Properties and Oxidative Stabilities of Enzymatically Interesterified Chicken Fat and Sunflower Oil Blend

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1 Journal of Oleo Science Copyright 2013 by Japan Oil Chemists Society Properties and Oxidative Stabilities of Enzymatically Interesterified Chicken Fat and Sunflower Oil Blend Magdalena Kostecka, Dorota Kowalska, Mariola Kozłowska and Boleslaw Kowalski * University of Life Sciences, Department of Food Sciences, Warsaw, Poland Abstract: Chicken fat and sunflower oil 2:3 m/m blend was enzymatically interesterified at 60 with and without microwaves assistance. As the catalyst a commercial preparation of the immobilized lipase from Rhizomucor miehei (Lipozyme RM IM) containing 2% of water was used, and the catalyst load was 8% in each case. The starting mixture and the interesterified products were separated by column chromatography into pure triacylglycerols fraction (TAG) and a non-triacylglycerol fraction, which contained free fatty acids (FFA), mono- and diacylglycerols (MAG and DAG). The oxidative stabilities (OS) of fats studied and TAG derived from them were assessed by Rancimat at 100 and by Pressure Differential Scanning Calorimetry (PDSC) under oxygen at Interesterification reduced the OS of chicken fat and sunflower oil blend. The main factors influenced on the OS of fats studied were concentrations of tocopherols and presence of FFA, MAG- and DAG. The structures of TAGs were of minor importance. From the resulting PDSC exotherms their times to reach the onset (τon) and peak maximum (τmax) were measured and used for calculations of parameters of the Arrhenius type kinetics for thermaloxidative decomposition of fats studied. Key words: enzymatic interesterification, chicken fat, Lipozyme RM IM, pressure differential scanning calorimetry (PDSC), sunflower oil 1 INTRODUCTION Fats are essential components of the diet as they are a concentrated source of energy and contain essential fatty acids and soluble witamins. They also contribute to the flavour, texture and palatability of a food 1. Fats can be used in native form or after blending but for some purposes they have to be modified by fractionation, hydrogenation and interesterification. The interesterifications of fats are used when an avoiding of trans isomers formation is a decisive factor. There are two types of interesterification presently in use, viz. chemical and enzymatic. Accordingly to these interesterifications different types of catalysts are employed. For chemical interesterifications typically metal alcoholates and for enzymatic interesterifications lipases both native or immobilized are used. The chemistry of chemical and enzymatic interesterifications was discussed by Marangoni and Rousseau 2. The present and potential applications of interesterification to the strategies of fat modifications were also reviewed 3, 4. Since a short time fat interesterification processes are assisted by microwaves irradiation 5, 6. The interesterification changes the molecular structure of fat acylglycerols involv- ing the changes in physical properties of fats. It is known that melting and crystallizing temperatures, solid fat content, polymorphic forms of interesterified fats differ substantially from these ones before interesterification. One of the most important property of fats is their resistance on autoxidation termed as oxidative stability. Several methods for the assessments of fat autoxidation have been developed in the past 7. Nowadays the method and the devices called Metrohm Rancimat Europe and OSI oxidative stability index USA are widely used for an assessment of oils and fats designated for high-temperature application 8, 9. Recently the kinetic approach to the data on fats oxidation obtained by Rancimat has been presented 10, 11. The thermaloxidative decomposition of fats is an exothermal process, so it could be quantified by thermal analysis techniques. Among them the DSC is the most frequently used. The DSC studies on the oxidative stability of fat can be performed in non-isothermal dynamic or isothermal modes at normal or at elevated kpa pressure of oxygen or air In 2011 the annual production of poultry meat in Poland was on the level of 2038 thousand tonnes. This production * Correspondence to: Boleslaw Kowalski, Laboratory of Food Chemistry, University of Life Sciences (SGGW), Warsaw, Poland bkowal@astercity.net ; boleslaw_kowalski@sggw.pl Accepted May 28, 2013 (received for review May 11, 2013) Journal of Oleo Science ISSN print / ISSN online

2 M. Kostecka, D. Kowalska, M. Kozłowska et al. gradually increased as in 2010 it was 3.8 lower. The prognoses not verified yet for 2012 showed that the poultry meat production will be 6 higher than in As poultry fat is a by-product of poultry meat production it is also produced in huge quantities and the chicken fat CHF is the main product. Chicken fat traditionally plays an important role in Polish meat industry and cuisine; it is mainly used as a component of sausages, pastries, cooking fats for poultry processing etc. Some quantities of chicken fat are used in animal feeding, in cosmetics and chemical industry. As the concentration of unsaturated fatty acids in CHF is relatively low it is blended with vegetable oils. Such blends can be directly used for human nutrition, as a cooking fats or the new fats can be obtained by interesterification 22. In this way the nutritional properties of fats are improved but it involves the changes in oxidative stabilities of interesterified fat. It was the purpose of this work to study the oxidative stability of a model blend of chicken fat with sunflower oil before and after enzymatic interesterification without and with microwaves assistance. 2 EXPERIMENTAL 2.1 Materials Sunflower oil SFO and Chicken fat CHF were refined, food grade products obtained from local market. Their parameters are listed in Table 1. The blend containing 40 CHF and 60 of SFO was prepared by mixing and homogenizing of CHF with SFO at 65 under nitrogen. The parameters of the components and their blend are listed in Table 1. The fatty acids composition of CHF, SFO and their blend 40 CHF 60 SFO are also listed in Table Catalyst and chemicals As the catalyst for enzymatic interesterification a commercial preparation Lipozyme RM IM Novozymes A/S, Bagsvaerd, Denmark distributed as biochemical reagent was used. The Lipozyme RM IM contains pure lipase from Rhizomucor miehei immobilized on an anionic type resin. The preparation contained 2 of water. All chemicals and solvents used were of analytical grade. 2.3 Enzymatic interesterification EI After thermal equilibration of fat blend at 60 8 of Li- Table 1 Parameters of chicken fat, sunflower oil and the mixture containing 40% chicken fat and 60% sunflower oil. Parameter Chicken fat (CHF) Sunflower oil (SFO) Mixture (40% CHF + 60% SFO) Acid value (AV; mg KOH/g) 0.28± ± ±0.08 Free fatty acids (FFA; %) 0.14± ± ±0.04 Peroxides (mmoles O 2-2 / kg) 14.5± ± ±0.26 MAG + DAG # (%) 10.9± ± ±0.1 TAG # (%) 88.8± ± ±0.1 Oxidative stability index (OSI; h) at ± ± ±0.14 OSI/TAG (h) at ± ± ±0.21 PDSC, τon at 120 (min) 9.07± ± ±1.05 PDSC, τmax at 120 (min) 19.78± ± ±1.02 Main fatty acids (%) Total/sn-2* Total/sn-2* Total/sn-2* Palmitic 22.0±0.2/18.5± ±0.2/8.8± ±0.2/12.6±0.1 Oleopalmitic 3.5±0.0/1.3± ±0.0/0.1± ±0.1/0.6±0.0 Stearic 6.5±0.1/8.3± ±0.1/4.8± ±0.1/6.1±0.1 Oleic 45.6±0.2/50.7± ±0.2/32.2± ±0.3/39.6±0.3 Linoleic 18.8±0.1/18.3± ±0.1/52.8± ±0.3/39.0±0.3 Linolenic 2.4±0.0/2.3± ±0.0/0.5± ±0.0/1.3±0.0 Σ SFA $ (%) 28.5± ± ±0.2 Σ UFA $ (%) 69.7± ± ±0.3 # MAG = monoacylglycerols, DAG = diacylglycerols, TAG = triacylglycerols OSI/TAG = oxidative stability index for TAG derived from samples studied * sn-2 = 3 TAG (total) 2 sn-1,3 $ Σ SFA = sum of saturated fatty acids, Σ UFA = sum of unsaturated fatty acids 894

3 Properties and Oxidative Stabilities of Enzymatically Interesterified Chicken Fat and Sunflower Oil Blend pozyme RM IM was added. After predetermined time 2, 4, 8, 24 hours of interesterification filtering off the catalyst stopped the reaction. As filtering bed contained drying agent SiO 2 MgSO 4 water from catalyst and higroscopic was removed from interesterified fat. 2.4 Enzymatic interesterification assisted with microwaves EI/MWA Blends were prepared as for enzymatic interesterification, but the reactions were performed in microwave reactor, Ertec Magnum Nova 09, Ertec Wroc aw, Poland. Fats were mixed using magnetic bar and after addition of 8 of Lipozyme RM IM they were heated 60 and irradiated by microwaves power adjustable W, frequency 2.45 GHz, glassware heater 300W, contactless IR thermometer. The EI/MWA interesterification times were 5, 10 and 20 minutes. The post-reaction mixtures PRM were treated in the same manner as for EI products. 2.5 Determinations and analyses The acid values were determined by titration of fat sample dissolved in a mixture of ethanol : diethyl ether 1:1 vol/vol with 0.1 M ethanolic potassium hydroxide solution. Free fatty acids were calculated from AV using the results of fatty acids compositions. The same alkaline KOH/Et-OH solution was used for deacidification of crude interesterified fats.the peroxide values of fats were determined by iodometric technique in accordance with Polish Standard PN-ISO 3960:2012. Fats before and after interesterifications were separated into TAG and non-tag fractions, referred to as polar fraction PF, by column chromatography on silica gel SG 60, mesh, Merck, Germany. The TAG were eluted with a mixture of petroleum ether:diethyl ether 87:13 vol/vol, and the polar fraction, which contained FFA, MAG and DAG, was eluted with diethyl ether. The weight percent of TAG and PF were determined after evaporation of solvent Polish Standard PN-ISO 8420:1995. The TAG fractions for PDSC determinations were also purified from prooxidants metal ions and peroxides by adsorption on activated neutral Al 2 O 3 column chromatography as reported elsewhere 23. The fatty acids composition of fats was determined by GLC after conversion of fats to fatty acid methyl esters. A Shimadzu GC 17A apparatus equiped with BPX-70 capillary column was used 17. The fatty acids distribution between the sn-1,3 and sn-2 positions of TAG, before and after interesterification, was determined by pancreatic lipase hydrolysis, separation of products on SiO 2 covered plates and GLC analysis as described elsewhere 24. The tocopherols content in sunflower oil and in the mixtures of sunflower oil with chicken fat before and after interesterification were determined by HPLC. The analysis was performed on a Waters 600 HPLC instrument equipped with Symmetry C18 column fitted with a Bondpak C10 guard column. Mobile phase was acetonitrile and methanol 1:1, v/v at a flow rate of 1 ml/min. Injection volume was 20 μl and the eluate was monitored using Waters 474 scanning fluorescence detector set for emission at 325 nm and excitation at 295 nm. The details are reported by Gliszczynska- Swiglo et al Oxidative stability by rancimat measurements The induction times OSI for oxidation of fats studied were measured at 100 with sample mass of g and air bubbling through the oil at the rate of 6 L/h. The Metrohm Rancimat apparatus model 679, Herisau, Switzerland was used. The experimental procedure is given elsewhere 10. The OSI values 10 h were determined with precision of 0.01 h. The OSI values 10 h were measured with precision of 0.1 h. 2.7 Oxidative stabilities by PDSC isothermal measurements The oxidative stabilities of fats before interesterification and interesterified pure TAG fraction were studied by Isothermal PDSC. The Q20P pressure differential scanning calorimeter TA Instruments, New Castle, Delaware, USA with high pressure DSC cell Q series DSC Pressure Cell, TA Instruments was used. The isothermal PDSC measurements for fats sample mass 3-4 mg, aluminium pans open were carried out at five selected temperatures from the range of and under 1400 kpa of oxygen flowing at the rate of 6L/h. From the resulting PDSC heat flow curve the time of onset oxidation τon and the time to reach the heat flow maximum τmax were determined by apparatus software exactly to 0.01 of a minute. For each fat at each temperature three experiments were performed and the average τon and τmax values were collected. 2.8 Statistical analysis The data were assessed statistically by ANOVA and Duncan`s multiple range test using commercially available package of SPSS software program SPSS Inc., Chicago,. IL, USA. A p value 0.5 was considered significant. 3 RESULTS AND DISCUSSION 3.1 Fats before enzymatic interesterification The properties of chicken fat, sunfower oil and their 2:3 m/m mixture are listed in Table 1. The values for CHF and SFO shown that fats were fresh with parameters acceptable by suitable standards. Additionally the determinations of tocopherols TOC α, β γ, δ shown that the SFO contains in total mg/kg of TOC α TOC mg/ kg, β γ TOC 29 1 mg/kg, δ TOC 12 2mg/kg. In contrary chicken fat contains practically no tocopherols so 895

4 M. Kostecka, D. Kowalska, M. Kozłowska et al. their contents in 40 CHF 60 SFO mixture depend on the proportion of SFO used. 3.2 Rancimat and PDSC determinations for fats before interesterification For technological and quality control purposes the determination of rancimat induction times at selected temperatures oxidative stability index OSI are one of the most frequently used parameters for characterization of fats oxidative stability. The results of the OSI for compositional fats and their blend 2:3, m/m at 100 are given in Table 1. Commercial chicken fat displayed rather low oxidative stability OSI h although it contains more saturated fatty acids than SFO OSI h and the mixture CHF SFO, OSI h. The chicken fat contained relatively large quantities of Peroxides, FFA, and MAG DAG. Such compounds belong to the prooxidants group 26. Taking into account that CHF does not contain any significant quantities of tocopherols its relatively low oxidative stability is justified. Removing of prooxidants from CHF improved its oxidative stability increasing the OSI up to h. On the other hand SFO, despite of high content of unsaturated fatty acids, but with lower than for CHF contents of FFA, Peroxides and MAG DAG, shown higher value of OSI. The natural defence system formed by antioxidants seems to be responsible for this property. When the SFO was mixed with CHF antioxidants covered whole system; and the mixture displayed OSI h. After separation of TAG fractions from CHF, SFO and their 2:3 m/m blend the τon and τmax as a function of temperature were measured. The data obtained at 120 are listed in Table 1. For each fat the dependencies τon and τmax against temperature were linear and they were correlated by the equations: log τ ON or log τ max a t b log τ ON or log τ max A T 1 B Eq.1 Eq.2 where a, b and A, B are adjustable coefficients and t and T are the temperatures in, and in K, respectively. From Eq.1 the τon or τmax at selected temperatures can be calculated. The PDSC data correlated by the Eq. 2 were used for kinetic analysis. The activation energy E was calculated from equation 27 : E 2.19 R d log τ /dt 1 Eq.3 where: R is the gas constant. Then the Arrhenius equation was applied and its preexponential factor Z was calculated. The results for regression and kinetic analyses of PDSC data for CHF, SFO and 40 CHF 60 SFO are presented in Table Fats after enzymatic interesterification After EI and EI/MWA of 40 CHF 60 SFO blend the Table 2 Regression analysis of τ on and τ max PDSC data (a, b, r 2 and A, B, r 2 ), activation energies (E, kj/mol), preexponential factors (Z, min -1 ) and rate constants (k, min -1 ) for oxidation at PDSC conditions of TAG separated from chicken fat (CHF), sunflower oil (SFO) and their mixture 40% CHF + 60% SFO. Parameters Calculations based on τ on measurements CHF/TAG SFO/TAG Mixture/TAG 40% CHF+60 % SFO Calculations based on τ max measurements CHF/TAG SFO/TAG Mixture/TAG 40% CHF+60 % SFO Equation 1 -a b r Equation 2 A B r E 115.4± ± ± ± ± ±2.4 Z k at k at k at k at

5 Properties and Oxidative Stabilities of Enzymatically Interesterified Chicken Fat and Sunflower Oil Blend analyses of post-reaction mixtures indicated that their proximate composition have changed if compare with nonesterified blend. The parameters of PRM after EI and EI/ MWA are summarized in Table 3. Due to interesterification of the blend the contents of FFA, Peroxides and MAG DAG have substantially increased. Although the EI/MWA reaction times were shorter 5 20 minutes than these ones applied for EI 2 24 hours the differences between some properties of the products obtained in both types of interesterifications were not so large. On the other hand it should be stressed that the products of EI/MWA displayed lower values of FFA, Peroxides and MAG DAG than their counterparts of EI products. After interesterifications of 40 CHF 60 SFO blend and then alkaline deacidification of the PRM the concentrations of tocopherols were determined. It has appeared that the total content of tocopherols have decreased to the of their initial content. The loss of α-tocopherol was in the range of for EI and for EI/MWA. For the sum of β γ -tocopherols and for δ-tocopherol the losses after EI were and , respectively. For EI/MWA reaction the losses of tocopherols were 12.9 lower that for EI and it is probably due to substantially shorter time of reaction. The losses of tocopherols during enzymatic interesterifications of fats were reported and explained as the result of esterification of -OH group in chromanol ring of tocopherol by fatty acid 28 and by loss of tocopherols due to rafination processes of PRM 29, 30. For the deacidified products FFA 0.1 their oxidative stabilities by Rancimat were measured. The results are listed in Table 4 and they show that the EI and EI/MWA reduced the oxidative stabilities of fats if compared with nonesterified products. The OSI values were reduced from h to h for EI products and to h for EI/MWA products. These findings are in agreement with general opinion that interesterification of fats reduces their oxidative stabilities 31. Presence of antioxidants, prooxidants, fatty acid composition FAC and their structure unsaturation and chemical structure of TAG influence on the oxidative stabilities. During enzymatic interesterification and product purification antioxidants and prooxidants can be removed or at least their concentrations are reduced. The FAC and unsaturation of the FA remain practically constant. Separation of TAG`s from PRM and determinations of their oxidative stabilities can provide additional information on participation of the TAG fraction in total stability of the system. The OSI results obtained by Rancimat for TAG fractions Table 4 show that they do not differ substantially from those for PRM. The OSI values for TAG`s separated from PRM after EI/MWA h are practically the same as OSI values for nonesterified 40 CHF 60 SFO blend h and are higher than these ones obtained for fats after EI. The TAGs obtained from fats after IE and EI/MWA by Al 2 O 3 column chromatography were studied by PDSC. The times for onset of oxidation and time to reach the heat flow maximum were correlated with equations Eq. 1 and Eq.2. Based on the experimental data the Arrhenius model kinetic analysis was employed. The results are tabularized in Tables 5 and 6. It has appeared that there were differences in oxidation rates for TAGs obtained from non-interesterified and interesterified by EI and EI/MWA mixtures of CHF and SFO. The TAGs separated from mixtures after EI/MWA have appeared to be more stable than these ones obtained from mixtures after EI. Rhizomucor miehei lipase, an active component of the Table 3 Propertiess of enzymatically interesterified mixture. Catalyst: Lipozyme RM IM containing 2% of water. Symbols and abbreviations as in Table 1. Fat sample Parameters Interesterification AV FFA Peroxides MAG + DAG TAG time (h) (mg KOH/g) (%) (mmoles O 2-2 /kg) (%) (%) ±0.01 ab 3.9±0.0 b 12.60±0.01 a 12.7±0.4 b 83.4±0.4 a 40% CHF ±0.04 b 4.1±0.1 b 12.91±0.03 a 11.4±0.3 a 84.5±0.3 b 60% SFO ±0.02 (interesterified) 4.4±0.1 b 13.62±0.06 ab 13.4±0.3 c 82.2±0.3 a ±0.01 a 3.4±0.1 a 14.90±0.04 b 11.9±0.4 a 85.1±0.4 b 40% CHF + 60% SFO 5* 4.89±0.03 a 2.4±0.0 a 10.68±0.13 a 8.3±0.1 a 89.3±0.1 b (interesterified 10* 4.80±0.01 a 2.4±0.0 a 11.05±0.14 ab 9.7±0.1 b 87.9±0.1 a with assistance of microwaves) 20* 5.26±0.01 b 2.6±0.1 b 11.87±0.10 b 10.1±0.1 c 87.3±0.2 a Values in columns with the same letter are not significantly different (p < 0.05) * Interesterification time in minutes 897

6 M. Kostecka, D. Kowalska, M. Kozłowska et al. Table 4 Oxidative stability measured by Rancimat at 100 for enzymatically interesterified mixtures of 40% chicken fat + 60 % sunflower oil and for triacylglycerol fraction separated from interesterified fat. Catalyst: 8% Lipozyme RM IM. Oxidative stability ( OSI h) Interesterification time (h) Fraction of TAG separated from Interesterified fat mixture interesterified mixture ±0.07 a 6.08±0.11 c ±0.21 ab 5.77±0.07 a ±0.10 a 4.12±0.06 ab ±0.04 b 4.36±0.08 b 5* 6.08±0.14 a 6.28±0.11 a 10* 6.67±0.01 a 6.02±0.06 a 20* 6.59±0.10 a 5.89±0.10 a Interesterification assisted by microwaves, interesterification times in minutes # Values in columns with the same letter are not significantly different (c p < 0.05) Table 5 Regression analysis of τ on and τ max PDSC data (a, b, r 2 and A, B, r 2 ), activation energies (E, kj/mol), pre-exponential factors (Z, min 1 ) and rate constants (k, min 1 ) for oxidation at PDSC isothermal conditions of TAG s separated from mixtures 40% CHF + 60% SFO after interesterification catalyzed by Lipozyme RM IM containing 2% of water. Parameters 40% CHF + 60 % SFO; isothermal PDSC calculations based on τ on measurements Interesterification time (h) 40% CHF + 60 % SFO; isothermal PDSC calculations based on τ max measurements Interesterification time (h) Equation 1 -a b r Equation 2 A B r E 112.7± ± ± ± ± ± ± ±2.7 Z k at k at k at k at catalyst, operates only on the sn-1,3 TAGs linkages. The sn-2 compositions of main fatty acids Table 1 after interesterification of the 40 CHF 60 SFO blend remained almost unchanged. Any changes arrising from acyl migrations from sn-1,3 to sn-2 and vice versa were negligible. There were 1.0 differences between sn-2 compositions of FA before and after interesterification. So, the structures of TAGs before and after interesterification are similar. Their resistance on oxidation is also similar and the contributions of TAGs into the total oxidative stability of fats before and after EI are not much different each other. Although the time of EI/MWA was relatively short the properties of the products obtained from EI/MWA and EI processes were not so much different. The TAGs isolated from 898

7 Properties and Oxidative Stabilities of Enzymatically Interesterified Chicken Fat and Sunflower Oil Blend Table 6 Regression analysis of τ on and τ max PDSC data (a, b, r 2 and A, B, r 2 ), activation energies (E, kj/mol), pre-exponential factors (Z, min 1 ) and rate constants (k, min 1 ) for oxidation at PDSC isothermal conditions of TAG s separated from mixtures 40% CHF + 60% SFO after interesterification assisted with microwaves and catalyzed by Lipozyme RM IM containing 2% of water. Parameters 40% CHF + 60 % SFO; isothermal PDSC calculations based on τ on measurements Interesterification time 40% CHF + 60 % SFO; isothermal PDSC calculations based on τ max measurements Interesterification time 5 min 10 min 20 min 5 min 10 min 20 min Equation 1 -a b r Equation 2 A B r E 118.6± Z k at k at k at k at nonesterified fats and interesterified PRM oxidised with comparable rates. Some differences can be observed at higher temperatures 140 where the TAGs from EI/ MWA oxidized slower than these ones after EI. 4 CONCLUSIONS Interesterification of chicken fat and sunflower oil mixtures can involve the production of a new fat with valuable functional and nutritional properties. The studied, interesterified fat is less stable against oxidation than starting 40 CHF 60 SFO blend, but more stable than rendered chicken fat. The oxidative stability of starting mixture comes from SFO that contains antioxidants tocopherols. Although a substantial part of tocopherols is deactivated during interesterification the remaining tocopherols still protect interesterified fats against oxidation. The changes in TAG structures resulting fom Rhizomucor miehei lipase catalyzed interesterification of chicken fat sunflower oil blend are of minor importance. The interesterified fats can be used for nutritional purposes and for food processing cooking, particularly when herbs and spices that have natural antioxidative properties are also used. The PDSC method has appeared a valuable tool for studying oxidative stability of native and interesterified fats, especially for highly purified TAG fraction. REFERENCES 1 Przybylski, R.; Ramamurthi, S. Interesterification Current Status and Future Prospects. In: Development and Processing of Vegetable Oils for Human Nutrition. Editors: R Przybylski and BE McDonald, ISBN , AOCS Publishing 1996, USA. 2 Marangoni, AG.; Rousseau, D. Engineering triacylglycerols: The role of interesterification. Trends Food Sci. Technol. 6, Gunstone,F. D. Movements towards tailor-made fats. Prog. Lipid Res. 37, Willis, W. M.; Lencki, R. W.; Marangoni, A. G. Lipid modification strategies in the production of nutritionally functional fats and oils. Crit. Rev. Food Sci. Nutr. 38, Roy, I.; Gupta, M. N. Applications of microwaves in biological sciences. Current Science 85, Saxena, R. K; Isar, J; Saran, S; Kaushik, R; Davidson, W. S. Efficient microwave-assisted hydrolysis of triolein 899

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