| Purple sweet potatoes are rich in nutrition with all-sided amino acids and natural pigments. They are the excellent raw materials for food processing because of their biological functions. Purple sweet potato flours (PSPF) not only keep nutrition, color and flavor of fresh potatoes, but also extend the storage period. Blending wheat flours with PSPF can offer a balanced nutrition. Application of blends in producing noodles is important for development of new products and making full use of purple sweet potatoes in food processing areas.Firstly, the differences of properties between purple sweet potato starches and wheat starches were studied. The results showed that purple sweet potato starches had low amylose content and small particle size (19.38μm) compared with wheat starches. Purple sweet potato starches started to swell at 75℃with high pasting viscosity, and were easy to retrogradate during cooling process. Hard and elastic gels were generated. Purple sweet potato starches were belonged to type -A high crystalline starch with high diffraction intensity. Swelling, pasting viscosity and gel elasticity properties of purple sweet potato starches were better than that of wheat starches.Secondly, properties of rheological, thermal and pasting characteristics of blends were investigated. It was showed that dough stability decreased from 18.0 to 7.8 min with increasing substitution levels of PSPF up to 30%. The dough was weakened obviously and no more to fit for processing when the proportion of PSPF increased to 20%. Replacement of PSPF increased gelatinization temperature from 66.31 to 67.73℃and decreased pasting viscosity of blends from 344 to 197 RVU. Results indicated that non-starch polysaccharides and anthocyanins played more important roles than purple sweet potato starches.Thirdly, the influence of PSPF levels on noodle making quality was discussed in this paper. Increasing levels of PSPF in the blends led to decrease in cooking and textural quality of noodles. Noodle making quality decreased significantly (p<0.05) when the addition of PSPF up to 20%. Sensory evaluation results showed that purple sweet potato noodles with darker color were popular, but the noodles were unpopular for their poor chewing as the levels of PSPF increased beyond 20%. As a result, the appropriate addition proportion of PSPF was 5%–15%. Improving the processing quality of purple sweet potato noodles by vacuum mixing technology was studied. Results showed that the best process parameters were: 38% water content, -0.06 MPa vacuum degree, stirring at 140 r/min for 4 min, and then stirring at 70 r/min for 6 min. Vacuum mixing could exhaust the air from dough and make the gluten network more closely. In addition, vacuum mixing could improve the flexibility and extensibility of dough and cooking quality of noodles.Finally, color stability problems of purple sweet potato noodles were solved. Pigments of purple sweet potatoes were vulnerable to pH and heating. The color of pigments turned from red to green with the pH increased, and turned to brown during heating. Cu2+ and Fe3+ showed great influence on pigment color at alkaline condition. PSPF suspension showed the color of blue-green without brown when heated at alkaline condition because pigment molecules were embedded by gelatinized starches of PSPF. The soup color of purple sweet potato noodles was blue-green because the boiled water was alkaline. Acidity regulators such as sodium dihydrogen phosphate, potassium dihydrogen phosphate and disodium dihydrogen pyrophosphate could improve the soup color stability of purple sweet potato noodles, and showed better effects when used in compound modality. |
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