HPLC/UV determination of organic acids in fruit juices and nectars

Transcription

1 Eur Food Res Technol (2002) 214:67 71 DOI /s ORIGINAL PAPER Sara C. Cunha José O. Fernandes Isabel M.P.L.V.O. Ferreira HPLC/UV determination of organic acids in fruit juices and nectars Received: 6 June 2001 / Revised version: 6 August 2001 / Published online: 5 October 2001 Springer-Verlag 2001 Abstract A reversed phase HPLC method for separation and determination of organic acids in fruit juices and nectars is presented. The method is based on the reaction of free organic acids with O-(4 nitrobenzyl)-n,n -diisopropylisourea (PNBDI) in presence of dioxane. Excess of reagent was removed with a strong cation-exchange resin. The p-nitrobenzyl esters were separated on a C 18 reversed phase column using gradient elution with water and acetonitrile and UV detection at 265 nm. Benzylmalonic acid was used as internal standard. The determinations were performed in the linear range of g/l for lactic, acetic and succinic acids, g/l for tartaric, malic and citric acids. The detection limits were g/l, g/l, g/l, g/l, g/l and g/l, respectively for lactic, acetic, tartaric, malic, succinic and citric acids. Validation of the method was carried out by the standard additions method, the recoveries ranged from 91.4 to 103.0%. The precision of the method was also evaluated and reported a CV% as less than 3.1%. Organic acid profiles of citrus fruit, pineapple and apple natural juices (home-made), commercial juices and nectars were established. Keywords Organic acids Fruit juices Nectars HPLC/UV Introduction Natural fruit juices, commercial juices and nectars contain different organic acids, components from the fruits used (tartaric, malic, citric and ascorbic acids) or incorporated as additives, namely antioxidants (tartaric, malic and citric) or preservatives (sorbic and benzoic acids). S.C. Cunha J.O. Fernandes I.M.P.L.V.O. Ferreira ( ) CEQUP/Serviço de Bromatologia, Faculdade de Farmácia, Universidade do Porto, Rua Aníbal Cunha, 164, Porto, Portugal bromato@ff.up.pt Present address: S.C. Cunha, Instituto Jean Piaget de Mirandela, Av. 25 Abril, 5730 Mirandela, Portugal Separation, identification and quantification of the major organic acids present in a fruit juice is of considerable importance, since these compounds influence the organoleptic properties of the product under examination and provide useful information regarding not only its authenticity but also microbiological alterations that may have occurred previously. Several methods have been used to determine organic acids in fruit juices, including enzymatic methods [1], chromatographic [2, 3, 4, 5, 6, 7] and capillary electrophoresis [8]. The standard methods, using enzymatic analyses, require specific kits for each organic acid [1], they are time-consuming and use large amounts of reagents, which make them expensive. Traditional HPLC methods without derivatisation [3, 4, 5, 6] require purification techniques to eliminate matrix interferences (e.g. sugars or phenolic compounds). Therefore, the use of derivatising reagents such as 2-nitrophenylhydrazides [6] or p-nitrobenzyl compounds [2] and detection at 265 nm offers better sensitivity and selectivity. The aim of our research work was the optimisation and validation of a rapid and specific method for simultaneous determination of the major organic acids (citric, malic, succinic, lactic and tartaric) in natural or commercial juices of citrus fruit, pineapple and apple. The sample pre-treatment with a strong cation-exchange resin was simple. Materials and methods Chemicals. Organic acids (or their sodium salts) were purchased from Fluka (Buchs, Switzerland) and Aldrich (Steinheim, Germany). PNBDI was obtained from Sigma (St. Louis, MO, USA). The cation-exchange resin Dowex-50W-X8 ( mesh) was obtained from Fluka. Before use the resin was activated, by washing successively with methanol, water, 0.1 mol/l HCl and water. Acetonitrile, methanol and dioxane were of HPLC grade and were obtained from Merck (Darmstadt, Germany). Water was prepared by purifying demineralised water in a Seral system. The eluents were filtered through 0.45-µm filters and degassed under reduced pressure in an ultrasonic bath.

2 68 Table 1 Identification of the analysed samples Type of fruit Natural juices Commercial juices Nectars Quantity of Abbreviation Numeration Quantity of Abbreviation Numeration Quantity of Abbreviation Numeration analysed analysed analysed samples samples samples Orange Three varieties of citrus fruit Orange ONJ 1 16 OCJ ON Grape fruit GNJ 2 Tangerine TNJ 3 Pineapple Three varieties PNJ PCJ PN of pineapples Apple Golden apple ANJ 37 7 ACJ AN 45 Sampling. Forty five samples were assayed, which included natural juices of citrus fruit, pineapple and apple and commercial juices and nectars of these fruits. To simplify discussion of the results each of the 45 samples was identified with an appropriate abbreviation (Table 1). Natural juices were home-made and obtained by expression of orange (Citrus sinensis var. valencia late), grapefruit (Citrus aurantium var. start ruby) and tangerine (Citrus reticulata var. encore), pineapples (Ananas comosus) obtained from Costa Rica, from Acorian Islands and from South Africa and golden apples. Commercial juices and nectars were randomly purchased from the Portuguese market. Sample clean-up and derivatisation procedure. The sample cleanup and the subsequent derivatisation procedure including the final clean-up of the crude reaction mixture were performed according to the proposal of Badoud and Pratz [2] with minor modifications. Briefly, 5.0 ml of sample was mixed with 100 µl of 100 g/l aqueous benzylmalonic acid solution (I.S., final concentration=2 g/l). The mixture was gently shaken for 15 min with 0.5 g of activated strong cation-exchange resin (Dowex-50W-X8). A portion of the clear supernatant was decanted and filtered through a 0.22-µm disposable filter disk. A 25-µl aliquot of the mixture was then placed in a 4-ml PTFE lined screw capped amber vial (Supelco, Bellefonte, PA, USA) and 0.5 ml of a freshly prepared solution of PNBDI in dioxane (40 mg/ml) were added. Acids were derivatised by heating the vial in a thermostatic block (Pierce, Rockford, IL, USA) for 60 min at 80 C. After cooling, the solution was diluted by addition of 2 ml of acetonitrile and 0.5 g of Dowex 50W-X8 was added. The mixture was shaken and left in contact with the resin for 15 min and finally, the supernatant was decanted and filtered through a 0.22-µm disposable filter disk. All the standard solutions underwent the same type of treatment. HPLC apparatus. Chromatographic analyses were carried out using a two-pump delivery system with gradient capability (Gilson Medical Electronics, Middleton, WI, USA) equipped with a variable wavelength UV-VIS detector (Gilson 118), a Rheodyne 7125 injection valve (Rheodyne, Cotati, CA, U.S.A.) with a 10-µl sample loop and a Varian 4290 integrator (Varian, Harbor City, CA, USA). The chromatographic separation was performed with a Spherisorb 3-µm ODS, mm analytical cartridge (Waters Corp., Milford, MA, USA) preceded by a Nucleosil guard-column C 18, 4 30 mm (Macherey-Nagel, Düren, Germany). Chromatographic conditions. The PNB ester derivatives of the organic acids were separated using a gradient of solvents water (A) and acetonitrile (B). The gradient was as follows: 30 to 45% of B within 10 min, 45 to 55% of B within 10 min and 55 to 80% of B within 10 min and returning to the initial conditions within 2 min. Elution was performed at a solvent flow rate of 1 ml/min. Detection was accomplished with a UV/VIS detector at 265 nm. The compounds were identified by their peak retention times. Results and discussion Linearity Under the assay conditions described, a linear relationship between the concentration of organic acids and UV absorbance at 265 nm was obtained. This linearity was maintained over the concentrations range of g/l for lactic, acetic and succinic acids, g/l for tartaric, malic and citric acids. The calibration curves were obtained by triplicate determinations of each of the calibration standards. Table 2 lists the calibration curve parameters for each organic acid.

3 69 Table 2 Calibration curves parameters and detection limit (LOD) of the method Organic acids y=a+bx r LOD (g/l) Lactic, LA a= b= Acetic, AA a= b= Tartaric, TA a= b= Malic, MA a= b=0.302 Succinic, SA a= b= Citric, CA a= b= All values correspond to the mean of three replicated determinations. a intercept, b slope, r correlation coefficient LOD Detection limit Detection limits The detection limit was based on a signal-to-noise ratio of three. The results, ranging between g/l (AA) and g/l (CA) are presented in Table 2. Quantification limits Considering that in HPLC methods the quantification limit typically requires peak heights at least ten times higher than the baseline noise [9] the values obtained varied between (AA) and g/l (CA). However, a standard solution with g/l of each acid injected for six times gave the following relative standard deviations: LA: 5.6%; AA: 2.5%; TA: 5.8%; MA: 4.5%; SA: 2.0%; CA: 5.0%. Precision The precision of the method was also evaluated by six complete analyses of the same sample of fruit juice. The % CV were 2.7, 3.1 and 3.0 for lactic, malic and citric acids, respectively (with concentrations of 0.15±0.006, 3.41±0.013 and 1.87±0.018 g/l, respectively). Recovery studies To demonstrate the accuracy of the method, a sample of fruit juice was analysed before and after the addition of known amounts of the organic acids under study. The results obtained (Table 3) clearly demonstrate the accuracy of the method. Table 3 Recovery of organic acids added to spiked fruit juice sample Organic acids Initial Added Found Recovery amount (g/l) (g/l) (%) (g/l) Lactic Acetic n.d Tartaric n.d Malic Succinic n.d Citric Analytical results and chromatograms for fruit juices The chromatograms presented in Figs. 1, 2 and 3 show the characteristic profiles of organic acids for orange, pineapple and apple natural juices. Results for natural and commercial citrus fruit juice samples are shown in Table 4. As can be observed, natural juices of orange (ONJ1), grapefruit (GNJ2) and tangerine (TNJ3) presented different profiles of organic acids. Orange juice presented similar contents of malic and citric acids (2.7 and 2.8 g/l, respectively), and the same occurred in tangerine juice, although in higher contents (4.3 and 4.0 g/l, respectively). In contrast, grapefruit juice contained 12.0 g/l of citric acid and traces of malic acid. Succinic acid was quantified in orange and grapefruit juices (0.16 and 0.18 g/l, respectively) and tartaric acid was not detected. Nectars (ON17, ON18, ON19) presented contents of malic acid similar to those present in orange natural juice and higher contents of citric acid, added as acidulant and antibrowning agent. The presence of tartaric acid in ON19, indicates that this nectar probably contains grape juice, not mentioned in the label. Only this nectar con-

4 70 Fig. 1 Chromatogram of a sample of natural orange juice containing g/l of malic acid (1), g/l of succinic acid (2), g/l of citric acid (3) and 2 g/l of benzylmalonic acid P.I. (4) Fig. 3 Chromatogram of a sample of natural apple juice containing g/l of malic acid (1), g/l of citric acid (2) and 2 g/l of benzylmalonic acid P.I. (3) Table 4 Results obtained for orange juices and nectars Sample Organic acids (g/l) Malic Citric Succinic Tartaric Fig. 2 Chromatogram of a sample of natural pineapple juice containing g/l of malic acid (1), g/l of citric acid (2) and 2 g/l of benzylmalonic acid P.I. (3) tains a small amount of succinic acid. According to European legislation [10] nectars are obtained by the addition of sugar and water to fruit juices and the addition of citric acid is allowed. However the concentrations found are above the 5 g/l established for that compound in nectars [11]. Commercial orange juices (OCJ4 OCJ16) contain lower amounts of malic acid when compared with natural orange juices, this ranged between not detected (OCJ6, OCJ15) and 0.8 g/l (OCJ8). According to legislation these juices can have added water, and thus malic acid is diluted. Citric acid contents were in general similar to those of natural orange juice; probably part of this ONJ n.d. GNU n.d. TNJ n.d. n.d. OJC n.d. n.d. OJC n.d. n.d. OJC 6 n.d n.d. n.d. OCJ n.d. n.d. OCJ n.d. OCJ n.d. n.d. OCJ n.d. OCJ n.d. n.d. OCJ n.d. n.d. OJC n.d. OCJ n.d. n.d. OCJ 15 n.d n.d. n.d. OCJ n.d. n.d. ON n.d. n.d. ON n.d. n.d. ON Organic acids under study that are not tabulated were not detected acid was added as acidulant. Five samples (OCJ4, OCJ8, OCJ9, OCJ12, OCJ14) contained higher levels of citric acid than the 3 g/l allowed by legislation for fruit juices [11]. Succinic acid was present in three samples in levels slightly higher than that found in natural orange juice. Table 5 presents malic and citric acid content of pineapple juices and nectars. Succinic and tartaric acids were not detected in those juices. No significant differences

5 Table 5 Results obtained for pineapple juices and nectars Sample Organic acids (g/l) Organic acids under study that are not tabulated were not detected Malic Citric PNJ PNJ PNJ PCJ PCJ PCJ n.d. PCJ PCJ PCJ PCJ PCJ PCJ PCJ PCJ n.d. PCJ n.d. PN PN Table 6 Results obtained for apple juices and nectars Sample Organic acids (g/l) Lactic Malic Citric ANJ 37 n.d ACJ ACJ ACJ ACJ ACJ ACJ ACJ AN Organic acids under study that are not tabulated were not detected were found between citric acid content of pineapple juices (PNJ20, PNJ21, PNJ22) of three different origins (p<0.05). Nectars (PN35, PN36) presented similar contents of malic acid and lower contents of citric acid when compared with natural juices. However, the concentration of citric acid is above the 5 g/l established for that compound in nectars [11]. Commercial juices of pineapple contained lower amounts of malic acid when compared with natural juices, concentrations ranging between 0.3 g/l (PCJ 32) and 1.5 g/l (PCJ 26), lower than the maximum of 3 g/l established for that compound in fruit juices. Greater variation was observed for citric acid content. In three samples this acid was not detected, but another three samples exceeded the 3 g/l allowed by legislation. With respect to apple juices, the major organic acid of apple natural juice (ANJ37) was malic acid. As expected only traces of citric acid were quantified. Apple nectar and apple commercial juices (except ACJ38) contained lactic, malic and citric acids within the levels allowed by legislation (Table 6). A lower amount of lactic acid was detected in the nectar and in all commercial juices. Conclusions The described procedure seems to fulfil the criteria of selectivity, sensitivity, reproducibility and convenience of a method suitable for routine assay of various organic acids in fruit juices. The levels of citric and malic acids found for some samples of orange and pineapple commercial juices and nectars were above the levels allowed by European legislation. References 1. Boehringer (1992)Methods of enzymatic food analysis 82/83. Boehringer, Mannheim 2. Badoud R, Pratz G (1986) J Chromatogr 360: Cámara MM, Diez C, Torija ME, Cano MP (1994) Z. Lebensm Unters-Frosch 198: Cano MP, Torija E, Marían M, Cámara M (1994) Z Lebensm Unters-Frosch 199: Lee HS (1993) J Agric Food Chem 41: Miwa HJ (2000) J Chromatogr A 881: Saccani G, Gherardi S, Trifirò A, Bordini CS, Calza M, Freddi C (1995) J Chromatogr 706: Saavedra L, García A, Barbas C (2000) J Chromatogr A 881: Huber L (1998) LC-GC Int 11: European Directives (1975) Directive no. 75/726/CEE of 17 November, O. J. L311/40, of 01 December European Directives (1995) Directive no. 95/2/CE of 20 February, O. J. L61/1, of 18 March