Study On The Influence Of Powder Characteristics On The Extraction Process And Flavor Of Coffee Brews

Coffee is a typical hobby beverage and is popular among consumers because of its unique flavor.The variety and origin of green coffee beans and the steps of its roasting,grinding,and extraction,any change in parameter will affect the sensory flavor of coffee brews and determine consumers’likes or dislikes.Coffee powder characteristics are the significant factor that affecting the extraction kinetics,extraction rate of flavor substances,and sensory quality.This study is based on the analysis of coffee powder characteristics（particle morphology and filling correlation coefficient）under different grinding conditions.And it discusses the critical powder characteristics and extraction process kinetic parameters that affect the extraction yields and flavor of coffee brews;analyze the influence rules of critical factors on the extraction kinetics of key aroma compounds in coffee and related mechanisms.And it provides essential theoretical guidance for optimization of coffee grinding process and extraction process control for making coffee with different flavors.The influence of the morphology of coffee powder on the extraction yields of coffee brews were studied.Medium roasted Guatemalan coffee beans were ground in different degrees by two coffee grinder,flat knife and cone knife,to obtain the coffee powder with different powder characteristics.Espresso coffee extraction process（constant pressure 900 k Pa,40 g coffee brews/17 g coffee powder）was used to obtain coffee brews.The results showed that the extraction yields of coffee solids ranged from 16%to 27%.Principal component analysis（PCA）analysis showed that the particle size distribution was the key characteristic parameter affecting the extraction yields,and the extraction yield increased firstly and then decreased with the increase of particle size distribution.At the same time,the relative contents of characteristic flavor compounds in the coffee brews with different extraction yields were determined.It was found that when the extraction yield was the highest,the concentrations of succinic acid,citric acid,and lactic acid also reached the highest,which were 5.57 mg/m L,3.43 mg/m L,and 1.69mg/m L,respectively.PCA of electronic nose,GC-MS heat map clustering and Napping sensory analysis all showed that although coffee powder extraction with different powder characteristics could obtain coffee brews with the same extraction yield,the aroma of coffee brews was still significantly different due to the different composition and concentration of aroma compounds,which was greater than the difference in taste.It was mainly reflected in the nutty,chocolate,and cigarette properties.The filling-related parameters and kinetic extraction parameters that affect the extraction yield were analyzed.The flowability,tapped density,compressibility of coffee powder was measured,and the dynamic parameters（permeability coefficient and consolidation degree,etc.）were calculated,to explore the influence of factors other than the particle size on the extraction yield.The results showed that compressibility and tapped density are the key filling-related parameters that affect the extraction yield.They affected the extraction yield by affecting the changes in permeability coefficient and consolidation degree during the extraction process.When the degree of compression decreased by 37%and the tapped density increased by 21.43%,the permeability coefficient increased significantly from 2.74×10^-12 m² to 1.47×10^-10 m²（p<0.05）,a magnitude change occurred,while the degree of consolidation decreased from 6.25%to 0%.Furthermore,the factors affecting the characteristics of the powder and the extraction process were analyzed.The roasted beans with different physical properties and chemical compositions were obtained by roasting Guatemalan coffee beans at different degrees.The hardness of the roasted coffee beans decreased,the brittleness increased,and the fat content increased significantly（p<0.05）from light to dark roast.Then the same grinding condition was used to grind roasted coffee beans.Coffee powder characteristics were measured and the kinetic parameters during extraction were calculated.The results showed that the lower the hardness of roasted coffee beans,the greater the brittleness,the smaller the d₁₀ of ground coffee powder,and the more regular the particle shape.The correlation analysis of the properties of roasted coffee beans,the characteristics of the coffee powder and the kinetic parameters of the extraction process showed that the flowability of the powder was not related to the properties of roasted coffee beans,but the degree of compression was significantly negatively correlated with the 100-grain weight of baked beans（p<0.01）.The tapped density,permeability coefficient and consolidation degree were significantly positively correlated with the density and hardness of roasted coffee beans（p<0.01）,and negatively correlated with brittleness and fat content of roasted coffee beans（p<0.01）.The dynamic extraction process of key aroma compounds and its influencing factors were studied.In the previous study,there were significant differences in aroma compounds and sensory properties when the extraction yield of coffee brews was the same.Therefore,SPME-GC-MS was used to measures the dynamic content changes of 10 key aroma compounds including pyridine,2-methyl pyrazine,2-ethylpyrazine,2-ethyl-6-methyl pyrazine,2-propylpyrazine,furfural,2,6-diethyl pyrazine,5-methylfuranal,β-macarone and 4-vinyl guaiacol in the constant pressure of coffee brews 900 k Pa extraction process.The results showed that the variation of the normalized extraction rate of key aroma compounds with the amount of extraction volume complied with the power-law equation y=ax^b.Correlation analysis was carried out between a and b values representing extraction rate of compounds and compound polarity,powder characteristics and kinetic extraction parameters.The results showed that the higher the polarity,the higher the permeability coefficient and the faster the initial extraction rate of compounds.Based on relevant odor activity value analysis,combined with the golden cup extraction rules of the Specialty Coffee Association Jinbei（18%～22%）,adjust the final extraction volume:light and light-medium roasted coffee could control the extraction volume to 25～30 g brews/17 g coffee powder,for coffee with a deeper roasting degree,it could appropriately increase the volume of the extraction volume to 40～50 g brews/17g coffee powder.