J. Agric. Food Chem. 2003, 51, 715−721
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Quality Assessment of Strawberries (Fragaria Species) RAMIN AZODANLOU,† CHARLY DARBELLAY,‡ JEAN-LUC LUISIER,§ JEAN-CLAUDE VILLETTAZ,§ AND RENATO AMADOÅ *,† Swiss Federal Institute of Technology Zurich, Institute of Food Science and Nutrition, ETH-Zentrum, Schmelzbergstrasse 9, CH-8092 Zurich, Switzerland, Swiss Federal Research Station for Plant Production, CH-1964 Conthey, Switzerland, and Department of Food Technology and Biotechnology, University of Applied Sciences Valais, CH-1950 Sion, Switzerland
Several cultivars of strawberries (Fragaria sp.), grown under different conditions, were analyzed by both sensory and instrumental methods. The overall appreciation, as expressed by consumers, was mainly reflected by attributes such as sweetness and aroma. No strong correlation was obtained with odor, acidity, juiciness, or firmness. The sensory quality of strawberries can be assessed with a good level of confidence by measuring the total sugar level (°Brix) and the total amount of volatile compounds. Sorting out samples using the score obtained with a hedonic test (called the “hedonic classification method”) allowed the correlation between consumers’ appreciation and instrumental data to be considerably strengthened. On the basis of the results obtained, a quality model was proposed. Quantitative GC-FID analyses were performed to determine the major aroma components of strawberries. Methyl butanoate, ethyl butanoate, methyl hexanoate, cis-3-hexenyl acetate, and linalool were identified as the most important compounds for the taste and aroma of strawberries. KEYWORDS: Strawberry; Fragaria sp.; quality; total volatiles; SPME; sensory evaluation; hedonic; flavor; aroma
INTRODUCTION
Consumers often criticize the organoleptic quality of strawberries. According to the information obtained from a large Swiss food retailer (Federation of Migros Cooperatives, Bussigny, Switzerland), 26% of consumers are often disappointed and 33% sometimes disappointed with the quality of strawberries. Agronomic research has so far set its priorities on appearance, and storage and transport resistance as well as on yield increase. Thus, it is not surprising that the sensory properties of strawberries only partly satisfy the expectations of consumers. Compounds contributing to the flavor of strawberries, especially the volatile ones, have been extensively studied. Nijssen (1) identified >360 volatile compounds. About 15-20 of them are believed to be essential for the sensory quality of strawberries, together with the nonvolatile sugars and organic acids (2-8). Flavor intensity and fruitiness persistence are influenced by the concentrations of sugars and acids (9, 10). Adding strawberry flavor compounds to a sucrose solution induces an increase in the perception of sweetness (11). Alavoine and Crochon (12) have shown that the total sugar content is correlated with strawberry taste. * Corresponding author (telephone 0041 1 632 32 91; fax 0041 632 11 23; e-mail
[email protected]). † Swiss Federal Institute Technology Zurich. ‡ Swiss Federal Research Station for Plant Production. § University of Applied Sciences Valais.
Despite extensive research done on strawberry flavor, the responsible substances for aromatic distinction among cultivars have not been fully characterized yet (2, 13). The differences in flavor of the three strawberry cultivars described by Ulrich et al. (7) are due to different concentrations and ratios of the key flavor compounds: wild strawberry, with a spicy odor derived from anthranilic acid methyl ester; Fragaria Virginia, with a fruity aroma characterized by esters; and Fragaria ananassa, characterized by Furaneol and 2,5-dimethyl-4-methoxy-3(2H)-furanone (14). The typical strawberry aroma is not due to a single compound but is rather the result of a complex multicomponent relationship among many aromatic constituents (15). The interactive effects of these compounds are still poorly understood. The aim of the present work was to assess the quality of strawberries. Consumer tests and sensory evaluations by a semitrained panel were performed to establish quality criteria. In addition, a newly developed concept (16, 17) was used to determine the amount of total volatile compounds in strawberries. Sensory evaluation and physicochemical analyses were used as complementary tools to determine and set quality acceptance limits. MATERIALS AND METHODS Fruit Samples and Sample Preparation. During three growing seasons (1997, 1998, and 1999), 80 samples representing 24 strawberry cultivars, grown in fields and/or under plastic tunnels, were harvested
10.1021/jf0200467 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/24/2002
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Figure 1. Equipment for the determination of total volatile compounds as specified by Azodanlou et al. (17). at the ripe stage and used immediately for sensory evaluation and instrumental analyses. The following cultivars were analyzed by consumer tests as well: Mara des bois, Carezza, Pegasus, Madeleine, Elsanta, and Marmolada. The samples were obtained either from the Swiss Federal Research Station for Plant Production in Conthey (Switzerland) or from a large Swiss food retailer (Federation of Migros Cooperatives). Intact fruits were used for sensory evaluation and for determination of total volatile compounds. Strawberries classified by sensory evaluation were homogenized at high speed in a professional blender (Kenwood Professional, Dublin, Ireland) for ∼30 s to produce a homogeneous puree, which was directly used for instrumental analyses. To inactivate the endogeneous enzymes, 50 g of a saturated ammonium sulfate (purum, Fluka AG, Buchs, Switzerland) solution was added to 50 g of fruit, directly into the blender. Finally, 2-methyl-1-pentanol (Fluka purum; 1 mg/100 g of homogenate) was added as internal standard. Sensory Evaluation. Consumer Tests. Standard hedonic consumer tests with an average of 120 participants were carried out in supermarkets (Federation of Migros Cooperatives) in different Swiss cities (La Chaux de Fonds, Pully, Sion, and Bienne). The test persons were asked to give an overall appreciation of strawberry quality on a 1-9 scale (1 ) extremely bad to 9 ) extremely good). In the 1999 campaign a modified procedure was adopted. Each fruit was divided into halves; half was used to assess the sensory quality, and the other half was assigned to different baskets according to the score obtained (1-9). The pooled samples were homogenized as described above and used for instrumental analyses. This way of classifying samples is hereafter called “hedonic classification”. Sensory Panel. The sensory panel consisted of 10-15 semitrained subjects. The subjects were asked to rate the following sensory attributes: odor, aroma, sweetness, acidity, firmness, and juiciness. They were also asked to give their overall appreciation. The volatile compounds were evaluated in two ways: first, through the nose (odor) and then by the retronasal way through the pharynx after masticating the sample (aroma). The panel rated the different parameters on a 1-9 scale (e.g., 1 ) very weak aroma intensity and 9 ) very strong aroma intensity). The same scale was used for the overall appreciation (extremely bad to extremely good). Panelists were given water (Volvic, Puy-de-Dome, France) as a neutralizing beverage between sample
testings. The evaluation was carried out in a standard sensory laboratory under well-controlled conditions using red light to mask any color differences. Instrumental Analyses. Determination of Total Volatile Compounds. Fresh intact strawberries (400 ( 1 g) were carefully placed in a 2 L headspace flask with wide opening (NS/160/100) as shown in Figure 1. The flask was sealed with a Teflon lid, allowing the simultaneous recovery of the volatile compounds by several SPME fibers. Different types of SPME fibers were used: poly(dimethylsiloxane) (PDMS) with 100 µm thickness (catalog no. 5-7300-U); polyacrylate (PA), 85 µm (catalog no. 5-7304); porous fibers of Carbowax/divinylbenzene (CW/ DVB), 65 µm (catalog no. 5-7312); and bipolar fibers PDMS/DVB, 65 µm (catalog no. 5-7310-U), Carboxen/PDMS (CAR/PDMS), 75 µm (catalog no. 5-7318), and CW/CAR/PDMS, 50/30 µm (5-7328-U), all obtained from Supelco Co. (Bellefonte, PA). The fruits were left at 25 °C for 5 min to obtain the necessary gas equilibrium in the headspace. Aliquots of the volatile compounds were then collected by inserting the SPME needle through a Teflon-coated silicone septum into the headspace of the flask. After 5 min (sampling time), the adsorbed substances were desorbed into a gas chromatograph HRGC-5300 (Carlo Erba S.p.A., Milano, Italy) equipped with a splitless injector port, directly coupled to the flame ionization detector, using a fused silica transfer tube (20 cm in length, 0.1 mm i.d., no. 160-2630, J&W, New Brighton, MN). The following GC conditions were used: helium carrier gas pressure, 150 kPa at a flow rate of ∼5 mL/min; hydrogen and air pressure for the FID, 50 and 80 kPa, respectively; oven temperature, 250 °C; injection port and detector temperatures, 200 and 250 °C, respectively. A mixture (3 mg/kg) of 1-methoxy-2-propylacetate (Merck, for synthesis), 2-methyl ethyl ketone (Fluka, purum), and butanol (Fluka, puriss.) was used as external standard. The total volatile peak (µV‚min) was measured with a Borwin integrator (JMBS Developpements, Grenoble, France). Between analyses, the headspace flask was cleaned by purging with filtered air that had previously passed through a charcoal trap (Supelpure-HC trap, Supelco Co). Measurements on strawberry puree were carried out by spreading the sample into a crystallizing dish (10 cm diameter, 3 cm height), which was then placed in the 6 L headspace flask. Total analysis time was ∼15 min, including 5 min for both equilibration and sampling. Each sample was analyzed in triplicate. With the above-described
Quality of Strawberries experimental conditions measurements of the total volatile fraction gave a coefficient of variation of
