| Highland barley(HB)is a special highland cereal grown in southwest China.It contains a wide range of nutrients,especially phenolic substances.HB is a typical whole grain diet with the bran and germ retained to a large extent and bioactive substances such as phenolics.The common processed HB foods are stir-fried HB noodles,HB noodles and HB biscuits,which involve thermal processing such as stir-frying,roasting and boiling,thus affecting the texture,flavor and nutritional composition of HB.At present,the effect of thermal treatment on the phenolic and flavor substances of HB and their relationship with the physicochemical properties during thermal treatment are still unclear.In addition,there is a relative lack of studies on the digestive properties of HB phenolics after thermal processing treatment.Therefore,in this paper,we investigated the effects of three different thermal treatments(boiling,roasting and stir-frying)on the phenolic and flavor substances of HB and their relationships with physicochemical properties were investigated.The optimal parameters of each thermal treatment were screened to further investigate the in vitro digestive properties of phenolics.The main research contents and results are as follows:(1)Twenty-one phenolics in HB were identified by UPLC Q-TOF-MS technique,among which 8 ferulic acid oligomers were identified in HB for the first time.Flavanols and phenolic acids were the main components of free phenolics and bound phenolics in HB,respectively.The content of phenolics and antioxidant capacity in HB were improved by boiling and roasting.The content of total free phenolics and total bound phenolics in HB was increased by 8.04%and 6.28% respectively under the condition of 1:4 material to water ratio.The contents of total free phenolics and total bound phenolics were increased by 44.82% and 13.30% respectively at160°C.Under the condition of 120°C,the phenolics in HB can be best preserved by stir-frying,and the total bound phenolic content was increased by 9.89%.By analyzing the composition of phenolics,it was observed that the procyanidin and ferulic acid polymers were depolymerized into monomers during thermal treatment,and the change of cereal matrix by thermal treatment might improve the extraction ability of phenolics.In addition,it was found that dry thermal treatment of roasting and stir-frying might produce substances that can promote the antioxidant activity of free extracts,such as Maillard reaction products.(2)The physicochemical properties of HB treated by thermal processing were analyzed.It was found that no Maillard reaction occurred in boiled HB,while the degree of browning deepened and the degree of Maillard reaction gradually increased with the increase in temperature of roasted and stir-fried HB.The free amino acids of the three kinds of roasted HB decreased significantly.The loss of free amino acids of the boiled HB was due to its thermal degradation and the formation of the characteristic flavor substances of boiling.The loss of free amino acids of the roasted and stir-fried HB was probably due to its participation in the Maillard reaction,which might be an important source of the flavor of the roasted and stir-fried HB.Microstructure analysis showed that the grain volume of the three thermal-treated HB expanded and the substrate was loose,thus improving the extractability of phenolics.The endosperm cell wall was deformed and damaged,and the starch and protein of roasted and stir-fried HB were cross-linked in a filamentous way,which was conducive to the development of Maillard reaction and the formation of flavor substances.(3)Furthermore,electronic nose,GC-MS and GCO techniques were used to explore the variation of flavor components of thermal-treated HB.With the increase of the ratio of material to water,the flavor types of boiling HB gradually stabilized into vanilla flavor,grass flavor and caramel flavor,among which aldehydes,2-amylfuran,2-furanyl alcohol and vanillin were the characteristic flavor substances of boiling HB.With the increase of roasting temperature,the relative content of pyrazine substances gradually increased,and the intensity of roasting flavor,caramel flavor,burnt flavor and vanilla flavor gradually increased.The roasted HB at 160°C had the best roasting flavor.The relative content of pyrazines in the stir-fried HB was relatively stable at 120-160°C.The HB under 120 and 140°C stir-frying conditions already had strong roasted and caramelized flavors,which gave the unique roasted flavor of stir-fried HB.Pyrazines,1-(2-furan methyl)-1h-pyrrole,2-furan methyl alcohol and 5-methyl-2-furan formaldehyde are characteristic flavor substances of roasted HB and stir-fried HB.(4)According to the study of the effects of three thermal treatments on phenolics and flavor substances of HB,the 1:4 ratio of material to water boiling HB,160°C roasting HB and120°C stir-frying HB were selected to study the in vitro digestion characteristics of phenolics.The results showed that the three thermal treatments improved the bioaccessibilities of phenolics after gastrointestinal digestion of HB.The bioaccessibilities of phenolics in HB were increased from 11.90% to 36.03%,22.75% and 31.34% by boiling,roasting and stir-frying,respectively.Phenolics were degraded to other small molecules during gastrointestinal digestion and might also combine with digestive enzymes.Molecular docking simulations showed that phenolics can also bind to α-amylase,pepsin and trypsin through hydrogen bonding,π-π interactions and van der Waals forces. |
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