Abstract
Sweet potato varieties, Tanzania (Tz), Nasport (Nsp) and Spk004 (Spk), identified as potential raw material for baby food formulation, were characterised for proximate composition, mineral content and physicochemical characteristics. The interaction of sweet potato flour and proteins (soya and fish) were investigated. Starch was the most abundant nutrient in sweet potato (58 to 79 g/100g flour dry weight). Resistant starch ranged between 17 and 29%; these values were lower in sun-dried samples compared with freeze-dried ones and were reduced significantly on boiling. The in vitro starch digestibility was high, particularly in the sun-dried samples (80-85%), indicating sweet potato carbohydrates may be utilised efficiently. Lipids (1.1-2.3 g/100g) and proteins (2.7-5.0 g/100g dry weight) were low. Aspartic acid and glycine were the most abundant amino acids, cysteine and methionine were limiting. The orange-fleshed varieties Nsp and Spk contained high levels of beta-carotene (provitamin A), which were reduced in sun-dried and oven-dried samples. The main elements were K, Na, Mg and Ca with an average of 1190, 408, 698 and 576 mg/kg, dry weight respectively and moderate iron (37 mg/kg) and Zn (40 mg/kg). The in vitro digestion method showed that Ca, Mn, Mg and Zn in sweet potato are moderately bioavailable (55, 51, 45 and 65 % respectively). Polyphenols identified in sweet potato were caffeoylquinic acids, p-coumaroylquinic acids, feruloyquinic acids, dicaffeoylquinic acids and caffeoylferuloylquinic acids. Caffeoylferuloylquinic acids have not been reported before in sweet potato. Small deformation rheology indicated that sweet potato flour had lower elastic modulus, G' values compared with extracted starch. A mixture of sweet potato and soya (5% sp: 15 soya) and that of soya and fish (10%: 10%) w/w in water had lower G' and G" values than a mixture of sweet potato and fish (10% sp: 10% fish), which could be useful in making soft products. A sweet potato-based infant food using the Microdiet program was developed with nutritional and rheological characteristics similar to commercial baby foods. Lipid oxidation increased on storage of the dried product for 16 weeks at 22 °C, particularly in product containing fish but reduced in the presence of antioxidants (350, 500 mg vitamin C, 350,500 mg vitamin E and 100 mg citric acid/ Kg). Products containing sweet potato and soya (recipe B) were more stable in the absence of antioxidants.