The physical, chemical and physicochemical properties of twenty-one Caribbean sweet potato cultivars were investigated. Hunter L a b colour parameters were measured, chroma, colour intensity and hue angle were calculated. Proximate composition, vitamin C, amylose, reducing and non-reducing sugars contents were determined using standard methods. Pasting properties were determined using Rapid Visco Analyser. Tuber size (length, r = 0.72, n = 21, P < 0.0015; width, r = 0.85, n = 21, P < 0.0002) was highly correlated with weight of the tubers. External colours of the tubers (L = 28.7-63.1, a = +5.9 to +18.0, b = +3 to +19.3) were lower than the flesh colours (L = 60.8-84.0, a = )2.4 to +27.8, b = +9.9 to +28.5). Proximate composition is typical of sweet potato cultivars but significant differences exist among cultivars. Total sugar, amylose and amylopectin contents ranged from 1.8 to 4.7%, 15.3-31.2% and 68.8-84.7%, respectively. Cooking profiles of the sweet potato flours showed similar trend. Pasting temperature, peak time, swelling power and solubility ranged from 78.3 to 93.0°C, 3.3-4.8 min, 5.6-23.5 and 11.7-45.7%, respectively.
The effect of storage on yield and modification of starch using hydrochloric acid and acetic anhydride were studied. Yield of starch from fresh roots (24.4%) was significantly greater than yield (16,2%) from roots stored at room temperature for six days. On a dry weight basis the amount of starch in the roots decreased over a six day storage period. The proximate composition of cassava starch on dry weight basis was 0.24% ash, 0.13% fat, 0.49% protein, 0.15% crude fibre and 98.4% starch. The isolated starch was treated with different concentration of hydrochloric acid to produce acid thinned and dextrinized starches and different concentration of acetic anhydride to produce acetyl substituted starches. The alkali number of acid thinned and dextrinized starches increased with acid treatment while the viscosity decreased.
The degree of substitution increased with the concentration of acetic anhydride used. The cold water solubility of the acid thinned and acetyl substituted starches were similar to that of native starches while the solubility of the dextrinized starches increased with the acid concentration. The viscosity of the acetyl substituted starches increased with the degree of substitution.
Some physicochemical properties of two cultivated species of Digifaria (D exifis and D ihurua) were studied. Physical properties such as 1000-kernel weight, kernel size and water absorption rate of the grains were determined. Water absorption by both cereals reached a maximum after 24 h steeping. The starch from both cereals exhibited a two-stage pattern of swelling and solubility properties similar to other non-waxy cereal starches. Starch from acha swelled less than iburu, 7.3 compared with 7.8 for iburu. Chemical constituents such as protein, oil, crude fibre, ash, nitrogen-free extract and amylose were also determined. An amylose content of 280 g kg-' was obtained for both cereal starches. Brabender amylograms obtained from starch and flour were typical of most normal, non-waxy cereals. Maize starch had relatively higher hot-paste and cold-paste viscosities than starches of the two Digifaria species.
A laboratory method was developed and used to prepare ogi from seven sorghum cultivars. Mean yields of ogi, bran and solubles were 72.9%, 15.9%, and 7.5% respectively. Mean ogi composition was 84.3% starch, 8.3% protein, 2.5% fat, 0.59% ash, and 1.3% soluble sugars. The protein, fat, ash, and soluble sugar content of laboratory ogi was within 2% of values obtained from analysis of three commercial Nigerian ogi samples. Yields of ogi were significantly affected by variety of sorghum.
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