The determination of iron in grapes and their alcoholic

Transcription

1 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, RESIDUES AND TRACE ELEMENTS Optimized Determintion of Iron in Grpe Juice, Wines, nd Other Alcoholic Beverges by Atomic Absorption Spectrometry OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, 2000 MANUEL OLALLA, MARIA CRUZ GONZÁLEZ, CARMEN CABRERA, nd MARIA CARMEN LÓPEZ University of Grnd, School of Phrmcy, Deprtment of Nutrition nd Bromtology, E-18071, Grnd, Spin This pper describes study of the different methods of smple preprtion for the determintion of iron in grpe juice, wines, nd other lcoholic beverges by tomic bsorption spectrometry with electrotherml tomiztion; results re lso reported for the prcticl ppliction of these methods to the nlysis of commercil smples produced in Spin. The methods exmined include delcoholiztion nd dry nd wet minerliztion tretment using different cids nd/or mixtures of them, both with nd without heting. The sensitivity, detection limit, ccurcy, precision, nd selectivity of ech method were estblished. The best results were obtined for wet minerliztion with heted cid (HNO 3 H 2 SO 4 ); the results for tble wines hd n ccurcy of %, reltive stndrd devition of 3.51%, detection limit of 19.2 g/l, nd determintion limit of 32.0 g/l. The method ws lso sufficiently sensitive nd selective. It ws pplied to the determintion of iron in grpe juice, different types of wines, nd beverges with high lcoholic content, ll of which re produced nd widely consumed in Spin. The vlues obtined rnged from ± 2.15 mg/l for the juice, ± 1.47 mg/l for the white wines, ± 5.43 mg/l for the sweet wines, ± 0.07 mg/l for the brndies, nd ± 0.12 mg/l for the nisettes. Thus, the method is useful for routine nlysis in the qulity control of these beverges. The determintion of iron in grpes nd their lcoholic derivtives is question of gret interest in the fields of grpe-growing nd wine-mking, prticulrly regrding the conservtion nd stbility of the products. Iron hs the cpcity to form compounds with wide vriety of substnces, giving rise to turbidity or chnges in color (blue or white shift). It lso cts s ctlyst in the oxidtion processes involved in ging nd hs n importnt effect on the sensoril evlution of the product (1, 2). Iron cn originte in rw mterils or be provided in considerble mounts during the mking nd bottling of wine. It Received My 4, Accepted by JS July 27, normlly ppers in the grpe s prt of certin enzymes, but its content in wines cn be rised by the type of soil nd its ph, the mturity of the grpe, climte conditions, grochemicl residues, proximity to sources of contmintion nd, especilly, by the processes involved in mking the wine (3). The vrious technicl processes tht cn ffect the iron content of wines nd lcoholic beverges in generl re the pressing, which is probbly the most significnt, followed by contct with continers nd the metl cps tht cover the corks during bottling (prticulrly if the cork is worn), nd the use of colored glss bottles (which provide iron slts) to void the ction of ultrviolet rys (3, 4). Iron lso plys n importnt role during fermenttion, depending on the ertion of the ferments, the presence or bsence of coloring gents, the reltive mounts of yest to which the iron ttches, nd the mount of loss through ttchment to the dregs. The most commonly used technique for determintion of iron in this type of product is tomic bsorption spectrometry (AAS) becuse of its verstility, precision, nd ccurcy (5 8). However, question yet to be settled is the prior tretment of the smple, which is criticl step in the nlyticl procedure. In its clssifiction of officil methods of nlysis, the Europen Union requires delcoholiztion of wine smples prior to ppliction of direct method AAS (9). Although this requirement ensures the elimintion of interferences from ethnol, it does not remove interferences from the orgnic mtrix, which hve considerble effect on the ccurcy nd precision of the determintions. Severl uthors hve proposed cid minerliztion tretment of smples prior to determintion of elements in wines, juice, nd grpes, using different cid mixes with or without ddition of ctlysts. Some of the most commonly used re HNO 3 /H 2 O 2 (10, 11) nd HNO 3 (with H 2 SO 4 or with V 2 O 5 ; 3, 5, 11, 12). Spnish legisltion on officil methods of nlysis (13) recommends dry minerliztion of the smple nd dissolution of the shes in cid. Some of the disdvntges of this method re the likely contmintion of the smples nd the possibility of nlyte losses through voltiliztion (14). The reserch presented here compres different smple preprtion techniques tht llow determintion of iron by AAS with electrotherml tomiztion. The methods exmined include delcoholiztion of the smples, for direct determintion of the element, nd dry nd wet minerliztion using different cids nd/or mixtures of cids with nd without heting

2 190 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, 2000 for both wines nd other lcoholic derivtives. We hve studied ll the prmeters involved in testing the vlidity of the proposed method. Experimentl Apprtus () Double-bem tomic bsorption spectrophotometer. Model 1100B, equipped with deuterium rc bckground corrector nd hollow cthode lmp of Fe (Perkin-Elmer Corp., Norwlk, CT) nd D-500 chrt recorder (Busch & Lomb, Rochester, NY). (b) Grphite furnce. HGA-700 (Perkin-Elmer Corp.), with pyrolyticlly coted grphite tubes nd pyrolyticlly coted L vov pltforms fixed in grooves (Perkin-Elmer Corp.). (c) Digestion block. Select (Rotterm, Brcelon, Spin) nd 25 ml Pyrex tubes. (d) Electric oven. Afor (Model 12-PR/200 Series 8B) with digitl temperture control nd PXW-9 heting control. (e) Drying stove. Select with nturl ir circultion. (f) Rotry evportor. B chi Model R110 with digitl monitoring of rottion speed. (g) Wter bth. Select with digitl temperture red-out immersion thermostt nd shking system. (h) Milli-Q purifier system. Model R015 (Millipore, Gif-sur-Yvette, Frnce). (i) Micropipettors. 10 µl Selpette universl pipettor (Selpette, Jencons, UK); Micropipette Microtrnsferpettor, Type Digitl Brnd, µl (Brnd, Wertheim, Germny). Regents () Iron stndrd solution. Titrisol solution, 1000 mg/l (E. Merck, Drmstdt, Germny). (b) Nitric cid. Suprpure, 65% (E. Merck). (c) Sulfuric cid. Regent grde, 97% (E. Merck). (d) Perchloric cid. Regent grde, 70% (E. Merck). (e) Hydrogen peroxide. Regent grde, 30% (E. Merck). (f) Wter. All solutions were prepred with deionized wter with specific resistivity of 18 M cm. Mterils To eliminte contmintion due to detergents nd smples, glsswre nd polyethylene smple continers were wshed with tp wter fter ech use, soked in 6M HNO 3, nd rinsed severl times with ultrpure wter. Smples Iron ws determined in 4 different types of wine: sweet wine (high sugr content) from the Málg Origin Appelltion (Spin) rosé wine (obtined by mixing white nd red grpes) from the L Mnch Origin Appelltion (Spin) red wine (obtined by mcertion of the solid prts of the grpe) from the Penedés Origin Appelltion (Spin) nd white wine (obtined from different vrieties of white grpes mcerted without skins) from the Rued Origin Appelltion (Spin). Tests were Tble 1. Sensitivity, linerity, precision, nd limits of detection, decision, nd determintion for the different methods tested Method Wine (S y/b) b, c, d [1 (S b /b)] 100 e RSD, % f (3 S ) g mg/l (6 S ) Sensitivity, mg/l Linerity, % Decision limit, mg/l Detection limit, Determintion limit, mg/l (10 S ) A B b A B c.1 A B c.2.1 A B c.2.2 A B c.2.3 A B b c d e f g A = rosé tble wine; B = strong sweet wine. S = stndrd devition. y = y-xis (vector of n observtions for the dependent vrible). b = slope of stndrd clibrtion. S b = stndrd devition of the slope. RSD = reltive stndrd devition (precision). S = stndrd devition of the intercept.

3 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, Tble 2. Selectivity of the different methods tested Method F test (F tb = Student s t Men 19.00) b Men DE c (t tb = 4.604) d slope Stndrd blnk Addition blnk Stndrds clibrtion Addition clibrtion Accurcy Accurcy (t cl = 2.571) e Rosé tble wine Yes b No c No c Yes c Yes c Yes Sweet wine Yes b Yes c Yes c Yes c Yes c Yes b c d e Results re in mg/l. F tb = F vlues from sttisticl tbles (F test) DE = stndrd determintion error. t tb = t vlues follow the Student s t distribution. t cl = t vlue clculted experimentlly. lso performed on grpe juice nd lcoholic beverges obtined from wine distilltes: brndy (wine distillte ged in ok wood) from the Jerez-Sherry Origin Appelltion (Spin) nisette (obtined by mcertion of Pimpinell nissum L., with lcoholic content between 45 nd 55%, v/v); gin (obtined by distilltion of hydrolcoholic mixture contining berries of Juniperus communis, with lcoholic content between 38 nd 50%, v/v); nd pchrán (obtined by mcertion of distilled lcohols nd sloe berries, with lcoholic content between 24 nd 30%, v/v). Smple Preprtion The following were the 4 techniques tested with the different smples: () Delcoholiztion (officil technique of the Europen Union). The lcohol ws eliminted from 25 ml smple by using rotry evportor in wter bth t 50 C. The volume of the smple ws djusted to the originl 25 ml with deionized bidistilled wter, nd the iron ws determined by electrotherml tomiztion AAS. Tble 3. Accurcy of the different methods evluted by recovery tests Smple Dry minerliztion Delcoholiztion Unheted digestion with HNO 3 Heted digestions with HNO 3 nd H 2 SO 4 HClO 4 H 2 O 2 White wine Rosé wine Sweet wine Pchrán Anisette Brndy Grpe juice Results re percentges.

4 192 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, 2000 Tble 4. Comprison of iron concentrtions (mg/l) in the 2 types of wine studied Method Wine Wter stright-line clibrtion % Vrition b Wine stright-line clibrtion % Vrition b Direct A B Ashes A B H 2 SO 4 A B HClO 4 A B H 2 O 2 A B Unheted A B Delcoholiztion A 2.57 B b A = rosé tble wine; B = strong sweet wine. In comprison with the officil technique of the Europen Union (delcoholiztion). Figure 1. Comprtive study of the different methods exmined with the result for heted digestion with nitric sulfuric cids s the reference vlue.

5 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, Tble 5. Comprison of some sttisticl vlidtion prmeters pplied to the different tretments studied Prmeter Delcoholiztion Dry minerliztion Unheted digestion with HNO 3 Heted digestion with HNO 3 nd H 2 SO 4 HClO 4 H 2 O 2 Rosé wine Sensitivity! #!!!!!!!! Uncceptble Linerity ## Uncceptble!!!!!!!!! Uncceptble Detection limit! ##!!!!!!! Uncceptble Precision!!!!!!! Uncceptble!!!!!!!! Sweet wine Sensitivity!!!!!!! Uncceptble!! ## ## Linerity!!!!!!! Uncceptble!!!!!!! Detection limit!!!!!!! Uncceptble!!!!! Precision ## ## Uncceptble!!!!!!! ## Reltive clssifiction bsed on the best method for ech of the prmeters nlyzed:!!!!, f = 1.00;!!!, 1.00<f<1.20;!!, 1.20<f< 1.50;!, 1.50 < f <1.75; ##, 1.75<f<2.50; ####, 2.50<f<3.00; f > 3.00, uncceptble; f = reltion between the evluted method nd the method with best results. (b) Dry minerliztion. A 25 ml portion of smple ws heted in wter bth t 130 C until syrupy consistency ws chieved.a1mlportion of H 2 SO 4 ws dded, nd the smple ws chrred in n electric oven t 525 C for 180 min. The shes obtined were llowed to cool to room temperture nd then dissolved in 1 ml 0.2N HCl; the volume ws djusted to 25 ml with deionized bidistilled wter. (c) Wet minerliztion. (1) Without heting. A 10 ml portion of HNO 3 ws dded to 12.5 ml smple, nd the solution ws left t room temperture for 2 h. The volume ws djusted to 25 ml with deionized distilled wter. (2) With heting. Different cids were tested with 25 ml portions of smple. These cids were mixture of 10 ml HNO 3 nd1ml H 2 SO 4, (method c.2.1), 1 ml HClO 4, (method c.2.2), nd 1mLH 2 O 2 (method c.2.3). In ll 3 cses, het ws pplied t 250 C for 30 min. The solutions obtined were diluted to finl volume of 25 ml with bidistilled deionized wter. All determintions were performed in triplicte. Smple Anlysis Iron ws determined by AAS with electrotherml tomiztion t nm, using pyrolytic grphite tubes nd L vov pltform under the nlyticl conditions optimized by Cbrer et l. (5). Results nd Discussion Anlytic Vlidtion of the Techniques To evlute the pplicbility of ech of the techniques tested, we vlidted their nlyticl chrcteristics. This process ensured the suitbility of the nlyticl requirements nd the relibility of the results. The sttisticl tretment ws performed with the Sttgrphic sttisticl softwre 6.0 pckge (STSG, Inc., Rockville, MD, 1991). Detection nd determintion limits must be estblished in trce nlysis, s well s the ccurcy, precision, linerity, nd selectivity of the method (14, 15). Anlyticl sensitivity ws clculted on the bsis of the slope of the clibrtion line. Tble 1 shows tht, for ordinry tble wines, delcoholiztion (method ) gives results similr to those obtined by heted minerliztion with nitric sulfuric nd perchloric cids (methods c.2.1 nd c.2.2) nd, indeed, better results thn those obtined by using hydrogen peroxide (c.2.3). However, for strong wines with high sugr content, delcoholiztion is surpssed in sensitivity only by dry minerliztion (method b). Attention should be given to the poor results obtined for wines with sugr-free mtrix. The linerity rnge ws estblished from the stndrd devition of the slope nd indictes higher or lower dispersion of the dt round the clibrtion line. The dt re summrized in Tble 1. For sweet wines, the linerity vlues re not cceptble for unheted digestion (88.2%) nd very cceptble for the officil method of the Europen Union, delcoholiztion (96.9%), nd dry minerliztion (97.4%). Minerliztion with mixture of nitric nd sulfuric cids gives fvorble results. We should point out the significnt differences in linerity vlues ccording to the wine mtrix in nonheted digestion with nitric cid, which does give stisfctory results for wines with sugr-free mtrix. The officil method of delcoholiztion (method ) hs decision limit (defined s the concentrtion corresponding to signl level bove which there is certinty tht it is due to presence of the nlyte) tht is lmost twice s high s the vlue for heted digestion with nitric nd sulfuric cids. However, dry minerliztion gives better results for sweet wines. Nonheted digestion nd heted digestion with perchloric cid re suitble for ordinry tble wines such s rosé.

6 194 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, 2000 The precision of the method indictes the degree of dispersion obtined with series of determintions with the sme smple. Ten determintions were performed with ech of the 2 wines studied for ech of the techniques exmined, nd the stndrd determintion errors were clculted. The results obtined (Tble 1) show tht in ll cses precision is better for tretments pplied to rosé tble wines thn for those pplied to sweet wines. In both cses, digestion with nitric nd sulfuric cids nd dry minerliztion give good precision. On the other hnd, nonheted digestion with nitric cid hs significntly poor precision. For strong sweet wines, only digestion with nitric nd sulfuric or perchloric cids gives cceptble results for precision. To evlute the selectivity of the method, sttisticl protocol hd to be performed tht showed the bsence of ny mtrix/nlyte interction, which is the methodologicl bsis of stndrd dditions. The men stndrd devition ws determined for ddition nd for clibrtion in queous medium, nd the slopes were compred by using Student s t-test. The men slope ws clculted between clibrtion nd ddition to estblish the new equtions. The results re shown in Tble 2. Hving obtined the new equtions, we clculted the blnk provided by the mtrix (to see if the mtrix cused ny errors) nd the mount of nlyte in the solution on the bsis of clibrtion by stndrds nd clibrtion by ddition, with sttisticl control of the concentrtions. If the clibrtion nd ddition slopes hd been different, we would hve hd to use recovery processes such s ddition clibrtion. Determintion of the ccurcy, defined s the proximity between the vlue obtined nd the true vlue, llows us to ensure tht no loss or contmintion occurred during the process. This determintion ws performed for ech of the tretments by studying the recovery percentges for the problem smples. We exmined 3 wine smples (white, rosé, nd sweet), 3 smples of different strong lcoholic beverges (nisette, brndy, nd pchrán), nd 1 smple of grpe juice. The results obtined (Tble 3) were pproprite for the rnge of iron concentrtions in the smples nlyzed (16 18). It cn be seen tht heted digestion using nitric nd sulfuric cids provides men recovery vlues of prcticlly 100%, wheres the other 2 heted cid minerliztion methods give slightly higher vlues (101.3% with hydrogen peroxide nd 104.3% with perchloric cid), with the ltter method giving the best results when pplied to grpe juice. The delcoholiztion method gives men recovery vlue of 102.1%. Tking the iron content obtined by using the officil method of delcoholiztion s the reference vlue (Tble 4), we exmined the percentge vrition of the tretments pplied to rosé wines. The concentrtion of iron ws found to vry between n increse of 7% for direct mesurement nd decrese of 40% for unheted digestion with nitric cid, with other negtive vritions of 9 26% for the 3 heted digestion methods. Figure 1 represents the sme study, but is bsed on the heted digestion method with the nitric sulfuric cids mixture. In this cse, the vrition percentges re much more uniformly distributed (3 positive, 3 negtive), vrying from Tble 6. Determintion of iron in commercilly vilble smples using the officil method of delcoholiztion nd heted cid digestion Smple n Men Fe found ± SD, mg/l White wine ± 1.47 Rosé wine ± 1.52 Sweet wine ± 5.43 Pchrán ± 1.14 Anisette ± 0.12 Brndy ± 0.07 Grpe juice ± 2.15 SD = stndrd devition. decrese of 23% with unheted nitric cid digestion to n increse of 24% with the direct method. Something similr occurs if we exmine the results of sweet wine nlysis with respect to the officil delcoholiztion tretment. In this cse, however, the vritions rnge from 3% decrese for direct mesurement to increses for ll the other tretments (bout 37% for the unheted digestion nd % for the heted digestions). The vlidtion study of the methods ccording to the different nlyticl prmeters (Tble 5) shows tht for tble wines the method with by fr the best nlyticl results for ll the prmeters except selectivity is heted digestion with nitric nd sulfuric cids. These results gree with those obtined in other studies (19 21) tht show the pplicbility of heted cid minerliztion to the determintion of trce elements in wines nd other lcoholic beverges. In the cse of sweet wines, choice of method is somewht more difficult, becuse dry minerliztion gives the best nlyticl results, except for ccurcy nd precision. This is understndble given the opertive difficulty of the technique, but nonetheless dvises ginst its use. The most pproprite of the heted digestion techniques seems to be tht using the nitric sulfuric cids mixture. Appliction of the Proposed Method As prcticl ppliction of this study, Tble 6 shows results for the determintion of iron in commercilly vilble smples of grpe juice, wines, nd other lcoholic beverges mde in Spin nd widely consumed there. The nlyticl method used ws heted cid digestion.the results given in Tble 6 gree with those found in the literture. Conclusions We believe tht heted cid digestion using the nitric sulfuric cids mixture should be used for ll types of smples nlyzed. The officil technique of delcoholiztion is vlid only for wines with mtrixes rich in lcohol nd sugrs nd, even then, it does not led to optimum results becuse no single method is outstnding in efficiency. Dry minerliztion for t-

7 OLALLA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 1, ble wines nd unheted digestion with nitric cid for either mtrix re quite uncceptble under ny circumstnces. Acknowledgment This pper is prt of Project ALI finnced by the Spnish Ministry of Eduction nd Science (CICYT). References (1) Mrec, I. (1983) Origen, Composición y Evolución del Vino, Editoril, Alhmbr, Mdrid, Spin (2) Gonzles, M., Gonzles, A., & Medin, B. (1987) Conniss. Vigne Vin 21, (3) Fernndez, P. (1988) Z. Lebensm. Unters. Forsch. 186, (4) Etievnt, P., Schlich, P., Bouvier, J.C., Symonds, P., & Bertrnd, A. (1988) J. Sci. Food Agric. 45, (5) Cbrer, C., De Men, C.M., Lorenzo, M.L., & Lopez, M.C. (1995) Ars Phrm. 36, (6) Gonzlez, M.J., Mrtinez-Pr, M.C., & Aguilr, M.V. (1988) Z. Lebensm. Unters. Forsch. 187, (7) Gorinstein, S., Goldblum, A., Kitov, S., & Deutsch, J. (1984) J. Food Sci. 49, (8) Lzos, E.S., & Alexkis, A. (1989) Int. J. Food Sci. Technol. 24, (9) Dirio Oficil de ls Comuniddes Europes (3110/90) Reglmento CEE No. 2676/90 de l Comisión 17 de Septiembre de 1990, Comunidd Econ mic Europe, Brusselles, Belgium (10) Teissedre, P.L., Cbnis, M.T., Chmpgnol, F., & Cbnis, J.C. (1994) Am. J. Enol. Vitic. 45, (11) Teissedre, P.L., Cbnis, M.T., & Cbnis, J.C. (1993) Anlusis 21, (12) Officil Methods of Anlysis (1990) 15th Ed., AOAC, Arlington, VA (13) Ministerio de Agricultur, Pesc y Alimentción (1986) Metodos Oficiles de Anlisis, Tomo II, Dirección Generl de Polític Alimentri, Mdrid, Spin (14) Pomernz, Y., & Melon, C.E. (1994) Food Anlysis: Theory nd Prctice, Chpmn nd Hll, New York, NY (15) Cudros, L., Grci, A.M., Alés, F., Jimenez, C., & Román, M. (1995) J. AOAC Int. 78, (16) Horwitz, W., Kmps, L.R., & Boyer, F.W. (1980) J. Assoc. Off. Anl. Chem. 63, (17) Horwitz, W., Albert, R., Deustch, M.J., & Thompson, J.N. (1990) J. Assoc. Off. Anl. Chem. 73, (18) Smrt, N.A. (1993) Anl. Proc. 30, (19) Lendinez, E., Lopez, M.C., Cbrer, C., & Lorenzo, M.L. (1998) J. AOAC Int. 81, (20) Cbrer, C., Teissedre, P.L., Cbnis, M.T., & Cbnis, J.C. (1998) J. Agric. Food Chem. 45, (21) Pedersen, G.A., Mortesen, G.K., & Lrsen, F.H. (1994) Food Addit. Contm. 11,