The Study On The Spray Freeze Drying Process Of Purple Yam Flour And It's Quality Characteristics

Purple yam is rich in nutrients.But it has strong seasonal and intolerable storage characters.Processing purple yam into flour can effectively extend its storage period,reduce transportation and storage costs,and expand its application.In this study,the spray-freeze drying process for flour preparation was optimized using single factor orthogonal array testing method and purple yam from Xinshao,Hunan,China as raw materials and compared with other processes including hot-air drying,spray drying and vacuum-freeze drying in terms of whole flour physical properties,nutrient content,antioxidant capacity and quality stability.The results showed that the optimum process of spray-freeze drying was injection rate of 25 mL/min,solid to liquid ratio of 1:1 and freezing at-30oC.The produced whole flour under the optimal condition is better with water content of 3.89%,solubility of 23.12% and anthocyanin content of 13.28 mg/100 g.Different drying processes have different effects on the physical properties,nutrient composition and antioxidant capacity of purple yam flour.Spray-freeze drying and vacuum-freeze drying can best maintain the original color of purple yam;Hot-air drying and spray drying,especially hot-air drying deepen the color of purple yam;The bulk density of whole purple yam flour prepared using spray-freeze drying method is the smallest,only 0.24 g/mL.The iodine content of whole purple yam flour prepared using spray-freeze and spray drying methods is the highest of 6.27.The water holding capacity of whole purple yam flour prepared using spray drying method is poor,only 2.29 g/g.The oil content of whole purple yam flour prepared using the four kinds of technologies is similar.The solubility of purple yam flour prepared using spray drying is the highest of 23.34%,followed by 22.87% of that prepared using spray-freeze drying.Purple yam flour prepared using spray-freeze drying retains the most nutrients.Its protein,allantoin and anthocyanins contents are 15.54%,1.31% and 14.17 mg/100 g,respectively.Overall,the quality of purple yam flour obtained using spray-freeze drying method is the best.Sensory assessment showed that exposure to air rapidly declines the quality of purple yam flour.By contrast,vacuum packaging could best retain the nutrients of purple yam flour including total sugar and anthocyanins.