Starch-based coatings for colon-specific drug delivery. Part I: the influence of heat treatment on the physico-chemical properties of high amylose maize starches.

In this study, the changes in the physico-chemical properties of different high amylose maize starches, i.e., Hylon VII, Hylon V and IM-DS acetate starch, were studied prior and after heat treatment used in the preparation of film coatings (WO 2008/012573 A1). Characterisation of the unprocessed maize starches was carried out with regard to the outer particle morphology, particle size distribution, specific surface area, moisture content, apparent particle density, swelling, polarised light microscopy, Fourier Transform Infrared (FT-IR), X-ray powder diffraction and modulated Differential Scanning Calorimetry (mDSC). Pure amylopectin and low amylopectin samples (LAPS) were also used to aid the interpretation of the results. The effect of heat processing was evaluated in terms of degree of crystallinity, FT-IR and mDSC. Enzymatic digestibility of both processed and unprocessed maize starches was estimated qualitatively using various alpha-amylases resembling those present under in vivo conditions. A significant decrease in the degree of crystallinity of the dried samples after processing was observed, in particular for amylopectin. Only LAPS and Hylon VII samples showed differences in their thermal behaviour upon heat treatment, thus suggesting that a minimum amount of amylose is required for an effect to be detectable. High amylose starches maintained a well-ordered arrangement of their macromolecular chains, as was seen by X-ray and FT-IR studies. This effect could be explained by a formation of retrograded forms of the starches. The retrograded starches were found to be less digestible by various types of amylase, in particular those found in the upper intestines, indicating that the formation of a butanol complex as claimed elsewhere is not essential in the preparation of colon delivery devices.