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.     

稻谷水热加工过程中镉迁移规律研究

目的研究稻谷在蒸谷米水热加工过程中镉的迁移规律。方法以蒸谷米小试装置及生产线采集样品为研究对象,在优化碾米时间的基础上,在排除水分含量差异对测定结果的前提下,利用统计分析考察水热工艺(浸泡和蒸煮)对稻谷各部分(稻壳、米糠和精米)镉含量的影响。结果由实验数据推断,在水热过程中,镉的迁移主要发生在稻谷的内部,且主要是在浸泡时从精米向米糠迁移富集,而蒸煮工艺则对镉的分布变化影响很小;经过浸泡,精米中的镉含量可降低约40%;推测热水的浸泡一方面使胚乳淀粉分子间游离的镉部分溶出,一方面促使镉向络合能力更强的米糠蛋白迁移,最终导致其分布发生明显变化。结论该研究为在稻谷加工过程中实现重金属镉的消减提供了科学依据。     

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.     

Microstructure of olbyeossal,partially milled parboiled glutinous rice made by modified parboiling method

The microstructure of partially milled parboiled glutinous rice (PMPGR) before and after cooking was investigated using scanning electron microscopy to identify the changes in the microstructure due to parboiling treatments. Parboiling was performed in two different ways: conventional and modified. The conventional parboiling method involved soaking at room temperature for 24 h, followed by steaming and drying. The modified parboiling method involved tumbling to replace soaking of the rice grains, tempering, retorting, and drying. The microstructures of the PMPGR made using both methods were compared. The modified parboiling method significantly changed the microstructure of the rice, but cooking did not cause considerable changes in the microstructure of PMPGR.     

Key factors affecting Fe density in Fe-fortified-parboiled rice: Parboiling conditions,storage duration,external Fe-loading rate and genotypic differences

The present study evaluated the key factors affecting the efficiency of iron (Fe) penetration into the endosperm in parboiled rice of different varieties. It also investigated effects of storage time on Fe bio-accessibility, rice colour and Fe retention after rinsing. Rice grains of three varieties were fortified with an increasing range of Fe-fortification rates during the parboiling process, under two typical parboiling conditions, which are ambient soaking temperature for 24 h and 60 °C soaking temperature for 6 h at neutral (6.0–6.5) and acidic pH (3.0–3.5). Soaking of paddy rice, at 60 °C in acidic water for 6 h before steaming, was found to be better for maximising the Fe concentration in white-parboiled rice than the former ambient soaking. Under this parboiling condition, adding 250 mg Fe kg⁻¹ of paddy rice, at soaking, produced the most desirable Fe concentration in white rice, ranging from 17.5 to 25.4 mg kg⁻¹ among the rice varieties tested. The concentrations of Fe in parboiled white rice exhibited an exponential increase with increasing concentrations of Fe in the soaking water in all varieties, which were linearly related to Fe concentration of brown rice (r = 0.96∗∗, p < 0.01). The colour of the parboiled rice fortified with Fe was initially light yellow, with variation among rice varieties, but it did become slightly darker after 16 weeks of storage, probably because of Fe oxidisation. This may be related to decreasing bio-accessibility after 20 weeks of storage. Storage, however, did not affect the total Fe retention after rinsing, though the retention rate was variety-dependent. Information about parboiling will provide the basis for formulating an optimal industry protocol for producing Fe-fortified-parboiled rice, which can be further refined in pilot studies on the industrial scale.     

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.     

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.     

Migration of mycotoxins into rice starchy endosperm during the parboiling process

Considering the occurrence of rice contamination by mycotoxins and the increase in rice consumption, the present work had the objective of assessing the migration of mycotoxins into the starchy endosperm during the parboiling process, as to propose conditions that provide lower contamination levels. The newly harvested rice grain sample was examined for the natural occurrence of mycotoxins (aflatoxin B₁, aflatoxin B₂, deoxynivalenol, ochratoxin A, and zearalenone); only the presence of aflatoxin B₁ was found (17 ng/g). The samples were then artificially contaminated with deoxynivalenol and zearalenone, and the parboiling process was conducted according to a 2³ factorial planning with central point, having as variables the contamination level deoxynivalenol 720, 1440, and 2160 ng/g, and zearalenone 476, 952, and 1428 ng/g the soaking time (4, 5, and 6 h) and autoclave time (15, 22.5, and 30 min). Mycotoxins aflatoxin B₁ (AFA B₁), deoxynivalenol (DON), and zearalenone (ZEA) were confirmed and determined through gas chromatography. Findings showed a lower migration trend for AFA B₁ under 6 h of soaking and 30 min of autoclaving, for DON under 6 h of soaking regardless of the autoclaving time, and for ZEA under 4 h of soaking and 15 min of autoclaving. This information can contribute to the choice of process parameters that limit the migration of these mycotoxins if they happen in the raw material.     

Changes in nutritional constituents, anthocyanins, and volatile compounds during the processing of black rice tea

The present study was undertaken to assess the changes in nutritional composition, anthocyanins, and volatile compounds during 3 successive processes (soaking, steaming, and roasting) for black rice tea production. Significant (p<0.05) decreases of cyanidin-3-glucoside and peonidin-3-glucosidetotal content were presented in black rice grains during soaking, steaming, and roasting. However, only steaming and roasting caused significant (p<0.05) decreases in the contents of total starch and crude fat, whereas accompanied with a stable level of total starch during all the processes. Furthermore, 94 of volatile compounds were identified in the raw and processed black rice. Nonanal, butylated hydroxytoluene, 1-hexanol, naphthalene, and 1-octen-3-ol were the main volatile compounds. These volative compounds extracted from raw black rice grain varied widely compared to the processed materials. The study indicated that processing could significantly affect the profiles of nutritional compositions, anthocyanins, and volatile compounds during the production of black rice tea.     

Zinc fortification of whole rice grain through parboiling process

The present study evaluated the effectiveness of zinc (Zn) fortification in a parboiling process for improving Zn density in parboiled-polished rice and its potential bioavailability in the human diet. Fortification of Zn in whole paddy rice grain with 50–400 mg Zn/kg paddy rice, during parboiling, increased Zn concentrations in polished-parboiled rice from 1.3 to 4.5 times those in unfortified parboiled rice. The added Zn rapidly penetrated into parboiled rice grains in the initial soaking process before saturation. There was an exponential correlation between Zn concentrations in unpolished (r = 0.63) (p < 0.01) and polished rice (r = 0.30) (p < 0.05) and soaking time. Zinc concentrations in unpolished rice were linearly correlated with Zn concentration in the polished rice (r = 0.60) (p < 0.01). Moreover, more than half of the added Zn is retained after a simulated washing process before cooking, ranging from 64–100%. In the Zn-fortified parboiled rice, 57–100% of Zn in polished rice grain was soluble in dilute acid, which was indicative of a high potential Zn bioavailability for human intake. The results suggest that parboiled rice has great potential for Zn fortification.     

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.     

Structural and chemical analysis of resistant starch effected by pre-treatments,cooking methods,reheating and storage condition in parboiled germinated brown rice (Oryza sativa)

Resistant starch (RS) content could be altered by the processing method, including pre-treatment, cooking method and storage conditions. This study determined the influence of RS formation in white rice (WR), brown rice (BR) and parboiled germinated brown rice (PGBR) as affected by various pre-treatment and processing conditions. This is the first report to chemically and structurally analyse WR, BR and PGBR of the same rice variety using X-ray diffraction, DSC and SEM. The results showed that the PGBR prepared by a non-soaking process cooked with steaming and stored cold contained RS content (92.1 mg/g, wet basis) followed by BR processed by soaking, steamed and then stored cold (91.8 mg/g, wet basis). The formation of RS3 as not affected by the parboiling and germination process compared to WR and BR. Reheating reduced resistant starch content by changing the RS structure conformation from crystalline to amorphous under higher temperatures. Moreover, the scanning electron microscope revealed that WR, BR and PGBR of the same rice variety show similar starch granule and microstructure patterns but slightly different in thermal conductivity value.     

速食发芽糙米的研究

以糙米为原料经过发芽处理,采用蒸煮糊化法制取速食发芽糙米。运用二次蒸煮糊化法进行糊化,将预蒸煮获得的速食发芽糙米进行浸泡和第二次蒸煮,糊化完成后进行干燥处理制成速食发芽糙米。结果表明:利用正交实验确定最佳糊化条件为:预蒸煮时间25min,浸泡温度60℃,浸泡时间35min,二次蒸煮时间30min;最佳干燥温度为80℃,时间为90min。     

Factors influencing antioxidant compounds in rice

Epidemiological and clinical studies suggest that the additive/synergistic effects of several bioactive compounds are responsible for the health benefits of rice. Among the leading contenders are phenolic acids, flavonoids, anthocyanins, proanthocyanidins, tocotrienols, tocopherols, λ-oryzanol, and phytic acid, which all possess strong antioxidant activities in vitro. In this review, data related to health effects of rice antioxidants using cultured cells, rodents and humans models are first summarized. The evidence is strong that consumption of rice tocotrienols translates into improved health outcomes. Current research, however, does not strongly support the health-promoting effects of rice tocopherols and phenolic acids. The crucial limitations in studies using rice flavonoids, anthocyanins, proanthocyanidins, λ-oryzanol and phytic acid appear to be the appropriateness of the substance tested (i.e., purity), and the scarcity of animal and human interventions. In a second part, rice antioxidants are reviewed with an emphasis on their composition and contents. Taking into account the bioavailability of these compounds, it is evident that a number of factors affect the antioxidant composition of rice, making it difficult to estimate dietary intake. Before harvest, factors including soil type, atmospheric CO₂, chemical inputs, temperature, and degree of ripening are important. After harvest, rice is subjected to processing methods that include drying, parboiling, storage, irradiation, milling, stabilization, soaking, germination, fermentation, boiling, steaming, roasting, baking, and extrusion. Quantitative knowledge about the effects of these processes is summarized in this review. Surprisingly, a high level of agreement was found among study results, which could be useful in manipulating the growing and processing techniques of rice grains to facilitate efficient and safe consumption of antioxidant compounds.     

Effect of Mashing Temperatures and Endo-β-Glucanase on β-Glucan Content of Malt Worts

Similar basal levels of β-D-glucans were released into worts produced at 45°C from enzymically active or inactivated flours of milled malts. In contrast, significantly higher levels of β-D-glucans were found in worts derived from either enzymically active or inactivated malt flours mashed at 65°C. In general, mashing temperature may play a more important role in releasing β-D-glucans during mashing than enzymes described as β-glucan-releasing. In this context, the physical release of β-D-glucan during mashing should be separated from the enzymic release and degradation of β-D-glucan which occur during malting.     

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.     

Purification and Properties of Saccharogenic Amylase from Piricularia oryzae

Piricularia oryzae, the pathogenic fungus of rice blast disease, produced a glucoamylase as a sole amylolytic enzyme with little concomitant activity of other related enzymes. The purified enzyme preparation showed a single band on electrophoresis. The enzyme had optimum pH at 6.5 and optimum temperature between 50 and 55°C. It was completely inactivated by heating for 15 min at 60°C. The molecular weight of the enzyme was estimated as about 94,000. The enzyme hydrolyzed various kinds of amylaceous substrates and some malto-oligosaccharides into glucose. Glucobioses such as isomaltose, nigerose, kojibiose, α,α-trehalose, β,β-trehalose as well as phenyl-α-glucoside were also hydrolyzed.     

Laboratory Process Development and Physicochemical Characterization of a Low Amylose and Hydrothermally Treated Ready-to-Eat Rice Product Requiring No Cooking

A low amylose and hydrothermally treated ready-to-eat rice product that requires no cooking was prepared in the laboratory. Hot soaking for 1–3 min with subsequent variable steaming at open and under pressures remarkably altered the kernel and flour properties. Increase in water absorption and lowering of cooking time with extent of steaming were prominent. Soaking of the product at 50 °C for 20 min gave texture values more similar to cooked samples. The viscosity parameters of hot soaking alone were in between those of hot soaking with open steaming and pressure steaming. Pressure steamed samples exhibited almost constantly increasing slurry viscosity throughout the heating and cooling phases of the rapid viscosity analyzer profile. Steaming variably altered the native A-type X-ray diffraction pattern. Pressure steaming of samples with 3 min hot soaking caused complete loss of the A-type conformation with feeble peaks for B- and V-type patterns. The open steamed samples showed peaks for all A-, B-, and V-type patterns. No endotherms for amylose-lipid complexes were however found in the differential scanning calorimetry of the pressure steamed samples. The raw rice flour was highly resistant to α-amylolysis. In open steamed samples, steaming severity decreased the hydrolysis rate indicating formation of enzyme-resistant fractions, while pressure steamed samples showed higher digestibility with treatment severity.     

Effects of hydrothermal treatment on the physicochemical properties of pregelatinized rice flour

Effects of hydrothermal treatment on the pasting, thermal and hydration properties of rice flour prepared from three Indica varieties of milled rice [Taichung Sen Glutinous (TCSW) 1, Taichung Sen (TCS) 10, and TCS 17], which contained 1.2, 17.9 and 28.8% amylose, were studied. Three hydrothermally-treated factors, soaking time, steaming temperature and steaming time were investigated. From the statistical results of an experimental design of L₂₇ orthogonal array, the soaking time is a significant factor that affects the pasting properties of three varieties of rice with various degrees of influence. However, the increase of pasting temperature, reduction of peak viscosity and elevation of final viscosity were investigated for all treated non-waxy rice and resulted in the reduction of its breakdown and total setback. The differences of pasting and hydration properties of treated rice flour were attributed to the changes of rigidity of starch granules under treatment. The results of DSC showed that the degree of gelatinization was higher in waxy rice than in non-waxy rice when subjected to the same hydrothermal conditions.     

苦荞米及萌动苦荞米加工工艺研究

研究了苦荞米及萌动苦荞米加工过程中浸泡和蒸煮时间对熟化度,以及熟化后苦荞含水量对脱壳率和整米率的影响。结果表明:苦荞浸泡时间≥4h,蒸煮≥30min和浸泡时间≥5h,蒸煮≥20min的各处理,熟化度都能达到100%;当熟化后的苦荞水分含量在24.0%～26.0%时,脱壳率达到100%,整米率>90%。萌动苦荞米与苦荞米的加工工艺,可以采用相同的熟化条件和脱壳条件。苦荞及萌动苦荞脱壳工艺条件的研究为苦荞米及萌动苦荞米的加工提供了一定的科学依据。