Hot Brew Coffee

The results from the hot brew coffee analyses are shown in Tables 1 and 2. The hot brew coffee samples analyzed in this study were found to have pH values ranging from 4.85 to 5.10. The Ethiopian-Ardi samples were observed to be the most acidic with a pH of 4.85 ± 0.09, whereas the Brazilian samples were the least acidic with a pH of 5.10 ± 0.02. Of the three CQA isomers analyzed, 5-CQA was found to have the highest concentration in all samples, in agreement with previous studies33,34,35,36,37,38,39. The Ethiopian-Ardi samples were also found to have the highest 5-CQA and total CQA concentration (1721 ± 99 mg/L of coffee and 3270 ± 90 mg/L of coffee, respectively). The Brazilian samples had the lowest 5-CQA and total CQA concentration (1261 ± 111 mg/L of coffee and 2503 ± 103 mg/L of coffee, respectively). The 3-CQA and 4-CQA concentrations were the highest in the Ethiopian-Ardi samples, while Myanmar samples contained the lowest concentration of these two isomers. Previous work by Moon et al.35 suggested that lower CQA concentration is correlated with a higher pH35. A similar trend was observed among the samples analyzed in this study, with a Pearson correlation coefficient of -0.70. These results agree well with pH data presented by Moon et al.35 for light roast hot brew coffees.

Table 1 Hot Brew Coffee Samples: concentration of 5-CQA, 4-CQA, 3-CQA, and total CQA concentration (milligrams per liter of brewed coffee) of hot brew coffee samples (Mean ± 95% Confidence Interval, n = 6). Full size table

Table 2 Hot Brew Coffee Samples: pH, total titratable acid concentration titrated to a pH of 6 and 8 (milliliters of 0.10 N NaOH per 40 milliliters of brewed coffee), and antioxidant activity (millimoles equivalence Trolox per liter of brewed coffee) of hot brew coffee samples (Mean ± 95% Confidence Interval, n = 6). Full size table

The total titratable acidity (TA) of the coffees is expressed in mL of 0.10 N NaOH required to titrate 40 ml of coffee to a pH of 6 and a pH of 8. There have been multiple attempts to understand the chemical characteristics of coffee that cause the perception of bitterness in coffee. Bähre et al. has demonstrated that TA shows better correlation to sourness than pH40. Maier et al. found that the sourness of coffee correlates well with TA titrated to pH 6.041. Balzer suggested that phenolic acids deprotonate at pH values greater than 842. Thus, TA titrated to pH 8.0 may be better end point for titration42. Although sourness is not the focus of this study, TA titrated to these two endpoints may provide some insights about the acid contents in coffee. An earlier study by Gloess et al.36 found no correlation between pH and TA36. For hot brew coffee samples, Columbia coffee was found to have the highest concentration of total titratable acids at both pH of 6 and pH of 8. Brazilian and Myanmar samples were observed to have the lowest concentrations of total titratable acids at both pH of 6 and pH of 8. Data collected in this study showed little correlation between the pH and TA titrated to pH 6 (Pearson correlation coefficient = -0.15) and TA titrated to pH of 8 (Pearson correlation coefficient = -0.09) for hot brew coffee, in support of findings by Gloess et al.36.

Ethiopian-Yirgz samples were observed to have the highest antioxidant activity and Brazilian samples were observed to have the lowest antioxidant activity. In general, the results of this study for hot brew coffee agree well with the general body of knowledge regarding the chemical characterization of light-to-medium roast coffees, including CQA content34,35 and antioxidant activity43,44,45,46.

Cold Brew Coffee

The results from the cold brew coffee analyses are shown in Tables 3 and 4. There is little published data to contextualize these results. However, comparison with the hot brew coffee characteristics in Table 1 point to the existence of chemical differences between cold and hot brew coffees prepared from the same coffee beans and extracted at the same ratio of water volume to grind weight. These data indicate that the temperature of the water used in brewing influences the release and diffusion of compounds in the resulting coffee beverage.

Table 3 Cold Brew Coffee Samples: concentration of 5-CQA, 4-CQA, 3-CQA, and total CQA concentration (milligrams per liter of brewed coffee) of cold brew coffee samples (Mean ± 95% Confidence Interval, n = 8). Full size table

Table 4 Cold Brew Coffee Samples: pH, total titratable acid concentration titrated to a pH of 6 and 8 (milliliters of 0.10 N NaOH per 40 milliliters of brewed coffee), and antioxidant activity (millimoles equivalence Trolox per liter of brewed coffee) of cold brew coffee samples (Mean ± 95% Confidence Interval, n = 6). Full size table

The pH values of cold brew samples ranged from 4.96 to 5.13, with Ethiopian-Yirgz being the most acidic (pH = 4.96 ± 0.08) and Myanmar being the least acidic (5.13 ± 0.03). Similar to the hot brew counterparts, 5-CQA was found to be the most abundant CQA isomer in cold brew coffee. Brazilian samples were observed to have the highest concentration of all three CQA isomers whereas Mexican samples had the lowest CQA isomer concentrations. The correlation between pH and total CQA concentration in cold brew coffee is somewhat weak (Pearson correlation coefficient = -0.52).

In terms of total titratable acids, Mexican samples had the lowest concentration of total titratable acids at both pH of 6 and pH of 8. Columbia samples had the highest concentration of total titratable acids (TA) at pH of 6 and Brazilian samples had the highest concentration of total titratable acids at pH of 8. Similar to the hot brew samples, no correlation between pH and TA were observed for the cold brew samples. Ethiopian-Ardi samples were observed to have the highest antioxidant activity, Myanmar and Ethiopian-Yirgz samples had the lowest antioxidant activity. In general, the cold brew extracts were found to have pH values comparable to those of the hot brew extracts, but lower total acidity measures, lower total CQA concentrations, and lower total antioxidant activities.

Hot and Cold Brew Comparisons

Total acidity and pH

Measurements of pH quantify the concentration of aqueous hydrogen ions at the time of analysis, providing a metric for the quantity of deprotonated acid molecules in a sample. Total titratable acidity (TA) is a measure of all acidic protons in a sample, including non-dissociated protons, that can be neutralized through the addition of a strong base.

Commercial vendors and coffee enthusiasts often suggest that cold brew and hot brew coffees boast different taste profiles due to differing acidity levels; and that cold brew coffee, being supposedly less acidic, may reduce gastrointestinal symptoms sometimes associated with coffee consumption6,47,48,49,50. This work found the pH measurements for all coffee samples tested to be comparable, ranging between 4.85 to 5.13. Varying the temperature of the extraction water did not result in distinguishable pH values between hot and cold brew coffees (Fig. 1).

Figure 1 pH values of six coffee samples brewed using both hot and cold brewing methods. The error bars represent 95% confidence level. Full size image

However, TA results indicate substantially different concentrations of total acidic compounds between hot and cold brew coffees. This research found hot coffee extracts to have larger measures of titratable acidity, indicating higher concentrations of extracted acids and/or additional acidic compounds not found in the cold brew coffee extracts (Fig. 2). The Pearson correlation coefficients for both hot and cold brew samples are less than 0.5. The lack of a correlation in this data agrees with the findings of Gloess et al.36 and suggests that pH is a poor measurement for the complex acid chemistry in both hot and cold brew coffee extracts.

Figure 2 Total titratable acids of six coffee samples brewed using both hot and cold brewing methods measure at (left) pH of 6.0 and (right) pH of 8.0. The values are reported as milliliters of 0.1 NaOH per 40 milliliters of brewed coffee. The error bars represent 95% confidence level. Full size image

In general, these results suggest that cold and hot brew coffees are similar in their total concentrations of deprotonated acid compounds, but differ in the concentration and possibly the complexity of protonated acids at the pH of extraction. The total CQA concentration data, shown in Tables 1 and 3, found hot brew extracts to have higher total CQA concentrations (Fig. 3). This is one source of the difference in total titratable acidities (TA). The compounds present in hot brew coffee but absent from cold brew coffee may be larger molecules with temperature-dependent solubilities, and/or compounds with significant intermolecular forces that result in strong coffee matrix-compound attraction.

Figure 3 (left) 3-CGA concentration in milligrams per liter of brewed coffee and (right) antioxidant activity in mmol equivalent of Trolox per liter of brewed coffee of the six coffee samples brewed using both hot and cold brewing methods. The error bars represent 95% confidence level. Full size image

Antioxidant activity and Total CQA Concentration

The family of chlorogenic acid compounds are known to contribute significantly to the antioxidant activity of coffee. Work by Daglia et al.51 and Stadler et al.52 have found the polyphenolic compounds in coffee to have antioxidant and antiradical activity in radical-mediated mutagenic pathways. Given the importance of this family of compounds, correlations between antioxidant activity and CQA concentrations were analyzed.

Similar to CQA data and TA, the data collected in this study indicated that hot brew extracts have higher antioxidant activity than their cold brew counterparts (Fig. 3). Figure 4 shows the relationship between antioxidant activity and total CQA concentration for hot and cold brew coffees. The cold brew samples were found to have a Pearson correlation coefficient of 0.82, indicating a relatively strong correlation between these two chemical characteristics. However, the antioxidant capacity and total CQA concentration of hot brew coffee were found to have a Pearson correlation coefficient of 0.22, indicating a much weaker relationship between antioxidant activity and chlorogenic acid concentration. Given that hot coffee extracts exhibited higher antioxidant activity than their cold brew counterparts, hot water must extract additional bioactive compounds. Hot brew coffees analyzed here were found to have increased concentrations of CQA isomers, and likely had increased concentrations of other chlorogenic acids. This may account for the difference in antioxidant activity between hot and cold brews, but there may be additional compounds responsible for this differential. The strong correlation between antioxidant activity and total CQA concentration in cold brew coffee suggests that CQA isomers are important drivers of cold brew coffee antioxidant activity.