Antioxidant effect of green-tea extracts on peroxidation of eels

Water and ethanol extracts of green tea were analyzed for their antioxidative properties on peroxidation of eel. As a pre-treatment, eels were treated with green tea extract (approximately two times the weight of eels) for 1 h and cleaned with water. The DPPH radical scavenging effect, ABTS scavenging effect, acid value, peroxide value, carbonyl value, and TBA value of eel, and antioxidant effect on linoleic acid and eel oil were investigated. Ethanol extract of green tea showed the highest DPPH radical scavenging. Regarding ABTS scavenging activity, ethanol extract of green tea was more effective than water extract (p<0.05). Both ethanol and water extracts showed higher ABTS scavenging effects than 10 mM vitamin C. Acid value and peroxide value were lowest for water extract, followed by ethanol extract and vitamin C. Carbonyl values of green tea extracts increased only 6-9 percent by the end of 21 days of refrigeration, indicating their ability to reduce carbonyl compounds. TBA values of green tea extractand vitamin C-treated eels were 4.6 and 5.6, respectively. Green tea extracts prevented oxidative rancidity and linoleic acid peroxidation. The ethanol extract was more effective than water extract in preventing linoleic acid peroxidation. In conclusion, pre-treatment with ethanolic extract (30°, Soju) of green tea was found to be effective in preventing peroxidation of eels in refrigeration.


Introduction
Fat and oil contained in food downgrade the food quality due to oxidation by oxygen, moisture, heat, and so on (1).
The rise of heart related diseases sparked the interest in and consumption of diet, including fishes containing high unsaturated fat. However, fishes with high unsaturated fat have been difficult to process, store, and distribute because of its easy rancidity. In order to resolve the problem of easy rancidity, the researches on peroxide prevention have been Pre-treatment of eels and eel oil extraction 80 g of eel and 150 mL of extracted solution were put together in a clean vinyl bag, and laid for one hour in a tied-up bag. After 1 h, eel was taken out to get a clean-up shower. Once cleaned and dried up, eel was used for eel oil extraction.
Eel oil was extracted using the method specified by Folch et al. (15). 80 g of chopped eel was mixed with 300 mL of blended solution of chloroform and methanol (2:1, v/v), then they were extracted using a homogenizer (SMG-G, Shinsang, Co., Ltd., Seoul, Korea), and filtered using a paper filter (No.1). Another 250 mL of blended solution of chloroform and methanol was added to the residue, then they were extracted using a homogenizer, which constitute the second time of filtration. All obtained solutions were poured into a separatory funnel and mixed with distilled water, and laid for 15 to 20 h. After that, the layer of chloroform was separated, dehydrated by Na 2 SO 4  The measurement of peroxide value After 1 to 2 g of lipid recovered using a method specified in Korean food code was taken into a flask, 25 mL mixed solution of acetic acid and chloroform (3:2, v/v) was added.
On top of it, 1 mL of KI saturated solution was also added.
Then, it was laid in the dark room for 10 min after being shaken for a min. With 30 mL of distilled water and starch indicator solution injected, the point without color was chosen as the end point through titration by 0.01 N Na 2 S 2 O 3 (17,18) The measurement of acid value

Statistical analysis
Comparative analysis among experiment groups was performed using IBM SPSS Statistics 20. After ANOVA was undertaken, Duncan's multiple range test was also performed with 5% confidence level (α=0.05).

DPPH and ABTS scavenging effect of green-tea extracts
In order to compare the DPPH and ABTS scavenging effects between water and ethanol extract of green-tea, Vitamin C 10 mM solution was used as a positive control.
The correlation coefficient between concentration and absorbance was more than 0.9 (r>0.9). Green-tea extracts were effective in DPPH radical scavenging, depending on the volume of extracts. Ethanol extract of green-tea had not only higher effects than water extract, but also as high scavenging radical scavenging effects. Both water and ethanol extracts of green-tea had higher scavenging effects than vitamin C 10 mM solution, and ethanol extract of green-tea was more effective than water extract (p<0.05). The findings that green-tea extracts were not so much effective as vitamin C in DPPH radical scavenging, but were more effective than vitamin C in ABTS radical were in accordance with the previous researches, and the difference is believed to come from the different antioxidant materials in each extract (17,21).

Acid value
In measuring lipid rancidity, acid value identified by free fatty acid analysis was used (1). While the acid vale of mackerel pre-treated with green-tea extract and also widely known for ease of lipid rancidity due to high unsaturated fatty acid was 2.3 mg KOH/g in pre-refrigeration stage (2), and commercial semi-dried conger eel's acid value was 8.5 mg KOH/g on average (22), the pre-refrigeration acid value of eel in this research was 2.7 mg KOH/g. Storage at refrigerator (4-8℃). Previously, the researchers found out water extract of green-tea prevented the peroxide of anchovy oil (3) and green-tea was used to improve the quality of sea-products in process (5,23).  Storage at refrigerator (4-8℃).  and 5.6, respectively, which also were 118% and 164% respective increase compared to the values on the zeroth day.
There was no meaningful difference, but the TBA value of green-tea extracts were lower than vitamin C solution; Storage at refrigerator (4-8℃).

2)
Superscripts A-C values are significantly different at p<0.05 within the same storage period.

3)
Superscripts a-c values are significantly different at p<0.05 within all storage periods.

Antioxidant effect on linoleic acid and eel oil
For the purpose of measuring the peroxide prevention effects of green-tea extracts, they were added to linoleic acid