| Summary: | 碩士 === 中國文化大學 === 生活應用科學研究所 === 91 === Browning is commonly observed in cut surfaces of yams. Tubers of some varieties are more sensitive to browning than others. It is a great deal of interest in which parameters are responsible for browning. This study was undertaken to attain the following aims: (1) to evaluate the influences of total phenol and of the activities of polyphenol oxidase (PPO) in fourteen cultivars of Taiwanese yams on browning; (2) to determine the antioxidant properties in these yam tubers; (3) to investigate the effect of food processing on total phenol retention and on antioxidant properties.
Yam tuber purees were submitted to incubate at 30ºC during 8 hours. Through spectrocolorimetric measurements, the color development of yam puree was expressed as browning index (BI) and lightness difference (∆L). According to the value of BI, the 14 cultivars of fresh yams could be clustered into 3 groups: (1) BI > 40: Yang-Ming-Shan Native and Jung-Guo-Chang; (2) 15 > BI > 25: Heng-chuen, Kee-lung yellow, Kee-lung white and Ja-I; (3) BI < 10: Tai-Nung 1, Tai-Nung 2, Da-san 2, Da-san 3, Yang-Ming-Shan white, Yang-Ming-Shan red, and Hua-lian 3. The tubers with greater ∆L (20 ~ 24) were Kee-lung yellow and Hua-lian 3, on the contrary, Tai-Nung 2 and Da-san 2 were with lesser ∆L (5 ~ 6). Found in the most yam tubers, the values of ∆BI as well as ∆L increased by increasing incubation time. When these values were plotted versus incubation time, it was observed that the curve of browning followed a linear correlation (r2 > 0.7, p < 0.05) within the first hour and then reached a plateau. The browning development was analyzed in relation to the total phenol and to the PPO activity. Of the analyzed variables, the influences on browning were insignificant (r2 = 0.288, p < 0.05). It might be complicated parameters leading to browning. The highest content of total phenol (55 mg/100g dry matter) was analyzed in white tuber of Yang-Ming-Shan Native. The content varied between 10 and 38 mg/100g dry matter in 11 cultivars. Yang-Ming-Shan white contained smallest number of total phenol (6 mg/100g dry matter). It was worthy of note that abundant total phenol (116 mg/100g dry matter) presented in a particular cultivar with purple flesh, Ming-jiang-chang-hung.
Antioxidant properties of methanol extracts from yam tubers were determined by using four evaluation systems: radical scavenging activity on 1,1-diphenyl-2-picrylhydrazyl (DPPH), Fe++-chelating effect, reducing power and inhibition on formation of conjugate diene. At a concentration of 10 mg/ml, all test yams have shown antioxidant ability and there were significant differences among them. Yang-Ming-Shan Native scavenged about 90% DPPH radical, which was comparable to BHT (100 ppm). The other yam extracts have exerted on scavenging DPPH in the range of 10 to 85%. In term of Fe++-chelating effect, Tai-Nung 2 showed the strongest activity (94%) among cultivars and no significant difference from the known cheating agent, EDTA (97% at 1000 ppm) at p < 0.05. The cheating powers of the remainder 12 cultivars were in the range of 40% to 90%, whereas that of Kee-lung yellow was 25%. Reducing power was expressed as the absorbance value at 700 nm (A700). The higher A700 represented stronger reducing power. Yang-Ming-Shan white showed a highest value of 0.54 among all cultivars, nevertheless, which was only one fifth of that of ascorbic acid (2.43 at 100 ppm). Comparison with BHT (62% at 100 ppm), yams possessed lesser inhibitory effect on formation of conjugate diene. Ming-jiang-chang-hung showed significantly higher suppression (46%) than the other yams. The results obtained from the antioxidant assay were insignificantly correlated to the amount of total phenol. It indicated that other compositional variables also contributed to its antioxidant properties.
Minimal processing included operations such as washing, peeling, slicing, and freeze-drying, which resulted in minor loss of total phenol. Blanching, heating and drying processes caused the loss of total phenols to a great different extent. Also, there were significant differences in antioxidant activity among processed yam tubers. It has shown that radical scavenging activity was in parallel with total phenol retention in processed yams.
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