Yeast Cells in Microencapsulation. General Features and Controlling Factors of the Encapsulation Process

Abstract

Besides their best-known uses in the food and fermentation industry, yeasts have also found application as microcapsules. In the encapsulation process, exogenous and most typically hydrophobic compounds diffuse and end up being passively entrapped in the cell body, and can be released upon application of appropriate stimuli. Yeast cells can be employed either living or dead, intact, permeabilized, or even emptied of all their original cytoplasmic contents. The main selling points of this set of encapsulation technologies, which to date has predominantly targeted food and-to a lesser extent-pharmaceutical applications, are the low cost, biodegradability and biocompatibility of the capsules, coupled to their sustainable origin (e.g., spent yeast from brewing). This review aims to provide a broad overview of the different kinds of yeast-based microcapsules and of the main physico-chemical characteristics that control the encapsulation process and its efficiency.

Keywords: diffusion phenomena; drug delivery; food technology.

Figure 1

In the structure of a yeast cell (here represented in a post-budding state), the elements potentially acting as barriers to encapsulation (cell wall and cell membrane) are depicted in a magnified fashion on the right hand side of the figure.

Figure 2

YBMCs can be grouped into four main classes, depending on whether intact or permeabilized cells, or their cell wall remnants are used for passive (diffusion-based encapsulation processes) or whether they are genetically engineered to produce one or more active compounds intracellularly. Examples of actives are provided for each class, and the corresponding references can be found in the text.

Figure 3

The yeast ultrastructure is dramatically altered by the encapsulation of hydrophobes; the TEM and confocal pictures show encapsulation in intact cells (limonene, 30% in relation to yeast dry weight, T = 60 °C, 4 h), but permeabilized cells behave similarly. The most striking result is the large increase in the number and dimensions of bodies that can be colored by the lipid stain Nile Red (in the insets, confocal images where the cell wall is stained by Concanavalin A, and Nile Red fluorescence highlights the lipidic bodies). However, as can be noted in the TEM images below, upon encapsulation of hydrophobes, the ‘periplasmic’ space (highlighted in light orange both in schemes and images) greatly increases; it is indeed difficult to define this area accurately, because the cell membrane has disappeared, but at this point, this is an area located between the wall and residual cytoplasmic content/lipid bodies (which localize predominantly in the center of the cell body, see inset on the right). Importantly, cytoplasm organization becomes more granular and—bar the lipidic bodies and the nucleus—hardly any organelle is recognizable. TEM images are courtesy of Federico Catalano, IIT Electron Microscope Imaging Facility (Genova, Italy).

Figure 4

(A). The process of diffusional encapsulation of hydrophobes into an (intact) yeast cell can be broken down into adsorption on the wall, permeation through the cell wall and periplasmic space, penetration in the cell membrane, and finally the disruption of the latter (and of all other membrane-limited organelles) to form large lipid bodies. (B). Mechanistically, it could be hypothesized that the hydrophobes permeating through cell wall do so both from adsorbed droplets and from the (saturated) external solution, or that they pass the membrane as small hydrophobic aggregates or as individual molecules. Experimental evidence and simple considerations lead to the conclusion that the hydrophobes should permeate as individual molecules originating from oil droplets (highlighted in the red box). Furthermore, there are two main partition equilibria: from adsorbed droplets to the cell wall, and from the periplasmic space to a (swollen) membrane/lipid body; the stabilization offered by the cell surfactants to the latter is likely the main thermodynamic driving force of the whole process.