A study on the functional properties of taro starches from Tonga : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University

Abstract

This study compared the functional properties of three taro starches extracted from selected cultivars, one from each of the three most commonly grown taro genera in Tonga. The selected cultivars were Alocasia macrorrhiza var 'Fohenga', Colocasia esculenta var 'Lau'ila', and Xanthosoma saggitifolium var 'Mahele'uli'. Cassava starch, a commercial product from Thailand, was studied together with the taro starches for comparison purposes. Freshly harvested taro corms/cormels were peeled, washed, ground into pulp. The taro pulp was washed with excess water and filtered with a cheese cloth. The solid pulp was discarded, and the water-starch mixture (starch milk) was collected in a settling tank. The starch was held for 10-24 hours to allow the starch to settle, and then the supernatant liquid was discarded. The Xanthosoma starch was successfully isolated using this method. For the Alocasia and Colocasia, the starch could not be isolated from the starch milk due to the presence of a mucilaginous material, and it was separated using a bowl centrifuge. The starches were dried, in a hot-air drier and then purified to remove trace of protein, fat, and fibre. All the taro starch granules were similarly polygonal in shape but the granule sizes were different. The Xanthosoma starch granule size (5-30μm) was similar to that of cassava starch granules (5-35μm). The granule sizes of Alocasia (0.5-3μm) and Colocasia (0.5-6μm) were very small, smaller than rice starch granules. The amylose contents, determined using an iodometric blue value colorimetry method, were 12.1, 13.6, 19.8, and 27.4% for Alocasia, Colocasia, cassava, and Xanthosoma starches respectively. The gelatinization temperatures for the starches were determined using sensory evaluation, hot stage microscopy, Brabender Amylograph, and Differential Scanning Calorimetry (DSC) methods. The gelatinization temperatures were approximately 69, 70, 75 and 80°C for cassava, Alocasia, Xanthosoma and Colocasia starches respectively. The gelatinization temperature ranges for Xanthosoma and Colocasia were similar to that of cassava starch, but Alocasia starch showed relatively wider temperature range. The viscosity of the Xanthosoma gelatinized starch paste was much higher than the other starches but showed greater breakdown on heating. The strengths of the starch gels were determined by measuring the rheological modulus G* of the gels using a Bohlin Rheometer, and the penetration strength test using an Instron. Both tests showed that the Xanthosoma starch produced a much stronger and higher viscosity gel than all of the cassava, Alocasia and Colocasia starches which produced gels with similar strength. The relative order of gel clarity from qualitative sensory evaluation, from highest to poorest clarity, was cassava, Xanthosoma, Colocasia, then Alocasia. The storage stability of the starch gels was evaluated by studying the crystallisation using DSC, and measuring the syneresis occurring during storage at 5 and 22°C. The Xanthosoma starch gel was extremely susceptible to crystallisation and syneresis during storage, compared with cassava, Colocasia, and Alocasia gels which had similar stabilities on storage. The freeze-thaw stability of the starch gels was studied by subjecting the starch gels to repeated freeze-thaw cycles. The Xanthosoma starch gel was extremely unstable with freeze-thaw treatment. The Alocasia and Colocasia starch gels were similar to cassava starch gel which was more stable with freeze-thaw treatment. The Xanthosoma starch, because of extremely high viscosity and gel strength, could be used in food products that need high viscous texture but require no further storage. The Colocasia and Alocasia starches, because of high digestibility due to very small granule sizes can be used in baby food formulations, which are either heat treated or frozen.