GRASAS Y ACEITES, 59 (1) ,ENERO-MARZO, 45 -50 , 2 0 0 8 ,

ISSN: 0017-3495

Analysis of used frying fats for the production of biodiesei

By M.V. Ruiz-Méndez^, S. Marmesat, A. Liotta and M.C. Dobarganes

Instituto de la Grasa (CSIC). Avda Padre García Tejero. 4. 41012 Sevilla*Corresponding author: mvruiz@cica.es

RESUMEN

Análisis de aceites y grasas de fritura para produc-ción de biodiesei

Los aceites y grasas de fritura, que se caracterizan por te-ner una calidad muy variable, se utilizan como material primapara la producción de biodiesel. El objetivo de este estudio esdefinir la utilidad de los métodos analíticos desarrollados paralos aceites y grasas de fritura para caracterizar el biodiesel ob-tenido. Veinticuatro aceites de fritura procedentes del sectorde restauración y de fritura doméstica fueron analizados antesy después de su transesterificación a esteres metílicos de áci-dos grasos. A partir de un análisis detallado mediante croma-tografías de adsorción y exclusión, se deduce la importanciacuantitativa de los compuestos de polimerización tanto en elanálisis directo de los aceites como en el análisis de los este-res metílicos. Se encontró una excelente correlación lineal en-tre los compuestos polares y los ásteres metílicos polares(R ^ 0.9768). Se definen las interferencias de los compuestosformados durante la fritura en el análisis estándar para cono-cer la calidad del biodiesel y se propone la determinación deesteres metílicos no polares como una buena alternativa a ladeterminación cromatográfica estándar.

PALABRAS - CLAVE: Aceites y grasas de fritura -Análisis - Biodiesel - Compuestos polares - Esteres metí-licos de ácidos grasos polares.

SUMMARY

Analysis of used frying fats for biodiesel production

Used frying fats and oils with highly variable anduncontrolled quality are used for the production of biodiesel .The objective of this study was to define the analyticalmethods useful to obtaining information on the quality of theused frying oils as raw material for biodieseis as well as for thecharacterization of the biodiesels obtained from them. Twenty-four used frying oils from restaurants and domestic fryers wereanalyzed before and after transesterification to fatty acidmethyl esters (FAME). From a detailed analysis of the samplesby means of a combination of adsorption and size exclusionchromatography, the quantitative importance of polymericcompounds was deduced both from the direct analysis of theoiis and from their FAME. Excellent linear correlation betweenpolar compounds and polar FAME (R = 0.9768) was found.The possibilities of interferences from polar fatty acid in thestandard method to determine the ester content are defined.Finally determination of non-polar FAME by silica column isproposed as a good alternative to the gas chromatographymethod.

KEY-WORDS: Analysis - Biodiesel - Polar compounds -Polar fatty acid methyl esters - Used frying oiis.

1. INTRODUCTION

The base-catalyzed transesterification method isnormally applied for biodiesel production. However,biodiesel produced from crude or refined oils isusually more expensive than petroleum-based dieselfuel. The recent concerns over sustainability,environmental issues, and raw material costs havemade the use of used frying oils very attractive to theindustry. In this respect, there have been a significantnumber of studies over the last decade with the aimof establishing the optimum conditions for biodieselproduction using frying oils as raw material {CostaNeto et al.. 2000; González Gómez et al., 2002;Leung, 2001; Al-Widyan ef a/., 2002; Supple et al.,2002; Tashtoush et al, 2004; Tomasevic and Siler-Marinkovic, 2003; Zhang et al.. 2003 a, b; Feiizardoetal., 2006; Leung and Guo, 2006) or evaluating theirperformance as fuels (Mittelbach and Tritthart, 1988;Al-Widyan and Al-Shyoukh, 2002; Zäher, 2003;Dorado eía/.,2003; Ulusoy el al., 2004).

However, used frying oils from restaurants andfood industries have a wide variety of qualities.During the frying process, the oil or fat is exposedto high temperatures in the presence of air andmoisture. Under these conditions, they mayundergo important changes due to hydrolytic,oxidative and thermal reactions resulting in the lossof quality of the frying oil and of the fried food.Changes in the main fat constituents are known,although it is not easy to foresee the rate of oildegradation due to the high number of variablesinvolved in the frying process. Some of them arelinked to the process itself, such as temperature,length of heating, continuous or discontinuousheating, turnover period, etc.; others to the foodsubjected to frying, i.e. lipid composition, main andminor constituents, etc.; or else to the fat or oil usede.g., unsaturation degree, initial quality andadditives (Várela, 1985; Rodrigues Machado etal.,2007).

The new products formed during frying arepolymers, dimers, oxidized triglycérides, as well asdigiycerides and fatty acids (Dobarganes andMárquez-Ruiz, 2006). All these groups possesshigher polarity than that corresponding to the initialtriglycérides and can be easily quantified by meansof adsorption chromatography (lUPAC, 1992). Thus,polar compound determination gives a goodmeasurement of the total degradation of frying oils

M.V. RUIZ MÉNDEZ, S. MARMESAT, A. LIOTTA AND M.C. DOBARGANES

and, for this reason, it is not only the most acceptedmethod in the evaluation of frying fats but also thelimitation of the alteration of frying fats for humanconsumption in polar compound levels around 25 %has been included in many official regulations(Firestone, 1996). Even more, analyses of usedfrying oils sampled by the Food Inspection Servicesin different countries have clearly demonstrated thatthe stated 25% polar compounds is overpassed in asignificant number of used frying oils {Dobarganesand Marquez Ruiz, 1995a, 1995b, 1998).

Thus, used frying oils can be highlyheterogeneous as compared to crude or refined oils.In general, it is found that crude oils have more than95% of triglycérides + partial glycerides whichwould give more than 98% of fatty acid methyl esters(FAME) supposing that total tranesterification takesplace. However, in the case of used frying oils, non-altered FAME depends on the variable quality of theused frying oil (Dobarganes et ai., 1984; MárquezRuiz et ai., 1995). At present, the quantification ofthe main groups of degradation compounds in usedfrying oils (Dobarganes et al, 1988, 2000) as well asin the FAME obtained after oil transesterification(Dobarganes et al, 1984; Márquez Ruiz e/a/., 1990)is not a difficult task and takes advantage of the twomain properties differentiating the new compoundsformed during frying from the initial TG, i.e. polarityand molecular weight (MW).

On the other hand, studies are necessary toclarify the new compounds present in used fryingfats and oils as many of the new compounds formedduring frying might interfere in the standard gasChromatographie determinations for evaluating thequality of biodiesel (European Standard, 2003a;2003b).

The objective of this study was to define theanalytical methods useful to obtaining informationon the compounds present in used frying oils and tocharacterize the biodiesels obtained from them.

2. EXPERIMENTAL PART

2.1. Samples

Twenty-four used frying fats and oils were selectedfrom those collected in restaurants and domesticfryers. Most of the samples had degradation levelsclose to that established for discarding used fryingfats and oils for human consumption (25% polarcompounds)

2.2. Analytical techniques

Quantification of poiar compounds and theirdistribution in oxidation, poiymerization andhydroiytic compounds. Non-polar and polarfractions were separated from Ig of oil sample bysilica column chromatography (20 g silica-H20, 95:5w/w). Non-polar fraction containing unaltered TGwas eluted with 150 mL of n-hexane/diethyl ether(90:10, v/v). A second fraction, comprized of total

polar compounds, was eluted with 150 mL of diethylether. Efficiency of the separation by adsorptionchromatography was checked by TLC usinghexane/diethyl ether/acetic acid (80:20:1, v/v/v) fordevelopment of plates and exposure to iodine vaporto reveal the spots. After evaporation of solvents,both fractions were weighed and dissolved indiisopropyl ether (25 mg/mL) for analysis by high-performance size exclusion chromatography(HPSEC), using a Rheodyne 7725i injector with 10-mL sample loop, a Waters 510 pump (Waters,Milford, MA, USA), a HP 1037 A refractive indexdetector and a HP 3392 A integrator (Hewlett-Packard, Avondale, PA, USA). The separation wasperformed on two 100 and 500Â Ultrastyragelcolumns (25 cm x 0.77 cm i.d.) packed with porous,highly cross linked styrene-divinylbenzenecopolymers (film thickness: 10 mm) (Hewlett-Packard, Avondale, PA, USA) connected in series,with tetrahydrofuran (1 ml_/min) as the mobilephase (Dobarganes ei ai., 2000). The groups ofcompounds separated were oligomeric triglycérides(OTG), dimeric triglycérides (DTG), oxidizedmonomeric TG (oxMTG), diglycerides (DG) andFatty acids (FA).

Quantification of poiar fatty acid metiiyi esters andtheir distribution in oiigomeric, dimeric and oxidizedmonomeric FAhAE. FAME were obtained bytransesterification of oil samples with sodiummethoxide and hydrochloric acid-methanol andsubsequent recovery of methyl esters (Dobarganeset ai., 1984). Methyl esters were separated by silicacolumn chromatography, using hexane/diethyl ether(95:5) to elute a non-polar fraction and diethyl etherand methanol to obtain the polar fraction. Analysis ofthe polar fraction was performed by HPSEC, usingthe Chromatographie conditions described above. Thegroups of compounds separated were oligomericFAME (OFA), dimeric FAME (DFA) and oxidizedmonomeric FAME (OxMFA). The methodology wasdescribed in detail, including calibration andreproducibility data, in an earlier publication(Márquez-Ruiz et ai., 1990).

Determination of fatty acid methyi esters wasperformed following the lUPAC Standard Method(lUPAC, 1992).

3. RESULTS AND DiSCUSSiON

Table 1 shows the evaluation of polarcompounds and their distribution in the used fryingoils selected. It is interesting to note that sampleswith the same percentage of total polar compoundsshowed different patterns of compound distribution.Thus, samples 8, 17, 21 and 22 had high contentsof DG formed by hydrolysis while for the rest of thesamples the compounds formed by oxidation andhigh temperature stood out. Such differencesbetween samples are not unusual, considering thatprevious treatment is unknown and they may havebeen subject to very different procedures ,especially in terms of frying temperature, frying

46 GñASAS Y ACEITES, 59 (1), ENERO-MARZO. 45-50, 2008, ISS 4̂: 0017-3495

ANALYSIS OF USED FRYING FATS FOR THE PRODUCTION OF BIODIESEL

Table 1Quantitative determination of total poiar compounds and their distribution in used frying oils

Sample

123456789

101112131415161718192021222324

Totai

14,517,421,122,823,023,125,525,525,726,426,627,527,627,628,930,833,135,237,339,239,743,547,149,7

OTG

1.12,13,73,33,34.04,72,76,35,54,96,26,53,75,05,96,58,6

10,610,57,8

12,112,321,1

Polar Compounds and

DTG

5,47,78,87,38,29,29,07.19,48,9

11,39,8

10,97,3

12.59,87,2

14,014,313,69,8

13,913,313,5

distribution (%)

OxMTG

5,65.96.18,99.16,88.68.65.96.48.77.77.29,49,8

11,55,4

10,89,09,8

13,310,015,210,9

DG

2,11,21,93,11,92,52,66,93,55,11,23,42,36,21.23,2

12,31.42.84,57,66,95,73,4

FA

0,40.50.60.30.40.60.70.10.60,50.40.40,61,10.40,41,70,40,70,91,20,70,70,8

Abreviatiorys: OTG, Olimeric triglycérides; DTG. dimeric triglycérides: OxMTG, oxidized monomeric triglycérides; Dg. diglycerides; andFA. fatty acids.

periods and total period of use. However, polymericTG, i.e. oligomeric + dimeric TG were the majorconstituents in aii the samples as previouslyreported (Marmesat etal., 2007).

Table 2 summarizes the content of poiar FAMEand their distribution in oiigomeric, dimeric andoxidized monomeric TG after tranesterification of thesamples. In the second column the percentage ofnon-polar FAME has also been inciuded. Figure 1shows typical chromatograms obtained by exclusionchromatography of the two fractions previouslyobtained by silica column. As can be observed, non-

Figure 1Partial high-performance size-exclusion chromatograms

of non-polar fatty acid methyl esters (A) and polar fatty acidmethyl esters (B). Retention times : 13.5 min, Ofigomeric

FAME; 14.1 min, Dimeric FAME; 15.2 min, oxidizedmonomeric FAME; 15.5 min, non-poiar FAME.

polar fraction is a single peak constituted by non-polar FAME; while in the polar fraction three maingroups of compounds were separated: oligomericTG {MW > 900), dimeric TG (MW around 600) andoxidized monomeric TG (MW around 300).

As compared to the poiar compounds in Table 1,the percentages of polar FAME were much lower asthey only include the modified fatty acyl groupspresent in theTG molecules. Non-polar FAME includenot only non-altered fatty acids from all the TGmolecules but also those present in DG and fattyacids. Consequently, poiar FAME is a directmeasurement of the rmox i dative degradation affectingunsaturated fatty acids. The results in Table 2 showthat polymeric fatty acids {oligomeric + dimericFAME) were the major compounds. Regressionbetween polar compounds and polar FAME arepresented in Figure 2 where the equation of the lineas well as the high correlation coefficient found havebeen included. From the equation it can be deducedthat a ievel of around 10% poiar FAME is expectedfrom samples around the level of used frying oilrejection (25% poiar compounds).

It is important to note that analysis of polarFAME can be directly applied to biodiesels and thatthe results obtained would be paraiiel to those givenin table 2 for used frying oils of a similar level ofdegradation. Thus, after biodiesel productionpolymeric FAME, i.e. oligomeric + dimeric FAME,as well as oxidized monomeric FAME are expected.Determination of non-polar FAME by adsorptionchromatography is an interesting method due to itsaccuracy in obtaining information on the quality of

GRASAS Y ACEn^S, 59 (1), ENERO-MARZO, 45*50, 2008, rSSN: 0017-3495 47

M.V. RUIZ MÉNDEZ, S. MARMESAT. A. LIOTTA AND M.C. DOBARGANES

Table 2Quantitative determination of total polar fatty acid methyi esters and their distribution in

used frying oils

Sample

123456789

101112131416181718192021222324

Non-poisrFAME

94,593,291,491,391,090,989,690,689,589,287,989,388,790,388,486,385,182,784,484,983,783,779,077,0

Totai

5,56,88,68,79,09.1

10,49,4

10,510,812,110,711,39,7

11,613,714,917,315,615,116,316,321,023,0

Polar FAME and

OligomericFAME

0,70,90,81,20,60,81,10,51,30,71,21,11,10,51,12,73,53,82,42,92,54,15,36,1

their distribution (*

DimericFAME

2,03,35,04,05,15,15,95,25,76,76,26,26,55,57,46,27,48,68,88,49,58,39,8

10,3

Yo)

Oxidizedmonomeric

FAME

2M2,62,83,63,33,33,43,73,53,44,73,43.73.83.14.74,04.94.43,8

3.96,06,6

the biodiesei as an alternative to the standardmethod by gas chromatography,

On the other hand, as stated for animal fats andlauric oils (Schober ei a/., 2006), the standardmethod EN14103 by GC (European Standard,2003b) may give misleading results depending onthe level of polar FAME, as many compoundsformed may elute between fatty acids ranging fromC 12:0 and C 24:1. In this respect, even if polymericFAME are the major compounds and they are non-

eluted materials in FAME analysis by GC, oxidizedfatty acid monomers are also important from aquantitative point of view and they elute in the GCanalysis.

Separation and identification of the mainstructures in FAME from used frying fats indicatethat compounds with molecular weight lower thanor similar to that of the original fatty acids arepresent. Among the first group, the short-chainFAME, the short-chain n-oxo FAME and diacid

25

r 20

15

10

Polar FAME (%) = 0,46 Polar Compounds (%) -1,4R= 0,9768

10 15 20 25 30 35 40

Polar Compounds (%)

45 50

•' Figure 2Linear regression between polar compounds and polar fatty acid methyl esters.

48 GRASAS y ACEITES. 59 (1), ENERO-MARZO, 45-50, 2008. ISSN: 0017-3495

ANALYSIS OF USED FRYING FATS FOR THE PRODUCTION OF BIODIESEL

FAME, originally TG-bound aldehydes and acidsresulting from hydroperoxide breakdown, are themost representative compounds (Márquez Ruizand Dobarganes, 1996; Velasco et ai., 2005)Concerning oxidized compounds of molecularweight similar to that of the starting fatty acids,the main groups present corresponded toepoxyacids, ketoacids and hydroxyacids {Velascoet ai., 2004).

Figure 3 shows a chromatogram of usedsunflower frying oil. The zone corresponding toretention times including C22:0 and C24:0 hasbeen widened to observe the differences betweenFAME obtained from a refined sunflower oil andfrom the same oil used in frying until the rejectionlimit (25%). The oxidized fatty acids formed andeluting would be included in the ester content givenby the standard method and may suppose anoverestimation between 1 and 2%.

Interferences from monomeric and polymericFAME can be also found in the application of thestandard method (European Standard, 2003b)where a high temperature is applied for the elutionof TG and partial glycerides. Studies on this pointare being carried out and the results will bereported shortly .

4. CONCLUSIONS

a) The determination of polar fatty acids inbiodiesets allows for the exact determination ofnon-polar esters and is an interesting alternativeto the GC standard method due to its accuracyand to the fact that no instrumental techniquesare required.

b) From the percentage of polar compounds inused frying fats and oils, the percentage of non-polar and polar methyl esters after theproduction of biodiesel can be roughly deducedgiven the excellent correlation between bothdeterminations.

c) Compounds from polar FAME could be presentin the chromatograms obtained from presenttechniques for the quality evaluation of biodieselwhen used frying fats and oils are used forbiodiesel production.

ACKNOWLEDGEMENT

This research work was supported by Junta deAndalucía and MCYT (Project AGL 2004-00148).The authors thanks Ms Mercedes Giménez forassistance.

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Recibido: 26/09/07Aceptado: 19/11/07

50 GRASAS Y ACEITES, 59 (1), ENERO-MAHZO. 45-50, 2008, ISSN! 001 7-3495