New approaches on the analyses of thermolabile coffee diterpenes by gas chromatography and its relationship with cup quality

A new gas chromatography method using pulsed split injector (PS-GC) was validated to quantify thermolabile diterpenes cafestol, kahweol and isokahweol in methanolysed Arabica coffee oils. Linearity was 0.99 from 8 to 69 mg mL(-1), recovery ranged from 99% to 101% and precision of less than 4% was obtained. Besides, Soxhlet extraction time was evaluated and Tukey's test indicated that the mass of diterpenes obtained in 4 h is equivalent to a 16 h period, which represents a space-time yield four times higher. The microwave assisted methanolysis proved to be efficient to quantitatively convert the natural diterpene esters in their respective alcohols and fatty acid methyl esters, accompanied by PS-GC. Also, the intact diterpene esters were analyzed by GC for the first time by the comparison between cold on-column (COC) and PS injection techniques. In all these stages, the molecular integrity of the thermolabile furokaurane diterpenes was maintained. The methanolysed oils from 13 samples of green Brazilian Arabica coffees were analyzed by PS-GC and the diterpenes composition varied from 8 to 12% w/w in oil and 0.7-1% in coffee beans. The ratio between cafestol and kahweol was successfully used to predict the quality of coffee even before the roasting and brewing processes. (C) 2015 Elsevier B.V. All rights reserved.