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Abstract

The effects of alum pretreatments (0, 2, 3, and 4%) on the phenolic compounds and color characteristics of fried-cut eggplants were determined. The fried-cut eggplant treated with 2% alum showed higher phenolic (1.68 mg/g fried-cut eggplants) and anthocyanin (0.08 mg/g fried-cut eggplants) contents than the fresh-cut eggplants and the other alum pre-treatments. In addition, the lightness of the fried-cut eggplants that were pretreated with 2% alum were higher than the other alum pretreatments. The fried-cut eggplants were kept at -20 °C for 6 months. It was found that the color of the fried-cut eggplants did not change. In addition, the effects of the alum pretreatments (0, 2, 3, and 4%) on the used frying oil were also determined. The chemical parameters (free fatty acid, acid value, peroxide value and conjugated diene) of the used frying oil significantly increased, when the frying cycle increased. However, the alum pretreatments did not affect the chemical parameters of the frying oil, but had an effect on the color of the used frying oil. The colors (L*, a* and b*) in increased with an increase of the alum concentration. The effect of different times (32, 40 and 45 sec), temperatures (170, 180 and 190 °C) and types of oils (palm oil, soybean oil and a mixture of both) on the fried-cut eggplants and the used frying oil were also studied. The frying period revealed no effect on the fried-cut eggplants and the used frying oil; whereas, the frying temperature affected both the fried-cut eggplants and the used frying oil. The anthocyanin and phenolic contents of the fried-cut eggplants that were fried at 170 °C displayed less value than the fried-cut eggplants that were fried at 180 °C and 190 °C. As a result, the polyphenol oxidase at 170 °C was active. In addition, the chemical parameters and color of the used frying oil that was heated to 170 °C revealed less changes than the used frying oil that was heated to 180 °C and 190 °C. The type of oil had no effect on the fried-cut eggplants, but did affect the used frying oil. The soybean oil revealed a higher chemical change than the palm oil. Additionally, the extension of the life of vegetable frying oil by treatment with activated charcoal (AC) and bentonite (BE) was also considered. Three treatments (15% w/v) including single adsorbent (AC or BE), adsorbent combinations (BE:AC ratio 2:1, 1:1 and 1:2) and two-step adsorbent (AC and then BE, BE and then AC) were investigated for the purposes of improving the physico-chemical parameters (free fatty acids, FFA; acid value, AV; peroxide value, PV; total polar materials, TPM; conjugated dienes, CD and color) of used vegetable frying oil. The two-step treatment (AC and then BE) exhibited the highest ability to improve the physico-chemical parameters of the vegetable oil when compared with the single adsorbent (AC or BE), three adsorbent combinations (BE:AC ratio 2:1, 1:1 and 1:2) and the two-step application of BE and then AC.