Chemical, functional and quality properties of Japanese plum (Prunus salicina Lindl.) as affected by mulching

Scientia Horticulturae 134 (2012) 114–120

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Chemical, functional and quality properties of Japanese plum (Prunus salicina Lindl.) as affected by mulching Pablo Melgarejo a , Ángel Calín-Sánchez b,∗ , Francisca Hernández a , Antoni Szumny c , Juan José Martínez a , Pilar Legua a , Rafael Martínez a , Ángel A. Carbonell-Barrachina b a Universidad Miguel Hernández, Departamento de Producción Vegetal y Microbiología, Grupo de Fruticultura y Técnicas de Producción, Carretera de Beniel, km 3,2. 03312-Orihuela, Alicante, Spain b Universidad Miguel Hernández, Departamento de Tecnología Agroalimentaria, Grupo Calidad y Seguridad Alimentaria, Carretera de Beniel, km 3,2. 03312-Orihuela, Alicante, Spain c Department of Chemistry, Wrocław University of Environmental and Life Sciences, ul. Norwida 25, 53-375 Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 8 June 2011 Received in revised form 12 October 2011 Accepted 14 November 2011 Keywords: Mulching Hydrodistillation Antioxidant activity Polyphenols Volatile composition Sensory evaluation

a b s t r a c t The aim of this study was to contribute to the chemical, functional and qualitative characterisation of Japanese plum (Prunus salicina L., cv. Red Beaut, genotype “606”) cultivated with and without a mulching plastic by investigating the organic acids and sugars profiles, the total antioxidant activity and total polyphenols content, the volatile composition and the sensory profile. Results indicated that total sugars, organic acids contents and the total concentration of volatile compounds were slightly higher in plums from trees with the mulching plastic film being used as a cultural practice, although differences were not statistically significant. Sixteen compounds were isolated from the volatile fraction from six different chemical families: aldehydes, monoterpenes, monoterpenoids, phenylpropanoids, acids and ketones. The major compounds in the volatile profile of plum juices were trans-linalool oxide, terpinolene, nonanal and ␣-terpineol. However, total antioxidant activity, total polyphenols content and the sweetness intensity were decreased by the use of the mulching plastic. Therefore, mulching with plastic materials with high light reflectance (e.g. white) seemed to be required to improve the quality of plums. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Plums (Prunus salicina Lindl.) are an important group among stone fruit grown commercially in Spain. Harvest date of plums is an essential determinant of consumer acceptability (Crisosto et al., 2004) and a number of parameters are generally used to establish the optimum ripening stage including, skin colour, soluble solids concentration, acidity, flavour and volatile composition. However, large variations in these parameters can be found depending on cultivar, production, production area, climatic conditions and harvest season (Kader and Mitchell, 1989). Increased intake of fruit and vegetables has been associated with reduced incidence of degenerative diseases due to their antioxidant potential (Prior, 2003; Schreiner and Huyskens-Keil, 2006). In this sense, plums are considered a fruit class with high amounts of bioactive compounds or phytochemicals such as vitamins (A, C and E), anthocyanins and other phenolic compounds, and carotenoids (Stacewicz-Sapuntzakis et al., 2001), which contribute to the antioxidant capacity. On the other hand, large variations in the concentration of bioactive compounds at commercial

∗ Corresponding author. Tel.: +34 966749738; fax: +34 966749677. E-mail address: [email protected] (Á. Calín-Sánchez). 0304-4238/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2011.11.014

harvesting depending on cultivar have been reported (Los et al., 2000; Tomás-Barberán et al., 2001a; Gil et al., 2002). Volatiles directly affect the sensorial quality of fresh and processed fruit products, the aroma of which is formed by a complex group of chemical substances (e.g., aldehydes, alcohols, ketones, esters, lactones, terpenes). The concentration of these volatile compounds is generally low (␮g/L) and can be affected by a number of agronomic (variety, climatological conditions, ripening stage) (Visai and Vanoli, 1997; Vendramini and Trugo, 2000) and technological (harvest, post-harvest treatments, storage and processing conditions) factors (Lin et al., 2002; Botondi et al., 2003). The main advantages associated with mulching are: (i) less water is required for irrigation (Trenor et al., 1998), (ii) advance of harvest (Ferrer Talón et al., 2004), and (iii) the bigger size of plants (Melgarejo et al., 1998). Several authors have described the influence of mulching on different fruits and quality parameters. For instance, Richardson and Money (1992) obtained mandarins with a reduction in the water consumption, while Nakhalla and Ghali (1996) obtained higher yield and bigger size in oranges cultivated using mulching. Other studies dealing with mulching have reported higher amount of soluble solids in peach (Layne et al., 2001) and plum (Kim et al., 2008) and better colour in “Mondial Gala” apple (Iglesias and Alegre, 2009). In general, mulching can be considered as a cultural practice that improves the global quality

P. Melgarejo et al. / Scientia Horticulturae 134 (2012) 114–120

of fruits but reduces the post-harvest shelf life (Pliakoni and Nanos, 2010a). Up to this time, there is no research literature on the effect of the cultural practices, such as mulching on volatile composition and sensory quality of plums. Therefore, the general aim of this study was to quantify the effects of mulching on the chemical composition (antioxidant capacity and total polyphenols content), volatile composition and sensory quality of plum cv. Red Beaut, genotype “606”. 2. Materials and methods 2.1. Plant material The experiment was carried out during the developmental cycle of the 2010 Spring at a commercial farm located in Ojós (latitude 38◦ 4 52.6 N × 1◦ 21 11.6 W, 120 m above see level), Murcia (South-Eastern Spain). Plums of cv. Red Beaut, genotype “606” (a bud sport from the plum cultivar Red Beaut), with dark-purple skin and yellow flesh, grafted on to hybrid GF-677 were selected for this experiment; plum trees were 9 year-old and their plantation frame was 6 m × 2 m. The culture was conducted on ridges with or without plastic mulching (from now T1 and T0, respectively). For T1, Sotrafilm NG (SOTRAFA, El Ejido, Almería, Spain) was used as mulching plastic; the main characteristics of this film are: black LLDPE (Lineal low density polyethylene) of 200 gauges (50 ␮m) of thickness; 3.5 m wide; light transmission 1%, absorption 95% and reflectance 4%. This particular plastic material was selected because it is one of the most widely used in the farming of plums in Spain and there is an urgent need of scientific data stating whether it is appropriate or not for this particular fruit. Mulching started one month before harvest and remained in place until the end of August. The irrigation system consisted of auto-compensated drips; T0: 4 drips/tree of 4 L/h and T1: 2 drips/tree of 4 L/h. From the fruit set until the harvest, a daily irrigation was applied in order to maintain the matric potential between 10 and 30 cb. Besides, standard cultural practices (pruning, thinning, fertilisation and treatments) were carried out for both treatments. Weather data from the previous 50 years were compared with the climatic conditions for the current year, 2010, and no significant differences were found. The mean temperature, humidity and accumulated rainfall for 2010 were 18 ◦ C, 64.2% and 426 mm. Summarizing two treatments were evaluated: (i) T0 or control treatment, which used traditional cultivation without mulching and with a high volume of irrigation water and (ii) T1 cultivation with mulching (black LLDPE) using a lower volume of water. 2.2. Sampling and sample processing Plums were picked (3rd June, 2010) at commercial ripening according to maturity indexes (fruit width 52 mm, fruit height 49 mm, fruit weight 83.5 g, total soluble solids 12%). Ten trees were selected for each treatment and twenty fruits, of the same stage of ripening, were randomly harvested from each tree. The effect of mulching on ripening time was out of the objective of this particular study. Once in the laboratory, fruits were visually inspected and ten groups of four homogeneous fruits per treatment (40 fruits were finally selected out of the 200 picked at the farm) were peeled and the skins were removed manually; juice of the flesh was prepared using a domestic blender and stored in individual bags (100 mL of fruit per bag) at −80 ◦ C until the analyses (less than two months). Saturated CaCl2 (150 mL kg−1 plum) was added to avoid enzymatic reactions during storage or manipulation of plums samples during analyses.

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2.3. Extraction of volatile compounds Hydrodistillation (HD) using a Deryng apparatus was used for the extraction of the volatile compounds from plums (Alonso et al., 2009); analyses were run in triplicate using samples from three different storage bags. 150 mL of plum juice were placed in the sample flask together with 300 mL of ultrapure water, 10 g NaCl and 10 ␮L of -anethole (1 mg mL−1 ), as internal standard. Sample flask was heated, after boiling, during 1 h. The vapours were condensed by means of a cold refrigerant, maintained at −5 ◦ C by a cryostat, model Frigiterm (Selecta). After 60 min of extraction, the solvent, 1 mL of cyclohexane (Panreac), containing the aroma compounds, was collected and analysed by GC–MS. 2.4. Chromatographic analyses The isolation and identification of the volatile compounds were performed on a gas chromatograph, Shimadzu GC-17A (Shimadzu Corporation, Kyoto, Japan), coupled with a Shimadzu mass spectrometer detector GC–MS QP-5050A. The GC–MS system was equipped with a TR-820262 Meta.X5 column (Teknokroma S. Coop. C. Ltd, Barcelona, Spain; 60 m × 0.25 mm × 0.25 ␮m film thickness). Analyses were carried out using chromatographic conditions previously described by Vázquez-Araújo et al. (2008). Most of the compounds were identified using three different analytical methods: (1) kovats indices, (2) GC–MS retention times (authentic chemicals), and (3) mass spectra (authentic chemicals and Wiley spectral library collection). Identification was considered tentative when it was based on only mass spectral data (McLafferty, 2000). The quantification of the volatile compounds was performed on a gas chromatograph, Shimadzu 2010, with a flame ionization detector (FID). The column and chromatographic conditions were those previously reported for the GC–MS analysis. The injector temperature was 250 ◦ C and nitrogen was used as carrier gas (1 mL min−1 ). For the quantification of the volatile compounds, trans-anethole was added as internal standard; this chemical was used as internal standard after checking that it was absent in plum juices and under the proposed conditions, it separates well from other volatile compounds. Data included in this study should be considered as semi-quantitative, because no standard curves were carried out for each one of the quantified volatile compounds. However, relative values are useful to compare differences between juices. 2.5. Extraction and determination of sugars and organic acids Sugars and organic acids analyses were run in triplicate using samples from three different storage bags. 1 mL of the centrifuged juice (15,000 rpm for 20 min) was passed through a 0.45 ␮m Millipore filter and then injected into a Hewlett-Packard HPLC series 1100. The elution system consisted of 0.1% phosphoric acid with a flow rate of 0.5 mL min−1 . Organic acids were separated on a Supelcogel TM C-610H column (30 cm × 7.8 mm i.d., Supelco, Bellefonte, PA, USA) and Supelguard column (5 cm × 4.6 mm, Supelco, Inc., Bellefonte, PA) and detected by absorbance at 210 nm. For sugar determinations, the same HPLC, elution system, flow rate and columns were used. The detection of sugars was obtained with a refractive index detector (HP 1100, G1362A). Standard curves for pure organic acids (oxalic, citric, tartaric, malic, acetic, fumaric, succinic and ascorbic acids) as well as for pure sugars (glucose, maltose, fructose, sucrose and sorbitol) purchased from Sigma (Poole, Dorset, UK), were used for quantification. Results for both individual organic acids and sugars were expressed as concentrations g 100 g−1 .

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2.6. Total antioxidant activity and total phenolic determination

3. Results and discussion

Total antioxidant activity (TAA) and total phenolic (TP) were quantified as described by Díaz-Mula et al. (2008) which enables to determine TAA due to both hydrophilic and lipophilic compounds in the same extraction. Briefly, for each sub-sample, 5 g of flesh and 1 g of skin tissues were homogenized in 5 mL of 50 mM phosphate buffer pH 7.8 and 3 mL of ethyl acetate, then centrifuged at 10,000 × g for 15 min at 4 ◦ C. The upper fraction was used for total antioxidant activity due to lipophilic compounds (L-TAA) and the lower for total antioxidant activity due to hydrophilic compounds (H-TAA) and total phenolic. TAA was determined in triplicate in each extract using the enzymatic system composed of the chromophore 2,2 -azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), the horse radish peroxidase enzyme (HRP) and its oxidant substrate (hydrogen peroxide), in which ABTS+ radicals are generated and monitored at 730 nm. The decrease in absorbance after adding the extract was proportional to TAA of the sample. A calibration curve was performed with Trolox ((R)-(+)-6-hydroxy-2,5,7,8-tetramethyl-chroman-2carboxylic acid) (0–20 nmol) from Sigma (Madrid, Spain), and results (mean ± SE) are expressed as mg of Trolox equivalent 100 g−1 . The TP compounds were quantified using Folin-Ciocalteu reagent (Singleton et al., 1999). Absorption was measured for the sample at 760 nm using spectrophotometer (ThermoSpectronic Heyios, made England). Results were expressed as mg gallic acid equivalent 100 g−1 fresh weight.

3.1. Organic acids and sugars

2.7. Sensory evaluation with trained panel Sensory evaluation with trained panel was used to describe fresh plum juices. A panel of 8 panellists, ages 20–55 years (4 female and 4 male) was trained in descriptive evaluation of fresh fruits, including plums. All panellists work at Miguel Hernández University and have a wide expertise in sensory evaluation of foods. Details about panel selection and training could be found in Ruiz et al. (2005). The study was carried out at UMH facilities (individual booths with controlled illumination and temperature) during three different sessions; samples were evaluated in triplicate. In each session, panellists tested both T0 and T1 plum juices; the samples order for each panellist was randomised. Approximately 20 mL of juice were served, at 10 ◦ C together with the appropriate questionnaire, one at a time and waiting 5 min between samples. Unsalted crackers and water were provided to consumers for palate cleansing between samples. In each questionnaire, panellists were asked, to evaluate the intensity of the following juice attributes: colour, vegetal odour, fresh plum odour, sourness, sweetness, bitterness, astringency, fresh plum flavour, vegetal flavour and the amount of pulp. Panellists used for the evaluation a 11-point scale, where 0 was extremely low intensity, 5 was regular intensity of fresh plum juice and 10 was extremely high intensity.

2.8. Statistical analyses Data from plums analyses were examined by analysis of variance (ANOVA) using STATGRAPHICS Plus 5.0 software (Manugistics, Inc., Rockville, MD). Significance was defined at p ≤ 0.05. Graphics were created using Sigma Plot 9.0 (SPSS Science, Chicago, IL, USA).

Malic, citric, tartaric, ascorbic, shikimic and fumaric acids were found in “606” cv. (Table 1); the organic acids profiles were not significantly affected by mulching. Malic acid was the major organic acid, representing about 90% of the total acids concentration. The organic acids profile reported here agreed with those from previous studied carried out by Robertson et al. (1992), Usenik et al. (2008), Singh and Singh (2008) and Singh et al. (2009). The total contents of organic acids were not significantly different between treatments (p < 0.05), with mean values being 21.4 and 23.5 g kg−1 for T0 and T1, respectively. The sugars profile was not significantly affected by the treatment (Table 2). Fructose, glucose and sucrose were identified as main sugars of “606” Red Beaut plums. Fructose presented the highest concentration followed by glucose and sucrose. These results are in concordance with previous reports in Japanese and European plums (Singh and Singh, 2008; Usenik et al., 2008; Singh et al., 2009). The total sugars contents showed no significant differences between treatments and the mean values were 75.0 and 86.5 g kg−1 for T0 and T1 fruits, respectively. 3.2. Total antioxidant activity (TAA) and total polyphenols content (TP) The total antioxidant activity was measured separately as hydrophilic (H-TAA) and as lipophilic (L-TAA) fractions by the ABTS method. It could be observed that TAA (H-TAA plus L-TAA) was significantly higher (p < 0.05), approximately one order of magnitude, in the skin than in the flesh of both control (T0) and treated plums (T1) (Table 3). T0 treatment provided plums with significantly (p < 0.05) higher antioxidant activity than T1. It is observed that the antioxidant activity in the hydrophilic fraction was always much higher than the lipophilic fraction; for instance, H-TAA of T0 was twice higher than L-TAA for the same treatment, while in the case of T1, H-TAA was four times higher than in its L-TAA. These results are in concordance with several studies which confirm that antioxidant capacity of fruits and vegetables is mainly due to the presence of water-soluble compounds, such as polyphenols (Kaur and Kapoor, 2001; Wu et al., 2004). The difference between the treatments could be explained considering that the black LLDPE film used in T1 has a lower sun-light reflection and as a consequence less coloured fruits were obtained. Several natural pigments, such as anthocyanins and carotenoids, have a direct influence on the total antioxidant activity of fruit and vegetables (Wang et al., 1997; Gardner et al., 2000). Pliakoni et al. (2010b) obtained higher concentration of phenolic compounds of nectarines with the application of deficit irrigation and mulching; however the plastic film used by these authors was white. Therefore, our main hypothesis is that the reduction in the total antioxidant activity in plum under study was mainly linked with the use of a black film and not because of the lower irrigation volume or mulching. Besides, it has been reported that climatic conditions and cultural factors can affect the nutritional composition including bioactive compounds (Scalzo et al., 2005; Tomás-Barberán and Espín, 2001b; Valero and Serrano, 2010). The concentration of TP (Table 4) was higher in the skin than in the flesh, with significant differences being found between treatments. The treatment T0 produced the plums with the highest phenolic concentration in the skin (241 mg 100 g−1 ) and in the flesh (62.4 mg 100 g−1 ) while that treatment T1 showed the plums with the lowest phenolic concentration in both the flesh and skin. The contents of TP in the “606” plum cultivar was within the range reported in other dark-purple skin plum cultivars (Díaz-Mula

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Table 1 Organic acids contents (g kg−1 ) in plums from “606” and cultivar. Treatment T0 T1 ANOVA†

Citric (g kg−1 ) ‡

0.62 ± 0.10 0.69 ± 0.18 N.S.

Tartaric (g kg−1 )

Malic (g kg−1 )

Ascorbic (g kg−1 )

Shikimic (g kg−1 )

Total

0.58 ± 0.10 0.67 ± 0.07 N.S.

19.7 ± 0.6 21.6 ± 1.1 N.S.

0.47 ± 0.01 0.43 ± 0.01 N.S.

0.06 ± 0.01 0.06 ± 0.01 N.S.

21.4 ± 0.2 23.5 ± 0.3 N.S.

†

N.S., not significant F ratio (p < 0.05). Treatment means of the ANOVA test (values are the mean value of 3 replications ± standard error). *Significant at p < 0.05. **Significant at p < 0.01. ***Significant at p < 0.001. ‡

Table 2 Sugars content (g kg−1 ) in plums from “606” cultivar. Treatment

Glucose (g kg−1 )

Fructose (g kg−1 )

Sucrose (g kg−1 )

Total

T0 T1 ANOVA†

21.2 ± 0.6‡ 20.2 ± 2.0 N.S.

39.4 ± 0.9 45.3 ± 3.0 N.S.

14.4 ± 2.1 21.0 ± 0.4

75.0 ± 1.0 86.5 ± 6.0 N.S.

*

†

N.S., not significant F ratio (p < 0.05). Treatment means of the ANOVA test (values are the mean value of 3 replications ± standard error). * Significant at p < 0.05. **Significant at p < 0.01. ***Significant at p < 0.001. ‡

Table 3 Total antioxidant activity (mg of trolox eq. 100 g−1 ) in plums from “606” cultivar. Treatment

ANOVA (fruit part)†

ANOVA (fruit part)†

L-TAA Flesh

T0 T1 ANOVA (mulching)† † ‡ * ** ***

‡

*** ***

H-TAA

Skin

14.4 ± 0.7 10.5 ± 0.1

206 ± 1 134 ± 2

*

***

*** ***

Flesh

Skin

40.1 ± 0.4 38.9 ± 1.3 N.S.

486 ± 2 429 ± 1 **

N.S., not significant F ratio (p < 0.05). Treatment means of the ANOVA test (values are the mean value of 3 replications ± standard error). Significant at p < 0.05. Significant at p < 0.01. Significant at p < 0.001.

Table 4 Total polyphenols contents (mg gallic acid eq. 100 g−1 ) in plums from “606” cultivar. Treatment

ANOVA (fruit part) †

Total polyphenols content (mg gallic acid equivalent 100 g−1 ) Flesh Skin

T0 T1 ANOVA (mulching)†

***

241 ± 7‡ 85.2 ± 2.9

62.4 ± 2.2 36.4 ± 0.4

***

***

**

†

N.S., not significant F ratio (p < 0.05). Treatment means of the ANOVA test (values are the mean value of 3 replications ± standard error). *Significant at p < 0.05. ** Significant at p < 0.01. *** Significant at p < 0.001. ‡

et al., 2008). Besides dark-purple plums generally have higher phenolic contents than yellow cultivars (Gil et al., 2002; Chun et al., 2003; Los et al., 2000; Usenik et al., 2008). The results also show the influence of polyphenols in the total antioxidant activity since T0 plums showed higher content of TP and TAA compared with those obtained by T1 plums. 3.3. Volatile composition A total of 16 compounds were isolated using the hydrodistillation, HD (Table 5), which was previously used by our research group to study the volatile composition of Spanish tomatoes (Alonso et al., 2009) and Spanish pomegranates (Calín-Sánchez et al., 2011). The results of volatile compounds differ from those of other authors perhaps because of the isolation technique or because the type of cultivar. Krammer et al. (1991) found 31 compounds in Prunus domestica, cv. Nancy, using simultaneous enzyme catalysis

extraction, while Sabarez et al. (2000) found only 6 volatile compounds in cultivar D’Agen using solid phase microextraction (SPME). Other researchers have determined the volatile composition of different cultivars of P. salicina using simultaneousdistillation–extraction (Gomez and Ledbetter, 1993) and using distillation–extraction with solvents (Lozano et al., 2009). However the present study is the first which employs hydrodistillation with a Deryng apparatus in P. salicina cv. “606” hybrid and seemed to be a technique with lower yield than others applied in different Prunus species due to lower number of volatile compounds found. The volatile compounds found in fresh plums (Table 5) can be grouped in 6 chemical groups: (a) aldehydes: benzaldehyde, phenylacetaldehyde, nonanal and decenal, (b) monoterpenes: limonene, terpinolene and ocimene, (c) monoterpenoids: trans-linalool oxide, linalool, trans-linalool oxide, myrcneol, ␣-terpineol and geraniol, (d) phenylpropanoids: eugenol, (e) organic acids: nonanoic acid and (f) ketones: demascenone.

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Table 5 Identification, quantification (mg kg−1 ) and characteristics of volatile compounds found in plums from “606” cultivar. Compound

RT (min)

Kovats Indexes Lit.

Exper.

Benzaldehyde Limonene Phenylacetaldehyde trans-Linalool oxide Linalool cis-Linalool oxide Terpinolene Nonanal Myrcenol Ocimene ␣-Terpineol Geraniol Nonanoic acid Decenal Eugenol Damascenone

10.13† 12.20 12.57 13.73 14.14 14.35 14.68 14.89 15.25 16.93 19.14 20.74 21.60 21.92 26.23 27.51

970 1031 1045 1074 1080 1091 1098 1102 1090 1155 1172 1255 1260 1261 1348 1384

969 1032 1042 1072 1083 1089 1098 1103 1117 1149 1199 1234 1252 1259 1352 1379

Total

ANOVA

Concentration (mg kg−1 ) T0

Descriptor T1

N.S.‡ N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S. N.S.

0.02 0.01 0.02 0.11 0.02 0.06 0.32 0.25 0.03 0.05 0.19 0.04 0.02 0.03 0.05 0.02

0.03 0.01 0.01 0.17 0.01 0.07 0.38 0.27 0.03 0.06 0.25 0.07 0.02 0.04 0.05 0.02

N.S.

1.24

1.49

Bitter almond, fragrant Mild, citrus, orange, lemon Floral Leafy, earthy Lemon, orange, citrus, floral Sweet, floral, creamy Sweet, fresh, pine, citrus Floral, citrus, orange, rose Fresh, floral, lavender, citrus Fresh, citrus, lemon, lime Lilac Rose Cheese, waxy Orange, slightly fatty, floral Cinnamon, clove, spicy Apple, woody, nutty, citrus

†

RT, retention time; Lit., literature; Exper., experimental. ‡ Treatment means of the ANOVA test (values are the mean value of 3 replications ± standard error). N.S., not significant F ratio. *Significant at p < 0.05. **Significant at p < 0.01. ***Significant at p < 0.001.

Several of the volatiles identified in fresh plums were present in very low concentrations, for instance benzaldehyde, limonene, phenylacetaldehyde and linalool (Table 5). In general, the major compounds were trans-linalool oxide (leafy, earthy), terpinolene (sweet, fresh, pine, citrus), nonanal (floral, citrus, orange, rose, fatty) and ␣-terpineol (lilac). Several authors have studied the flavour of different plums cultivars and their profiles were different from the ones reported here. For instance, Ismail et al. (1981) found linalool and ethyl butanoate as the most important compounds; Etievant et al. (1986) showed that hexyl, butyl and ethyl esters were very abundant in the headspace of cv. Insitia; Sabarez et al. (2000) obtained hexanal, 2-hexenal and 1-hexanol as the major components. Monoterpenoids was the most abundant group for both treatments and ranged from 0.45 up to 0.60 mg kg−1 for T0 and T1, respectively, followed by monoterpenes with values ranged

from 0.38 up to 0.45 mg kg−1 for T0 and T1, respectively. The third group was aldehydes with concentrations ranging from 0.32 to 0.35 mg kg−1 , respectively. In general, monoterpenes and monoterpenoids can be related to pine and citrus notes, while aldehydes can be related to green, grassy and herbaceous notes. The total concentrations of volatile compounds ranged from 1.24 to 1.49 mg kg−1 , with no statistically significant differences (Table 5). These results agreed with data obtained for organic acids and sugars in fresh plums, in which no statistically significant differences were found between T0 and T1 plums. The total concentration of volatiles found in the present experiment was higher than that reported by Gomez and Ledbetter (1993), who reported a content of 0.423 mg kg−1 in P. salicina, cultivar Blackamber. No quantification of volatiles from P. domestica species were found in the literature. These differences could be

Fig. 1. Aroma profiles of plums from “606” cultivar as evaluated by a trained panel.

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related, among other factors, to the plant species, the cultivar and the cultural practices. The results obtained for cv. “606” showed that the treatment employed in plum farming with the black LLDPE film seemed to be an appropriated cultural practice because it maintains the total concentration of volatiles. It is reported that volatiles directly affect the sensory quality of fresh and processed fruit products (Visai and Vanoli, 1997; Vendramini and Trugo, 2000). 3.4. Sensory evaluation The present study was the first experiment dealing with the use of mulching as a cultural practice and its influence on the sensory quality of plums. Sensory results from the trained panel (Fig. 1) proved that fresh juices from P. salicina cv. “606” provided very good results because of the high intensities of colour (9.4 and 7.9 for T0 and T1, respectively), fresh plum flavour (7.1 and 7.4, respectively), vegetal flavour (6.6 for both treatments), vegetal odour (6.0 and 6.1, respectively) and fresh plum odour (5.8 and 5.9, respectively); medium intensities of bitterness (3.4 and 4.0, for T0 and T1, respectively), astringency (3.7 and 4.0, respectively); and low scores of pulp content (3.1 and 2.6, respectively). The sourness values were higher than desired (8.5 and 8.6 for T0 and T1, respectively) and at the same time the sweetness values were too low (1.2 and 1.4, respectively). Other studies carried in fresh juices such as pomegranate juices determined that Spanish consumers are not willing to consume sour juices (Vázquez-Araújo et al., 2010). The sensory results obtained proved that black LLDPE film provided fruits with only slightly lower intensity of colour but higher intensity of bitterness; these differences could be related to the high light absorbance of the film used in this study and consequently of its low reflectance, which significantly reduced the colour of plums. 4. Conclusions The use of mulching in the farming of P. salicina cv. Red Beaut genotype “606” could be considered as an appropriate cultural practice (production of fruits of high quality, similar to that of controls, but with a significant reduction in the volume of irrigation water) but based on the data obtained in the present experiment the use of black LLDPE film of low reflectance cannot be recommended. The contents of both organic acids and sugars together with the total concentration of volatile compounds of plums from trees cultivated using film of black LLDPE were statistically similar to those of control fruits. However, the antioxidant activity and polyphenols content were decreased with the use of the PE film. Therefore, better light reflectance films (mainly white) must be used as mulching materials to increase the sun reflection and improve the colour, antioxidant activity and polyphenols contents. References Alonso, A., Vázquez-Araújo, L., García-Martínez, S., Ruiz, J.J., Carbonell-Barrachina, A.A., 2009. Volatile compounds of traditional and virus-resistant breeding lines of Muchamiel tomatoes. Eur. Food Res. Technol. 203, 315–323. Botondi, R., DeSantis, D., Bellicontro, A., Vizovitis, K., Mencarelli, F., 2003. Influence of ethylene inhibition by 1-methylcyclopropene on apricot quality, volatile production, and glycosidase activity of low- and high-aroma varieties of apricots. J. Agric. Food Chem. 51, 1189–1200. Calín-Sánchez, A., Martínez, J.J., Vázquez-Araújo, L., Burló, F., Melgarejo, P., Carbonell-Barrachina, A.A., 2011. Volatile composition and sensory quality of Spanish pomegranates (Punica granatum L.). J. Sci. Food Agric. 91, 586–592. Chun, O.K., Kim, D.O., Moon, H.Y., Kang, H.G., Lee, C.Y., 2003. Contribution of individual polyphenolics to total antioxidant capacity of plums. J. Agric. Food Chem. 51, 7240–7245. Crisosto, C.H., Garner, D., Crisosto, G.M., Bowerman, E., 2004. Increasing ‘Black Amber’ plum (Prunus salicina Lindell) consumer acceptance. Postharvest Biol. Tec. 34, 237–244. Díaz-Mula, H.M., Zapata, P.J., Guillén, F., Castillo, S., Martínez-Romero, D., Valero, D., Serrano, M., 2008. Changes in physicochemical and nutritive parameters and

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