The effect of microwave irradiation on the structure of selected plant tissues

Abstract

Microwave-assisted extraction (MAE) is a green technique for the extraction of natural products with the advantages of shorter extraction time, less solvent consumption and higher quality of target compounds compared with conventional techniques, which has been proven to be a superior approach to extract active compounds from plant materials. In this thesis, two typical examples of plant tissues, namely dried mulberry leaf and fresh pomelo fruit peel tissues were selected, and two microwave extraction methods including MAE and solvent-free microwave extraction (SFME) were performed. Some techniques such as optical microscope and scanning electron microscope (SEM) were applied to investigate the structure of plant tissues. The target compounds were analyzed by high performance liquid chromatography with an ultraviolet-visible detector (HPLC-UV) and gas chromatograph-mass spectrophotometer (GC-MS). Firstly, this thesis presents a study of the influence of operation conditions on the extraction of flavonoids from mulberry leaves by MAE, and the effects of microwave power, extraction time and temperature on the yield of flavonoids. An orthogonal test has been designed in order to optimize the operating conditions efficiently and determine what factors are significant. An efficient MAE process has been developed for the fast extraction of flavonoids from mulberry leaves. The highest yield of flavonoids is obtained under an optimal extraction condition, i.e. microwave power 500 W, extraction temperature 60 °C, extraction time 4 min, aqueous ethanol concentration 50% and solvent to material ratio 40 ml/g. Compared to heat reflux extraction (HRE), the obtained MAE process can reduce extraction time and gain higher yield of flavonoids extracted from mulberry leaves without any degradation of flavonoids. Therefore, MAE is an alternative extraction method due to its fast extraction of flavonoids from mulberry leaves and should be promising for industrial application. Secondly, SFME was used to extract the essential oil from pomelo fruit peels. SFME is a recently developed green technique which combines microwave heating and dry distillation, performing in an atmospheric condition without the addition of any solvent or water. The effects of the microwave power, extraction time and temperature on the yield of essential oil from peels are investigated. The most efficient operational condition for SFME is found as the microwave power of 150 W and extraction time of 90 min. Compared to hydrodistillation (HD), SFME can be an alternative extraction method because of its shorter extraction time, higher yield and quality of essential oil without any degradation. Lastly, two distinct extraction mechanisms of MAE are suggested to be involved in the extraction of active compounds from dried mulberry leaf and fresh pomelo fruit peel tissues. One is related to the diffusion of target compounds across unbroken cell walls, and another one is related to the exudation of target compounds to the surrounding solvents from broken cell walls.