| Sweet potato flour is a novelty raw material of food products which is produced bydehydration of the fresh sweet potato and owns the nutrition, flavor and original taste of thefresh sweet potato after rehydration. Enzyme browning during processing would have anegative effect on the color of the sweet potato flour and the broken of the sweet potato cellscould also lead to the losing of nutrition and flavor substances as well as a high degree ofviscosity. In order to solve these problems and develop a new craft for manufacturing sweetpotato flour with better color, intact cell wall structure and a high retention of nutrition andflavor substances, compound color fixative without sulfur and low temperature blanchingwith Ca2+addition technologies were researched in this study to inhibit enzyme browning andreduce free starch rate respectively.Firstly, five color fixatives (critic acid, disodium stannous citrate, phytic acid, sodiumsulfite, ascorbic acid) were used to inspect their effects on the color protecting of sweetpotato flour, the order of their functions could be listed as follows: sodium sulfite> criticacid> disodium stannous citrate> ascorbic acid> phytic acid. In order to develop food safetyand flavor, reducing the dosage of the single color fixative, critic acid and ascorbic acid wereselected as the preferred compound color fixative. Through regression orthogonal design weconfirmed that better quality of sweet potato flour could be obtained through soaking the8mm slices in the compound color fixative which contains mass fraction of0.3%critic acidand mass fraction of0.1%ascorbic acid for30min. The L*value of the sweet potato flourdealed with the compound color fixative could reach90.58; the polyphenol oxidase (PPO)inhibition rate could also reach88.13%. The addition of critic acid could significantlyenhance the thermal stability of ascorbic acid, through the UV spectrum research of PPOenzymatic products, we could clearly acknology that with the addition of compound colorfixative, quinones were reduced to phenolics and the browning products were transformed tonon-color compounds due to the reaction with compound color fixative. Circular dichroism ofPPO informed that compound color fixative could change the formation of enzyme structurewhich contribute to the decrease of α-helix and increase of β-sheet, and inhibit the enzymaticbrowning finally.Then, the research of low temperature blanching together with Ca2+addition wasconducted to reduce the free starch rate of sweet potato flour. The optimum two step thermaltreatment was achieved by single factor experiments including blanching temperature,blanching time, steam-cooking time and Ca2+concentration followed by response surfaceexperiment. The theoretical value of free starch rate could be decreased to about7.25%underthe optimum low temperature blanching condition (blanching temperature,59.4℃, blanchingtime,29.7min, steam-cooking time,14.6min, Ca2+concentration,400mg/kg). Lowtemperature blanching could clearly activated PME to catalyze the hydrolysis of esterifiedcarboxyl groups in pectin molecules and induce cross-linking between carboxyl groups andcalcium ions. Blanching sweet potato slices at60℃for30min was confirmed as theoptimum condition for low temperature blanching through the assay of PME, PG andβ-amylase activity at various temperatures and time. Tissue firmness increased almost2.5 times of its original firmness and tissue degree of methyl esterfication decreased significantlyafter low temperature blanching treatment. Ca2+and PME could significantly enhance thehardness of the pectin gel which plays an important role in cell wall strengthening. Therefore,low temperature blanching assisted by Ca2+could significantly increase the tissue firmnessand strengthen the cell wall to resist the shear destruction and reduce the free starch rateduring flour processing.Finally, the physicochemical and application characteristics of sweet potato flour wereassayed and compared as well. The results showed that: Sweet potato flour fromself-producing owned attractive color and sensory characteristics, free starch rate wasdecreased by40.77%than sweet potato flour from market saled. Sweet potato flour from selfproducing exhibited an intact cell wall structure and the starch was wrapped within the cellwall which could better preserve the nutrition and flavor substances as well as contribute to asuitable viscosity. So that when rehydration, sweet potato flour owned the original flavor andnutrition of fresh sweet potato and had a wide market prospect. |
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