Mechanisms Based On Acoustic Effects For Green Extraction Of Polyphenols

Ultrasonic technology has been applied in the fields of food,medicine,and chemical industry,including emulsification,freezing,sterilization,extraction,and purification.Especially in the extraction of bioactive components,parametric optimization(ultrasonic frequency,temperature,time,duty cycle,solid-liquid ratio,ultrasonic amplitude,ultrasonic intensity,solvent type,solvent concentration,reactor shape and size,etc.)and the combination of a variety of innovative technologies(ultrasound-microwave,ultrasound-instantaneous controlled pressure drop process,ultrasound-supercritical fluid,etc.)have been developed comprehensively.Recently,the newly developed green solvents,including deep eutectic solvent,bio-based solvent,and polyethylene glycol solvent,have been used in extraction.However,the property difference among the solvents will affect the acoustic effects by affecting the size,life span and type of cavitational bubble,so how the solvent properties affect the acoustic effect has not been further investigated.Furthermore,the degradation of bioactive components often occurs in extraction,but the reason of degradation has not been investigated in detail.Therefore,based on the ultrasonic acoustic effects,taking the most common phenols in fruits and vegetables as the target compounds,this paper introduces the commonly used characterization technologies in the acoustic field,including Rayleigh-Plesset equation,sonochemiluminescence,iodometry,calorimetry and micromorphology observation technology,comprehensively explores the dependence of acoustic effect on the properties of a variety of green solvents,and analyzes the degradation of phenolic compounds during extraction.Meanwhile,the impact of sonochemical effect on the extraction of phenols in countercurrent dynamic extraction was discussed.The main results of this experiment are as follows:1.Response surface methodology(RSM)was used to analyze the effects of temperature,duty cycle and solid-liquid ratio on the production of polyphenols.Then,taking the RSM data as the sample,the genetic algorithms-artificial neural network(GA-ANN)was used to deeply learn and optimize the sample data.The results showed that high temperature(70℃)can effectively promote the release of polyphenols,but high duty cycle can significantly cause the degradation of polyphenols.The combination of RSM and GA-ANN has a better prediction ability.The optimum conditions are: temperature 67℃,duty cycle 1/2,solid-liquid ratio 1:25g/ml.The total phenol yield was 8.21 mg GAE/g,which was 84% that of the methanol water.The feasibility of ultrasound-assisted water extraction of polyphenols and the excellence of RSM combined with GA-ANN in the optimized extraction were proved;2.Based on the optimal parameters,the acoustic effects of green solvents(deep eutectic solvent,bio-based solvent,polyethylene glycol solvent)were further analyzed.The bubble behavior was analyzed by Rayleigh-Plesset equation(bubble life,collapse radius,collapse pressure),the sonochemiluminescence phenomenon(ability to produce free radicals)was analyzed by using luminol solution,and the microstructure of citrus peel were analyzed by the scanning electron microscope.The results showed that it was not feasible to predict the extraction ability of solvent only by Hansen solubility parameter,because the influence of solvent on cavitation characteristics,especially on bubble behavior,was more significant.There was a significant correlation between the bubble collapse pressure and the yield of flavonoids,but there was no significant correlation between the bubble collapse pressure and phenolic acids,which may be due to the different positions of flavonoids and phenolic acids in citrus peel.The results of microstructure analysis showed that the relatively high collapse pressure of bubble was helpful to the fragmentation of skin.Green solvent,especially ethyl lactate,had strong extraction ability,and the extract also had excellent antioxidant ability;3.Based on the temporal and spatial dynamic changes of acoustic effect,the degradation behavior of polyphenols in enhanced extraction was analyzed and proposed.The results showed that the sonochemical and mechanical effects were significantly affected by ultrasonic parameters and can be controlled by adjusting ultrasonic parameters.For example,increasing the irradiation distance(10-50 mm)can significantly inhibit the formation of free radicals and enhance the mechanical effects.Secondly,there was a dose-response relationship between the degradation of polyphenols and free radicals under ultrasonic field.The existence of free radicals does not necessarily lead to the degradation of polyphenols.Only when the concentration of free radicals reaches a certain threshold,will the degradation occur.Caffeic acid showed higher stability than rutin and cyanidin-3-glucoside.The degradation kinetics showed that caffeic acid and rutin followed the zero-order kinetics,while cyanidin-3-glucoside followed the second order kinetics;4.Through correlation analysis,it was found that the degradation mechanism of polyphenols caused by ultrasonic parameters can be attributed to the hydroxyl radicals,and different ultrasonic parameters can cause different production of hydroxyl radicals.At the same time,based on acoustic effects,the parameters with moderate sonochemical effects are selected and the correlation between extraction yield and mechanical effects was further analyzed.It is found that there was also a good correlation between extraction yield and ultrasonic parameters.Therefore,the different impacts of mechanical effects and sonochemical effects on enhanced extraction were further verified;5.Based on the static ultrasound,the impacts of dynamic extraction assisted by ultrasound on the polyphenols yield were studied.Increasing flow rate can indeed promote the oxidation of potassium iodide and the enhancement of sonochemiluminescence signal,thus increase the production of hydroxyl radicals.In addition,the higher flow rate induced higher yield of hydroxyl radicals but the yield of phenolic acids and flavonoids was not significantly affected by the hydroxyl radicals.With the increase of flow rate from 25 V to 100 V m L/min,the yield of phenolic acids increased by 25%.Since the increase of flow rate can enhance sonochemical activity,the impacts of sonication time and duty cycle on sonochemical activity were significant.The results showed that there may be synergistic effects between sonication time and flow rate as well as duty cycle and they can jointly promote the degradation of polyphenols during extraction.Compared with the statically ultrasound-assisted extraction,the degradation can appear at 15 minutes during dynamically ultrasound-assisted extraction.Therefore,during actual extraction,shortening extraction time as much as possible,reducing duty cycle,and improving flow rate to promote the extraction of bioactive components were needed to be considered.