The Physicochemical Properties of Yeast Microcarriers

摘要

Understanding the physicochemical properties of an encapsulation system is critical to enhance the encapsulation and modulate the release properties of labile bioactives; this is particularly necessary for novel systems which are derived from complex, natural structures. Yeast, although one of the most well-studied eukaryotic organisms, have had limited utilization as microcarriers for bioactive compounds. This study aims to evaluate the physicochemical properties of yeast, specifically how the cell wall and intracellular constituents influence encapsulation, release during in vitro digestion and oxidative and thermal stability of encapsulated bioactives.;In this research, proposed to use vacuum infusion, a method solely employed for vascular and animal tissues, on single-cellular cells of S. cerevisiae . Native, intact yeast cells and plasmolyzed cells, i.e. yeast cell wall particles (YCWPs), were selected as model microbial microcarriers. Multiple processing parameters such as concentration of ethanol, octanol/water partition coefficient (log P), and applied vacuum pressure were modified to probe the role of the physical properties of yeast microcarriers on encapsulation of model bioactives. Spectrophotometric methods were coupled with fluorescent imaging techniques to further elucidate this understanding.;To evaluate the influence of the physicochemical properties of yeast microcarriers on the release of encapsulated bioactives, in vitro digestion models incorporating gastric, intestinal and sequential phases were selected. Transmission electron microscopy (TEM) and confocal, fluorescence imaging techniques were employed to evaluate changes in the cell wall and intracellular constituents to comprehend the rates of retention of encapsulated bioactives during simulated digestion.;This study also evaluated the effect of the interfacial composition of emulsions on the chemical stability of encapsulated bioactives. The results of this section were used as a comparison for yeast microcarriers under both thermal and AAPH-induced oxidative stress. The results of the oxidative stress measurements were explored further with kinetic modeling.;The results of this research demonstrated that the intracellular constituents within native, intact yeast cells contribute significantly to encapsulation. The binding of bioactives within the membrane-bound compartments, DNA and protein of native yeast cells resulted in four to five-fold higher encapsulation efficiency/yield than YCWPs. In addition, yeast cell walls were recalcitrant to the action of proteolytic enzymes during digestion, which suggests that the cell wall did not play a critical role in the release of compounds. However, the action of acid in the gastric phase denatured the bioactive-biopolymer complexes, allowing for the release and micellization of the bioactives by bile salts. The composition of the interface and the bulk phase of emulsions contributes most to the thermal and oxidative stability of bioactives during thermal and nonthermal processing. Moreover, there was no correlation between the physicochemical stability of the emulsions and the chemical stability of the encapsulated bioactive. The absence of cytoplasmic material improves the stability of heat labile compounds; however, the endogenous antioxidant systems such as glutathione and catalase within native, intact cells.;Overall, the results of this research have enabled the detailed understanding of the physicochemical properties of unicellular microcarriers. The comprehensive approach will lead to the development of novel encapsulation systems with enhanced barrier properties utilizing other natural systems such as algae, bacteria, etc.