Microencapsulation facilitates incorporating bioactive volatile compounds into products throughout the food, health, and cosmetics industries. To minimize the number of ingredients when microencapsulating volatile oils, we examined surfactant-free encapsulation of d-limonene in cross-linked alginate microcapsules (CLAMs) via in situ cross-linking during spray drying. Surfactant-free CLAMs (SF-CLAMs) were prepared by forming a Pickering d-limonene emulsion stabilized by calcium carbonate nanoparticles (CaCO3-NPs), combining with alginates, and then spray drying. CaCO3-NPs served as both the emulsifier and the reservoir of alginate cross-linking agent. SF-CLAMs, with a volatile retention of 73.1 ± 4.7% and total limonene content of 13.6 ± 8.6% (w/w, d.b.), exhibited core-shell morphology where CaCO3-NPs surrounded large emulsion cores (∼5 μm) encased in densely cross-linked alginate shells. Limonene was fully retained for up to 4 h in SF-CLAMs in water at 37 °C. Moreover, microencapsulation in SF-CLAMs minimized release in simulated gastric fluid (2.2 ± 0.3% in 2 h) while fully releasing in simulated gastric fluid at 37 °C.

Microencapsulation of plant-beneficial bacteria, such as pink pigmented facultative methylotrophs (PPFM), may greatly extend the shelf life of these Gram-negative microorganisms and facilitate their application to crops for sustainable agriculture. A species of PPFM designated Methylobacterium radiotolerans was microencapsulated in cross-linked alginate microcapsules (CLAMs) prepared by an innovative and industrially scalable process that achieves polymer cross-linking during spray-drying. PPFM survived the spray-drying microencapsulation process with no significant loss in viable population, and the initial population of PPFM in CLAMs exceeded 10¹⁰ CFU/g powder. The PPFM population in CLAMs gradually declined by 4 to 5 log CFU/g over one year of storage. The extent of alginate cross-linking, modulated by adjusting the calcium phosphate content in the spray-dryer feed, did not influence cell viability after spray-drying, viability over storage, or dry particle size. However, particle size measurements and light microscopy of aqueous CLAMs suggest that enhanced crosslinking may limit the release of encapsulated bacteria. This work demonstrates an industrially scalable method for producing alginate-based inoculants that may be suitable for on-seed or foliar spray applications.

Alginates gel rapidly under ambient conditions and have widely documented potential to form protective matrices for sensitive bioactive cargo. Most commonly, alginate gelation occurs via calcium mediated electrostatic crosslinks between the linear polyuronic acid polymers. A recent breakthrough to form crosslinked alginate microcapsules (CLAMs) by in situ gelation during spray drying ("CLAMs process") has demonstrated applications in protection and controlled delivery of bioactives in food, cosmetics, and agriculture. The extent of crosslinking of alginates in CLAMs impacts the effectiveness of its barrier properties. For example, higher crosslinking extents can improve oxidative stability and limit diffusion of the encapsulated cargo. Crosslinking in CLAMs can be controlled by varying the calcium to alginate ratio; however, the choice of alginates used in the process also influences the ultimate extent of crosslinking. To understand how to select alginates to target crosslinking in CLAMs, we examined the roles of alginate molecular properties. A surprise finding was the formation of alginic acid gelling in the CLAMs that is a consequence of simultaneous and rapid pH reduction and moisture removal that occurs during spray drying. Thus, spray dried CLAMs gelation is due to calcium crosslinking and alginic acid formation, and unlike external gelation methods, is insensitive to the molecular composition of the alginates. The 'extent of gelation' of spray dried CLAMs is influenced by the molecular weights of the alginates at saturating calcium concentrations. Alginate viscosity correlates with molecular weight; thus, viscosity is a convenient criterion for selecting commercial alginates to target gelation extent in CLAMs.