Molecular structure and functional properties of starches on the East African market
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Starches have an unrivalled potential as raw materials for several food and non-food uses such as bakery, confectionary, brewery, textiles, pharmaceuticals, paper and laundry. Unravelling the potential of commercial starches for use in the food and non-food industries in East Africa calls for a better understanding of their unique physicochemical, functional and molecular structural properties. This will enable their full exploitation and bring economic benefit to both local farmers and starch based industries in the sub region. To understand the properties of commercial starches on the East African market, a study was undertaken in Uganda, Kenya and Tanzania to determine the botanical sources of commercial starches on the East African market. High performance liquid chromatography (HPLC) and high performance size exclusion chromatography (HPSEC) were used to determine the amylopectin molecular structure, differential scanning calorimetry was used in the determination of gelatinisation and retrogradation properties while rapid visco analyser to determine pasting properties. Six starch samples were selected for laboratory analysis: Maize 4, TME 14, NASE 10, Bamunanika, LEB8004 and Kenya tapioca. The study revealed that the East African market had two botanical sources of commercial starches; maize and cassava. Considerable variations were observed in a number of starch parameters. Amylose content was higher (28.55%) in maize than in cassava (18.95-24.67%). Analysis of amylopectin molecular structure revealed that maize starch had a higher (48.04%) proportion of A chains (DP<12) than cassava starches (42.27-46.63%). A higher proportion of A chains has been associated with low gelatinisation temperature, low retrogradation rates and low pasting temperature hence suitable in most industries due to reduced energy costs in the production processes. Cassava starches had a higher proportion (19.66-23.55%) of B3+ chains (DP >37) than maize starch 18.84% which is a typical characteristic for root starches. Maize amylopectin had higher molecular weight 3.33x108 g/mol while cassava starches had a range of 2.42x108-3.19x108 g/mol. The lower amylopectin molecular weight in cassava starches indicates their high solubility in water hence suitable in pharmaceuticals, baby food, yoghurt and dessert like products. Maize starch had a higher gelatinisation temperature (69.90oC) while Bamunanika and NASE10 exhibited lower gelatinisation temperatures (63.76 and 63.60oC) respectively which makes them more suitable in numerous food and non food industries that need low temperature in production processes. Maize starch had a higher retrogradation percentage (39.85%) while cassava starches ranged from (14.26-18.05%) which makes cassava starches more suitable in the production of dessert-like products and cakes to maintain a soft texture with a slow staling process yielding an extended shelf life. Bamunanika (cassava) had high final viscosity 2477.66 mPa s which makes it suitable in the production of sauces, soups, dressings and textiles. Kenya tapioca (cassava) with a low final viscosity (1532.54 mPa s) was more suitable in the dry stage of paper production. NASE10 and Kenya tapioca had the highest set back viscosities (494.50 and 492.00 mPa s) respectively, which limits their use in the food and textile industries while maize starch had a lower setback viscosity (354.00 mPa s) which is suitable in textiles industries. Maize had a higher pasting temperature (76.7oC) while in cassava starches it ranged from (65.23-70.22oC) indicating that cassava starches with a lower pasting temperature were more suitable in the textile, food and paper industries as the lower pasting temperature reduces the energy costs. Significant correlations existed between the amylopectin molecular structure and different functional properties. There was a significant negative correlation between final viscosity and A chains (r = -0.64) indicating the weak interactions of the short branch chains which do not hold the integrity of the swollen granules resulting into granule disruption during heating, pasting temperature and A chains (r = - 0.77) suggesting that short chains do not contribute to a high crystalline quality hence less energy is required to melt the starch crystallinity resulting in a low pasting temperature, A, B1 chains and gelatinisation temperature of retrograded starches (r = -0.82, r = -0.87) suggesting that a high proportion of short branch chains do not easily reassociate on cooling and therefore inhibit retrogradation, gyration radius and onset temperature of gelatinisation for the retrograded starch (r = -0.56) attributed to the more free movement of longer linear chains in the amylopectin polymer hence readily forming double helices during cooling leading to high rates of retrogradation and a significant positive correlation between B2 chains and pasting temperature (r = 0.94), indicating a high crystallinity quality as a result of long branch chains which require more energy to melt hence high pasting temperature, amylopectin molecular weight and percentage retrogradation (r = 0.66) suggesting decreased degree of branching from increased amylopectin molecular weight affecting the amylopectin cluster stability leading to increased retrogradation rates. These findings revealed considerable variations in the molecular structure of the starches and functional properties indicating possible improvement through breeding for particular starches with desired qualities for various uses.