Effect of corona discharge plasma on microbial decontamination of dried squid shreds including physico-chemical and sensory evaluation

LWT - Food Science and Technology 75 (2017) 323e328

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Effect of corona discharge plasma on microbial decontamination of dried squid shreds including physico-chemical and sensory evaluation Soee Choi, Pradeep Puligundla, Chulkyoon Mok* Department of Food Science & Biotechnology, Gachon University, Seongnam, 13120, 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 7 July 2016 Received in revised form 23 August 2016 Accepted 30 August 2016 Available online 31 August 2016

Non-thermal techniques for microbial decontamination in foods are becoming more promising. This work aims to evaluate the suitability and effectiveness of corona discharge plasma jet (CDPJ) for the inactivation of microbial contaminants of dried squid shreds. CDPJ was generated using 20 kV pulsed DC voltage and at a 58 kHz frequency. Upon the CDPJ treatment (0e3 min) of dried shreds, contaminants namely aerobic bacteria, marine bacteria and Staphylococcus aureus were inactivated by 2.0, 1.6, and 0.9 log units, respectively. Also, a 0.9 log reduction of yeasts and molds contaminants was observed. The inactivation pattern fitted well to the pseudo-first-order model rather than first-order kinetic model. The CDPJ treatment did not exert statistically significant (P > 0.05) changes in color characteristics and volatile basic nitrogen content of dried squid shreds as compared with untreated controls. In contrast, the moisture and thiobarbituric acid reactive substances levels of shreds were significantly (P < 0.05) altered by the plasma exposure. However, the treatment exerted no significant (P > 0.05) impact on the sensory characteristics of dried squid shreds. The CDPJ was found to be effective for microbial decontamination of real-world samples of dried squid. This technology can readily be applied to commercial dried squid processing. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Corona discharge plasma jet Dried squid shreds Microbial decontamination Kinetic modeling Physicochemical properties

1. Introduction In countries with long coastlines, seafood is a major source of proteins and unsatured lipids for human nutrition. The popular seafood squid (Todarodes pacificus) contains a significant (13.0e19.2%) amount of protein with all the essential amino acids in a good balance required by the human body, and thus squid meat may be considered nutritionally a very good source of protein (Bano, Shakir, Begum, & Qadri, 1992; Deng et al., 2012). As the fresh squid contains a moisture level of more than 80%, they are often preserved in the dry form for food use. Dried squid, made simply by drying after removal of the internal organs, has been highly prized in the Far East (Japan, Korea, China, Philippines and Vietnam, etc.) for its characteristic flavor and often eaten directly as a snack food, side dishes or refreshments (Lee, Park, & Ha, 2015). Traditionally, fresh squid are processed by hand and sun-dried (Sheehy & Vik, 1980), therefore, there is a greater risk of crosscontamination of product with foodborne pathogens apart from

* Corresponding author. Department of Food Science & Biotechnology, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea. E-mail address: [email protected] (C. Mok). http://dx.doi.org/10.1016/j.lwt.2016.08.063 0023-6438/© 2016 Elsevier Ltd. All rights reserved.

natural bio-contaminants. In a study, the microbial contamination during seasoned and dried squid processing, through the apparatus, machines, and employee's gloves at each step in processing, was demonstrated (Choi, Park, & Shin, 2012). The results obtained indicate that sanitation standard operating procedures (SSOP) must be developed for control of microbial contamination in seasoned and dried squid processing. It has also been reported that dried sliced squid distributed in supermarkets and traditional markets in Korea have tested positive for coliforms, S. aureus and Bacillus cereus (KMFDS 2012; Lee et al., 2015). Furthermore, a relatively high level of bacterial contamination in dried and seasoned seafood products compared with other food products was reported in a study (Kim, Kim, Kang, Hwang, & Rhee, 2013). Such high contamination levels are believed to originate from the raw materials (such as squid, octopus and filefish) and/or the manufacturing process. Therefore, various strategies to reduce microbial populations have received greater importance to render seafood products safe for human consumption. In recent years, much research has been focused on non-thermal methods of bio-decontamination of foods, such as use of high hydrostatic pressure, pulsed electric fields, ionizing radiation, high intensity ultrasound, and non-thermal plasma. Earlier, some non-

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thermal decontamination techniques have been applied to improve the microbiological quality of dried squid. Electron beam irradiation at dosages of 2, 4, 8, 12 and 16 kGy was successfully applied to improve the microbial safety and qualities of sliced dried squid. In that study, the decimal reduction dose (D10 value) of total bacteria count, yeast and mold, coliforms in sliced dried squid were 8.57, 4.60, and 8.10 kGy, respectively (Ko, Ma, & Song, 2005). Lee et al. (2015) investigated the decontaminating effects of different doses of UV-C light at 253.7 nm (0e18 kJ/m2) on E. coli, S. aureus and B. cereus in contaminated sliced squid surfaces. The counts of all the three bacteria were significantly (P < 0.05) reduced by the increase of UV-C dosage. Upon using the UV dosage of 18 kJ/m2, the E. coli, S. aureus and B. cereus bacteria were reduced by 1.35, 0.54 and 1.05 log CFU/g, respectively. Recently, non-thermal plasmas (NTPs) for biodecontamination of foods have also received substantial attention owing to their excellent antimicrobial activity against a wide range of microorganisms (Kim, Puligundla, & Mok, 2015; Ma et al., 2015; Misra, Tiwari, Raghavarao, & Cullen, 2011; Puligundla, Kim, & Mok, 2015). Nonthermal plasma is an ionized gas that consists of charged particles, electric fields, UV photons and reactive species (Deng, Shi, & Kong, 2006; Ma et al., 2015). Corona discharge (a pulsed DC discharge plasma) is one of several approaches for NTPs generation under atmospheric pressure conditions. These discharges in air have a complex chemical composition and are not been fully understood (Timoshkin et al., 2012). Corona discharges are known to produce chemically active species, namely oxygen ions and other charged species, which act as very strong oxidizers (Deng et al., 2007). Corona discharges generated in atmospheric air have a strong bactericidal effect. Several attempts have been made in the past to study the inactivation effect of different corona discharges on microbes (Fletcher et al., 2007, pp9; Julak, Kriha, & Scholtz, 2006; Kim et al., 2015; Korachi, Turan, Senturk, Sahin, & Aslan, 2009; Machala, Chladekova, & Pelach, 2010, pp7). Recently, we have shown the effectiveness of corona discharge plasma jet (CDPJ) for inactivation of Escherichia coli O157:H7 and Listeria monocytogenes on both fresh and frozen pork (Choi, Puligundla, & Mok, 2016). In the present study, CDPJ was used to decontaminate dried squid shreds, and possible changes in physicochemical and sensory properties of the shreds due to the plasma treatment were evaluated.

melted agar (~15 ml), and incubated at 37
C for 24 h. General purpose media namely plate count agar (PCA) and potato dextrose agar (PDA) were used for mesophilic aerobic bacteria, and yeasts and molds detection and enumeration, respectively. Selective enrichment media (Difco, Becton Dickinson and Co., Sparks, USA) used include marine agar (MA) for marine bacteria, eosinmethylene blue agar for E. coli, Baird-Parker agar for S. aureus, xylose-lysine-deoxycholate (XLD) agar for Salmonella spp., thiosulfate-citrate-bile salts-sucrose (TCBS) agar for Vibrio spp., and Oxford Listeria-selective agar supplemented with Oxford Listeriaselective supplement (Merck, Darmstadt, Germany) for Listeria monocytogenes. 2.3. CDPJ generation and squid shreds treatment CDPJ was generated as discussed in our earlier publications (Choi et al., 2016; Kim et al., 2015). The plasma generation system consisted of power supply, electrode assembly, air blower and sample treatment plate. The CDPJ used for experimentation was generated using 220 V AC power with an output voltage of 20 kV DC, at a current of 1.50 A, and a frequency of 58 kHz. A centrifugal air blower operating at a constant rotational speed of 3312 rpm was used for plasma jet/plume creation. Air velocity at the electrodes tip (with 5 mm inter-electrode gap) was 2.5 m/s. The size of emission slit in electrode housing unit was 6  35 mm. The shreds were treated with the CDPJ for 0, 1, 2 and 3 min. A span length of 25 mm was maintained between the plasma electrode and sample. The sample size for each treatment condition was 10 (n ¼ 10). For each treatment cycle, shreds (10 g) were aseptically taken in a glass petri dish and exposed to the CDPJ for a predetermined amount of time. Immediately after the plasma treatment, samples were tested for surviving microbes, and subjected to instrumental color measurement and sensory evaluation. The physicochemical analyses were conducted within 24 h post-treatment. All samples were stored in airtight dark bottles and kept at 25
C until analysis. 2.4. Modeling of inactivation Microbial inactivation models were developed based on pseudo-first-order kinetics or Singh-Heldman (2009) model, as described in our earlier publication (Kim et al., 2015).

2. Materials & methods

2.5. Physicochemical analyses

2.1. Dried squid samples

2.5.1. Moisture content and water activity Moisture content was measured using the 105
C drying method (AOAC, 2000). The water activity of shreds was analyzed using a humidity sensor (TR-77Ui, T&D Corporation, Nagano, Japan).

Unpackaged dried squid shreds were purchased at Moran market (Seongnam-si, Gyeonggi-do, Korea). The samples were then sealed in plastic bags to protect the product from moisture gain and stored at refrigeration temperature (4
C) until use. Shreds were measured about 7e10 cm long, 0.3e0.5 cm in width and 0.1e0.2 cm thick. 2.2. Identification & enumeration of microbial contaminants Microbial contaminants of dried squid shreds were detected using general and selective growth media, and viable counts were enumerated using standard plate count method (KFDA, 2011). Shreds (10 g) were taken in a sterile sample bag (3M Korea, Seoul, Korea), and sterile saline solution (90 ml) was added to it. Then, the sample was homogenized in a paddle blender for 3 min (Masticator, IUL Instruments, Barcelona, Spain). Under sterile conditions, aliquots (1 ml each) from filtrate were removed from the stomacher bag, serially diluted with 0.85% sterile saline, and aliquots of each dilution were pipetted into petri plates (pour plate method) with

2.5.2. Instrumental color measurement Color characteristics of untreated and CDPJ-treated squid shreds were determined using a colorimeter (CR-200, Konica-Minolta Inc., Tokyo, Japan) with an 8.0 mm aperture and D65 illuminant; and values are expressed in terms of the L* (brightness/darkness), a* (redness/greenness), and b* (yellowness/blueness). Also, the total color difference (DE) was calculated. 2.5.3. pH measurement For pH measurement, sample weighing 3 g was homogenized with 30 ml of distilled deionized water for 1 min. The homogenate was filtered through Whatman No. 2 filter paper, and then the pH of filtrate was measured using a Mettler Toledo 320 pH-meter. 2.5.4. Volatile basic nitrogen (VBN) VBN content of dried squid shreds was determined using the

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Conway's micro-diffusion technique as modified by Miwa and Iida (1973). Results were expressed as mg% (dry mass basis). 2.5.5. Thiobarbituric acid reactive substances Lipid oxidation of shreds was measured in terms of thiobarbituric acid reactive substances (TBARS) contents using a spectrophotometer (Model T60 U, PG Instruments, Leicestershire, UK) as described by Buege and Aust (1978). Results were expressed as mg malondialdehyde (MDA)/kg (dry mass basis). 2.6. Sensory evaluation The sensory characteristics that were evaluated include appearance, color, flavor, taste and texture of the product, using a 9point hedonic scale (9 ¼ like extremely; 1 ¼ dislike extremely). The dried squid shreds (untreated and CDPJ-treated) were sensory evaluated by a ten-member panel consisting of staff members from the Dept. of Food Science & Biotechnology, Gachon Univ., who were previously experienced and familiar in quality evaluation. 2.7. Statistical analyses The data are presented as the mean value ± standard deviation (SD) from triplicate experiments. All statistical analyses were carried out by using the SAS software package (version 9.2; SAS Institute Inc., Cary, NC). Microbial counts are expressed as log10 colony-forming units (CFUs) per gram of the dried samples. The statistical significance (P < 0.05) of the physicochemical and sensory evaluation data was analyzed by a one-way ANOVA test (Duncan's multiple range test). 3. Results and discussions 3.1. Microflora of dried squid shreds Dried shreds of squid were found to contain aerobic and marine bacteria as predominant contaminants. Other than these, S. aureus and yeasts and molds were determined in significant numbers (Table 1). However, the common foodborne bacterial pathogens, namely E. coli, Vibrio spp., L. monocytogenes and Salmonella spp., were not detected in the dried squid samples. The microbial contamination of dried squid products during their processing has been reported (Choi et al., 2012). The study showed that, in domestic and imported daruma (a semi-processed product of seasoned and dried squid) at companies A and B, the numbers of Staphylococcus aureus (3.6e6.0 log CFU/g) and Escherichia coli (1.3e1.4 log MPN/100 g) exceeded the regulatory limits of the Food Sanitary Law of Korea (S. aureus, 2.0 log CFU/g; E. coli, negative). An another study showed that the contamination levels of coliforms, S. aureus and Bacillus cereus bacteria in supermarket squid were 0.40, 1.00 and 1.00 log CFU/g, respectively; and their

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levels in traditional market squid were 0.90, 1.00 and 1.48 log CFU/g, respectively (KMFDS, 2012; Lee et al., 2015). Seasoned dried squids have been reported to contain total aerobic bacteria in the range of 3.00e6.45 log10 CFU/g (Yoon et al., 2009). All these studies indicate that microbial contaminants, including some pathogens, are generally exist in commercially available dried squid products.

3.2. CDPJ inactivation of microorganisms On CDPJ treatment of squid shreds, time-dependent reduction of counts of all the detected microbial contaminants was observed. Over 0e3 min treatment, >2.0 log reduction of initial aerobic bacterial count was noted; followed by marine bacteria, which were decreased by 1.6 log units (Fig. 1). Further, S. aureus, and yeasts and molds were reduced by 0.9 and 0.9 log units, respectively, during the same treatment period. In an earlier study, it was shown that CDPJ at the same treatment conditions was effective in reducing E. coli O157:H7 and L. monocytogenes on pork samples by 1.5 log and >1.0 log units, respectively, within 2 min (Choi et al., 2016). The results indicate that the extent of inactivation by CDPJ depends on the type of microorganism and the nature of the substratum.

3.3. Modeling of inactivation For quantitative microbial risk assessment, modeling the inactivation kinetics provides very useful information. It also provides the tools to compare the impact of different process technologies on reduction of microbial populations (FDA, 2013). Generally, microbial inactivation follows first-order kinetics. Therefore, it can be characterized by a single rate constant ‘k’ or its reciprocal, the D-value, which is considered a measure of resistance to an applied lethal agent. The scatter plot shows a linear relationship between the two variables, log inactivation and time of plasma exposure. However, using first-order kinetic inactivation model, satisfactory explanation cannot be given for inactivation pattern (data not shown) over the entire treatment period of 3 min. Therefore, pseudo-first-order kinetics or Singh-Heldman model was applied. The Singh-Heldman model exhibited a strong fit to rate of inactivation (Fig. 2). The coefficients of determination closer to 1 imply that inactivation patterns are better explained using pseudo-firstorder kinetics.

Table 1 Microbial contaminants of dried squid shreds (Unit: log CFU g1). Microorganisms

Mean ± SD (n ¼ 10)

Aerobic bacteria Marine bacteria Staphylococcus aureus Yeasts and molds Escherichia coli Vibrio spp. Listeria monocytogenes Salmonella spp.

6.69 6.63 5.25 4.83 ND ND ND ND

ND-not detected.

± ± ± ±

0.05 0.09 0.06 0.34

Fig. 1. Log reduction of microorganisms on dried squid shreds by CDPJ treatment.

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Fig. 2. Inactivation kinetics of shreds contaminants upon exposure to CDPJ (based on pseudo-first-order kinetics).

3.4. D0 -values D0 -value is similar to, if n ¼ 1, decimal reduction time (D-value) in case of heat sterilization, which is the time required at a certain temperature to kill 90% of the organisms being studied. Aerobic and marine bacteria of dried squid shreds were inactivated relatively quickly compared to Staphylococcus aureus, yeasts and molds by the CDPJ treatment (Table 2). Staphylococcus aureus bacteria seem to be more resistant to the CDPJ-induced inactivation as their D0 -value is the highest among the inactivated microbes. 3.5. Physicochemical properties 3.5.1. Moisture content and water activity (aw) Statistically significant (P < 0.05) reductions in the moisture content and water activity of dried shreds were observed on the CDPJ treatment. Reductions of about 10% and 15% of initial moisture content were observed after 2 and 3 min of the plasma treatment (Table 3). The water activity of shreds followed a similar pattern. The water activity was decreased significantly (P < 0.05) with increasing the plasma treatment time; about 19%, 25% and 28% reductions were noted after 1, 2 and 3 min treatment, respectively. The loss of moisture could mainly be due to the localized heat accumulation during the CDPJ treatment. However, the product quality was not negatively affected by low post-treatment moisture. The low moisture content can prevent the deterioration of squid. It has been reported that the spoilage of squid is retarded when its water content falls below 50e55% (Benjakul et al., 2000). 3.5.2. Color The color coordinates (L*, a*, and b*) of dried squid shreds did not change significantly due to the CDPJ exposure for up to 2 min. However, the brightness was significantly decreased (P < 0.05) compared to control, beyond 2 min exposure. The average ‘L*’ values were in the range of 75.68e77.74; ‘a*’ values were in the range of 2.29 to 2.40; and ‘b*’ values were in the range of 8.27e9.55 (Table 4). The magnitude of color change after the CDPJ

Table 2 D0 -values of microbial contaminants of dried squid by CDPJ treatment. Microorganisms

D0 -value (min)

Aerobic bacteria Marine bacteria Yeasts and molds Staphylococcus aureus

1.47 1.83 2.74 3.12

Table 3 Moisture content and water activity (aw) of CDPJ-treated dried squid shreds. Treatment time (min)

Moisture content (%)

0 1 2 3

26.91 25.51 24.12 22.88

± ± ± ±

0.15a 0.76ab 0.92bc 0.81c

aw 0.71 0.57 0.53 0.51

± ± ± ±

0.01a 0.02b 0.01c 0.01d

Values are given as mean ± SD (n ¼ 10). Distinct letters within the same column indicate significant differences (P < 0.05).

treatment was indicated by DE. An exponential increase in DE values with respect to treatment time was observed. Differences in perceivable color can be analytically classified as very distinct (DE > 3), distinct (1.5 < DE < 3), and small differences (DE < 1.5) (Tiwari, Muthukumarappan, O'Donnell, & Cullen, 2008). In this study, DE values were found to be ‘small differences’ for 1 min, and ‘distinct’ for 2 and 3 min plasma treatment times. According to Francis and Clydesdale (1975), color differences are obvious to the human eye when DE > 3. Therefore, the color differences due to the CDPJ plasma treatment are not visually noticeable. 3.5.3. pH, VBN and TBARS The pH values of dried squid shreds were slightly but insignificantly (P > 0.05) decreased upon the CDPJ exposure (Table 5). The pH values of untreated and the plasma-treated samples indicated slight acidic nature of shreds. pH is an important indicator of the level of lipid and protein hydrolysis in dried fish products

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treatment.

Table 4 Changes in CIE color values of dried squid shreds by CDPJ treatment. Treatment time (min)

Color coordinates

0 1 2 3

77.74 77.24 76.01 75.68

L*

3.6. Sensory characteristics a*

± ± ± ±

1.67a 0.62ab 0.65ab 0.49b

b*

2.40 2.32 2.31 2.29

± ± ± ±

0.24a 0.20a 0.24a 0.17a

8.27 8.38 9.00 9.55

DE ± ± ± ±

0.23a 0.54a 1.33a 0.92a

0a 0.79 ± 0.44a 2.20 ± 0.48b 2.53 ± 0.58b

Values are given as mean ± SD (n ¼ 10). Means in the same column followed by the same letter are not significantly (P > 0.05) different by Duncan's multiple range test.

Table 5 Changes in volatile basic nitrogen (VBN) content, 2-thiobarbituric acid reactive substances (TBARS) of dried squid shreds on CDPJ treatment. Treatment time (min)

pH

0 1 2 3

5.98 5.97 5.95 5.93

VBN (mg %) ± ± ± ±

0.07a 0.03a 0.04a 0.03a

51.67 52.21 52.61 53.41

± ± ± ±

2.06a 2.63a 2.51a 2.01a

1.75 1.85 2.02 2.07

± ± ± ±

0.01a 0.02b 0.04c 0.01d

Table 6 Sensory properties of CDPJ-treated dried squid shreds. Sensory properties

Treatment time (min)

Appearance Color Flavor Taste Texture Overall acceptance

8.17 8.00 8.50 8.67 6.92 7.58

0

1 0.72a 0.74a 0.52a 0.49a 0.67a 0.51a

8.25 8.08 8.33 8.42 7.08 7.42

2 ± ± ± ± ± ±

0.75a 0.67a 0.49a 0.51a 0.90a 0.51a

8.17 7.92 8.25 8.42 7.25 7.33

All the tested sensory properties, namely appearance, color, flavor, taste and texture, of dried squid shreds were not altered significantly (P > 0.05) upon the plasma exposure as compared with untreated controls (Table 6). All the tested sensorial qualities of squid shreds except texture were decreased minorly by the CDPJ treatment for 0e3 min. The texture of the plasma-treated shreds becomes more crispier, and this could be due to slight moisture loss. However, the overall acceptance scores of shreds remain unchanged upon the plasma treatment. 4. Conclusion

TBARS (mg MDA/kg)

Values are given as mean ± SD (n ¼ 10). Distinct letters within the same column indicate significant differences (P < 0.05). MDA, Malondialdehyde.

± ± ± ± ± ±

327

3 ± ± ± ± ± ±

0.83a 0.67a 0.75a 0.51a 0.62a 0.49a

8.00 7.67 8.17 8.58 7.50 7.50

± ± ± ± ± ±

0.92a 0.65a 0.72a 0.51a 0.52a 0.52a

Values are given as mean ± SD (n ¼ 10). Means in the same row followed by the same letter are not significantly (P > 0.05) different by Duncan's multiple range test.

In conclusion, a wide variety of microorganisms, from pathogenic and non-pathogenic bacteria to yeasts and molds, were found in dried squid shreds. The detection of Staphylococcus aureus from the shreds is certainly a cause for concern in terms of food safety; however, determination of pathogenicity is needed before any firm conclusions. The CDPJ used in the study inactivated all of the detected microorganisms in shreds by up to 90e99%. The inactivation kinetics of the microorganisms can be better explained using Singh and Heldman model. Except moisture level and TBARS, all of the other tested physicochemical and sensory properties of shreds were insignificantly affected by the CDPJ exposure. However, those factors significantly affected by the plasma treatment had exerted minimal-to-no effect on the product quality. Finally, the plasma treatment for 2 min was found optimal, after taking into account the physicochemical and sensory characteristics of plasma-treated squid shreds. Acknowledgements This research was supported by the Ottogi Foundation.

(Kim,Choi, Lee, Hong, & Kim, 2007). Based on the observed pH values, it is clear that the proteins and lipids of shreds were not considerably hydrolyzed due to the plasma exposure. Total volatile basic nitrogen (VBN) is a measure of protein degradation. The VBN includes trimethylamine, basic amines, and basic nitrogenous compounds such as ammonia. The concentrations of VBN of dried squid shreds were insignificantly (P > 0.05) increased upon the CDPJ treatment as compared with untreated control (Table 5). The average levels of VBN were in the range 51.67e53.41 mg%. The thiobarbituric acid reactive substances (TBARS) are formed as a byproduct of lipid peroxidation. TBARS values, which represent the content of the secondary lipid oxidation, are regarded as the most important nonmicrobial factor responsible for meat deterioration. Compared to untreated control, the TBARS levels of CDPJtreated squid samples were progressively (P < 0.05) increased with increasing exposure time. The average TBARS levels of shreds were in the range 1.75e2.07 mg MDA/kg dried sample (Table 5). The increase in TBARS value during the CDPJ treatment may be attributed to the increased oxidation of unsaturated fatty acids and to the partial dehydration in dried squids. Although generally dried squid exhibit low TBA levels soon after drying, levels significantly increase during further storage. It has been shown that TBA values of dried-seasoned squid increased progressively from the initial value of 0.886 mg MDA/kg to the final value of 1.95 mg MDA/kg during storage at 25
C for 120 days (Dong, Zhu, Li, & Li, 2013). An air-tight or vacuum packaging could prevent oxidative rancidity in dried squid after the plasma

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