Research On Antioxidative Properties Of Grape Peel Ethanol Extract In Soybean And Sunflower Oils During Deep Frying

Grape peels are the main by-product of the grape processing industry.However,these grape peels have strong antioxidant activity due to variety and high content of phenolic and natural active substances.Herein,the ethanol-soluble compounds were ultrasonically assisted extracted from grape peel with 50%ethanol and further purified by NKA-9macroporous resin.Soybean and sunflower oils added with a certain concentration（0,0.1,0.3,0.5,0.7 and 1.4 g/L）of grape peel ethanol extract（GPEE）were individually selected as frying medium to deep-fry intermittently French fries at 180℃ for 16 h.The antioxidant activity and heat-stable property of GPEE was evaluated by determining the changes in acid value（AV）,carbonyl value（CV）,total polar component（TPC）content,the composition of fatty acids,Fourier transform infrared spectroscopy（FTIR）and total phenol content of frying oil.The main results are exhibited as follows:（1）The total phenol content of GPEE was 22.04 mg GAE/g,in which the relative contents of chlorogenic acid,（+）-catechin,（-）-epicatechin and rutin were 276.68±15.24μg/g,628.66±1.06μg/g,340.27±80.85μg/g and 925.50±97.59μg/g,respectively.Within the concentration range of 0.02-1.00 mg/m L,the scavenging activity of DPPH and ABTS⁺free radicals,and the FRAP reduction ability were 16.94%-95.49%,22.57%-94.24%and 0.16-4.01 mmol Fe²⁺,respectively.（2）Compared to frying oil without antioxidants,the color difference of soybean or sunflower oils added 0.5 g/L,0.7 g/L and 1.4 g/L GPEE were reduced by 14.13%,-13.04%and 42.39%,or-97.5%,-42.5%and 22.5%,respectively.The viscosity were reduced by3.58%,-13.25%and 5.44%,or 15.08%,24.36%and 26.77%,respectively.The increased percentage of AV were reduced by-28.43%,10.70%and 7.32%,or 39.65%,30.38%and38.59%,respectively.The CV were reduced by 18.85%,12.00%and 31.31%,or 38.72%,47.22%and 54.42%,respectively.The TPC content were reduced by 13.17%,25.40%and37.40%,or 30.75%,32.58%and 37.83%,respectively.However,soybean or sunflower oils added with 0.1 g/L and 0.3 g/L GPEE had weaker protective effects than those added with TBHQ due to the thermal decomposition of the GPEE.（3）The polyunsaturated fatty acid content of soybean or sunflower oils added with 0.5g/L,0.7 g/L and 1.4 g/L GPEE reduced by 5.14%,4.29%and 3.72%,or 6.43%,5.80%and5.04%.The reduction of content of linoleic acid of soybean and sunflower oils gradually decreased with the increase of GPEE concentration.However,the decrease in the content of linolenic acid of soybean oil tend to decrease with the decrease of GPEE concentration.（5）FTIR coupled with partial least squares（PLS）was developed to predict AV,CV,and TPC content of frying oil.The best model was second derivative combined with multiple scattering correction spectrum pretreatment in soybean oil,and the correlation coefficients of verification set were 0.9143,0.9632 and 0.9203,respectively.However,the best model was second derivative spectrum pretreatment in sunflower oil,and the correlation coefficients of verification set were 0.8200,0.8152 and 0.7889,respectively.（5）Increasing GPEE content caused an increase in the total phenolic content of the oils.The decomposition rate of the total phenol content of soybean or sunflower oils added with0.5 g/L,0.7 g/L,and 1.4 g/L GPEE were 100%,84.30%and 78.08%,or 93.73%,81.53%and 73.56%,respectively.However,the decomposition rate of the total phenol content of soybean or sunflower oils added with 0.1 g/L and 0.3 g/L GPEE were both 100%.In summary,addition of GPEE has stronger antioxidant effects on frying performance of soybean or sunflower oils during frying.Wherein,sunflower oil added with 1.4 g/L GPEE exhibited the most excellent heat-stable and antioxidant properties and might be a good natural alternative to existing synthetic antioxidants in the frying oil industry.