The Key Factors Affecting The Oil Absorption Properties Of Fried Potato Strips And Its Underlying Mechanism

Fried foods are very popular because of their crispy taste and unique fried aroma.However,the high oil content of fried foods is contrary to the low-fat diet advocated by today’s society.The influencing factors and mechanisms of oil absorption of fried foods during the preparation procedure should be understood for controlling the oil content of fried foods scientifically and effectively.Most of the current related researches only used simulation systems or conducted theoretical analysis based on ideal conditions.There is a lack of systematic research on the influence of various factors involving oil absorption during the actual fried food production process.Therefore,from the perspectives of different pretreatments（changing food properties）and the properties of frying oil,this thesis studied the effects of cooking,drying and freezing pretreatment on the oil absorption in fried potato strips comprehensively and systematically.The differences in the physicochemical properties of different frying oils and their effects on the oil absorption of potato strips were analyzed comparatively.On this basis,further study was conducted to explore the changing laws of the content of different types of oils in fried potato strips during continuous frying.The key factors in the fatty acid composition and physicochemical properties of frying oil affecting the oil absorption of fried potato strips were also explored.It is hoped to understand deeply the factors and mechanisms of the oil absorption in fried foods during the preparation procedure so as to provide a theoretical basis for enterprises to take corresponding oil control measures.The main contents of this research are as follows:First,the effects of different cooking and drying pretreatments on the oil absorption of fried potato strips were compared.The reasons for the changes in oil content were analyzed from the changes in the moisture content,moisture state,starch properties and microstructure of the potato strips after different pretreatments.Compared with the control group,the oil content of the fried potato strips pretreated by blanching significantly increased by 2.26%（p<0.05）;and the oil content of fried potato strips pretreated by microwave heating at 20 s,40 s,and 60 s was significantly reduced by 3.16%,11.29%and 17.93%（p<0.05）,respectively;and the oil content of fried potato strips pretreated by drying in the vacuum oven at 60 min,120 min,and 240 min significantly decreased by 5.40%,8.45%,and 19.53%（p<0.05）,respectively.Further analysis revealed that the moisture content of the potato strips significantly increased（p<0.05）after the blanching pretreatment,the moisture content of the potato strips did not change significantly（p>0.05）after the microwave pretreatment,and the moisture content of the potato strips significantly decreased（p<0.05）after the vacuum drying pretreatment.After blanching or microwave pretreatment,the content of the semi-bound water in the potato strips increased,and the content of free water was greatly reduced.However,after vacuum drying pretreatment,only the free water of the potato strips decreased.X-ray diffraction（XRD）results showed that the content of V-type starch-lipid complex was the highest in the fried potato strips pretreated by microwave,which distributed on the surface of the fried potato strips and hindered the penetration of oil.Scanning electron microscopy（SEM）showed that the structure of potato strips became loose after blanching,and many pores and cavities appeared.The structure of potato strips after microwave pretreatment was dense with few holes,and the crust formed after frying was denser and thicker than others.The volume of the potato strips shrunk after vacuum drying pretreatment,and the pores became small.The changes in moisture content,moisture state,starch properties and microstructure of potato strips caused by different pretreatments affected the oil absorption of fried potato strips.Afterthat,the influence of conventional freezing pretreatment on the oil absorption of potato strips during frying was analyzed.The results showed that,compared with those of the fried potato strips without frozen pretreatment,the total oil content,surface oil content and matrix oil content of the potato strips pretreated by conventional freezing significantly increased by 10.05%,0.24%and 10.29%after frying（p<0.05）,respectively.Magnetic resonance imaging（MRI）technology found that oil signals appeared at the cores of unfrozen potato strips and conventional frozen pretreated potato strips when they were fried for 210 s and 150 s,respectively indicating that conventional freezing enabled frying oil to penetrate into potato strips earlier.The SEM images showed that the surface of the fried potato strips with conventional freezing pretreatment was rougher,and the internal structure was seriously damaged.The effect of different cooking methods（blanching,microwave）combined with different freezing methods（conventional freezing,ultra-low temperature freezing）on the oil absorption of fried potato strips was further compared and analyzed.It was found that,compared with blanching combined with conventional freezing pretreatment,the total oil content of fried potato strips was significantly reduced by 15.86%（p<0.05）when blanching was changed to microwave heating.The total oil content was significantly reduced by 6.09%（p<0.05）when conventional freezing was changed to ultra-low temperature freezing.Microwave combined with ultra-low temperature freezing pretreatment reduced the total oil content of fried potato strips by 20.46%significantly（p<0.05）.The results of confocal laser scanning microscopy（CLSM）,differential scanning calorimeter（DSC）and SEM together confirmed that microwave combined with ultra-low temperature freezing pretreatment promoted the formation of V_II type starch-lipid complexes in the crust of fried potato strips,and the surface structure of the crust was denser and smoother with less oil attached,and less damage in the internal of fried potato strips than others.Combined with the results of the color and sensory evaluation,it showed that microwave combined with ultra-low temperature freezing pretreatment can reduce oil absorption of fried potato strips as well as improving their quality.Next,ten different frying oils were used to fry potato strips,and the relationship between the physicochemical properties of frying oils and the oil absorption of fried potato strips was analyzed.The results showed that the surface oil content,surface penetrating oil content,and total oil content of the potato strips fried by different frying oils had significant differences（p<0.05）,while the matrix oil content was not significantly different（p>0.05）.Among them,the total oil content of potato strips fried in sunflower oil and corn oil was lowest（34.07%,34.32%）,while the total oil content of potato strips fried in high oleic sunflower oil,high oleic rapeseed oil and rapeseed oil was highest（40.11%,40.09%,39.91%）.The results of orthogonal projections to latent structures（OPLS）model analysis showed that the viscosity of frying oil is the most critical factor affecting the oil absorption of the fried potato strips.Moreover,it was found that the viscosity of frying oil is significantly positive correlated with oleic acid content（r=0.974）,and negatively correlated with linoleic acid content（r=-0.894）.Then,the differences in oil content,oil distribution,interaction between components as well as microstructure characteristics of potato strips fried with fresh oil and used 36 h oil were compared and analyzed.The results showed that,compared with the potato strips fried with fresh oil,the oil content in crust and overall oil content of the potato strips fried with used 36 h oil increased,while the oil content in core decreased;the overall oil content of potato strips fried in used 36 h oil of high oleic sunflower oil,sunflower oil,rapeseed oil and corn oil increased by 2.23%,11.95%,3.05%and 11.10%,respectively.The LF-NMR results further confirmed the difference in the oil content of the potato strips fried with oils of different freshness.Compared with the potato strips fried with fresh oil,the surface oil content of potato strips fried with used 36 h oil increased,but the matrix oil content remained unchanged,indicating that the oil after 36 h frying is more likely to adhere to fried potato strips surface,and also is hard to drip during cooling stage.The potato strips fried with fresh oil had complete cell morphology and structure,with fewer damaged cells,while the potato strips fried with used 36 h oil showed a large number of damaged and structurally deformed cells.The results of total polar compound（TPC）content and the CLSM fluorescence recovery after bleaching（FRAP）experiment confirmed that the frying oil with lower freshness produced the fried potato strips with higher oil content.Finally,the changing rules of the content of different type oil in potato strips during continuous frying experiment were explored.The OPLS model was used to study the key factors affecting the oil absorption of fried potato strips based on the physicochemical properties of frying oil.The results showed that,during the prolonged continuous frying process,the content of surface oil in fried potato strips gradually increased.The content of surface penetrating oil presented a fluctuating decreasing trend from 0 h to 20～24 h,and a fluctuating increasing trend from 20～24 h to 36 h.The changes of content of matrix oil had no obvious change trend.The total oil content showed a trend of first decreasing slightly and then rising rapidly.OPLS analysis results showed that changes in the physicochemical properties of frying oil had a significant impact on the content of surface oil,surface penetrating oil and matrix oil in fried potato strips.Among the three types of oil,the change in surface oil content mainly led to the increase in total oil content.The TPC content is the most critical factor affecting the oil absorption.The increase in TPC content increased the viscosity of frying oil,which enhanced the surface adhesion of the frying oil,resulting in a continuous increase in the surface oil content.When the TPC content exceeded 22%～25%,oil absorption increased rapidly.