Composition of the Low Temperature Precipitate from Commercial High Dextrose Equivalent Maltodextrins and the Kinetics of its Formation

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Abstract
Analysis of the precipitate formed from 50% w/w solutions of maltodextrins with high dextrose equivalents shows that oligosaccharides with degrees of polymerization of 11 and above are the major components with no evidence for the presence of small oligosaccharides with degrees of polymerization up to 7. Fast precipitating batches of commercial high dextrose equivalent maltodextrins give precipitates which contain almost constant amounts of oligosaccharides with degrees of polymerization 8 to 10, irrespective of the time of storage. For slower precipitating batches the contribution of these oligosaccharides increases with the time of storage. Analysis of the water content of the precipitate resulted in values within the range 40 to 48% w/w. This indicates that a constant gel/solvated precipitate structure exists for both fast and slow precipitating samples. Photomicroscopy of the precipitate, stained with iodine/potassium iodide, indicates that it consists of essentially spherical particles ranging from 1.0 to 8.5 μm diameter. The particle distribution is non Gaussian with approximately 90% of the particles having diameters less than 3.5 μm. The mean diameter was found to be 2.4 μm.A study of the kinetics of the precipitation reaction showed that it cannot be considered as a single chemical reaction with zero or first order kinetics. By considering the reaction as the formation and growth of crystals in mother liquor an Avrami exponent of between 2 and 3 was obtained indicating that the reaction represents three dimensional crystal growth with instantaneous nucleation. The effect of the mean molecular weight and molecular weight distribution on the kinetics is discussed and compared to the better documented systems of bread staling and amylose retrogradation.