3.1. Sensory Evaluation
A total of 46 fully completed responses was achieved. Mean intensity ratings for each descriptor, ANOVA p
-value and Tukey’s test results are presented in Table 1
. The evaluation revealed statistically significant (p
< 0.001) effects of development time on every descriptor, except Body.
The ‘Fast’ sample scored statistically significantly higher for the attributes Acidity, Fruit+Berry and Clean Cup. Longer development times of ‘Slow’ and ‘Baked’ lead to statistically significantly stronger perceptions of Astringency, Bitterness, Nutty+Chocolate, and Roasted notes. Sweetness was found to be highest in samples with a short development time. Considering the subtle modulations and identical roast degree, the differences were substantial.
A high degree of co-variance was observed between several descriptors, indicating a one-dimensional effect on flavour by modulating development time in the roasting process. It should be questioned whether the panel is able to differentiate between certain descriptors that appear to be highly correlated, i.e., logical error [26
], or whether the attributes truly do modulate in the same manner. However, these effects were to some extent expected, since development time modulation of the roast profiles is naturally a one-dimensional parameter.
Body does not appear to vary depending on development time, as commonly believed in the specialty coffee industry. Body is one of the core concepts used when professionals describe the sensory impression of coffee, especially in regard to roast profiles. The present study defined the concept of Body and provided references (Appendix A
) to promote vocabulary development and calibration of the participants, yet no difference was found in the evaluation. It has long been speculated how Body modulates through roast profiles [27
], yet the current findings suggest that development time has no impact on modulating the sensory attribute. The elusiveness of the descriptor is highly likely to contribute to the difficulty of finding a significant difference despite the efforts made towards vocabulary development on the panel. Individual understandings of Body are prevalent in the industry, as indicated by internal studies in CoffeeMind, which lead to complications in sensory evaluations. The lack of alignment is expected to be reflected as an incoherence in the data, which has also been found for other descriptors [28
The current trend in specialty coffee emphasises the development of high levels of acidity and fruitiness in the coffee, which was favoured by the ‘Fast’ roast. This is interesting, as extremely fast roasts are traditionally considered roasting defects, due to a theoretically more pronounced gradient of roast degree from the surface to the centre of the coffee bean. In contrast, the baked roast appeared to gain characteristics akin to darker roasts. These typically favour increased bitterness, along with roasty or even burnt notes [7
3.4. Correlation between Sensory and Instrumental Variables
Multivariate analysis was performed using the analytical software LatentiX, version 2.12 (LatentiX, Frederiksberg, Denmark). A Partial Least Squares (PLS) model on NMR and GC–MS-data (X, autoscaled) and sensory data (Y) was created using full cross-validation to investigate correlations between sensory attributes and chemical compounds. The first two components were sufficient to explain 85% of the variance. Most of the variation was explained by Component 1 (76%), which created a clear separation along the x-axis, as shown in the Bi Plot in Figure 2
. The most extreme samples on this dimension were ‘Baked’ and ‘Fast’, corresponding to the extremes of the roast profiles. ‘Slow’ shared the characteristics of the ‘Baked’ sample, whereas ‘Medium’ and ‘Fast’ had slight differences, yet high correlation.
The PLS model created a clear separation of the NMR- and GC–MS-data, predicting the sensory attributes that characterise the ‘Fast’ and ‘Baked’ roasts, respectively. Thus, two main groups of compounds were found for the corresponding two groups of sensory attributes. These are presented in Table 4
A faster development time promoted more hexanal, (E)-2-pentenal, and benzeneacetaldehyde, which exhibit green, apple-like, fruity, and floral aroma notes (thegoodscentscompany.com). The effect of an increased concentration of aldehydes such as hexanal in fast roasts is supported by Baggenstoss, J. et al. (2008), whose research indicates that hexanal formation depends on high temperatures in the roasting process. The NMR analysis revealed a higher presence of various acids, i.e., malic, citric, and formic acid in the ‘Fast’ roast. In combination, these compounds are likely to contribute to the sensory perception of Acidity in the ‘Fast’ sample. Both 2,3-butanedione and 2,3-pentanedione were present to a greater extent in ‘Fast’ and are generally agreed to have a butter-like aroma quality.
Chlorogenic acids are a major constituent of green coffee [29
] and were found to a higher extent in the ‘Fast’ roast. These are important precursors for the bitter-tasting compounds of quinic acid and quinide, and are degraded with an increased roast degree [25
]. The present study showed continuous degradation of the chlorogenic acids 5-CQA and 3-CQA with increased development time and this is likely to increase the perceived Bitterness of the ‘Baked’ roast.
A longer development time favoured a slightly higher presence of Maillard-derived pyrazines with roasty or nutty aroma qualities. This may correlate to the sensory descriptors Nutty+Chocolate and Roasted, which were perceived as more intense in the ‘Slow’ and ‘Baked’ samples by the panel. An example is 2.5-dimethylpyrazine, which is typically characterised by a roasty or hazelnut-like aroma. This compound has been reported in other studies as an important contributor to the characteristic aroma of coffee, and is furthermore found in higher concentrations in slow-roasted coffee [30
]. Trigonelline in ‘Fast’ roasts may also act as a precursor for the pyridine found in ‘Baked’ roasts, supported by previous studies of pyridine’s continuous increase with roast duration [12
]. Pyridine has previously been proposed as a marker of the baked roasting defect [31
], consistent with the present study. Pyridines and alcohols like 2-methyl-1-propanol, 3-methyl-1-butanol, and 2-methyl-1-butanol were more dominant in the ‘Baked’ profile and can further contribute to a fusel or roasty aroma character. Certain volatiles in the ‘Baked’ profile such as 3-methyl-3-buten-1-ol and 3-hexanone may still contribute to a slight fruitiness; however, from sensory analysis, the subtle fruity notes seem obscured in the presence of intense roasty Maillard derivatives. Other studies, not specific to development time, have shown that low-temperature roasting with a longer duration produces a coffee with less headspace intensity and acidity when compared to its more quickly roasted counterpart [11
]. It is important to acknowledge that most volatile compounds exist in all samples, whereas the proportion of each compound varies to a large extent, leading to a shift in the perceived flavour profile of the coffee. The importance of each compound may shift due to the formation dynamics in the roasting process, consistent with other studies [32
The degradation of acids with longer development times was correlated with a reduced perception of Acidity in the Baked coffee. It is notable that all acids appear to be degraded to the same extent, meaning the ratio of particular acids does not change as an effect of development time. A popular theory in the coffee roasting community is that certain roast profiles may favour a particular composition of acids, allowing the roaster to highlight a specific acid. The findings of this study indicate that development time does not allow for such alterations. This is consistent with other authors suggesting a decrease in acids with prolonged overall roast times without changes to the relative composition of the acids [11
Contrary to popular belief in the specialty coffee industry, a sweet perception in the brew is very unlikely to be due to the presence of sugars. A significant difference in the sensory perception of Sweetness was found between the coffee samples, yet no identifiable simple sugars were found from the NMR spectra. A concentration of 1 mmol/L of sugar, e.g., glucose or fructose, could be identified if present. Taste recognition thresholds of sugars are generally higher than 20 mmol/L [29
]. Furthermore, roasting has previously been shown to drastically degrade sucrose by up to 99% depending on the roast profile [25
]. Reducing sugars are formed from the hydrolysis of long-chain carbohydrates during the roasting process, but may rapidly enter as reactants in the Maillard reaction [33
]. It is thus unlikely for the carbohydrates to have a significant role in the sweet perception of the brew, considering the low concentration and the complexity of coffee substances inducing other sensations that may suppress a sweet taste.
The sweet perception in coffee could hypothetically be induced by aromas that exhibit characteristics of sweet foods and drinks, rather than an actual sweet taste from sugars. The ketones 2,3-butanedione (diacetyl) and 2,3-pentanedione are both described as exhibiting pleasant, buttery, caramel-like or butterscotch sensations [32
] and are both found in high concentrations in samples with shorter development times. In particular, diacetyl is a widely used compound in the food flavouring of sweet items [35
] and may partially explain the higher perceived sweetness found by the sensory panel in Fast and Medium. Schenker et al. (2002) and Baggenstoss et al. (2008) found 2,3-butanedione and pentanedione concentrations of fast roasts to be higher when compared to slower roasts, although these studies focused on overall roast time and not development time specifically. The compounds were found to originate from different sugar fragments, and both showed drastic degradation with longer roast duration [12
]. Furthermore, 2,3-butanedione has been shown to be stable even at high temperatures in the roasting process [32
]; hence, degradation in the present study is likely due to excessively extended roast development times. Other unknown compounds exhibiting a sweet taste may also play a role; however, they are yet to be identified in coffee.
Roast development time modulations facilitate a rather large alteration in the overall flavour profile, considering the uniformity of roast degree between the samples. Generalisation of the data is naturally limited due to the sample size and the vast diversity of coffee species. However, the present study sets a solid foundation for further research in coffee roasting with practically applicable results to aid the industry in their craft. Coffee roasters may benefit greatly from including development time as a process parameter in quality control programmes and product development processes, as the results illustrate that roast colour alone is not a sufficient indicator of the chemical and sensory properties of the coffee. Thus, an improved quality control process should include both colour readings and development time data when evaluating coffee roasting consistency. In addition, the results support the relevance of training the skill of modulating development time in certification programmes of the specialty coffee industry.
Whether development time changes are positive or negative is a question of consumer research that should be addressed by the specific segment targeted by the coffee roaster. Furthermore, the present study does not provide any information with regard to the ability of consumers in detecting flavour differences between development time modulations in coffee.
The study was limited to investigating the effects of roast development time at the specific roast degree of Agtron 76 ± 1. Whether these effects persist at different roast degrees is an interesting area for further research. The current roast degree was chosen as it is deemed relevant as a ‘Light roast’ in commodity roasting and a ‘Dark roast’ in speciality roasting.