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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2023)

This dissertation focuses on gaining fundamental understanding of the extraction dynamics and sensory quality of full immersion brewed coffee. The overall goal is to use a diffusion-limited mass transfer model following a “universal brewing curve” to capture and predict the strength, measured in total dissolved solids (TDS), and extraction yield, denoted E, of full immersion brews under various brewing conditions, and to relate these physical properties (TDS and E) to sensory properties using sensory descriptive analysis to help achieve desired cup quality of full immersion brewed coffee.The dissertation is composed of four main stages. First, the equilibrium values of TDS and E for full immersion brews were investigated over a range of brewing parameters and interpreted using an equilibrium desorption model. The results indicate that the equilibrium TDS is approximately inversely proportional to the water/coffee mass brew ratio, while E is independent of the brew ratio. Surprisingly, the species-averaged equilibrium constant K was insensitive to grind size, roast level, and hot brew temperature over the range 80-99°C. Furthermore, an analysis of the standard oven-drying method for measuring E demonstrated that it yields significant underestimates of the true value at equilibrium, due to retained brew within the spent moist grounds. The second main stage involves the dynamics of extraction. A diffusion-limited mass transfer model was updated and extended to predict the TDS and E of full immersion brewed coffee in terms of three parameters: an initial “rinse” concentration (C_0), an equilibrium concentration (C_eq), and an effective mass transfer coefficient (k). The model was validated with a series of systematic brewing experiments using various coffee preparation factors, including varied coffee types, roast levels, brewing temperatures, grind sizes, and agitation rates. The experimental results indicated that (i) increasing brew temperature strongly increases the extraction rate, (ii) light roasted beans extract more quickly than dark roast, and (iii) coffee types from different origins using different post-harvest processing methods yielded no statistically significant differences in extraction kinetics. Furthermore, our agitation experiments demonstrated that occasional and slow manual stirring was as effective as continuous high-speed agitation in terms of increasing the extraction rate, and that the overall extraction rate was primarily limited by diffusion through the interior of the coffee ground. The third stage involved the impact of coffee ground sedimentation on the dissolved solids concentration in ostensibly quiescent (non-externally agitated) brews. Time-lapse photography observations indicated that dark roasted coffee grounds sediment more slowly than light roasted. Simultaneous measurements of the TDS versus both time and vertical position in the brew revealed that full immersion brews undergo a concentration gradient inversion midway through the brewing process. The time scale of the inversion depends sensitively on the roast level of the coffee grounds, which occurs up to a factor of three slower for darker roasts. A key implication is that variations in TDS due to different sedimentation rates could possibly confound sensory evaluations of coffees with different roast levels. Finally, the fourth stage explored the relationship between coffee sensory quality and extraction dynamics of full immersion brewed coffee. A descriptive analysis method was used to explore how the sensory profile of full-immersion brews evolved versus time for different roast levels and brewing temperatures. Our results indicate roast level had the greatest impact on the sensory profile of the coffees, followed by brewing temperature. Surprisingly the effect of brew time, particularly the longer brew time as the TDS approaches equilibrium, had subtler impact than expected at least for the range of brew temperatures and brew time points studied here. The intensity of sweetness was negatively correlated with TDS, while 19 other attribute intensities were positively correlated with TDS. To increase efficiency, coffee brewing practitioners can reduce brew time beyond equilibrium without changing their products. Interestingly, certain long time cold brews were found to have similar sensory profiles to those of some short time hot brews in light roast, suggesting the potential of achieving similar sensory profiles of long-time cold brews using short-time chilled hot brews through controlled preparation method. Overall, this dissertation aims to advance the understanding of how coffee preparation factors impact the extraction dynamics, and corresponding TDS and E, of full immersion brewed coffee, with the ultimate goal of helping coffee practitioners best obtain desired sensory qualities.

    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    The sensory qualities of brewed coffee are known to be strongly correlated with the total dissolved solids (TDS) and extraction yield (E) of the brew. Here, we derive a predictive model for the TDS and E of full immersion brewed coffee using a pseudo-equilibrium desorption approach. Assuming a single, species-averaged equilibrium constant [Formula: see text] yields theoretical predictions indicating that the TDS is approximately inversely proportional to the water/coffee mass brew ratio, while E is independent of the brew ratio. Our experimental results strongly accord with both theoretical predictions, and indicate that E is approximately 21% over a wide range of brew ratios. An analysis of the standard oven-drying method for measuring E indicates that it yields significant underestimates of the true value at equilibrium, due to retained brew within the spent moist grounds. We further demonstrate that [Formula: see text] is insensitive to grind size, roast level, and brew temperature over the range 80-99 °C. Taken together, our results indicate that full immersion brewing offers precise control over the TDS at equilibrium but little control over E, and that practitioners should pay careful attention to their brew ratio as the most important parameter for full-immersion brewing.
      Cover page: An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2024)
      With the growing popularity of cold brewed coffee comes a need for brewing efficiency while preserving the desirable flavor profile. Despite the wide use of full immersion brewing techniques, the effect of brew time on the dynamic sensory profiles of full immersion brewed coffee remains underexplored. Here, we investigated the relationship between coffee sensory quality and extraction dynamics, measured as Total Dissolved Solids (TDS) and Extraction (E) of full immersion brewed coffee at various roast levels, and brewing temperatures (4 °C, 22 °C and 92 °C), over brew time using a generic descriptive analysis method. Specifically, different brew time points were selected for different temperatures based on five targeted coffee extraction stages. Furthermore, the unique experimental design also explored a sensory-driven engineering research process. Roast level had the greatest impact on the sensory profile of the coffees, followed by brewing temperature, but brew time, especially the longer brew times as TDS plateaued, had subtler impacts than expected. Twenty-five of 28 sensory attributes were significantly different among the 30 coffee samples, indicating a single source green coffee blend can produce a wide range of complex sensory profiles using different combinations of roast level, temperature, and brew time. Specifically, the intensity of sweetness was negatively correlated with TDS, and 19 other attribute intensities were positively correlated with TDS. Interestingly, we found that certain long time cold brews had similar sensory profiles to those of some short time hot brews, suggesting the sensory profiles of certain hot brews and cold brews could possibly be matched through controlled preparation. Overall, our study demonstrated an approach of integrating food engineering and sensory analysis for product development, and our findings provide valuable insights into the extraction dynamics and sensory quality of full immersion brewed coffee and opens new brewing avenues for the coffee industry.
        Cover page: Sensory analysis of the flavor profile of full immersion hot, room temperature, and cold brewed coffee over time.
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2022)
        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.
          Cover page: Sensory Analysis of Full Immersion Coffee: Cold Brew Is More Floral, and Less Bitter, Sour, and Rubbery Than Hot Brew.
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2022)
          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.
            Cover page: Roast level and brew temperature significantly affect the color of brewed coffee.
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