Microstructure and lactose crystallization properties in spray dried nanoemulsions

•Powders prepared from nanoemulsions crystallized more quickly than those prepared from conventional emulsions.•PLM was used to confirm onset of crystallization and was consistent with DVS data.•CLSM and cryo-SEM showed the distribution of fat globules within the particles in situ. Cryo-SEM was particularly useful for characterizing nano-sized fat droplets (<200 nm) which were below the resolution limit of confocal microscopy.•It is suggested that reduced protein in the continuous phase with lactose contributed to the observed increase in crystallization rate in powders prepared from nanoemulsions.

The objective of this study was to characterize lactose crystallization behaviour and microstructure of spray dried nanoemulsions with different fat globule sizes (FGS). Powders of the same composition (57.7% w/w lactose, 12.3% w/w sodium caseinate, 27.7% w/w sunflower oil, and 2.3% w/w water) but different FGS (mean diameters 1100 nm and 155 nm) prior to spray drying were manufactured. Differences in lactose crystallization were studied using dynamic vapour sorption (DVS) and polarized light microscopy (PLM). Crystallization kinetics was modelled using the Avrami and Yang equations. Results showed that lactose crystallized in three dimensions and more rapidly in powders with a smaller FGS. PLM images showed a higher rate of lactose crystal formation for smaller FGS powders when stored for 4 days at 55% relative humidity. Confocal laser scanning microscopy (CLSM) and cryo-scanning electron microscopy (Cryo-SEM) images indicated the more evenly distributed small fat globules inside powder particles prepared from spraydried nanoemulsions. The surface of powder particles was uneven and ruptured post lactose crystallization. Crystals appeared after humidification and were assumed to be anhydrous α- and β-lactose in a 5:3 molar ratio. Results showed powder particles of the same composition were altered in lactose crystallization characteristics by changing the FGS of emulsions pre spray drying.

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