Study And Application Of Modified Yeast Cell As A Novel Microencapsulation Wall Material

Phytochemicals of antioxidant property, especially phenolic compounds, areattracting considerable interest for their beneficial effects on human health.Hydrophilic chlorogenic acid and hydrophobic resveratrol, are two typical phenolicswith extensive biological and pharmacological activities. However, their labileproperties prevented them from application widespread. Microencapsulationtechniques have been widely used in pharmaceutical, cosmetic, food industries, and soon, due to its extension of shelf-life, protection against oxidation and control releaseof active components. The wall materials commonly used are polymers or expensivelecithin. It is now a true challenge to prevent active core substances from changesolely by using food-grade wall materials.A novel microencapsulation technique, based on the light colour, bland taste, andavailability in large volumes of yeast cells, has been developed for the encapsulationof essential oil and flavour. The eukaryotic structure of yeast cell wall makes it apotential excellent encapsulating wall material and its natural properties make it manybenefits over other microencapsulation technology. Additionally, no other additivesbut the yeast cell wall material, the core substance and solvent were involved duringthe encapsulation. This dissertation focused on the culture and modification of yeastcells (Saccharomyces cerevisiae Han.) as wall materials to encapsulate antioxidant.The main results are as following,1. The optimization of culture conditions for S. cerevisiaeBy employing central composite design, the relationship between the biomassdensity of yeast cells and nutrients including carbon, inorganic nitrogen resource andyeast extract was established, and the optimum culture medium was obtained.Analysis of variables indicated that the nutrients significantly influenced the biomassdensity of yeast cells. Further analyses of the fitted mode and its contour plot revealedthat, under our experimental conditions, carbon resource and yeast extract hadsignificant positive effects on the biomass density of yeast cells, while inorganicnitrogen had negative effect. The optimal culture conditions were obtained with thecoded concentrations of carbon, inorganic nitrogen resource and yeast extract, +1, -1and+1, respectively, with a maximum biomass density of 16.50 g·L^-1.Meanwhile, both the culture media and culture time affected the chemical compositions and the penetration of S. cerevisiae cells. Cells from the optimal culturemedium (culture mediumⅠ) had the highest encapsulation yield for both chlorogenicacid and resveratrol, and the encapsulation yields were 10.2%, 12.6% and 26.4%higher for chlorogenic acid encapsulation as well as 12.9%, 32.9% and 17.6% higherfor resveratrol encapsulation, respectively, as compared with the three commonculture media selected.2. Modification of yeast cells as wall materials for antioxidant encapsulationThe results indicated that all the physical or chemical modification treatments,except freezing, had similar effects on the infrared spectra of S. cereviase. After allthese treatments we observed decreases in characteristic infrared bands for proteinand nucleic acid, and increases in the characteristic infrared bands for polysaccharide.The contents of total nitrogen, total phosphorous, soluble polysaccharide and proteinin the cell suspensions were found to be significantly different among variouschemical treatments, being consistent with the IR spectra. All of the modified yeastcells can be used as wall materials for the encapsulation of chiorogenic acid orresveratrol, with different encapsulation yields. The largest encapsulation yields werepromoted by 52.6% for chlorogenic acid and by 14.3% for resveratrol through themodification of yeast cells.3. Study on the correlations between the compositions of the chemically treatedcell suspensions and the encapsulation yield of chiorogenic acid or resveratrolPartial correlation analysis revealed that the encapsulation yield of chlorogenic acidwas positively correlated with the protein content (p＜0.01) and negatively correlatedwith the total nitrogen content (p＜0.05) while the encapsulation yield of resveratrolwas negatively correlated with the total nitrogen protein content (p＜0.01) andnegatively correlated with soluble polysaccharide content (p＜0.05). Comparisonsamong the FT-IR spectra of microcapsules, yeast-cell wall material, physical mixtureof chlorogenic acid (or resveratrol) and yeast cells, and chlorogenic acid (orresveratrol) indicated the successful encapsulation of chlorogenic acid or resveratrolwithin the modified yeast cells.4. Preparation of yeast-cell-based microeapsnles of chlorogenie acid, with goodstability, promoted antioxidant activities and satisfactory release properties, andresveratrol microcapsules, with a sustained release profile and improved watersolubility and bioavailabilityThe analyses of HPLC and UV spectra suggested that no chemical change hadtaken place during the encapsulation of chlorogenic acid or resveratrol. The comparisons of FT-IR spectra, fluorescence and confocal micrographs between themicrocapsules and yeast cells confirmed that the water-soluble chlorogenic acid andhydrophobic resveratrol had been successfully encapsulated within yeast cells.Analyses of involved variables indicated that the encapsulating conditionssignificantly influenced the encapsulation efficiency and encapsulation yield ofchlorogenic acid or resveratrol, and the encapsulation efficiency and encapsulationyield were 68.41%,17.04% for chlorogenic acid and 74.44%, 13.22%for resveratrol,respectively, under the optimal encapsulating conditions. Further analyses of the fittedmode and its contour plot revealed that, under our experimental conditions, theconcentration of chlorogenic acid or resveratrol was the most significant factor andthe encapsulation time was the least one for the encapsulation efficiency andencapsulation yield of chlorogenic acid or resveratrol. The higher the encapsulatingtemperature, the lower the encapsulation efficiency and encapsulation yield ofchlorogenic acid, while the greater the concentration of chlorogenic acid, the higherthe encapsulation yield of chlorogenic acid but the lower the encapsulation efficiencyof chlorogenic acid. However, the higher the encapsulating temperature andresveratrol concentration, the bigger the encapsulation efficiency and encapsulationyield of resveratrol.The obtained yeast-cell-encapsulated chlorogenic acid or resveratrol exhibited goodstability after stored under 25℃/75% relative humidity (RH), 25℃/90% RH and60℃for 10 days. Their scavenging capacities on DPPH and hydroxyl radicals werestronger than those of non-encapsulated chlorogenic acid or resveratrol. Theyeast-encapsulated chtorogenic acid showed improved anti-denaturation naturewithout the sacrifice of its release rate, and the encapsulation yield of chlorogenic acidwas promoted by 68% as the purity of chlorogenic acid used increased from 35% to98%. The sustaining release profile suggested that the poor bioavailability due to therapid metabolism and elimination of resveratrol can be supplemented, to some extent,by the yeast cell encapsulation technology, and thus its biological activities were wellmaintained. The yeast cell encapsulated resveratrol showed 2-3 times higher the watersolubility and the slower photodecomposition. The obtained microencapsules have aunique size, without non-encapsulated chlorogenic acid or resveratrol on the surfaceof yeast cells.