Quality Changes and Tocopherols and γ-Orizanol Concentrations in Rice Bran Oil During the Refining Process

The quality changes and the concentrations of tocopherols and γ-oryzanol, during successive steps of rice bran oil refining (RBO), were studied. For this purpose, samples of crude, degummed, neutralized, bleached, dewaxed and deodorized RBO were taken from an industrial plant and analyzed. The moisture, pH, acidity, peroxide value and unsaponifiable matter, were determined. The fatty acid composition was evaluated by GC, and the concentrations of tocopherols and γ-oryzanol were determined using HPLC with fluorescence and UV–Vis detection, respectively. To identify γ-oryzanol components, fractions of the HPLC eluant were collected and analyzed using mass spectrometry. Oil refining reduced the peroxide value and acidity to 1 and 3% of the values obtained in crude RBO, respectively. The fatty acid composition were not significantly altered during refining. The concentrations of the tocopherols in RBO followed the order α > (β + γ) > δ. The total concentration of tocopherols was 26 mg/100 g, and remained practically unaltered during refining. Up to nine components were distinguished in γ-oryzanol. After collecting the elution fractions, up to six components were identified by electrospray mass spectrometry. Refining reduced the total concentration of γ-oryzanol to 2% of its initial value.     

Study on the composition of rice bran oil and its higher free fatty acids value

The compositions of rice bran oils (RBO) and three commercial vegetable oils were investigated. For refined groundnut oil, refined sunflower oil, and refined safflower oil, color values were 1.5–2.0 Lovibond units, unsaponifiable matter contents were 0.15–1.40%, tocopherol contents were 30–60 mg%, and FFA levels were 0.05–0.10%, whereas refined RBO samples showed higher values of 7.6–15.5 Lovibond units for color, 2.5–3.2% for unsaponifiable matter, 48–70 mg% for tocopherols content, and 0.14–0.55% for FFA levels. Of the four oils, only RBO contained oryzanol, ranging from 0.14 to 1.39%. Highoryzanol RBO also showed higher FFA values compared with the other vegetable oils studied. The analyses of FA and glyceride compositions showed higher palmitic, oleic, and linoleic acid contents than reported values in some cases and higher partial glycerides content in RBO than the commonly used vegetable oils. Consequently, the TG level was 79.9–92% in RBO whereas it was >95% in the other oils studied. Thus, refined RBO showed higher FFA values, variable oryzanol contents, and higher partial acylglycerol contents than commercial vegetable oils having lower FFA values and higher TG levels. The higher oryzanol levels in RBO may contribute to the higher FFA values in this oil.     

Optimization of Physical Refining to Produce Rice Bran Oil with Light Color and High Oryzanol Content

Crude rice bran oil (RBO) is rich in valuable minor components such as tocotrienols, phytosterols and γ-oryzanol. These compounds are well preserved during physical refining, but in current industrial practice, RBO is mostly refined chemically because this results in a lighter color. However this process removes most of the γ-oryzanol. The challenge is to develop a refining process which combines a high γ-oryzanol retention with the commercially desired light color. A modified physical refining process was developed, consisting of an acid degumming, prebleaching, dewaxing, physical removal of free fatty acids using packed column technology, a modified washing step, conventional bleaching and deodorization. A RBO with acceptable oryzanol retention of 39% had a Lovibond red color value (measured with a 5.25-inch cell) of 2.8, approaching very close the color of a chemically refined RBO (red = 2). At the process step where high (94%) retention of γ-oryzanol was achieved, a somewhat darker Lovibond red value of 5.2 was obtained.     

Effect of refining of crude rice bran oil on the retention of oryzanol in the refined oil

The effect of different processing steps of refining on retention or the availability of oryzanol in refined oil and the oryzanol composition of Indian paddy cultivars and commercial products of the rice bran oil (RBO) industry were investigated. Degumming and dewaxing of crude RBO removed only 1.1 and 5.9% of oryzanol while the alkali treatment removed 93.0 to 94.6% of oryzanol from the original crude oil. Irrespective of the strength of alkali (12 to 20° Be studied), retention of oryzanol in the refined RBO was only 5.4–17.2% for crude oil, 5.9–15.0% for degummed oil, and 7.0 to 9.7% for degummed and dewaxed oil. The oryzanol content of oil extracted from the bran of 18 Indian paddy cultivars ranged from 1.63 to 2.72%, which is the first report of its kind in the literature on oryzanol content. The oryzanol content ranged from 1.1 to 1.74% for physically refined RBO while for alkali-refined oil it was 0.19–0.20%. The oil subjected to physical refining (commercial sample) retained the original amount of oryzanol after refining (1.60 and 1.74%), whereas the chemically refined oil showed a considerably lower amount (0.19%). Thus, the oryzanol, which is lost during the chemical refining process, has been carried into the soapstock. The content of oryzanol of the commercial RBO, soapstock, acid oil, and deodorizer distillate were in the range: 1.7–2.1, 6.3–6.9, 3.3–7.4, and 0.79%, respectively. These results showed that the processing steps—viz., degumming (1.1%), dewaxing (5.9%), physical refining (0%), bleaching and deodorization of the oil—did not affect the content of oryzanol appreciably, while 83–95% of it was lost during alkali refining. The oryzanol composition of crude oil and soapstock as determined by high-performance liquid chromatography indicated 24-methylene cycloartanyl ferulate (30–38%) and campesteryl ferulate (24.4–26.9%) as the major ferulates. The results presented here are probably the first systematic report on oryzanol availability in differently processed RBO, soapstocks, acid oils, and for oils of Indian paddy cultivars.     

Comparison of supercritical fluid and solvent extraction methods in extracting γ-oryzanol from rice bran

Organic solvents were compared with supercritical CO₂ relative to efficiency for extracting lipid and γ-oryzanol from rice bran. A solvent mixture with 50% hexane and 50% isopropanol (vol/vol) at a temperature of 60°C for 45–60 min produced the highest yield (1.68 mg/g of rice bran) of γ-oryzanol among organic solvents tested. The yield of γ-oryzanol without saponification was approximately two times higher (P<0.05) than that with saponification during solvent extraction. However, the yield (5.39 mg/g of rice bran) of γ-oryzanol in supercritical fluid extraction under a temperature of 50°C, pressure of 68,901 kPa (680 atm), and time of 25 min was approximately four times higher than the highest yield of solvent extraction. Also, a high concentration of γ-oryzanol in extract (50–80%) was obtained by collecting the extract after 15–20 min of extraction under optimized conditions.     

Influence of chemical refining on the major and minor components of rice brain oil

The effects of each individual step of the chemical refining process on major and minor components of rice bran oil were examined. In comparison with common vegetable oils, rice brain oil contains a significantly higher level of several bioactive minor components such as γ-oryzanol, tocotrienols, and phytosterols. Alkali treatment or neutralization results in a significant loss of oryzanol. In addition, it gives rise to a change in the individual phytosterol composition. After bleaching, some isomers of 24-methylenecycloartanol were detected. Because of their relatively high volatility, phytosterols and tocotrienols are stripped from the rice brain oil during deodorization and concentrated in the deodorizer distillate. At the same time, oryzanol is not volatile enough to be stripped during deodorization; hence, the oryzanol concentration does not change after deodorization. Complete refining removed 99.5% of the FFA content. Depending on the applied deodorization conditions, trans FA can be formed, but the total trans content generally remains below 1%.     

Recovery of γ-oryzanol from rice bran oil with silica-based continuous chromatography

A simulated moving bed chromatography separator was tested for recovery of γ-oryzanol from degummed and dewaxed rice bran oil that contained 1.2 to 1.6% γ-oryzanol. A crude product with 12–15% of γ-oryzanol was obtained and a 90 to 95% pure product was recovered from the concentrate by crystallization from heptane. With the recycling of the crystallization liquor, an overall γ-oryzanol recovery of 85 to 90% is feasible and is potentially higher than the recovery in the conventional soapstock-based process. Approved for publication by the Director, Louisiana Agricultural Experiment Station as publication No. 97-60-0276.     

Review on Recent Trends in Rice Bran Oil Processing

Rice bran oil (RBO) is popular in several countries such as Japan, India, Korea, China and Indonesia as a cooking oil. It has been shown that RBO is an excellent cooking and salad oil due to its high smoke point and delicate flavor. The nutritional qualities and health effects of rice bran oil are also established. RBO is rich in unsaponifiable fraction (unsap), which contains the micronutrients like vitamin E complexes, gamma oryzanol, phytosterols, polyphenols and squalene. However, the high FFA and acetone-insoluble content of RBO made it difficult for processing. Therefore, in recent years, research interest has been growing in RBO processing to obtain good quality oil with low refining loss. This review article deals with detailed reports on RBO processing including membrane-based techniques from the production and quality point of view.     

Refining of rice bran oil by neutralization with calcium hydroxide

The applicability of calcium hydroxide (lime) in the neutralization of rice bran oil (RBO) was investigated. Crude RBO samples of three different free fatty acids (FFAs) (3.5–8.4 wt%) were degummed, dewaxed, bleached, and neutralized with lime and deodorized. The oils obtained thus were characterized by determining the color, peroxide value (PV), content of unsaponifiable matter (UM), and FFA. Conventionally practiced caustic soda neutralization (at 80–90°C) of FFA has in the present investigation been replaced by a high temperature (150–210°C) low pressure (2–4 mm Hg) reaction with lime. It was observed that neutralization with Ca(OH)₂ at high temperature (210°C) and under low pressure (2–4 mm Hg pressure) may substantially reduce the FFA content (0.8 wt%, after 2 h). The deodorized oil was found to be of acceptable color, PV, and content of UM and FFA. Neutralization of oil was also carried out by using NaHCO₃ and Na₂CO₃, nonconventional alkalies for neutralization, and the results were compared with NaOH and Ca(OH)₂. Overall recovery of oil in Ca(OH)₂ refining process (88.5 ± 0.6 wt%, for Sample 1 containing 8.4%‐wt FFA) was found to be more than other competitive processes studied.     

Extrusion deactivation of rice bran enzymes by pH modification

Rice bran is considered in Mexico as “waste”, useful only for feeds. As considerable amounts of oil are available in rice bran, it might be worthwhile to stabilize it and extract the edible oil before using it for feedstuffs. Precisely these oils are responsible for rice bran rapid deterioration, particularly in climatic conditions such as those prevalent in Mexico's tropical areas (high humidity and high temperature). This paper deals with the study of the effect of pH during extrusion of fresh rice bran in order to inactivate lipid‐breaking enzymes. Hydrochloric acid or calcium hydroxide, Ca(OH)₂, were added at 0, 1, 5, 10% (dry basis), and moisture content of the bran samples was varied (20, 30, 40%, dry basis) in a 3² factorial design to corroborate its effect at acid and alkaline pH range. Free fatty acids (FFA) increase was the control variable. Extruded samples were stored at room temperature (between 20 and 28 °C) using a non‐extruded sample as control to assess the shelf life effects. Results indicate that in acid‐extruded samples, the increase in FFA concentration after 98 days was much less than in the unmodified‐pH or alkaline samples. The lowest FFA increase after 3 months of storage time was <10 mg FFA/g rice bran using extrusion with no water or chemicals added or using extrusion adding HCl, irrespective of the moisture content of rice bran.     

Deacidification of high-acid rice bran oil by reesterification with monoglyceride

Autocatalytic esterification of free fatty acids (FFA) in rice bran oil (RBO) containing high FFA (9.5 to 35.0% w/w) was examined at a high temperature (210°C) and under low pressure (10 mm Hg). The study was conducted to determine the effectiveness of monoglyceride in esterifying the FFA of RBO. The study showed that monoglycerides can reduce the FFA level of degummed, dewaxed, and bleached RBO to an acceptable level (0.5±0.10 to 3.5±0.19% w/w) depending on the FFA content of the crude oil. This allows RBO to be alkali refined, bleached, and deodorized or simply deodorized after monoglyceride treatment to obtain a good quality oil. The color of the refined oil is dependent upon the color of the crude oil used.     

Reducing Oil Losses in Alkali Refining

Neutralization is an important step in the chemical refining of edible oils. Free fatty acids (FFA) are generally removed in neutralization as sodium soaps but neutral oil is also entrapped in the emulsion and removed with the soap during centrifugation. Thus, alkali neutralization causes a major loss of neutral oil in the chemical refining of edible oils. The effects of demulsifiers (NaCl, KCl, Na₂SO₄ and tannic acid) on reducing alkali refining losses of refined palm, soybean, and sunflower oils (used as model oils) incorporated with FFA from rice bran oil were investigated. Adding small amounts of demulsifiers to the alkali neutralization step significantly reduced neutral oil loss of these model oils. All demulsifiers except for tannic acid had similar effects on refining losses in all oil model systems. The optimum demulsifier content was 1.0 % (w/w of oil).     

Comparison of bio- and autocatalytic esterification of oils using mono- and diglycerides

The purpose of this study was to investigate enzymatic and autocatalytic esterification of FFA in rice bran oil (RBO), palm oil (PO), and palm kernel oil (PKO), using MG and DG as esterifying agents. The reactions were carried out at low pressure (4–6 mm Hg) either in the absence of any added catalyst at high temperature (210–230°C) or in the presence of Mucor miehei lipase at low temperature (60°C). The reactions were carried out using different concentrations of MG, and the optimal FFA/MG ratio and time were 2∶1 (molar) and 6 h, respectively, in both auto- and enzyme-catalyzed processes. With DG as the esterifying agent in the autocatalytic process, the optimal temperature was 220°C, and the optimal FFA/DG ratio was 1∶1.25. For both MG and DG, the enzymatic process was more effective in reducing FFA and produced more favorable levels of unsaponifiable matter and color in the final product. The PV of the final products were also lower (1.8–2.9 mequiv/kg) by using the enzymatic process. To produce edible-grade oil, a single deodorization step would be required after enzymatic esterification; whereas, alkali refining, bleaching, and deodorization would be required after autocatalytic treatment.     

Recovery of γ-oryzanol from rice bran oil byproduct using supercritical fluid extraction

The residue of fatty acids distillation from rice bran oil soapstock (RFAD-RBOS) is a byproduct from rice bran oil industry. It contains a large amount of γ-oryzanol, which is a valuable antioxidant. The main objective of this work was to investigate the recovery of γ-oryzanol from the RFAD-RBOS using supercritical fluid extraction (SFE). The Soxhlet technique was conducted in order to compare results with SFE. The influence of process parameters over SFE was evaluated in terms of global yield, γ-oryzanol content, γ-oryzanol recovery rate, and fatty acids composition. The mathematical modeling of SFE overall extraction curve (OEC) was also investigated. The condition of 30 MPa/303 K presented the maximum global yield (39 ± 1%, w/w), maximum γ-oryzanol recovery rate (31.3%, w/w), relatively high γ-oryzanol content (3.2%, w/w), and significant presence of monounsaturated and polyunsaturated fatty acids. The logistic model presented the best fit to experimental OEC.     

Acidity of oryzanol and its contribution to free fatty acids value in vegetable oils

Model oil systems containing physically refined rice bran oil to which oryzanol was added were examined to determine the effects of oryzanol concentration on FFA values. When oryzanol was added to the model oils at a 0.5% level and FFA was determined, increases in FFA value were 0.28% as determined with phenolphthalein, 0.58% with thymolphthalein, and 0.07% with alkali blue 6B. Oils containing added oryzanol at 0.5–1.5% showed a proportionate increase in FFA values with an average increase of 0.413% per gram of oryzanol. A direct titration of purified oryzanol showed an acidity of 42.5% expressed as FFA. In spectroscopic studies, the phenolic group in the ferulic acid moiety of oryzanol was titrated by sodium hydroxide. Based on these data, indicator correction factors for oryzanol's acidity and a formula for calculating real FFA content of vegetable oils containing oryzanol were developed.     

Effects of Rice Bran Oil Enriched with n-3 PUFA on Liver and Serum Lipids in Rats

Lipase-catalyzed interesterification was used to prepare different structured lipids (SL) from rice bran oil (RBO) by replacing some of the fatty acids with α-linolenic acid (ALA) from linseed oil (LSO) and n-3 long chain polyunsaturated fatty acids (PUFA) from cod liver oil (CLO). In one SL, the ALA content was 20% whereas in another the long chain n-3 PUFA content was 10%. Most of the n-3 PUFA were incorporated into the sn-1 and sn-3 positions of triacylglycerol. The influence of SL with RBO rich in ALA and EPA + DHA was studied on various lipid parameters in experimental animals. Rats fed RBO showed a decrease in total serum cholesterol by 10% when compared to groundnut oil (GNO). Similarly structured lipids with CLO and LSO significantly decreased total serum cholesterol by 19 and 22% respectively compared to rice bran oil. The serum TAGs level of rats fed SLs and blended oils were also significantly decreased by 14 and 17% respectively compared to RBO. Feeding of an n-3 PUFA rich diet resulted in the accumulation of long chain n-3 PUFA in various tissues and a reduction in the long chain n-6 PUFA. These studies indicate that the incorporation of ALA and EPA + DHA into RBO can offer health benefits.     

Chemistry of Color Fixation in Crude,Physically Refined and Chemically Refined Rice Bran Oils upon Heating

Compared to other vegetable oils, rice bran oil (RBO) has a characteristic dark color which further deepens upon heating or frying of foods in the oil. Darkening of the oil during heating has been studied. The dark color‐causing material in crude, chemically refined and physically refined rice bran oils was separated using a silica gel column for a hexane‐eluted oil fraction and a methanol eluted fraction. The methanol eluted fraction for all the above three types of RBO produced a dark color upon heating, hence the physically refined RBO methanol fraction was investigated further and contained monoglycerides (23.4 %) and diglycerides (67.4 %) of linoleic + linolenic acids in its methanol fraction as analyzed by column chromatography and HPLC which decreased in concentration after heating. The linoleic acid level of 37.7 % in the methanol fraction was reduced significantly to 18 % after heating (52.3 % reduction). The IR and NMR spectra were similar to those of a monoglyceride/diglyceride with NMR spectra indicating a lower amount of olefinic protons for the heated sample. These results showed that the darkening of RBO was due to the oxidation and polymerization of monoglycerides/diglycerides containing linoleic acid/linolenic acid.     

Influence of viscosity on wax settling and refining loss in rice bran oil

The role of viscosity on was settling and refining loss in rice bran oil (RBO) has been studied with model systems of refined peanut oil and RBO of different free fatty acids contents. Wax was the only constituent of RBO that significantly increased the viscosity (81.5%) of oil. Monoglycerides synergistically raised the viscosity of the oil (by 114.2%) and lowered the rate of wax settling. Although a reduction in the viscosity of the oil significantly decreased the refining loss, the minimum loss attained was still 20% more than the theoretically predicted value. This led us to conclude that some chemical constituents, such as monoglycerides, must be removed before dewaxing; thereafter, oryzanol and phospholipids have to be removed. One can get an oil free of wax, recover other by-products and reduce processing losses.     

A study of rice bran oil refining

Examination of a number of rice bran oils revealed the presence of monoglycerides (0.5–1.4%) and other hydroxylated compounds such as diglycerides and glucosides. The hydroxyl numbers of the samples ranged from 8.5 to 27, depending on their acidity. On the assumption that the inordinately high refining losses of rice bran oil are due, along with the acidity, to the presence of hydroxylated compounds, the hydroxyl numbers of several samples of that oil were reduced by progressive acetylation with acetic anhydride. This was accompanied by gradual reduction of the refining losses, which seems to support the above mentioned assumption.     

Rice bran FFA determination by diffuse reflectance IR spectroscopy

Rice bran with FFA levels above 0.1% cannot be used as a food ingredient due to oxidative off-flavor formation. However, extracting high FFA oil from bran by in situ methanolic esterification of rice bran oil to produce methyl ester biodiesel produces greater yields relative to low-FFA rice bran oil. Therefore, high-FFA bran could be exploited for biodiesel production. This study describes an FTIR spectroscopic method to measure rice bran FFA rapidly. Commercial rice bran was incubated at 37°C and 70% humidity for a 13-d incubation period. Diffuse reflectance IR Fourier transform spectra of the bran were obtained and the percentage of FFA was determined by extraction and acid/base titration throughout this period. Partial least squares (PLS) regression and a calibration/validation analysis were done using the IR spectral regions 4000-400 cm⁻¹ and 1731-1631 cm⁻¹. The diffuse reflectance IR Fourier transform spectra indicated an increasing FFA carbonyl response at the expense of the ester peak during incubation, and the regression coefficients obtained by PLS analysis also demonstrated that these functional groups and the carboxyl ion were important in predicting FFA levels. FFA rice bran changes also could be observed qualitatively by visual examination of the spectra. Calibration models obtained using the spectral regions 4000-400 cm⁻¹ and 1731-1631 cm⁻¹ produced correlation coefficients R and root mean square error (RMSE) of cross-validation of R=0.99, RMSE=1.78, and R=0.92, RMSE=4.67, respectively. Validation model statistics using the 4000-400 cm⁻¹ and 1731-1631 cm⁻¹ ranges were R=0.96, RMSE=3.64, and R=0.88, RMSE=5.80, respectively.