Effect of storage temperature and relative humidity on long-term colloidal stability of reconstitutable emulsions stabilised by hydrophobically modified starch

Dried emulsions leading to formulations exhibiting a high level of colloidal stability post rehydration would have many potential industrial applications and are of significant interest to food scientists in that the dry formulations can be easily stored and more cheaply transported. The influence of powder storage time and conditions on the long-term colloidal stability of reconstituted oil-in-water emulsion has been examined here. Emulsion systems of 20% oil were prepared with 2.5% hydrophobically modified starch acting as the emulsifier. These were subjected to freeze drying followed by up to 3 weeks of powder storage under different conditions varying in relative humidity and temperature. Rehydration was performed at specific time intervals during storage for each set of powders. The change in droplet size and morphology of the reconstituted emulsion showed that powder storage temperature has a significant effect on the long-term colloidal stability of reconstituted emulsions. Powders stored under the lowest temperature condition produced the smallest droplet size and were the most colloidally stable emulsions once rehydrated, whereas those stored at higher temperatures showed inferior performance in this respect. Freeze-dried emulsion powder, stored at −30 ± 1 °C for 3 weeks, once rehydrated gave liquid emulsions that were stable for at least 2 weeks. In contrast, flocculation was observed upon reconstitution of dry powders that were stored at relatively high storage temperatures (4 °C and 20 °C), but neither creaming nor extensive coalescence were present, post rehydration. It is often assumed that little change to the colloidal state of the system occurs during storage, once the system has been fully dried. Our results indicate otherwise. Even in the dried form, emulsion droplets still undergo substantial changes in their surface properties, impacting the subsequent colloidal interactions and thus their colloidal stability during storage and particularly post reconstitution.