Rice bran oil is cooking that has high oxidative stability and with high smoke point which is suitable for high temperature cooking such as deep-frying . RBO are made of the bran of Oryza sativa, which compose of pericarp, seed coat, nucellus, Aleurone layer and the rice embryo. These components make up 8% of the whole paddy weight and contain three quarter of the total oil in paddy seed (Ceciro & Derosa, 2005).http://www.ricebranoil.info/images/grafix/rice.gif
Figure 1: The composition of Oryza sativa
Based on several research on RBO, major compound found in RBO such as oryzanols and tocotrienols. Oryzanols helps to reduce the level of total plasma cholesterol by altering the activity of acid cholesterol esterase enzyme and the activity of acyl-Coenzyme A: cholesterol acyl transferase enzyme to balance out the lipid deposition in the body by accelerating the excretion of lipid in blood (Ceciro & Derosa, 2005). Besides that, tocotrienols also helps to reduce the uptake of cholesterol in the body by inhibiting HMGCoA-R enzyme that responsible for synthesizing cholesterol by decreasing the translation of HMGCoA-R mRNA and by increasing the degradation of reductase protein (Ceciro & Derosa, 2005).
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However, RBO need to be extracted and processed instantly to prevent changes in the odor and the color of the RBO and degradation of the free fatty acid found in RBO (Kusum et al, 2011). This is because of the presence of lipase enzyme that already presence in the Oryza sativa will deteriorate or hydrolyze the free fatty acid (FFA) triglyceride into fatty acid and glycerol, which found in the RBO and form dark color oil and rancid smell. Therefore, several modifications and refining techniques should be used to treat the RBO in order to sustain and improve the quality of RBO.
Refining and Modifications of RBO
There are two methods in refining RBO, which includes physical and chemical refining. Nevertheless, physical refining has been chosen as the best method to refine crude rice bran oil (CRBO). This is because, physical refining techniques help to preserve the FFA in RBO and help in conserving the bioactive compound such as oryzanol and tocotrienols in RBO as well as help to reduce lose of neutral oil that can be lost if chemical refining method is used (Manjula & Subramaniam, 2009).
Physical refining process through degumming and de-waxing
First example of physical refining procedures are degumming and de-waxing. De-waxing is the process of refining CRBO to remove waxes, long ester fatty acid chain, and long chain of primary alcohol in CRBO. Degumming is the process of removing leftover waxes after de-waxing and phosphate group in the CRBO. During degumming process, the phosphorous compound presence in the CRBO that responsible for color fixation and rancid smell in final product of RBO will be removed. However, two methods has used in the industry to remove phosphorous-containing components in the CRBO, which are water degumming and addition of degumming agent. However, addition of degumming agent which constitute of additional of 3% of water, and 0.25-1% of phosphoric acid is choose as the best to remove both hydratable phospholipids as well as non-hydratable phospholipids with minimum lost of FFA compared to using water degumming (Tyagi et al., 2012). Meanwhile, in de-waxing process, the CRBO is treated with hexane to separate the wax and other cell debris found in the CRBO. The de-waxed CRBO is then centrifuged to remove the wax and unwanted substance in the pellet. After de-waxing process, the oil is degummed by pre-treated the CRBO with phosphoric acid and steam the pre-treated CRBO in 80-100Â°C to prevent crystallization and to remove the waxes and the gum (Manjula & Subramaniam, 2009). This will eventually deactivate and rupture the structure of highly active lipase, lipoxygenase, and peroxidase that will reduced the quality of the RBO (Orthoefer, 2005). This procedure is repeated for several times until the wax are fully removed. The removing of phosphate in the RBO will help to increase the shelve life and the quality of the RBO.
Incorporation of capric acid (CA) in RBO
RBO is known for its extra advantages that helps to reduced the level of plasma cholesterol and helps to accelerate the excretion of lipid in the body as well as helps in reducing the cholesterol uptake by the body which due to the presence of oryzanol and tocotrienolsin RBO. However, there are some problem regarding the uptake of the active ingredients, oryzanol and tocotrienols for human body especially to infants that suffer cystic fibrosis, colitis that cause lack of fatty acids and fat-soluble in their diets due to fat abnormal absorption of long-chain fatty acid (LCFA). Meanwhile, for medium-chain triacylglycerols is normally used for those who are interest in health and fitness that do not want deposition of fat in their adipose tissue, since it is include in high-energy diets that helps athletes to boost their energy without the need to eat much. However, medium-chain triacylglycerols does not meet the human requirements for essential fatty acids.
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Therefore, medium-chain fatty acids such as capric acid (CA) is incorporated into the RBO which has the same glycerol backbone to help increasing the uptake of essential fatty acid for patient with fat abnormal absorption and for athletes and high-energy diets patient to have essential fatty acid without concern of forming deposited fat at adipose tissue (Jennings & Akoh, 2000).
In order to incorporate medium-chain fatty acid CA, immobilized lipase (IM) enzyme from Rhizomucor miehei is used as catalyst. Firstly, the RBO is mixed with CA at mole ratio of 1 triacylglycerol to 2 CA fatty acid in 3mL of n-hexane. IM enzyme is added at 10% of the weight of the reactant and incubated in shaking water bath at 55Â°C for 24hours at 200rpm (Jennings & Akoh, 2000). Based on the Jennings & Akoh (2000) findings, shows that the modified RBO is successful, which produce RBO that share glycerol backbone with the CA. However, several types of FFA such as Î´-tocopherol and Î´-tocotrienois lost during chemical modifications. Nevertheless, the effect of the RBO that helps reducing plasma cholesterol level are not affected.
Through this modification, the modified RBO with CA has new function that has the low calorie intake property and less uptake of cholesterol in the body.
Methanolysis and transesterification of RBO in biodiesel production
Recently, the use of RBO as the raw material of biodiesel (BD) has increasing since rice bran is cheap renewable and reusable material that has high desirable fatty acid content that turn into BD through several steps (Lai et al., 2005). The production of BD is somewhat similar to the production of RBO that used for cooking. However, several extra steps are needed to be used on RBO to obtain BD, which are lipase-catalyzed methanolysis, esterfification of FFA in RBO and transeterification of acylglycerols remaining in the RBO.
RBO may also undergo two-step enzymatic methanolysis of RBO, which includes esterfification of FFA and transeterification of acylglycerols remaining in the RBO to completely converting FFA and triacylglycerol (TG) in RBO into methyl esters. The procedure starts with adding 1.0g of lipase Novozym 435 enzyme into 20g refined RBO in a screw-capped vessel at 50Â°C at 150rpm of stirred water bath. 0.8moles of methanol in every 30minutes interval for 2hours is added. Then, the lipase is removed through filtration and the organic phase mixture is filtered using sodium sulfate bed and dried using rotary-evaporator at 90Â°C for 1hour. This mixture is then undergoes second methanolysis of lipase-catalyze by treating the 10g of refined RBO with 0.5 pretreated immobilized lipase suspended at 50Â°C at 150rpm of shaking. The mixture is incubated and 0.9moles methanol is added in the mixture for every 30minutes interval for 2hours to allow full conversion of RBO. Then, the mixture is diluted in 200mm3 of hexane and the enzyme is removed by filtration (Lai et al., 2005).
By having modifications of RBO into BD, unutilized source of fatty acid of RBO can have different function that help in reducing the emission of net atmospheric CO2 level by photosynthetic carbon fixation due to its high oxygen content (Zullaikah et al., 2005).
In conclusion, modifications of fatty acid mainly involve stabilization of RBO in order to suppress and inhibits the reaction of active lipase enzyme that will increase the FFA concentration and darkening the colour of the RBO, in which causes the reduction of the quality and shelf life of the RBO. Therefore, modifications such as degumming, dewaxing, and moist heat treatment will remove phosphate compound that responsible for darkening the RBO and denaturing the very active enzyme that responsible for hydrolyzing RBO into FFA and glycerol. Besides that, modifications of RBO may also involve additional of extra functions that have low calorie intake property and less uptake of cholesterol in the body, which is suitable for abnormal fat absorption patient, and patient with high energy diets especially athletes. Moreover, modifications of fatty acid may involve extension of RBO process in order to produce biodiesel. Through this modifications, the cheap raw materials that overlook by many can be used to produce something that can helps to improve the quality of the environment as well as reducing the use of natural resources such as petroleum.
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