Improvement on structure and adsorption of insoluble dietary fiber from garlic straw induced by ultrasound

Abstract: Dietary fiber (DF) has drawn the attention of researchers due to its significant health benefits. Over the past decade greater DF materials from fruits and vegetables are being introduced in the market because of the presence of associated bioactive compounds and balanced compositions. The physicochemical and functional properties of DFs always depend on the food sources, extraction methods and structure of DF. Garlic bulb is consumed worldwide because of its widespread medicinal and nutritional value. During harvesting period, garlic bulb yields a considerable amount of straw which is simply thrown or disposed, causing a severe environmental problem. Garlic straw, with similar compounds to garlic bulb, might be used as a novel source of functional DF. Composition analysis reveals garlic straw consists of 7.15% soluble dietary fiber (SDF) and 72.20% insoluble dietary fiber (IDF), which exhibit different physiological effects on human health. The health effects exhibited by IDF are often not as good as that of SDF due to their different physicochemical and functional properties. Therefore, the aim of this study was to improve the functional properties of IDF from garlic straw with ultrasonic processing technology. IDF was put into a 500 mL beaker and dispersed with 300 mL deionized water. The beaker containing the sample was placed in a thermostatic water bath at initial temperatures of 25℃ for 10 min. And then the mixture was sonicated in ultrasound processor with different liquid-sample ratios, ultrasonic powers and ultrasonic time. After the pretreatment, the mixture was stored at?20℃ and freeze-dried for later analysis. The optimum ultrasonic conditions were obtained based on the responses of glucose absorption capacity from single factor experiments and response surface methodology. The functional properties including cholesterol binding capacity, water holding capacity, oil holding capacity and water swelling capacity were determined. Scanning electron microscope (SEM) and Fourier transformed infrared spectroscopy (FTIR) were used to analyze the microstructure of IDF. Results suggest that liquid-sample ratio of 30:1, ultrasonic power of 700 W and ultrasonic time of 40 min were considered as the optimum ultrasonic conditions for preparation of IDF with good glucose absorption capacity. Under these conditions, the glucose absorption capacity and cholesterol binding capacity were increased by 12.8% and 45.0% compared to those without ultrasound treatment (P<0.05). Ultrasonic-treated IDF exhibited better color, functional and physicochemical properties than untreated IDF with significant difference (P<0.05). FTIR of control and ultrasonic-treated IDF was basically the same. Structural analysis from SEM indicated that ultrasonic treatment destroyed the microstructure of IDF from garlic straw. Findings from structural analysis revealed that the porosity and surface area of IDF were effectively improved by ultrasound, which proved to enhance functional and physicochemical properties of modified IDF. The positive effect of ultrasound may be useful for functional modification and utilization of DF from garlic straw.