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.     

Nature of Starch Crystallinity in Parboiled Rice

A-type X-ray diffraction spectra of raw rice was weakened upon mild parboiling and completely disappeared in moderately severe parboiled rice. A feeble V pattern of lipid-amylose complex appeared in its place. There was no unambiguous evidence of a B-type spectra of retrograded starch, although partial and weak B-like spectra could be seen. Differential scanning calorimetry showed that raw rice flour and starch had a large gelatinization and a small lipid-amylose endotherm, the latter disappearing after defatting. The gelatinization endotherm first decreased and eventually disappeared in samples with moderately severe parboiling, but a small lipid-amylose endotherm was visible in most parboiled rice samples. On the whole, parboiled rice appeared largely amorphous with only minor crystallinity, and the peculiar properties of parboiled rice could not be definitely ascribed to a definite crystalline form of starch.     

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.     

RHEOLOGICAL AND HYDRATION PROPERTIES OF POPPED RICE

The cold-slurry viscosity, sediment volume and hydration power of popped rice, along with those of dry-heat parboiled rice and expanded rice for comparison, were determined. Values of all these properties increased with increasing severity of processing treatment but decreased with starch reassociation. The cold slurries of these products showed time-dependent thinning, indicating thixotropic nature. A yield value appeared beyond a certain minimum concentration. The viscosities of the products, their hydration power and sediment volume were all interrelated. The popped rice was well gelatinized with little or no starch reassociation.     

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.     

COLD-SLURRY VISCOSITY OF PROCESSED RICE FLOUR

Viscosities of dispersions of raw and various processed rice flours in water were examined with a coaxial cylinder viscometer. The measured viscosity increased with increasing degree of processing and with a reduced level of apparent starch reassociation in the processed rice. Measurement of the apparent viscosity of cold slurries could thus be a simple practical test of processed, including parboiled, rice and of the degree of processing. The system showed time-dependent thinning and a typical loop in the shear rate-viscosity curve, showing that it was thixotropic in nature. The viscosities were proportional to the hydration power and slurry-sedimentation behaviour of the samples.     

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.     

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.     

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.     

CHANGES IN SUGARS AND AMINO ACIDS DURING PARBOILING OF RICE

ABSTRACT Parboiled brown rice contained considerably more reducing sugars (RS) but less sucrose and free amino acids (FAA) than raw brown rice. On milling, there was considerably loss of sucrose and FAA from raw rice, but very little from parboiled rice; reducing sugars changed little in either. Processing conditions affected the contents of sugars and FAA. Maximum increase in RS and decrease in sucrose content occurred after soaking at 60°C. Controlled incubation of rice flour, intact grain, separated germ and degermed rice in water showed that considerable changes in sugars and FAA occurred in all cases, the magnitude depending on the circumstances, but a greater part of the sugars leached out into the water during soaking.     

Antistaling of rice starch in a gel model system and Korean rice cake: the role of wheat flour in retrogradation-retardation technology

This study focused on the mechanisms of retrogradation-retardation technology for applications in garaetteok (steamed and extruded cylinder-shaped rice cake) production. Based on the comparison of conventional and retrogradation-retardation methods, the addition of raw wheat flour was selected as a key process. The rheological properties of gelatinized rice starch pastes after treatment with raw or autoclaved wheat flour and Novamyl L were investigated. Among them, raw wheat flour and Novamyl L resulted in significantly lowered the elastic modulus. The textural properties of garaetteok treated with raw or autoclaved wheat flour were examined by texture profile analysis (TPA) during storage for 3 days at 4°C. The hardness of garaetteok made with raw wheat flour was maintained for up to three days. In contrast, autoclaved wheat flour did not affect elastic modulus or hardness. As a result, raw wheat flour effectively retards starch retrogradation through starch hydrolysis induced by amylase in wheat flour.     

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.     

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.     

Varietal differences in properties of extrusion-cooked rice flour

Ten milled rices differing in starch properties were extrusion cooked through a twin screw extruder at 15% moisture and 120, 135 and 150°C. Instron hardness values of extrudate were lowest at 150°C. Extrusion cooking drastically reduced viscosity in 0·2n KOH and in water without any decrease in amylose content and with little increase in reducing sugars. Spherical protein bodies were intact in the protein masses uniformly dispersed in the gelatinized starch matrix. Amylose content and gel consistency of raw rice flour were significantly correlated with expansion ratio only at 135°C, water absorption and solubility indexes at all temperatures and with gel viscosity of extrudates. Only gel consistency of raw flour correlated with cold paste viscosity of extrudate. Final gelatinization temperature of starch was not significantly correlated with any extrudate property.     

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.     

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.     

A comparative study on the physicochemical,cooking and textural properties of fortified rice kernels prepared from raw and parboiled rice

Fortified rice kernels (FRK) are extruded rice-shaped kernels manufactured by blending broken rice flour and vitamin–mineral premix. In this study, four parboiled and three raw rice with intermediate amylose content (20–24%) were processed to manufacture FRK. The system parameters (torque and die pressure), physicochemical properties (length, lightness, water absorption index and water solubility index), cooking properties (cooking time, solid loss, elongation) and textural properties of FRK were analysed and correlated using the Pearson correlation coefficient. The torque and die pressure were 16.76–22.76 Nm and 92.56–136.41 bar, respectively. Compared to raw rice, the parboiled rice showed the highest torque, die pressure and axial expansion in uncooked FRK while longer cooking times, higher elongation ratio and hardness; lesser water absorption ratio and solid loss in cooked FRK. The raw rice FRK showed higher lightness. There was a significant difference between all FRK for water absorption index (1.74–2.52) and water solubility index (2.06–3.53%).     

不同储藏年限稻谷的品质及蒸谷米加工适应性分析

为了解不同储藏年限的稻谷加工蒸谷米的品质变化,为蒸谷米加工原料的选择提供依据。以不同储藏年限的籼稻、粳稻各40 份样品为研究对象,将其加工成蒸谷米,并对蒸谷米的蒸煮特性、外观及其制备米饭的食用品质与质构特性等指标进行测定,采用描述统计分析、方差分析、主成分分析等方法对蒸谷米的品质指标进行分析,采用聚类分析探讨稻谷的蒸谷米加工适应性。结果表明：随着原料稻谷储藏年限的延长,蒸谷米吸水率、体积膨胀率均呈上升趋势,与储藏1 年的稻谷相比,储藏4 年的籼稻和粳稻加工成的蒸谷米吸水率分别增加49.1%和35.9%,体积膨胀率分别增加70.6%和66.6%;米汤pH值、碘蓝值和固形物含量均无显著变化;感官评价总分均降低,米粒色泽加深,外观结构变差,米饭的适口性、滋味均降低,其中籼型蒸谷米的感官品质较粳型蒸谷米好;蒸谷米米饭质构特性各指标中除硬度、咀嚼性呈上升趋势以及粳型蒸谷米米饭的黏着性呈下降趋势外,其他各项指标无显著变化。在感官品质评价的基础上,引入质构特性指标分析,对蒸谷米的品质进行综合评价;并利用主成分分析,构建了蒸谷米品质评价的数学评价模型;再对综合评分进行聚类分析得出品质分类,并统计各储藏年份的样品数量,由此证实籼稻加工蒸谷米适应性优于粳稻,且储藏1、2 年的籼稻加工蒸谷米品质较好。     

Techniques for Canning Instant Parboiled Rice

The technology to produce an acceptable canned rice was investigated. Conventional raw and parboiled rice exhibit excessive starch leaching, loss of structural integrity, discoloration, unpalatable odors, and poor cooking qualities when canned. Instant parboiled rice which has been hydrated in boiling water before canning yields a superior product Objective color values are improved by flushing the hydrated rice with tap water before canning. Canned grains are free flowing and yield a product similar to freshly cooked rice. Without hydration, the product tends to be a solid mass of chalky and crumbly grains. A direct relation exists between subjective values for appearance and texture of canned rice.