Studies On Degradation Mechanism Of Lycopene From Red Grapefruit During Processing And Quantitative Structure-activity Relationship

Red grapefruit(Citrus paradise Macf.) is an important kind of orange with high nutritional value and pretty red color. The quality of red grapefruit degenerated during processing and storage, which result in a reduced market value. This study originated from the degradation of red grapefruit lycopene. On the basis of isolation and identification of lycopene from red grapefruit, we investigated the degradation pathway of lycopene obtained from red grapefruit, and the effect of intrinsic factors of red grapefruit juice on the degradation of lycopene. The results help us further understand the browning mechanism of red grapefruit. In addition, we analysed the stability of different lycopene isomers with quantum chemistry, and established the quantitative structure-activity relationship (QSAR) model of antioxidant activity for various carotenoid isomers. Making generalization and summary of the research results, the main conclusion derived from this study are:1. The nutrient compositions of red grapefruit were analyzed, and the changes of composition of red grapefruit during storage were studied. Red grapefruit juice was abundant in lycopene, other carotenoids, ascorbic acid and sugars etc. There are also rich in trace elements such as Cu, Fe, Zn, Mg, Ca, which are necessary to human. The sugars in red grapefruit juice are glucose, fructose and sucrose. The characteristic flavor compounds in red grapefruit are limonene and 1-caryophyllene.During the storage of red grapefruit juice, the attractive red color was losed along with the decreasing of lycopene content, and low temperature was conducive to the stability of lycopene and its color; total acid content increased smoothly and reducing sugar content increased first and then decreased; the content of ascorbic acid declined first and then increased smoothly.2. The major carotenoids in red grapefruit were identified as β-carotene, 15-cis-lycopene,13-cis-lycopene,9-cis-lycopene and all-trans-lycopene and the highest content was all-trans-lycopene. Response surface methodology was used to optimize the extractive craft of lycopene from red grapefruit and the optimal extraction processing parameters were petroleum ether as extraction solvent, extraction temperature of 30℃, extraction time of 3.8 h and liquid-material ratio of 3.5:1. Separation and purification of lycopene by the silica gel column chromatography with elution phase of petroleum ether and acetone was established. A combination of UV-Visible spectra, chemical qualitative determination, infrared spectra, HPLC analysis and HPLC-MS analysis was used to identify the structure of lycopene extracted from red grapefruit.3. The ultrasound extraction principle of all-trans-lycopene was comprehensively investigated. The UAE can obtained much more lycopene yield with less extraction time and lower extraction temperature than CSE; The UAE had obvious advantage in the enhancement of lycopene yield at different solvent and material ratio; the appropriate ultrasonic intensity was conducive to extract lycopene; compared to continuous ultrasonic, pulsed ultrasound had better extraction efficiency with suitable pulse interval. The HPLC-PAD analysis of lycopene isomerization in UAE found the major isomers were 15-cis-,13-cis-,9-cis-,9,13’-di-cis-and 9,13-di-cis-lycopene.4. The study confirmed the effects of different processing and storage factors on the stability of lycopene from red grapefruit. The degradation rate of lycopene was fastest at pH4.0. In addition, lycopene were unstable for heat and light. Mg2+, Zn2+, Ca2+ had protective effect on the thermal degradation of lycopene. Cu2+ and Fe3+ had significant destructive effect on lycopene stability, and the more content of Cu+ and Fe+, the faster degradation rate of lycopene. Glucose had a protective effect on the thermal degradation of lycopene, whereas sucrose and fructose could promote the degradation of lycopene, and the effect of sucrose was greater than fructose.5. The thermal degradation kinetics of red grapefruit lycopene and color were investigated. The results indicated that the degradation of lycopene and color were first-order degradation kinetics. The relationships between lycopene content and visual colo during thermal processing at selected temperatures could be expressed by the same equation:C/Co= 1.7707(a*/a*0)+0.8012. The HPLC-PAD analysis of lycopene degradation was found the content of mono-cis isomers increased first and then decreased; the content of di-cis isomer generated after then was decreased when heating for 4h because of the change to mono-cis isomer or degradation. During the whole heating processing, the content of all-trans-lycopene was sustained declined and continuously heating can cause generated isomers to degrade to aldehydes or ketones, which resulted in the decrease content of total lycopene.6. Degradation kinetics of lycopene in model systems and the effect of intrinsic factors on lycopene degradation were investigated. The results indicated that ascorbic acid and sugars significantly accelerated lycopene degradation. However, β-carotene had a protective effect on degradation of lycopene, and the protective effect of P-carotene played a more important role in the degradation of lycopene comparing to the negative effect of ascorbic acid or sugars. The promoting effects of sugars on lycopene degradation correlated with the temperature, and the degradation of lycopene in the presence of sugars followed complex reaction kinetics at comparatively higher temperature (above 70 ℃). The stimulative effects of sugars on lycopene degradation were according to the following descending order:fructose> sucrose> glucose. In addition, Cu2+ showed a destructive effect on lycopene degradation, it could significantly accelerate lycopene degradation in the presence of ascorbic acid. So the effect of Cu2+ on lycopene degradation was a coupled oxidation.7. The total energy values of different lycopene isomers had been calculated by AM1 of semi-empirical approach in quantum chemistry. The stability of various lycopene isomers were according to the following descending order: 5-cis>9-cis>all-trans>13-cis>15-cis>1-cis>11-cis. The molecular mechanics and quantum chemistry methods were used to establish the regression and neural network model for building quantitative structure-activity relationship (QSAR) of antioxidant activity of various carotenoid isomers. The MLP model had high fitting ability and predictive ability. The constructor parameters of Ehl and HF were the major affecting factors for the antioxidant activities of various carotenoid isomers.