Effects of Heating and Drying Processes on Physicochemical Properties of Yam (Dioscorea species) Flour

• Main Authors: Yaw-Ming Liao, 廖耀銘
• Other Authors: Ting-Jang Lu, Ph.D.
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/48992338919738036197

碩士 === 國立臺灣大學 === 食品科技研究所 === 90 === Tubers of Chinese yam (Dioscorea spp.) are high in starch content with significant level of (3~7% on dry basis) mucilage. The starch may gelatinize and retrograde and the mucilage may lose its viscosity during the drying and heating processes. Tubers of Tainung 2 cultivar (D. alata) were selected for its low enzymatic browning tendency and high harvest yield to study the effects of drying and heating processes on the properties of the flour. Two uncooked flour samples were prepared from freeze- and hot-air (60℃) dried of peeled tubers. The values of average particle-size of uncooked samples were about 33μm. The rehydrated uncooked flour dispersed well in water and the values of dispersibility were 82 and 97% respectively for freeze- and hot-air dried sample. The starch granules could be observed in those samples. The rehydrated both samples formed viscous pastes and showed capabilities of emulsifying and foaming. The values of emulsion stability of freeze- and hot-air dried samples were 54 and 52%, respectively; the values of half-time drainage of foams were 132 and 49 minutes. Freeze drying was found to be the better method to maintain the hydration properties of mucilage than hot-air drying. The uncooked samples as well as yam starch showed a restricted pasting behavior that did not have an obvious gelatinization peak measured on a Rapid Visco Analysis (RVA). Drum dried yam flour like gelatinized starch dispersed well in cold water and formed viscous paste. The values of swelling power and water-soluble index at 55~95℃ of the solution were in the range of 12.5 ~ 17.4 g H2O/g and 20.1 ~ 25.4%, respectively. It was observed on RVA of drum dried flour that high viscosity values at 35℃ and shear thinning behavior. Baking, boiling and steaming processes reduced the dispersibility of flour in water, although the starch in the flour was partially gelatinized (38~44%). The values of swelling power and water-soluble index of the three samples were significantly lower than drum dried sample. The average particle-size of pre-gelatinized flour samples were similar within the range of 91~115μm. The properties of emulsion of soybean oil and crude mucilage solution were also evaluated. The value of emulsion stability was 78.1% and it indicated a stable emulsion as 0.5% of crude mucilage solution was tested. The emulsion was an oil-in-water system and was a thixotropic (shear thinning) fluid. The values of average-diameter of emulsion droplet and protein load were 4.5μm and 6.9mg/m2, respectively. The enzymatic browning tendency and activities of polyphenol oxidase (PPO) of five common yam cultivars grown in Taiwan were also included in this thesis. There not significant relationship between enzymatic browning and activities of PPO been found.