Increasing yield of soluble dietary fiber from sweet potato peel by using subcritical water extraction

Sweet potato is an important economic crop growing in mountainous area of Zhejiang Province. Sweet potato was processed into traditional vermicelli and starch products, and dried sweet potato becomes more and more popular in recent years. Sweet potato peels is one of the main by-products during dried sweet potato processing, and it accounts for 20%-30% of the total fresh material. Only a small amount of sweet potato peels is used as animal feed because of low protein content, and the majority is dumped in the trash, leading to environmental pollution and waste of resources which restricts the sustainable development of the sweet potato industry chain. Therefore, improving the utilization rate of sweet potato peels is an urgent problem. Dietary fiber is known as the seventh nutrient with the characteristics of fermentability, water and oil retention, promoting gastrointestinal motility and lowering blood cholesterol. On the other hand, subcritical water extraction has the advantages of high efficiency, environmental protection and convenient operation as an emerging physical extraction method. In order to utilize sweet potato peels effectively, soluble dietary fiber (SDF) was extracted from sweet potato (Zheshu 13) peels by subcritical water. Effect of solid-liquid ratio, extraction temperature and extraction time on SDF yield was investigated, and orthogonal application was used to optimize the extraction conditions of SDF from sweet potato peels based on the individual factor experiment. The physicochemical properties of SDF and total dietary fiber (TDF) were evaluated by water holding capacity, swelling capacity, oil holding capacity, peak viscosity and glucose adsorption capacity. The functional properties of sweet potato peels SDF and TDF were evaluated by hydroxyl radical scavenging rate and DPPH radical scavenging activity. Meanwhile, the extraction yield and all the above properties of SDF and TDF extracted by subcritical water were compared with those under other extraction conditions (hot water extraction, ultrasonic-assisted extraction and microwave-assisted extraction). The results showed that the influence of subcritical water extraction factors on SDF yield was in the order of solid-liquid ratio>extraction time>extraction temperature, and SDF yield reached to 10.43% under the optimal extraction condition (the solid-liquid ratio 1:30, 120 ℃, 30 min), which was 769.17%, 27.04% and 34.75% higher than that in the cases of unassisted hot water extraction, ultrasonic-assisted extraction and microwave-assisted extraction respectively. The ratio of SDF in TDF (SDF/TDF) reflects the quality of TDF and it was the highest when subcritical water extraction was adopted (42.26%), which was 665.58%, 14.43% and 19.24% higher than that in the cases of unassisted hot water extraction, ultrasonic-assisted extraction and microwave-assisted extraction. The glucose adsorption capacity (4.79 mmol/g), hydroxyl radical scavenging capacity (54.28%) and DPPH free radical scavenging capacity (69.28%) of SDF extracted by subcritical water were higher than that extracted by unassisted hot water, ultrasonic-assisted extraction and microwave assisted-extraction. The swelling capacity (7.89 mL/g), oil holding (4.74 g/g) and peak viscosity (27.18 mPa/s) of TDF extracted by subcritical water were significantly higher than those under the other extraction conditions. However, there was no significant difference in the swelling capacity and the peak viscosity for SDF extracted by different methods. The results of this study will provide a theoretical basis for effective utilization of byproduct during sweet potato processing, and it will be a reference for further research on subcritical water in the field of dietary fiber extraction. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.