Optimization of Coffee Micro-water Degumming Process Based on Principal Component Analysis

To optimize the micro-water degumming process, this study investigated the effects of different processing techniques including mechanical peeling degumming+sun drying (A), mechanical peeling degumming+heat pump drying (B), manual peeling+heat pump drying (C), mechanical peeling degumming+water soaking degumming+heat pump drying (D), and manual peeling+water soaking degumming+heat pump drying (E) on the content of aroma precursors (protein and crude fat), taste-producing substances (caffeine, chlorogenic acid and caffeic acid), and volatile substances in green coffee beans. The results showed that compared to the manual peeling degumming+heat pump treatment group, the mechanical peeling degumming+heat pump drying group exhibited a significantly increased chlorogenic acid and caffeic acid contents in raw coffee beans, a significantly decreased (P<0.05) protein content with the increase in pectin residue (P<0.05). The content of crude fat and caffeic acid in raw coffee beans significantly higher in the mechanical peeling degumming+water soaking degumming+heat pump drying treatment group than in the mechanical peeling degumming+heat pump drying treatment group (P<0.05). Chlorogenic acid and caffeic acid contents in raw coffee beans were significantly higher in the mechanical peeling degumming+heat pump drying treatment group than in the mechanical peeling degumming+sun-drying treatment group (P<0.05). The contents of protein, crude fat, and caffeine in green coffee beans were lower in manual peeling+heat pump drying (C) and manual peeling+water soaking degumming+heat pump drying (E). The protein content in green coffee beans was the highest (13.71%) in mechanical peeling degumming+water soaking degumming+heat pump drying(D), and its crude fat content was only second (9.84%) to mechanical peeling degumming+sun drying (A). According to the overall score of quality indicators from high to low, the 5 treatment groups were ranked as D>B>A>E>C. The PCA score plot grouped samples B and D into one quadrant, and the samples in other treatment groups were located in different quadrants, which indicated a relatively small difference in the quality of raw coffee beans between the mechanical peeling degumming+heat pump drying group and mechanical peeling degumming+water soaking degumming+heat pump drying treatment group, while the difference in quality of raw coffee beans among the other treatment groups was dramatic. The analysis results of the relative content of volatile substances showed that raw coffee beans in the manual peeling+water soaking degumming+heat pump drying treatment group contained more main flavor compounds such as n-octadecane (4.80 µg/g), damascenone (1.85 µg/g), dibutyl phthalate (20.28 µg/g) than those in the mechanical peeling degumming+water soaking degumming+heat pump drying treatment group (1.63, 0.94, 4.07 µg/g), and compounds in raw coffee beans exhibited an overall decreasing trend with decreasing pectin residues. Taken together, rich flavor could be obtained from green coffee beans by taking advantage of pectin, and the optimal comprehensive quality could be obtained from the micro-water degumming process, namely, mechanical peeling degumming+water soaking degumming+heat pump drying. The findings would provide new perspectives for the processing of the Yunnan fine coffee beans and fermented flavor coffee beans.