Study On Structural Changes And Oil Absorption Of Starches With Different Crystal Types During Frying

Fried food is deeply loved by consumers because of its golden shell,crisp texture,and unique flavor.Reducing the oil content of fried food and improving the nutritional value of fried food have become the focus of fried food research,and the key to the regulation of oil content lies in the study of the oil absorption mechanism of basic components of food.Starch is an important component of deep-fried high-temperature food.Understanding the relationship between starch structure and oil absorption during fried high-temperature treatment is essential for the study of oil absorption mechanisms in complex food and the production of low-fat healthy food.In this study,three kinds of starch with different crystalline types,namely A-type wheat starch（NWS）,B-type potato starch（PS）,and C-type pea bean starch（PBS）,were used to investigate the hierarchical structure,physicochemical properties,and oil absorption properties of different crystalline starches during deep-frying,including the changes of granule morphology and surface morphology,the evolution of lamellar structure and crystalline characteristics;and the physicochemical properties of different crystalline starch during frying,including thermodynamic properties,gelatinization properties and rheological properties.Furthermore,the oil absorption patterns of different crystalline starch during frying will be systematically studied,with focus on the content of lipids absorbed by starch and oil distribution in starch.The relationship between the microstructure of different crystalline starches and their oil absorption properties during frying was further investigated,including chain length distribution,pore structure,single/double helices structure,etc.,it provides an important scientific basis for revealing the oil absorption common mechanism of starch under high-temperature frying conditions.1.The granule structure of three kinds of crystal starch under different initial moisture content was studied by scanning electron microscope.The results showed that the fragmentation degree of starch granules increased with the increase of moisture content,and the fragmentation of B-type potato starch was the most obvious at the same initial moisture content.The small-angle X-ray scattering（SAXS）results showed that PS had the lowest crystalline lamella thickness（D_c=5.49 nm）and mass fractal index（D_m=1.61）,while PBS had the highest crystalline lamella thickness（D_c=6.20 nm）and mass fractal index（D_m=2.24）at the same moisture content（20%）.The X-ray diffraction（XRD）results showed that at an initial moisture content of 40%,the NWS crystal form changed from A-type to A+V type,the PBS crystal form changes from C-type to A-type,and the B-type characteristic peak disappeared,and the size relative crystallinity was PBS>NWS>PS.The infrared spectroscopy results showed that the 1047/1015 cm^-1 value of all three crystalline starches decreased when the initial moisture content increased from 20%to 50%,with the smallest change in the 1047/1015 cm^-1value for PBS（a decrease of 0.099）,and the 1047/1015 cm^-1 value for NWS and PS decreased by 0.188 and 0.113,respectively.The above results show that the hierarchical structure of PS is more susceptible to damage than NWS and PBS during frying,resulting in the oil absorption of PS more easily than NWS/PBS,which was attributed to the loosely arranged structure of molecules within PS.2.The physical and chemical properties of the three crystalline starches were investigated by a differential calorimetric scanner（DSC）,rapid viscosity analyzer（RVA）,and rheometer under different initial moisture contents during deep frying.DSC results showed that the gelatinization transition temperature of fried NWS and PBS samples increased with increasing initial moisture content,while the gelatinization transition temperature of fried PS samples changed little.At the same initial water content,ΔH values:PBS（8.06 J/g）>NWS（7.80 J/g）>PS（5.52 J/g）,indicating that the structural order of PS is weak.The RVA results showed that frying significantly changed the gelatinization properties of the samples.When the moisture content increased from 20%to 50%,the peak viscosity of NWS,PS,and PBS decreased by852.50 m Pa s,2826.00 m Pa s,1452.67 m Pa s,respectively.The dynamic rheological results showed that after frying,the storage modulus（G’）of PS was significantly lower than that of NWS and PBS,and the loss modulus of PS was significantly higher than that of NWS and PBS,indicating that the PS starch granules were more severely damaged than those of NWS and PBS.3.Low-field nuclear magnetic resonance（LF-NMR）,attenuated total reflection-Fourier transform infrared spectroscopy（ATR-FTIR）and confocal laser scanning microscopy（CLSM）were used to study the changes in oil content and distribution of three kinds crystalline starch under different initial water content during frying.The results of LF-NMR and ATR-FTIR showed that the oil content increased at first and then decreased with the increase of initial water content in the sample.When the initial moisture content increases from 20%to 30%,PS absorbed the most oil（0.5558 g/g）.CLSM results showed that compared with NWS and PBS,the fluorescence intensity of PS was the strongest,indicating that PS had the highest content of lipid molecules.4.The relationship between the structure and oil absorption properties of starch with different crystal forms during frying was studied by ion chromatography,low-temperature nitrogen adsorption instrument,and ¹³C solid nuclear magnetic resonance.The ion chromatography results showed that the frying treatment increased the proportion of short chains and decreased the proportion of long chains（B₂+B₃）in different crystalline starch,with the PS side chain length degraded more significantly than NWS and PBS,resulting in a higher oil content.The results of nitrogen adsorption at low temperature showed that the specific surface area(S_BET=0.8948 m²/g),total pore volume(V_BJH=0.9048 m²/g),and average pore size(D_BJH=14.5884 nm)of PS were the largest when the initial water content was 40%,which favoured the adsorption of oil by PS.¹³C CP/MAS NMR results showed that when the initial water content is 40%,PS had the lowest double helices content（54.6%）compared to NWS and PBS,indicating that the double helices structure was more severely disrupted,making the internal molecular structure more sparse,and this loose structure facilitated the penetration of oil molecules.