Curcumin-Based Photodynamic Sterilization for Preservation of Fresh-Cut Hami Melon

Transcription

1 molecules Article Curcumin-Based Photodynamic Sterilization for Preservation of Fresh-Cut Hami Melon Yilin Lin 1,, Jiamiao Hu 1,, Shiyang Li 1, Siti Sarah Hamzah 2, Huiqin Jiang 1, Arong Zhou 1, Shaoxiao Zeng 1 and Shaoling Lin 1, * 1 College of Food Science, Fujian Agriculture and Forestry University, Fuzhou , Fujian, China 2 Institute for Medical Research, Jalan Pahang, Kuala Lumpur, Malaysia * Correspondence: shaoling.lin@fafu.edu.cn; Tel.: These authors contributed equally to this work. Academic Editors: Derek J. McPhee and Francisco J. Barba Received: 24 May 2019; Accepted: 25 June 2019; Published: 27 June 2019 Abstract: Fresh-cut fruits and vegetables are the main sources of foodborne illness outbreaks with implicated pathogens such as Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes. This study aimed at investigating the influence of two key parameters (concentration of curcumin and illumination time) on the effects of curcumin-based photodynamic sterilization on the preservation of fresh-cut Hami melons. The results indicated that illumination with 50 µmol/l curcumin for 60 min using a blue LED lamp reduced the total aerobic microorganism count by ~1.8 log CFU/g in fresh-cut Hami melons. Besides this, the effects of photodynamic sterilization on the soluble solids content, color, water content, firmness, and sensory indices of the fresh-cut Hami melons were also evaluated. Compared to the control group, photodynamic sterilization can effectively delay the browning rate and maintain the luminosity, firmness, water content, and soluble solids content of fresh-cut Hami melon. The sensory quality was indeed preserved well after 9 days of storage in a fridge. These results showed that photodynamic sterilization is an effective and promising technology to prolong the shelf life of fresh-cut Hami melons. Keywords: Hami melon; curcumin; photodynamic; colony number; storage quality 1. Introduction Fruits are an integral part of our daily diet and are rich sources of vitamins, micronutrients, and antioxidants [1]. Driven by increasing demand for convenient, ready-to-eat foods, the market of fresh-cut fruits (also known as pre-cut fruits) has grown rapidly. However, alongside this trend, foodborne disease outbreaks associated with fresh fruits have also risen globally. Recent statistics showed that the pathogens that most frequently contribute to foodborne illness outbreaks linked to fresh fruits and vegetables are Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes. Hami melon (also known as cantaloupe) is a type of muskmelon, well known for its aesthetic appearance and sweet flavor. This melon is considered the king of melons [2]. Due to its richness in nutrients and good taste, Hami melon is a popular fruit item used to make fresh-cut fruits. However, mechanical damage during fresh-cut processing often destroys the tissues of Hami melon, leading to the loss of nutrients and aromatic odor. Furthermore, mechanical damage also facilitates microbial growth, speeding the process of fruit degradation [3]. In addition, the high sugar content of Hami melon further accelerates the growth of microorganisms, which greatly shortens its shelf life. Besides this, long storage periods will also cause fresh-cut fruits to lose water content and become brownish in color, degrading the taste [4]. Presently, a number of approaches including coating [5], ultra-violet and violet-blue light irradiation [6 8], modified atmosphere packaging [9], and chlorine dioxide treatment [10] have been Molecules 2019, 24, 2374; doi: /molecules

2 Molecules 2019, 24, of 10 developed for preserving fresh-cut fruits, but these processes are relatively expensive. Therefore, developing novel approaches that are cheaper alternatives for fresh-cut fruit preservation has great potential in the food industry [11]. Photodynamic sterilization technology (PDS) is an environmentally friendly, non-thermal sterilization method. The underlying mechanism involves photosensitizers being activated by lights of certain wavelengths and absorbing the energy of photons to produce reactive oxygen species. These active substances then kill pathogenic microorganisms without damaging the adjacent tissues and cells. PDS was originally used for treating malignant tumors [12], papillomatosis [13], endodontic infection [14], and blood product disinfection [15], and then successfully applied to foods in the 1980s [16,17]. Compared to traditional sterilization technology, PDS possesses a stronger germicidal effect and is less expensive. In addition, this novel technology may also preserve the respective color, fragrance, and freshness of the foods. Currently, a number of novel photosensitizers have been identified or synthesized, including both antimicrobial and anti-cancer photosensitizers [18]. Among these photosensitizers, curcumin, a natural plant phenolic food additive, is widely used as a photosensitizer in PDS due to its low cost and safety [17,19]. Studies have already shown that the photodynamic action of curcumin can effectively kill malignant tumor cells [20 22]. Recently, scientists also demonstrated curcumin as a photosensitizer that can kill a range of pathogenic microorganisms in food even at low curcumin concentration ( µmol/l) [23 26]. Furthermore, this method has already shown great potential in the preservation of fresh-cut fruits. For example, curcumin-mediated photodynamic sterilization was successfully applied in the preservation of fresh-cut Fuji apples [27]. Therefore, in this study, curcumin-mediated photodynamic treatment was conducted to determine its potential application for the preservation of fresh-cut Hami melon during storage at 4 C. 2. Results and Discussion 2.1. Antibacterial Activity of PDS Treatment The effects of photosensitizer concentration and exposure time on the antibacterial activity of curcumin-mediated PDS for fresh-cut Hami melon preservation are shown in Tables 1 and 2. The data showed that the bacterial count of the fresh-cut Hami melon markedly increased with storage time. Meanwhile, the curcumin-mediated photodynamic treatment significantly dampened the bacterial growth compared to the control group (p < 0.05) (Tables 1 and 2). Illumination with 50 µmol/l curcumin decreased the bacterial count in the fresh-cut Hami melon from 3.97 (log CFU/g) in the control group to 2.59 (log CFU/g) at 1 day post-treatment. This effect was still obvious till the ninth day of storage as evidenced by the decrease in the bacterial count from 6.81 (log CFU/g) in the control group to 4.95 (log CFU/g) in the fresh-cut Hami melon treated with 50 µmol/l curcumin and blue light, indicating that PDS treatment resulted in ~1.8 log CFU/g reduction when compared to the control after 9 days of storage (Table 1). These results showed that curcumin as a photosensitizer can effectively inhibit bacterial growth in fresh-cut Hami melon. Indeed, our results here are consistent with a previous report in which 2 µmol/l of curcumin and 510 s of illumination was demonstrated to reduce E. coli counts by 0.95 logs at 4 C in fresh-cut apple [27].

3 Molecules 2019, 24, of 10 Table 1. Effects of photosensitizer curcumin concentrations on the antibacterial activity of curcumin-based photodynamic sterilization for the preservation of fresh-cut Hami melons. Storage (day) Bacterial Count (log CFU/g) Curcumin Concentration 0 µmol/l 10 µmol/l 20 µmol/l 40 µmol/l 50 µmol/l ± 0.04 a 3.76 ± 0.01 b 3.04 ± 0.11 d 3.26 ± 0.05 c 2.59 ± 0.09 e ± 0.02 a 4.94 ± 0.02 a 4.91 ± 0.09 b 4.67 ± 0.01 c 3.81 ± 0.01 d ± 0.01 a 5.38 ± 0.09 b 5.23 ± 0.07 c 5.00 ± 0.08 d 4.23 ± 0.03 e ± 0.07 a 5.91 ± 0.04 b 5.75 ± 0.02 b 5.04 ± 0.03 c 4.77 ± 0.04 c ± 0.04 a 6.20 ± 0.08 b 5.87 ± 0.05 c 5.11 ± 0.07 c 4.95 ± 0.02 c Note: The data in the table indicate the mean ± standard deviation, and significant differences exist among the data with different superscript letters in the same row (p < 0.05). Table 2. Effects of different exposure time on the antibacterial activity of curcumin-based photodynamic sterilization for the preservation of fresh-cut Hami melons. Storage (day) Bacterial Count (log CFU/g) Exposure Time 0 min 5 min 30 min 60 min 90 min ± 0.03 a 3.45 ± 0.08 b 3.28 ± 0.01 c 2.65 ± 0.04 d 2.68 ± 0.06 d ± 0.02 a 4.26 ± 0.01 b 4.00 ± 0.08 c 3.67 ± 0.02 d 3.68 ± 0.01 d ± 0.05 a 5.32 ± 0.08 b 4.94 ± 0.12 c 4.20 ± 0.01 c 4.26 ± 0.09 c ± 0.10 a 5.99 ± 0.11 b 5.40 ± 0.01 c 4.61 ± 0.07 d 4.65 ± 0.08 d ± 0.07 a 6.11 ± 0.10 b 5.90 ± 0.01 b 4.86 ± 0.08 c 4.90 ± 0.01 c Note: The data in the table indicate the mean ± standard deviation, and significant differences exist among the data with different superscript letters in the same row (p < 0.05). Hami melon samples sprayed with curcumin solution at 50 µmol/l and illuminated for different time periods were next analyzed for the photodynamic antibacterial effect. Generally, the germicidal efficacy increased with prolonged exposure time (Table 2), but beyond 60 min, no further significant increases were obtained. Notably, even the shortest illumination time in our study (5 min) already led to a significant decrease in the bacterial count compared to the control group (p < 0.05). Therefore, the optimal illumination time was chosen as 60 minutes, which was used throughout this study. In a previous study, similar findings were observed that the bacterial killing effect of PDS was more pronounced with prolonged illumination time but leveled off when irradiation time was further increased [28] Effects on Soluble Solids Content The soluble solid content is an important quality indicator of fresh-cut fruits during storage [29] and is closely associated with the flavor, taste, and nutrition of the fruits. As shown in Figure 1, the content of soluble solids in fresh-cut Hami melon decreased with increasing storage time in both the control and photodynamic-treated Hami melon samples (Figure 2). In the control group, the contents of soluble solids were 13.44%, 12.83%, 12.54%, 11.84%, and 10.92% after 1 day, 3 days, 5 days, 7 days, and 9 days of storage, respectively. The indices of the contents of the soluble solids in the PDS-treated group after 1 day, 3 days, 5 days, 7 days, and 9 days of storage were 13.37%, 13.14%, 12.66%, 12.06%, and 11.71%, respectively. The contents of the soluble solids included sugar, acid, vitamins, and minerals which were influenced by a number of factors, including microbial metabolic activities [30]. Figure 1 indicates that PDS significantly changed the content of soluble solids compared to the control group only at Day 9 (p < 0.05). It is possible that microbial respiration affected by PDS, especially during long-term storage of the fruit, contributed to this phenomenon. Indeed, a previous study also demonstrated that applying an antimicrobial coating on fresh-cut mango not only inhibited

4 The water content of fresh-cut fruits is an important factor that influences the sensory quality and consumer perception [30]. In fact, it also affects the physiological metabolism of the fresh-cut fruits, in addition to making fresh-cut Hami melon stiff and full. As shown in Figure 3, the water content decreased under storage conditions. After 9 days of storage, the water contents decreased to 71.93% and 89.04% of the initial moisture content in the control and PDS-treated groups, Molecules 2019, 24, of 10 Molecules 2019, 24, x 4 of 11 mango the growth not only of microorganisms inhibited the growth but also of slowed microorganisms down the decrease but also of slowed the soluble down solids the decrease content during of the soluble storage solids [31]. content during storage [31]. Figure 1. Effects of photodynamic sterilization (PDS) technology on the content of soluble solids in Figure fresh-cut 1. Hami Effects melons. of photodynamic The PDS conditions sterilization included (PDS) 50 technology µmol/l curcumin on the content and 60 of min soluble of illumination solids in fresh-cut Hami melons. The PDS conditions included 50 μmol/l curcumin and 60 min of Molecules with2019, LED24, light. x **p < 0.01 compared to the control group without photosensitizer and light treatment. 5 of 11 illumination Data points represent with LED the light. mean **p and < 0.01 standard compared deviation to the ofcontrol three experiments. group without photosensitizer and light treatment. Data points represent the mean and standard deviation of three experiments Effects on Color Color is also an important characteristic which determines the sensory quality of fresh-cut fruits [29,32]. To quantify the color changes of fresh-cut Hami melon samples, the L*a*b* coordinates were measured, in which the L* value is the lightness of fruit, while the a* and b* values represent the redness and yellowness, respectively. As shown in Figure 2A, the L* values of the Hami melon samples decreased slowly during storage due to enzymatic browning on the cut surface. Besides this, the L* values of samples with PDS treatment were slightly higher than those of the control group. Furthermore, the downward trend with a sharper slope in samples without PDS suggests that PDS can slow the reduction of the L* value in fresh-cut Hami melon. A similar phenomenon was also reported in a previous study in which results showed that PDS treatment may not only inhibit bacterial growth but also show anti-browning effects and prevent changes in the surface color of fresh-cut apple [27]. Besides this, the a* (redness) value and b* values (yellowness) of the fresh-cut Hami melon showed a similar pattern during storage, in which both decreased sharply after the fifth day (Figure 2B and C). Additionally, significant differences were also observed in the a* and b* values between the treated and control samples after 5 days of storage (p < 0.05). Taken together, these results demonstrate that PDS can effectively maintain the surface color of fresh-cut Hami melon throughout the storage period. Figure 2. Effect of photodynamic sterilization technology on the L* (lightness) value (A), a* (redness) value Figure (B), 2. Effect and b* of (yellowness) photodynamic value sterilization (C) of fresh-cut technology Hami on melons. the L* (lightness) The PDS conditions value (A), a* used (redness) were 50 value µmol/l (B), curcumin and b* (yellowness) and 60 minvalue exposure (C) of to fresh-cut LED light. Hami *p < 0.05, melons. **p < The 0.01, PDS ***p conditions < , ****p used < were compared μmol/l curcumin to the control and 60 group min exposure without photosensitizer LED light. *p and < 0.05, light**p treatment. < 0.01, ***p Data < , points represent ****p < the mean compared and standard to the control deviation group of three without experiments. photosensitizer and light treatment. Data points represent the mean and standard deviation of three experiments Effects on the Water Content

5 Molecules 2019, 24, of Effects on Color Color is also an important characteristic which determines the sensory quality of fresh-cut fruits [29,32]. To quantify the color changes of fresh-cut Hami melon samples, the L*a*b* coordinates were measured, in which the L* value is the lightness of fruit, while the a* and b* values represent the redness and yellowness, respectively. As shown in Figure 2A, the L* values of the Hami melon samples decreased slowly during storage due to enzymatic browning on the cut surface. Besides this, the L* values of samples with PDS treatment were slightly higher than those of the control group. Furthermore, the downward trend with a sharper slope in samples without PDS suggests that PDS can slow the reduction of the L* value in fresh-cut Hami melon. A similar phenomenon was also reported in a previous study in which results showed that PDS treatment may not only inhibit bacterial growth but also show anti-browning effects and prevent changes in the surface color of fresh-cut apple [27]. Besides this, the a* (redness) value and b* values (yellowness) of the fresh-cut Hami melon showed a similar pattern during storage, in which both decreased sharply after the fifth day (Figure 2B,C). Additionally, significant differences were also observed in the a* and b* values between the treated and control samples after 5 days of storage (p < 0.05). Taken together, these results demonstrate that PDS can effectively maintain the surface color of fresh-cut Hami melon throughout the storage period Effects on the Water Content The water content of fresh-cut fruits is an important factor that influences the sensory quality and consumer perception [30]. In fact, it also affects the physiological metabolism of the fresh-cut fruits, in addition to making fresh-cut Hami melon stiff and full. As shown in Figure 3, the water content decreased under storage conditions. After 9 days of storage, the water contents decreased to 71.93% and 89.04% of the initial moisture content in the control and PDS-treated groups, respectively. The higher water content percentage in the photodynamically sterilized fresh-cut Hami melon might also result from, at least partially, the antibacterial effects of PDS. This can be explained by the fact that rapid microbial growth can result in reduced water content of the fruit. Similar results were also found by Tao et al., who reported that fresh-cut apple with PDS treatment showed less water loss during storage time compared to control samples [27]. These results suggest that PDS treatment may Molecules 2019, 24, x 6 of 11 contribute to the maintenance of the water content in fresh-cut Hami melons during their storage. Figure 3. Effects of photodynamic sterilization technology on the water content of fresh-cut Hami melons. Figure 3. The Effects PDSof conditions photodynamic used sterilization were 50 µmol/l technology curcumin on the andwater 60 min content exposure of fresh-cut to LED Hami light. **p melons. < 0.01, The ****p PDS < conditions compared used were to the50 control μmol/l group curcumin without and photosensitizer 60 min exposure and to light LED treatment. light. **p < Data 0.01, points ****p < represent compared the meanto and the standard control group deviation without of three photosensitizer experiments. and light treatment. Data points represent the mean and standard deviation of three experiments Effects on the Firmness 2.5. Effects Firmness on the is an Firmness important parameter reflecting the texture of a fruit [29,32] and may be affected by the ripening, Firmness softening, is an important and aging parameter of the fruits reflecting [33]. As the shown texture inof Figure a fruit 4, [29,32] duringand themay first three be affected days, an by upward the ripening, trendsoftening, was observed and aging theof firmness the fruits of the [33]. fresh-cut As shown Hami in Figure melon, 4, which during may the be first due three to days, an upward trend was observed in the firmness of the fresh-cut Hami melon, which may be due to an increase in toughness during storage. Then, the firmness of the fresh-cut Hami melon decreased with increasing storage duration. Several possible mechanisms may be related to this phenomenon, including the rapid growth of microorganisms which convert starch into soluble sugars, transforming the protopectin and pectin into pectic acids and destroying the spatial structure

6 points represent the mean and standard deviation of three experiments Effects on the Firmness Firmness is an important parameter reflecting the texture of a fruit [29,32] and may be affected Molecules 2019, 24, of 10 by the ripening, softening, and aging of the fruits [33]. As shown in Figure 4, during the first three days, an upward trend was observed in the firmness of the fresh-cut Hami melon, which may be due to an increase an increase toughness in toughness during during storage. storage. Then, the Then, firmness the firmness of the fresh-cut of the fresh-cut Hami melon Hami decreased melon with decreased increasing with storage increasing duration. storage Several duration. possible Several mechanisms possible mechanisms may be related may to this be related phenomenon, to this phenomenon, including the rapid including growththe of microorganisms rapid growth of which microorganisms convert starch which into soluble convert sugars, starch transforming into soluble sugars, the protopectin transforming and pectin the protopectin into pectic acids and pectin and destroying into pectic the acids spatial and structure destroying of Hami the spatial melonstructure cells [34]. of In Hami addition, melon water cells loss [34]. may In addition, also cause water the softening loss may also of Hami cause melon the softening on the surface. of Hami Here, melon we on also the surface. observedhere, that the we rate also of observed decrease inthat firmness the rate in the of control decrease group in firmness was significantly in the control higher than group that was in significantly the PDS group. higher A significant than that in difference the PDS group. can be observed A significant on the difference ninth day can between observed the PDS-treated on the ninth day and between control groups. the PDS-treated The firmness and in control the PDS groups. The was firmness 9.79% higher in the than PDS that group in was the control 9.79% higher group (p than < 0.05), that suggesting the control that group PDS can (p < delay 0.05), the suggesting decline of that firmness PDS can in Hami delay melon. the decline Indeed, of firmness a previous in Hami study melon. demonstrated Indeed, that a previous 1-methylcyclopropene study demonstrated treatment that 1-methylcyclopropene inhibited the growthtreatment of microorganisms inhibited the in fresh-cut growth of Hami microorganisms melon and suppressed in fresh-cut its Hami softening melon during and suppressed storage [35], its softening suggesting during that slowing storage [35], downsuggesting the deterioration that slowing of fresh-cut down Hami the melon deterioration by inhibiting of fresh-cut microorganisms Hami may melon help by toinhibiting maintain microorganisms its firmness. may help to maintain its firmness. Figure 4. Effects of photodynamic sterilization technology on the firmness of fresh-cut Hami melons. The Figure PDS 4. conditions Effects of photodynamic used were 50 µmol/l sterilization curcumin technology and 60on min the exposure firmness toof LED fresh-cut light. Hami ***p < melons , ****p The PDS < conditions compared used towere the control 50 μmol/l group curcumin without and photosensitizer 60 min exposure and light to LED treatment. light. ***p Data < , points ****p represent < the mean compared and standard to the control deviation group of three without experiments. photosensitizer and light treatment. Data points represent the mean and standard deviation of three experiments Effects on the Sensory Indices The sensory qualities of fresh-cut Hami melon, including color, smell, appearance, and mouthfeel (Table 3), were evaluated in the PDS-treated and control groups during storage (Figure 5). On the fifth day, the color and mouthfeel scores of the fresh-cut Hami melon were not significantly different between the two groups, while the scores of appearance and smell were significantly higher in the PDS-treated group than those in the control group (Figure 5A), and no samples were rotten across these two groups. With the increase in the storage time, the sensory indices of the fresh-cut Hami melon were significantly decreased (Figure 5B). On the ninth day, the scores of the smell and appearance of the fresh-cut Hami melon were higher in the PDS group than those in the control group (Figure 5B). In the control group, visible plaque appeared in the Hami melon, and the surface was softened and rotten, which decreased the quality of the Hami melon. Indeed, a number of studies have suggested that application of PDS treatment in food preservation can result in higher sensory scores upon storage [27,36]. In summary, photodynamic sterilization can effectively delay the deterioration of fresh-cut Hami melon.

7 Molecules 2019, 24, of 10 Molecules 2019, 24, x 7 of Effects on the Sensory TableIndices 3. Sensory and quality evaluation of fresh-cut Hami melon. Score The sensory Color qualities Flavorof and fresh-cut Aroma Hami melon, Surface including color, smell, Feel appearance, and mouthfeel (Table 3), were Characteristic evaluated in the PDS-treated Fresh, plaque-free, and control and nogroups The during fleshstorage is firm, sweet, (Figure 5). 9 Bright On the fifth day, the color and aroma mouthfeel scores of the softening. fresh-cut Hami melon crispy, were and not juicy. significantly different between the two groups, while the Fresh, scores no plaque, of appearance and no and Thesmell pulp is were moresignificantly compact, higher 7 in the Slightly PDS-treated group Normal than those in softening, the control slight group surface (Figure harder, 5A), and no crispy samples with awere rotten across these two groups. With the increase water in the loss. storage time, the thicker sensory sweetness. indices of the fresh-cut Hami melon were significantly decreased Lack of freshness, (Figure 5B). obvious On the The ninth sweet day, taste the isscores light, the of the smell 5 and appearance Normal of the Scented fresh-cut Hami melon surfacewere waterhigher loss, and in the PDS pulpgroup is soft, than less juice, those and in the softening. not crispy. control group (Figure 5B). In the control group, visible plaque appeared in the Hami melon, and the surface Local diseased plaque and 3 was softened Dim and Loss rotten, of aroma which decreased the quality of the Hami melon. NotIndeed, edible. a number touch with a slimy feeling. of studies have suggested that application of PDS treatment in food preservation can result in higher sensory Fermentative, More plaque and more 1 scores Black upon storage [27,36]. In summary, photodynamic sterilization can Not edible. effectively delay acidic, and rotten serious softening. the deterioration of fresh-cut Hami melon. Figure 5. Effect of photodynamic sterilization technology on the sensory indices of fresh-cut Hami Figure 5 Effect of photodynamic sterilization technology on the sensory indices of fresh-cut Hami melons. The PDS conditions used were 50 µmol/l curcumin and 60 min exposure to LED light. (A) The melons. The PDS conditions used were 50 μmol/l curcumin and 60 min exposure to LED light. (A) sensory evaluation scores of fresh-cut Hami melon samples in the control groups and photodynamic The sensory evaluation scores of fresh-cut Hami melon samples in the control groups and treatment group after five days of storage. (B) The sensory evaluation scores of fresh-cut Hami melon photodynamic treatment group after five days of storage. (B) The sensory evaluation scores of samples in the control groups and photodynamic treatment group after nine days of storage. fresh-cut Hami melon samples in the control groups and photodynamic treatment group after nine 3. Materials days of storage. and Methods 3.1. Materials Preparation and of Fresh-Cut Methods Hami Melon Ripe Hami melon (~3 kg) in pest-free condition was washed with water and disinfected with 3.1. Preparation of Fresh-Cut Hami Melon 0.21% sodium hypochlorite, followed by rinsing in distilled water. The Hami melon was peeled and then cut Ripe into Hami 16 parts melon a(~3 HEPA kg) in clean pest-free bench, condition placed intowas plastic washed boxes, with andwater storedand at 4disinfected C prior to PDS with treatment 0.21% sodium withinhypochlorite, 2 h. followed by rinsing in distilled water. The Hami melon was peeled and then cut into 16 parts on a HEPA clean bench, placed into plastic boxes, and stored at 4 C prior to 3.2. PDS Photosensitizer treatment within and Light 2 h. Source Curcumin was purchased from Hefei Bomei Biotechnology Co., Ltd. (Anhui, China). A curcumin stock 3.2. Photosensitizer solution was and prepared Light Source at the concentration of 10 mmol/l. A blue light-emitting diode with a maximum Curcumin irradiation was wavelength purchased from of 460Hefei nm was Bomei used as Biotechnology light source inco., all PDS Ltd. treatments. (Anhui, China). A curcumin stock solution was prepared at the concentration of 10 mmol/l. A blue light-emitting 3.3. Photosensitizer and Irradiation diode with a maximum irradiation wavelength of 460 nm was used as light source in all PDS treatments. Fresh-cut Hami melon was sprayed with curcumin solution at different concentrations (10 50 µmol/l) and then exposed to LED illumination for 60 min at room temperature. Saline was used instead of 3.3. curcumin Photosensitizer for the and treatment Irradiation of Hami melon in the control group. For the 50 µmol/l concentration of curcumin, Fresh-cut the effect Hami of melon LED illumination was sprayed time with (5 curcumin min, 30 min, solution 60 min, at different and 90 min) concentrations on the fresh-cut (10 50 Hami μmol/l) melon and was then also exposed evaluated. to LED Then, illumination the fresh-cut for Hami 60 min melon at room was stored temperature. in the fridge Saline at was 4 C. used instead of curcumin for the treatment of Hami melon in the control group. For the 50 μmol/l concentration of curcumin, the effect of LED illumination time (5 min, 30 min, 60 min, and 90 min)

8 Molecules 2019, 24, of Microbiological Analyses The quantification of microorganisms (mainly live, aerobic bacteria) in the fresh-cut Hami melon was performed by the plate colony counting method according to a previous report [37]. Briefly, the samples (~5 g) were homogenized in 100 ml sterile water, sequentially tenfold diluted, and inoculated on the bacteriological substrate plate count agar (PCA). After incubating for 48 h at 37 C, the number of colonies was counted. The total number of surviving colonies on the plate was counted Quality Characteristics and Sensory Evaluation The soluble solids of the fresh-cut Hami melon were evaluated using a WZ Hand-held saccharometer (Chennuo Biotechnology Co., Ltd., Shanghai, China) according to the manufacturer s instruction. The color of the fresh-cut Hami melon was measured using an ADCI full-automatic color difference instrument (ADCI, Beijing Chen Taike Instrument Technology Co., Ltd., Beijing, China). The changes of color were represented by values of L* (lightness from black (0) to white (100)), a* (redness from green (-) to red (+)), and b* (yellowness blue (-) to yellow (+)). The moisture content of the fresh-cut Hami melon was determined using an SFY-20A Halogen Rapid Moisture Analyzer (Shenzhen Guanya Electronic Technology Co., Ltd., Shenzhen, China). The firmness of the fresh-cut Hami melon was tested using a TA.XTplus texture apparatus equipped with a P5 cylindrical probe (Beijing Chen Taike Instrument Technology Co., Ltd.). The pre-test speed, test speed, and post-test speed was set to 5 mm/s, 2 mm/s, and 2 mm/s, respectively. The nine-point evaluation method was used to evaluate the sensory scores, including the aroma, taste, and appearance of the fresh-cut Hami melon (Table 3) Statistical Analysis Results are presented as the mean ± SEM for at least three independent experiments. Statistical significance was determined using Student s t-test when comparing two groups or using one-way ANOVA followed by post hoc Tukey s multiple comparisons when comparing three or more groups. The difference between the means was considered statistically significant when p < All statistical calculations were performed using GraphPad Prism Conclusions Curcumin can be used as a photosensitizer in photodynamic sterilization for food preservation. Here, our results also showed that curcumin-mediated PDS can effectively sterilize residual bacterial contamination in fresh-cut Hami melon and delay their growth. The antibacterial property of the photodynamic sterilization was affected by the length of time of LED exposure and the curcumin concentration. The results showed that microbes in fresh-cut Hami melon could be effectively killed by 50 µ mol /L of curcumin under exposure to the light from a blue LED lamp for 60 min. A previous study demonstrated that in fresh-cut fruit with a neutral ph, such as Hami melon (cantaloupe), bacteria are the main source of spoilage [38]. Therefore, the effects of PDS in preserving the fresh-cut Hami melon in our study may be largely due to its bactericidal activity. However, we cannot rule out that fungicidal activity may also be involved in this process. Furthermore, compared to the control group, color changes were also inhibited effectively by photodynamic sterilization. The firmness, water content, and soluble solid content in the fruits were more stable in the PDS-treated fresh-cut Hami melon. In addition, the sensory qualities of the fresh-cut Hami melon were effectively maintained. Therefore, photodynamic sterilization may be a promising technology that can be utilized conveniently for the preservation of fresh-cut Hami melon and of other fresh-cut fruits in industry. Author Contributions: S.L. (Shaoling Lin) and S.Z. conceived and designed the experiments; Y.L. and S.L. (Shiyang Li) performed the experiments; Y.L., H.J. and A.Z. analyzed the data; Y.L., J.H. and S.S.H. wrote the manuscript. All authors read and approved the final manuscript. Acknowledgments: The authors are grateful to the National Natural Science Foundation of China ( ) and the China Postdoctoral Science Foundation (2018M63072, 2019) for their financial support.

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