Rheology and Physicochemical Properties of Spray Dried Glucomannan Extracted From Elephant Foot Yam (Amorphophallus oncophyllus) Tuber

Abstract

Indonesia has many kinds of cereals and tubers which are rich in carbohydrates. One of the potential tubers in Indonesia is elephant foot yam, also known as iles-iles. Elephant foot yam has two important compounds, glucomannan and calcium oxalate. Mannan flour is made from elephant foot yam tuber and has more glucomannan content than the other component in that flour. Glucomannan has many special characteristics, such as capable to form thick solution in water, capable to form gel and capable to form thin and water impermeable film by adding sodium hydroxide or glycerin. First stage in this research was glucomannan extraction in water, then the glucomannan extract (also called slurry) was dried using spray dryer. During drying process, the solid compounds including glucomannan became flour. Then, some analysis were performed to evaluate the physicochemical and rheological characteristics of the mannan flour. Based on result of the research, grating was the chosen treatment used for tuber size reduction. Meanwhile, gradual water changing for 4 times (ratio of tuber: water was 1:5 for one time water changing, thus the total ratio of tuber : water was 1:20) was the chosen treatment for glucomannan extraction. Browning prevention was performed by soaked the size reduced tuber in 1000 ppm sodium metabisulfite solution for 10 minutes. The highest yield and lightness value on mannan flour obtained from laboratory scale production is from slurry which came from the grating treatment and gradual water changing. The yield and lightness value are 3.10% and 98.74, respectively. Output of mannan flour obtained from pilot plant scale production is 3.15%. Based on result of proximate analysis, mannan flour obtained from pilot plant scale production has 9.96% of water content, 6.70% of ash content, 5.64% of protein content, 1.16% of fat content, and 86.49% of carbohydrate content (by difference). The lightness value of mannan flour obtained from pilot plant scale production is 90.37, while the whiteness value is 87.64. The lightness and whiteness value of mannan flour, both from laboratory scale and pilot plant scale production, are higher than the lightness and whiteness value of commercial mannan flour. Glucomannan content and glucomannan yield of mannan flour are 42.99% and 21.39%, respectively. The elephant foot yam tuber which is used as raw material has 6.29% glucomannan content. Mannan flour obtained from spray drying process has good solubility when rehydrated in water. According to rheology characteristics, viscosity of 10% (b/v) mannan flour solution showed no dependency to shear rate and time while viscosity of 11% (b/v) mannan flour solution showed pseudoplastic and thixotropic behavior. Viscosity of 10% (b/v) and 11% (b/v) mannan flour solution decreased due to the heating process until it reached temperature 90OC. Due to the low measured viscosity, mannan flour obtained from spray drying process cannot be used as thickener and gelling agent. Same rheology measurements were also done to 7 mannan flour obtained from different sources, such as drum drying method, conventional (oven drying) method, and commercial mannan flour.