自然與微生物發酵木薯(Manihot esculenta, Crantz) 粉的理化性質

Abstract

Cassava (Manihot esculenta Crantz) is one of the most important staple food crops grown in tropical Africa. It plays a major role in efforts to alleviate the African food crisis because of its efficient production of food energy, year-round availability, tolerance to extreme stress conditions, and suitability to present farming and food systems in Africa. Thus, the objectives of this study were to investigate natural and microbial fermentation techniques to produce and to compare the physiochemical properties nutritionally-enhanced modified flour. The flours were compared in terms of their proximate composition, thermal and physicochemical properties. Proximate composition revealed that there were no significant differences for all parameters between both flours for different treatments at α=0.05; however, significant difference was observed for total acid and pH between treatments, with values of the microbial fermented cassava flour (MFC) higher for the total acid and lower for the pH, which is an indication that more acid is contained in the flour sample. The total moisture contents of both flours are within the 10-20% moisture level recommended for commercial flours. There was high moisture content for the first (24 hours) and final (72 hours) treatments of the natural fermented cassava flour, but lower than the microbial flour for the second (48 hours) treatments. The moisture content overall is generally low and this is an indication of stable shelf life if properly packaged and stored. Low moisture is necessary in flour for good keeping quality and longer shelf life. The protein contents for both flours were 1.20, 1.07, 1.30 and 1.24, 1.23, 1.17 for the treatments 24, 48, and 72 hours, respectively. There was no significant difference between both flours; however, slightly higher values are observed at the 72 hour treatment for both flours. The ash contents of natural fermented cassava flour (0.25%) and microbial fermented flour (0.25%) were low, and within the limit, 0.5%, recommended for grade A industrial flour and starches. Nevertheless, there was no significant difference for ash between the flour of natural and microbial fermented cassava, although slightly higher ash content for the last treatments (72 hours) of both flours were recorded. The fibre content was low for the microbial fermented cassava flour (1.45) at treatment 72 hours, but no significant difference were observed at 24, 48, and 72 hours for all other treatments. Starch content was higher in natural fermented cassava (NFC) and microbial fermented cassava flours at 72 hours (98.6%) and at 24 hours (98.4) and lowest at 24 hour (84.15%), and 72 hours (85.1), respectively. Gelatinization in NFC flour occurred at a lower temperature range (62.03-71.75 oC) compared with MFC flour (61.98-71.55 oC) with the endothermic gelatinization enthalpy having slightly higher values in flour of microbial than NFC flour, although minimal differences were observed. Swelling and solubility patterns indicated lower relaxation temperature and higher swelling and solubilization rates in MFC flour compared with NFC flour. However, no significant differences were observed for solubility for both treatments at temperatures 60, 70, and 80 oC, respectively; although significant differences were observed at 90 oC, with values of microbial fermented flour having higher values than those of the natural fermented flour at both treatments. The pasting characteristics of 9% (db, dry basis) flour slurry of natural fermented cassava showed higher final viscosity, trough, and setback but lower breakdown ratios compared with flour of microbial fermented. This indicates that flour of natural fermented cassava paste might be better in withstanding processing conditions and would present a slightly superior thickening characteristic than flour of microbial fermented cassava paste. The differences in the viscoelastic properties and physico-functional characteristics of the natural and microbial fermented cassava flour could be used in their selection for specific food and industrial processing applications.