Coffee is one of the most important beverage commodities traded on the global market [1
]. With increasing demand worldwide, particularly in the specialty coffee segment, scientific sensory evaluation of coffee is increasingly sought after to understand and meet consumer demands [3
]. The sensory quality of coffee is a function of all the links in the coffee production chain—plant genetics, terroir, transportation, storage, roasting, grinding, brewing—which, together with social, psychological, and situational factors, determine the final consumer experience [5
]. A clear picture of how each link, as well as the subprocesses in each link, modulates the flavour of the coffee is necessary in order to inform product development and quality control, that is to have the opportunity to create high-quality products that consistently give a satisfying consumer experience [5
From a sensory perspective, roasting has a major influence on the quality of coffee and is thus an important aspect for product differentiation in the marketplace. Deliberate modulation of the roasting process based on knowledge of the target consumer is therefore an important part of the product design process and quality control (QC) protocols. This calls for reliable evidence-based knowledge of which input factors modulate the coffee flavour, and specifically how input (roast control parameters) and output (sensory attributes) factors are related.
The main non-volatile constituents of Arabica green beans are water (≈12%), cellulose (≈50%), protein (≈10%), mono- and oligo-saccharides (≈10%), free amino acids (≈1%), chlorogenic acid (≈7%), and aliphatic acids such as citric, malic, and tartaric (≈2%) [12
]. The beans also contain ≈15% lipids, which are not reactive during the roasting process other than being pushed to the surface during dark and/or fast roasts. During coffee roasting, the green beans undergo a transformation caused by a drastic increment in temperature from room temperature to temperatures around and often exceeding 200
C. The heat brings the green bean into pyrolysis, where volatiles and other non-volatiles are developed from the non-volatile precursors. The majority of the coffee flavour during roasting is developed via non-enzymatic browning reactions such as the Maillard reaction and caramelisation, leading to a range of coffee specific aromas such as sweet/caramel, earthy, roasted, smoky, fruity, and spicy [13
]. The reactions also lead to non-volatile substances, of which two are the main contributors to bitterness in coffee: melanoidins from the browning reactions and the fragmentation of chlorogenic acid into quinic and caffeic acid [15
]. Caffeine is another source of bitterness, but the natural content of caffeine in the green beans is hardly affected by the roasting process, due to the heat stable nature of the molecule [16
]. In addition to the natural content of aliphatic acids already mentioned, carbohydrates are fragmented into smaller aliphatic acids such as formic, acetic, glycolic, and lactic acid, which contribute to acidity in different phases of the roast [17
]. During the roasting process, the beans lose between 15 and 22% of their weight, of which most is water as it ends up as a rather dry product with between 1 and 2.5% moisture [18
]. From a physical perspective, it gains from around 50% to almost 100% increased volume [13
The application of various time-temperature relationships in the roasting process, and the following effects on chemical composition, have been investigated by several authors in the past [18
] (see also [7
] for recent reviews). In the scientific literature, the roasting process lasts for as low as 4 min to almost 15 min, depending on the roast style [14
]. Commodity coffee is sometimes roasted in 1–3 min, which is referred to as “flash roasting” [13
From a roast control perspective, most existing research has focused exclusively on overall roasting time by running isothermal high temperature short time (HTST) and low temperature long time (LTLT) roast profiles (a term indicating that the time-temperature path of the process is monitored and considered important) and related this to the resulting aroma chemistry of the coffee [18
]. To the best of our knowledge, no previous research has focused on distinct sub-phases of the roasting process, and none of them evaluated sensory properties directly using sensory descriptive analysis. We recently addressed this topic in two papers where we demonstrated the important effect of roasting time and temperature on volatile formation and the sensory quality of coffee [26
]. While these previous studies focused on common roasting-related defects, research on subtler levels of process variation is scant. Moreover, there currently seems to be a gap between the scientific literature and the growing industry trend in the last two decades where roasters, in addition to total roasting time and roast colour, have additionally placed much emphasis on the occurrence of the so-called “first crack” [28
]. The first crack is an event between the onset of roasting and the termination of the roasting process where the accumulated steam pressure causes the beans to crack and expel steam along with other volatile compounds [18
]. This first phase is primarily endothermic due to the heating of water, which gives a high demand for energy input in the beginning of the roasting process. Conversely, at the end of the coffee roasting process, there is a much lower need for heat input, as the process itself becomes progressively more exothermic. The added heat takes the material into a state of pyrolysis where the first important chemical reaction is the Maillard reaction in which amino acids and sugars react to form various components and particularly coffee specific aromas [6
]. Thus, the time elapsed from the first crack to the end of the roasting process should be expected to be the time where the aroma formation speed is the highest [18
The other major gap in the literature is that most results of roast profile modulation experiments were only reported in terms of the effects on aroma chemistry (e.g., [17
]), with neither sensory descriptive analysis, nor consumer test data, save for very few exceptions [5
]. This severely limits the usefulness of this research for the specialty coffee community: in part because it may be hard for coffee professionals (who may lack a deep background in flavour chemistry) to make practical sense of a list of reported aroma compounds and most importantly because of the inherent difficulty in reliably predicting the flavour of coffee based on its chemical make-up [5
Clarke and Vitzthum [13
] suggested a framework for reporting all relevant information in research on coffee roasting; here, the colour of the roasted coffee and total roast time were included in the necessary category of information to report, whereas “time of bean popping” (i.e., time to first crack) was included in the optional category for information to report. In the coffee industry, however, there seems to be no consensus over whether roast colour measurement is in and of itself an important indicator, or whether focus should instead be exclusively placed on the temperature evolution over time, or finally whether time of popping is particularly interesting compared to other aspects of the roasting time curve. The problem with any of these viewpoints is that in most cases, they are based on the individual opinions of professional roasters, rather than on carefully planned experiments with trained sensory panels (e.g., [32
]). This could lead to commercial failures as, for instance, colour measurements are often left out of QC procedures in specialty coffee roasteries; if colour is indeed an important flavour modulator, this could result in consumer relevant, but unknown (not checked) batch-to-batch variations. The notion that colour has a larger impact on the flavour of coffee than different timing aspects is consistent with the results reported in [26
], where multivariate analyses of sensory profiles consistently showed the main variation in the data to be between light and dark roast, whereas variation in timing conditions was less well explained. However, as previously mentioned, this study featured rather large variation in roasting conditions between the samples, so further research with subtler differences is needed to corroborate these initial findings.
To summarize, the overall aim of this research is to understand the relationship between the technical input parameters of coffee roasting and the resulting sensory properties of the coffee. To this end, we will present data from seven sensory profiling studies focusing on coffee roasted with different time and temperature profiles. All studies adopted the same methodological approaches and focused on the same set of sensory attributes, providing a solid basis for quantifying the magnitude of influence each roasting process control parameter (input) has on the sensory properties of the resulting coffees (output).
We will, in detail, focus on the following three objectives:
Compare the overall impact of roast colour modulation versus roast timing modulation on the sensory profile of coffee;
Assess the effect (magnitude and direction) of colour modulations on individual sensory attributes;
Assess the effect (magnitude and direction) of timing modulations on individual sensory attributes, with a focus on distinct phases of the roasting process.
The overall aim of this research was to assess the relative importance of two roasting parameters—colour and time—on the sensory properties of coffee. Drawing on data from eight studies, the results clearly indicated that, while both parameters were significantly related to coffee flavour, colour was the stronger predictor of the two. The direction of the effects for both colour and time were similar, with darker roasts/longer roasting times associated with an increase in bitterness and a decrease in acidity, fruitiness, and sweetness. An interesting finding was that variation in roasting timing, keeping colour constant, had a systematic effect on flavour. Here, we distinguished two phases, “time to first crack”, corresponding to the time between the onset of roasting and the moment where the accumulated steam pressure causes the beans to crack, and “development time”, corresponding to the time elapsed from the first crack to the end of the roasting process. The latter was expected to be the primary aroma development phase, and accordingly, the results indicated that development time had a larger impact on coffee flavour than time to first crack.
This research represents a first attempt to evaluate the effects of roasting parameters using sensory descriptive analysis with trained assessors, as opposed to “cupping” evaluation from coffee experts, which from a sensory science perspective, may be seen as unreliable due to the very small sample size, lack of sufficient calibration, the lack of replicates, etc. Future research on a wider range of roasting conditions is advised to gain a fuller picture of how roasting modulates coffee flavour. From a practical perspective, this research should provide an initial framework for the coffee industry to understand how the roast degree (i.e., colour) and roasting time affect coffee flavour. In particular, our results can be used, e.g., in evidence-based certification systems and to aid coffee roasters in their product development projects.