Combined effects of thermosonication and slightly acidic electrolyzed water on the microbial quality and shelf life extension of fresh-cut kale during refrigeration storage

Food Microbiology 51 (2015) 154e162

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Combined effects of thermosonication and slightly acidic electrolyzed water on the microbial quality and shelf life extension of fresh-cut kale during refrigeration storage Ahmad Rois Mansur, Deog-Hwan Oh* Department of Food Science and Biotechnology, School of Bioconvergence Science and Technology, Kangwon National University, Chuncheon, Gangwon 200-701, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2014 Received in revised form 16 May 2015 Accepted 20 May 2015 Available online 28 May 2015

This study evaluated the efficacy of thermosonication combined with slightly acidic electrolyzed water (SAcEW) on the shelf life extension of fresh-cut kale during storage at 4 and 7
C. Each kale (10 ± 0.2 g) was inoculated to contain approximately 6 log CFU/g of Listeria monocytogenes. Each inoculated or uninoculated samples was dip treated at 40
C for 3 min with deionized water, thermosonication (400 W/L), SAcEW (5 mg/L), sodium chlorite (SC; 100 mg/L), sodium hypochlorite (SH; 100 mg/L), and thermosonication combined with SAcEW, SC, and SH (TS þ SAcEW, TS þ SC, and TS þ SH, respectively). Growths of L. monocytogenes and spoilage microorganisms and changes in sensory (overall visual quality, browning, and off-odour) were evaluated. The results show that lag time and specific growth rate of each microorganism were not significantly (P > 0.05) affected by treatment and storage temperature. Exceeding the unacceptable counts of spoilage microorganisms did not always result in adverse effects on sensory attributes. This study suggests that TS þ SAcEW was the most effective method to prolong the shelf life of kale with an extension of around 4 and 6 days at 4 and 7
C, respectively, and seems to be a promising method for the shelf life extension of fresh produce. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Fresh-cut kale Thermosonication Slightly acidic electrolyzed water Microbial quality Sensory shelf life

1. Introduction Kale (Brassica oleracea L. var. acephala) is a leafy green vegetable belonging to the Brassica group, which is well recognized as a rich source of vitamins, flavonoids, minerals, dietary fibre, and antioxidative compounds (Lisiewska et al., 2008). Thus, it becomes one of the famous green leafy vegetable juices consumed in South Korea (Kim et al., 2008) and the fastest growing superfood in the US (Wong, 2014). The availability of fresh kale, however, is seasonal (Lisiewska et al., 2008) and there has also been an evident concern about its microbial safety and quality due to it is mostly consumed raw (USDA, 2012). Listeria monocytogenes is one of the most common foodborne pathogens that contaminate leafy green vegetables or their readyto-eat salads, which cause foodborne illnesses (Taban and Halkman, 2011). The high presence of natural occurring microorganisms (aerobic bacteria, Enterobacteriaceae, Pseudomonas spp.,

* Corresponding author. E-mail address: [email protected] (D.-H. Oh). http://dx.doi.org/10.1016/j.fm.2015.05.008 0740-0020/© 2015 Elsevier Ltd. All rights reserved.

yeasts, and molds) on fresh vegetables may lead to the rejection of the product by consumers due to spoilage, even though they are usually nonpathogenic for humans. Also, the damage of the fresh vegetable surfaces during peeling, cutting, and shredding may lead to the worse contamination with spoilage microorganisms. Consequently it may reduce the microbial and sensory shelf life of the end product (Ahvenainen, 1996; Ragaert et al., 2007). Therefore, it is necessary to develop an effective preservation method that can enhance the shelf life of fresh-cut kale during storage. Storage under refrigerated condition is one of the most commonly employed methods to inhibit deterioration of fresh fruits and vegetables as microbial growth and chemical and biochemical reactions, which may reduce microbial and sensory shelf life, slow down when temperature is reduced (Alexandre et al., 2012). To improve the safety and quality of fresh vegetables prior to the refrigeration storage, certain sanitizing processes complementary to refrigeration have been developed and used. Traditionally, wash water and several sanitizer agents such as chlorine solutions have often been used to rinse fresh vegetables, with the main objective of reducing microbial contamination and extending shelf life of the final product. The chlorination process

A.R. Mansur, D.-H. Oh / Food Microbiology 51 (2015) 154e162

usually consists on adding sodium hypochlorite or sodium chlorite (NaOCl or NaClO2) to wash waters, at concentrations between 100 and 1000 mg/L and for a contact time of 1e15 min (Allende et al., 2009; Inatsu et al., 2005; Stopforth et al., 2008). The excessive use of chlorine, however, can lead to several environmental and human health effects. In recent years, therefore, most of the studies on decontamination approaches for the fresh-cut industry have been focused on alternative sanitizing treatments to chlorine (Gil et al., 2009). Slightly acidic electrolyzed water (SAcEW) is well recognized as one of alternative sanitizers, which has a pH of 5.0e6.5 and contains approximately 95% of hypochlorous acid (Cui et al., 2009). The low available chlorine concentration (ACC) of SAcEW leads to the maximized use of hypochlorous acid, reduction of the corrosion on the surfaces, and reduction of the human health and safety issues from Cl2 gas (Guentzel et al., 2008). Recently, the food industry has also discovered ultrasound technology which has a variety of applications in processing, preservation, and extraction (Chemat et al., 2011). Bilek and Turantas¸ (2013) reported that combination of ultrasound and aqueous sanitizers effectively reduced microbial contamination in fresh fruits and vegetables. To enhance its efficacy, however, it would most likely have to be used in conjunction with heat treatment (thermosonication) (Awad et al., 2012; Chemat et al., 2011; Mansur and Oh, 2015). Although there have been numerous studies on the application of SAcEW, alone or in combination with ultrasound, they mostly focus on microbial reduction and do not discuss the effects of the treatment on the fate of microorganisms and sensory characteristics of produce during storage. This study, therefore, was conducted to evaluate the efficacy of thermosonication combined with SAcEW on the shelf life extension of fresh-cut kale during storage under refrigerated conditions at 4 and 7
C. 2. Materials and methods 2.1. Culture preparation L. monocytogenes (ATCC 19114 and ATCC 19115) obtained from the Korean National Institute of Health (Seoul, Korea) were separately cultured in tryptic soy broth (TSB; Difco, BD) at 37
C with two consecutive transfers after 24 h, for a total culture period of 48 h. Each culture was separately centrifuged at 4000  g for 10 min at 4
C, and the supernatant was removed. The obtained cell pellet was washed twice with 0.1% buffered peptone water (BPW; Difco, BD) pH 7.1 and re-suspended in 10 mL of the same solution to obtain a final cell level of approximately 8 log CFU/mL. The two strains of L. monocytogenes were then combined in a cocktail with the final cell population of approximately 8 log CFU/mL. The inoculum population in each culture cocktail was confirmed by plating 0.1 mL portions of appropriately diluted culture on tryptic soy agar (TSA; Difco, BD), and then incubated at 37
C for 24 h. The prepared culture cocktails were then used in the subsequent experiments. 2.2. Sample inoculation Fresh-cut kale (B. oleracea L. var. acephala) purchased from a local supermarket (Lotte Mart, Chuncheon, Korea) were stored at 4
C and used within 24 h. Damaged portions were removed aseptically by hand and prior to inoculation with L. monocytogenes, each sample (10.0 ± 0.2 g) was placed on sterile aluminum foil in a laminar flow hood and treated with UV light (TUV 15 W; Philips Lighting, Roosendaal, Netherlands) for 50 min (25 min for each side) to reduce the background microflora. Each sample was then inoculated by pipetting 0.1 mL of culture cocktail (approximately 8 log CFU/mL) onto the leaf surfaces to contain an initial level of

155

approximately 6 log CFU/g. Each sample was air dried for 1 h in a laminar flow hood with the fan running at 23 ± 2
C to allow for bacterial attachment, and then immediately exposed to sanitization treatments. 2.3. Preparation of slightly acidic electrolyzed water (SAcEW) SAcEW was produced according to Mansur and Oh (2015), by electrolysis of a dilute HCl solution (6%) in a chamber without a membrane using an electrolysis device (BIOCIDER, model BC-360, Rvd Corp., Gyeonggi, Korea). The obtained SAcEW was then diluted in deionized water (DW) to produce SAcEW with an available chlorine concentration (ACC) of 5 mg/L. The sodium chlorite (SC) and sodium hypochlorite (SH) solutions (each 100 mg/L) were prepared with the addition of 0.1 g of NaClO2 (Kanto Chemical Co., Tokyo, Japan) and NaOCl (DC Chemical Co., Seoul, Korea) in 1 L of DW, respectively. The oxidation reduction potential (ORP) and pH of sanitizer solutions were measured immediately before treatment with a dual-scale pH meter (Accumet model 15, Fisher Scientific Co., Fair Lawn, NJ) bearing ORP and pH electrodes. The ACC was determined by a colorimetric method using a digital chlorine test kit (RC-3F, Kasahara Chemical Instruments Co., Saitama, Japan) with the detection range of 1e300 mg/L. The physicochemical properties of sanitizer solutions used in this study are summarized in Table 1. 2.4. Experimental procedure Inoculated and uninoculated kale (10 g each) were separately placed in sterile stomacher bags (Whirl-Pak, Nasco Janesville, WI) and immersed in 200 mL of each sanitizing solution (DW, SAcEW [5 mg/L], SC [100 mg/L], and SH [100 mg/L]) at 40
C for 3 min. For the combined treatment, kale samples were immersed in each sanitizing solution placed in a rectangular tank of ultrasonic cleaner device (model JAC-4020, Kodo Technical Research Co., Ltd., Hwaseong, Gyeonggi, Korea) with a fixed frequency of 40 kHz and operated at an acoustic energy density (AED) of 400 W/L, 40
C, and for 3 min (Mansur and Oh, 2015). Each of kale treated with SAcEW, SC, and SH was then washed for 1 min with 200 mL of neutralizing solution (0.85% NaCl containing 0.5% Na2S2O3) in order to stop the microbicidal effect of the treatment, and excess sanitizing solutions on each treated kale was removed with sterile paper towel. The unwashed sample was used as control. For determination of shelf life, each kale (untreated or treated and uninoculated or inoculated with L. monocytogenes) was air packaged in a stomacher bag (Whirl-Pak, Nasco Janesville, WI) and stored at 4 and 7
C. During storage, subsamples were analyzed at 2 day intervals and the mean microbial populations (log CFU/g) at each sampling interval were then calculated. The modified Gompertz model (Zwietering et al., 1990) was used to describe the microbial growth curves, and growth parameters (specific growth rate

Table 1 Physicochemical properties of tested sanitizer solutions.a Sanitizing solutions c

DW SAcEWd 5 mg/L NaClO2 100 mg/L NaOCl 100 mg/L

ORP (mV)b

pH 6.98 6.28 9.00 9.82

± ± ± ±

0.16b 0.18a 0.21c 0.14d

428 895 563 620

± ± ± ±

18a 20c 26b 27b

a Values are means ± standard deviation (n ¼ 6). Within the same column, values not followed by the same letter are significantly different (P < 0.05). b Oxidation reduction potential. c Deionized water. d Slightly acidic electrolyzed water.

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[SGR]: log CFU/h; lag time [LT]: h) were analyzed by the GraphPad prism software (version 5.01, GraphPad Software, Inc., San Diego, CA). The sensory parameters: overall visual quality (OVQ; visual defects and textural breakdown), browning, and off-odour were employed to determine the end of sensory shelf life since these parameters reflected the major deterioration in sensory quality of minimally processed vegetables during storage (Ragaert et al., 2007). The sensory parameters were scored using a numeric ratmez-Lo  pez et al. (2008). The evaluated ing scale proposed by Go sensory attributes in this study were: OVQ (1 ¼ excellent, 5 ¼ acceptable, 9 ¼ extremely poor), browning and off-odour (1 ¼ none, 3 ¼ acceptable, 5 ¼ severe). Sensory evaluation of kale (10 g each) was carried out at 2 day intervals by a semi-trained panel of six persons in a special room with individual booths. On each day of testing, panelists were presented with fresh-cut kale as a reference. The end of shelf life was determined if the population of a group of spoilage microorganisms (Aerobic mesophilic bacteria [AMB], Enterobacteriaceae, Pseudomonas spp., and yeast and mold counts [YMC]) or when the sensory score of the samples reached the unacceptable limit. The end of microbial shelf life was determined at 7 to 8 log CFU/g for AMB, Enterobacteriaceae, and psychrotrophic counts (Pseudomonas spp.), and 5 log CFU/g for YMC (Ragaert et al., 2007), while the end of sensory shelf-life was reached when at least one of the mean scores was above the middle point of the  mez-Lo  pez et al., 2008). Each experiment respective scale (Go consists of two independent trials with triplicate samples. 2.5. Microbial enumeration Each 10 g of kale was mixed with 90 mL of 0.1% sterile BPW and homogenized for 2 min in a stomacher (400 Circulator, Seward, London, UK). Each mixture was serially diluted and plated onto plate count agar (Difco, BD), violet red bile glucose agar (Difco, BD), Pseudomonas agar base plus CFC supplement (Oxoid), potato dextrose agar with 10% tartaric acid solution (Difco, BD), and modified Oxford agar base with the addition of Oxford antimicrobic supplement (Difco, BD) to enumerate AMB, Enterobacteriaceae, Pseudomonas spp., YMC, and L. monocytogenes, respectively. All plates were incubated at 37
C for 24 h, except the plates for Pseudomonas spp. and YMC, which were incubated at 25
C for 24 h and 45 days, respectively. 2.6. Statistical analyses The experimental data (mean ± standard deviation) were analyzed using One-way ANOVA in SPSS ver. 13.0 (Statistical Package for the Social Sciences, Chicago, IL, USA). The analysis included treatment, storage temperature, and microorganisms as fixed effects. Differences between effects were assessed using the Tukey's test at P < 0.05. 3. Results 3.1. Effect of decontamination treatments on microbial growths and quality The growth curves of L. monocytogenes and spoilage microorganisms (Aerobic mesophilic bacteria [AMB], Enterobacteriaciae, Pseudomonas spp., and yeast and mold counts [YMC]) on kale at 4 and 7
C are shown in Figs. 1 and 2, respectively. After 14 days of storage at 4 and 7
C, the microbial populations on kale treated with combined treatment of thermosonication and SAcEW (TS þ SAcEW) were much lower compared to those on untreated

Fig. 1. Effect of various decontamination treatments on growth of L. monocytogenes on kale during storage at 4 and 7
C. Values shown are mean ± standard deviation. (Control: Untreated samples; DW: Deionized water; TS þ DW: Thermosonication; SAcEW: Slightly acidic electrolyzed water; TS þ SAcEW: Thermosonication combined with slightly acidic electrolyzed water; SC: Sodium chlorite; TS þ SC: Thermosonication combined with sodium chlorite; SH: Sodium hypochlorite; TS þ SH: Thermosonication combined with sodium hypochlorite).

kale (control) and kale treated with other decontamination treatments (deionized water [DW], thermosonication [TS þ DW], slightly acidic electrolyzed water [SAcEW], sodium chlorite [SC], sodium hypochlorite [SH], and thermosonication combined with SC and SH [TS þ SC and TS þ SH]). However, in many cases, lag time (LT; Table 2) and specific growth rate (SGR; Table 3) of each microorganism on kale were not significantly (P > 0.05) affected by decontamination treatments. LT and SGR of each microorganism were also not significantly (P > 0.05) affected by storage temperatures. Moreover, SGR of L. monocytogenes and YMC were found to be significantly lower (P < 0.05) compared to those of other spoilage bacteria (AMB, Enterobacteriaciae, and Pseudomonas spp.). In Fig. 2, the populations of spoilage bacteria on treated (TS þ SAcEW) kale did not reach the unacceptable limit (7 log CFU/g) after 14 days of storage at 4 and 7
C, except Pseudomonas spp. on TS þ SAcEW treated kale stored at 7
C (14 days). The populations of spoilage bacteria on kale treated with other decontamination treatments (DW, TS þ DW, SAcEW, SC, TS þ SC, SH, and TS þ SH) reached the unacceptable limit in less than 6e12 days of storage, depending on the type of bacteria, treatment, and storage temperature. In addition, the final populations of YMC in treated kale after 14 days of storage at 4 and 7
C were all at the acceptable limit (<5 log CFU/g). According to shelf life derived from microbial growths (Table 7), TS þ SAcEW resulted in more prolonged the microbial shelf life (14 days) of kale during storage at 4 and 7
C compared to those of untreated kale and kale treated with other sanitization treatments, which were limited to 6 to 12 days depending on the treatment and storage temperature.

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Fig. 2. Effect of various decontamination treatments on growths of spoilage microorganisms on kale during storage at 4 and 7
C. Values shown are mean ± standard deviation. (Control: Untreated samples; DW: Deionized water; TS þ DW: Thermosonication; SAcEW: Slightly acidic electrolyzed water; TS þ SAcEW: Thermosonication combined with slightly acidic electrolyzed water; SC: Sodium chlorite; TS þ SC: Thermosonication combined with sodium chlorite; SH: Sodium hypochlorite; TS þ SH: Thermosonication combined with sodium hypochlorite; : The end of shelf life).

3.2. Effect of decontamination treatments on sensory shelf life The mean values of sensory parameters (overall visual quality [OVQ], browning, and off-odour) of untreated and treated kale are summarized in Tables 4e6, respectively. The OVQ, browning, and off-odour values of treated kale were not significantly (P > 0.05) different from those of untreated kale on day 0, except the off-

odour values of kale treated with SH and TS þ SH. The values of all sensory parameters of untreated and treated kale increased over time during storage at 4 and 7
C. The OVQ, browning, and off-odour values of TS þ SAcEW treated kale were at the acceptable limits on day 14. On the contrary, the OVQ, browning, and off-odour of untreated kale and kale treated with other decontamination treatments (except SH and TS þ SH) were limited

158

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Table 2 Lag time (LT) of Listeria monocytogenes and spoilage microorganisms on treated kale during refrigeration storage. Storage (
C)

Treatmentsd

Control DW

e

4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7

f

TS þ DWg SAcEWh TS þ SAcEWh SCh TS þ SCh SHh TS þ SHh

Lag time (h)a,b LMc

AMBc

EBc

PSDc

YMCc

100.2 ± 26.4Bbc 60.6 ± 8.1Babc 88.1 ± 32.8Aabc 71.4 ± 32.0ABabc 86.6 ± 22.8BCabc 65.3 ± 22.5ABabc 87.7 ± 11.9Cabc 70.1 ± 20.2Aabc 82.5 ± 15.0ABabc 52.1 ± 7.4Aab 46.6 ± 2.7Aab 30.6 ± 12.0Aa 61.9 ± 15.9Aabc 41.9 ± 14.9Aab 111.3 ± 37.8Abc 82.3 ± 4.7Babc 121.5 ± 43.1BCc 99.2 ± 21.8Bbc

61.1 ± 17.0ABabc 30.6 ± 6.9ABa 109.7 ± 21.6Abcd 95.0 ± 42.4Babcd 74.3 ± 16.7Babcd 73.2 ± 5.2ABabcd 49.6 ± 15.4Aab 44.0 ± 29.2Aab 86.8 ± 2.4ABabcd 68.3 ± 4.1ABabcd 137.3 ± 23.0Bd 93.7 ± 12.4Babcd 61.1 ± 11.9Aabc 48.9 ± 11.7Aab 119.0 ± 8.7Acd 71.6 ± 20.1ABabcd 134.1 ± 16.3Cd 100.0 ± 24.6Bbcd

64.0 ± 9.3ABa 60.8 ± 20.3Ba 82.9 ± 17.5Aabc 74.3 ± 4.2ABabc 93.6 ± 8.6BCabcd 79.0 ± 11.8ABabc 67.7 ± 11.6ABab 63.1 ± 4.3Aa 79.6 ± 8.9Aabc 88.4 ± 5.9BCabcd 109.2 ± 17.3Bbcd 83.9 ± 8.5Babc 78.8 ± 2.7ABabc 58.6 ± 17.0Aa 102.4 ± 17.1Acd 88.1 ± 7.8Babcd 120.4 ± 12.0BCd 72.5 ± 12.7ABabc

117.7 ± 30.0Bb 62.4 ± 15.7Ba 102.3 ± 9.6Aab 86.7 ± 16.4ABab 110.0 ± 16.6Cb 90.0 ± 5.4Bab 98.4 ± 11.5Cab 74.3 ± 12.7Aab 107.5 ± 20.6Bab 100.6 ± 12.5Cab 110.7 ± 12.8Bb 83.9 ± 6.0Bab 99.6 ± 3.8Bab 63.5 ± 7.0Aa 92.3 ± 9.3Aab 87.8 ± 20.5Bab 71.8 ± 12.2Aab 71.0 ± 10.1ABab

58.3 ± 18.9Aabcd 17.1 ± 11.7Aa 87.4 ± 17.5Acdef 38.0 ± 9.6Aab 45.1 ± 17.1Aab 39.9 ± 16.0Aab 82.0 ± 2.4BCbcdef 38.5 ± 31.9Aab 75.0 ± 12.3Aabcdef 55.7 ± 15.3Aabcd 120.4 ± 17.1Bf 111.2 ± 7.2Cef 92.6 ± 6.0ABdef 63.5 ± 5.8Aabcde 111.2 ± 21.3Aef 47.8 ± 13.1Aabc 81.5 ± 26.1ABbcdef 67.5 ± 18.1Aabcde

a

Within the same column, values not followed by the same lowercase letter are significantly different (P < 0.05). Within the same row, values not followed by the same uppercase letter are significantly different (P < 0.05). c LM: Listeria monocytogenes; AMB: Aerobic mesophilic bacteria; EB: Enterobacteriaciae; PSD: Pseudomonas spp.; YMC: Yeast and mold counts. d All treatment was conducted at 40
C for 3 min. e Untreated samples. f Samples washed with deionized water (Positive control). g Samples treated with thermosonication. h Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication). b

to 10 to 14, 8 to 12, and 8 to 14 days, respectively, except the browning values of untreated kale stored at 4
C, which were still at the acceptable limit on day 14. The OVQ, browning, and offodour values of treated (SH and TS þ SH) kale were more limited to 6 to 8, 8 to 10, and 2 days, respectively. Table 7 shows that TS þ SAcEW extended the sensory shelf life (>14 days) of kale during storage at 4 and 7
C, while those of untreated kale and kale

treated with other decontamination treatments were limited to 2 to 12 days depending on the treatment and storage temperature. 4. Discussion The application of aqueous sanitizers either alone or in combination with thermosonication could potentially affect the quality

Table 3 Specific growth rate (SGR) of Listeria monocytogenes and spoilage microorganisms on treated kale during refrigeration storage. Treatmentsd

Storage (
C)

Specific growth rate (log CFU/h)a,b LMc

Control

e

DWf TS þ DWg SAcEWh TS þ SAcEWh SCh TS þ SCh SHh TS þ SHh a

4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7

0.011 0.015 0.004 0.009 0.013 0.011 0.012 0.020 0.015 0.015 0.010 0.011 0.009 0.011 0.008 0.018 0.009 0.011

AMBc ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.001ABab 0.003BCab 0.003Aa 0.004Aab 0.008ABab 0.002Aab 0.004ABab 0.002Bb 0.006Aab 0.001Aab 0.000Aab 0.001Aab 0.002Aab 0.005Aab 0.002Aab 0.004Bb 0.009Aab 0.005Aab

0.016 0.008 0.018 0.024 0.020 0.026 0.022 0.023 0.025 0.029 0.023 0.038 0.024 0.026 0.024 0.022 0.023 0.032

EBc ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006Bab 0.001ABa 0.003Bab 0.004Bbc 0.006Bbc 0.004BCbcd 0.005Cbc 0.002BCbc 0.003BCbcd 0.001CDbcd 0.003Bbc 0.009Cd 0.003Bbc 0.002Bbcd 0.002Bbc 0.003BCbc 0.006Bbc 0.005Bcd

0.017 0.016 0.026 0.028 0.025 0.029 0.023 0.020 0.017 0.023 0.023 0.027 0.024 0.029 0.026 0.034 0.020 0.025

PSDc ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.002Bab 0.003BCa 0.002Cbcde 0.003Bcde 0.002Babcde 0.003Cde 0.001Cabcd 0.001Babc 0.002ABab 0.001BCabcd 0.003Babcd 0.004BCbcde 0.003Babcd 0.002Bde 0.004Bbcde 0.009De 0.003Babc 0.003ABabcde

0.013 0.019 0.023 0.027 0.023 0.020 0.021 0.025 0.030 0.034 0.021 0.029 0.023 0.027 0.028 0.029 0.018 0.021

YMCc ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.003Ba 0.007Cab 0.001Cabcd 0.004Bbcd 0.000Babcd 0.001Babc 0.001BCabc 0.002Cbcd 0.004Ccd 0.005Dd 0.001Babc 0.005BCbcd 0.003Babcd 0.003Bbcd 0.005Bbcd 0.005CDbcd 0.002ABab 0.002ABabc

0.006 0.006 0.007 0.009 0.011 0.010 0.008 0.009 0.014 0.017 0.014 0.016 0.013 0.012 0.005 0.009 0.009 0.031

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.001Aa 0.002Aa 0.001Aab 0.001Aab 0.001Aab 0.002Aab 0.002Aab 0.001Aab 0.002Aab 0.002ABb 0.001Aab 0.004ABab 0.001Aab 0.001Aab 0.001Aa 0.001Aab 0.000Aab 0.014Bc

Within the same column, values not followed by the same lowercase letter are significantly different (P < 0.05). Within the same row, values not followed by the same uppercase letter are significantly different (P < 0.05). LM: Listeria monocytogenes; AMB: Aerobic mesophilic bacteria; EB: Enterobacteriaciae; PSD: Pseudomonas spp.; YMC: Yeast and mold counts. d All treatment was conducted at 40
C for 3 min. e Untreated samples. f Samples washed with deionized water (Positive control). g Samples treated with thermosonication. h Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication). b c

A.R. Mansur, D.-H. Oh / Food Microbiology 51 (2015) 154e162

159

Table 4 Overall visual quality (OVQ)a of treated kale during refrigeration storage. Treatmentsc

Storage (
C)

Time (weeks)b 0

Control DW

d

e

TS þ DWf SAcEWg TS þ SAcEWg SCg TS þ SCg SHg TS þ SHg

4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7

1.2 1.2 1.3 1.3 1.4 1.4 1.3 1.3 1.2 1.2 1.4 1.4 1.4 1.4 1.5 1.5 1.7 1.7

2 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.3a 0.3a 0.2a 0.2a 0.3a 0.3a 0.4a 0.4a 0.4a 0.4a 0.4a 0.4a 0.2a 0.2a 0.4a 0.4a 0.8a 0.8a

1.6 1.8 1.7 2.1 1.8 2.2 1.7 1.9 1.5 1.4 1.6 1.8 1.6 1.5 2.4 2.5 2.4 2.7

4 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5 ab 0.2abc 0.4abc 0.4bcde 0.3abc 0.2cdef 0.3abc 0.3abcd 0.2a 0.3a 0.4 ab 0.4abc 0.3 ab 0.3a 0.4def 0.5ef 0.3def 0.4f

6

1.8 2.7 2.5 2.8 2.2 2.7 2.0 2.4 1.8 1.8 2.0 2.2 2.0 2.2 3.4 3.6 3.0 3.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4a 0.4defg 0.5cdef 0.4efg 0.4abc 0.3defg 0.4 ab 0.5bcdef 0.3a 0.4a 0.5 ab 0.5abc 0.3 ab 0.3abc 0.3ghi 0.8hij 0.4gh 0.4j

2.7 3.1 3.3 3.8 2.6 3.4 2.5 2.8 2.2 2.3 2.5 3.0 2.5 2.8 4.0 4.6 4.0 5.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4abcd 0.6def 0.5efg 0.4gh 0.6abc 0.5 fg 0.6abc 0.7bcde 0.4a 0.7 ab 0.4abc 0.3cdef 0.5abc 0.5bcde 0.4h 0.3i 0.4h 0.5i

8

10

12

14

3.3 ± 0.3def 3.7 ± 0.4 fg 4.1 ± 0.4gh 4.5 ± 1.1hi 3.0 ± 0.4abcd 3.5 ± 0.9ef 2.7 ± 0.4 ab 3.2 ± 0.5cde 2.6 ± 0.7a 2.7 ± 0.5 ab 3.3 ± 0.6def 3.2 ± 0.5cde 3.0 ± 0.4abcd 2.9 ± 0.4abc 5.1 ± 0.6j 5.7 ± 0.7k 4.6 ± 0.7ij 5.6 ± 0.4k

4.9 ± 0.6de 4.7 ± 0.5d 4.8 ± 0.5d 5.1 ± 0.4de 3.5 ± 0.6bc 3.6 ± 0.3bc 3.3 ± 0.7 ab 3.4 ± 0.6abc 3.0 ± 0.8a 3.2 ± 0.3 ab 4.0 ± 0.6c 3.5 ± 0.7bc 3.5 ± 0.9bc 3.1 ± 0.4 ab 6.0 ± 0.6 fg 6.4 ± 0.8g 5.5 ± 0.6ef 6.2 ± 0.8g

5.9 ± 0.7f 6.2 ± 0.7fgh 6.0 ± 0.5 fg 6.5 ± 0.6gh 4.8 ± 0.8c 5.5 ± 1.0de 4.7 ± 0.8c 5.0 ± 1.2cd 3.3 ± 0.7a 4.0 ± 0.5b 5.2 ± 0.6cd 5.7 ± 0.7ef 5.0 ± 0.4cd 5.2 ± 0.3cd 6.6 ± 0.4h 6.6 ± 0.6h 5.8 ± 0.3ef 6.4 ± 0.8gh

6.9 ± 0.9g 7.0 ± 1.4g 7.0 ± 2.0g 7.0 ± 1.5g 5.3 ± 1.0cd 5.6 ± 0.5de 5.1 ± 0.5c 5.5 ± 0.8cde 3.5 ± 0.5a 4.5 ± 0.6b 6.2 ± 1.0f 6.8 ± 0.3g 5.6 ± 0.6de 6.0 ± 0.8ef 7.0 ± 1.0g 7.0 ± 1.2g 6.8 ± 0.7g 7.0 ± 1.3g

a

OVQ (overall visual quality): 1 ¼ excellent, 5 ¼ acceptable, 9 ¼ extremely poor. Numbers in bold indicate scores above the acceptability limit. Within the same column, values not followed by the same lowercase letter are significantly different (P < 0.05). c All treatment was conducted at 40
C for 3 min. d Untreated samples. e Samples washed with deionized water (Positive control). f Samples treated with thermosonication. g Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication). b

microorganisms on kale were observed. The growths of L. monocytogenes and YMC in all kale samples were more retarded as compared to those of spoilage bacteria (AMB, Enterobacteriaciae, and Pseudomonas spp.). Brackett (1999) reported that cold storage reduces the growth rate of L. monocytogenes, but it is still able to grow on a wide variety of fresh vegetables at storage temperatures used to maintain product quality. Due to low temperature storage,

and shelf life of kale during refrigeration storage. To investigate this possibility, the microbial and sensory shelf life of kale treated for 3 min at 40
C with DW, TS þ DW, SAcEW, TS þ SAcEW, SC, TS þ SC, SH, and TS þ SH were evaluated during storage under refrigerated temperatures at 4 and 7
C. Our study reveals that no significant influences of the decontamination treatments and both refrigerated temperatures on the growth kinetics (LT and SGR) of

Table 5 Browninga of treated kale during refrigeration storage. Treatmentsc

Storage (
C)

Controld

4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7

Time (weeks)b 0

DWe TS þ DWf SAcEWg TS þ SAcEWg SCg TS þ SCg SHg TS þ SHg e

1.3 1.3 1.6 1.6 1.3 1.3 1.5 1.5 1.3 1.3 1.3 1.3 1.2 1.2 1.5 1.5 1.4 1.4

2 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2a 0.2a 0.5a 0.5a 0.4a 0.4a 0.3a 0.3a 0.2a 0.2a 0.6a 0.6a 0.2a 0.2a 0.2a 0.2a 0.3a 0.3a

1.5 1.5 1.6 1.6 1.6 2.0 1.6 1.6 1.3 1.4 1.3 1.5 1.5 1.6 1.8 1.9 2.0 2.0

4 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2abc 0.3abc 0.7abcd 0.3abcd 0.4abcd 0.3def 0.3abcd 0.3abcd 0.2a 0.5 ab 0.4a 0.5abc 0.2abc 0.4abcd 0.2bcdef 0.2cdef 0.6def 0.3def

1.6 1.7 1.8 2.0 1.9 2.5 1.8 2.0 1.3 1.6 1.6 1.9 1.7 2.0 2.1 2.4 2.7 2.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 ab 0.4abc 0.3bc 0.2bcd 0.4bc 0.3ef 0.5bc 0.2bcd 0.2a 0.2 ab 0.2 ab 0.6bc 0.3abc 0.2bcd 0.3cde 0.4def 0.2f 0.6f

6

8

10

12

14

1.7 ± 0.5 ab 2.4 ± 0.5cde 2.3 ± 0.2cde 2.3 ± 0.6cde 2.2 ± 0.2cd 2.9 ± 0.3ef 2.0 ± 0.2bcd 2.4 ± 0.2cde 1.5 ± 0.2a 1.9 ± 0.3abc 1.9 ± 0.3abc 2.1 ± 0.3bcd 2.1 ± 0.3bcd 2.4 ± 0.6cde 2.5 ± 0.2de 2.8 ± 0.3ef 3.5 ± 0.3g 3.2 ± 0.2 fg

1.8 ± 0.4a 3.2 ± 0.3f 2.4 ± 0.2bcd 2.5 ± 0.3d 2.5 ± 0.3d 2.9 ± 0.6ef 2.6 ± 0.2de 2.4 ± 0.3bcd 1.7 ± 0.2a 2.0 ± 0.4 ab 2.0 ± 0.3 ab 2.4 ± 0.2bcd 2.4 ± 0.4bcd 2.4 ± 0.2bcd 3.3 ± 0.2f 3.8 ± 0.5g 3.9 ± 0.5g 4.0 ± 0.5g

2.0 ± 0.3 ab 3.8 ± 0.2gh 2.9 ± 0.8de 3.6 ± 0.2 fg 2.9 ± 0.8de 3.1 ± 0.3ef 2.9 ± 0.7de 2.5 ± 0.2bcd 1.8 ± 0.2a 2.2 ± 0.6abc 2.5 ± 0.3bcd 2.9 ± 0.2de 2.7 ± 0.5cde 2.5 ± 0.6bcd 4.0 ± 0.2ghi 4.6 ± 0.3j 4.2 ± 0.6hij 4.5 ± 0.2ij

2.2 ± 0.2 ab 4.4 ± 1.0h 3.5 ± 0.5cd 4.4 ± 0.4h 3.7 ± 0.2cdef 4.2 ± 0.4fgh 3.4 ± 0.3c 3.8 ± 0.5defg 2.0 ± 0.3a 2.5 ± 0.3b 3.3 ± 0.7c 3.7 ± 0.5cdef 3.6 ± 0.5cd 3.5 ± 0.3cd 4.0 ± 0.4efgh 5.0 ± 0.8i 4.3 ± 0.4gh 5.0 ± 0.8i

2.5 5.0 3.8 4.6 3.9 4.4 3.5 4.2 2.0 2.8 3.8 4.3 3.9 4.0 4.2 e 4.4 e

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7b 0.8i 0.3cd 0.3hi 0.4cdef 0.2gh 0.4c 0.3efgh 0.5a 0.4b 0.6cd 0.3fgh 0.5cdef 0.5defg 0.7efgh

± 0.8gh

Not measured. a Browning: 1 ¼ none, 3 ¼ acceptable, 5 ¼ severe. Numbers in bold indicate scores above the acceptability limit. Within the same column, values not followed by the same lowercase letter are significantly different (P < 0.05). c All treatment was conducted at 40
C for 3 min. d Untreated samples. e Samples washed with deionized water (Positive control). f Samples treated with thermosonication. g Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication). b

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A.R. Mansur, D.-H. Oh / Food Microbiology 51 (2015) 154e162

Table 6 Off-odoura of treated kale during refrigeration storage. Treatmentsc

Storage (
C)

Time (weeks)b 0

Control DW

d

e

TS þ DWf SAcEWg TS þ SAcEWg SCg TS þ SCg SHg TS þ SHg

4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7 4 7

1.3 1.3 1.5 1.5 1.7 1.7 1.6 1.6 1.4 1.4 1.6 1.6 1.8 1.8 2.3 2.3 2.5 2.5

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5a 0.5a 0.4a 0.4a 0.3 ab 0.3 ab 0.3a 0.3a 0.2a 0.2a 0.3a 0.3a 0.4abc 0.4abc 0.6bc 0.6bc 0.8c 0.8c

2

4

6

1.3 ± 0.5a 1.5 ± 0.3abc 1.7 ± 0.3abcd 2.0 ± 0.2d 1.7 ± 0.8abcd 1.8 ± 0.2bcd 1.7 ± 0.6abcd 1.7 ± 0.4abcd 1.4 ± 0.4 ab 1.4 ± 0.2 ab 1.7 ± 0.4abcd 1.8 ± 0.5bcd 1.8 ± 0.4bcd 2.0 ± 0.5d 3.4 ± 0.6e 3.5 ± 0.3e 3.3 ± 0.3e 3.4 ± 0.4e

1.5 1.8 2.0 2.1 2.0 2.0 1.8 1.8 1.4 1.5 1.8 2.0 2.0 2.1 3.5 3.6 3.3 3.6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.8 ab 0.6abc 0.1c 0.7c 0.2c 0.2c 0.2abc 0.3abc 0.3a 0.6 ab 0.5abc 0.5c 0.3c 0.5c 0.5d 0.6d 0.2d 0.3d

1.7 2.1 2.2 2.9 2.0 2.1 1.8 2.2 1.5 1.6 1.8 2.3 2.0 2.4 3.5 4.1 3.6 4.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2abc 0.4bcde 0.3cde 0.3f 0.2abcde 0.4bcde 0.5abcd 0.3cde 0.3a 0.4 ab 0.5abcd 0.3de 0.2abcde 0.6e 0.4g 0.4h 0.4gh 0.4h

8

10

12

14

2.4 ± 0.2cde 2.4 ± 0.4cde 2.7 ± 0.2e 3.5 ± 0.5f 2.1 ± 0.2bcd 2.6 ± 0.3e 2.0 ± 0.6abc 2.4 ± 0.3cde 1.6 ± 0.4a 1.8 ± 0.3 ab 2.0 ± 0.5abc 2.6 ± 0.4e 2.4 ± 0.4cde 2.7 ± 0.6e 4.0 ± 0.5g 4.7 ± 0.6h 3.8 ± 0.4 fg 4.2 ± 1.1g

3.2 ± 0.8ef 3.2 ± 0.5ef 3.4 ± 0.2 fg 3.7 ± 0.5g 2.5 ± 0.6bcd 2.8 ± 0.2cde 2.3 ± 0.8bc 2.6 ± 0.6bcd 1.8 ± 0.7a 2.1 ± 0.4 ab 2.5 ± 0.2bcd 3.0 ± 0.2def 3.0 ± 0.7def 3.2 ± 0.6ef 4.7 ± 0.6hi 5.0 ± 0.4i 4.5 ± 0.5hi 4.3 ± 0.5h

3.5 ± 0.3cd 4.2 ± 0.4ef 3.8 ± 0.3de 4.6 ± 0.6 fg 3.4 ± 0.3bcd 3.2 ± 0.3bc 3.5 ± 0.3cd 3.5 ± 0.2cd 2.1 ± 0.4a 2.3 ± 0.2a 3.0 ± 0.3b 3.2 ± 0.5bc 3.4 ± 0.4bcd 3.7 ± 0.8de 5.0 ± 0.7g e 4.7 ± 0.3g 5.0 ± 0.6g

3.8 ± 0.7bc 4.9 ± 0.7d 3.9 ± 0.5bc 5.0 ± 0.8d 3.6 ± 0.5bc 3.6 ± 0.3bc 3.7 ± 0.6bc 4.0 ± 0.4c 2.3 ± 0.6a 2.6 ± 0.3a 3.8 ± 0.7bc 3.5 ± 0.3b 3.5 ± 0.7b 3.9 ± 1.2bc 5.0 ± 0.5d e 5.0 ± 0.4d e

e

Not measured. a Off-odour: 1 ¼ none, 3 ¼ acceptable, 5 ¼ severe. Numbers in bold indicate scores above the acceptability limit. b Within the same column, values not followed by the same lowercase letter are significantly different (P < 0.05). c All treatment was conducted at 40
C for 3 min. d Untreated samples. e Samples washed with deionized water (Positive control). f Samples treated with thermosonication. g Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication).

the microbiological population in vegetables is expected to be dominated by natural psychrotrophic bacteria such as Pseudomonas spp.. Whilst, yeasts and molds grow slower in fresh and minimally processed (MP) vegetables due to the intrinsic properties such as a slightly acid to neutral pH favoring bacterial growth (Ragaert et al., 2007; Tournas, 2005). The efficacy of decontamination methods is reflected not only in the obtained microbiological reduction but also in the maintenance of this reduction during storage (Ragaert et al., 2007). In this study, the microbial populations on TS þ SAcEW treated kale were much lower compared to those on untreated kale (control) and kale treated with other decontamination treatments, which were also at

Table 7 Shelf life of treated kale during refrigeration storage. Treatmentsc

Controld DWe TS þ DWf SAcEWg TS þ SAcEWg SCg TS þ SCg SHg TS þ SHg a

SL

microbial growth

(days)a

SL

sensory

(days)b




4 C




7 C




4 C

7
C

6 8 10 12 >14 10 10 8 10

6 8 10 10 14 10 10 8 8

10 10 12 12 >14 12 12 2 2

8 8 10 12 >14 12 10 2 2

Shelf life based on microbial growth. Shelf life based on sensory panel evaluation. All treatment was conducted at 40
C for 3 min. d Untreated samples. e Samples washed with deionized water (Positive control). f Samples treated with thermosonication. g Samples treated with the individual treatments (SAcEW [Slightly acidic electrolyzed water], SC [Sodium chlorite], and SH [Sodium hypochlorite]) and their combinations with TS (Thermosonication). b c

the acceptable limits after 14 days of storage at 4 and 7
C, except Pseudomonas spp. on TS þ SAcEW treated kale stored at 7
C (14 days). These results suggest that TS þ SAcEW treatment was the most effective method to control L. monocytogenes and spoilage microorganisms on kale during refrigeration storage. Also, maintaining storage temperature at 4
C would enhance more effectively the microbial shelf life of kale following decontamination treatment as Rediers et al. (2009) reported that storage of fresh-cut vegetables should be maintained below 5
C to reduce the proliferation of pathogenic and spoilage microorganisms. The efficacy of decontamination methods also depends on the degree to which adverse effects occur in sensory qualities, such as visual defects, textural breakdown, browning, and off-odour. The effect of microbiological and physiological activity on sensory parameters of fresh and minimally processed vegetables has been investigated in several previous studies. Some of these studies are conducted to establish a limit for the psychrotrophic microbial count on vegetables in which degradation in sensorial quality occurs obviously (Ragaert et al., 2007). The current study reveals that the sensory parameters (OVQ, browning, and off-odour) of kale were not affected by decontamination treatments on day 0, except the off-odour on SH and TS þ SH treated kale. All the acceptability of sensory parameters of untreated and treated kale, however, decreased over time during storage at 4 and 7
C. TS þ SAcEW treatment was found to be the most effective method to prevent the undesirable effects in sensory parameters of kale. The OVQ of the TS þ SAcEW treated kale was extended up to >14 days, which was much longer compared to those of other decontamination treatments and control. The OVQ of untreated kale and kale treated with other decontamination treatments were limited to 8 to 14 days, corresponding to the population of spoilage bacteria (7 log CFU/g) and/or YMC (5 log CFU/g). Similar mez-Lo pez et al. (2007) shows that the OVQ of result from Go neutral electrolyzed oxidizing water (NEW) treated MP cabbage stored under equilibrium modified atmosphere at 4
C is prolonged

A.R. Mansur, D.-H. Oh / Food Microbiology 51 (2015) 154e162

up to >14, while that stored at 7
C is limited to 9 days with corresponding to psychrotrophic and aerobic plate counts exceeding 7 log CFU/g. The more rapid visual defects and textural breakdown in stored kale was likely due to higher activity of aerobic psychrotrophic bacteria. These bacteria are mainly pectinolytic species such as Enterobacteriaceae and Pseudomonas spp., which are the major spoilage agents and also responsible for bacterial soft rot (Escalona et al., 2010; Lund, 1992; Nguyen-the and Carlin, 1994). In addition, higher chemical and biochemical reactions due to metabolic or respiration rates in stored kale might also result in more rapid quality deterioration such as visual defects and textural breakdown (Kader, 1992) since the TS þ SAcEW treated kale stored at 7
C had a good OVQ after 14 days of storage even though Pseudomonas Spp. counts reached >7 log CFU/g. The rate of browning also determines the sensory shelf life of many vegetable commodities (Jacxsens et al., 2003). The browning reaction on TS þ SAcEW treated kale at 4 and 7
C was much slower compared to those on other treated kale, which was retarded up to >14 days. Interestingly, this also occurred in untreated kale stored at 4
C. These evidences, therefore, were more likely due to the higher metabolic or respiration rates of the stored kale rather than the activity of spoilage microorganisms. Kader and Ben-Yehoshua (2000) reported that tissue browning can be due to oxidation of phenolic compounds by polyphenol oxidase (PPO) which results from loss of compartmentalization within the cells when exposed to physical and/or physiological stresses. Different results, however,  mez-Lo pez et al. (2007) and Koseki and Itoh were observed by Go (2002) where NEW and acidic electrolyzed water (AcEW) treated MP cabbage showed less browning than untreated sample and  mez-Lo  pez et al. (2007) rewater washed control, respectively. Go ported that PPO could have been inhibited by ions derived from NaCl used to produce NEW due to the formation of a complex between the halide and copper ion in the enzyme, while Koseki and Itoh (2002) postulated that PPO might have been oxidized and weakened by the strong ORP of AcEW. The off-odour on TS þ SAcEW treated kale was also retarded up to >14 days, which was much longer compared to those on other treated kale and control. The off-odour on untreated kale and kale treated with other decontamination treatments occurred in 8 to 14 days, while off-odour on kale treated with SH and TS þ SH occurred in just 2 days. We found that the off-odour was not always observed as the populations of spoilage microorganisms reached the unacceptable limits and/or the product started to visually deteriorate. This might not only be due to the high ethanol producing fermentation reactions, which then caused alcoholic odour (Hagenmaier and Baker, 1998) but also due to the application of SH treatment was likely to have an important role in leading the more rapid off-odour on kale. Generally, the end of shelf life derived from microbial growth was not always same with that from sensory panel evaluation. TS þ SAcEW treatment was the most effective method to prolong the microbial and sensory shelf life of kale up to 14 days when stored at 4 and 7
C. Accordingly, a shelf life extension of around 4 and 6 days in kale stored respectively at 4 and 7
C was achieved by treating fresh-cut kale with TS þ SAcEW. In conclusion, the application of TS þ SAcEW treatment could prolong the microbial and sensory shelf life of kale during storage under refrigerated temperatures at 4 and 7
C. Exceeding the microbiological limits did not always result in adverse effects on sensory attributes as both microbiological and physiological activity play a role in spoilage of stored kale. This study, therefore, may provide some valuable insights that help to the shelf life extension of fresh produce. The limitation of this study, however, is that we used a small scale sample which was air packaged in a stomacher bag. This condition does not adequately represent industrial

161

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