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.     

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.     

Changes in physico‐chemical and rheological properties of rice during flaking

Hydration, solubility, pasting and rheological properties of roasted‐parboiled rice and flakes were examined in comparison to raw rice. There was an increase in the hydration capacity, swelling power and solubility during roasting‐parboiling of rice as a result of gelatinization of its starch. Flaking caused further increase due to damage of starch by application of mechanical force. The changes were higher in flakes from roller‐flaker as compared to those from edge‐runner. Pasting characteristics of flour slurries in Rapid Visco Analyzer showed a typical profile for pregelatinized products viz. a higher initial viscosity but a lower peak viscosity for roasted‐parboiled rice and flaked rice than for raw rice. The viscosity curves and flow curves of the products over a wide range of shear rate indicated a non‐Newtonian, pseudoplastic behaviour for all the samples. All samples showed typical hysteresis loop in their viscosity curves indicating their thixotropic nature. Flakes from roller‐flaker exhibited lower viscosity but more thixotrophy indicating higher starch breakdown in them than in edge‐runner flakes, which seemed to have more of heat damaged starch.     

Studies on parboiled rice: Part 1—Comparison of the characteristics of raw and parboiled rice

Various properties of raw and parboiled rice were compared in an effort to elucidate the factors responsible for the changes induced by parboiling.The parboiled rice was less prone to disintegration on cooking, the kernels remaining well separated and less sticky than the non-parboiled sample. The solids leached into the cooking water and the extent of solubilisation of the kernels on cooking were both considerably lowered by parboiling. Amylograms of flour prepared from the rice revealed that this was due to the resistance of the starch in the parboiled rice to swelling and solubilisation in hot water.From the results of X-ray diffraction spectra it was concluded that the behaviour of parboiled rice is influenced by the presence of an insoluble helical amylose complex and not, as is generally assumed, by retrogradation.     

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.     

The Significance of Starch Polymorphism in Commercially Produced Parboiled Rice

A study of eight commercial parboiled samples derived from two varieties of rice produced by four different processes has shown that depending on the parboiling process, the starch component itself can be present as native and/or retrograded starch in addition to the amylose-lipid complex. Further, it was demonstrated that the polymorphic states of starch can influence the texture and behaviour of cooked rice. The parboiled rice samples which had all three states of starch (i.e. ungelatinized and recrystallized amylopectin plus the amylose-lipid complex) possessed the hardest eating property but the lowest solubility. A negative linear relationship was demonstrated between the hardness and the solubility of cooked, parboiled rice. Overall, the observations suggest the existence of different forms of starch in parboiled rice which vary with the different parboiling protocols. The conditions governing their formation need to be established before investigating the specific functionality of individual forms within the rice. This study further confirmed that retrograded starch (amylopectin) in parboiled rice did not exhibit a B-tye X-ray pattern but mixed A+V patterns.     

Parboiling of rice. Part II: Effect of hot soaking time on the degree of starch gelatinization

A differential scanning calorimetric study was done on raw and parboiled rice to determine the degree of gelatinization. Unparboiled rice absorbed the highest amount of endothermic heat, the enthalpy change gradually decreasing with increasing hot soaking time. The highest degree of gelatinization was achieved when the paddy was soaked for 120 min at 80 °C. With increasing degree of gelatinization, the yield point in a compression test also increased. During the parboiling process internal fissures were healed, resulting in higher head rice yield during milling.     

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.     

Cooking qualities of parboiled rices produced at low and high temperatures

The cooking characteristics of parboiled rice are related to (i) its hydration behaviour at temperatures above and below the gelatinisation point; (ii) to kernel elongation on cooking; and (iii) to the extent of amylose solubility. These properties differ among samples, depending on the parboiling conditions. Samples were prepared by parboiling paddy at 70, 80, 90, 100, 110 and 120°C. The kernel elongation on cooking and the amount of soluble amylose in the gruel were then determined. The water uptake values for raw and parboiled rice samples were determined by hydrating them at room temperature (25-30), 60 and 98°C (boiling temperature) for optimal cooking times. The rate of hydration at temperatures below the gelatinisation point increased on parboiling and, conversely, a reverse pattern above this point. Close correlations existed between the temperature of parboiling and the properties studied. The different properties studied also correlated well. The temperature of parboiling influenced the linear elongation of the kernel after cooking. The soluble amylose content was negatively correlated with the temperature of parboiling. Though the hydration properties of different parboiled samples differed among themselves, depending on the degree of parboiling, they fell into two distinct classes, viz. the samples parboiled at a temperature close to the gelatinisation point having cooking qualities similar to raw rice, and above this point qualities differing from raw rice. The water uptake values at room temperature and at 60°C, and the ratio of water uptake at 98°C and optimum cooking time to that at 60°C were found to be useful in differentiating the parboiled rices into the two classes.     

INFLUENCE OF PARBOILING ON RICE STARCH STRUCTURE AND COOKED-RICE TEXTURE

Four varieties of rice having very high (29.4%) to low (17.6%) amylose-equivalent (AE) were parboiled by steaming at atmospheric (PB-O) and at 3 kg/cm2 (PB-3) gauge pressure. The cooked rice texture of the raw, PB-0 and PB-3 rice was determined by Viscoelastographe. Gel permeation chromatography of rice flour starch on Sepharose CL-2B, in general, indicated thermal breakdown of starch. The extent of starch breakdown and the firmness values of cooked PB-0 and PB-3 rices were reasonably well correlated suggesting that starch breakdown during parboiling may have some role to play in varying the texture of rice by parboiling. The chain profile of rice starch remained unaltered after parboiling.     

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.     

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.     

Impact of parboiling conditions on Maillard precursors and indicators in long-grain rice cultivars

The effect of steaming conditions (mild, intermediate and severe) during parboiling of five different long-grain rice cultivars (brown rice cultivars Puntal, Cocodrie, XL8 and Jacinto, and a red rice) on rice colour, and Maillard precursors and indicators was investigated. Rice colour increased with severity of parboiling conditions. Redness increased more than yellowness when parboiling brown rice. Parboiling turned red rice black. It changed the levels of glucose, fructose, sucrose, and maltose. Losses of the non-reducing sugar, sucrose were caused by both leaching into the soaking water and enzymic conversion, rather than by thermal degradation during steaming. Concentrations of the reducing sugars, glucose and fructose, in intermediately parboiled rice were higher than those of mildly parboiled rice. After severe parboiling, glucose levels were lower than those of intermediately parboiled rice, while fructose levels were higher. These changes were ascribed to the sum of losses in the Maillard reaction (MR), formations as a result of starch degradation and isomerisation of glucose into fructose. It was clear that the ε-amino group of protein-bound lysine was more affected by parboiling conditions and loss in MRs, than that of free lysine. Low values of the MR indicators furosine and free 5-hydroxymethyl-2-furaldehyde (HMF) in processed brown and red rices were related to mild parboiling, whereas high furosine and low free HMF levels were indicative of rices being subjected to intermediate processing conditions. High furosine and high free HMF contents corresponded to severe hydrothermal treatments. The strong correlation (r = 0.89) between the free HMF levels and the increase in redness of parboiled brown rices suggested that Maillard browning was reflected more in the red than in the yellow colour.     

Effect of hydrothermal treatment varying in time and pressure on the properties of parboiled rices with different amylose content

Effects of mild, moderate and severe hydrothermal treatments at open and under steam pressure on four rice varieties differing in amylose content were studied. Degree of gelatinization was markedly high in the severely pressure parboiled samples. Pressure parboiled waxy samples showed extensive fall of gelatinization temperature and increased hydration at lower temperatures. Formation of short amylopectin fine structures in these samples was indicated by sediment volume and viscosity patterns. The pattern of change in the infrared spectroscopic bands may be due to shift in stretching mode. Wide angle X-ray diffractographs of raw samples showed peaks at 2 θ values near 20 which indicated ‘in situ’ points for amorphous amylose complex formation. A, B and V-type polymorphs were seen in pressure parboiled samples and only A and V-type polymorphs were observed in open steamed samples. Loss in crystallinity with simultaneous increase in water uptake can be attributed to the amorphous fractions in parboiled rice.     

Effect of soaking medium on the physicochemical properties of parboiled glutinous rice of selected Laotian cultivars

The effect of various soaking mediums, viz. water (control), 3% NaCl and 0.2% acetic acid, and without soaking on the physicochemical properties of parboiled selected glutinous (TDK8 and TDK11) and non-glutinous (Doongara) was investigated in the present study. Results showed that the chemistry of soaking had a significant effect on the head rice yield (HRY), grain hardness, crystallinity, color, pasting and thermal properties, textural attributes, and glycemic index of these rice varieties. Soaking with NaCl and acetic acid significantly increased the grain hardness and HRY than control and without soaking treatments. Acetic acid and NaCl soaking significantly affected crystalline regions of starch resulting in reduced crystallinity in X-ray diffraction analysis and thermal endotherms in DSC analysis. NaCl soaking induced swelling of starch granules resulting in high peak and final viscosities. However, acetic acid restricted swelling resulting in reduced peak and final viscosities. NaCl and acetic acid soakings also resulted in increased hardness and adhesiveness of cooked grains than normal water soaked and un-soaked parboiled rice samples. Interestingly, change in textural attributes was prominent in parboiled glutinous rice. The color difference value for fresh parboiled samples was significantly lower for acetic acid soaked samples compared to NaCl soaked and un-soaked samples probably due to bleaching effect of acetic acid. Moreover, parboiling also resulted in significant reduction in glycemic index of glutinous rice. These findings revealed the potential application of parboiling with modified soaking techniques to improve the grain quality.     

Thermal Degradation of Starch in Parboiled Rice

Raw and variously parboiled Intan rice (intermediate-amylose variety) starch was fractionated in Sepharose CL-2B column into a larger-molecule (Fr I) and a smaller-molecule (Fr II) fraction. The amount of Fr I decreased and that of Fr II increased in parboiled as compared to raw rice, the extent of change increasing with increasing severity of processing. The proportion of polysaccharide and “amylose” in Fr I decreased from 70% and 40% of total, respectively, in raw rice to 16% in parboiled rice prepared by 3 kg/cm² gauge steam pressure. Simultaneously, the λ_max of its iodine complex increased from 570 to 588 nm in Fr I and decreased from 635 to 620 nm in Fr II. The change was marginal or low after steaming at ambient pressure or after sand roasting. The data show that thermal degradation of starch must be considered as another contributor, in addition to starch gelatinization and reassociation, to the peculiar properties of parboiled rice.     

Effect of parboiling on thiamin, protein and fat of rice

Parboiling decreased the thiamin content of rice. Nevertheless, milled parboiled rice contained more thiamin than milled raw rice at the same degree of milling because parboiling caused thiamin to diffuse inwardly. Both the degree of loss and the degree of diffusion depended on the severity of the heat treatment. Bran-polish of parboiled rice had a higher fat and protein content than bran-polish of raw rice at the same degree of milling because the starchy endosperm of parboiled rice had a greater resistance to milling and so the bran and the germ were more effectively removed. Milled parboiled rice tended to have lower protein content than milled raw rice. Starch content was lower in bran-polish from parboiled rice.     

Milling breakage susceptibility and mechanical properties of parboiled brown rice kernels

Parboiling, a hydrothermal treatment of paddy or brown rice, impacts both head rice yield and texture and nutritional characteristics of cooked rice. Here, milling breakage susceptibility of parboiled brown rice was investigated on both bulk and kernel level. Brown rice was parboiled by soaking at 40, 55 or 65 °C and steaming at 106, 120 or 130 °C. The breakage susceptibility and changes in starch and proteins of bulk samples were related to the properties of individual rice kernels. An increase in milling breakage susceptibility from 1% to 11% corresponded to a decrease in average bending force of individual kernels from 34.9 to 14.6 N. Furthermore, both white bellies and fissured parboiled rice grains were more breakage susceptible. Their average bending force was respectively 14.1 N and 17.6 N compared to an average of 39.6 N for intact parboiled rice grains. Whereas the level of proteins extractable with sodium dodecyl sulfate containing medium had no impact, the degree of starch gelatinization was critical in determining the presence of both white bellies and fissured parboiled rice grains. More in particular, complete starch gelatinization ensured the absence of white bellies and minimized fissuring in the parboiled end-product, thereby decreasing milling breakage susceptibility.     

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.     

INFLUENCE OF PARBOILING AND FREEZING CONDITIONS ON TEXTURAL PROPERTIES OF CALIFORNIA COOKED RICE

The textural properties of thawed samples of cooked parboiled, long (Cal Belle) and short (S201) grain rice varieties were evaluated using an Instron Universal Testing Instrument (Model 1122). In general, the parboiling treatments resulted in a significant increase of hardness but a significant decrease in stickiness of both long and short grain cooked rice when freezing conditions were pooled. Freezing increased hardness and decreased stickiness of long grain cooked parboiled rice significantly regardless of parboiling conditions, however, it did not decrease the stickiness of short grain cooked parboiled rice significantly. The long grain rice was harder and less sticky than the short grain rice when cooked regardless of treatments used. Hardness was negatively correlated with stickiness indexes (r =?0.819, P < 0.001).