The Study On Path And Mechanism Of Orientation Degradation Of Pectin Polysaccharide With Ultrasound

Pectin, as an acid polysaccharide, is one of the major components of the primary cell walls and middle cellular of higher plants. Pectin has been widely used in food, medicine and cosmetic industry for its natural, non-toxic and cheaper characters. The structure of pectin varies according to the source species, original tissue type and location, developmental or metabolic stage, environmental state and conditions of extraction. These variations together with its larger molecular weight, limited the wide use of pectin. Recently, more and more researches are focused on modification of pectin (modifing molecular weight or structure) to adapt to different needs. Generally, modified pectin is prepared by chemical or enzymatic depolymerization processes. These methods are hard to control and generally not oriented. Meanwhile, the chemical method was time consuming and lead to environment pollution. The "green and innovative" technique in pectin modified is preferred. Ultrasound, as a green and physical technique, has been widely used in polymer degradation. However, there were rarely reports on the degradation of pectin polysaccharide by ultrasound.This research was the first time to investigate the degradation behavior pathway and mechnism of pectin degradation by ultrasound in detail. The ultrasound degradation kinetics of pectin was established. This study revealed the degradation pathway of pectin by atomic force microscopy at nanolevel. The degradation mechnism was also revealed through investigation of radical and mechanical effects of ultrasound. Compared with the acid degradation methods of pectin, ultrasound is a simple, rapid, cheaper and green method.The main results and conclusions were listed as follows:(1)The degradation behaviour of pectin polysaccharide with ultrasound:The study established the degradation behaviour of pectin polysaccharide with ultrasound by determined the molecular mass and its distribution of pectin polysaccharide. The results indicated that the molecular mass and degradation rate of pectin polysaccharide decreased with ultrasound time. The molecular mass of pectin polysaccharide decreased with increasing ultrasound intensity. However, the ultrasound efficiency could not increase indefinitely. Lower temperature could enhance the degradation process. The ultrasound effect was most significant when the duty cycle was50%.(2) The degradation path of pectin polysaccharide with ultrasound:The degradation path of pectin polysaccharide with ultrasound was determined by the structure investigation. The distribution of molecular mass of pectin polysaccharide was more uniform after ultrasound treatment. The pectin fractions which below100kDa were accounted47%when ultrasound90min. The monosaccharide types and main chain of pectin polysaccharide were not changed after ultrasound treatment. However, the pectin sidechains were degraded. The break chain sequence of pectin polysaccharide with ultrasound was Ara>Gal>Rha>GalUA. AFM analysis indicated that the ultrasound degraded pectin was mainly composed of smaller polymers and shorter linear chains. The branched structure was reduced. There was a hypothesis that the branches and linear chains are homogalacturonans composed solely of galacturonic acid. The ration of short length and wider chains was increased and the hight of the chains was reduced. There was a positive correlation among length, width and molecular mass and its distribution of pectin polysaccharide.(3)The degradation kinetic of pectin polysaccharide with ultrasound:The study established the degradation kinetic of pectin polysaccharide with ultrasound by determining the dynamic change of molecular mass and distribution of pectin polysaccharide. Degradation kinetics model of pectin fitted to1/Mt-1/Mo=kt from5to45℃. Ultrasound could decrease the activation energy and accelerate the pectin degradation.(4) The degradation mechanism of pectin polysaccharide with ultrasound:The radical and mechanical effects were studied to investigate the degradation mechanism of pectin polysaccharide with ultrasound. The degradation rate and chain-breaks were decreased with reducing chain length of pectin. O2and Ar could enhance the production of radical, while N2has no significant effect on the radical yields. The degradation rate and chain-breaks of pectin with ultrasound were significant increased when O2existed. Ar and N2had no significant influence on the degradation rate and chain-breaks of pectin. There was a negative correlation between radical yields and degradation rate and chain-breaks. Gases could induce sidechain reaction of pectin with ultrasound. Therefore, mechanical effect was the main factor to affect the degradation of pectin with ultrasound.(5)The functional properties of degraded pectin polysaccharide with ultrasound: The appearance viscosity of pectin significantly reduced with ultrasound. The pectin solution showed viscous properties after ultrasound treatment. Chain length of pectin was one of the reasons resulted the decrease of appearance viscosity of pectin. The pectin solutions showed Newtonian fluid properties (n=l). The lipase inhibition activity of pectin increased after ultrasound treatment.(6)Establishment of the degradation method of pectin polysaccharide with ultrasound:The study compared the degradation methods (ultrasound and acid) of pectin. The results indicated that ultrasound degradation did not alter the structure of pectin. Meanwhile, the ultrasound method has the advantages of mild, efficiency, environmentally friendly and so on. Ultrasound method is a green technique for pectin modification.This study could provide theoretical basis on the green modification technique of polysaccharide. It could expand the application of ultrasound in food industry, and enlarge the uses of pectin in food and medicine industry. It is meaningful to utilize the waste resources of citrus and apple pomace to protect environment.