Effects of micro-perforated film on the quality and shelf life improvements of pork loins during chilled storage

Effects of micro-perforated film on the quality and shelf life improvements of pork loins during chilled storage

Meat Science 66 (2003) 77–82 www.elsevier.com/locate/meatsci Effects of micro-perforated film on the quality and shelf life improvements of pork loins ...

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Meat Science 66 (2003) 77–82 www.elsevier.com/locate/meatsci

Effects of micro-perforated film on the quality and shelf life improvements of pork loins during chilled storage Keun-Taik Lee*, Won-Sun Choi, Chan-Suk Yoon Department of Food Science, Kangnung National University, 123 Jibyun-Dong, Kangnung, Kangwon 210-702, South Korea Received 13 August 2002; received in revised form 9 December 2002; accepted 9 December 2002

Abstract The effects of micro-perforated film (MPF) packaging method on the quality and shelf life of pork loin during chilled storage were investigated, compared with non-packaging (control) and non-perforated film (PPF) packaging. Loins were removed from five hogs after slaughter and chilling, and each loin was portioned into four parts. These samples (40 sections) were allocated to each of five intervals (0, 1, 4, 7 and 14 days) to minimize variation among treatments and stored for 14 days under 55  10% relative humidity and 0  1  C. Total aerobic counts of control sample were significantly lower than those from MPF or PPF loins after 14 days of storage. At 14 days, the counts of Pseudomonas and Enterobacteriaceae in the samples from MPF were significantly lower than those from PPF. At 7 days, ‘L’ value for the control and ‘a’ value for PPF were significantly lower, and E values for the control were significantly higher than other samples. During storage time, percentage weight loss was the most in the control samples with MPF following and PPF the least. Accordingly, water contents decreased highest in the control samples followed by those from MPF and PPF. TBA values increased with storage time, however no significant differences were observed among treatments. Sensory evaluation analyses showed that MPF samples were generally better evaluated in all parameters over the storage time. Therefore, it has been concluded that MPF could be used as an effective packaging technology since it extends the shelf life of fresh meat by controlling the microbial growth and evaporation at a moderate level. # 2003 Elsevier Ltd. All rights reserved. Keywords: Packaging; Micro-perforation; Quality; Shelf life; Pork loin

1. Introduction Raw meat is one of the most perishable foodstuffs and thus susceptible to undergo physicochemical changes during storage. Contamination by micro-organisms is one of the major causes that shorten the shelf life by accelerating the spoilage of meat. Especially, evaporative weight loss lowers the quality of the meat and brings economical loss. By introducing an appropriate packaging, shelf life of meat can be extended by excluding the sources of additional microbial contamination and reducing evaporation during chilled storage and distribution. In this connection, major problems with the chilling cabinet have been enumerated as follows: too high heat loads for the cabinet, extreme temperature cycles, * Corresponding author. Tel.: +82-33-640-2333; fax: +82-33-6474559. E-mail address: [email protected] (K.-T. Lee). 0309-1740/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0309-1740(03)00017-2

excessive weight loss in the products, excessive defrost cycles and inaccurate temperature sensors, etc. (Gormley & Zeuthen, 1990). In most of the current refrigerators for storage of fresh meat, humidity control is poorly conducted, which can lead to relative humidity change in refrigerator depending on the quantity and condition of the meat being loaded. When the relative humidity in the refrigerator is too high, putrefaction proceeds rapidly due to the microbial growth. Bem and Hechelmann (1995) reported that the film wrapping of fresh meat caused the increase of the relative humidity resulting in the shortening of shelf life. Contrarily, weight reduction or discolouration occurs when the relative humidity is too low. Moerman (1972) has therefore recommended that the required relative humidity for storage of fresh meat should be 80–90% and high as possible, but at least 60% for display. However, according to Lee, Lee, Lee, and Chung (1998), the relative humidity of refrigerators and display cabinets in retail butchers decreases below 50% occasionally.

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Therefore, development of a new and novel preservation technology that would not demand additional investment and yet be efficient and practicable to solve the earlier mentioned problem is being required. In this regard, the perforation-mediated package is one such alternative, where the regulation of water vapour and gas exchange is achieved (Fonseca, Fernanda, Oliveira, & Chau, 1999). This study was carried out to prove the effectiveness of micro-perforated film for minimizing microbial contamination and evaporative weight loss of fresh meat during chilled storage.

2. Materials and methods 2.1. Sample and treatment After pigs were slaughtered, loin meat was selected from a total of five carcasses. pH was determined 1 and 24 h postmortem before choosing a carcass for use in the experiment. The sample loin was obtained by sanitary sampling methods and transported to the laboratory in iceboxes. A loin section (two obtained from one pig) was divided into four pieces and thus totally forty samples were obtained from five pigs. These samples were put on the table covered with a plastic pouch that had been previously disinfected with 70% ethanol solution for 30 min and turned inside out. Then, the samples were sufficiently mixed with inherent microorganisms by hand wearing sterile plastic gloves for ensuring the homogeneous contamination of the sample surface and randomly allocating samples. The subsequent sample treatment and packaging methods were as follows; (1) not packaged: the control group, (2) packaged with micro-perforated oriented polypropylene film (MPF), thickness 30 mm, water vapour transmission rate of ca. 2000 g/m2/day, about 1600 perforations in 1 mm diameter per 1 dm2 of film and (3) packaged with non-perforated oriented polypropylene film (PPF), thickness 30 mm, water vapour transmission rate of 8.6 g/m2 day, oxygen permeation rate of 2450 cm3/m2/day/atm. The samples were then stored in a convection type refrigerator (LSS-370RD, LG, Korea) that was maintained at 0  1  C and 55 10% relative humidity for five different periods: 0, 1, 4, 7 and 14 days. Temperature and humidity changes during storage were observed with a data logger (Agent HT-1, Switzerland). For the determination of thiobarbituric acid (TBA) value and water content, the samples were obtained by excising ca. 30 g (10 mm thick) from the meat sample surface on the appropriate day of storage. Then, they were minced and vacuum packaged in polyamide and polyethylene laminated film before freezing at 18  C.

2.2. Microbial analysis Total aerobic bacteria (Standard-1 agar, Merck), Pseudomonas (GSP agar, Merck), Enterobacteriaceae (DHL agar, Merck), lactic acid bacteria (MRS agar, Merck), and yeast and mould (Malt extract agar, Merck) were analysed during storage of pork loin samples. The procedure for the sample selection, inoculation and incubation was followed according to the method of Lee and Yoon (2001). 2.3. Colour evaluation On the appropriate day of storage, a Colour difference meter (CR-300, Minolta, Japan) with a 38-mm diameter aperture, ‘D65 illuminant’ and 10 standard observers were used for the determination of colour characteristics. For the packaged samples, colour was measured immediately upon opening the package. Hunter ‘L’ (lightness), ‘a’ (redness) and ‘b’ (yellowness) were measured and thereafter the total colour difference E’s value was calculated using the following equation (CIE, 1976):  1=2 E ¼ ðLo Lx Þ2 þðao ax Þ2 þðbo  bx Þ2 where Lo, ao and bo were ‘L’, ‘a’ and ‘b’ at 0 day and Lx, ax and bx were ‘L’, ‘a’ and ‘b’ at x days. 2.4. Measurement of physicochemical properties The weight loss was determined as the difference between the initial weight and weight after chilled storage, and expressed as a percentage of the original meat weight. Changes in TBA, using the method of Witte, Krause, and Bailey (1970) were used to assess lipid oxidation. Absorbance was read at 538 nm, and malonaldehyde concentration in the meat was determined. The TBA value was expressed as mg malonaldehyde per kg meat. pH was measured by inserting a spear type electrode connected to pH meter (720A, Orion, USA) directly into samples. The water content of the samples was determined by oven drying at 105  C. 2.5. Sensory evaluation A trained, 10-member panel evaluated meat samples by using a five-point hedonic scale with 0.5 intervals for discolouration (5, no, to 1, total), off-odour (5, no, to 1, abundant), outer appearance (5, no, to 1, extreme), and overall acceptability (5, extremely desirable, to 1, extremely undesirable). For packaged meat samples off-odour was assessed immediately after opening the package. Shelf life criteria assumed that rejection would occur when the sensory attributes declined below 3.0.

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2.6. Statistical methods This experiment was repeated in triplicate and the mean values were obtained from three measurements at each day. Data were analysed by using SAS statistical package (SAS Institute, Cary, NC). Furthermore, Duncan’s multiple range test was used to compare means and significance which was established at P < 0.05.

3. Results and discussion 3.1. Changes in microbial counts Table 1 is the comparison of changes in microbial counts of pork loin during storage between the control and the treated samples that were either packed with MPF or PPF. The aerobic plate count (APC) in all three sample treatments was initially 5.50 log10cfu/cm2 and was maintained at this level until the seventh day of storage. However, after 14 days the increase of APC was significant and was more pronounced in MPF and PPF samples than in the control group. It has been concluded that the meat surface of control group was getting dry during storage and thereby the microbial growth was relatively retarded compared with packaged samples. Catsaras (1973) has reported that the meat Table 1 Changes in microbial counts (log10cfu/cm2) of pork loins which were not packaged (control) and wrapped with micro-perforated polypropylene film (MPF) or non-perforated polypropylene film (PPF) after 0, 1, 4, 7, and 14 days of storage at 0  C Microorganism

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surface becomes dry, leading to inhibited growth of microorganisms, when the relative humidity of the refrigerator is lowered from 95 to 75% and the temperature is maintained at 0–2  C. Between MPF and PPF samples, the APC showed no significant differences (P < 0.05). Pseudomonas was detected from MPF and PPF samples at 3.74 log10 cfu/cm2 initially. However, on the 14th day, the count was increased to 5.79 and 6.52 log10 cfu/ cm2, respectively, which represented the more dominant growth of proteolysis spoilage bacteria in PPF rather than in MPF samples (P< 0.05). Enterobacteriaceae was initially 3.75 log10cfu/cm2 but after the fourth day, PPF sample tended to have higher counts than the control and MPF samples. As with Pseudomonas, the count of Enterobacteriaceae of PPF sample at day 14 was significantly higher than the others (P < 0.05). The growth of lactic acid bacteria, and yeast and mould were not significantly different among samples during storage (P > 0.05). It has been proved that the packaging with plastic film of low water vapour permeability would not be advantageous to retard the microbial growth of fresh meat, especially in extended storage. Previously, similar results were reported that wrapping carcass or wholesale cut beef with PVC film was more beneficial for preserving colour of lean meat and fat, and for the reduction of weight loss, but accelerated microbial growth due to the condensation of water vapour inside the film when the meat was stored longer than 9 days (Marriot, Smith, Hoke, & Carpenter, 1977; Rea, Smith, & Carpenter, 1972). 3.2. Change of meat colour

Treatments Storage time (days) 0

1

4

7

14

5.47 a 5.78 a 5.88 a

6.94 bAb 7.79 bB 8.04 bB

Total aerobes

Control MPF PPF

5.50 aa 5.38 a 5.68 a 5.50 a 5.42 a 5.48 a 5.50 a 5.42 a 5.40 a

Pseudomonas

Control MPF PPF

3.74 a 4.03 a 4.02 a 3.96 aA 5.59 bA 3.74 a 3.56 a 4.48 b 4.61 bB 5.79 cA 3.74 a 3.12 a 4.05 ab 4.90 bB 6.52 cB

Entrobacteriaceae Control MPF PPF

3.75 a 3.48 a 3.79 a 4.02 a 3.75 a 3.54 a 3.98 a 4.10 a 3.75 a 3.54 a 4.02 ab 4.56 b

5.52 bA 5.76 bA 6.40 cB

Lactic acid bacteria

Control MPF PPF

4.40 a 4.57 a 4.24 a 4.40 a 4.22 a 4.31 a 4.40 a 4.27 a 4.35 a

4.52 a 4.61 a 5.01 a

6.41 b 6.32 b 6.82 b

Yeast and Mould

Control MPF PPF

4.66 a 4.55 a 4.38 a 4.66 a 4.18 a 4.45 a 4.66 a 4.22 a 4.48 a

4.46 a 4.88 a 4.73 a

6.21 b 6.61 b 6.45 b

a Means with different small letters in the same row represent significant difference at P < 0.05. b Means with different capital letters in the same column represent significant difference at P < 0.05.

Table 2 is the result of Hunter ‘L’, ‘a’, ‘b’ and E of the pork loin wrapped with MPF and PPF compared with control samples. The ‘L’ value of control sample showed slight decrease during the storage period and was significantly lower since the seventh day when compared to MPF and PPF samples (P< 0.05). On the other side, no significant change for the ‘L’ value could be observed with PPF over the storage period (P > 0.05). The significant difference between MPF and PPF sample for the ‘L’ value also was not established (P > 0.05). The ‘L’ value of the control sample lowered and was even more pronounced as the storage period extended. This can be explained by the fact that the excessive water evaporation from the meat surface caused the colour to become darker owing to the reduced light’s diffusion. The changing curve of ‘a’ value during storage appeared differently as that of ‘L’ value, that is, a gradual increase followed by bending down. The peak point of the ‘a’ value was reached at day 7 for the control and MPF samples, and at day 4 for the PPF sample. After 7

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Table 2 Changes in colour values of pork loins which were not packaged (control) and wrapped with micro-perforated polypropylene film (MPF) or non-perforated polypropylene film (PPF) after 0, 1, 4, 7, and 14 days of storage at 0  C Parametera Treatments Storage time (days) 0

1

4

7

L

Control MPF PPF

51.21ab 46.93 a 46.22 a 45.55 a 49.75 a 47.75 a

a

Control MPF PPF

14.62 a 16.01 ab 17.87 ab 18.99 bB 14.04 a 15.24 a 16.62 a 17.19 aB 14.47 a 16.15 a 16.48 a 14.52 aA

b

Control MPF PPF

6.23 a 5.64 a 5.38 a

9.03 b 8.36 b 9.00 b

E

Control MPF PPF

– – –

5.30 a 3.04 a 4.46 a

14

45.36 a 38.45 bAc 28.59 cA 43.47 ab 43.72 abB 40.08 bB 46.31 a 47.96 aB 45.56 aB

10.68 bc 11.01 c 9.85 b 10.02 b 9.37 b 8.94 b 8.03 b 5.65 b 5.63 b

14.30 cB 5.94 bA 3.98 aA

17.54 abB 14.13 aA 12.36 bA 10.15 bB 8.59 bAB 7.32 abA 23.14 dB 6.81 cA 5.07 bA

a

L, lightness; a, redness; b, yellowness; E, total colour difference. Means with different small letters in the same row represent significant difference at P< 0.05. c Means with different capital letters in the same column represent significant difference at P < 0.05. b

days ‘a’ value of PPF sample was significantly lower, compared with the control and MPF samples (P < 0.05). It can be assumed that the evaporation has occurred at a minimum from the meat surface packed with water vapour tight film as in the case of PPF. On the other side, the condensed water vapour inside the package extracted the water-soluble meat pigment, myoglobin, which ultimately led to a pale surface colour. It has been observed with the naked eye that the meat surface discoloured palely and light red meat juice was formed inside the film. Meanwhile, the ‘b’ value rapidly increased in all samples at the first day and steadily increased until the seventh day for the control and MPF sample, and fourth day for PPF sample, but started decreasing afterwards. Furthermore, the significant difference was not shown until day 7 of storage (P > 0.05), but PPF sample showed the lowest value among the samples on day 14 (P < 0.05). E indicates the total colour difference between the samples at the day 0 and the each measurement day. The increase of E implies that the greater change in the absolute values of colour space coordinates as compared to those of the day 0 has occurred. The trend of the inconsistent increase or decrease of E value during storage could not be confirmed in the PPF samples (P > 0.05). On the contrary, with the MPF samples, E value tended to increase gradually, and notably a significantly sharp increase could be confirmed with the control group after 7 days (P < 0.05). Boakye and Mittal

(1996) supported our result that the total colour difference increased linearly with an increase in ageing time for chilled beef without packaging or packaged under vacuum indicating that the sample was becoming darker in colour which associated with the decrease of ‘L’ value. The significant differences of E among samples could not be confirmed until day 4, however the control samples showed significant difference after day 7 compared with MPF and PPF samples (P < 0.05). 3.3. Physicochemical change Table 3 shows the comparison of weight loss, water content, TBA and pH values of pork loin packed in MPF and PPF with the control samples during storage. The weight loss of the control group consistently showed higher value compared with PPF or MPF sample during storage. That is, the weight loss of control group increased from 2.1% at day 1 to 21.8% at day 14 (P < 0.05). On the contrary, the weight loss of MPF and PPF samples showed a low level of 1.7% and 0.7% each on day 1, but increased gradually in both groups as the storage period extended. After 14 days, the weight loss of MPF and PPF samples reached 9.4 and 5.4%, respectively (P < 0.05). According to Malton (1972), the beef and pork stored in a convection type refrigerator for 24 h showed an average of 3% of weight loss. Furthermore, Taylor (1972) showed a different weight loss level from 1.95 to 5.07%, when Large White breed was slaughtered and slowly cooled for 24 h, and subse-

Table 3 Changes in weight loss, water contents, TBA and pH values of pork loins which were not packaged (control) and wrapped with microperforated polypropylene film (MPF) or non-perforated polypropylene film (PPF) after 0, 1, 4, 7, and 14 days of storage at 0  C Parameter

Treatments Storage time (days) 0

Weight loss (%)

1

Control MPF PPF

0.0 a 0.0 a 0.0 a

Water Control contents (%) MPF PPF

73.8 a 73.4 a 73.3 a

a

4 b

2.1 bB 1.7 bB 0.7 bA 72.7 a 74.2 a 74.3 a

7 5.7 cC 4.3 cB 2.5 cA

14

10.0 dB 21.8 eB 7.2 dB 9.4 eA 2.7 cA 5.4 dA

69.7 abA 65.3 bA 53.1 cA 73.5 aB 72.7 aB 66.7 bB 73.3 aB 73.4 aB 72.3 aB

TBA (ppm)

Control MPF PPF

0.29 a 0.29 a 0.29 a

0.37 b 0.36 b 0.33 ab

0.38 b 0.37 b 0.32 ab

0.41 b 0.37 b 0.41 b

0.48 c 0.47 c 0.48 c

pH

Control MPF PPF

5.7 5.8 5.7

5.7 5.7 5.8

5.8 5.8 5.8

5.8 5.7 5.8

5.8 5.9 5.9

a Means with different small letters in the same row represent significant difference at P < 0.05. b Means with different capital letters in the same column represent significant difference at P < 0.05.

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quently different parts of meat were stored for 2 days at 0  C without packaging. Newton’s diffusion law determines the evaporation rate of the raw meat surface. In other words, the rate of water vapour diffusion through the air film of meat surface direct proportions to the exposed surface, inverse proportions to the air film’s thickness and direct proportions to the air layer’s pressure difference (Bailey, 1972). Therefore, the evaporation of the raw meat stored in the refrigerator is affected by the meat’s surface area and water vapour pressure, storage temperature, the air velocity and relative humidity in the refrigerator (James & Bailey, 1990). As shown in Table 3, the higher result is obtained in MPF sample compared to PPF over the storage period. This is conjectured to be the evaporative weight loss from the meat surface packed in micro-perforated film while storing in the refrigerator with low relative humidity. Nevertheless, the evaporative weight loss from MPF samples was higher than expected. In consideration of this result, further research needs to be performed for reduction of the evaporative loss by reducing the size and number of perforations rather than the conditions of this experiment. The water content showed a level of 73.3–73.8% in samples initially. However, it fell significantly after 14 days in the control and MPF samples, while changing little in PPF sample during storage. Regarding the difference of water content among the samples, the control samples had significantly lower values after day 4 than those of MPF and PPF samples. This could be attributed to the excessive evaporative weight loss in control samples. The TBA value steadily increased from 0.29 ppm of day 0 to MPF sample 0.47 ppm, and PPF and control samples 0.48 ppm each on day 14. The significant increase of TBA values in all samples was observed after 14 days. However, the significant difference between samples was not determined. Numerous factors affect lipid oxidation including light, oxygen concentration, temperature, presence of anti- and pro-oxidants, degree of unsaturation of the fatty acids and the presence of enzymes (Skibsted, Mikkelsen, & Bertelsen, 1998). However, above factors would basically not be different from the type of packaging or treatment as in this study, and was assumed that lipid oxidation was not affected even by drying-up (as in the control) or by wetting (as in the PPF) the meat surface of sample. Other researchers reported that the increase of TBA values with storage was observed for modified atmosphere packaged beef (Jacobsen & Bertelsen, 2000) and pork packaged with polyethylene film (Juncher et al., 2001). However, lipid oxidation is not normally considered to be a limiting factor for shelf life of aerobic packed chill stored meat, as lipid oxidation occurs at a slower rate than discolouration from microbial growth (Zhao, Wells, & McMillin, 1994).

The pH of sample meat did not change much and maintained the level of 5.7–5.9 until the end of storage, allowing us to assume that the proteolysis degradation in all samples has not fully developed to increase the pH, apparently. 3.4. Sensory evaluation Table 4 is a comparison of the sensory quality changes of pork loins during storage between the control group and those packaged with MPF and PPF. The evaluation of the discolouration showed a tendency of declining as the storage period extended and was lower than 3.0 in all samples at day 14 losing its market value. However, the MPF sample was assessed as higher than 4.2 until day 7, and 2.7 at the 14th day receiving the highest evaluations among three samples. On the contrary, the PPF sample obtained 1.5 in discolouration score after day 14 indicating a pronounced discolouration. The off-odour score was over 3.9 until the storage day 7, but decreased below 3.0 in all samples after day 14. Tewari, Jayas, and Holley (1999) reported that off-odour develops at 7 log10cfu/cm2, which was also confirmed in this study as shown in Table 1. At this time, off-odour was described as stale and slightly putrid. Table 4 Sensory evaluation scores of pork loins which were not packaged (control) and wrapped with micro-perforated polypropylene film (MPF) or non-perforated polypropylene film (PPF) after 0, 1, 4, 7, and 14 days of storage at 0  C Parameter

Treatments Storage time (days) 0

1

4

7

14

Discolourationc Control MPF PPF

5.0 aa 4.8 a 4.2 b 5.0 a 4.9 a 4.5 b 5.0 a 4.8 a 4.2 b

3.5 cAb 2.1 dB 4.2 cB 2.7 dC 3.4 cA 1.5 dA

Off-odourd

Control MPF PPF

5.0 a 5.0 a 5.0 a

4.9 a 4.5 b 4.9 a 4.6 b 4.9 a 4.5 b

3.9 c 4.0 c 3.9 c

Outer appearancee

Control MPF PPF

5.0 a 5.0 a 5.0 a

4.7 a 4.1 bA 4.8 a 4.4 bB 4.6 b 4.0 cA

3.6 cB 2.5 dB 4.1 cC 3.0 dC 3.2 dA 1.8 eA

Overall acceptabilityf

Control MPF PPF

5.0 a 5.0 a 5.0 a

4.7 b 4.2 cAB 3.6 dA 2.1 eB 4.9 a 4.5 bB 4.1 cB 2.9 dC 4.7 b 4.1 cA 3.4 dA 1.7 eA

2.6 d 2.8 d 2.7 d

Sensory panel rating of each treatments a Means with different small letters in the same row represent significant difference at P <0.05. b Means with different capital letters in the same column represent significant difference at P<0.05. c 1=Total discolouration; 5=no discolouration. d 1=Abundant off-odour; 5=no off-odour. e 1=Extremely undesirable; 5=extremely desirable. f 1=Extremely undesirable; 5=extremely desirable.

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The outer appearance showed over 3.0 of MPF sample until day 14, maintaining the market value, whereas the control group and PPF showed significant difference with MPF after 4 days (P < 0.05). MPF sample obtained the highest-ranking overall acceptability throughout the storage period, while PPF sample obtained lower evaluation than the control group after 4 days. Especially, the PPF sample at day 14 appeared to be unattractive owing to the bleached colour, a little slimy surface and excessive purge loss. Generally speaking, the samples packaged with MPF were better evaluated in sensory attributes to other samples. Consequently, it can be concluded that the use of MPF proved to be an effective packaging technology for preserving the raw pork loin during chilled storage.

Acknowledgements The authors acknowledge the Kangnung National University for financial support to carry out this study.

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