Changes in Flavour and Texture During the Ripening of Strawberries

Changes in Flavour and Texture During the Ripening of Strawberries

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Eur Food Res Technol (2004) 218:167–172 DOI 10.1007/s00217-003-0822-0 ORIGINAL PAPER Ramin Azodanlou · Charly Darbellay · Jean-Luc Luisier · Jean-Claude Villettaz · Renato Amad Changes in flavour and texture during the ripening of strawberries Received: 17 June 2003 / Published online: 26 November 2003 Springer-Verlag 2003 Abstract The amount of total volatile compounds, total Keywords Ripeness · Total volatile · Solid phase acidity, total sugar content (degrees Brix) and fruit microextraction · Strawberry · Flavour · Texture firmness were used to characterize the degree of ripeness of three strawberry varieties (Carezza, Darselect and Marmolada). A novel concept using solid phase microex- Introduction traction (SPME) and measurement of total volatile compounds to distinguish between various stages of The quality of strawberries is defined by sensory strawberry ripeness was applied. The carboxene/poly- attributes such as flavour and texture. Plant selection, dimethylsiloxane SPME fibre was found to be best suited growing conditions (soil, climate, etc.), harvest date, and to differentiate between the stages of ripeness. The post-harvest treatments all influence these quality at- amount of total volatile compounds rapidly increased tributes [1]. near to maturity (between the three-quarters red stage and Fruit ripening is a very complex process. It is the dark-red stage). Most of the volatile compounds influenced by the synthesis and action of hormones identified were esters, followed by aldehydes, and responsible for the rate of ripening, the biosynthesis of alcohols. The most abundant volatile compounds were pigments (carotenoids, anthocyanins, etc.), the metabo- propyl butanoate, 3-phenyl-1-propanol, butyl butanoate, lism of sugars, acids and volatile compounds involved in isobutyl butanoate, 3-methyl butyl butanoate and isopro- flavour development. In addition the modifications of the pyl hexanoate. The concentration of green aroma com- structure and composition of the cell wall are known to ponents such as hexanal, trans-2-hexenol and cis-3- affect the texture of the fruit [2, 3]. hexenyl acetate progressively decreased during the mat- Glucose, fructose and sucrose are by far the most uration process until they became minor components in abundant soluble components in strawberries. The sugars mature strawberries. constitute precursors of flavour compounds (e.g. fura- nones), and are primarily used as an energy source in the R. Azodanlou · R. Amad ()) ripening process [4]. Acids can affect flavour as well, but Swiss Federal Institute for Technology of Zurich, they are more important in processing [5]. Further, it has Institute for Food Science and Nutrition, ETH Zentrum, been shown that volatile fatty acids play an important role Schmelzbergstrasse 9, CH-8092 Zurich, Switzerland in ester formation in strawberries [6]. e-mail: [email protected] A small number of researchers have focused their work Tel.: +41-1-632-3291 on changes in the profile of volatile compounds during Fax: +41-1-632-1123 fruit ripening [6, 7, 8, 9, 10]. During the ripening process C. Darbellay the capacity to esterify 1-pentanol is developed by the Swiss Federal Research Station for Plant Production, fruit, presumably by enzymatic reactions, although the CH-1964 Conthey, Switzerland enzymes responsible have not yet been characterized [6]. Alternatively, some flavour components could be the J.-L. Luisier · J.-C. Villettaz products of non-enzymatic reactions (e.g. the reaction of Department of Food Technology and Biotechnology, an alcohol with an acid), where the formation of Ecole d’ Ingnieurs du Valais, CH-1950 Sion, Switzerland precursors is enzymatically controlled. The authors Ya- mashita et al. pointed out that esters and furanones are key Present address: compounds of the strawberry flavour. An increase in the R. Azodanlou, Syngenta Biotechnology Inc., total amount of volatile compounds during ripening has 3054 Cornwallis Rd., Research Triangle Park, P.O. Box 12257, also been observed [7]. Esters, in particular large amounts NC 27709–2257, USA 168 of methyl acetate and methyl butanoate, were predomi- strawberries was quantified by GC-FID. The volatiles present in the nant. The concentration of volatile compounds increased headspace (250 ml) were analysed using the procedure described in [11]. Extraction was performed using the carboxene (CAR)/ during ripening of Chandler strawberries, and ethyl polydimethylsiloxane (PDMS) SPME fibre (Supelco Co., Belle- hexanoate, ethyl butanoate and methyl butanoate were fonte, USA). The volatile components were identified using the predominant components present in the fully ripe fruit reference compounds, Kovats retention indices data and GC-MS [8]. The relationship between the degree of maturity and library data [11]. the concentration of 2,5-dimethyl-4-hydroxy-3(2H)-fura- Determination of total sugar and acid contents. For the determi- none and its derivatives has been investigated in seven nation of total sugar content (degrees Brix) and total acidity, strawberry varieties by Sanz et al. [9]. 200€1 g of strawberry puree was used to ensure sample homoge- The influence of sampling on flavour analysis was neity. Total sugar content was measured by refractometry (Atago discussed by Parliament, who stated that liquid-liquid PR1, Atago, Tokyo, Japan). Total acidity was determined by titrating a 10€0.1 g sample to pH 8.0 using 0.1 M NaOH in a extraction and simultaneous distillation may lead to the Mettler DL25 titrator (Mettler-Toledo, Greifensee, Switzerland). production of artefacts, whereas solid phase microextrac- The titrated volume (in millilitres) expressed directly the total tion (SPME) allowed a more reliable flavour analysis acidity in grams/litre of citric acid. [10]. Texture analysis. The firmness of strawberries (100€1 g) was In the present study a quantitative analysis of flavour measured with a shear test equipment (VersaTest+Advanced Forces compounds and an assessment of texture changes during Gauge, Memesin, Brtsch & Regger, Zurich, Switzerland) fitted strawberry ripening were carried out. Three strawberry with a Kramer shear cell. Strawberries were divided into two equal varieties were investigated. Flavour analysis was per- parts prior to measurements. The fruits were sheared at a speed of 250 mm/min at ambient temperature. The measurements were formed after SPME, measurement of total volatile performed in triplicate. compounds, GC-FID and GC-MS. For each strawberry variety the chromatographic aroma profiles at six differ- Statistical evaluation. The Statview program (Abacus Concepts ent ripening stages were compared. The results were Inc., Berkeley, Calif.) was used for analysis of variance (ANOVA). processed by principal component analysis (PCA). Total Significant differences in instrumental measurements between samples were determined by protected least significant difference acidity, total sugar content (degrees Brix) and fruit (PLSD) with P0.05. The Statbox program (Grimmer, Logiciels firmness (using a Kramer shear cell) were determined Corp., Paris, France) was used for the Pearson’s correlation as well. (P0.05) and the PCA to identify the interdependence between different stages of ripeness of the fruits and the instrumental data (P0.05). Materials and methods Fruit samples and sample preparation Results and discussion Three commercial varieties of strawberries (Carezza, Darselect and Total sugar and total acidity Marmolada) grown under plastic tunnels were obtained from the Swiss Federal Research Station for plant production in Conthey The flavour of strawberries is greatly influenced by the (Switzerland). The fruits were harvested at six different ripening stages (green, white, half red, three-quarters red, red-mature and presence of sugars and acids. Several authors have shown dark-red). the total sugar content to increase during ripening and to stimulate the formation of secondary metabolites such as anthocyanins and furanones [12, 13]. Further, a decrease Preparation of fruit puree in acidity with maturity was observed, particularly for Strawberries were homogenized at high speed in a professional citric acid, malic acid and quinic acid, which decreased blender (Kenwood Professional, Dublin, Ireland) for approximately considerably [12, 14]. In the present study the two first 30 s. The endogenous enzymes were inactivated by adding 50 g of a stages of ripeness were not analysed, because the blender saturated ammonium sulfate (purum, Fluka AG, Buchs, Switzer- land) solution to 50 g of fruit directly into the blender. Finally 2- could not produce a puree with green and white straw- methyl-1-pentanol (1 mg/100 g of puree, Fluka, purum) was added berries. In agreement with previous studies, a general as internal standard. The puree was directly used for instrumental increase in the total sugar content (degrees Brix) for the analysis. three strawberry varieties investigated at the more advanced stages of ripeness (Table 1) was observed. In Instrumental analyses addition, a significant decrease in total acidity (TA) during different stages of ripening was observed. The Determination of total volatile compounds. Total volatile com- Darselect variety contained more sugar than the two other pounds of strawberries were analysed as described by Azodanlou et varieties. al. [11]. The determinations on fresh intact strawberries (400€1 g) were carried out in a 2 l headspace

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