Vol. 65, No. 7, 2000—JOURNAL OF FOOD SCIENCE 1107 © 2000 Institute of Food Technologists Food Chemistry and Toxicology JFS: Food Chemistry and Toxicology Influence of Blanching-osmotic Dehydration Treatments on Volatile Fraction of Strawberries I. ESCRICHE, A. CHIRALT, J. MORENO AND J.A. SERRA ABSTRACT: The effects of steam (S) and microwave (MW) blanching and osmotic treatments, applied either singu- larly or combined, on the volatile fraction of strawberry have been evaluated. Osmotic dehydration was carried out at atmospheric conditions (OD) and by applying a vacuum pulse at the beginning of the process (PVOD). Volatiles were obtained by a simultaneous distillation-extraction procedure and identified/quantified by gas chromatogra- phy-mass spectrometry. Esters and 2,5-dimethyl-4-hydroxy-3(2H)-furanone were the major compounds. Differ- ences in volatile concentration promoted by blanching and/or osmotic processes were evaluated. Osmotic treat- ments promote formation of esters and furanones, differently for either OD or PVOD treatments, but this effect was greatly inhibited when osmosis was preceded by blanching. The kind of blanching (MW or S) also affected the sample final volatile profile. Key Words: volatile compounds, blanching, osmotic dehydration, minimally processed fruit, strawberry Introduction S TRAWBERRIES ARE HIGHLY VALUED DUE TO THEIR SENSORY characteristics. Fresh fruit commercialization and limited pro- cessing to juices and jams can not absorb all the production. Suit- able technologies, such as osmotic dehydration at mild tempera- ture or blanching, considered as minimal processing, that pre- serve the fresh-like characteristics of fruit may be used to obtain strawberry products, of interest as food ingredients in confection and dairy industries. Processed strawberry quality is greatly influenced by color and flavor preservation. Peroxidase (POD) and polyphenoloxi- dase (PPO) activities cause deterioration of these parameters. PPO present in strawberry tissue provokes loss of intense red col- or and browning due to the degradation of anthocyanins (Cano and others 1997). Therefore, blanching is essential to inactivate the enzymes causing alterations. Strawberry aroma profiles have been studied by several au- thors, using gas chromatography-mass spectrometry (GC-MS) (Lesschaeve and others 1991; Larsen and others 1992) and high- performance liquid chromatography (HPLC) (Sanz and others 1994; Pérez and others 1999). Strawberry aroma is mainly formed by a complex mixture of esters, aldehydes alcohols and sulfur compounds. Although no character impact compound has been found for strawberry aroma, some researchers consider esters and furanones as key compounds of strawberries flavor (Larsen and others 1992; Pérez and others 1996). Nevertheless, these aroma compounds are subjected to important modifications throughout ripening and the different stages of processing (Gui- chard and others 1991; Lesschaeve and others 1991; Kimura and others 1994) Osmotic dehydration is a usual operation to reduce the product water activity in minimal processing, this being car- ried out either at atmospheric pressure (OD) or at vacuum condi- tions, vacuum osmotic dehydration (VOD). Recently, pulsed vac- uum osmotic dehydration (PVOD) has been described (Fito and others 1994). In this operation vacuum is applied at the begin- ning of the process for a short time, the process following at at- mospheric conditions. During the vacuum pulse, expansion and subsequent compression of occluded gas in the product pores, due to pressure changes, promote the exchange of the pore gas/ liquid for the external liquid (Chiralt and others 1999). Sample vacuum impregnation with the osmotic solution accelerates mass transfer rate, thus reducing the temperature requirements to reach short process time (Fito and others 1994). In this work, the effect of blanching and osmotic treatments on strawberry volatile fraction has been analyzed. Steam or mi- crowave blanching as well as osmotic dehydration (OD and PVOD) were applied either singularly or combined. Materials and Methods Raw material Strawberries (Fragaria ananassa, c.v. Chandler) from Huelva (Spain) were obtained from commercial sources. Fruits were se- lected according to their ripeness, size and color, washed and the stalk was removed. Sample treatments Strawberry samples were submitted to blanching and osmotic processes in separated or combined treatments. Blanching was carried out by steam treatment (S) or by microwaves (MW). In S treatments, samples were exposed to steam for 0.5 min at atmo- spheric pressure and then cooled in water at 15 °C. In MW treat- ments, 400 g of fruit were heated in a microwave oven at 400 W for 2.5 min and afterwards cooled in water at 15 °C. S and MW blanching conditions were established in a previous work, where the effectiveness of both S and MW treatments was analyzed. In the above mentioned conditions, enzyme activity was reduced to 80% with a minimum effect on fruit sensory quality (Moreno and others 2000). Sample temperatures in the fruit center and sur- face throughout S and MW treatments were controlled by using a LUXTRON 790 Flouroptic thermometer with optic fiber, probe Mod. SIW SN-2. Osmotic treatments were carried out at 30 °C in 65 Brix sucrose solution for 4 h, at atmospheric pressure (OD treatments) and by applying a vacuum pulse (5 min at 50 mbar) at the beginning of the process (PVOD treatments). In the design of combined blanching-osmotic treatments, all possible combinations were considered (S-OD, S-PVOD, MW-OD, MW-PVOD, OD-S, PVOD-S, OD-MW, PVOD-MW). To perform all these treatments, and the subsequent analysis, 4 batches of fruit were required due to the difficulty to carry out all of the experi- ments at the same time. Batch 1 is used for treatments S, S-OD