Coffee contains a variety of organic acids (OAs) and chlorogenic acids (CGAs) that contribute to overall sensory properties. Large variations in preparation and measurement methodology across the ...literature complicate interpretation of general trends. Here, we perform a systematic review and meta-analysis of the published literature to elucidate the concentrations of OAs and CGAs in both Coffea arabica (arabica) and Coffea canephora (robusta), for both green coffee and roasted coffee at multiple roast levels. A total of 129 publications were found to report acid concentration measurements, yielding 8,634 distinct data points. Analysis of the full data set reveals several trends. First, roasted robusta has considerably more acidic compounds than arabica with 2 to 5 times as much total OAs, and much larger amounts of formic and acetic acid. As for CGAs, in both arabica and robusta 5-CQA is the major component, and progressive roasting decreases the concentration of all CGAs. The total amount of CGA present was more dependent on roast level than the type of coffee (arabica vs. robusta). Overall, this meta-analysis suggests that the increases in certain OAs with roast level might play more of a role in the sensory profile of dark roast coffees than previously suspected.
The brew temperature is widely considered a key parameter affecting the final quality of coffee, with a temperature near 93 °C often described as optimal. In particular, drip brewers that do not ...achieve a minimum brew temperature of 92 °C within a prescribed time period fail their certification. There is little empirical evidence in terms of rigorous sensory descriptive analysis or consumer preference testing, however, to support any particular range of brew temperatures. In this study, we drip-brewed coffee to specific brew strengths, as measured by total dissolved solids (TDS), and extraction yields, as measured by percent extraction (PE), spanning the range of the classic Coffee Brewing Control Chart. Three separate brew temperatures of 87 °C, 90 °C, or 93 °C were tested, adjusting the grind size and overall brew time as necessary to achieve the target TDS and PE. Although the TDS and PE both significantly affected the sensory profile of the coffee, surprisingly the brew temperature had no appreciable impact. We conclude that brew temperature should be considered as only one of several parameters that affect the extraction dynamics, and that ultimately the sensory profile is governed by differences in TDS and PE rather than the brew temperature, at least over the range of temperatures tested.
Brewing is the final and key step in the production of the coffee beverage. Extraction related metrics such as the total dissolved solids (TDS), percentage extraction yield (PE) of solutes, and brew ...temperature (BT) are widely believed to govern the flavor and corresponding consumer acceptance of the resulting brew, as summarized in the industry standard “Coffee Brewing Control Chart.” In this study, we investigated how the three factors of TDS, PE, and BT affected consumer acceptance of a medium roast, single‐origin coffee and whether consumer preference segmentation would be observed based on these variables. A cohort of 118 mostly college‐age, self‐reported consumers of black coffee tasted coffees that varied in BT, TDS, and PE. For each coffee, consumers rated overall acceptance on the 9‐point hedonic scale; the adequacy of serving temperature, flavor intensity, acidity, and mouthfeel using 5‐point just‐about‐right (JAR) scales; and described the flavor using a check‐all‐that‐apply list of 17 attributes. Cluster analysis revealed two consumer segments whose preferences varied most strongly with TDS. Response surface methodology relating liking to TDS and PE produced dome‐ and saddle‐shaped surfaces for the two segments, respectively. External preference mapping and penalty analysis indicated that overall flavor intensity as well as acidity heavily influenced the preferences of the two clusters. The Coffee Brewing Control Chart's “ideal” coffee should therefore be reconsidered to reflect consumer preference segmentation.
Practical Application
This research informs the way coffee brewers manipulate brew strength and extraction of drip brew coffee for optimal consumer acceptance; and justifies a reform of the standard “Coffee Brewing Control Chart” in its representation of an “ideal” coffee as we uncovered two consumer preference segments with different positive and negative sensory drivers of liking.
Beverage color significantly affects perceived sensory quality and consumer preference. Although the color of coffee beans is well known to vary strongly with roast level, little work has examined ...how roast level and brewing conditions affect the color of the final beverage. Here, we report that the color of full immersion brewed coffee is significantly affected by both roast level and brewing temperature. Coffees from three different origins were each roasted to three different levels (light, medium, and dark) and then brewed at three different temperatures (4, 22, and 92°C). Each sample was brewed toward full extraction and then diluted to precisely 2% total dissolved solids so that differences in concentration would not confound color measurements. Absorbance spectra (UV‐vis) and color tristimulus values (L*a*b*) were then collected and analyzed. We find that roast level had the strongest impact on brew color, and that brew temperature had a significant impact on color for light and medium roasts, with less impact on dark roasts. Qualitatively, the cold brewed coffees tended to be redder, while the hot brewed coffees were blacker. The results suggest that there is an opportunity to manipulate and brand brewed coffee color through judicious choices of roast level and brewing temperature.
Practical Application
Color serves as an indicator of coffee quality and potentially could affect perceived sensory characteristics. Our results suggest that appropriate control of roast level and brew temperature could yield desired colors for novel coffee products.
Cold brew coffee is often described as sweeter or less acidic than hot brew coffee. Such comparisons, however, are potentially confounded by two key effects: different brew temperatures necessarily ...change the extraction dynamics and potentially alter the resulting brew strength, and different consumption temperatures are well known to affect perceived flavor and taste. Here, we performed a systematic study of how extraction temperature affects the sensory qualities of full immersion coffee. The investigation used a 3 × 3 × 3 factorial design, with coffee from three different origins representing different post-harvest methods (washed, honey-processed, and wet-hulled), each roasted to three different levels (light, medium, and dark), and each brewed at three different temperatures (4 °C, 22 °C, and 92 °C). All coffees were brewed to equilibrium, then diluted to precisely 2% total dissolved solids (TDS) and served at the same cold temperature (4 °C). We find that four attributes exhibited statistically significant variations with brew temperature for all origins and roast levels tested, with bitter taste, sour taste, and rubber flavor all higher in hot brewed coffees, and floral flavor higher in cold brewed coffee. However, there were strong interactions with origin and roast, with several additional attributes significantly impacted by temperature for specific origins and roast levels. These results provide insight on how brew temperature can be used to modulate the flavor profile of full immersion coffee.
(10,8)CASPT2/6-31G**//(10,8)CASSCF/6-31G** and CCSD(T)/cc-pVDZ//(10,8)-CASSCF/6-31G** calculations have been performed on the potential surface for the 3,3 sigmatropic allyl ester rearrangements ...of cis-3-penten-2-yl acetate (16) to trans-3-penten-2-yl acetate (17) and 3-buten-2-yl acetate (21) to trans-2-buten-1-yl acetate (22). The results are compared to DFT (B3LYP/6-31G**) calculations on the known 16 → 17 rearrangement that reported it to be concerted and pseudopericyclic through a boat-shaped transition structure (Birney, D. M. et al. J. Am. Chem. Soc. 2009, 131, 528−537 ). The CASSCF calculations, on the other hand, uncovered competitive concerted pathways for both the 16 → 17 and 21 → 22 rearrangements, though it was necessary to apply certain approximations in the former case. While one CASSCF pathway in each case involves a boat-shaped transition structure, similar to the one located through DFT calculations, the other pathway involves a chair-shaped transition structure. Preference for chair or boat is shown to be method dependent. Moreover, examination of the CASSCF active-space orbitals clearly confirms that the boat-shaped transition structures are pseudopericyclic but significantly also established that the chair-shaped transition structures are clearly pericyclic. Conclusions based on these results, and regarding our understanding of pericyclic vs pseudopericyclic reactions, are proffered.
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