Structure and properties of modified starch films

Recrystallisation, complexation of amylose with lipids [26] or flavourings [47], polymorphic transitions, amorphisation, etc. - controlling these is essential to optimising the properties of food and non-food materials. The recrystallisation of films from hydroxypropylated pea starch (HPPS) fractions during the drying of solutions has clearly shown the impact of molecular weight on the crystallinity and morphology of the crystalline structures formed [43]. When mixed with k-carrageenan, HPPA provides a base composition for pharmaceutical capsules by governing the glass transition and mechanical properties of the films obtained by casting, while the former governs the flow properties of the mixture [42].

All these results can be interpreted in terms of self-association and typical entanglement/crystallisation behaviour of biopolymers. The kinetics of crossing the Tg is also a factor determining the porosity of biopolymer materials. Indeed, we have shown that the rate of drying, from 50 to 10% water content (total wet basis), of protein tablets such as zein governs their porosity by the eventual collapse of the structure [57].

These biopolymers, starch and zein, are incompatible, which leads to mixtures with a morphology of the continuous phase (or matrix) / particle dispersion type, whose mechanical behaviour is similar to that of polymer / filler composite materials [48]. In the glassy state, the material based on flour or starch/zein mixtures is brittle, whereas the isolated constituents are ductile, suggesting a lack of adhesion between starch and zein.