Title

Identification and quantification studies on structures, dynamics and mechanism for thermal and photo-degradation products of [beta]-carotene

Author

Yuan Zhao

Date of Award

2011

Degree Name

M.S. in Chemistry

Department

Department of Chemistry

Advisor/Chair

Advisor: David W. Johnson

Abstract

Beta-carotene is comprised of isoprene units, and hence contain methyl-substituted polyene chromophores, which contains 11 highly unsaturated conjugated double bonds, nine of which are in a planar polyene chain and the other two of which are located in terminal β-ionylidene rings. The first objective is to determine the kinetics of β-carotene (βC) thermal and photo-degradation rate and evaluate the correlation among content, bleaching time and temperature and solvent effects. The second objective is to figure out the chemical structure of unknown thermal and photo-degradation extents and products structures by instruments of UV-Vis spectrometry, NMR and GC-MS. Solid βC was heated in sealed pyrex tubes to ensure isothermal heating and no oxygen impacts in open tubes with vents to detect oxygen impacts. Analysis of degradation kinetic data suggested a first-order reaction for the thermal-degradation of βC. The thermal-degradation of β-Carotene was studied at oven temperatures of 100, 150 , 200, 250, 300 , 350 °C for 30min, 60min, 90min, 120min, 4h, 8h, 24h, 72h separately. The kinetics parameters, rate constant k, activation energy Ea =11.5763kJ/mol, and pre-exponential factor A (1.4041h-1) have been calculated based on the linear regression of concentration vs temperature (100-350°C). β-carotene is heat sensitive as expected and the degradation fate displayed a relative heat instability and temperature dependence. The higher the treatment temperature, the longer time exposure period, the faster the degradation rate. The degradation products, retinol, β- ionone etc are confirmed by GC-MS, most of other products are believed to be apocarotenals and epoxy compounds. The kinetics from photochemical experiments using UV irradiation are compared with similar experiments conducted with two-photon laser irradiation. The kinetics of photochemical reactions of β-carotene was studied in hexane, carbon tetrachloride and percentages of carbon tetrachloride in hexane below 5%. At low percentages of carbon tetrachloride, the reaction is first order in both carbon tetrachloride and β-carotene. The activation energy of the reactions at temperatures around 25°C was found to be positive but very small. The photochemical results are also compared with thermal-degradation experiments conducted at temperatures between 250 and 350°C. In addition to the kinetic results, some products of the reaction have been identified by nuclear magnetic resonance spectroscopy. It is interesting to note that under thermal conditions, retinal and retinol with vinyl hydrogens are identified as products, while under UV irradiation, there are no identifiable products that contain double bonds.

Keywords

Beta carotene Effect of heat on Testing, Beta carotene Effect of light on Testing, Beta carotene Decay Testing

Rights Statement

Copyright 2011, author

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