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Effect of cutting styles on quality and antioxidant activity in fresh-cut pitaya fruit
Postharvest Biology and Technology 124 (2017) 1–7
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Effect of cutting styles on quality and antioxidant activity in fresh-cut pitaya fruit Xiaoan Li, Qinghong Long, Fan Gao, Cong Han, Peng Jin, Yonghua Zheng* College of Food Science and Technology, Nanjing Agricultural University, Nanjing 210095, Jiangsu, PR China
A R T I C L E I N F O
Article history: Received 5 June 2016 Received in revised form 22 September 2016 Accepted 23 September 2016 Available online xxx Keywords: Pitaya fruit Cutting styles Quality Reactive oxygen species Antioxidant activity
A B S T R A C T
The effect of different cutting styles on the quality and antioxidant activity of pitaya fruit during 4 d of storage at 15 C was investigated. Pitaya fruit was cut into slice, half-slice and quarter-slice, all in 1 cm of thickness, with corresponding wounding intensity (A/W) of 2.0, 2.9 and 3.7 cm2 g 1, respectively. Results: showed that cutting styles had little influence on fruit quality parameters such as vitamin C, soluble solids, titratable acidity and flesh color. While total phenolic content, antioxidant activity, and phenylalanine ammonia-lyase activity increased significantly with cutting wounding intensity at the first 2 d of storage. In addition, fresh-cut processing induced the reactive oxygen species (ROS) generation and enhanced the activity of antioxidant enzymes including catalase, superoxide dismutase and glutathione reductase at the initial storage time. These results demonstrated that cutting styles didn’t have much adverse effect on the organoleptic quality, but significantly induced the biosynthesis of phenolics and improved the antioxidant activity of fresh-cut pitaya fruit. Moreover, ROS may act as signaling molecules in the accumulation of phenolics in fresh-cut pitaya fruit. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Cutting is an essential procedure that divide intact products into smaller pieces in fresh-cut fruits and vegetables processing. This cutting operation will inevitably cause the tissue to suffer from wounding stress, which may accelerate the deterioration processes including water loss, oxidative browning, tissue softening and development of off-flavours, thus limiting the shelf-life of fresh-cut produce (Gil et al., 2006; Hodges and Toivonen, 2008). It is generally recognized that two types of phenolic metabolism responses will be triggered in plant tissues when a wounding stress occurs (Rhodes and Wooltorton, 1978). Firstly, the breakage of the plasma membrane induces the oxidative enzyme systems to react with the existing phenolic compounds, causing the oxidation of phenolics and the browning of tissues, which is adverse for maintaining of the produce quality (Saltveit, 2000). Secondly, when wounding stress occurs, plants produce injury signals to induce the production of more secondary metabolites including phenolic antioxidants to defense and heal the wounding damage (Ryan, 2000; Rakwal and Agrawal, 2003; Cisneros-Zevallos, 2003). Therefore, the wounding stress caused by cutting may increase the
* Corresponding author. E-mail address: [email protected] (Y. Zheng). http://dx.doi.org/10.1016/j.postharvbio.2016.09.009 0925-5214/ã 2016 Elsevier B.V. All rights reserved.
phenolic content and improve the antioxidant activity of fresh-cut fruits and vegetables depending on the balance between phenolic synthesis and oxidation (Reyes et al., 2007). This phenomenon has been confirmed in a number of fresh-cut produce such as carrot (Torres-Contreras et al., 2014; Surjadinata and Cisneros-Zevallos, 2012), celery (Vina and Chaves, 2006), lettuce (Zhan et al., 2012), broccoli (Benito Martinez-Hernandez et al., 2013), mushroom (Oms-Oliu et al., 2010), onions (Berno et al., 2014) and mangoes (Maribel Robles-Sanchez et al., 2013). The fruit of pitaya is a non-climacteric fruit with green scales on the rosy-red peel. The pulp is delicate and juicy and is interspersed with numerous small seeds (Sim et al., 2012). In recent years, pitaya fruit have drawn more attention worldwide, not only because of its sensorial properties and economic importance, but also for its high antioxidant activity owing to its high phenolic antioxidants content (Beltran-Orozco et al., 2009). Since the size of pitaya fruit is relatively large, fresh-cut produce may be more convenient for consumers. Previous study has shown that the increase of phenolic content and antioxidant activity in fresh-cut fruits and vegetables relies on the type of produce tissue (Reyes et al., 2007). Furthermore, the increase of phenolic compounds and enhancement of antioxidant activity in fresh-cut carrot increased with wounding severity (Surjadinata and Cisneros-Zevallos, 2012). However, no study has been done about the effect of cutting styles on quality and antioxidant activity in fresh-cut pitaya fruit.
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Therefore, we investigated the effect of cutting styles with different intensities on main quality parameters, total phenolics content, antioxidant activity and the production of ROS in fresh-cut pitaya fruit. 2. Materials and methods 2.1. Fruit material and processing Pitaya fruit (Hylocereus undatus cv. Shuijing) were obtained from local market in Nanjing, selected, washed and sterilized in 0.02% (v/v) of sodium hypochlorite (pH 6.5). Three different cutting styles were performed as shown in Fig. 1. The fruit were peeled manually, cut into slice (1 cm thickness), half-slice (1/2 section from a slice of 1 cm thickness) and quarter-slice (1/4 section from a slice of 1 cm thickness), with whole fruit serving as the control. The wounding intensity calculated by the method of Surjadinata and Cisneros-Zevallos (2012) was 2.0, 2.9 and 3.7 cm2 g 1 for slice, halfslice and quarter-slice, respectively. All the fruit were then packaged in 15 cm 10 cm 4 cm polypropylene containers and stored for 4 d at 15 C. Lower temperature would be more appropriate for pitaya fruit storage, but pitaya is a chilling sensitive fruit which displays chilling injuries including water-soaking, wilting and pulp browning when storing at 6 C (Nerd et al., 1999) and 8 C was recommended as the appropriate storage temperature for fresh-cut pitaya fruit (Chien et al., 2007). Thus the high storage temperature (15 C) used in this study is not recommended to keep the quality of fresh-cut pitaya fruit in industry, it was applied only to expedite the response to wounding stress (Reyes et al., 2007). Fruit samples were collected daily for analysis of quality parameters, total phenolics content, antioxidant activity and ROS production. 2.2. Quality parameters and total aerobic bacterial count assays The Vitamin C content was analyzed by the procedure of Arakawa et al. (1981). Frozen tissue sample (2 g) was extracted in 5 mL of 5% trichloroacetic acid (TCA) solution. The homogenate extracts were centrifuged at 12,000 g for 20 min at 4 C. And the extract supernatants were used for VC analysis. VC content was expressed as g kg 1 fresh weight, based on a standard curve. Total soluble solid (TSS) was measured by an Abbe refractometer (14081 S/N, USA). Titratable acidity (TA) was determined by the procedure of Jin et al. (2009). Flesh color was evaluated by
measuring L*, a*, and b* values using a colorimeter (Konica Minolta, Japan). In our previous experiments neither L* nor a* value of fresh-cut pitaya fruit showed significant variations during 4 d of storage, while b* value (yellow degree) increased gradually during storage, corresponding to flesh browning. Thus, b* value was used to reflect flesh color in this study. Total aerobic bacterial count (TABC) was analysed according to a standard enumeration method by Tomas-Callejas et al. (2012). TABC was expressed as log10 colony-forming unit per kilogram based on fresh weight (log CFU kg 1). 2.3. Total phenolics content measurement The total phenolics (TP) content was analysed according to the Folin-Ciocalteu procedure of Swain and Hillis (1959) with slight changes. Frozen tissue samples (5 g) were extracted in methanol (25 mL). The obtained homogenates were preserved in covered centrifuge tubes for 12 h in darkness at 4 C, and centrifuged at 12,000 g for 20 min. The extract supernatants were used for TP analysis. TP content was expressed as g kg 1 of GAE on a fresh weight basis. 2.4. Antioxidant activity (AOX) assay The antioxidant activity was analyzed through the method of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging. The extraction method was the same as the extracts prepared for the TP assay and the determination was carried out by the procedure of Brand-williams et al. (1995). Results were calculated with the formula as follows: DPPH radical scavenging activity (%) = [(A0
A1)/A0] 100
With A0 refers to absorbance of the control, A1 refers to absorbance of the samples (Gorinstein et al., 2004). 2.5. Phenylalanine ammonia lyase (PAL) activity assay The activity of PAL was measured according to the procedure of Ke and Saltveit (1986) with minor changes. Frozen tissue sample (1 g) was mixed and extracted in 5 mL of ice-cold borate buffer (50 mM, pH 8.5), which contained PVPP (40 g L 1), b-mercaptoethanol (5 mM) and EDTA (2 mM). The homogenate extracts were centrifuged at 12,000 g for 20 min at 4 C. The extract supernatant was prepared for enzyme analysis. Fifty millimole of borate buffer (2.8 mL) was blended with 0.5 mL of 20 mM L-phenylalanine, incubating for 10 min at 37 C, 0.7 mL of the enzyme extraction solution was mixed with the reaction system and the absorbance at 290 nm was determined promptly (OD0). The absorbance of the mixture (OD1) was measured again after another one hour incubation at 37 C. A unit of PAL activity was equivalent to a variation of 0.1 at 290 nm per second and it was expressed as U kg 1 based on protein content. 2.6. O2 and H2O2 measurements
Fig. 1. Different cutting styles applied in pitaya fruit. Whole (A), slice (B), half-slice (C) and quarter-slice (D).
The measurement of O2 production was based on the procedure of Elstner (1976) with slight modifications. Frozen sample (1 g) was extracted in 5 mL of phosphate buffer (100 mM, pH 7.8). The homogenate was centrifuged at 12,000 g for 20 min at 4 C. One milliliter of the supernatant was mixed with 1 mL of 1 mM hydroxylamine hydrochloride solution and incubated at 25 C for 1 h. Then 1 mL of 7 mM a-naphthylamine and 1 mL of 17 mM 4-Aminobenzene sulfonic acid were added to the reaction system and the mixture were incubated at 25 C for another 20 min, and the absorbance of the mixture after reaction was
X. Li et al. / Postharvest Biology and Technology 124 (2017) 1–7
measured at 530 nm. The O2 generation rate was calculated according to a standard curve based on sodium nitrite and expressed as mmol kg 1 s 1 NO2 produced based on protein content. The level of H2O2 was determined referred to the method of Patterson et al. (1984). Frozen tissue (1 g) was extracted in 5 mL of ice cold acetone and the homogenates were centrifuged at 12,000 g at 4 C for 20 min. Extract supernatants were prepared for H2O2 determination. The level of H2O2 was expressed as mmol kg 1 based on fresh weight. 2.7. Antioxidant enzymes activity determination The extraction of antioxidant defense system enzymes were performed according to Kang and Saltveit (2001) and the obtained enzyme supernatants were prepared for analysis. The activity of SOD was determined based on Rao et al. (1996). One unit of SOD activity was calculated as the quantity of enzyme which causes a 50% reduction of nitroblue tetrazolium under assay conditions and expressed as U kg 1 based on protein content. The activity of CAT was carried out by the procedure of Maehly and Chance (1954). One unit of CAT activity was calculated as the quantity of enzyme that deoxidized 1 mmol of H2O2 per second and expressed as U kg 1 based on protein content. The activity of APX was analyzed as described by Chen and Asada (1989). One unit of APX activity was calculated as the quantity of enzyme which oxidized 1 mmol of ascorbate per second and expressed as U kg 1 based on protein content. The activity of GR was measured based on the procedure of Klapheck et al. (1990). One unit of GR activity was calculated as the
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quantity of enzyme which deoxidized 1 mmol of GSSG per second and expressed as U kg 1 based on protein content. The protein contents of the samples were analyzed referred to the procedure of Bradford (1976), bovine serum albumin was used as the standard. 2.8. Statistical analysis Experiment results were analyzed by one-way analysis of variance (ANOVA) based on the SAS Version 9.2 (SAS Institute Inc., Cary, NC, USA). Duncan’s multiple range tests were used to compare means and differences at p < 0.05 were considered to be statistically significant. 3. Results 3.1. Effect of cutting styles on quality parameters and TABC in fresh-cut pitaya fruit All fresh-cut pitaya fruit showed slight decline in the content of vitamin C, titratable acidity and soluble solids content with storage duration, but no distinct differences were found among the tissues (Fig. 2A, B and C). Whole pitaya fruit showed no obvious change in b* value during storage, while for the fresh-cut tissues, the b* value stayed constant at the first 3 d of storage and then increased slowly (Fig. 2D). But samples with different cutting styles showed no dramatic differences in b* value (p > 0.05). TABC of whole pitaya fruit maintained at very low level during 4 d of storage, while in all fresh-cut tissues, this index showed an increasing trend during the storage period. No remarkable differences in TABC were observed among three cutting styles (p > 0.05, Fig. 3).
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Fig. 2. Effect of cutting styles on vitamin C (A), total soluble solids (B), total acids (C) and b* value (D) of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). VC content was expressed as g kg 1 based on fresh weight. TSS and TA ratio was expressed as percent (%). Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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As is presented in Fig. 4A, there was a very slow increase in TP content of whole pitaya fruit with storage time. Fresh-cut processing significantly (p < 0.05) induced the phenolics accumulation in pitaya fruit in comparison with whole fruit at the first 2 d of storage. This enhancement of TP content was intensified with the increase of wounding intensity (p < 0.05). On the second day of storage, TP contents reached their maximum values, it was increased by 63, 78, and 90% for slice, half-slice and quarterslice, respectively. While for the whole pitaya fruit, the increment was only 34%. After 2 d of storage, the fresh-cut pitaya tissues showed a decline in TP content, but it was still higher than the whole fruit, where the quarter-slice maintained the highest TP level. 3.3. Effect of cutting styles on the AOX in fresh-cut pitaya fruit The AOX of fresh-cut pitaya fruit was determined during storage as shown in Fig. 4B. For the whole pitaya fruit, only a slight increase of the AOX was observed. While all the three cutting styles showed much higher AOX (p < 0.05). After 2 d of storage, the fresh-cut pitaya fruit reached their maximum peaks in AOX and then began to decrease. The quarter-slice showed the highest increase (70%) as compared with whole pitaya fruit (15%) at the second day, followed by half-slice and slice (59% and 47% increment). The dynamics of the AOX was in consistent with that of TP content. 3.4. Effect of cutting styles on the activity of PAL in fresh-cut pitaya fruit The PAL activity of whole pitaya fruit showed very slight changes during storage, while for the fresh-cut tissues, the changes of PAL activity were much more intensive (p < 0.05, Fig. 4C). At day 2, whole, slice, half-slice and quarter-slice showed a 23%, 48%, 70%, 86% increase compared to day 0, respectively. Fresh-cut processing apparently induced the activity of PAL, which was increased with increasing wounding intensity. 3.5. Effect of cutting styles on the contents of O2 and H2O2 in fresh-cut pitaya fruit In whole pitaya fruit, O2 production rate increased very slowly with the storage duration. However, fresh-cut processing caused a rapid increase of O2 production. The rate of O2 production
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Fig. 3. Effect of cutting styles on total aerobic bacterial count of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). TABC was expressed as log10 CFU kg 1 based on fresh weight. Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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reached their maximum at the second day and decreased then (Fig. 5A). Quarter-slice showed slightly lower O2 production rate than slice and half-slice at the early stage of the storage. H2O2 content of whole pitaya fruit increased slightly during the whole storage, whereas a sharp increase at the first day and a following reduction was observed for all fresh-cut pitaya tissues (Fig. 5B). The quarter-slice showed the highest H2O2 level, followed by half-slice and slice (p < 0.05). 3.6. Effect of cutting styles on the antioxidant enzymes activity in fresh-cut pitaya fruit To understand the effect of cutting styles on the antioxidase system, the activities of SOD, CAT, APX and GR were determined on the whole pitaya fruit and fresh-cut tissues during storage (Fig. 6). The SOD activity of whole pitaya fruit showed a slow increase through storage time, while fresh-cut processing induced a much higher increase of SOD at the first 2 d of storage and a slight
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decrease then. During the whole storage time, the quarter-slice showed the highest SOD activity enhancement, followed by halfslice and slice (p < 0.05, Fig. 6A). For the whole pitaya fruit, CAT activity showed a similar variation compared to that of SOD. Cutting styles had no obvious effect on CAT activity during the first day, but after then, the three kind of fresh-cut tissues showed higher enzyme activity compared with the control (p < 0.05, Fig. 6B). APX activity had no significant change for whole pitaya fruit during the storage, while fresh-cut processing caused a decrease in APX activity at the first day and an increase later on. For the freshcut tissues, the APX activity declined with the increase of wounding intensity (p < 0.05, Fig. 6C). No remarkable change of GR activity was observed in whole pitaya fruit. Fresh-cut processing significantly induced an increase of GR at the first 2 d of storage and this effect was enhanced with the increase of wounding intensity (p < 0.05, Fig. 6D).
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The quality of pitaya fruit deteriorated gradually during postharvest storage, but fresh-cut processing had little adverse effect on the contents of VC, TSS and TA, all of which are important quality properties of pitaya fruit. Flesh color is another important quality characteristic of fresh-cut pitaya fruit, the increase in b* value is probably owing to the oxidation of total phenolics to quinones, which can be spontaneously polymerized to form brown pigments (Degl’Innocenti et al., 2005). In our experiment, browning color appeared on the surface of fresh-cut pitaya fruit and made them unacceptable after 4 d of storage. As shown in the TABC assay, fresh-cut processing inevitably increased the growth of microorganism in fresh-cut products. Previous studies have
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Fig. 6. Effect of cutting styles on the activity of pitaya SOD (A), CAT (B), APX (C) and GR (D) during storage at 15 C. Data are expressed as the mean SD (n = 3). One unit of each enzyme activity was expressed as U kg 1 based on protein content. Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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reported that commonly the total aerobic bacterial count in freshcut vegetable products varied from 6 to 9 log CFU kg 1 (Ragaert et al., 2007). All the results of quality evaluation suggested that different cutting styles didn’t have much adverse effect on the quality of pitaya fruit during 4 d of storage at 15 C. Cutting styles influenced the accumulation of TP in fresh-cut pitaya tissues at 15 C, and greater degrees of wounding intensity caused more enhancement of TP content. These results were similar to previous reports on fresh-cut produce such as carrots (Surjadinata and Cisneros-Zevallos, 2012), mushrooms (Oms-Oliu et al., 2010), and mangos (Maribel Robles-Sanchez et al., 2013). Phenol compounds are widely considered to be important secondary metabolites which make great contribution to AOX in fruits and vegetables (Heo et al., 2007; Kenny and O’Beirne, 2010). In this study, higher wounding intensity caused by cutting induced more production of total phenols, thus enhanced the antioxidant activity and showed higher AOX value. During the later stage of storage, TP content began to decrease possibly because of its decrease in synthesis rate and increase in utilization rate, which resulted in lower AOX level as well (Reyes et al., 2007). PAL is a critical enzyme in the phenylpropanoid metabolism (Hahlbrock and Scheel, 1989). The activation of PAL induced by wounding stress resulted in the synthesis and accumulation of TP (Peiser et al., 1998; Surjadinata and Cisneros-Zevallos, 2012). In our experiment, the PAL activity showed a similar variation trend to that of TP content and AOX. The increase of PAL activity during the early stage of the storage indicated that wounding stress can activate the biosynthetic pathway of phenolic compounds, thus accumulate higher level of TP and enhance AOX in fresh-cut pitaya fruit. After 2 d of storage, PAL activity decreased, being one of the reasons why TP contents and AOX reduced. Du et al. (2012) also discovered similar results in the research about fresh-cut carrot. ROS were considered as unavoidable by-products of aerobic metabolism (Dat et al., 2000). The ROS production in plant tissue is low under normal condition, but wounding stress can disorder the homeostasis in cells, enhance the respiration rate and trigger other sources of ROS such as amine oxidases and NADPH oxidases, leading to the increase of ROS production in the site or adjacent of wounded cells (Mittler, 2002). Previous researches on fresh-cut carrots have proposed that low levels of ROS may play a role as signaling molecules, triggering the activation of phenylpropanoid metabolism and inducing the synthesis of soluble phenolics (Jacobo-Velazquez et al., 2011; Jacobo-Velazquez and CisnerosZevallos, 2012). In our experiment, the level of ROS increased immediately after fresh-cut processing, and higher wounding intensity led to higher H2O2 level, in accordance with the activation of PAL and the accumulation of total phenolics, confirming the viewpoints of ROS as a signal for the stress-induced synthesis of phenolics in plant tissues (Dixon and Paiva, 1995; Razem and Bernards, 2003; Jacobo-Velazquez et al., 2015). Interestingly, lower O2 contents was observed in quarter-slice than in slice and halfslice at the early stage of the storage, probably because higher SOD activity caused by wounding stress converted more O2 into H2O2, similar to the observations on potato tubers and carrots (Razem and Bernards, 2003; Torres-Contreras et al., 2014). At the early storage after cutting process, CAT activity stayed unchanged, and APX activity was inhibited, but when ROS reached toxic levels, CAT and APX were activated to finely modulate ROS together. The decrease of ROS during the later storage may be attributed to the interaction among these detoxifying enzymes. These results indicated that ROS may play a role as signaling molecules in the wound induced accumulation of phenolics in fresh-cut pitaya fruit, and the antioxidant enzymes could finely modulate the ROS level, maintaining higher antioxidant activity of fresh-cut pitaya fruit (Jacobo-Velazquez et al., 2011, 2015).
In conclusion, cutting styles didn’t have much adverse effect on the organoleptic quality of pitaya fruit, however, cutting could induce the production of ROS, increase the accumulation of phenolics and enhance the antioxidant activity of fresh-cut pitaya fruit during the early storage time. These phenomena were more obvious in the cutting style with higher degree of wounding intensity. These results suggested that wounding stress caused by cutting can increase phenolics level and enhance the antioxidant activity of fresh-cut pitaya fruit, which may improve the nutritional value of the produce. Moreover, ROS may play a role as signaling molecules in the phenolic accumulation in pitaya fruit induced by wounding stress. Thus, by simply changing cutting styles and increasing wounding intensity, it is possible to provide consumer with fresh-cut pitaya fruit rich in phenolic antioxidants in regular diets. Acknowledgment This research was supported by Natural Science Foundation of China (No. 31471632). References Arakawa, N., Tsutsumi, K., Sanceda, N.G., Kurata, T., Inagaki, C., 1981. A rapid and sensitive method for the determination of ascorbic acid using 4,7-diphenyl1,10-phenanthroline. Agric. Biolog. Chem. 45, 1289–1290. Beltran-Orozco, M.C., Oliva-Coba, T.G., Gallardo-Velazquez, T., Osorio-Revilla, G., 2009. Ascorbic acid, phenolic content, and antioxidant capacity of red, cherry, yellow and white types of pitaya cactus fruit (stenocereus stellatus riccobono). Agrociencia 43 (2), 153–161. Benito Martinez-Hernandez, G., Artes-Hernandez, F., Gomez, P.A., Formica, A.C., Artes, F., 2013. Combination of electrolysed water: UV-C and superatmospheric O2 packaging for improving fresh-cut broccoli quality. Postharvest Biol. Technol. 76, 125–134. Berno, N.D., Tezotto-Uliana, J.V., dos Santos Dias, C.T., Kluge, R.A., 2014. Storage temperature and type of cut affect the biochemical and physiological characteristics of fresh-cut purple onions. Postharvest Biol. Technol. 93, 91–96. Bradford, M.M., 1976. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72, 248–254. Brand-williams, W., Cuvelier, M.E., Berset, C., 1995. Use of a free radical method to evaluate antioxidant activity. LWT—Food Sci. Technol. 28 (1), 25–30. Chen, G.X., Asada, K., 1989. Ascorbate peroxidase in tea leaves-occurrence of 2 isozymes and the differences in their enzymatic and molecular-properties. Plant Cell Physiol. 30 (7), 987–998. Chien, P.J., Sheu, F., Lin, H.R., 2007. Quality assessment of low molecular weight chitosan coating on sliced red pitayas. J. Food Eng. 79, 736–740. Cisneros-Zevallos, L., 2003. The use of controlled postharvest abiotic stresses as a tool for enhancing the nutraceutical content and adding-value of fresh fruits and vegetables. J. Food Sci. 68, 1560–1565. Dat, J., Vandenabeele, S., Vranova, E., Van Montagu, M., Inze, D., Van Breusegem, F., 2000. Dual action of the active oxygen species during plant stress responses. Cell. Mol. Life Sci. 57, 779–795. Degl’Innocenti, E., Guidi, L., Pardossi, A., Tognoni, F., 2005. Biochemical study of leaf browning in minimally processed leaves of lettuce (Lactuca sativa L. var. acephala). J. Agric. Food Chem. 53, 9980–9984. Dixon, R.A., Paiva, N.L., 1995. Stress-induced phenylpropanoid metabolism. Plant Cell 7 (7), 1085–1097. Du, W.X., Avena-Bustillos, R.J., Breksa III, A.P., McHugh, T.H., 2012. Effect of UV-B light and different cutting styles on antioxidant enhancement of commercial fresh-cut carrot products. Food Chem. 134, 1862–1869. Elstner, E.F., 1976. Inhibition of nitrite formation hydroxylam-mouium-chloride: a simple assay for superoxide dismutase. Anal. Biochem. 70, 616–620. Gil, M.I., Aguayo, E., Kader, A.A., 2006. Quality changes and nutrient retention in fresh-cut versus whole fruits during storage. J. Agric. Food Chem. 54, 4284– 4296. Gorinstein, S., Haruenkit, R., Park, Y.S., Jung, S.T., Zachwieja, Z., Jastrzebski, Z., Katrich, E., Trakhtenberg, S., Belloso, O.M., 2004. Bioactive compounds and antioxidant potential in fresh and dried jaffa(r) sweeties, a new kind of citrus fruit. J. Sci. Food Agric. 84 (12), 1459–1463. Hahlbrock, K., Scheel, D., 1989. Physiology and molecular-biology of phenylpropanoid metabolism. Annu. Rev. Plant Physiol. Plant Mol. Biol. 40, 347– 369. Heo, H.J., Kim, Y.J., Chung, D., Kim, D.O., 2007. Antioxidant capacities of individual and combined phenolics in a model system. Food Chem. 104 (1), 87–92. Hodges, D.M., Toivonen, P.M.A., 2008. Quality of fresh-cut fruits and vegetables as affected by exposure to abiotic stress. Postharvest Biol. Technol. 48, 155–162.
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Ragaert, P., Devlieghere, F., Debevere, J., 2007. Role of microbiological and physiological spoilage mechanisms during storage of minimally processed vegetables. Postharvest Biol. Technol. 44, 185–194. Rakwal, R., Agrawal, G.K., 2003. Wound signaling-coordination of the octadecanoid and mapk pathways. Plant Physiol. Biochem. 41 (10), 855–861. Rao, M.V., Paliyath, C., Ormrod, D.P., 1996. Ultraviolet-B and ozone-induced biochemical changes in antioxidant enzymes of arabidopsis thaliana. Plant Physiol. 110 (1), 125–136. Razem, F.A., Bernards, M.A., 2003. Reactive oxygen species production in association with suberization: evidence for an nadph-dependent oxidase. J. Exp. Bot. 54 (384), 935–941. Reyes, L.F., Villarreal, J.E., Cisneros-Zevallos, L., 2007. The increase in antioxidant capacity after wounding depending on the type of fruit or vegetable tissue. Food Chem. 101, 1254–1262. Rhodes, M.J., Wooltorton, L.S.C., 1978. The biosynthesis of phenolic compounds in wounded plant storage tissue. In: Kahl, G. (Ed.), Biochemistry of Wounded Tissues. Walter de Gruyter & Co, Berlin,German, pp. 243–286. Ryan, C.A., 2000. The systemin signaling pathway: differential activation of plant defensive genes. Biochim. Biophys. Acta 1477 (1–2), 112–121. Saltveit, M.E., 2000. Wound induced changes in phenolic metabolism and tissue browning are altered by heat shock. Postharvest Biol. Technol. 21 (1), 61–69. Sim, H.L., Hong, Y.K., Yoon, W.B., Yuk, H.G., 2012. Behavior of Salmonella spp. and natural microbiota on fresh-cut dragon fruits at different storage temperatures. Int. J. Food Microbiol. 160, 239–244. Surjadinata, B.B., Cisneros-Zevallos, L., 2012. Biosynthesis of phenolic antioxidants in carrot tissue increases with wounding intensity. Food Chem. 134 (2), 615– 624. Swain, T., Hillis, W.E., 1959. The phenolics constituents of Prunus domestica: I. The quantitative analysis of phenolics constituents. J. Sci. Food Agric. 10, 63–68. Tomas-Callejas, A., Oton, M., Artes, F., Artes-Hernandez, F., 2012. Combined effect of UV-C pretreatment and high oxygen packaging for keeping the quality of freshcut Tatsoi baby leaves. Innovative Food Sci. Emerg. Technol. 14, 115–121. Torres-Contreras, A.M., Nair, V., Cisneros-Zevallos, L., Jacobo-Velazquez, D.A., 2014. Plants as biofactories: stress-induced production of chlorogenic acid isomers in potato tubers as affected by wounding intensity and storage time. Ind. Crops Prod. 62, 61–66. Vina, S.Z., Chaves, A.R., 2006. Antioxidant responses in minimally processed celery during refrigerated storage. Food Chem. 94 (1), 68–74. Zhan, L.J., Li, Y., Hu, J.Q., Pang, L.Y., Fan, H.P., 2012. Browning inhibition and quality preservation of fresh-cut romaine lettuce exposed to high intensity light. Innovative Food Sci. Emerg. Technol. 14, 70–76.
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Effect of cutting styles on quality and antioxidant activity in fresh-cut pitaya fruit Xiaoan Li, Qinghong Long, Fan Gao, Cong Han, Peng Jin, Yonghua Zheng* College of Food Science and Technology, Nanjing Agricultural University, Nanjing 210095, Jiangsu, PR China
A R T I C L E I N F O
Article history: Received 5 June 2016 Received in revised form 22 September 2016 Accepted 23 September 2016 Available online xxx Keywords: Pitaya fruit Cutting styles Quality Reactive oxygen species Antioxidant activity
A B S T R A C T
The effect of different cutting styles on the quality and antioxidant activity of pitaya fruit during 4 d of storage at 15 C was investigated. Pitaya fruit was cut into slice, half-slice and quarter-slice, all in 1 cm of thickness, with corresponding wounding intensity (A/W) of 2.0, 2.9 and 3.7 cm2 g 1, respectively. Results: showed that cutting styles had little influence on fruit quality parameters such as vitamin C, soluble solids, titratable acidity and flesh color. While total phenolic content, antioxidant activity, and phenylalanine ammonia-lyase activity increased significantly with cutting wounding intensity at the first 2 d of storage. In addition, fresh-cut processing induced the reactive oxygen species (ROS) generation and enhanced the activity of antioxidant enzymes including catalase, superoxide dismutase and glutathione reductase at the initial storage time. These results demonstrated that cutting styles didn’t have much adverse effect on the organoleptic quality, but significantly induced the biosynthesis of phenolics and improved the antioxidant activity of fresh-cut pitaya fruit. Moreover, ROS may act as signaling molecules in the accumulation of phenolics in fresh-cut pitaya fruit. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Cutting is an essential procedure that divide intact products into smaller pieces in fresh-cut fruits and vegetables processing. This cutting operation will inevitably cause the tissue to suffer from wounding stress, which may accelerate the deterioration processes including water loss, oxidative browning, tissue softening and development of off-flavours, thus limiting the shelf-life of fresh-cut produce (Gil et al., 2006; Hodges and Toivonen, 2008). It is generally recognized that two types of phenolic metabolism responses will be triggered in plant tissues when a wounding stress occurs (Rhodes and Wooltorton, 1978). Firstly, the breakage of the plasma membrane induces the oxidative enzyme systems to react with the existing phenolic compounds, causing the oxidation of phenolics and the browning of tissues, which is adverse for maintaining of the produce quality (Saltveit, 2000). Secondly, when wounding stress occurs, plants produce injury signals to induce the production of more secondary metabolites including phenolic antioxidants to defense and heal the wounding damage (Ryan, 2000; Rakwal and Agrawal, 2003; Cisneros-Zevallos, 2003). Therefore, the wounding stress caused by cutting may increase the
* Corresponding author. E-mail address: [email protected] (Y. Zheng). http://dx.doi.org/10.1016/j.postharvbio.2016.09.009 0925-5214/ã 2016 Elsevier B.V. All rights reserved.
phenolic content and improve the antioxidant activity of fresh-cut fruits and vegetables depending on the balance between phenolic synthesis and oxidation (Reyes et al., 2007). This phenomenon has been confirmed in a number of fresh-cut produce such as carrot (Torres-Contreras et al., 2014; Surjadinata and Cisneros-Zevallos, 2012), celery (Vina and Chaves, 2006), lettuce (Zhan et al., 2012), broccoli (Benito Martinez-Hernandez et al., 2013), mushroom (Oms-Oliu et al., 2010), onions (Berno et al., 2014) and mangoes (Maribel Robles-Sanchez et al., 2013). The fruit of pitaya is a non-climacteric fruit with green scales on the rosy-red peel. The pulp is delicate and juicy and is interspersed with numerous small seeds (Sim et al., 2012). In recent years, pitaya fruit have drawn more attention worldwide, not only because of its sensorial properties and economic importance, but also for its high antioxidant activity owing to its high phenolic antioxidants content (Beltran-Orozco et al., 2009). Since the size of pitaya fruit is relatively large, fresh-cut produce may be more convenient for consumers. Previous study has shown that the increase of phenolic content and antioxidant activity in fresh-cut fruits and vegetables relies on the type of produce tissue (Reyes et al., 2007). Furthermore, the increase of phenolic compounds and enhancement of antioxidant activity in fresh-cut carrot increased with wounding severity (Surjadinata and Cisneros-Zevallos, 2012). However, no study has been done about the effect of cutting styles on quality and antioxidant activity in fresh-cut pitaya fruit.
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Therefore, we investigated the effect of cutting styles with different intensities on main quality parameters, total phenolics content, antioxidant activity and the production of ROS in fresh-cut pitaya fruit. 2. Materials and methods 2.1. Fruit material and processing Pitaya fruit (Hylocereus undatus cv. Shuijing) were obtained from local market in Nanjing, selected, washed and sterilized in 0.02% (v/v) of sodium hypochlorite (pH 6.5). Three different cutting styles were performed as shown in Fig. 1. The fruit were peeled manually, cut into slice (1 cm thickness), half-slice (1/2 section from a slice of 1 cm thickness) and quarter-slice (1/4 section from a slice of 1 cm thickness), with whole fruit serving as the control. The wounding intensity calculated by the method of Surjadinata and Cisneros-Zevallos (2012) was 2.0, 2.9 and 3.7 cm2 g 1 for slice, halfslice and quarter-slice, respectively. All the fruit were then packaged in 15 cm 10 cm 4 cm polypropylene containers and stored for 4 d at 15 C. Lower temperature would be more appropriate for pitaya fruit storage, but pitaya is a chilling sensitive fruit which displays chilling injuries including water-soaking, wilting and pulp browning when storing at 6 C (Nerd et al., 1999) and 8 C was recommended as the appropriate storage temperature for fresh-cut pitaya fruit (Chien et al., 2007). Thus the high storage temperature (15 C) used in this study is not recommended to keep the quality of fresh-cut pitaya fruit in industry, it was applied only to expedite the response to wounding stress (Reyes et al., 2007). Fruit samples were collected daily for analysis of quality parameters, total phenolics content, antioxidant activity and ROS production. 2.2. Quality parameters and total aerobic bacterial count assays The Vitamin C content was analyzed by the procedure of Arakawa et al. (1981). Frozen tissue sample (2 g) was extracted in 5 mL of 5% trichloroacetic acid (TCA) solution. The homogenate extracts were centrifuged at 12,000 g for 20 min at 4 C. And the extract supernatants were used for VC analysis. VC content was expressed as g kg 1 fresh weight, based on a standard curve. Total soluble solid (TSS) was measured by an Abbe refractometer (14081 S/N, USA). Titratable acidity (TA) was determined by the procedure of Jin et al. (2009). Flesh color was evaluated by
measuring L*, a*, and b* values using a colorimeter (Konica Minolta, Japan). In our previous experiments neither L* nor a* value of fresh-cut pitaya fruit showed significant variations during 4 d of storage, while b* value (yellow degree) increased gradually during storage, corresponding to flesh browning. Thus, b* value was used to reflect flesh color in this study. Total aerobic bacterial count (TABC) was analysed according to a standard enumeration method by Tomas-Callejas et al. (2012). TABC was expressed as log10 colony-forming unit per kilogram based on fresh weight (log CFU kg 1). 2.3. Total phenolics content measurement The total phenolics (TP) content was analysed according to the Folin-Ciocalteu procedure of Swain and Hillis (1959) with slight changes. Frozen tissue samples (5 g) were extracted in methanol (25 mL). The obtained homogenates were preserved in covered centrifuge tubes for 12 h in darkness at 4 C, and centrifuged at 12,000 g for 20 min. The extract supernatants were used for TP analysis. TP content was expressed as g kg 1 of GAE on a fresh weight basis. 2.4. Antioxidant activity (AOX) assay The antioxidant activity was analyzed through the method of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging. The extraction method was the same as the extracts prepared for the TP assay and the determination was carried out by the procedure of Brand-williams et al. (1995). Results were calculated with the formula as follows: DPPH radical scavenging activity (%) = [(A0
A1)/A0] 100
With A0 refers to absorbance of the control, A1 refers to absorbance of the samples (Gorinstein et al., 2004). 2.5. Phenylalanine ammonia lyase (PAL) activity assay The activity of PAL was measured according to the procedure of Ke and Saltveit (1986) with minor changes. Frozen tissue sample (1 g) was mixed and extracted in 5 mL of ice-cold borate buffer (50 mM, pH 8.5), which contained PVPP (40 g L 1), b-mercaptoethanol (5 mM) and EDTA (2 mM). The homogenate extracts were centrifuged at 12,000 g for 20 min at 4 C. The extract supernatant was prepared for enzyme analysis. Fifty millimole of borate buffer (2.8 mL) was blended with 0.5 mL of 20 mM L-phenylalanine, incubating for 10 min at 37 C, 0.7 mL of the enzyme extraction solution was mixed with the reaction system and the absorbance at 290 nm was determined promptly (OD0). The absorbance of the mixture (OD1) was measured again after another one hour incubation at 37 C. A unit of PAL activity was equivalent to a variation of 0.1 at 290 nm per second and it was expressed as U kg 1 based on protein content. 2.6. O2 and H2O2 measurements
Fig. 1. Different cutting styles applied in pitaya fruit. Whole (A), slice (B), half-slice (C) and quarter-slice (D).
The measurement of O2 production was based on the procedure of Elstner (1976) with slight modifications. Frozen sample (1 g) was extracted in 5 mL of phosphate buffer (100 mM, pH 7.8). The homogenate was centrifuged at 12,000 g for 20 min at 4 C. One milliliter of the supernatant was mixed with 1 mL of 1 mM hydroxylamine hydrochloride solution and incubated at 25 C for 1 h. Then 1 mL of 7 mM a-naphthylamine and 1 mL of 17 mM 4-Aminobenzene sulfonic acid were added to the reaction system and the mixture were incubated at 25 C for another 20 min, and the absorbance of the mixture after reaction was
X. Li et al. / Postharvest Biology and Technology 124 (2017) 1–7
measured at 530 nm. The O2 generation rate was calculated according to a standard curve based on sodium nitrite and expressed as mmol kg 1 s 1 NO2 produced based on protein content. The level of H2O2 was determined referred to the method of Patterson et al. (1984). Frozen tissue (1 g) was extracted in 5 mL of ice cold acetone and the homogenates were centrifuged at 12,000 g at 4 C for 20 min. Extract supernatants were prepared for H2O2 determination. The level of H2O2 was expressed as mmol kg 1 based on fresh weight. 2.7. Antioxidant enzymes activity determination The extraction of antioxidant defense system enzymes were performed according to Kang and Saltveit (2001) and the obtained enzyme supernatants were prepared for analysis. The activity of SOD was determined based on Rao et al. (1996). One unit of SOD activity was calculated as the quantity of enzyme which causes a 50% reduction of nitroblue tetrazolium under assay conditions and expressed as U kg 1 based on protein content. The activity of CAT was carried out by the procedure of Maehly and Chance (1954). One unit of CAT activity was calculated as the quantity of enzyme that deoxidized 1 mmol of H2O2 per second and expressed as U kg 1 based on protein content. The activity of APX was analyzed as described by Chen and Asada (1989). One unit of APX activity was calculated as the quantity of enzyme which oxidized 1 mmol of ascorbate per second and expressed as U kg 1 based on protein content. The activity of GR was measured based on the procedure of Klapheck et al. (1990). One unit of GR activity was calculated as the
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quantity of enzyme which deoxidized 1 mmol of GSSG per second and expressed as U kg 1 based on protein content. The protein contents of the samples were analyzed referred to the procedure of Bradford (1976), bovine serum albumin was used as the standard. 2.8. Statistical analysis Experiment results were analyzed by one-way analysis of variance (ANOVA) based on the SAS Version 9.2 (SAS Institute Inc., Cary, NC, USA). Duncan’s multiple range tests were used to compare means and differences at p < 0.05 were considered to be statistically significant. 3. Results 3.1. Effect of cutting styles on quality parameters and TABC in fresh-cut pitaya fruit All fresh-cut pitaya fruit showed slight decline in the content of vitamin C, titratable acidity and soluble solids content with storage duration, but no distinct differences were found among the tissues (Fig. 2A, B and C). Whole pitaya fruit showed no obvious change in b* value during storage, while for the fresh-cut tissues, the b* value stayed constant at the first 3 d of storage and then increased slowly (Fig. 2D). But samples with different cutting styles showed no dramatic differences in b* value (p > 0.05). TABC of whole pitaya fruit maintained at very low level during 4 d of storage, while in all fresh-cut tissues, this index showed an increasing trend during the storage period. No remarkable differences in TABC were observed among three cutting styles (p > 0.05, Fig. 3).
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Fig. 2. Effect of cutting styles on vitamin C (A), total soluble solids (B), total acids (C) and b* value (D) of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). VC content was expressed as g kg 1 based on fresh weight. TSS and TA ratio was expressed as percent (%). Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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As is presented in Fig. 4A, there was a very slow increase in TP content of whole pitaya fruit with storage time. Fresh-cut processing significantly (p < 0.05) induced the phenolics accumulation in pitaya fruit in comparison with whole fruit at the first 2 d of storage. This enhancement of TP content was intensified with the increase of wounding intensity (p < 0.05). On the second day of storage, TP contents reached their maximum values, it was increased by 63, 78, and 90% for slice, half-slice and quarterslice, respectively. While for the whole pitaya fruit, the increment was only 34%. After 2 d of storage, the fresh-cut pitaya tissues showed a decline in TP content, but it was still higher than the whole fruit, where the quarter-slice maintained the highest TP level. 3.3. Effect of cutting styles on the AOX in fresh-cut pitaya fruit The AOX of fresh-cut pitaya fruit was determined during storage as shown in Fig. 4B. For the whole pitaya fruit, only a slight increase of the AOX was observed. While all the three cutting styles showed much higher AOX (p < 0.05). After 2 d of storage, the fresh-cut pitaya fruit reached their maximum peaks in AOX and then began to decrease. The quarter-slice showed the highest increase (70%) as compared with whole pitaya fruit (15%) at the second day, followed by half-slice and slice (59% and 47% increment). The dynamics of the AOX was in consistent with that of TP content. 3.4. Effect of cutting styles on the activity of PAL in fresh-cut pitaya fruit The PAL activity of whole pitaya fruit showed very slight changes during storage, while for the fresh-cut tissues, the changes of PAL activity were much more intensive (p < 0.05, Fig. 4C). At day 2, whole, slice, half-slice and quarter-slice showed a 23%, 48%, 70%, 86% increase compared to day 0, respectively. Fresh-cut processing apparently induced the activity of PAL, which was increased with increasing wounding intensity. 3.5. Effect of cutting styles on the contents of O2 and H2O2 in fresh-cut pitaya fruit In whole pitaya fruit, O2 production rate increased very slowly with the storage duration. However, fresh-cut processing caused a rapid increase of O2 production. The rate of O2 production
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Fig. 3. Effect of cutting styles on total aerobic bacterial count of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). TABC was expressed as log10 CFU kg 1 based on fresh weight. Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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Storage time(day) Fig. 4. Effect of cutting styles on total phenolic contents (A), antioxidant capacity (B) and PAL activity (C) of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). TP content was expressed as g kg 1 of GAE on a fresh weight basis, AOX was expressed as percent (%) and a unit of PAL activity was expressed as U kg 1 based on protein content. Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
reached their maximum at the second day and decreased then (Fig. 5A). Quarter-slice showed slightly lower O2 production rate than slice and half-slice at the early stage of the storage. H2O2 content of whole pitaya fruit increased slightly during the whole storage, whereas a sharp increase at the first day and a following reduction was observed for all fresh-cut pitaya tissues (Fig. 5B). The quarter-slice showed the highest H2O2 level, followed by half-slice and slice (p < 0.05). 3.6. Effect of cutting styles on the antioxidant enzymes activity in fresh-cut pitaya fruit To understand the effect of cutting styles on the antioxidase system, the activities of SOD, CAT, APX and GR were determined on the whole pitaya fruit and fresh-cut tissues during storage (Fig. 6). The SOD activity of whole pitaya fruit showed a slow increase through storage time, while fresh-cut processing induced a much higher increase of SOD at the first 2 d of storage and a slight
X. Li et al. / Postharvest Biology and Technology 124 (2017) 1–7
decrease then. During the whole storage time, the quarter-slice showed the highest SOD activity enhancement, followed by halfslice and slice (p < 0.05, Fig. 6A). For the whole pitaya fruit, CAT activity showed a similar variation compared to that of SOD. Cutting styles had no obvious effect on CAT activity during the first day, but after then, the three kind of fresh-cut tissues showed higher enzyme activity compared with the control (p < 0.05, Fig. 6B). APX activity had no significant change for whole pitaya fruit during the storage, while fresh-cut processing caused a decrease in APX activity at the first day and an increase later on. For the freshcut tissues, the APX activity declined with the increase of wounding intensity (p < 0.05, Fig. 6C). No remarkable change of GR activity was observed in whole pitaya fruit. Fresh-cut processing significantly induced an increase of GR at the first 2 d of storage and this effect was enhanced with the increase of wounding intensity (p < 0.05, Fig. 6D).
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Storage time(day) Fig. 5. Effect of cutting styles on O2 production (A) and H2O2 content (B) of pitaya fruit during storage at 15 C. Data are expressed as the mean SD (n = 3). The O2 production was expressed as mmol kg 1 s 1 NO2 produced based on protein content and the level of H2O2 was expressed as mmol kg 1 based on fresh weight. Data points carrying different letters for the same storage time indicate significant statistically differences (p < 0.05).
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The quality of pitaya fruit deteriorated gradually during postharvest storage, but fresh-cut processing had little adverse effect on the contents of VC, TSS and TA, all of which are important quality properties of pitaya fruit. Flesh color is another important quality characteristic of fresh-cut pitaya fruit, the increase in b* value is probably owing to the oxidation of total phenolics to quinones, which can be spontaneously polymerized to form brown pigments (Degl’Innocenti et al., 2005). In our experiment, browning color appeared on the surface of fresh-cut pitaya fruit and made them unacceptable after 4 d of storage. As shown in the TABC assay, fresh-cut processing inevitably increased the growth of microorganism in fresh-cut products. Previous studies have
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Fig. 6. Effect of cutting styles on the activity of pitaya SOD (A), CAT (B), APX (C) and GR (D) during storage at 15 C. Data are expressed as the mean SD (n = 3). One unit of each enzyme activity was expressed as U kg 1 based on protein content. Data points carrying different letters for the same storage time indicate statistically significant differences (p < 0.05).
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X. Li et al. / Postharvest Biology and Technology 124 (2017) 1–7
reported that commonly the total aerobic bacterial count in freshcut vegetable products varied from 6 to 9 log CFU kg 1 (Ragaert et al., 2007). All the results of quality evaluation suggested that different cutting styles didn’t have much adverse effect on the quality of pitaya fruit during 4 d of storage at 15 C. Cutting styles influenced the accumulation of TP in fresh-cut pitaya tissues at 15 C, and greater degrees of wounding intensity caused more enhancement of TP content. These results were similar to previous reports on fresh-cut produce such as carrots (Surjadinata and Cisneros-Zevallos, 2012), mushrooms (Oms-Oliu et al., 2010), and mangos (Maribel Robles-Sanchez et al., 2013). Phenol compounds are widely considered to be important secondary metabolites which make great contribution to AOX in fruits and vegetables (Heo et al., 2007; Kenny and O’Beirne, 2010). In this study, higher wounding intensity caused by cutting induced more production of total phenols, thus enhanced the antioxidant activity and showed higher AOX value. During the later stage of storage, TP content began to decrease possibly because of its decrease in synthesis rate and increase in utilization rate, which resulted in lower AOX level as well (Reyes et al., 2007). PAL is a critical enzyme in the phenylpropanoid metabolism (Hahlbrock and Scheel, 1989). The activation of PAL induced by wounding stress resulted in the synthesis and accumulation of TP (Peiser et al., 1998; Surjadinata and Cisneros-Zevallos, 2012). In our experiment, the PAL activity showed a similar variation trend to that of TP content and AOX. The increase of PAL activity during the early stage of the storage indicated that wounding stress can activate the biosynthetic pathway of phenolic compounds, thus accumulate higher level of TP and enhance AOX in fresh-cut pitaya fruit. After 2 d of storage, PAL activity decreased, being one of the reasons why TP contents and AOX reduced. Du et al. (2012) also discovered similar results in the research about fresh-cut carrot. ROS were considered as unavoidable by-products of aerobic metabolism (Dat et al., 2000). The ROS production in plant tissue is low under normal condition, but wounding stress can disorder the homeostasis in cells, enhance the respiration rate and trigger other sources of ROS such as amine oxidases and NADPH oxidases, leading to the increase of ROS production in the site or adjacent of wounded cells (Mittler, 2002). Previous researches on fresh-cut carrots have proposed that low levels of ROS may play a role as signaling molecules, triggering the activation of phenylpropanoid metabolism and inducing the synthesis of soluble phenolics (Jacobo-Velazquez et al., 2011; Jacobo-Velazquez and CisnerosZevallos, 2012). In our experiment, the level of ROS increased immediately after fresh-cut processing, and higher wounding intensity led to higher H2O2 level, in accordance with the activation of PAL and the accumulation of total phenolics, confirming the viewpoints of ROS as a signal for the stress-induced synthesis of phenolics in plant tissues (Dixon and Paiva, 1995; Razem and Bernards, 2003; Jacobo-Velazquez et al., 2015). Interestingly, lower O2 contents was observed in quarter-slice than in slice and halfslice at the early stage of the storage, probably because higher SOD activity caused by wounding stress converted more O2 into H2O2, similar to the observations on potato tubers and carrots (Razem and Bernards, 2003; Torres-Contreras et al., 2014). At the early storage after cutting process, CAT activity stayed unchanged, and APX activity was inhibited, but when ROS reached toxic levels, CAT and APX were activated to finely modulate ROS together. The decrease of ROS during the later storage may be attributed to the interaction among these detoxifying enzymes. These results indicated that ROS may play a role as signaling molecules in the wound induced accumulation of phenolics in fresh-cut pitaya fruit, and the antioxidant enzymes could finely modulate the ROS level, maintaining higher antioxidant activity of fresh-cut pitaya fruit (Jacobo-Velazquez et al., 2011, 2015).
In conclusion, cutting styles didn’t have much adverse effect on the organoleptic quality of pitaya fruit, however, cutting could induce the production of ROS, increase the accumulation of phenolics and enhance the antioxidant activity of fresh-cut pitaya fruit during the early storage time. These phenomena were more obvious in the cutting style with higher degree of wounding intensity. These results suggested that wounding stress caused by cutting can increase phenolics level and enhance the antioxidant activity of fresh-cut pitaya fruit, which may improve the nutritional value of the produce. Moreover, ROS may play a role as signaling molecules in the phenolic accumulation in pitaya fruit induced by wounding stress. Thus, by simply changing cutting styles and increasing wounding intensity, it is possible to provide consumer with fresh-cut pitaya fruit rich in phenolic antioxidants in regular diets. Acknowledgment This research was supported by Natural Science Foundation of China (No. 31471632). References Arakawa, N., Tsutsumi, K., Sanceda, N.G., Kurata, T., Inagaki, C., 1981. A rapid and sensitive method for the determination of ascorbic acid using 4,7-diphenyl1,10-phenanthroline. Agric. Biolog. Chem. 45, 1289–1290. Beltran-Orozco, M.C., Oliva-Coba, T.G., Gallardo-Velazquez, T., Osorio-Revilla, G., 2009. Ascorbic acid, phenolic content, and antioxidant capacity of red, cherry, yellow and white types of pitaya cactus fruit (stenocereus stellatus riccobono). Agrociencia 43 (2), 153–161. Benito Martinez-Hernandez, G., Artes-Hernandez, F., Gomez, P.A., Formica, A.C., Artes, F., 2013. Combination of electrolysed water: UV-C and superatmospheric O2 packaging for improving fresh-cut broccoli quality. Postharvest Biol. Technol. 76, 125–134. Berno, N.D., Tezotto-Uliana, J.V., dos Santos Dias, C.T., Kluge, R.A., 2014. Storage temperature and type of cut affect the biochemical and physiological characteristics of fresh-cut purple onions. Postharvest Biol. Technol. 93, 91–96. Bradford, M.M., 1976. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72, 248–254. Brand-williams, W., Cuvelier, M.E., Berset, C., 1995. Use of a free radical method to evaluate antioxidant activity. LWT—Food Sci. Technol. 28 (1), 25–30. Chen, G.X., Asada, K., 1989. Ascorbate peroxidase in tea leaves-occurrence of 2 isozymes and the differences in their enzymatic and molecular-properties. Plant Cell Physiol. 30 (7), 987–998. Chien, P.J., Sheu, F., Lin, H.R., 2007. Quality assessment of low molecular weight chitosan coating on sliced red pitayas. J. Food Eng. 79, 736–740. Cisneros-Zevallos, L., 2003. The use of controlled postharvest abiotic stresses as a tool for enhancing the nutraceutical content and adding-value of fresh fruits and vegetables. J. Food Sci. 68, 1560–1565. Dat, J., Vandenabeele, S., Vranova, E., Van Montagu, M., Inze, D., Van Breusegem, F., 2000. Dual action of the active oxygen species during plant stress responses. Cell. Mol. Life Sci. 57, 779–795. Degl’Innocenti, E., Guidi, L., Pardossi, A., Tognoni, F., 2005. Biochemical study of leaf browning in minimally processed leaves of lettuce (Lactuca sativa L. var. acephala). J. Agric. Food Chem. 53, 9980–9984. Dixon, R.A., Paiva, N.L., 1995. Stress-induced phenylpropanoid metabolism. Plant Cell 7 (7), 1085–1097. Du, W.X., Avena-Bustillos, R.J., Breksa III, A.P., McHugh, T.H., 2012. Effect of UV-B light and different cutting styles on antioxidant enhancement of commercial fresh-cut carrot products. Food Chem. 134, 1862–1869. Elstner, E.F., 1976. Inhibition of nitrite formation hydroxylam-mouium-chloride: a simple assay for superoxide dismutase. Anal. Biochem. 70, 616–620. Gil, M.I., Aguayo, E., Kader, A.A., 2006. Quality changes and nutrient retention in fresh-cut versus whole fruits during storage. J. Agric. Food Chem. 54, 4284– 4296. Gorinstein, S., Haruenkit, R., Park, Y.S., Jung, S.T., Zachwieja, Z., Jastrzebski, Z., Katrich, E., Trakhtenberg, S., Belloso, O.M., 2004. Bioactive compounds and antioxidant potential in fresh and dried jaffa(r) sweeties, a new kind of citrus fruit. J. Sci. Food Agric. 84 (12), 1459–1463. Hahlbrock, K., Scheel, D., 1989. Physiology and molecular-biology of phenylpropanoid metabolism. Annu. Rev. Plant Physiol. Plant Mol. Biol. 40, 347– 369. Heo, H.J., Kim, Y.J., Chung, D., Kim, D.O., 2007. Antioxidant capacities of individual and combined phenolics in a model system. Food Chem. 104 (1), 87–92. Hodges, D.M., Toivonen, P.M.A., 2008. Quality of fresh-cut fruits and vegetables as affected by exposure to abiotic stress. Postharvest Biol. Technol. 48, 155–162.
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