Starch microsphere preparation and phase behaviour in aqueous two-phase systems – effect of continuous-phase polymer

When producing starch microspheres in aqueous two-phase systems (ATPS), a continuous-phase polymer is employed to induce segregative phase separation. This creates a continuous polymer-rich phase, in which starch-rich phase droplets can be dispersed and crystallised into solid, semi-crystalline starch microspheres. This study aims to explore how polymers with different chemical structures and sizes affect phase behaviour in starch-based ATPS and the formation of starch microspheres. The polymers studied were polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), poly(2-ethyl-2-oxazoline) (PEtOx) and hydroxypropyl methylcellulose (HPMC). To reach a starch concentration in the starch-rich phase (55 %) that yielded solid starch microspheres, less PEG (32 %) than PVP (39 %) or PEtOx (42 %) were required in the continuous phase. The sizes of the polymers had little or no effect on the water distribution in the ATPS. The governing factor is the water activity within the system. The water activity in the starch-rich phase must be low enough to allow for crystallisation within the predetermined time frame. Accordingly, the polymer concentration must be selected so that this water activity condition is reached. The viscosity of the continuous phase significantly impacted the microsphere production. It needs to be high enough to prevent the coalescence of starch droplets as they transition to solid gels, but not so high that it hinders emulsification. While polymer choice affected particle size and aggregation, it did not notably change the interior structure, surface morphology, crystal type, or thermal behaviour of the dried starch microspheres. This opens up the possibility to optimise the production of microspheres by changing polymers, while tuning the starch crystallisation through control of the water activity in the starch phase.