The compositional basis of coffee flavour

Abstract

Coffee trade is a billion-dollar industry and coffee remains one of the most traded commodities of economic importance to both the countries producing, and the countries consuming coffee. To ensure demand, it is of great importance to constantly maintain and, where possible, improve its quality. Flavour is crucial to coffee quality. However, describing coffee flavour is a very complex task as it is influenced by numerous factors from the farm to cup. These factors influence compositional properties of coffee and have a large impact on the perceived flavour. The ultimate aim of this project was to understand how different components in coffee influence flavour perception. The approach involved an exploration on the sensory and physicochemical profiles of sensorily diverse 26 commercial single origin ‘specialty’ coffees. Sensory evaluation of these 26 selected coffees involves assessment on the brews while coffee ground and extracts were analysed for physicochemical profiles. An analytical method was developed and applied for rapid quantification of the targeted volatiles compounds in coffee. The results of sensory and physicochemical evaluation were modelled using a multivariate analysis to explore the relationship between attributes and to know potential sensory markers that could contribute to coffee flavour. Coffee proved to be a highly challenging matrix to study and required special preparation and presentation of individual samples at consistent temperature during sensory evaluation. Nevertheless, sensory profiles for the diverse range of coffees was achieved. Certain coffees were clearly distinctive: Ethiopian coffees registered fruity, citrus and aromatic spice sensory profile notes, Australian coffees exhibited milder profiles, while India Robusta coffees generally possess smoky, woody, earthy and cereals profile. Three analytical stable isotope dilution analysis/gas chromatography-mass spectrometry (SIDA/GC-MS) methods were developed and applied to quantify 27 key volatile compounds which were targeted due to their reported importance to coffee flavour. These methods involved a headspace-solid phase microextraction/gas-chromatography (HS-SPME/GC-MS) of coffee, steam distillation extraction of coffee followed by a HS-SPME/GC-MS and direct liquid injection of a steam distilled extract of coffee to GC-MS. The 26 medium-roasted single origin coffees investigated were quite diverse in physicochemical properties. It was clear that some coffees showed distinct physicochemical characters, for example, Robusta coffee contained doubled the caffeine content, higher concentrations of dicaffeoylquinic acids (diCQAs), higher pH, and higher L* value (a measure of lightness) but was lower in crude fat and trigonelline than the Arabica coffees. Principal component analysis (PCA) and partial least squares (PLS) regression were used as multivariate modelling techniques using compositional variables to mathematically predict coffee sensory attributes scoring. The models could be validated (Rv2≥0.4) for sensory attributes: aroma intensity, citrus, earthy, sourness, bitterness, flavour intensity and residual. For example: the prediction of sourness using caffeic acid, titratable acidity (TA), L* value and pyrazines; the prediction of citrus using ketones and aldehydes. The results indicated that those chemical components could be potential and contribute to certain sensory attributes of coffee. An additional study was conducted on the evolution of important chlorogenic acids from the green coffee beans to the coffee brews and to know the potential role to coffee flavour, specifically to volatile phenols. Chlorogenic acids were progressively lost during coffee processing with approximately only a fifth of the level in green coffee beans remains in a cup of coffee. The evolution of chlorogenic acids during coffee processing involves biochemical degradation and synthesis pathway that include acyl migration, isomerisation, lactonisation, epimerisation, hydrolysis, and polymerisation mechanism. Since there was no relationship found between the volatile phenols and chlorogenic acids, further investigation on the contribution of chlorogenic acids to coffee flavour will be beneficial. Since not all important components of coffee have been measured in this study, not every predictive model performed well. However, this is the first study to investigate sensory and physiochemical properties of a broad range of commercially diverse single-origin ‘specialty’ coffees as well as to quantify volatile monoterpenes especially geraniol. This study provided a new knowledge on an Australian coffees and specific coffee types such as Indonesia Luwak (Arabica) and Indian Monsooned Malabar. The present study also provides some interesting insight on coffee flavour. It has shown the potential for further coffee and flavour researches while offering advantages for industrial applications.