The effect of temperature on rice oil bleaching to reduce oxidation and loss in bioactive compounds

M.M. Strieder^a, J.I. Engelmann^a, R.S. Pohndorf^b, P.A. Rodrigues^a, R.S. Juliano^a, G.L. Dotto^c and L.A.A. Pinto^a,*

^aSchool of Chemistry and Food, Federal University of Rio Grande, FURG, Italia Avenue, km 08, 96203–900, Rio Grande, RS, Brazil.

^bDepartment of Agroindustrial Science and Technology, Federal University of Pelotas, Pelotas, RS, Brazil.

^cChemical Engineering Department, Federal University of Santa Maria, UFSM, 1000 Roraima Avenue, 97105–900, Santa Maria, RS, Brazil

*Corresponding author: dqmpinto@furg.br

1. INTRODUCTIONTOP

Vegetable oils are a good source of lipids and antioxidant compounds for a healthy human diet, and rice oil is unique because it contains a great deal of γ-oryzanol (Pestana-Bauer et al., 2012). γ-oryzanol is a mixture of ferulic acid esters of sterol and triterpene alcohols, and has been studied because of its antioxidant, anti-inflammatory, anticancer and anti-diabetic properties (Juliano et al., 2005; Jha and Panchal, 2017). Maintaining the carotenoids found in rice oil during the refining steps is important as they have antioxidant effects and prevent oil rancidity (Vaskova and Buckova, 2015; Silva et al., 2014). In addition, β-carotene is an essential vitamin which is required for proper body functioning and a deficiency in this vitamin is linked to many chronic diseases.

Crude oils contain non-glyceride compounds which must be removed to obtain oil with a desirable light color, mild taste and oxidative stability (González-Tovar et al., 2005). The refining process can be carried out either physically or chemically (Torres-González et al., 2009). A degumming step using phosphoric acid (chemical refinement) or hot water (physical refinement) to remove phospholipids and gums (Kreps et al., 2014) is typically applied. In a neutralization step, the free fatty acids are removed through the addition of an alkali solution of sodium hydroxide. A bleaching step aims to remove pigments, oxidation products and traces of metals, using adsorbent substances (Marrakchi et al., 2015). Some vegetable oils have a high wax content, which is separated from the oil in the winterization step (de-waxing) by cooling the oil and allowing the waxes to crystallize or through the use of solvents (Baümler et al., 2007). In physical refining an alkaline solution is not used to neutralize the oil. I It also prevents nutrient losses that occur in chemical refining (Pestana-Bauer et al., 2012). The degumming–neutralization (chemical refining) steps lead to the greatest reduction in bioactive components (Zhu et al., 2016). According to Paucar-Menacho et al., (2007), at least 90% of the γ-oryzanol present in crude rice oil is lost during the neutralization step.

Bleaching is the third step in the classical refining process of vegetable oil. This is an adsorption operation aimed at the elimination of substances which provide color to vegetable oil, such as chlorophyll and carotenoids. This process also results in the removal of oxidation products, thus promoting its stability against oxidation (Strieder et al., 2017; García-Moreno et al., 2013). The temperature, the type and amount of adsorbent, the contact time and vacuum are variables of the bleaching process and temperature is an important factor because heat promotes interactions between the adsorbent and the adsorbate (Pohndorf et al., 2016b). The composition and antioxidant activity of the oil can change the thermal condition used during refining and heat can promote oxidative reactions in oil (Olmedo et al., 2015; Güneşer et al., 2017). In the oil industry, the temperature of rice oil bleaching is approximately 110 °C, but the process consumes large amounts of energy which represent high costs (Pestana-Bauer et al., 2012; Köseoglu and Engelgau, 1990).

The aim of this work was to evaluate the refining type (chemical or physical) and the effect of temperature on oil bleaching in carotenoid reduction and γ-oryzanol loss. In addition, the aspects related to hydrolytic and oxidative rancidity were investigated.

2. MATERIALS AND METHODSTOP

2.1. MaterialsTOP

Degummed and neutralized rice oil, and activated earth (Tonsil Supreme 110 FF with the following characteristics: average particle diameter of 68 ± 7 mm, specific surface area of 245 ± 12 m²·g^-1, pore diameter of 2.00 ± 0.05 nm, bulk density (460 ± 10 kg·m^-3) and real density of 2150 ± 10 kg·m^-3) were provided from a rice oil processing industry located in the city of Pelotas, Brazil. The oil samples were stored at -20 °C in amber containers to avoid oxidation until use.

2.2. Bleaching assaysTOP

The parameters used in the bleaching process, such as time, temperature and amount of adsorbent, depend on the oil type and adsorbent characteristics. The optimum temperature range for the bleaching of traditional vegetable oil reported by Shahidi (2005) was from 85 to 110 °C. The bleaching assays were performed using 30 g of degummed (physical refining) or neutralized oil (chemical refining). The oil samples were heated at temperatures of 80, 95 or 110 °C, under constant stirring (40 rpm) and vacuum. Afterward, 1% (w/w) of activated earth was added in each assay, and the bleaching was performed for 20 min (Pohndorf et al., 2016a). The adsorbent was separated from the rice oil at 60 °C by filtration in a vacuum, with a pre-coat of diatomaceous earth, for 15 min. The experimental assays were performed in triplicate.

2.3. Oils quality analyses TOP

The oxidative quality of the oil was determined in terms of free fatty acids (FFA), peroxide value (PV) and anisidine value (AnV) according to the methods: Ca 5a-40, Cd 8-53 and Cd 18-90 (AOCS, 2017), respectively. The oil color was determined according to the method Cc 13e-92 (AOAC, 2017) using a Lovibond Tintometer (Lovibond Color STALER Tintometer, model F, UK), with a 5¹/₄” cell, by varying the yellow (Y) and red (R) colors and fixing the blue (B) color at unit zero. All analyses were carried out in triplicate. The total oxidation value (TOTOX value) was determined according to Equation 1:

where PV is the peroxide value (meq kg^-1) and AnV is the anisidine value.

The chlorophyll content was obtained through the absorbance, performed in triplicate, according to Sabah (2007), in Equation 2:

where Cl is the chlorophyll content (mg_pheophytin-a kg^-1_oil), A₇₁₀, A₆₇₀ and A₆₃₀ are the absorbance wavelengths, respectively, of 670, 630 and 710 (nm), V is the volume of hexane (mL), L the length of the cuvette (cm) and, M is the mass of the oil sample (g).

2.4. Quantification of bioactive compounds TOP

The γ-oryzanol content was determined according to Bucci et al., (2003). The oil samples were diluted in isopropyl alcohol and read in a spectrophotometer at wavelengths of 327 nm, in triplicate, according to Equations 3 and 4:

where C_GO is the γ-oryzanol content in the oil solution (g_oryzanol L^-1), A₃₂₇ is the absorbance at 327 nm, e is the specific extinction coefficient (L g_oryzanol cm^-1), which is 33.82 L·g^-1·cm^-1 (in isopropyl alcohol solvent), TGO_E-UV is the γ-oryzanol content (%m m^-1) and C_DIL is the oil concentration in the diluted solution (g·L^-1).

The carotenoid content in the rice oil was determined by the spectrometric method, taking readings of the oil diluted in hexane (10% m·v^-1) at 446 nm, in tripicate, according to Equation 5 (Mustapa et al., 2011):

where C is the carotenoid content (mg kg^-1), A₄₄₆ is the absorbance at 446 nm, L is the length of the cuvette (cm), c is the oil concentration in hexane (g·100 mL^-1) and 383 is the extinction coefficient for carotenoids.

2.5. Identification of functional groups TOP

The identification of functional groups of the degummed, neutralized and bleached oils at the best temperature was analyzed by Fourier transform infrared spectroscopy with attenuated total reflectance (FT-IR-ATR) (Prestige 21, 210045, Japan). The spectra were recorded in the 4.000–600 cm^–1 region, with a resolution of 4 cm^–1.

2.6. Thermal analyses TOP

The melting curves of the neutralized, degummed and bleached oils (at the best temperature) were analyzed by differential scanning calorimetry (DSC). The melting point was obtained by DSC (Shimadzu, DSC-60, Japan) equipped with a liquid nitrogen cooling system. Samples (5 mg) were hermetically sealed in aluminum pans and cooled to -50 °C and then heated to 60 °C at a rate of 10 °C·min^-1.

The thermogravimetric curves were obtained in a thermobalance with a nitrogen flow of 50 mL·min^-1 and a heating rate of 10 °C·min^-1. The samples were placed in aluminum pans and heated in the temperature range of 20 to 500 ºC (Garcia et al., 2004). All thermal analyses were carried out in triplicate.

2.7. Statistical analysisTOP

Comparisons among the bleached, neutralized and degummed oil samples were made by analysis of variance and Tukey’s test at 95% confidence (p < 0.05), using the Statistic 7.0 software. The assays were made in triplicate.

3. RESULTS AND DISCUSSIONSTOP

3.1. Characterization of the raw materialsTOP

Some aspects related to the quality of degummed and neutralized oil prior to the bleaching step are shown in Table 1. The free fatty acids, peroxide and anisidine values, chlorophyll content and γ-oryzanol content were lower in the neutralized oil, with the exception of the carotenoid content. This occurred because of the addition of the sodium hydroxide solution in the neutralization step, which reacted with the free fatty acids forming the soaps, which carried peroxides, aldehyde and γ-oryzanol. The loss in γ-oryzanol was 87% in the neutralization step. A high amount of free fatty acids in the degummed oil was verified, which indicates that the rice bran was not effectively inactivated, allowing the action of the lipases in the triacylglycerols (Paucar-Menacho et al., 2007). The crude oil showed a Lovibond color of 57.5Y/12.1R (5¹/₄” cell), and for the degummed oil and the neutralized oil the colors were of 70.0Y/3.8 and 77.0Y/2.5R, respectively.

3.2. Quality aspects of the bleached oilTOP

The effects of temperature and refining method on some quality characteristics of rice oil are shown in Table 2. The initial concentration of free fatty acids (FFA) in the oil is important in the refining process due to its influence in the neutralization and bleaching steps, and the different levels are due to variations in sources of crude oil (Lin and Lin, 2005). The bleached oil from degummed oil (BDO) showed a reduction of about 30% in FFA (Table 2), indicating the adsorption of this compound. However, the bleached oil from the neutralized oil (BNO) showed a small increase in FFA. A similar result was obtained for soybean oil by Lin and Lin (2005), who verified that the initial adsorption rate of FFA was increased by the increase in the initial FFA concentration. Silva et al., (2014) also verified a small increase in the FFA values in the bleaching step of neutralized palm oil. This can be explained by the heating, which favored oil oxidation during bleaching.

Despite the reduction in FFA in the BDO, the content was high and may have favored oxidation reactions which generated peroxides, mainly in the temperature of 80 °C (Table 2). Oil oxidation is favored by several factors, such as the composition of fatty acids, type of oxygen, and compounds such as pigments, metals, antioxidants, and free fatty acids (Choe and Min, 2006). Therefore, the presence of free fatty acids along with the heat provided by the operation could have favored the formation of peroxides at 80 °C. At other temperatures, there was no increase or decrease in this value. This can be due to the acid-activated earth, which catalytically decomposes peroxides in secondary oxidation products (Silva et al., 2014) and, thus, at other temperatures, decomposition may have occurred. For the BNO it was observed that a reduction of about 75% occurred in the peroxide value for any temperature analyzed. The same behavior was verified by Strieder et al., (2017), who found the value of 2.7 meq·kg^-1 for rice oil bleached at 110 °C. According to Moigradean et al., (2012), the amount of peroxide in vegetable oil indicates its oxidative level and, thus, its tendency to become rancid.

In the oxidation reaction, the primary oxidation products are decomposed into minor substances such as aldehydes, which lead to a rancid smell and taste in the oil. These compounds are determined by the anisidine value (Moigradean et al., 2012; Marina et al., 2009). In Tables 1 and 2, an increase in the anisidine value can be seen for the BNO, indicating the formation of secondary oxidation products during the bleaching step. The temperature could have favored secondary oxidation because the peroxide content decreased and formed aldehydes in this step (Silva et al., 1999). The high anisidine values in the BDO can be associated with the high content presented in the degummed oil. It can be seen that the anisidine value was not increased by the bleaching step, and was even reduced at temperatures of 80 and 95 °C, indicating that aldehyde was absorbed at these temperatures.

The TOTOX value indicates total oxidation in the oil, which includes primary and secondary compounds formed in the oxidative reactions (Moigradean et al., 2012). It can be observed in Table 2 that the lowest values for the BDO and BNO were obtained at the temperature of 95 °C, indicating that this temperature favored adsorption and promoted less oxidation than the other temperatures. The leaching of rice oil is an endothermic process (Pohndorf et al., 2016a), and at temperatures of 80 ºC and 110 ºC the adsorption capacities were lower and, consequently, the TOTOX values were higher.

Some pigments, such as chlorophylls, favor oxidation reactions in oil, and therefore, it is important to remove it (Choe and Min, 2006). The chlorophyll content was reduced by about 26% in all bleaching assays of neutralized oil (Tables 1 and 2), but in the BDO there was a small increase. This may be justified by the exposure time and heat applied in the bleaching step, which, according to Sabah (2007), promotes pheopigment formation from chlorophylls. The temperature of 80 °C was less efficient to remove chlorophyll, due the fact that the heat supply was not sufficient for the operation. According to Ponhdorf et al., (2016a), who studied the bleaching step at 100, 110 and 120 °C, chlorophyll molecules require more energy to be removed from rice oil. The Lovibond colors for BDO and BNO presented values of around 50.0Y/10.5R and 11.4Y/2R, respectively.

Chemical refining was more efficient for removing oxidation compounds, especially because the degummed oil presented a great amount of these compounds. In addition, the temperature of 95 °C was the best for promoting a good interaction between adsorbent and adsorbate, which is not so high a temperature to cause oxidation.

3.3. Bioactive compounds in the bleached oilTOP

The presence of carotenoids in the oil is interesting because they are natural antioxidants which assist in the maintenance of quality and increase nutritional value (Vaskova and Buckova, 2014). Tables 1 and 2 show that the carotenoid values were reduced in BNO but remained constant in BDO. Similar results for the carotenoid content in bleached rice oil were found by Strieder et al., (2017), using 1% of an activated earth/activated carbon blend at 110 °C, obtaining an oil with 5.7 mg·kg^-1. According to their study, activated carbon is better than activated earth for maintaining carotenoid levels in rice oil.

In Table 2, the bleaching step presented a loss of approximately 40 % in the γ-oryzanol content in the BDO, but this loss was well below what occurred when the oil was degummed and neutralized (90%). At 110 °C there was a greater reduction in γ-oryzanol for the bleached oil obtained from degummed oil. γ-oryzanol was indicated by Spiazzi et al., (2013) as being one of the main antioxidant components in rice oil, thus, it is very important for preserving the quality of the oil; however, the refined commercial oil presents only about 0.16 g of γ-oryzanol per kg of oil due to losses during chemical refining (Paucar-Menacho et al., 2007). Thus, it can be said that the better process for maintaining carotenoids and γ-oryzanol was physical refining at temperatures of 80 and 95 °C, because at 110 °C more loss occurred.

3.4. Identification of functional groups by FTIR spectra TOP

The spectra of the degummed, neutralized and bleached oils at the best temperature studied (95 °C) is shown in Figure 1. The spectra for the oils were very similar, with the exception of the band of 3200-3600 cm^-1, which is visible in the oils before the bleaching step. This can be attributed to the vibration stretching of OH, which indicates the presence of polar compounds in the oil, such as peroxides, hydroperoxides and traces of moisture. After bleaching, the peak was less evident because this operation removed these compounds. In addition, the functional groups of the rice oil were observed, with the band at 3005 cm^–1 corresponding to stretching CH of the cis double bands of unsaturated fatty acids. The bands at 2917 and 2854 cm^–1 refer to the asymmetric and symmetric vibration stretching of the CH₂ functional group, respectively. The band at 1744 cm^–1 is related to the carbonyl ester group C=O of triglycerides. The band at 1653 cm^–1 is associated with the stretching vibration of the double bands between carbons C=C. The band at 1456 cm^–1 refers to the bending (scissoring) of the CH₂. The bands at 1161 and 1097 cm^–1 are related to the stretching vibration of the ester groups C–O (Ponhdorf et al., 2016b).

3.5. Thermal properties of rice oil TOP

Figure 2 shows the melting temperatures of the degummed oil, BDO, neutralized oil and BNO at -7.22, -19.44, -15.99 and -18.94 °C, respectively. A reduction in the melting temperature in the bleached oils was observed. The melting temperature indicates the oil composition in triacylglycerols, and therefore, it is possible that the pigments, free fatty acids and oxidation compounds increased the melting temperature of the oil. The melting temperatures of BDO and BNO were similar, and they were close to the values found by Strieder et al., (2017), who found -17.53 °C for bleached rice oil.

The thermogravimetric curves of the oils are shown in Figure 3I. It was verified that the decomposition of triacylglycerols occurred in the temperature range from 220 °C to 450 °C. The degummed oil and BDO (curves A and B) started the degradation at a lower temperature than the neutralized oil and BNO (curves C and D). This can be due to the oxidation compounds present in degummed oil. These compounds absorb part of the energy incited on the sample, causing a decrease in the degradation temperature (Huang and Sathivel, 2010). Besides that, the first stage of oil’s decomposition is the most important value for the characterization of thermal stability according to Szabo et al., (2012). Thus, it can be inferred that the oil was more stable after the bleaching step and that the BNO was the most stable.

In Figure 3II, the curves show that the temperature of maximum mass loss rate was 400 °C for all oils. It can also be observed that the curves of degummed oil and DBO presented a second degradation point between 200 and 250 °C, associated with the presence of oxidation compounds.

4. CONCLUSIONSTOP

In this work it was determined that the best temperature for the bleaching step of neutralized and degummed industrial rice oil was 95 °C. In addition, it was possible to verify that for the reduction in oxidation compounds (free fatty acids, peroxides and aldehydes) it was necessary to perform a neutralization step, even though a degumming step was performed physically. It was possible to determine through DSC curves that the pigments, free fatty acids and oxidation compounds increased the melting temperature of the oil. Based on TGA curves, it can be verified that the oils were more stable after the bleaching step and that the bleached oil obtained with neutralized oil (BNO) was the most stable. The FTIR spectra of the degummed, neutralized and bleached oils at the best temperature studied (95 °C) were very similar. However, after the bleaching step the vibration stretching peak of OH was less intense because this peak indicates the presence of polar compounds in the oil, such as peroxides, hydroperoxides and traces of moisture. It was observed that the bleached rice oil obtained with degummed oil provides an oil which is more rich in γ-oryzanol, indicating that the bleaching step without a previous neutralization step is viable for maintaining this compound, but it is necessary to use an industrial crude oil with a lower grade of oxidation.

ACKNOWLEDGMENTSTOP

The authors are grateful to the CAPES/Brazil (Coordination for the Improvement of Higher Education Personnel) and CNPq/Brazil (National Council for Scientific and Technological Development) for their financial support.

REFERENCESTOP