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Peeling has no effect on respiration and ethylene production and only minimal effect on quality of fresh white asparagus spears
Postharvest Biology and Technology 50 (2008) 224–227
Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Research Note
Peeling has no effect on respiration and ethylene production and only minimal effect on quality of fresh white asparagus spears Anastasios S. Siomos a,∗ , Dimitrios Gerasopoulos b , Pavlos Tsouvaltzis a , Athanasios Koukounaras a a b
Department of Horticulture, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Food Science and Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 20 December 2007 Accepted 19 May 2008 Keywords: Light processing Fresh cut Antioxidants Phenolics Color Toughness
a b s t r a c t Fresh white asparagus spears were subjected to peeling or left unpeeled before storage at 10 ◦ C for 7 days. CO2 and ethylene production rates were determined during storage, while color, toughness and DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content were determined before and after storage in both peeled and unpeeled spears. Peel CO2 and ethylene production rates were also determined during storage. Spears subjected to peeling were more tender, and had lower lightness and chroma and higher hue angle values compared to the unpeeled ones. However, both peeled and unpeeled spears had similar DPPH radical scavenging activity, soluble solids, total soluble phenols and nitrate contents. Peeling resulted in increased CO2 and C2 H4 production rates, but peel contributed to total increase of CO2 and C2 H4 production. As the measured metabolic activity of the peeled asparagus spears was similar to that of the unpeeled, peeling did not adversely affect quality during storage. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The market demand for lightly processed, ready-to-use vegetables is growing rapidly and asparagus is suitable for light processing, since it requires peeling before cooking. The physiology of the lightly processed fruit and vegetables is essentially the one of a wounded tissue and is characterized by increased respiration and ethylene production. Other consequences of wounding are chemical or physical in nature, such as oxidative browning reactions or enhanced rate of water loss. Moreover, in response to tissue wounding, a large number of secondary compounds are synthesized, many of which may affect the aroma, flavor, color and nutritional value of the product (Brecht, 1995). The intensity of the wounding response in the lightly processed plant tissues is proportional to the extent of wounding (Brecht, 1995). In asparagus, the wounding induced by peeling is expected to have a greater effect on spear metabolism compared to wounding induced by spear cutting at harvest (Papadopoulou et al., 2001). Therefore, peeled asparagus is expected to undergo accelerated deterioration and senescence.
∗ Corresponding author. Tel.: +30 2310 998646; fax: +30 2310 998609. E-mail address: [email protected] (A.S. Siomos). 0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.05.007
Color is one of the basic quality characteristics or specifications for sorting fresh white asparagus in commercial grades (Siomos et al., 2000; Siomos, 2003), but in recent years, consumers have become interested in nutritional and other compounds such as antioxidants and nitrates, respectively. Among 23 commonly consumed vegetables, asparagus has been reported to be the richest in total antioxidants, while the main components responsible for asparagus high bioactivity are phenols (Vinson et al., 1998). Although the effect of packaging materials on the quality of peeled asparagus has been extensively studied (Huyskens-Keil and Kadau, 2003; Kadau et al., 2003; Scheer et al., 2003; Simón et al., 2004; Albanese et al., 2007; An et al., 2007), the effect of peeling on respiration and ethylene production as well as on color, phenolic compounds, antioxidant components and nitrates of fresh white asparagus is still unknown. Therefore, this work investigates the effect of peeling on metabolic activity and quality aspects of fresh white asparagus spears. 2. Materials and methods White asparagus (Asparagus officinalis L.) spears of the cvs. ‘Dariana’ and ‘Grolim’ were harvested early in the morning from a commercial plantation near Gianitsa, Macedonia, Greece and placed into a styrofoam container to prevent exposure to light. After transportation for 3 h in darkness at ambient temperature (ca.
A.S. Siomos et al. / Postharvest Biology and Technology 50 (2008) 224–227
20 ◦ C), the spears were immersed in cool water (1–2 ◦ C) for 10 min, trimmed to 23 cm in length and washed thoroughly with tap water. Straight and undamaged spears, 16–24 mm in diameter with closed bracts were selected. The spears were divided in two groups: those of the first group were peeled with a sharp vegetable peeler starting 3 cm below the tip, while the spears of the second group were left unpeeled. The weight of the unpeeled and peeled spears as well as of the removed peel tissue was recorded. For each treatment three replications were used, each one consisting of five spears (200–220 g). The unpeeled and peeled spears as well as the removed peel of each replicate were placed in 3-L glass jars, ventilated with humidified air free of ethylene at a flow rate of 30 mL min−1 to ensure that the accumulated CO2 was always below the physiologically active levels of about 0.20%. The jars were kept in the dark at 10 ◦ C for 7 d. During storage, two 1 mL air samples were taken from the exit tube of the jars for the determination of CO2 and ethylene production rates. The CO2 concentration was measured using a CO2 /O2 analyzer (model Combo 280, David Bishop Instruments, UK), while the ethylene concentration was measured by using a Varian 3300 gas chromatograph (Varian Instruments, Walnut Creek, CA) equipped with a flame ionization detector. Before and after storage, the fresh weight, color, toughness and DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents were determined in unpeeled and peeled spears of each replication. Color measurements were made using a chromameter (Minolta CR-200, Minolta, Osaka, Japan). In the middle of each spear, two color measurements were made at two opposite sites. Spear toughness was measured using a Chatillon penetrometer (John Chatillon and Sons, New Gardens, NY) equipped with a plunger 3.2 mm in diameter and 9.5 mm in length (Siomos et al., 2000). In the middle of each spear, two toughness measurements were made at two opposite sites. After the determination of weight, color and toughness, the unpeeled and peeled spears of each replication were macerated in a blender for the determination of DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content. DPPH radical scavenging activity was determined according to Brand-Williams et al. (1995). Soluble solids content was measured in the juice of the blended material using a portable Atago PR-1 refractometer (Atago Co. Ltd., Tokyo, Japan). Total soluble phenol content was determined with the Folin–Ciocalteu procedure as described by Scalbert et al. (1989). Nitrate content was determined colorimetrically as described by Cataldo et al. (1975). Data were analyzed by analysis of variance (ANOVA) using a completely randomized design with three replications and the means were compared by LSD or by Duncan’s multiple range test at the 0.05 level with the MSTAT version 4.00/EM (Michigan State University). 3. Results and discussion 3.1. Effect of peeling on spear respiration rate during storage In cv. ‘Dariana’, the respiration rate of the unpeeled spears at the beginning of storage at 10 ◦ C was about 136 mg kg−1 h−1 CO2 , it decreased rapidly during the first 48 h, when it approached about 40% of the initial values and remained stable thereafter at 47–53 mg kg−1 h−1 CO2 (Fig. 1A). The respiration rate is best described by a hyperbolic relationship with storage time (R2 = 0.979, P < 0.000000007) and this pattern was very similar to that reported previously for unpeeled spears (Papadopoulou et al., 2001).
225
Fig. 1. Respiration rate (A) and ethylene production (B) of the unpeeled and peeled white asparagus spears of the cultivar ‘Dariana’ as well as of the peel during storage at 10 ◦ C for 7 d. Each data point is the mean of three replications and overall LSD at 5% is shown.
The respiration rate of the peeled spears at the beginning of storage at 10 ◦ C was about 148 mg kg−1 h−1 CO2 and showed a very similar pattern of change to that found in our study for unpeeled spears throughout the storage period (R2 = 0.935, P < 0.000001), without any significant differences between the peeling treatments (Fig. 1A). Lightly processed products have been reported to show either higher or lower respiration rates compared to the intact ones (Watada et al., 1996). At a storage temperature of 10 ◦ C, the increase of the respiration rate ranged from 8% for bell pepper to 152% for lettuce, while a decrease for zucchini, muskmelons and honeydew melons have been reported. The respiration rate of the peel at the beginning of storage at 10 ◦ C was high (328 mg kg−1 h−1 CO2 ), showing thereafter a very similar pattern of change with time (R2 = 0.935, P < 0.000001) to that found in our study for the unpeeled and peeled spears, but always remained at very high values (Fig. 1A). The respiration rate of peel was about 1.3–3.1 times higher than that of unpeeled spears, throughout the storage period. Agar et al. (1999) reported that the respiration rate of kiwifruit peel was about 2–4 times higher than that of unpeeled kiwifruit slices. That very high respiration rate of the peel is apparently due to the extensive wounding imposed by peeling (Brecht, 1995). In cv. ‘Grolim’, a similar pattern of spear respiration rate was observed during storage, although lower values for the respiration rate of the peel were recorded throughout the storage period (data not shown). 3.2. Effect of peeling on spear ethylene production during storage Ethylene production rate of the unpeeled spears at the beginning of storage at 10 ◦ C was 0.77 L kg−1 h−1 , decreasing rapidly by about 61% during the first 2 h, and remaining unchanged thereafter (Fig. 1B). Peeled spears had an ethylene production rate of about 0.56 L kg−1 h−1 at the beginning of storage at 10 ◦ C, which
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Table 1 Lightness, chroma, hue angle, fresh weight loss (%), toughness (N), soluble solids content (%), DPPH radical scavenging activity (mg of ascorbic acid equivalent antioxidant capacity 100 g−1 f.w.), total soluble phenols (mg of gallic acid equivalents g−1 f.w.) and nitrate contents (mg kg−1 f.w.) of unpeeled and peeled asparagus spears of two cultivars, before (Day 0) and after storage (Day 7) at 10 ◦ C for 7 days Lightness Day 0
Fresh weight loss
Toughness
Day 7
Chroma Day 0
Day 7
Hue angle Day 0
Day 7
Day 0
Day 7
Day 0
Day 7
cv. ‘Dariana’ Unpeeled Peeled
83.02 aa , b 80.17 d
77.82 e 77.29 e
6.25 c 5.24 d
11.80 a 11.33 a
106.26 c 109.20 ab
99.54 d 100.26 d
0.00 d 0.00 d
1.41 b 2.03 a
9.85 bc 4.77 d
8.66 c 4.70 d
cv. ‘Grolim’ Unpeeled Peeled
82.38 ab 80.43 cd
81.52 bc 80.81 cd
6.54 c 5.11 d
9.93 b 9.02 b
107.57 bc 110.45 a
98.93 d 100.05 d
0.00 d 0.00 d
0.71 c 1.18 bc
11.47 ab 8.84 c
11.99 a 8.86 c
Soluble solids Day 0
DPPH
Soluble phenols
Nitrates
Day 7
Day 0
Day 7
Day 0
Day 7
Day 0
Day 7
cv. ‘Dariana’ Unpeeled Peeled
7.03 a 7.17 a
5.87 de 6.03 cd
1.76 d 1.58 d
3.35 c 3.69 c
0.152 c 0.158 c
0.269 a 0.284 a
78.17 ab 78.27 ab
53.23 c 60.90 abc
cv. ‘Grolim’ Unpeeled Peeled
6.27 bc 6.40 b
5.87 de 5.57 e
5.51 b 5.74 b
7.21 a 7.16 a
0.214 b 0.204 b
0.267 a 0.265 a
70.23 abc 60.50 bc
79.50 a 65.80 abc
a b
Each value is the mean of three replicates. Different letters at two columns of each parameter denote significant differences (P < 0.05) between means according to Duncan’s multiple range test.
remained without significant change during the first 18 h and showed a slight decrease by 28 h, when it approached values of about 48% of the initial ones and remained unchanged thereafter (Fig. 1B). No significant differences between unpeeled and peeled spears were observed throughout the storage period. The peel had a very high ethylene production rate at the beginning of storage at 10 ◦ C (1.89 L kg−1 h−1 ), showing a decrease during the first 4 h. However, ethylene production increased again and peaked at 2.72 L kg−1 h−1 by 12 h, followed by a decrease to about 0.50 L kg−1 h−1 by 72 h and it remained unchanged thereafter (Fig. 1B). The peel had 1.5- to 11.5-fold higher ethylene production than the unpeeled spears throughout the storage period, indicating that the ethylene production was a proportional response to the extensive tissue wounding compared to the total tissue of the peel. Peel has also been found to be the largest source of ethylene production when compared to the unpeeled and peeled kiwifruit or compared to the unpeeled or peeled slices (Agar et al., 1999). In cv. ‘Grolim’, a quite similar pattern of spear ethylene production rate during storage was observed, although both unpeeled and peeled spears had lower ethylene production rates at the beginning of storage. Moreover, lower values for the ethylene production rate of the peel were recorded throughout the storage period (data not shown). 3.3. Effect of peeling on spear quality before storage The spears subjected to peeling had 81.29 ± 1.69% and 78.02 ± 2.80% of the weight of the unpeeled spears, while the peel represented 17.33 ± 1.50% and 20.81 ± 2.58% of it and 1.38 ± 0.19% and 1.17 ± 0.30% was lost as juice during peeling in cvs. ‘Dariana’ and ‘Grolim’, respectively. Before storage, the peeled spears showed a lower lightness and chroma and a higher hue angle, as well as being more tender than the unpeeled ones (Table 1), because the epidermis and the sclerenchyma cells were removed. The cell walls of the spear outer tissues (peel) contain more lignin than those of the inner tissues (Lipton, 1990). No significant effect of peeling on DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents before storage was observed (Table 1).
3.4. Effect of peeling on spear quality after storage The analysis of variance showed that peeling had a significant effect on lightness, chroma, hue angle, fresh weight loss and toughness, while the storage significantly affected all measured parameters with the exception of toughness and nitrates. As far as lightness, hue angle and fresh weight loss is concerned, a significant effect of the interaction of peeling and storage was also observed; however, most of the total variance was accounted due to storage (data not shown). At the end of storage, both unpeeled and peeled spears showed a decrease in lightness (cv. ‘Dariana’) and hue angle and an increase in chroma compared to spear color before storage, resulting in non-significant differences among them (Table 1). These results indicate that the peeled white asparagus spears do not exhibit browning, probably due to low tannin content as well as a low polyphenoloxidase activity. Fresh weight loss of the peeled spears after 7 d at 10 ◦ C was about 2.03%, that is about 44% higher than the loss of the unpeeled ones (Table 1). This is apparently due to the removal of the protective epidermal cells by peeling. However, in cv. ‘Grolim’ no significant differences between peeled and unpeeled spears were observed. In both unpeeled and peeled spears, toughness was not affected by storage; after 7 d at 10 ◦ C the peeled spears remained more tender than the unpeeled ones (Table 1). No significant differences among peeled and unpeeled spears regarding DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents were observed at the end of storage (Table 1). In both unpeeled and peeled spears, storage resulted in a decrease of soluble solids as well as in an increase of DPPH radical scavenging activity and total soluble phenols content. The decrease of soluble solids could be attributed, partially, to the respiratory activity. The respiration rate was similar for unpeeled and peeled spears, resulting in a similar consumption of carbohydrates reserves. Phenolic compounds have been reported to have multiple biological effects, including antioxidant activity (Kähkönen et al., 1999); a linear relationship between total phenolic content and total antioxidant activity was found (Yang et al., 2004). Although light processing has been reported to increase the phenolic content and the antioxidant capacity in carrot and lettuce (Howard and Griffin, 1993; Kang and Saltveit, 2002), this was not evident in the peeled spears of this experiment.
A.S. Siomos et al. / Postharvest Biology and Technology 50 (2008) 224–227
3.5. Effect of cultivar on spear quality The analysis of variance showed that cultivar had a significant effect on lightness, chroma, fresh weight loss, toughness, soluble solids, DPPH radical scavenging activity and soluble phenols. Moreover, its interaction with storage was significant on lightness, chroma, fresh weight loss, soluble solids, soluble phenols and nitrates. However, most of the total variance was accounted for due to cultivar only for DPPH radical scavenging activity as well as due to interaction of cultivar × storage only for nitrates (data not shown). DPPH radical scavenging activity was higher in both unpeeled and peeled spears, before and after storage, in cv. ‘Grolim’ than cv. ‘Dariana’ (Table 1). In conclusion, white asparagus spears subjected to peeling were more tender, they had a lower lightness and chroma and a higher hue angle compared to the unpeeled ones; however, both peeled and unpeeled spears had similar DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content. Peeling resulted in increased CO2 and C2 H4 production rates, but peel contributed to total increase of CO2 and C2 H4 production. As the measured metabolic activity of the peeled asparagus spears was similar to the one of the unpeeled, peeling did not adversely affect quality during storage. Acknowledgements This work was partially supported by the General Secretariat for Research and Technology (GSRT) of the Greek Ministry of Development, European Union-European Social Fund (EU-ESF), Operational Programme “Competitiveness” the 3rd Community Support Framework (3rd CSF) 2000–2006 and company Agrocom S.A., Krya Vrissi, Giannitsa, Greece. References Agar, I.T., Massantini, R., Hess-Pierce, B., Kader, A.A., 1999. Postharvest CO2 and ethylene production and quality maintenance of fresh-cut kiwifruit slices. J. Food Sci. 64, 433–440. Albanese, D., Russo, L., Cinquanta, L., Brasiello, A., Di Matteo, M., 2007. Physical and chemical changes in minimally processed green asparagus during cold-storage. Food Chem. 101, 274–280.
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An, J., Zhang, M., Lu, Q., 2007. Changes in some quality indexes in fresh-cut green asparagus pretreated with aqueous ozone and subsequent modified atmosphere packaging. J. Food Eng. 78, 340–344. Brand-Williams, W., Cuvelier, M.E., Berset, C., 1995. Use of free radical method to evaluate antioxidant activity. Food Sci. Technol. 28, 25–30. Brecht, J.K., 1995. Physiology of lightly processed fruits and vegetables. HortScience 30, 18–22. Cataldo, D.A., Haroon, M., Schrader, L.E., Youngs, V.L., 1975. Rapid colorimetric determination of nitrate in plant tissue by nitration of salicylic acid. Commun. Soil Sci. Plant Anal. 6, 71–80. Howard, L.R., Griffin, L.E., 1993. Lignin formation and surface discoloration of minimally processed carrot sticks. J. Food Sci. 58, 1065–1067, 1072. Huyskens-Keil, S., Kadau, R., 2003. Quality and shelf life of fresh-cut asparagus (Asparagus officinalis L.) in different packaging films. Acta Hortic. 600, 787–790. Kadau, R., Huyskens-Keil, S., Goßmann, M., Bütner, C., 2003. Postharvest quality dynamic of fresh-cut asparagus (Asparagus officinalis L.) in different film packaging. Acta Hortic. 599, 109–113. Kähkönen, M.P., Hopia, A.I., Vuorela, H.J., Rauha, J.P., Pihlaja, K., Kujala, T.S., Heinonen, M., 1999. Antioxidant activity of plant extracts containing phenolic compounds. J. Agric. Food Chem. 47, 3954–3962. Kang, H.M., Saltveit, M.E., 2002. Antioxidant capacity of lettuce leaf tissue increases after wounding. J. Agric. Food Chem. 50, 7536–7541. Lipton, W.J., 1990. Postharvest biology of fresh asparagus. Hortic. Rev. 12, 69– 115. Papadopoulou, P.P., Siomos, A.S., Dogras, C.C., 2001. Metabolism of etiolated and green asparagus before and after harvest. J. Hortic. Sci. Biotechnol. 76, 497– 500. Scalbert, A., Monties, B., Janin, G., 1989. Tannins in wood: comparison of different estimation methods. J. Agric. Food Chem. 37, 1324–1329. Scheer, C., Schonhof, I., Brückner, B., Schreiner, M., Knorr, D., 2003. Influence of a short-term storage on quality determining product characteristics of white prepared asparagus. Acta Hortic. 604, 437–441. Simón, A., Gonzáles-Fandos, E., Tobar, V., 2004. Influence of washing and packaging on the sensory and microbiological quality of fresh peeled white asparagus. J. Food Sci. 69, 6–12. Siomos, A.S., 2003. Quality, handling and storage of white asparagus. In: Dris, R., Niskanen, R., Jain, S.M. (Eds.), Crop Management and Postharvest Handling of Horticultural Products. Volume II. Fruits and Vegetables. Science Publishers, Inc., NH, pp. 65–88. Siomos, A.S., Sfakiotakis, E.M., Dogras, C.C., 2000. Modified atmosphere packaging of white asparagus spears: composition, color and textural quality responses to temperature and light. Sci. Hortic. 84, 1–13. Vinson, J.A., Hao, Y., Su, X., Zubik, L., 1998. Phenol antioxidant quantity and quality in foods: vegetables. J. Agric. Food Chem. 46, 3630–3634. Watada, A.E., Ko, N.P., Minott, D.A., 1996. Factors affecting quality of fresh-cut horticultural products. Postharvest Biol. Technol. 9, 115–125. Yang, J., Meyers, K.J., Van der Heide, J., Liu, R.H., 2004. Varietal differences in phenolic content and antioxidant and antiproliferative activities of onions. J. Agric. Food Chem. 52, 6787–6793.
Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Research Note
Peeling has no effect on respiration and ethylene production and only minimal effect on quality of fresh white asparagus spears Anastasios S. Siomos a,∗ , Dimitrios Gerasopoulos b , Pavlos Tsouvaltzis a , Athanasios Koukounaras a a b
Department of Horticulture, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Food Science and Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 20 December 2007 Accepted 19 May 2008 Keywords: Light processing Fresh cut Antioxidants Phenolics Color Toughness
a b s t r a c t Fresh white asparagus spears were subjected to peeling or left unpeeled before storage at 10 ◦ C for 7 days. CO2 and ethylene production rates were determined during storage, while color, toughness and DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content were determined before and after storage in both peeled and unpeeled spears. Peel CO2 and ethylene production rates were also determined during storage. Spears subjected to peeling were more tender, and had lower lightness and chroma and higher hue angle values compared to the unpeeled ones. However, both peeled and unpeeled spears had similar DPPH radical scavenging activity, soluble solids, total soluble phenols and nitrate contents. Peeling resulted in increased CO2 and C2 H4 production rates, but peel contributed to total increase of CO2 and C2 H4 production. As the measured metabolic activity of the peeled asparagus spears was similar to that of the unpeeled, peeling did not adversely affect quality during storage. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The market demand for lightly processed, ready-to-use vegetables is growing rapidly and asparagus is suitable for light processing, since it requires peeling before cooking. The physiology of the lightly processed fruit and vegetables is essentially the one of a wounded tissue and is characterized by increased respiration and ethylene production. Other consequences of wounding are chemical or physical in nature, such as oxidative browning reactions or enhanced rate of water loss. Moreover, in response to tissue wounding, a large number of secondary compounds are synthesized, many of which may affect the aroma, flavor, color and nutritional value of the product (Brecht, 1995). The intensity of the wounding response in the lightly processed plant tissues is proportional to the extent of wounding (Brecht, 1995). In asparagus, the wounding induced by peeling is expected to have a greater effect on spear metabolism compared to wounding induced by spear cutting at harvest (Papadopoulou et al., 2001). Therefore, peeled asparagus is expected to undergo accelerated deterioration and senescence.
∗ Corresponding author. Tel.: +30 2310 998646; fax: +30 2310 998609. E-mail address: [email protected] (A.S. Siomos). 0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.05.007
Color is one of the basic quality characteristics or specifications for sorting fresh white asparagus in commercial grades (Siomos et al., 2000; Siomos, 2003), but in recent years, consumers have become interested in nutritional and other compounds such as antioxidants and nitrates, respectively. Among 23 commonly consumed vegetables, asparagus has been reported to be the richest in total antioxidants, while the main components responsible for asparagus high bioactivity are phenols (Vinson et al., 1998). Although the effect of packaging materials on the quality of peeled asparagus has been extensively studied (Huyskens-Keil and Kadau, 2003; Kadau et al., 2003; Scheer et al., 2003; Simón et al., 2004; Albanese et al., 2007; An et al., 2007), the effect of peeling on respiration and ethylene production as well as on color, phenolic compounds, antioxidant components and nitrates of fresh white asparagus is still unknown. Therefore, this work investigates the effect of peeling on metabolic activity and quality aspects of fresh white asparagus spears. 2. Materials and methods White asparagus (Asparagus officinalis L.) spears of the cvs. ‘Dariana’ and ‘Grolim’ were harvested early in the morning from a commercial plantation near Gianitsa, Macedonia, Greece and placed into a styrofoam container to prevent exposure to light. After transportation for 3 h in darkness at ambient temperature (ca.
A.S. Siomos et al. / Postharvest Biology and Technology 50 (2008) 224–227
20 ◦ C), the spears were immersed in cool water (1–2 ◦ C) for 10 min, trimmed to 23 cm in length and washed thoroughly with tap water. Straight and undamaged spears, 16–24 mm in diameter with closed bracts were selected. The spears were divided in two groups: those of the first group were peeled with a sharp vegetable peeler starting 3 cm below the tip, while the spears of the second group were left unpeeled. The weight of the unpeeled and peeled spears as well as of the removed peel tissue was recorded. For each treatment three replications were used, each one consisting of five spears (200–220 g). The unpeeled and peeled spears as well as the removed peel of each replicate were placed in 3-L glass jars, ventilated with humidified air free of ethylene at a flow rate of 30 mL min−1 to ensure that the accumulated CO2 was always below the physiologically active levels of about 0.20%. The jars were kept in the dark at 10 ◦ C for 7 d. During storage, two 1 mL air samples were taken from the exit tube of the jars for the determination of CO2 and ethylene production rates. The CO2 concentration was measured using a CO2 /O2 analyzer (model Combo 280, David Bishop Instruments, UK), while the ethylene concentration was measured by using a Varian 3300 gas chromatograph (Varian Instruments, Walnut Creek, CA) equipped with a flame ionization detector. Before and after storage, the fresh weight, color, toughness and DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents were determined in unpeeled and peeled spears of each replication. Color measurements were made using a chromameter (Minolta CR-200, Minolta, Osaka, Japan). In the middle of each spear, two color measurements were made at two opposite sites. Spear toughness was measured using a Chatillon penetrometer (John Chatillon and Sons, New Gardens, NY) equipped with a plunger 3.2 mm in diameter and 9.5 mm in length (Siomos et al., 2000). In the middle of each spear, two toughness measurements were made at two opposite sites. After the determination of weight, color and toughness, the unpeeled and peeled spears of each replication were macerated in a blender for the determination of DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content. DPPH radical scavenging activity was determined according to Brand-Williams et al. (1995). Soluble solids content was measured in the juice of the blended material using a portable Atago PR-1 refractometer (Atago Co. Ltd., Tokyo, Japan). Total soluble phenol content was determined with the Folin–Ciocalteu procedure as described by Scalbert et al. (1989). Nitrate content was determined colorimetrically as described by Cataldo et al. (1975). Data were analyzed by analysis of variance (ANOVA) using a completely randomized design with three replications and the means were compared by LSD or by Duncan’s multiple range test at the 0.05 level with the MSTAT version 4.00/EM (Michigan State University). 3. Results and discussion 3.1. Effect of peeling on spear respiration rate during storage In cv. ‘Dariana’, the respiration rate of the unpeeled spears at the beginning of storage at 10 ◦ C was about 136 mg kg−1 h−1 CO2 , it decreased rapidly during the first 48 h, when it approached about 40% of the initial values and remained stable thereafter at 47–53 mg kg−1 h−1 CO2 (Fig. 1A). The respiration rate is best described by a hyperbolic relationship with storage time (R2 = 0.979, P < 0.000000007) and this pattern was very similar to that reported previously for unpeeled spears (Papadopoulou et al., 2001).
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Fig. 1. Respiration rate (A) and ethylene production (B) of the unpeeled and peeled white asparagus spears of the cultivar ‘Dariana’ as well as of the peel during storage at 10 ◦ C for 7 d. Each data point is the mean of three replications and overall LSD at 5% is shown.
The respiration rate of the peeled spears at the beginning of storage at 10 ◦ C was about 148 mg kg−1 h−1 CO2 and showed a very similar pattern of change to that found in our study for unpeeled spears throughout the storage period (R2 = 0.935, P < 0.000001), without any significant differences between the peeling treatments (Fig. 1A). Lightly processed products have been reported to show either higher or lower respiration rates compared to the intact ones (Watada et al., 1996). At a storage temperature of 10 ◦ C, the increase of the respiration rate ranged from 8% for bell pepper to 152% for lettuce, while a decrease for zucchini, muskmelons and honeydew melons have been reported. The respiration rate of the peel at the beginning of storage at 10 ◦ C was high (328 mg kg−1 h−1 CO2 ), showing thereafter a very similar pattern of change with time (R2 = 0.935, P < 0.000001) to that found in our study for the unpeeled and peeled spears, but always remained at very high values (Fig. 1A). The respiration rate of peel was about 1.3–3.1 times higher than that of unpeeled spears, throughout the storage period. Agar et al. (1999) reported that the respiration rate of kiwifruit peel was about 2–4 times higher than that of unpeeled kiwifruit slices. That very high respiration rate of the peel is apparently due to the extensive wounding imposed by peeling (Brecht, 1995). In cv. ‘Grolim’, a similar pattern of spear respiration rate was observed during storage, although lower values for the respiration rate of the peel were recorded throughout the storage period (data not shown). 3.2. Effect of peeling on spear ethylene production during storage Ethylene production rate of the unpeeled spears at the beginning of storage at 10 ◦ C was 0.77 L kg−1 h−1 , decreasing rapidly by about 61% during the first 2 h, and remaining unchanged thereafter (Fig. 1B). Peeled spears had an ethylene production rate of about 0.56 L kg−1 h−1 at the beginning of storage at 10 ◦ C, which
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Table 1 Lightness, chroma, hue angle, fresh weight loss (%), toughness (N), soluble solids content (%), DPPH radical scavenging activity (mg of ascorbic acid equivalent antioxidant capacity 100 g−1 f.w.), total soluble phenols (mg of gallic acid equivalents g−1 f.w.) and nitrate contents (mg kg−1 f.w.) of unpeeled and peeled asparagus spears of two cultivars, before (Day 0) and after storage (Day 7) at 10 ◦ C for 7 days Lightness Day 0
Fresh weight loss
Toughness
Day 7
Chroma Day 0
Day 7
Hue angle Day 0
Day 7
Day 0
Day 7
Day 0
Day 7
cv. ‘Dariana’ Unpeeled Peeled
83.02 aa , b 80.17 d
77.82 e 77.29 e
6.25 c 5.24 d
11.80 a 11.33 a
106.26 c 109.20 ab
99.54 d 100.26 d
0.00 d 0.00 d
1.41 b 2.03 a
9.85 bc 4.77 d
8.66 c 4.70 d
cv. ‘Grolim’ Unpeeled Peeled
82.38 ab 80.43 cd
81.52 bc 80.81 cd
6.54 c 5.11 d
9.93 b 9.02 b
107.57 bc 110.45 a
98.93 d 100.05 d
0.00 d 0.00 d
0.71 c 1.18 bc
11.47 ab 8.84 c
11.99 a 8.86 c
Soluble solids Day 0
DPPH
Soluble phenols
Nitrates
Day 7
Day 0
Day 7
Day 0
Day 7
Day 0
Day 7
cv. ‘Dariana’ Unpeeled Peeled
7.03 a 7.17 a
5.87 de 6.03 cd
1.76 d 1.58 d
3.35 c 3.69 c
0.152 c 0.158 c
0.269 a 0.284 a
78.17 ab 78.27 ab
53.23 c 60.90 abc
cv. ‘Grolim’ Unpeeled Peeled
6.27 bc 6.40 b
5.87 de 5.57 e
5.51 b 5.74 b
7.21 a 7.16 a
0.214 b 0.204 b
0.267 a 0.265 a
70.23 abc 60.50 bc
79.50 a 65.80 abc
a b
Each value is the mean of three replicates. Different letters at two columns of each parameter denote significant differences (P < 0.05) between means according to Duncan’s multiple range test.
remained without significant change during the first 18 h and showed a slight decrease by 28 h, when it approached values of about 48% of the initial ones and remained unchanged thereafter (Fig. 1B). No significant differences between unpeeled and peeled spears were observed throughout the storage period. The peel had a very high ethylene production rate at the beginning of storage at 10 ◦ C (1.89 L kg−1 h−1 ), showing a decrease during the first 4 h. However, ethylene production increased again and peaked at 2.72 L kg−1 h−1 by 12 h, followed by a decrease to about 0.50 L kg−1 h−1 by 72 h and it remained unchanged thereafter (Fig. 1B). The peel had 1.5- to 11.5-fold higher ethylene production than the unpeeled spears throughout the storage period, indicating that the ethylene production was a proportional response to the extensive tissue wounding compared to the total tissue of the peel. Peel has also been found to be the largest source of ethylene production when compared to the unpeeled and peeled kiwifruit or compared to the unpeeled or peeled slices (Agar et al., 1999). In cv. ‘Grolim’, a quite similar pattern of spear ethylene production rate during storage was observed, although both unpeeled and peeled spears had lower ethylene production rates at the beginning of storage. Moreover, lower values for the ethylene production rate of the peel were recorded throughout the storage period (data not shown). 3.3. Effect of peeling on spear quality before storage The spears subjected to peeling had 81.29 ± 1.69% and 78.02 ± 2.80% of the weight of the unpeeled spears, while the peel represented 17.33 ± 1.50% and 20.81 ± 2.58% of it and 1.38 ± 0.19% and 1.17 ± 0.30% was lost as juice during peeling in cvs. ‘Dariana’ and ‘Grolim’, respectively. Before storage, the peeled spears showed a lower lightness and chroma and a higher hue angle, as well as being more tender than the unpeeled ones (Table 1), because the epidermis and the sclerenchyma cells were removed. The cell walls of the spear outer tissues (peel) contain more lignin than those of the inner tissues (Lipton, 1990). No significant effect of peeling on DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents before storage was observed (Table 1).
3.4. Effect of peeling on spear quality after storage The analysis of variance showed that peeling had a significant effect on lightness, chroma, hue angle, fresh weight loss and toughness, while the storage significantly affected all measured parameters with the exception of toughness and nitrates. As far as lightness, hue angle and fresh weight loss is concerned, a significant effect of the interaction of peeling and storage was also observed; however, most of the total variance was accounted due to storage (data not shown). At the end of storage, both unpeeled and peeled spears showed a decrease in lightness (cv. ‘Dariana’) and hue angle and an increase in chroma compared to spear color before storage, resulting in non-significant differences among them (Table 1). These results indicate that the peeled white asparagus spears do not exhibit browning, probably due to low tannin content as well as a low polyphenoloxidase activity. Fresh weight loss of the peeled spears after 7 d at 10 ◦ C was about 2.03%, that is about 44% higher than the loss of the unpeeled ones (Table 1). This is apparently due to the removal of the protective epidermal cells by peeling. However, in cv. ‘Grolim’ no significant differences between peeled and unpeeled spears were observed. In both unpeeled and peeled spears, toughness was not affected by storage; after 7 d at 10 ◦ C the peeled spears remained more tender than the unpeeled ones (Table 1). No significant differences among peeled and unpeeled spears regarding DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate contents were observed at the end of storage (Table 1). In both unpeeled and peeled spears, storage resulted in a decrease of soluble solids as well as in an increase of DPPH radical scavenging activity and total soluble phenols content. The decrease of soluble solids could be attributed, partially, to the respiratory activity. The respiration rate was similar for unpeeled and peeled spears, resulting in a similar consumption of carbohydrates reserves. Phenolic compounds have been reported to have multiple biological effects, including antioxidant activity (Kähkönen et al., 1999); a linear relationship between total phenolic content and total antioxidant activity was found (Yang et al., 2004). Although light processing has been reported to increase the phenolic content and the antioxidant capacity in carrot and lettuce (Howard and Griffin, 1993; Kang and Saltveit, 2002), this was not evident in the peeled spears of this experiment.
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3.5. Effect of cultivar on spear quality The analysis of variance showed that cultivar had a significant effect on lightness, chroma, fresh weight loss, toughness, soluble solids, DPPH radical scavenging activity and soluble phenols. Moreover, its interaction with storage was significant on lightness, chroma, fresh weight loss, soluble solids, soluble phenols and nitrates. However, most of the total variance was accounted for due to cultivar only for DPPH radical scavenging activity as well as due to interaction of cultivar × storage only for nitrates (data not shown). DPPH radical scavenging activity was higher in both unpeeled and peeled spears, before and after storage, in cv. ‘Grolim’ than cv. ‘Dariana’ (Table 1). In conclusion, white asparagus spears subjected to peeling were more tender, they had a lower lightness and chroma and a higher hue angle compared to the unpeeled ones; however, both peeled and unpeeled spears had similar DPPH radical scavenging activity as well as soluble solids, total soluble phenols and nitrate content. Peeling resulted in increased CO2 and C2 H4 production rates, but peel contributed to total increase of CO2 and C2 H4 production. As the measured metabolic activity of the peeled asparagus spears was similar to the one of the unpeeled, peeling did not adversely affect quality during storage. Acknowledgements This work was partially supported by the General Secretariat for Research and Technology (GSRT) of the Greek Ministry of Development, European Union-European Social Fund (EU-ESF), Operational Programme “Competitiveness” the 3rd Community Support Framework (3rd CSF) 2000–2006 and company Agrocom S.A., Krya Vrissi, Giannitsa, Greece. References Agar, I.T., Massantini, R., Hess-Pierce, B., Kader, A.A., 1999. Postharvest CO2 and ethylene production and quality maintenance of fresh-cut kiwifruit slices. J. Food Sci. 64, 433–440. Albanese, D., Russo, L., Cinquanta, L., Brasiello, A., Di Matteo, M., 2007. Physical and chemical changes in minimally processed green asparagus during cold-storage. Food Chem. 101, 274–280.
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