High Frequency Ultrasonic Testing Of Shear Viscosity Of Edible Oils With Repeated Heating

Edible oil is one of the main edible materials in the human daily life,and it not only provides people with the energy needed by the human body,but also provides fat-soluble vitamins and other health-promoting ingredients,it is an indispensable nutrient element for human beings.Fried food is delicious because of its crispness,which is deeply loved by consumers.Frying as a means of food processing,in many cases,edible oil is often repeatedly fried to save money by human people.However,in the process of frying at high temperature for many times,edible oil will decompose and polymerize gradually with the extension of frying time and the increase of heating times.A series of changes in physical and chemical components of oils and fats will produce a large number of aldehydes,ketones,acids,polymers and other substances,which not only seriously affect the quality and flavor of food,but also have potential harm to human health,and even carcinogenic.Therefore,it is particularly important to monitor and control the quality of edible oil online in the process of high temperature frying.It is of great significance to determine the hygienic quality index of edible oil accurately and quickly.As a non-destructive testing and analysis technology,ultrasonic plays an increasingly important role in the quality testing of edible oil due to its advantages of convenience,time-saving,non-destructive and safety,and has shown a good application prospect.Therefore,this paper is mainly based on the reflection echo of ultrasonic wave at the interface between delay line and oil product.The echo amplitude and phase difference are detected by parameter optimization and appropriate signal processing technology,so as to determine the shear viscosity of oil,and at the same time explore the possibility of predicting oil quality by the viscosity.First of all,based on the theory of acoustic interfacial reflection,we studied the relationship between the complex reflection coefficient of echo at the solid-viscoelastic liquid interface and the viscoelastic modulus and viscosity coefficient of the liquid.And taking the solid-air interface as the reference interface,by comparing the ultrasonic reflections of solid-air interface and the solid-liquid interface,the effects of the center frequency of the probe,the impedance of the solid matching layer and the viscosity of the liquid on the amplitude and phase of the reflection coefficient are numerically analyzed,In particular,the time resolution of the instrument used in the experimental measurement is predicted.The results show that the high-frequency probe and low-impedance solid matching layer are may suitable for experimental measurement,in order to improve the detection sensitivity.Moreover,the measuring system has a high temporal resolution and can be measured in nanoseconds for viscoelastic liquids with low viscosity.Secondly,in order to evaluate the quality of edible oil heated at high temperature for several times,we selected the most representative repeatedly fried peanut oil,rapeseed oil as the research object.The viscosity coefficients of two kind of oils with different heating times at room temperature were measured by the method of shear wave reflection-amplitude method.The experimental results show that although the the ultrasonic shear wave and the rheometer test results have some measurement errors,they both can well reflect the relationship between the viscosity of oil samples and the heating times.This study proves that the ultrasonic method is feasible for on-line evaluation of oil deterioration.In addition,in order to reduce the measurement error of ultrasonic shear wave reflection method and improve the detection accuracy,based on the theory of shear wave reflection method of single parameter-amplitude method,the two kinds data post-processing methods(Central frequency method and Integral ratio in a frequency band method)of frequency domain were adopted.The results found that the Integral ratio in a frequency band method bandwidth is closer to the rheometer than the center frequency method.This extends the application of the ultrasonic shear wave multiple reflection method to low viscosity liquid viscosity detection.Finally,in order to solve the monotony of using viscosity as an indicator to evaluate the quality of edible oils,we try to find the relationship between viscosity and other evaluation indexes of edible oil,and we predict some detection indexes by using viscosity to further evaluate the deterioration degree of repeatedly heated oil.Therefore,we fitted with the the relationship between the viscosity and the number of heating times,and it is found that the number of heating times is highly correlated with the viscosity of edible oil.According to the relationship between viscosity and other indicator contents,we predicted indicator content through the relationship between viscosity and other test indicators,and then according to these indicators to further evaluate the deterioration degree of reheated oil.This study has laid a foundation for the further use of ultrasonic on-line dynamic monitoring of oil deterioration.