Evaluation of Aroma Differences between Hand-Squeezed Juices from Valencia Late and Navel Oranges by Quantitation of Key Odorants and Flavor Reconstitution Experiments Andrea Buettner and Peter Schieberle* Deutsche Forschungsanstalt fu¨ r Lebensmittelchemie, Lichtenbergstrasse 4, D-85748 Garching, Germany Twenty-five odor-active compounds were quantified in hand-squeezed juices of Valencia late and Navel oranges using stable isotope dilution assays. Odor activity values (OAVs, ratio of the concentration to odor thresholds) based on odor thresholds in water were calculated for the entire set of aroma compounds in both varieties. It was shown that due to their high OAVs, the fruity- smelling esters ethyl 2-methylpropanoate, ethyl butanoate, (S)-ethyl 2-methylbutanoate, and 3a,4,5,- 7a-tetrahydro-3,6-dimethyl-2(3H)-benzofuranone (wine lactone), the grassy smelling (Z)-hex-3-enal, and the citrus-like decanal were the most potent odorants in both juices. The weaker fruity note in the Navel oranges was clearly correlated with significantly lower OAVs of all fruity-smelling esters but a higher OAV of (Z)-3-hexenal compared to Valencia late. Model solutions simulating the odor of both orange varieties confirmed the findings of the quantitation studies. Keywords: Stable isotope dilution assay; orange aroma; quantitation; wine lactone; (Z)-3-hexenal; (R)-ethyl 3-hydroxyhexanoate INTRODUCTION Due to its pleasant aroma hand-squeezed, kitchen- made orange juice is very popular all over the world. Industrially processed juices, however, show a quite different aroma, which is mainly caused by the higher pressure used for squeezing and, also, the thermal treatment applied. The original delicate aroma of fresh orange juice was for a long time thought to be a complex mixture of many volatile constituents blended in the proper proportions (1). For this reason, numerous investigations dealt with the identification of many potential contributors, result- ing in a considerable number of volatiles identified in orange juice (2). On the basis of these data, extensive quantitations of volatiles in the fresh juices from a variety of orange cultivars such as Valencia, Navel, Pera, or Pineapple were performed to gain more ac- curate information about their contribution to orange flavor (3-5). Comparison of the quantitative data of some volatiles to odor threshold data suggested li- monene, acetaldehyde, ethyl butanoate, and decanal as possible contributors to fresh orange aroma (4-7). However, a great variance in both the quantitative and sensory data did not allow precise conclusions. There- fore, reconstitution experiments were performed to prove the contribution of several compounds to orange aroma by using pumpout orange juice as the matrix (8, 9). However, the typical aroma of fresh orange juice could not be reconstituted, thereby indicating that important odorants were missing. By application of aroma extract dilution analysis (AEDA) on an extract prepared from hand-squeezed juice of Valencia late oranges, we recently identified 42 odor-active compounds in the flavor dilution (FD) factor range of 4-1024 (10). Among them, ethyl butanoate, ethyl 2-methylpropanoate, (S)-ethyl 2-methylbutanoate, and 3a,4,5,7a-tetrahydro-3,6-dimethyl-2(3H)-benzofura- none (wine lactone) with fruity, sweet odor notes, the grassy smelling (Z)-hex-3-enal, the terpene-like (R)- limonene and (R)-R-pinene, and the metallic-smelling trans-4,5-epoxy-(E)-dec-2-enal showed the highest FD factors. By static headspace-olfactometry acetaldehyde was established as another important aroma contributor (10). AEDA is a useful screening method for the detection of potent odorants in foods (cf. review in ref 11). However, this method is based on odor thresholds of the compounds in air. On the other hand, possible losses of odorants during the isolation steps are not fully taken into account. Therefore, the contribution of single odor- ants to orange juice aroma, depending on odor thres- holds in aqueous media, has to be confirmed by accurate quantitations and aroma reconstitution experiments. In the present study, stable isotope dilution assays (SIDAs) in combination with mass chromatography were used for the determination of the important odorants in two different orange varieties. Beforehand, comparative AEDAs of both hand-squeezed juices were performed to objectify similarities and differences among the aroma compounds of both juices. EXPERIMENTAL PROCEDURES Material. Oranges [Citrus sinensis (L.) Osbeck cv. Valencia late and cv. Navel, grown in Argentinia and Italy, respectively] were purchased at a local market and used immediately for juice making. Chemicals. The following compounds were obtained from the suppliers given in parentheses: [ 13 C2]acetaldehyde (Pro- mochem, Wesel, Germany); acetyl chloride, dec-5-en-1-ol, [ 2 H6]- ethanol, lithium bis(trimethylsilyl)amide, 1.0 M solution in diethyl ether, and pyridinium chlorochromate (Aldrich, Stein- heim, Germany); platinum(IV) oxide hydrate (Merck-Schu- * Address for correspondence (telephone +49-89-289-13265; fax +49-89-289-14183; e-mail Peter.Schieberle@Lrz.tum.de). 2387 J. Agric. Food Chem. 2001, 49, 2387-2394 10.1021/jf001363l CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001