基于风味指纹图谱的咖啡豆微波真空干燥过程中风味品质变化机制及调控研究

Coffee is an important tropical beverage crops in China, as well as one of the pillar industry contributing to economic development of the Hainan and Yunnan provinces. After harvesting the coffee fruits are prone to generate heat, rancidity and spoilage when drying was not timely. Conventional sun drying and hot air drying could lead to the occurrence of browning, texture shrinkage, producing stewed flavor, etc. The mechanism of major flavor components evolution is unclear during drying, quality deterioration is a key problem which influenced the flavor quality of coffee seriously and has become a bottleneck problem limiting coffee industrial development. Coffee beans were selected as the experimental material in this project, the major flavor compounds are analyzed by UPLC-Q-TOF-MS, GC-O-MS, electronic nose and other modern instruments, establishing a reliable characteristic fingerprint of coffee flavor, exploring the effect of drying factors on coffee flavor during microwave vacuum drying (MCV), and modeling the kinetics of flavor component change. NMR, SEM and other technologies are applied to investigate the effect MCV on the water loss kinetics and color change kinetics during dehydration of coffee beans, to determine the distribution and status of water and microstructure of samples. Principal component analysis (PCA), partial least squares regression (PLSR) and artificial neural networks (ANN) are used to establish regression models to evaluate the relationship among drying kinetics, color change kinetics and flavor components change kinetics, to explain the mechanism of flavor quality evolution and offer strategies for coffee quality improvement regulation.

咖啡是我国重要的特色热带饮料作物，亦是海南和云南经济发展的支柱产业之一。咖啡鲜果采收后若不及时干燥，极易发热、酸败及霉变，传统日晒和热风干燥果实易色泽褐变、组织结构塌陷及产生“焖味”，主要风味成分变化机制不清楚，品质劣变是咖啡干燥过程中的关键问题，严重影响咖啡的风味品质，已成为限制咖啡产业发展的瓶颈问题。本项目以咖啡豆为试材，利用UPLC-Q-TOF-MS、GC-O-MS和电子鼻等现代仪器手段检测咖啡豆中主要风味物质，构建咖啡风味指纹图谱库，研究微波真空干燥过程中各干燥因素对风味物质的影响规律，建立风味物质变化动力学模型；通过核磁共振和扫描电镜等技术对水分迁移行为、色泽及微观结构进行分析，建立干燥动力学和色泽变化动力学模型；采用PCA、PLSR及ANN建立模型探索干燥动力学、色泽变化动力学与风味物质变化动力学间的相互关系，阐明咖啡豆微波真空干燥过程中风味品质变化机制，提出品质优化调控策略。

咖啡为世界三大饮料之一，在我国主要分布在海南和云南省，是海南和云南地区农民增收致富的支柱之一，传统干燥方法易导致发霉、酸败及品质劣变。本项目以咖啡豆为试材，首先以微波功率、物料质量为影响因素，探讨不同微波真空条件下生咖啡豆风味前体物及挥发性成分的变化规律；然后对咖啡豆微波真空干燥的干燥特性、微观结构进行研究，建立干燥动力学模型；最后分析其色泽动态变化规律，利用低场核磁和核磁共振影像揭示其水分迁移对咖啡豆风味品质的影响机制，比较微波真空干燥与传统干燥方法对产品品质的影响。结果表明：（1）不同组合功率干燥可以在缩短单一功率干燥时间的同时，对咖啡豆中色泽、脂肪、蛋白质、咖啡因、葫芦巴碱、新绿原酸、苹果酸保护较好；不同组合功率干燥条件下共定性出41种挥发性化合物，酸类、醛类、烷烃烯烃类及醇类为生咖啡豆中四类主要的挥发性化合物，酯类化合物和其它类别化合物含量相对较低；（2）当微波功率从2.0 kW降到0.3 kW时，咖啡豆的干燥用时增加了近8倍，大功率下（2.0 kW、1.5 kW、1.0 kW）咖啡豆的干燥速率为慢-快-慢，而小功率（0.5 kW、0.3 kW）下的其干燥速率几乎为匀速，且咖啡豆干燥动力学最佳拟合模型为Page模型；（3）低场核磁共振表明干燥过程中咖啡豆的水分状态变化也很大，其中束缚水和结合水的变化最大；色泽指标、总酚含量、抗氧化活性的动力学变化为：a*值变化较小，b*、L*、ΔE值、总酚含量、抗氧化活性随着干燥程度的增加而增大。相比其它干燥方式，微波真空干燥处理样品5-O-咖啡酰奎宁酸、3-O-阿魏酰奎宁酸、4-O-阿魏酰奎宁酸、5-O-阿魏酰奎宁酸、总酚含量较高；水分迁移显著影响了咖啡豆风味组分的含量和组成方式。本研究结果可为咖啡豆的产地加工、品质提升及质量控制提供理论基础。