Investigation On The Structural Changes And Oil Absorption Of Starch During Frying

Frying is an efficient,economic,and convenient food processing technology that produces foods with desirable color,aroma,and taste.Starch is the main component in many fried foods,and a better understanding of the relationship between the starch structure and oil absorption is crucial for producing healthier products with reduced oil contents.However,there is limited research on this aspect.In particular,there is a lack of study on the common mechanisms and regulation of oil absorption in starch-based fried foods.Based on this,a starch-oil-water model was used to simplify the complex fried foods.The evaluation methods,crucial influence factors,mechanism based on the relationship between structure and property,and the regulation strategy for oil absorption by starch during frying were systematically investigated in the present work.Firstly,a new LF-NMR based method was established to simultaneously determine the oil and water contents in the model fried system.The proton signals from oil and water were verified and distinguished by desiccation at 105°C.There was no superposition between oil and water signals in the fried starch.Fried samples exhibited two connected peaks in the T2range from 0.1 to 10 ms,and another peak in the T2 range from 20 to 1000 ms,corresponding to the proton signals from tightly bound water,weakly bound water,and oil,respectively.There was no significant difference between the water contents measured by LF-NMR analysis and that determined by oven method.However,compared with Soxhlet extraction,the LF-NMR analysis provided a more accurate result of oil content.Attenuated total reflection-Fourier transform infrared(ATR-FTIR)spectroscopy,together with the LF-NMR method,was further used to analyze the external and internal oil contents in fried starchy samples.The micromorphology of fried starchy samples was further investigated using scanning electron microscope(SEM),polarized light microscope(PLM)and confocal laser scanning microscopy(CLSM).The results indicated that large amounts of oil were absorbed in or within waxy maize starch,but the majority of oil was located near the surface of the starch granules.The proportion of external oil in total oil was as high as74.08%-97.80%.After defatting,the internal oil was thoroughly removed,while a small amount of external oil remained.With the help of X-ray diffraction(XRD),starch and lipids on the surface was found to form V-type complex compounds during frying at high moisture.CLSM images showed that the oil distribution in starch granules varied significantly from an external distribution at low moisture(10%and 20%)to a dispersed distribution at high moisture(40%and 80%).The hierarchical structure changes and oil absorption of normal maize starch(NMS)as affected by frying conditions were investigated.Initial moisture content had the most pronounced influence on the amount of oil absorbed by starch.The oil content first increased and then decreased with increasing moisture content,being 0.190,0.395,0.264,and 0.147 g/g for 20,40,60 and 80%moisture content.No significant difference(p>0.05)in samples fried at 120-180?was found,however,the oil content significantly decreased from 0.400 g/g to0.327 g/g when the temperature increased from 180 to 210?.As for frying time,the final oil content decreased from 0.478 g/g to 0.366 g/g as the frying time increased from 5 to 30 min.The fraction of oil at the exterior of the samples increased with increasing moisture content,frying temperature,and frying time.Above variations in oil absorption were attributed to changes in the hierarchical structures of the fried samples,including granule morphology,crystalline property,double helices,and molecular features,which were directly determined by frying conditions.The effects of initial water content on the pasting and rheological properties of fried starch were further studied.The results showed that the starch granules were partially gelatinized and expanded during frying.After frying,the gelating ability and thixotropy of NMS decreased obviously.The absorption of oil by starch during frying is also determined by the structural properties of starch itself,especially the amylose content.Therefore,waxy maize starch(WMS),normal maize starch(NMS),and high amylose maize starch(HAMS)were fried at different moisture content to evaluate the role of amylose content on the hierarchical structure and the oil absorption of starch.During frying,the changes in granular morphology,the crystalline structures,and the double helices happened to a lower extent in starch with high amylose contents.Amylose affected oil absorption by interfering with the structural evolution of the starch or directly interacting with the lipids during frying.At low moisture level(20%),the granular state was preserved after frying,the size and porosity of the granules therefore played a dominant role in the oil absorption process.The highly porous surface of the granules in WMS thus enabled WMS to absorb the highest level of oil(0.242 g/g).At moderate(40%)and high moisture(60%)levels,where the starch granules gradually fractured and merged together,the impact of the initial granule size was less important.Instead,the packing of the starch molecules inside the gelatinized starch samples and the interaction between amylose and lipids during frying were more important.At 40%moisture content,NMS absorbed the most oil(0.395 g/g)because of its granular morphology and lower crystallinity(6.520%).At 60%moisture content,HAMS absorbed more oil(0.336 g/g)than NMS(0.264 g/g),because more amylose molecules in HAMS provided more hydrophobic helical cavities available for lipids.The variation in granular size directly impacts the physicochemical properties of starch.Thus,potato starch(PS)granules were firstly separated into three fractions:small(S),medium(M)and large(L)granules.Then,the nanoscale surface features of fractionated potato starch granules were investigated using atomic force microscopy(AFM)and low-temperature nitrogen adsorption.The effect of granule size on the oil absorption by starch was further investigated by frying the fractionated popato starches under either low(25%)or high(50%)moisture.Interestingly,the oil content was inversely proportional to the starch granule size at low moisture but proportional to granule size at high moisture.Under low moisture,all the fractions maintained their overall starch granule morphology.The particle size effect will play a dominant role in oil absorption.Thus,PS-S with the largest specific surface area(0.50 m~2/g)absorbed the highest level of oil(0.166 g/g)during frying.Conversely,under high moisture,almost all the starch granules disintegrated and fused together.In this case,the oil absorption was determined by the components and structure characteristics of fried starchy samples.PS-L had the loose structure with the lowest crystallinity(4.2%)and mass fractal index(Dm=1.38),thus absorbing the highest level of oil(0.349 g/g).Finally,the oil-absorption behavior of NMS during frying were investigated in the presence or absence of pullulan(PUL)and pectin(PEC).Both PUL and PEC addition significantly reduced the oil content of the fried samples.However,PUL is better than PEC in reducing oil absorption.By adding 1%,3%and 5%PUL,the oil content of fried NMS decreased by 21.5%,30.6%and 35.9%,respectively.However,the oil content of fried NMS with 1%,3%and 5%PEC was only reduced by 0.0%,14.0%and 18.2%,respectively.SEM analysis showed that the intact granular morphology was preserved upon addition of hydrocolloids.The XRD,FTIR,and DSC results showed that hydrocolloids protected both the short-range double helices and long-range crystalline structure of the granules during frying.As a result,fried NMS-PUL and NMS-PEC mixtures were denser than fried NMS,thus inhibiting oil absorption during frying.