Structural Characterization of Microcapsules from Common Bee Pollen for the Development of Delivery Systems

Abstract

Exine, in the form of a natural microcapsule, refers to the outermost layer of the pollen grains and is composed of a complex mixture of sporopollenin, a highly resistant polymer, which makes it durable and able to withstand harsh conditions. Distinctive features of sporopollenin have attracted interest in the encapsulation of bioactive substances. Herein, we describe the pathway to producing sporopollenin microcapsules (SMCs) by exploiting bees and trapping common bee pollen pellets, offering a simple approach to acquiring substantial amounts of pollen grains for industrial application. Palynological results showed that separating bee pollen pellets by colour could lead to almost pure products ranging from 90 to 96%, depending on the pollen species. Subsequently, a single extraction technique removed around 82– 86% of the proteinaceous content, which could cause potential allergic reactions in humans. Detailed morphological analysis by scanning electron microscope (SEM), confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and laser diffraction particle size (LDPS) analysis proved that the purified SMCs retained their 3D micro-structures, besides being hollow and uniform micron-scale size. Fourier-transform infrared spectroscopy (FTIR) findings point out that the sporopollenin biopolymer structure of the pollen grain comprises distinct aliphatic and aromatic domains, and the purification of the SMCs resulted in the loss of nitrogen-related peaks. The hydrophobic/hydrophilic properties of the SMCs, evaluated by contact angle measurements, showed variability between pollen, depending on the specificities of their chemical structure. Simultaneous thermal analysis (STA) confirmed SMCs thermal stability up to 451 °C. Altogether, we showed that green microcapsules with various morphological properties could be produced by simply processing Castanea spp., Cistus spp., Erica spp., Olea spp, and Rubus spp, all common bee pollen pellets available in large quantities in the northeast of Portugal, but also many other countries. These microcarriers promise applicability to various fields, from pharmaceuticals to the food industry.