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Article

Sustainable Utilization Technology for Improving the Freshness of Oysters—Development of Alkaline Electrolysis Seawater Depuration System

by
Hsin-Shan Tsai
1,2,
Yu-Tien Hsiao
2,*,
Yih-Ming Weng
1 and
Jen-Ming Liu
3
1
Department of Food Science, National Chiayi University, No. 300, Syuefu Road, Chiayi City 60004, Taiwan
2
Department of Seafood Science, National Kaohsiung University of Science and Technology, No. 142, Haizhuan Road, Nanzi District, Kaohsiung City 81154, Taiwan
3
Department of Fishery Production and Management, National Kaohsiung University of Science and Technology, No. 142, Haizhuan Road, Nanzi District, Kaohsiung City 81154, Taiwan
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(1), 785; https://doi.org/10.3390/su15010785
Submission received: 22 November 2022 / Revised: 21 December 2022 / Accepted: 28 December 2022 / Published: 1 January 2023
(This article belongs to the Special Issue Food Processing Technology Applications for Health and Safety)
Browse Figures
Review Reports Versions Notes

Abstract

:
The main purpose of this study is to study the effect of live oysters on various quality indicators after alkaline electrolytic seawater depuration. The depuration treatments were carried out for 12 h at temperatures of 5 °C, 10 °C, 15 °C, and 20 °C, pH = 9, 10, and 11, respectively. The total aerobic plate count (TAPC) of oyster meat was reduced from about 5.2 ± 0.4 log CFU/g to below detection limits when the oysters were depurated in pH = 11 alkaline electrolytic seawater for 9 h at 5 °C and 12 h at 10 °C. At the same pH value, the lower the seawater temperature, the lower the amount of TAPC, and it decreased with the increase in depuration time. After the oyster had been depurated, the chemical components contained in the oyster meat, such as protein, crude fat, and glycogen, did not change differentially. On the other hand, each group of live oysters (4 individuals) spat out an average of about 690–695 ± 0.4 mg of impurities and dirt. These are new achievements and discoveries. When the depurated oyster meat was stored at a low temperature, the freshness period could be extended to 21 days, which is much longer than the 5 days of the un-depurated oyster meat, and this is a significant difference. If coupled with vacuum packaging, the effect will be even better.

1. Introduction

Seafood is the primary source of animal protein for billions of people worldwide. Oysters are one of the most widely distributed marine biological resources in the world, and are very rich in nutrients, important ingredients including such as essential and non-essential amino acids, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and taurine, and trace elements such as zinc, selenium, iron, copper, and iodine are also abundantly contained in it [1,2,3]. Since the waters inhabited by aquatic species are prone to contamination, once contaminated bivalve mollusks are eaten, disease may follow, especially when most bivalve mollusks are eaten raw or cooked in small amounts. That is, the disease may enter the mouth, and one has to pay attention and take care. As filter feeders, shellfish can absorb and concentrate a variety of pathogens, and viruses and bacteria may become trapped in the shellfish if the water environment is contaminated with human feces [4].
There is a considerable amount of human pathogenic viruses and bacteria in coastal waters, and in order to protect the health of consumers, aquaculture production areas are subject to some level of surveillance and control to avoid products that carry human pathogens. How to ensure consumers safely obtain pathogen-free mollusks, of course, requires everyone’s joint efforts. However, from a production standpoint, it is not feasible to farm and harvest in areas free from any type of pollution, as there are few such waters. Therefore, it is imperative to cut in from the perspective of food processing.
Oysters are known to be one of the most delicious seafoods, but cleaning up before people eat them can be nerve-racking. The main reason is that oysters are a filtering marine organism, they grow by filtering nutrients in seawater, so there is a lot of sand, mud, and undigested algae in the intestinal or filtering system, worms, and microorganisms, which we call impurities or dirt [5,6,7]. Once the oysters have been shucked, it is not easy to remove these impurities or dirt from the oyster meat; therefore, they will affect the taste and hygiene safety of raw or cooked meat. Therefore, the removal of impurities or dirt accumulated in oyster meat is one of the focuses of this study. When we observed that the clams had to spit out sand, and the prawns with dirt stuffed in their intestines excreted disgusting stools, we decided to try to use depuration methods to solve the problem of the accumulation of impurities or dirt in the oysters, in order to improve the quality and hygienic safety of oyster meat.
Currently, the most common method of reducing pollution is decontamination operations. Purification is a commercial practice in which harvested shellfish are placed in a tank filled with clean seawater for a period of time, allowing the shellfish to discharge contaminants out of the body [4,8].
Accordingly, how to stimulate the filtering ability of the shellfish itself through artificial condition setting or pretreatment in the processing process, so as to achieve the purpose of reducing or sterilizing bacteria and discharging impurities or dirt in the shellfish. Improving food quality and solving food hygiene and safety issues are important goals of this study. Although the depuration process appears to be a promising post-harvest treatment for minimizing risks of infections associated with raw oyster consumption, it is necessary to use this in combination with other sterilization treatments to achieve better decontamination efficacy.
The oyster meat after shelling is soft and easy to break. Environmental pollution of the inhabited sea area, which attaches a high quantity of microorganisms to the shellfish, coupled with improper storage or transportation after processing, can lead to corruption and quality degradation, increasing the risk of health and safety. In recent years, many scholars have published methods that can sterilize or reduce the total number of bacteria. These include high hydrostatic pressure [9,10,11,12] and rapid freezing with frozen storage [13]. In terms of sterilization, Riondet et al. [14] believe that changing the amount of electric current in cells will prevent microorganisms from living, as microampere electric currents were shown to suppress the growth of bacteria. Ohmic heating (OH) is an alternative food processing technology used to effectively inactivate microorganisms [15]. Kim et al. [16] also proposed that the use of HOCl can affect the protein synthesis of microorganisms and damage the DNA, killing microorganisms instantly. Manousaridis et al. [17] and Hernández et al. [18] claimed that the use of ozone water to treat oyster meat can achieve significant sterilization effects. Furthermore, Venkitanarayanan et al. [19] observed that acidic electrolyzed water can be used as a fungicide. Hsu et al. [20] and Pal et al. [21] sprayed acidic electrolyzed water on food to reduce bacteria on the food surface. In addition, Shen et al. [22] and Naka et al. [23] pointed out that acidic electrolyzed seawater can effectively kill pathogenic bacteria. Sorio et al. [24] published a research report on UV irradiation depuration treatment. In general, acidic electrolytic water is selected for food surface sterilization. In addition, there has recently been a rise in research related to reduce microbial numbers, using ultra-high-pressure or high hydrostatic pressures technology, and therefore, extending shelf-life and improving the microbiological safety of ready-to-eat meats and seafood, and also improving the processing of shellfish and crustaceans [25,26,27,28].
When applied to organisms such as shellfish, the above processes will take longer to eliminate microorganisms present in the gut, muscles, etc. In addition, once electrolyzed acidic water is used in organisms, acidic pH conditions also inactivate the physiological functions of the tissues, often resulting in the death of the organisms. Moreover, although ultra-high-pressure technology can sterilize, it cannot remove impurities or dirt from living organisms. In terms of alkaline electrolyzed water utilization, Shimamura et al. [29] pointed out that alkaline electrolyzed water can reduce beef and chicken by 5 log values (pH = 11.5 and 4 °C). Al-Haq et al. [30] and Forghani [31] also proposed using alkaline electrolyzed water for soil improvement and sterilization. Regrettably, there is no research report at present to propose a technology that can make live aquatic products reach a sterile state at the same time, with no impurity or dirt in the body. How to solve the above defects is the ultimate goal of this study.
Since the effect of alkaline electrolyzed water on oyster depuration has rarely been studied, this study utilized a self-developed alkaline electrolysis seawater purification system to understand the quality changes of live oysters during the purification process. These include changes in chemical composition, the total aerobic plate count, attached or hidden, and changes in dirt accumulated in the oyster. In addition, the laboratory also continues to conduct a series of depuration studies on various live aquatic products (including shrimp, shellfish, and crabs), expecting to obtain clean, sterile, safe, and high-quality aquatic products. This will be sustainable research on how to ensure hygiene and safety, extend the fresh-keeping period, and improve the quality of aquatic products, and of course we hope to contribute to the whole society.

2. Materials and Methods

2.1. Experimental Materials

Live Pacific oysters (Crassostrea gigas) were purchased from Baishui Lake Production and Marketing Class in Dongshi Township, Chiayi County (Taiwan) on the day of experiment. Seawater was used to clean the sludge and attachments on the surface of oyster shells at the seaside, and to exclude small and damaged oysters. Immediately, they were taken to the laboratory, where the shells were washed with seawater and they were stored in groups.

2.2. Oyster Depuration Process

Figure 1 shows that the structure used in this experiment for oyster depuration treatment is actually a self-assembled temperature-controllable and clean alkaline electrolysis seawater depuration circulation system. Therefore, the filter devices in the first stage (F1–F3) are used to remove large particles of impurities (sand, plankton, other impurities, etc.), and the filter device in the latter stage (F4) is used to remove impurities (microorganisms, etc.) with smaller volumes, so that clean seawater can be obtained. The clean seawater is then divided into acidic electrolytic seawater and alkaline electrolytic seawater through the electrolysis system. Among them, the former is stored for use and the latter is discarded, and the latter enters the temperature-controllable cooling circulation system. The pH value can be adjusted to the set conditions according to the electrolysis equipment, and the relevant experiments can only be carried out after all the conditions are in place.
First, the seawater (100 L; salinity 34.5‰) was pumped from the tank T1 to the filter system (Fl~F4) and then into the electrolytic device El. After the process of electrolysis, the acidic electrolyzed water was stored in T2, while alkaline electrolyzed water was stored in T3. The temperature and pH of the seawater were kept at the ranges of 5–20 °C and 9–11, respectively. While reaching the steady controlled conditions, the oysters (64 live oysters (4 in each 500 mL beaker, for a total of 16 beakers)) were placed into batches. Then, the batches were placed into T4 (containing 60 L alkaline electrolytic water) and maintained for up to 12 h. Meanwhile, the viability of oysters was visually examined by lightly tapping on each shell to check its reaction. As a result, all oysters were alive and vivid during the depuration process.

2.3. Refrigeration Storage of Depurated Oysters

The oysters were shelled after purifying at 5 °C, pH = 11.0 for 12 h, then the oyster meat was rinsed with ice-cold sterile distilled water and put into polyethylene vacuum bags (40 g per bag, 12 bags in total). In addition, the same number of bags of oysters were not vacuum-packed. Then, the same amount of raw shucked oyster meat was prepared for the same pretreatment as before. All bagged oyster meats were stored in low-temperature equipment at 5 °C for the following measurements.

2.4. pH Measurement

A sample of five grams of shucked oyster meat and 4 times the amount of distilled water was homogenized for 2 min at 12,000 rpm at ambient temperature. The pH values of the homogenate were measured using a pH meter (LAQUA F-70 pH meter, HORIBA, Kyoto, Japan). In addition, the pH values of alkaline electrolyzed water were examined.

2.5. Chemical Compositions Analyses

The un-depurated and depurated fresh oysters were shelled (each weighed about 5 g) and made ready for experimenting. In accordance with methods 950.46, 940.25, and 991.36 of AOAC [32], analyses of moisture, crude protein, crude fat, carbohydrates, etc. of the sampled oysters were performed.

2.6. Sensory Evaluation

The sensory evaluation of un-depurated and depurated oysters was carried out by 11 panelists of staff from the Department of Seafood Science, National Kaohsiung University of Science and Technology, using the modified guidelines from Lee et al. [33], Songsaeng et al. [34], and Cao et al. [35], with a scale from 1 to 9: 9—excellent; 8—extremely good; 7—very good; 6—good; 5—acceptable; 4—poor; 3—very poor; 2—extremely poor, and 1—unacceptable. The panelists were asked to score the intensity of each characteristic, describing appearance (shape and size) and taste (chewing elasticity, sweet, and refreshing).

2.7. Determination of Total Aerobic Plate Count (TAPC)

The determination of total aerobic plate count was conducted according to Maturin et al. [36] and AOAC [37]. Oyster meat (25 g) was homogenized with 225 mL of sterile 0.85% sodium chloride. Serial dilution was made, and the pour plate method was used with plate count agar (Merck, Banani, Germany). Colonies were counted after incubation at 37 °C for 48 h.

2.8. Volatile Basic Nitrogen (VBN) Measurement

Volatile basic nitrogen (VBN) of oyster meat was measured using the microdiffusion method and determined according to Zamir et al. [38]. Oyster meat (10 g) was homogenized with 20 mL of 7% trichloroacetic acid. The homogenate was centrifuged (8000 rpm; 10 min) at ambient temperature and filtered with Whatman no. 1 filter paper. The volume of the filtrate was made up to 100 mL with 7% trichloroacetic acid. An aliquot of 1 mL 10% boric acid solution was put in the inner chamber of a Conway’s dish, and an aliquot of 1 mL TCA extract and 1 mL saturated sodium carbonate solution was placed in the outer chamber of a Conway’s dish. Afterwards, the Conway’s dish was incubated at 37 °C for 90 min and the boric acid was titrated with 0.02 N HCl until light red. A control using 1 mL 7% trichloroacetic acid to replace oyster meat extraction was included for calculation. The VBN of oyster meat was calculated using the following equation:
VBN (mg/100 g) = 0.28 × (A − B) × F × 100/S
A: titration volume (mL) of 0.02 N HCl for the oysters;
B: titration volume (mL) of 0.02 N HCl for the blank;
F: titer of 0.02 N HCl titer;
S: weight of samples (g).

2.9. Quantitative Determination of Impurity and Dirt Expelled from Oyster

Four live oysters (with shell; about 160 g on average) whose shells had been cleaned were placed into each 500 mL glass cup, for a total of 16 cups. Then, all the cups loaded with oysters was placed in the purification tank of alkaline electrolyzed seawater with set conditions for experimental operation. During this period, cups were removed in order to measure the quantity of impurities or dirt at the bottom of the cup. The live oysters were taken out of the cup, and the filtrate was filtered with filter paper. Finally, the filter paper containing impurities or dirt was dried at a constant temperature of 70 °C.
Wet of impurities or dirt = W1 − W2
W1: Weight of filter paper plus impurities after drying;
W2: Weight of filter paper.

2.10. Glycogen Content Assay

The Anthrone method improved by Kamata et al. [39] and Qin et al. [40] was used to determine the oyster glycogen content.
Anthrone reagent preparation: mix 76 mL of distilled water and 196 mL of concentrated sulfuric acid, then add 250 mg of thiourea and 250 mg of anthrone (also known as 9, 10-dihydroanthracene-9-one), mix well and dissolve.
The experimental operation process of glycogen: take 0.5 g of oyster meat, add 1.5 mL of KCl to a glass bottle, cover with a water bath and heat at 100 °C for 20 min, add 0.25 mL of Na2SO4 and 95% 2.0 mL of ethanol, mix and stir, and then use a water bath. The mixture was heated to boiling, cooled with water, centrifuged at 3000 rpm for 5 min, and the precipitate was dissolved in distilled water into a diluent for colorimetry and quantification.
The process of colorimetric quantification of glycogen: place 1.0 mL of the diluent in a test tube, add 5 mL of Anthrone reagent, mix and stir, boil in a water bath for 15 min, cool in running water for about 10 min, and perform colorimetric quantification with a 620 nm spectrophotometer (U-5100, Hitachi, Tokyo, Japan) (standard curve: repeat the above operation with 0.5 mL of hepatic glycogen solution).

2.11. Statistical Analysis

In order to analyze the correlation between the experimental groups, the comparison of means was accomplished by LSD and Scheffe’s test for significant differences at p < 0.05.

3. Results and Discussion

3.1. Chemical Composition

The experimental data in Table 1 are the chemical composition content of Pacific oyster under various depuration conditions. The contents of these chemical components, moisture, crude protein, crude fat, and carbohydrates, were 83.2 ± 0.6, 9.0 ± 0.2, 2.3 ± 0.1, and 4.6 ± 0.2 before purification, which are very close to those reported by Songsaeng et al. [34]. The conditions set in this experiment were the temperatures of 5 °C, 10 °C, 15 °C, and 20 °C and the pH values of 9.0, 10.0, and 11.0. Under these conditions, the depuration of seawater by alkaline electrolysis was carried out for 0, 6, and 12 h, respectively. According to Celik et al. [41] and Grizzle et al. [42], the nutritional composition of oysters will change greatly due to the influence of wind and waves, earthquakes, water temperature, vibration, and other factors. However, it can be seen in the table that the temperature did not affect the change of chemical composition under various depuration conditions. That is, temperature, one of the external influencing factors, did not cause any significant difference in the chemical composition content of oyster meat.
In Table 2, it is observed whether the chemical composition was affected by the pH value of electrolyzed seawater at a low temperature of 5 °C; it was still not affected by the change of pH value. From the above results, it can be considered that even if the depuration conditions are changed during the depuration process, the chemical composition will not be affected. The content of glycogen is one of the indicators of oysters. Berthelin et al. [43,44] and Baccaa et al. [45] all pointed out that the content of glycogen is the main factor affecting the flavor quality of bivalve mollusks. Therefore, this study also further determined whether the glycogen content changed under the depuration conditions. From the experimental data in Table 1 and Table 2, it can be seen that the content of glycogen in the oyster did not change due to the temperature and pH of the seawater. That is, pH value, which was another external factor, still did not affect the flavor of oysters before and after depuration. From the results in Table 1 and Table 2, it can be found that neither the temperature effect nor the pH effect caused significant changes in the chemical composition of oysters during the depuration process.

3.2. The Effects of pH, and Temperature on Microbial Quality during Depuration

Table 3 shows the changes of the total aerobic plate count (TAPC) observed under the depuration conditions of 20 °C, 15 °C, 10 °C, and 5 °C and pH values of 9.0, 10.0, and 11.0. From the experimental results, it can be seen that the TAPC decreased significantly with the increase of depuration time, regardless of different temperatures or pH values. In addition, under the same depuration pH value, the lower the temperature, the smaller the value of TAPC, and the better the depuration effect. This also showed that the level of temperature can affect the effect of depuration. At the same depuration temperature, the higher the pH value, the smaller the TAPC value, and the better the depuration effect. This also showed that the level of pH still affects the depuration effect. It can be further found from the experimental data that for sterilization, the effects of low temperature or high pH exist simultaneously.
As far as the overall depuration results are concerned, under the conditions of 5 °C and pH = 11, the depuration time exceeded 9 h, and a completely sterile state could be achieved. At 5 °C, pH = 10.0, the depuration time exceeded 12 h, and the same depuration effect was also found. However, at 5 °C and pH = 9.0, the value of TAPC was still high at 3.6 ± 0.4 even if the depuration time exceeded 12 h. Therefore, to achieve a completely sterile state, the depurated seawater temperature, pH value, and depuration time must meet certain conditions to show the effect.

3.3. The Effects of pH and Temperature on the Amount of Discharge Spat out by Live Oysters during Depuration

Table 4 shows the amount of dirt or impurities (including worms, algae, sediment, etc.) spat out by live oysters under the depuration conditions of 20 °C, 15 °C, 10 °C, and 5 °C and pH = 9.0, 10.0, and 11.0. Judging from the experimental data, the longer the depuration time, the more that is spat out. At 3 h of decontamination time, the amount of discharge averaged from 580.5 ± 0.6 to 610.2 ± 0.4, but there was no significant difference between the groups. When the depuration time increased to 6 h, the average spit volume increased from 650.2 ± 0.5 to 672.2 ± 0.6 mg, and the number increased significantly, which means that the longer the depuration time, the better the purification effect. This trend can also be seen in 12 h, where the average amount of spit was 680.2 ± 0.6 to 692.2 ± 0.5 mg. However, when the depuration time was 9 h and 12 h, there was almost no difference. It is speculated that the dirt in the oysters should have been completely discharged at this time.

3.4. The Quality of Depurated Oysters during Storage

3.4.1. Effects of Depuration on Quality of Oysters during Storage

Depuration of oysters does reduce the number of microorganisms and also affects the quality. In this study, the effect of vacuum-packaging on preservation during low-temperature storage was also observed. Figure 2a shows the data obtained by the storage experiment of oysters at a low temperature of 5 °C. It can be seen from the figure that the TAPC of the un-depurated group increased exponentially (UD-VP (■) = un-depurated but vacuum-packed oyster meat; UD-NVP (□) = un-depurated and non-vacuum-packed oyster meat), and both soared to more than 9.0 ± 0.3 log CFU/g on the ninth day. However, the difference between the two groups was not obvious, that is, whether vacuum packaging was performed or not, its influence was not great. In the other two groups of depurated oysters, microorganisms were detected on the 15th day (D-VP (▲) = depurated but vacuum-packed oyster meat; D-NVP (△) = depurated and non-vacuum-packed oyster meat). From the initial undiscovered microorganisms to the final detection of their traces, it is speculated that under the depurated conditions of 5 °C, pH=11.0, and 10 hours, they were not completely sterilized, leading to the revival of microorganisms during subsequent cryopreservation. Even so, the TAPC of depurated oysters was much lower than that of un-depurated oysters.

3.4.2. Effects of Depuration on VBN during Storage

As mentioned by Javier et al. [46], it was speculated that the increase in VBN value with time during storage was caused by bacteria and endogenous protein hydrolase. VBN compounds in seafood are often derived from the degradation of nitrogen-containing components, especially proteins, by microorganisms or enzymes. When the animal dies, the microorganisms start to grow, which is accompanied by the initiation of the body’s own digestion, which eventually causes the VBN content to increase with the increase of storage time.
Figure 2b shows the un-depurated oyster meat and the depurated oyster meat under non-vacuum-packing (□UD-NVP, △D-NVP) or under vacuum-packing (■UD-VP, ▲D-VP), in a refrigeration temperature of 5 °C when stored for 24 days, to observe the change in freshness. The freshness index, VBN, of oyster meat takes 25 mg/g as the boundary, and if it exceeds 25 mg/g, it is judged to be spoiled. It can be seen from the graphic trend that the un-depurated oyster meat stored for more than 5–7 days had entered a state of corruption. Among them, un-depurated and non-vacuum-packed meats were the worst (□UD-NVP), followed by un-depurated but vacuum-packed ones (■UD-VP). The oysters that were depurated and vacuum-packed (▲D-VP) could maintain their freshness for the longest time, for 24 days. Only depurated but non-vacuum-packed oyster meat entered a state of spoilage at 21 days (△D-NVP).
Whether oysters are cleaned or not has a huge impact on the maintenance of freshness, and whether or not vacuum-packing oysters will also slightly affect the preservation time of freshness. Based on the above research results, the reason why the freshness of un-depurated oyster meat cannot be maintained for a long time is not only the effect of endogenous enzymes, but also the spoilage caused by the rapid growth of microorganisms. On the other hand, the implementation of vacuum-packaging can insulate the air, reducing the occurrence of food oxidation and preventing the growth of microorganisms. In addition, in the depurated oyster meat, the VBN value still increased with the accumulation of storage time, which was caused by the decomposition enzyme of the oyster meat itself.

3.4.3. Effects of Depuration of Oysters on Sensory Evaluation Score during Storage

Sensory evaluation is a useful indicator for the description of food quality [34,47]. Therefore, our main job is to investigate and examine the appearance, color, flavor, and taste of food or the product as a reference for food development and quality improvement.
The results of the nine-point scoring system by professional judges are shown in Figure 2c. The un-depurated and non-vacuum-packed oyster meat already scored less than 5 points at 7 days (□UD-NVP; ■UD-VP); that is, it was rated as unacceptable. As for those with vacuum-packaging, it was also below 5 points on the ninth day. However, under the same low-temperature storage conditions, the oyster meat after depuration, regardless of whether it was vacuum-packed or not, still maintained a score of more than 5 points on day 21 (▲D-VP; △D-NVP). That is, the oyster meat at this time was rated as acceptable.
The decision to carry out depuration operations and vacuum-packing storage should have a certain influence on the quality of oyster meat in refrigeration [48,49,50]. The scores of the un-depurated groups were lower than those of the depurated groups, showing significant differences (□UD-NVP, ■UD-VP and ▲D-VP, △D-NVP). As for whether to carry out vacuum-packaging, it can also be seen that there were slight differences between them (□UD-NVP, △D-NVP), (■UD-VP, ▲D-VP); however, the difference was smaller than the above-mentioned whether or not the depuration operation was performed.
Compared with Figure 2b,c, the overall quality change trends and evaluation scores are roughly similar. The VBN value, one of the quality indicators, decreased the fastest in the short term in the □UD-NVP group, followed by the ■UD-VP group. The score was also the lowest in the □UD-NVP group, followed by the ■UD-VP group. The best group was the ▲D-VP group, which remained in an acceptable (above score 5) or ready-to-eat (VBN value below 25) state for 24 days.

4. Conclusions

At the beginning of the promotion of this experiment, the preliminary experiment had been completed, using acidic electrolyzed seawater, and it was found that the last live oysters were in an open-shell dead state, and under the same conditions, the oysters were in a closed-shell living state. Therefore, this study was carried out under alkaline conditions. It was found from the experimental results that the total aerobic count of live oysters was reduced by about 5 log values (Table 3) during alkaline electrolytic seawater depuration. Shimamura et al. [29] and Donaldson et al. [51] pointed out that when seawater temperature drops rapidly, it may cause many physiological, behavioral, and health consequences to fishes, which is called the effect of cold shock or hypothermia. This cold stimulus generally occurs when fish have acclimatized to a specific water temperature or temperature range and are subsequently exposed to a rapid drop in temperature, resulting in a range of physiological and behavioral responses. In addition, Ghanbari et al. [52] also pointed out that the exposure of fish species to acidic and alkaline water will cause strong stimulation to fish, which will lead to the disorder of internal regulation function. According to the experimental results, the depurated oysters discharged lot of impurities or dirt from the body, and the oyster meat became very clean, which can also be seen from the high score of sensory evaluation. These phenomena or effects should be regarded as AES (alkaline environment stimulus or the impact of alkaline environment) or CS (cold stimulus or the impact of low temperature), that is, inner edge stimulation [53,54,55] results.
With the self-developed alkaline electrolysis seawater depuration system in this study, the TAPC of live oysters could be reduced to a relatively low level (pH = 11.0, 5 °C, over 9 h or pH = 10.0, 5 °C, over 12 h). Excellent results can be obtained by depurated with alkaline electrolysis of seawater without the use of chemicals, UV light, ultra-high pressure, etc. In addition, the depuration operation under various set conditions will not reduce or affect the chemical composition and glycogen content. Moreover, after 12 h of depuration, live oysters can completely remove and release the debris from the body, which greatly improves the quality of oyster meat, which is also one of the very important results of this research. In addition, the fresh-keeping period can be extended to 24 days in the state of vacuum-packaging, which also surpasses all the refrigerated storage methods of oysters.

Author Contributions

Conceptualization, investigation, methodology, data curation, formal analysis, writing—original draft, H.-S.T.; supervision, visualization, data curation, Y.-T.H.; writing—review and editing, Y.-M.W.; data curation, J.-M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Thanks to the three teachers for their guidance and help in research architecture, experimental equipment, experimental operation, data sorting, statistical analysis, and thesis writing.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Framework of the oyster depuration system. T1: Seawater tank; T2: Acidic electrolyzed water tank; T3: Alkaline electrolyzed water tank; T4: Oyster depuration system containing a seawater tank; F1~F4: Filter; E1: Electrolytic device; V1, V2, V3: Valve; C1: Cooling circulation system; P1, P2, P3: Pump.
Figure 1. Framework of the oyster depuration system. T1: Seawater tank; T2: Acidic electrolyzed water tank; T3: Alkaline electrolyzed water tank; T4: Oyster depuration system containing a seawater tank; F1~F4: Filter; E1: Electrolytic device; V1, V2, V3: Valve; C1: Cooling circulation system; P1, P2, P3: Pump.
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Figure 2. Changes in oysters during storage at 5 °C. (a) Total aerobic plate count; (b) VBN; (c) Score. Error bars represents standard deviations of data from three different trials. UD-VP (■) = un-depurated but vacuum-packed oyster meat, UD-NVP (□) = un-depurated and non-vacuum-packed oyster meat, D-VP (▲) = depurated but vacuum-packed oyster meat, and D-NVP (△) = depurated and non-vacuum-packed oyster meat.
Figure 2. Changes in oysters during storage at 5 °C. (a) Total aerobic plate count; (b) VBN; (c) Score. Error bars represents standard deviations of data from three different trials. UD-VP (■) = un-depurated but vacuum-packed oyster meat, UD-NVP (□) = un-depurated and non-vacuum-packed oyster meat, D-VP (▲) = depurated but vacuum-packed oyster meat, and D-NVP (△) = depurated and non-vacuum-packed oyster meat.
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Table
Table 1. Chemical composition changes of oyster during depuration at different temperatures and time with pH = 11.0.
Table 1. Chemical composition changes of oyster during depuration at different temperatures and time with pH = 11.0.
Composition
(% Wet Basis)		Depuration Time
(h)		20 °C15 °C10 °C5 °C
Moisture083.3 ± 0.6 aA83.2 ± 0.5 aA83.1 ± 0.6 aA83.0 ± 0.5 aA
683.3 ± 0.6 aA83.3 ± 0.6 aA83.3 ± 0.6 aA83.0 ± 0.6 aA
1283.4 ± 0.8 aA83.2 ± 0.6 aA83.0 ± 0.8 aA83.0 ± 0.6 aA
Crude Protein09.0 ± 0.2 aA9.0 ± 0.2 aA9.1 ± 0.3 aA9.1 ± 0.2 aA
69.1 ± 0.2 aA9.1 ± 0.2 aA9.1 ± 0.2 aA9.1 ± 0.2 aA
129.1 ± 0.2 aA9.1 ± 0.2 aA9.2 ± 0.3 aA9.2 ± 0.2 aA
Crude fat02.3 ± 0.1 aA2.3 ± 0.1 aA2.2 ± 0.1 aA2.2 ± 0.1 aA
62.3 ± 0.1 aA2.2 ± 0.1 aA2.2 ± 0.1 aA2.2 ± 0.1 aA
122.2 ± 0.1 aA2.2 ± 0.2 aA2.1 ± 0.1 aA2.1 ± 0.2 aA
Carbohydrate04.6 ± 0.2 aA4.7 ± 0.2 aA4.6 ± 0.2 aA4.6 ± 0.1 aA
64.6 ± 0.1 aA4.6 ± 0.1 aA4.6 ± 0.1 aA4.6 ± 0.1 aA
124.6 ± 0.1 aA4.8 ± 0.1 aA4.7 ± 0.1 aA4.8 ± 0.1 aA
Glycogen (mg/100g)0685.5 ± 0.6 aA678.5 ± 0.4 aA680.5 ± 0.6 aA683.5 ± 0.5 aA
6683.5 ± 0.4 aA679.5 ± 0.5 aA677.5 ± 0.5 aA681.5 ± 0.6 aA
12682.5 ± 0.5 aA675.5 ± 0.4 aA678.5 ± 0.6 aA680.5 ± 0.5 aA
Values are mean ± SD of triplicates. a Mean values in the same row followed by different letters are significantly different (p < 0.05). A Mean values in the same column followed by different letters are significantly different (p < 0.05).
Table
Table 2. Chemical composition changes of oysters during depuration at different pH values with 5 °C.
Table 2. Chemical composition changes of oysters during depuration at different pH values with 5 °C.
Composition
(% Wet Basis)		Depuration Time
(h)		pH = 9pH = 10pH = 11
Moisture083.2 ± 0.6 aA83.2 ± 0.4 aA83.1 ± 0.5 aA
683.3 ± 0.6 aA83.3 ± 0.5 aA83.3 ± 0.6 aA
1283.2 ± 0.5 aA83.1 ± 0.5 aA82.9 ± 0.6 aA
Crude Protein09.0 ± 0.2 aA9.0 ± 0.2 aA9.1 ± 0.3 aA
69.1 ± 0.2 aA9.1 ± 0.2 aA9.1 ± 0.2 aA
129.1 ± 0.2 aA9.1 ± 0.2 aA9.2 ± 0.3 aA
Crude fat02.3 ± 0.1 aA2.3 ± 0.1 aA2.2 ± 0.1 aA
62.3 ± 0.2 aA2.2 ± 0.1 aA2.2 ± 0.1 aA
122.2 ± 0.1 aA2.2 ± 0.2 aA2.1 ± 0.1 aA
Carbohydrate04.6 ± 0.2 aA4.7 ± 0.2 aA4.6 ± 0.1 aA
64.7 ± 0.2 aA4.6 ± 0.1 aA4.6 ± 0.1 aA
124.7 ± 0.1 aA4.8 ± 0.2 aA4.8 ± 0.1 aA
Glycogen (mg/100g)0682.5 ± 0.5 aA685.5 ± 0.4 aA683.5 ± 0.5 aA
6683.6 ± 0.6 aA684.5 ± 0.4 aA681.5 ± 0.6 aA
12681.5 ± 0.5 aA685.5 ± 0.5 aA680.5 ± 0.5 aA
Values are mean ± SD of triplicates. a Mean values in the same row followed by different letters are significantly different (p < 0.05). A Mean values in the same column followed by different letters are significantly different (p < 0.05).
Table
Table 3. The changes of TAPC in oyster meat observed during depuration at different pH values and temperatures from 5 to 20 °C.
Table 3. The changes of TAPC in oyster meat observed during depuration at different pH values and temperatures from 5 to 20 °C.
Depuration Time (h)
TACP (log/CFU/g)036912
pH = 9.0
20 °C5.3 ± 0.5 aA5.3 ± 0.3 aA5.2 ± 0.4 abA4.9 ± 0.4 abA4.7 ± 0.5 bA
15 °C5.3 ± 0.4 aA5.2 ± 0.5 abAB4.7 ± 0.4 bB4.5 ± 0.5 bcAB4.2 ± 0.4 cBC
10 °C5.2 ± 0.3 aA4.8 ± 0.3 abBC4.5 ± 0.3 bBC4.1 ± 0.3 cBC3.8 ± 0.5 cC
5 °C5.2 ± 0.4 aA4.6 ± 0.3 bC4.2 ± 0.4 cC3.8 ± 0.4 dC3.6 ± 0.4 dC
pH = 10.0
20 °C5.3 ± 0.5 aA5.1 ± 0.3 aA5.0 ± 0.2 abA4.8 ± 0.3 abcA4.0 ± 0.2 abcA
15 °C5.3 ± 0.4 aA5.1 ± 0.2 aA4.6 ± 0.3 aA4.0 ± 0.2 abB3.5 ± 0.2 abcB
10 °C5.2 ± 0.4 aA4.6 ± 0.3 bB3.6 ± 0.2 cB3.0 ± 0.3 dC1.5 ± 0.5 eC
5 °C5.2 ± 0.5 aA4.5 ± 0.3 bB3.0 ± 0.2 cC1.5 ± 0.4 dD1.0 ± 0.3 eD
pH = 11.0
20 °C5.2 ± 0.4 aA4.3 ± 0.3 bA3.6 ± 0.2 cA3.0 ± 0.3 dA2.4 ± 0.2 eA
15 °C5.3 ± 0.5 aA4.2 ± 0.2 bA3.1 ± 0.3 cB3.0 ± 0.2 cA2.3 ± 0.2 dA
10 °C5.2 ± 0.4 aA4.0 ± 0.3 bA2.5 ± 0.2 cB1.5 ± 0.3 dB0.0 ± 0.0 eB
5 °C5.3 ± 0.4 aA3.8 ± 0.3 bB2.0 ± 0.2 cD0.0 ± 0.4 dC0.0 ± 0.0 dB
Values are mean ± SD of triplicates. abcde Mean values in the same row followed by different letters are significantly different (p < 0.05). ABCD Mean values in the same column followed by different letters are significantly different (p < 0.05). For each temperature, data in the same column with different superscript lowercase letters are significantly different (p < 0.05).
Table
Table 4. Quantity of impurities or dirt in oysters observed during depuration at different pH values and temperatures from 5 to 20 °C.
Table 4. Quantity of impurities or dirt in oysters observed during depuration at different pH values and temperatures from 5 to 20 °C.
Weight
(mg)		Depuration Time
(h)		20 °C15 °C10 °C5 °C
pH = 9.03580.5 ± 0.6 aB585.2 ± 0.5 aB600.2 ± 0.5 aB602.2 ± 0.4 aB
6650.2 ± 0.2 aA658.5 ± 0.6 aA660.2 ± 0.4 aA666.2 ± 0.6 aA
9679.2 ± 0.5 aA686.2 ± 0.5 aA691.4 ± 0.6 aA689.6 ± 0.5 aA
12680.2 ± 0.6 aA687.2 ± 0.4 aA692.2 ± 0.5 aA690.2 ± 0.4 aA
pH = 10.03595.2 ± 0.5 aB590.2 ± 0.3 aB604.2 ± 0.3 aB600.2 ± 0.5 aB
6655.2 ± 0.6 bA660.2 ± 0.5 bA658.2 ± 0.5 bA669.2 ± 0.6 aA
9684.2 ± 0.5 aA686.5 ± 0.4 aA689.2 ± 0.5 aA694.2 ± 0.5 aA
12685.2 ± 0.6 aA687.2 ± 0.5 aA690.8 ± 0.4 aA695.2 ± 0.4 aA
pH = 11.03602.2 ± 0.6 aB606.2 ± 0.5 aB609.2 ± 0.4 aB610.2 ± 0.4 aB
6670.2 ± 0.5 aA666.2 ± 0.4 aA668.2 ± 0.6 aA672.2 ± 0.6 aA
9685.8 ± 0.6 aA685.2 ± 0.5 aA690.6 ± 0.4 aA691.4 ± 0.4 aA
12686.3 ± 0.5 aA685.6 ± 0.4 aA691.2 ± 0.3 aA692.2 ± 0.5 aA
Values are mean ± SD of triplicates. ab Mean values in the same row followed by different letters are significantly different (p < 0.05). AB Mean values in the same column followed by different letters are significantly different (p < 0.05).
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Tsai, H.-S.; Hsiao, Y.-T.; Weng, Y.-M.; Liu, J.-M. Sustainable Utilization Technology for Improving the Freshness of Oysters—Development of Alkaline Electrolysis Seawater Depuration System. Sustainability 2023, 15, 785. https://doi.org/10.3390/su15010785

AMA Style

Tsai H-S, Hsiao Y-T, Weng Y-M, Liu J-M. Sustainable Utilization Technology for Improving the Freshness of Oysters—Development of Alkaline Electrolysis Seawater Depuration System. Sustainability. 2023; 15(1):785. https://doi.org/10.3390/su15010785

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Tsai, Hsin-Shan, Yu-Tien Hsiao, Yih-Ming Weng, and Jen-Ming Liu. 2023. "Sustainable Utilization Technology for Improving the Freshness of Oysters—Development of Alkaline Electrolysis Seawater Depuration System" Sustainability 15, no. 1: 785. https://doi.org/10.3390/su15010785

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