Comparative studies on the effects of different modifications on the cold water solubility of starch from selected underutilized legumes

Thesis Abstract

The industrial utilization of native starch is limited by its imperfect nature, hence the need for modifications. The continued use of conventional crops such as cassava, maize, rice and potato as sources of starch for industrial uses adds to the demand pressure on these crops. On the other hand, the isolation of starch from underutilized legumes will not only reduce this pressure, but will also add value to boost the economic potential of these legumes. This work was aimed at comparing the effects of different modifications on the cold water solubility and functional properties of starch isolated from Vigna subterranea, Sphenostylis stenocarpa, Cajanus cajan, and Mucuna pruriens var pruriens. The recovered starch yield ranged between 70 99 %. Most of the modifications enhanced the desirable properties of the starches. The cold water solubility of the modified starches were in the range of 35 81 %, with acid-alcohol, alcohol- alkaline and acid hydrolysis modifications giving the best solubility, while heat moisture treatment and carboxymethylation modifications gave the least solubility. Most of the modified starches had high swelling power, with the exception of the acid treated starches. The pH of the modified starches were around neutrality (pH 7) excluding acid-treated which was neutralised with a 1 M NaOH solution. The water absorbing capacity of the starches increased with increasing solubility for most modified starches excluding the acid-treated starches. There were reductions in the amylose content of most of the modified starches with the exception of the acid hydrolysed, pyrodextrinized and osmotic pressure treated starches. The gelatinisation temperature of the modified starches reduced with increasing solubility. The gelatinisation temperature analyses of the starches ranged between 68 and 72 0C for the native starches and 34 and 60 0C for the modified starches. The moisture contents of the modified starches were relatively low. The clarity of the starches varied, with the oxidized and acid-alcohol treated starches giving the highest clarity. The modified starch yield rranged between 70 % and 99 %. Based on visual assessments, the samples were relatively clean and white with a minor contribution of colour from the seed coat of the grains. There were some variations in the transmittance properties of the pastes of the native and modified starches. These properties of modified starches suggest that starches from underutilized legumes could be used in the production of cold water soluble starch thereby increasing its value addition.