Effect Of Chestnut Inner Skin Flavonoids Extract On In Vitro Digestibility Of Starches

Starchy foods are the main energy and carbohydrate sources in the human diet,but excessive intake may lead to high blood sugar.Natural products,especially flavonoids,can effectively inhibit the activities of starch digestive enzymes and other blood glucose-related enzymes and be used to control postprandial blood glucose.Chestnut inner skin is rich in flavonoids.In the present study,flavonoids were extracted from chestnut inner skin and characterized by high-performance liquid chromatography-time-of-flight-mass spectrometry（HPLC-TOF-MS）.The physicochemical properties of chestnut inner skin flavonoids extract（CISFE）-chestnut starch complex and the effects of CISFE on the in vitro digestibility of starches with different crystallinity were analyzed.In addition,the mechanisms of inhibition ofα-amylase,α-glucosidase and dipeptidyl peptidase IV（DPP-IV）by CISFE were investigated.The main results are as follows:（1）The chemical compounds in chestnut inner skin.The total content of polyphenols and flavonoids in the CISFE was 620.0mg GAE/g and 519.8mg RE/g,respectively.Twelve flavonoids were identified,including naringin（50.23%）,isorhamnetin（15.71%）,luteolin（13.05%）,kaempferol（9.48%）,apiin（6.33%）,eriodictyol（1.47%）,quercetin（1.19%）,hyperoside（1.01%）,catechin（0.92%）,rhamnetin（0.40%）,epicatechin gallate（ECG）（0.14%）,and rutin（0.08%）.（2）The complex was prepared by adding CISFE into chestnut starch.The particles became smaller with increasing CISFE concentration.The crystal form of starch was not changed,but the relative crystallinity of the complex decreased.CISFE can slow down the retrogradation of starch.The ratio of 1047/1022 cm^-1 and 1000/1018cm^-1 of chestnut starch decreased,which means that flavonoids reduced the short-range molecular order and double helix structure of starch.Compared with the control,the addition of CISFE to starches with different crystallinity led to a slight increase in resistant starch（RS）.In contrast,the rapidly digestive starch（RDS）decreased significantly after the addition of CISFE.The starch treated with 2.5%CISFE had significantly（P<0.01）higher RS than the control.The RS content increased from 52.8 to 64.2%in maize starch,from 72.4 to 76.8%in potato starch,and from50.0 to 54.4%in chestnut starch.Maize starch,potato starch and chestnut starch are medium glycemic index foods.After adding CISFE,the e GI value of maize and chestnut starch decreased,but they were still medium glycemic index foods.The e GI value of potato starch was lower than 55,which was low glycemic index food.（3）Inhibitory mechanism of CISFE on postprandial blood glucose-related enzymes.The CISFE was a strong inhibitor ofα-amylase and a weak inhibitor ofα-glucosidase,but hardly inhibited DPP-IV,with an IC₅₀ of 27,2.3 and 1140μg/m L,respectively.ECG showed an inhibitory activity against the three enzymes,with an IC₅₀ of 1558,79.7 and 961μg/m L forα-amylase,α-glucosidase and DPP-IV,respectively.Apiin,quercetin and catechin showed low inhibitory activity onα-amylase,α-glucosidase and DPP-IV,respectively.Furthermore,the inhibition type of CISFE onα-amylase was a mixed-type,whereas ECG and apiin showed a non-competitive mechanism againstα-amylase.The inhibition type of CISFE,GCE and quercetin onα-glucosidase was non-competitive inhibition.Linagliptin exhibited a mixed-type inhibition mechanism for DPP-IV.The CISFE,ECG and catechin inhibited DPP-IV through an uncompetitive mode.Analysis by ultraviolet-visible spectroscopy,fluorescence spectroscopy and circular dichroism suggested that flavonoids altered the hydrophobicity and microenvironment of these enzymes.