Optimization of compound color fixative without sulfur during sweet potato flour processing

Sweet potatoes are highly nutritious vegetables that are rich in calories and biologically active phytochemicals such as β-carotene, polyphenols, ascorbic acid and dietary fiber. While the sweet potato is the seventh most important food crop grown in around 111 countries, China contributes about 80% of the world's production of sweet potatoes, where the roots are used as food, animal feed and as a raw material for starch and noodle manufacture. Sweet potatoes are highly perishable and difficult to store. In developing countries, there are many problems related to storage and transportation of the raw sweet potatoes. Sweet potato flour is regarded as the ideal product to solve the storage and transportation problem which could expand its utilization in various areas of food production as well. But using sweet potato flour for product development is hindered by the discoloration from enzymatic browning. Using a color fixative is the most convenient and effective method , the commonly used color fixatives are Critic acid, Disodium stannous citrate，Phytic acid, Sodium sulfite , Ascorbic acid and so on. Among these chemicals, Sodium sulfite is the most effective one, but using Sodium sulfite could lead to SO2 residue which may threat human health seriously, earlier in 1986, the FDA had banned its use in fresh fruit and vegetables. Therefore we urgently need to develop a safe and efficient color fixative without sulfur to reach the purpose of improving the food taste, nutrition and safety. Because of its more comprehensive and efficient color protecting effect, compound color fixative was regarded as the feasible scheme to take the place of Sodium sulfite . In this paper ,Critic acid, Disodium stannous citrate，Phytic acid, Sodium sulfite and Ascorbic acid were selected as the color fixative to inspect their effects on the L* value of the sweet potato flour. Critic acid and ascorbic acid were selected as the preferred color fixative. Through regression orthogonal design, we confirmed that a better quality of sweet potato flour could be obtained through soaking the 10mm slices in the compound color fixative that contains a mass fraction of 0.3% critic acid and a mass fraction of 0.1% ascorbic acid for 30 min. At the same that the mass fraction was (0.4%), the L* value of the sweet potato flour treated with the compound color fixative could reach 90.58, which was similar with the product treated with Sodium sulfite, the polyphenol oxidase inhibition ratio could also reach 88.13%, higher than others treated with single color fixative. Finally, there was a preliminary exploration of the browning inhibition mechanism of the compound color fixative. The results showed that the addition of critic acid could enhance the thermal stability of ascorbic acid. The absorption peak of enzymatic browning products of polyphenol oxidase with a compound color fixative was significantly weakened at 410 nm in UV spectrophotometric, which indicated that compound color fixative could effectively inhibit the enzymatic browning reactions to reduce the formation of precipitation recognized as the browning products. Circular dichroism showed that the compound color fixative might reduce polyphenol oxidase activity by changing its molecular conformation. The amount of α-helix in the secondary structure of polyphenol oxidase decreased as opposed to the increase of β-sheet. This study can provide references for the use of a compound color fixative without sulfur during sweet potato flour processing.