Hyun-Gyu Lee

Efficient visible-light-driven photocatalytic degradation of nitrophenol by using graphene-encapsulated TiO2 nanowires

In this work, a new hybrid nanocatalyst, namely titanium dioxide (TiO2) composite nanowires, encapsulated with graphene (G) and palladium nanoparticles (Pd NPs) (designated as G-Pd@TiO2-CNWs), was prepared. In preparing the nanowires, a combination of electrospinning and hydrothermal approaches was employed. The visible-light-driven photocatalytic performance of G-Pd@TiO2-CNWs was investigated using the reduction of 4-nitrophenol (4-NP) as a model reaction. The results showed that G-Pd@TiO2-CNWs converted nearly 100% of 4-NP under visible light irradiation. The reaction kinetics of the photocatalytic reduction of 4-NP was studied by UV-vis spectrophotometry and the apparent rate constant was determined and compared with those for other supported TiO2 catalysts. Furthermore, the spent G-Pd@TiO2-CNWs could be recovered by simple centrifugation and reused. The work is expected to shed new light on the development of G-incorporated hybrid nanostructures for harvesting light energy and on the development of new photocatalysts for the removal of environmental pollutants.

Optimization of microwave-assisted extraction of total extract, stevioside and rebaudioside-A from Stevia rebaudiana (Bertoni) leaves, using response surface methodology (RSM) and artificial neural network (ANN) modelling.

Stevia rebaudiana (Bertoni) consists of stevioside and rebaudioside-A (Reb-A). We compared response surface methodology (RSM) and artificial neural network (ANN) modelling for their estimation and predictive capabilities in building effective models with maximum responses. A 5-level 3-factor central composite design was used to optimize microwave-assisted extraction (MAE) to obtain maximum yield of target responses as a function of extraction time (X1: 1-5min), ethanol concentration, (X2: 0-100%) and microwave power (X3: 40-200W). Maximum values of the three output parameters: 7.67% total extract yield, 19.58mg/g stevioside yield, and 15.3mg/g Reb-A yield, were obtained under optimum extraction conditions of 4min X1, 75% X2, and 160W X3. The ANN model demonstrated higher efficiency than did the RSM model. Hence, RSM can demonstrate interaction effects of inherent MAE parameters on target responses, whereas ANN can reliably model the MAE process with better predictive and estimation capabilities.

Determination of Quality Index Components in High-Acidity Cider Vinegar Produced by Two-Stage Fermentation

In this study, high-acidity cider vinegar (HACV) was produced by alcoholic and acetic acid fermentation of apple concentrate without any nutrients and then the optimum alcohol concentration was determined through a qualitative study. HACV was fermented with different initial alcohol concentrations (6-9%) during the process of acetic acid fermentation. The highest content of reducing sugar, organic acids, and free amino acids was observed at 6% of initial alcohol concentration. Approximately 20 types of volatile compounds were identified by solid-phase microextraction (SPME) and GC-MS. The total volatile content was the highest at 6% of initial alcohol concentration, and the acid content was the lowest at 9% of the initial alcohol concentration. The HACV produced by a two-stage fermentation process was qualitatively better than commercial HACV presenting the highest value at 6% of initial alcohol concentration. Malic acid, aspartic acid, and hexyl acetate were selected as quality index components of HACV production by two-stage fermentation on the basis of correlation between their physicochemical properties and the sensory attributes of HACV.

Cold Pasteurization of Frozen Crushed Garlics using Electron Beam Irradiation and the Stability of Bioactive Components

Recently, the market for ready-to-use vegetables has increased largely due to consumer demands, which led to the production of minimally-processed frozen crushed garlic products. This study was designed to determine the effect of electron beam irradiation (0-7 kGy) on microbial decontamination, bioactive compounds, and antimicrobial activities of frozen crushed garlic obtained from Korea and China. The microbial counts (total bacteria, yeasts & molds) were reduced from log 3-4 CFU/g to non-detectable levels as a result of irradiation at 4 kGy. Irradiation treatment at 4 kGy did not affect the amount of alliin, allicin, total pyruvate, and total thiosulfinate of crushed garlic, however, this dose induced insignificant changes in antibacterial activities against pathogenic bacteria including Bacillus subtilis, Staphylococcus aureus, Salmonella enteritidis, Vibrio parahaemolyticus, and Yersinia enterocolitica. Therefore, electron beam irradiation less than 4 kGy can be considered suitable to improve the microbial decontamination without altering the biological activity of frozen garlics.