| With the improvement of the people's living standard,consumers are becoming more and more picky about the taste of French fries.Potato strips fried in soybean oil and rapeseed oil cannot meet the consumers'personalized needs of consumers due to poor flavor and soft texture respectively.Meanwhile,it also limits the development and utilization of different oils in the potato strips processing industry.The reason lies in the lack of understanding of the law and mechanism of the effect of different oils on the quality of French fries.Texture is one of the most important qualities for French fries,while little is known about the effect mechanism of different oils on the texture of French fries.Therefore,in this dissertation,the French fries and oils were taken as the research objects.Firstly,the method for evaluating the texture of French fries was determined.Secondly,the effect of different frying oils on the texture of French fries was analyzed,and then taking the fatty acid composition in the oil as the breakthrough point,the relationship between the fatty acid composition and the texture of French fries was explored.Thirdly,the effect mechanism of frying oil on the texture of French fries was clarified from the perspective of the heat and mass transfer.Finally,the rationality of the influence mechanism was demonstrated by experimental design.This study will lay a solid theoretical foundation for the development of special frying blend oil for the French fries which will integrate the advantages of various oils and cater to the needs of the consumers.The main research contents and results are as follows:Firstly,in order to optimize the appropriate method for determining the texture of French fries,the texture of ten commercial French fries were characterized by the methods of quantitative description analysis in sensory evaluation and five kinds of instrument tests,and the correlation between sensory data set and instrumental data set was explored.The results showed that three-point blending test had the largest coefficient of variation and the lowest precision.The overall hardness represented by the positive peak area that were obtained through the puncture test had the highest correlation coefficient with sensory hardness(r=0.88,p<0.01)and sensory crispness(r=0.84,p<0.01).Based on the comprehensive consideration of the coefficient of variation of texture measured by instrument test and its correlation with sensory attributes,the puncture test is the most suitable to determine of the texture of French fries,which provides an objective and economical evaluation method for characterizing the texture of French fries.Secondly,ten kinds of oils,including soybean oil,corn oil,peanut oil,rice bran oil,rapeseed oil,high oleic acid rapeseed oil,sunflower oil,high oleic acid sunflower oil,palm oil,and lard,were used as the frying medium,and the effects of different oils on the quality of French fries prepared with two ways(industrial production and household production)were analyzed and compared.The results showed that there were similar characteristics for the fries prepared with two ways.potato strips fried in lard had higher hardness,but the oil content was also higher;potato strips fried in peanut oil had lower oil content,but higher volume shrinkage rate;potato strips fried in rapeseed oil had good color(bright yellow),but lower hardness.In addition,the sensory evaluation found that the overall score of fries fried in rice bran oil was higher,but the overall score of fries fried in lard was lower.The assessor preferred the hardness of potato strips fried in rice bran oil and palm oil.Pearson correlation analysis showed that the hardness of fries prepared by the two methods mentioned above was significantly positively correlated with total saturated fatty acids,and the correlation coefficients were all above 0.662(p<0.05),indicating that the content of saturated fatty acids in oil was the main factor affecting the texture of French fries.Thirdly,in order to reveal the mechanism of the effect of oils on the texture of French fries from the perspective of heat transfer,four oils(high oleic acid sunflower oil,soybean oil,palm oil and rice bran oil)with low,medium,and high saturated fatty acid content and excellent sensory evaluation were selected from the above ten oils as the frying medium,and the effect mechanism of heat transfer characteristics of oils on the texture of potato strips during frying was explored.The results showed that the hardness of potato strips fried in high oleic acid sunflower oil was lower than that of the other three oils during the frying.In the first 60 s of frying,the convective heat transfer coefficient of potato strips fried in palm oil was the highest(914.17 W·m-2·°C-1),followed by rice bran oil(726.75 W·m-2·°C-1),while the convective heat transfer coefficient of potato strips fried in high oleic acid sunflower oil was the lowest(556.73W·m-2·°C-1);after 60 s,the change trend of convective heat transfer coefficient was opposite,but the difference was reduced gradually.The frying process of French fries was numerically simulated based on the heat transfer parameters.It was found that the difference of temperature response of French fries fried in different oil was attributed to the difference of convective heat transfer coefficient.In the first 25 s of frying,the surface and center temperature of fries fried in palm oil and rice bran oil were higher than those of high oleic acid sunflower oil and soybean oil.Correspondingly,the evolution process of the crust thickness of French fries was observed through the binary image of nuclear magnetic resonance and stereomicroscope,it was found the crust of potato strips fried in palm oil was preferentially formed,followed by fried in rice bran oil,and at last fried in high oleic acid sunflower.Moreover,the crust thickness of potato strips fried in palm oil and rice bran oil was significantly higher than the other two oils during the frying.In addition,the result of X-ray diffraction(XRD)showed that the relative crystallinity at the 20°(2?)diffraction peak(starch-lipid complex)of potato strips fried in palm oil was higher than that of the other three oils with the increase of frying time.The results of scanning electron microscopy(SEM)also showed that the crust of potato strips fried in high oleic acid sunflower oil and soybean oil were loose and more holes,however the crust of fries fried in rice bran oil had fewer holes and was denser,and the crust of fries fried in palm oil was denser than the other three oils.In summary,in the early stage of frying,the convective heat transfer coefficient of potato strips fried in palm oil and rice bran oil was higher than that of the other two oils,and the crust was formed on the surface of potato strips prior to the other two oils.As the extension of frying time,the crust thickness was increased,and the content of starch-lipid complex was also high.This series of changes contributed to the hardness of fries fried in palm oil and rice bran higher than the other two oils.In addition,in order to deeply understand the influence mechanism of oils on the texture of French fries from the perspective of mass transfer,the effect mechanism of the above four oils on the texture of the surface and core of French fries were explored.The results showed that the hardness of potato strips fried in high oleic acid sunflower was lower than that of the other three oils regardless of the surface of the core region.During the first 30 s of frying,the water loss in the core region of potato strips fried in high oleic acid sunflower oil was the slowest,followed by soybean oil,while the water loss in the core region of potato strips fried in palm oil and rice bran oil was the fastest.After 60 s,the trend was opposite.The moisture content in the surface region of potato strips fried in palm oil and rice bran oil was lower than that of the other two oils during the frying.Correspondingly,the results of water loss kinetics showed that the surface water loss rate of potato strips fried in palm oil was the fastest(0.0536s-1),followed by rice bran oil(0.0275 s-1),and high oleic acid sunflower oil was the slowest(0.0216 s-1).While the water loss rate in the core region of potato strips fried in palm oil was lower than the other three oils.The results of nuclear magnetic resonance imaging(MRI)also showed that the hydrogen proton density of potato strips fried in palm oil and rice bran oil was lower than that of the other two oils.Besides,the content of starch-lipid complex of potato strips fried in palm oil and rice bran oil was higher than that of the other two oils.The results of SEM showed that the crust region of potato strips fried in palm oil and rice bran oil was more compact than other two oils.Overall,fast surface water loss,high content of starch-lipid complex and compact surface structure played important roles to the increase the hardness of French fries.Finally,in order to further demonstrate the mechanism of the effect of oils on the texture of French fries from the perspective of oil saturation,the effects of oils with different saturation obtained by mixing refined rapeseed oil and fully hydrogenated rapeseed oil in different mass ratios(0%,20%,40%,60%,80%,100%)on the texture,temperature profile,water distribution,crust thickness,crystal structure and microstructure of French fries were studied.The results showed that the hardness,crust thickness,total relative crystallinity of French fries increased,and the microstructure of the crust region also became denser with the increase in the proportion of fully hydrogenated rapeseed oil(the degree of saturation).In the first 40 s of frying,the surface and center temperature of potato strips fried in high saturation oil were higher than those of potato strips fried in low saturation oil.After 50 s,the surface temperature curve of potato strips was opposite to that of the first 40 s.The MRI results showed that the extent of surface water loss of potato strips fried in high saturation oil was higher than that of potato strips fried in low saturation oil.These results confirmed the mechanism of the effect of oil saturation on the texture of French fries.In addition,combined with the previous sensory evaluation and the research results of this chapter,it is found that the oil saturation range of fried fries with better texture is between 37%?51%,which provides a useful reference for the development of special oil suitable for French fries in the fries processing industry. |
PDF Full Text Request