties of rice (average 37.5°) and paddy (average 36.5°).
• (b) With increasing moisture content, in rice, density decreased linearly; but the bulk density decreased twice as fast and the porosity increased owing to a concurrent progressive increase in the frictional property which decreased the degree of grain packing. In paddy, with increasing moisture content, density and bulk density increased, showing the presence of void space between the husk and the kernel; the angle of repose also increased moderately, but the porosity remained nearly constant.
• (c) Density of rice increased slightly with milling; but bulk density, porosity and angle of repose were markedly affected by the degree of milling. In general, with progressive milling, bulk density decreased at first and then increased, while porosity and angle of repose changed in reverse fashion—the changes being more pronounced in parboiled rice than in raw rice. Thus brown rice packed well and flowed freely; rice of intermediate polish, particularly parboiled rice, had poor packing and flow properties; and fully milled rice again packed and flowed well. The primary change in these was in frictional property, which affected the porosity and hence the bulk density. The frictional property in turn was related primarily to the surface fat content of the rice. The surface fat increased sharply on initial milling, more in parboiled rice, and then gradually disappeared on full milling.

     

Rice degree of milling effects on hydration,texture, sensory and energy characteristics. Part 1. Cooking using excess water

The purpose was to assess the effect of degree of milling (DOM) on cooking kinetics, sensory attributes, and energy requirements when cooking rice in excess water. Commercial milling equipment was adjusted to produce parboiled and non-parboiled rice samples that were milled to varying DOMs, including brown rice lots having no milling. Surface lipid content (SLC) ranged from 0.15% to 0.55% for non-parboiled rice and from 0.40% to 0.95% for parboiled rice. The percentage gelatinized kernels, moisture content, peak force and sensory attributes were determined as a function of cooking duration for all samples. The cooking duration required to attain ‘well-cooked’ rice was determined, after which the energy required for cooking was measured. Within the SLC range tested, DOM did not affect cooking kinetics, texture and flavor of rice. Non-parboiled brown rice required the most energy, expressed as energy per unit mass of uncooked rice, to be cooked, followed by parboiled brown, parboiled milled and non-parboiled milled rice.     

EFFECT OF ARTISANAL PARBOILING METHODS ON MILLING YIELD AND COOKED RICE TEXTURAL CHARACTERISTICS

ABSTRACT

One of the main objectives of artisanal rice parboiling is to reduce the levels of broken grains (brokens) on milling. Rice samples that had been parboiled using different regimes of soaking temperatures and steaming times were analyzed for their physical properties and cooked rice textures. It was established that inappropriate soaking and steaming regimes resulted in greater levels of brokens than raw‐milled paddy. Consequently, in artisanal parboiling, the initial soaking temperature should be about 90C and the steaming time should be more than 8 min, ideally, about 12 min. On cooking, more severely parboiled rice samples had firmer textures than mildly parboiled samples. The commercially parboiled sample and the more severely laboratory‐parboiled samples required a rice‐to‐water ratio of 1:3, while the raw‐milled sample and the mildly parboiled ones required a 1:2½ rice‐to‐water ratio for optimum cooking.

PRACTICAL APPLICATIONS

Artisanal rice parboiling is carried out mainly to reduce the levels of broken grains and increase the yield of milled rice in many countries. If this is carried out very well, there are economic benefits as more rice of better quality is available to be sold. This study provides information on optimum processing conditions, i.e., initial soaking temperature of about 90C and a steaming time of about 12 min. The study also provides recommendations on optimum cooking conditions, i.e., rice‐to‐water ratio, for the variably parboiled rice samples.     

Fatty acid composition of rice oils.

Analysis of rice oil methyl esters of two varieties by gas-liquid chromatography showed that oil from bran-polish and oil from milled rice extracted with petroleum ether had similar fatty acid composition and had oleic/linoleic acid ratios of about 1.0. However, milled rice oil extracted with chloroform/methanol had a higher linoleic acid content and a lower oleic acid content than oil extracted with petroleum ether.     

Status of dietary fiber contents in pigmented and non-pigmented rice varieties before and after parboiling

Five different varieties of paddy (four pigmented and one non-pigmented) were shelled and milled in pre and post parboiled form, their dietary fiber contents were estimated. Under similar conditions of milling, raw rice showed a high degree of polish (DOP), 9–12 g/100 g and parboiled rice showed low DOP, 4.6–6.6 g/100 g. Dietary fiber content was high in pigmented rice, 9–10 g/100 g compared to non-pigmented, ∼6 g/100 g. Soluble fiber content in pigmented head rice (dehusked) varied from 1 to 1.5 g/100 g and in its brokens varied from 0.45 to 1.45 g/100 g. Dietary fiber content was low by about 1% in parboiled rice. In the parboiled rice of pigmented varieties, the total fiber content varied from 7.95 ± 0.15 to 9.05 ± 0.25 g/100 g and the soluble fiber content varied from 0.7 to 0.9 g/100 g. In milled parboiled rice the respective values were 5 ± 0.4 to 6 ± 0.1 g/100 g and 0.85 ± 0.05 to 1.25 ± 0.05 g/100 g. However, the soluble fiber content in the non-pigmented brown rice, IR-64 remained same after parboiling, 0.75 ± 0.5 g/100 g. Milled parboiled rice showed higher soluble dietary fiber compared to milled raw rice. In conclusion, dietary fiber was high in pigmented rice varieties when compared with non-pigmented rice.     

Technological properties of microwave parboiled rice

Two rice varieties, a short grain (Giza 175) and a long grain (Giza 181), were parboiled by soaking in water at 80–85 °C for 1.5 h and then dried in the microwave oven for 3, 5, 6 and 8 min. The effect of such parboiling treatment on milling output and technological properties (cooking and eating quality) of milled rice were studied. There was a negative significant correlation between head rice and the drying time and a positive correlation between the drying time and the broken grains. The effect of such treatment on the chemical composition of milled parboiled rice, i.e. amylose, protein, fat and ash contents, showed that the amylose content of Giza 175 variety significantly decreased while not affect in Giza 181 variety. No significant differences were obtained in protein, fat and ash contents by increasing drying time. Microwave drying was more pronounced on Giza 175 variety rather than Giza 181 one. However, the optimum cooking time of the parboiled samples of the two varieties was not affected as a result of increasing the microwave drying time.     

Effect of processing conditions on the oil content of parboiled-rice bran

The oil content of residual milled kernel was consistently lower and of bran higher in parboiled as compared to raw rice at all degrees of milling (d.m.), showing that the rice oil migrated outwards upon parboiling. However, the d.m.-oil content curves for the residual kernel as well as the bran were largely unaffected by varying conditions of parboiling. The total oil content of the grain was also unchanged after parboiling. When the data for the fraction of the total grain oil coming into the bran were plotted against the d.m. for a variety of parboiling conditions, they again fell into a single line. These results showed that, contrary to many earlier claims, the processing condition during parboiling had no effect on the redistribution of the fat in the grain.     

Effect of milling on mineral and trace element composition of raw and parboiled rice.

The effect of milling on the mineral and trace element composition of raw and parboiled grain samples of 16 varieties of rice was investigated. Varietal, locational and seasonal differences in the nutrient composition of brown rice were noted. The mean values for different nutrients per 100 g brown rice were total ash 1.37 g, phosphorus 349 mg, magnesium 157 mg, calcium 18 mg and iron 2.16 mg and in μg per g samples, zinc 14.3, manganese 11.0, copper 2.39, molybdenum 0.775 and chromium 0.088. Parboiling per se had no effect on the composition of brown rice. The degree of milling and the initial concentration of the nutrient in the grain determined the magnitude of loss on milling both in raw and parboiled grain. Percent losses of different nutrients on 5 and 10% milling of raw rice respectively were total ash 40, 62; iron 51, 67; magnesium 40, 64; calcium 36, 57; iron 54, 64; copper 26, 45; manganese 48, 56; molybdenum 24, 34; chromium 57, 69; and zinc only 2.8, 4.6. Zinc in rice grain was uniformly distributed and a major portion of other nutrients was concentrated in the outermost 2.5% surface layers of the grain. Parboiling appeared to have altered the distribution of some nutrients in rice grain except for zinc, magnesium and copper. Milling losses for other nutrients were therefore significantly lower in parboiled than in raw rice.     

Parboiling of rice. Part I: Effect of hot soaking time on quality of milled rice

This study investigated the effect of soaking time on the quality of parboiled rice. The paddy was soaked in water at 25 and 80 °C for 15, 30, 45, 60 and 120 min. The soaked paddy was steamed, dried, stored and milled. With increasing soaking time a significant increase in water absorption and milling and head rice yield (hence reduction in broken rice) was observed. A significant difference in milling yield, at the 1% level, was obtained between the raw rice control and the hot soaked parboiled samples. A large reduction in fissured grain was observed after soaking. It is suggested that parboiling fills the void spaces and cements the cracks inside the endosperm, making the grain harder and minimizing internal fissuring and thereby breakage during milling.     

Studies on Expanded Rice. Optimum Processing Conditions

Optimum conditions of parboiling, milling and puffing for making expanded rice were studied on a small laboratory scale. Optimum puffing was obtained by heating milled parboiled rice at a moisture content of 10.5–11% with 15 times its weight of fme sand at 250°C for 10–11 sec. Raw and mildly parboiled rice gave minimal expansion, which increased with increasing severity of parboiling up to a steam pressure of 1.5 kg/cm². However, rice parboiled by heating with sand (250°C, 2.5 min) expanded best. Starch retrogradation after parboiling reduced expansion, as did cracked and broken grains and insufficient milling of the rice. Addition of salt increased expansion. Expansion initially increased and then decreased with increasing age of paddy after harvest.     

Swelling and solubility behaviour of parboiled rice flour

Parboiled rice flour swelled and dissolved more than raw rice flour in water at temperatures below 70°C, but less than raw rice at higher temperatures. This difference between raw and parboiled rice increased with an increasing degree of parboiling. A sample of parboiled rice produced by dry-heating soaked paddy in hot sand behaved differently; but when it was wetted and tempered to favour reassociation of starch, its properties fell in line with normal steam-parboiled rice. The above behaviours of raw and parboiled rice flour were similar to those of corresponding whole-grain rice. They also reinforce the earlier suggestion of starch reassociation in conventional parboiled rice.     

RETENTION OF IODINE IN FORTIFIED PARBOILED RICE AND ITS PASTING CHARACTERISTICS DURING STORAGE

Iodine fortification, as potassium iodide (KI) or potassium iodate (KIO₃), in parboiled milled rice was investigated. The fortified parboiled milled rice (9–10% moisture content) contained 11.6 × 10^?2 mol KI per 100‐g salt or 9 × 10^?2 mol KIO₃ per 100‐g salt. The iodine‐fortified parboiled milled rice after dialysis retained 80.50–84.87% iodine, while the milled rice retained 97–100% iodine in the samples. Test on in vitro starch digestion showed that most iodine was released within 15 min of starch digestion. The iodine‐fortified and nonfortified parboiled milled rice showed significant higher pasting temperature, peak time and setback viscosity than normal milled rice. After storage for 5 months, iodine content in fortified parboiled milled rice decreased significantly (P ≤ 0.01), while iodine content in milled rice did not change during storage. For the rice stored at ambient temperature, the rapid visco‐analyzer amylograph viscosity (peak viscosity, trough viscosity and final viscosity) increased during the first 2 months and decreased during storage. Iodine did not affect these attributes of parboiled milled rice.     

Starch Retrogradation and Starch Damage in Parboiled Rice and Flaked Rice

The equilibrium moisture content attained on soaking in water (EMC-S) of parboiled paddy was very high immediately after steaming and fell appreciably on storage, demonstrating starch retrogradation. The extent of retrogradation was dependent on the temperature of storage as well as the moisture content. It was maximum at about 25% moisture when stored at room temperature. Roasting of soaked raw paddy raised its EMC-S due to gelatinization. When stored at room temperature there was progressive retrogradation above 18% moisture. Flaking of roasted paddy and of moist milled rice, both raw and parboiled, also raised the EMC-S. This appeared to be caused by mechanical damage of starch granules. Flaked rice, too, showed retrogradation. — These results confirmed that many properties of parboiled rice are due to starch retrogradation while those of roasted and flaked rice are due to absence of retrogradation.     

Functional Properties as Affected by Laboratory‐Scale Parboiling of Rough Rice and Brown Rice

ABSTRACT: Rough rice (RR) is the conventional feedstock for parboiling. The use of brown rice (BR) instead of RR is gaining interest because it results in shorter processing time and lower energy requirement. This study compared the functional properties of milled parboiled rice under different parboiling conditions from RR and BR. Presoaked RR and BR from cultivars Bolivar, Cheniere, Dixiebelle, and Wells were parboiled under mild (20 min, 100 °C, 0 kPa) and severe (20 min, 120 °C, 98 kPa) laboratory‐scale conditions. Head rice yield improved on the RR and BR samples subjected to severe parboiling and was comparable to that of a commercially parboiled sample. Mild parboiling of BR resulted in lower head rice yields. Parboiling generally resulted in decreased head rice whiteness, decreased apparent amylose, increased total lipid, and sparingly changed protein content. Under the same parboiling conditions, the extent of starch gelatinization was higher for BR compared to RR as manifested by some distinct differences in pasting and thermal properties. The cooking characteristics (water uptake ratio, leached materials, and volumetric expansion) and cooked rice texture (hardness and stickiness) of RR and BR subjected to severe parboiling were fairly comparable. Differences in parboiled rice functional properties due to cultivar effect were evident.     

不同加工精度籼米的感官品质和营养品质

为保证大米良好的食用品质的条件下尽可能保留营养成分,寻找籼米的适度加工范围,以6种不同品种的籼糙米为研究对象,将其分别碾磨成不同加工精度的籼米,通过感官评价和喜好度评价对不同加工精度的籼米米饭进行实验,并对比适碾米和精碾米的部分营养成分含量的差异,进而评价不同加工精度籼米的感官品质和营养品质.结果表明:随着加工精度的逐...     

Effect of superheated-steam drying compared to conventional parboiling on chalkiness,head rice yield and quality of chalky rice kernels

Four rice cultivars (Ayutthaya 1, Khao Bahn Nah 432, Plai Ngahm Prachin Buri, and Prachin Buri 2) that usually have a major problem with chalkiness were processed by applying superheated-steam drying and conventional parboiling methods. The main objectives were: (1) to determine the possibility of applying superheated-steam drying to solve the chalkiness and low head rice yield problems and (2) to compare the properties of rice produced using superheated-steam drying and the conventional parboiling process. Both the initial moisture content and superheated-steam drying temperature significantly affected head rice yield. The higher moisture helped to increase starch gelatinization leading to a stronger rice structure and subsequently an increased head rice yield. The rice samples dried in the superheated-steam dryer using an initial moisture content of paddy at 32% w.b. for 6 h under a steam pressure of 1.2 bar and at three drying temperatures (120, 140, 160 °C) had higher milling quality than the conventionally parboiled rice samples. The darker color of the superheated-steam-dried samples was their main drawback. Both parboiling and superheated-steam drying could clearly lessen the percentage of chalky rice kernels compared to the raw paddy. The parboiled rice and superheated-steam-dried rice had more nutrients than normal white rice.     

不同加工程度大米食味变化分析

为研究大米加工程度对米饭食味的影响,对不同加工程度大米进行基本理化指标、质构、感官品质及风味测定。结果表明,随着加工程度提高,大米食味值增大,总脂肪、蛋白质含量降低,米饭硬度减小。蒸煮可显著增加米饭的水溶性蛋白质含量。大米加工程度越高,米饭风味成分含量损失越严重。相对于糙米,碾磨3 min、碾磨6 min、碾磨9 min的大米米饭挥发性成分总量分别减少了50.7%、73%、79%。碾磨6 min和碾磨9 min大米在理化性质（食味值、粗蛋白、直链淀粉、脂肪含量）、质构（黏度、平衡、弹性）均无显著差异。     

Nutrient content and distribution in milling fractions of rice grain.

Nutrient content and distribution in a low-protein (7.5%) rice and a high-protein (10.8 %) rice were studied by analysing successive abrasive milling fractions of brown rice. Non-starch constituents decreased from the surface to the centre of the grain in both rices, except that the highest protein fraction in high-protein rice was the sub-aleurone layer. Starch and amylose contents of starch increased progressively from the surface to the centre of the grain and were lower in high-protein grain. Although this low-protein rice had lower total ash content than the high-protein rice, other samples of the same low-protein rice had comparable ash content to the high-protein rice. Glutelin accounted for 87–93% of milled rice protein. Protein and protein bodies of the sub-aleurone layer and inner endosperm have similar amino-grams, and electrophoretic patterns using analytical and SDS-polyacrylamide disc gels.