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Caffeine content in coffee as influenced by grinding and brewing techniques
Food ResearchInternational,Vol. 29, No. 8, pp. 185189, 1996 0 1997 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology Printed in Great Britain PII:SO963-9969(97)00002-l
0963-9969/96 $15.00 + 0.00
Caffeine content in coffee as influenced by grinding and brewing techniques Leonard N. Bell,* Clinton R. Wetzel & Alexandra N. Grand Department of Nutrition and Food Science, Auburn University, AL 36849, USA
The caffeine content of coffee as influenced by various coffee preparation methods was investigated. The variables studied included the coffee solids to water
volume ratio, the volume of coffee prepared, home versus store grinding, and drip/filtered versus boiling. Caffeine contents per 177ml (6 oz) of coffee ranged from 50 to 143mg, depending upon the mode of preparation. As expected, more coffee solids and larger extents of grinding led to significantly higher caffeine contents in filtered coffee. Larger volumes of coffee prepared at a constant coffee solids to water ratio also yielded significantly higher caffeine contents. Homegrinding yielded caffeine contents similar to that of store-ground coffee. Boiled coffee had caffeine contents equal to or greater than filtered coffee, depending upon the length of boiling time. The variable caffeine contents in coffee resulting from the mode of preparation should be recognized and addressed by both food composition data bases and epidemiologists. (Q 1997 Published by Eisevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology Keywords: caffeine, coffee, brewing.
for coffee prepared by drip methods neglected to specify the amount of grounds and the ground size. Similarly, the USDA Nutrient Data Base for Standard Reference, an electronic version of Agriculture Handbook 8, lists brewed coffee as containing, on average, 58 mg caffeine/ 100 g coffee ( x 103 mg/ 177 ml), which may vary significantly depending upon how the coffee is prepared. To more accurately estimate caffeine consumption, more details about the coffee preparation need to be provided. Several differences exist in the preparation of coffee which may influence caffeine consumption. The coffee grounds to water ratio will influence the caffeine content of coffee. In addition, the volume of coffee prepared, at a constant coffee grounds to water ratio, may influence the caffeine content of the beverage and this has not been previously investigated. Recently, home coffee grinders have enabled consumers to purchase whole beans and grind them freshly at home. It is estimated that 15-20% of the coffee is now purchased as whole beans (McCue, 1995; Moukheiber, 1995). The effect of home-grinding, as compared to store-grinding, on the caffeine content of coffee has also not been evaluated. Many researchers (Bak & Grobbee, 1989; Pietinen et al., 1990; van Dusseldorp et al., 1991; Lindahl et al., 1991) have studied the effect of boiled coffee on heart disease,
INTRODUCTION Caffeinated beverages are consumed largely for the stimulatory effect of caffeine. While some research has linked caffeine to various health problems (Minton ef al. 1979; Bergman et al., 1990; Stanton & Gray, 1995), conclusive evidence is lacking, and caffeine remains generally recognized as safe by the US Food and Drug Administration. Part of these inconclusive findings may arise from the way epidemiological studies are performed. Epidemiological studies frequently equate caffeine with cups of coffee. Schreiber et al. (1988) discussed the errors of estimating caffeine consumption using the amount of coffee consumed, citing coffee preparation techniques as one factor which influences the amount of caffeine in the beverage. Studies demonstrating the influence of coffee preparation methods on its caffeine content are lacking. While Schreiber et al. (1988) point out how caffeine content varied with instant, percolated, and drip coffees, the value of 138 mg caffeine/6 oz cup *To whom correspondence should be addressed, at 328 Spidle Hall, Nutrition and Food Science, Auburn University, AL 36849, voice (334) 844-3272, fax (334) 844-3268. [email protected]. 785
786
L. N. Bell, C. R. Wetzel, A. N. Grand
but have failed to determine its caffeine content and compare it to that of drip/filtered coffee. Thus, the objective of this project was to determine how variations in the preparation of coffee influence its caffeine content. The variables evaluated included the coffee grind to water volume ratio, the volume of coffee prepared, home- and store-grinding, and boiling of the grounds. The caffeine contents from the various preparation techniques were determined and compared.
MATERIALS AND METHODS Sample preparation Whole coffee beans (Original Eight O’clock Bean Coffee, Compass Foods, Montvale, NJ) were purchased at a local grocery store. This coffee is a medium roasted 100% Arabica blend grown in Mexico and Brazil. Three bags were mixed to avoid variations between bags. Personal preference dictates how strongly coffee is prepared. In this study, coffee was prepared by using 1 tablespoon (T) to brew a 177 ml (6 oz) cup of coffee, which is similar to that recommended by most coffee brands (e.g. Maxwell House, Folgers, JFG), but less than the frequently used amount of 2T per 177ml water. Using a Braun model KSM2 home grinder (Lynnfield, MA), 2T of coffee beans and 8T of beans were ground for 8 and 18 s, respectively. The grounds were transferred into a coffee filter (Mr. Coffee, Bedford Heights, OH) and weighed. The 2 T of grounds weighed approximately 8 g while the 8T grounds weighed approximately 32g. Coffee was brewed using a Mr. Coffee automatic drip coffee maker (Bedford Heights, OH) with 355ml(12oz) or 1420ml(48 oz) of water per 2 or 8T coffee, respectively. Immediately after the brewing was completed, a 7 ml sample was removed for caffeine and total solids analysis. Coffee was ground and brewed in triplicate. Approximately one pint of whole beans was ground using a large store-type grinder (Grindmaster Corp., Louisville, KY) set on espresso (fine grind), auto-drip (medium grind), and percolator (coarse grind). Before each batch was ground, a small portion of beans was ground at the next setting to eliminate any of the previously ground coffee lodged in the grinder. To be consistent with the home ground coffee, 8 or 32g of coffee grounds were weighed into a filter. Coffee was prepared via the drip/filter method, at least in triplicate, as discussed above. Aliquots were removed for analysis. Samples of the finely ground store-ground coffee were also prepared in triplicate using 3 T (12 g) and 4 T (16 g) coffee grounds per 355 ml (12 oz) water. Caffeine contents were determined for these beverages. An 8 g portion of the coarsely ground coffee was also boiled. The water (355 ml) was brought to a boil, the
coffee was added, and boiling continued for either 1 or 2 min. The coffee was immediately filtered through cheesecloth. A 7ml aliquot of the resulting beverage was removed for analysis. Caffeine analysis Each of the triplicate coffee brews was diluted 1:lO in duplicate prior to caffeine analysis by high performance liquid chromatography (HPLC). The HPLC method was a variation of that used by Blauch and Tarka (1983). Similar HPLC methodologies have been used in our previous caffeine research (Grand & Bell, 1996; Hicks et al., 1996). A mobile phase consisting of 15% acetonitrile and 85% water (v/v) acidified to pH 3 with phosphoric acid was run isocratically at 1 ml/min through a 250 x 4.6 mm C-18 column having a 5 pm particle size and a 20% carbon load (Ultracarb 5 ODS 20, Phenomenex, Torrance, CA). Caffeine eluted around 7 min and was detected at 254nm. The filtered coffee was analyzed without further filtration of solids while the boiled coffee was passed through a 0.2pm syringe filter. Filtration of a caffeine standard through the syringe filter indicated caffeine did not bind to the filter. Caffeine (Sigma, St. Louis, MO) was dissolved in purified water to form standard solutions. These solutions were prepared in duplicate and serially diluted to prepare the standard curve. The standard curve had a linear correlation of 0.999. Total solids Approximately 5ml of each triplicate brew were weighed into glass vials which were placed in a drying oven set at 80°C for three to four days. The total solids (dissolved and suspended) were determined based on the amount of dry coffee residue. Particle size estimation Home-ground and store-ground coffee powders were sieved to estimate the particle size distribution. A portion of each powder was passed through a sieve having 25 holes per inch (approximately 0.6 mm pore size). The portion that did not go through was classified as “coarse” while that which did pass through was sieved through a smaller orifice (45 holes/inch or approximately 0.3mm pore size). The powder that passed through the 0.3 mm sieve was classified as “fine” while that which did not pass through was classified as “medium.” Sifting was performed in triplicate. Statistics The means with standard deviations were calculated for all analyses. Differences between means were tested
Cafeine content in coffee using a paired t-test with the level of confidence set at 95%.
RESULTS
m
8g(2T)/3!Xml
II
32 g (8 TM ,420 ml
AND DISCUSSION
Filtered coffee Figure 1 shows the mean caffeine contents of storeground coffee. When coffee was prepared using 8 T coffee grounds/l420ml water, the extent of grinding did not significantly (p 2 0.05) affect the caffeine content. All the caffeine contents, regardless of grinding, were approximately 70 mg/ 177 ml (6 oz cup). However, when only 2T of coffee grounds were brewed in 355ml of water, the finely ground coffee yielded a significantly (p < 0.05) higher caffeine content than the other grinds while the coarsely ground coffee yielded significantly @<0.05) lower caffeine values. This difference in caffeine content between finely and coarsely ground coffee is expected because more finely ground coffee would have a larger surface area, allowing for greater caffeine extraction. The larger the extent of grinding also led to more total solids being extracted from the coffee grounds (Fig. 2). Despite the ratio of coffee grounds to water being constant, the volume of coffee produced significantly (p < 0.05) influenced the caffeine contents of the medium and coarsely ground coffees (Fig. 1). Smaller volumes of coffee yielded less caffeine than larger volumes. Coffee prepared as 2T grounds/355ml water took approximately 3 min to brew while 8 T grounds/l420 ml water took over 10 min to brew. Lee et al. (1992) showed the extraction of caffeine was dependent upon the time of brewing. While consumers at home do not have the ability to control the flow rate of water in their coffee makers, using larger volumes of water accomplishes similar results. The longer brew time implies longer 90 m I
f
8o
E
70
warse
medium
fine
Extent of Grinding
Fig. 2. Total extracted solids of drip-prepared coffee as influenced by extent of store-grinding and volume of coffee prepared (n= 3, vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
contact time between the water and coffee grounds leading to more complete caffeine extraction. While caffeine content depended on both the extent of grinding and the volume of coffee prepared, the total extracted solids appears to depend to a greater degree on grinding. Figure 3 shows the mean caffeine contents in coffee as influenced by home-grinding. While the trend is less clear, the highest caffeine content (around 70 mg/ 177 ml) occurred in coffee prepared by brewing 8 T/1420 ml water while the lowest caffeine content (59mg/177ml) occurred in coffee prepared by brewing 2T/355ml water. The total extracted solids as a function of grinding in Fig. 4 resembles the pattern of caffeine in Fig. 3. Both trends are similar to that of the store-ground coffee. The lesser degree of significance in the home-ground coffees (Figs 3 and 4) could be due to less distinctive grinding patterns as shown by the particle size distribution (Table 1). As the extent of store-grinding increased, 80
8 g (2 T)/355 ml 32 g (8 T)/l,420 ml
b
P p0 9 550 0 .z 40 c LYI 20 coarse
medium
fine
Extent of Grinding
8g@T)
8g(2T)
8sec
18 sac
32 g (8 T) 8sac
32g(8T) 18 set
Extent of Grinding
Fig. 1. Caffeine content of drip-prepared by extent of store-grinding and volume (n = 3-5; vertical bars represent standard letters represent differences at
coffee as inthreneed of coffee prepared deviations; differing p < 0.05)
Fig. 3. Caffeine content of drip-prepared coffee as influenced by home-grinding (n = 3-5; vertical bars represent standard deviations; differing letters represent differences at p < 0.05)
L. N. Bell, C. R. Wetzel, A. N. Grand
788
b
10 8gt2T) 8 set
8gt2T) 18 set
32 g (8 9 8 set
32 g (8 T) 18 set
Extent of Grinding
Fig. 4. Total extracted solids of drip-prepared
coffee as influ-
enced by home-grinding (n= 3; vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
the percent of fine and medium grinds increased while the amount of coarse grinds decreased (Table 1). This distinctive grinding pattern was not evident in the home-ground coffees. Nonetheless, longer grinding times and coffee brewed in larger volumes resulted in higher caffeine contents in coffee prepared from homeground beans as was the case for store-ground beans. Increasing the amount of coffee grounds per given amount of water gave the expected increase in the caffeine content of the beverage. The caffeine content of coffee prepared from 2T of finely ground coffee per 355 ml water was around 70mg/177ml. This increased to 109mg and 143mg/177ml for coffee prepared using 3 T and 4 T grounds, respectively. These values translate into 17.4 to 18.1 mg of caffeine being extracted per gram of coffee powder, which is comparable to the value of 17.3 mg caffeine/g grounds derived from data presented by Bunker and McWilliams (1979). Using the data from Blauch and Tarka (1983), drip-prepared coffee contained approximately 15.9 mg caffeine/g grounds. Based on the previous discussion, more coarsely ground coffee brewed in small volumes would yield less than the 1718 mg caffeine/g coffee grounds due to less complete extraction.
Boiled coffee
Figure 5 shows the caffeine contents of boiled coffee as compared to filtered coffee. The caffeine content of coffee boiled for 1 min was not significantly different from that of filtered coffee prepared using 2 T/355 ml water. Boiling for 2 min yielded a significant @ < 0.05) increase in caffeine as compared to the filtered coffee prepared from 2T grounds. A different coarsely ground coffee was boiled for 3 min (the same time it takes 12 oz of filtered drip coffee to brew). The coffee boiled for 3 min contained over 40% more caffeine than a similar filtered coffee. However, the highest caffeine content remained that prepared by the drip method using a filter and 8T coffee grounds/l420 ml water. The interest in boiled coffee arises from its link to heart disease. Boiled coffee elevates serum cholesterol (Aro et al., 1987; Stensvold et al., 1989; Tverdal et al., 1990), and people who consumed filtered coffee had lower cholesterol values than those consuming boiled coffee (Bak & Grobbee, 1989; Pietinen et al., 1990; van Dusseldorp et al., 1991; Lindahl et al., 1991). The studies on boiled coffee did not address the possible differences in caffeine contents between boiled and filtered coffee. Results from the current study suggest that caffeine contents can be higher in boiled coffee than in filtered coffee, depending upon the length of boiling. However, the caffeine contents from boiled coffee are often less than filtered coffee prepared as large volumes (i.e. 1420ml), suggesting that the caffeine content of boiled coffee is not responsible for the beverage’s cholesterol-elevating effect, which is consistent with the findings of Weusten-Van der Wouw et al. (1994) who determined that the lipids cafestol and kahweol were responsible for the cholesterol-elevating effect of boiled coffee.
80 ,
Table 1. Average particle size distribution of coffee grounds (n=3)
Fine (%)
Medium (%)
Coarse (%)
Store (fine)
26
Store (medium) Store (coarse) Home (2 T, 8 s)
13 6 22
41 19 11 30
33 61 83 48
Home (2 (8 T, 818s)s) Home (8 T, 18 s)
46 14 32
39 23 40
:: 28
Grinding Method
filtered 8g
boiled 1 min
boiled 2 min
filtered 32 g
Extent of Grinding
Fig. 5. Caffeine content of boiled coffee as compared to drip/ filtered coffee (n = 3; vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
Caffeine content in coflee
CONCLUSION The use of more coffee grounds, larger extents of grinding, and larger volumes of coffee prepared yielded higher caffeine contents in filtered coffee. Home-grinding, which yields less distinctive grinding patterns, had caffeine contents similar to that of store-ground coffee. Boiled coffee had similar or higher caffeine contents than filtered coffee, depending upon the length of boiling time. Consumers desiring to moderate their caffeine intake, researchers performing epidemiological studies, and food composition data bases should recognize how the coffee preparation variables influence the caffeine content of the beverage.
REFERENCES Aro, A., Tuomilehto, J., Kostiainen, E., Uusitalo, U. and Pietinen, P. (1987) Boiled coffee increases serum low density lipoprotein concentration. Metabolism, 36, 1027-1030. Bak, A. A. A. and Grobbee, D. E. (1989) The effect on serum cholesterol levels of coffee brewed by filtering or boiling. New Engl. J. Med., 321, 1432-1437. Bergman, E. A., Massey, L. K., Wise, K. J. and Sherrard, D. J. (1990) Effects of dietary caffeine on renal handling of minerals in adult women. Life Sciences, 41, 557-564. Blauch, J. L. and Tarka, Jr., S. M. (1983) HPLC determination of caffeine and theobromine in coffee, tea, and instant hot wcoa mixes. Journal of Food Science, 48,745747,750. Bunker, M. L. and McWilliams, M. (1979) Caffeine content of wmmon beverages. J. Am. Diet. Assoc, 7428-32. Grand, A. N. & Bell, L. N. (1996) Caffeine content of fountain and private label store brand carbonated beverages. Journal of the American Dietetic Association, (in press). Hicks, M. B., Hsieh, Y-H. P. and Bell, L. N. (1996) Tea preparation and its influence on methylxanthine concentration. Food Research Znternational, (in press). Lee, T. A., Kempthome, R. and Hardy, J. K. (1992) Compositional changes in brewed coffee as a function of brewing time. Journal of Food Science, 57, 1417-1419. Lindahl, B., Johansson, I., Huhtasaari, F., Hallmans, G. and Asplund, K. (199 1) Coffee drinking and blood cholesterol-
789
effects of brewing method, food intake and life style. (1991) J. Intern. Med, 230, 299-305. McCue, N. (1995) Flavored coffees capture consumers. Prepared Foods, 164(10), 79. Minton, J. P., Foeking, M. K., Webster, D. J. T. and Matthews, R. H. (1979) Response of fibrocystic breast disease to caffeine withdrawal and correlation to cyclic nucleotides with breast disease. American Journal of Obstetrics and Gynecology, 135, 157-l 58. Moukheiber, Z. (1995) Oversleeping. Forbes, 78, 82. Pietinen, P., Aro, A., Tuomilehto, J., Uusitalo, U. and Korhonen, H. (1990) Consumption of boiled coffee is correlated with serum cholesterol in Finland. International Journal of Epidemiology, 19, 586-590. Schreiber, G. B., Maffeo, C. E., Robins, M., Masters, M. N. and Bond, A. P. (1988) Measurement of coffee and caffeine intake: implications for epidemiologic research. Prev. Med, 17,28&294. Stanton, C. K. and Gray, R. H. (1995) Effects of caffeine consumption on delayed conception. American Journal of Epidemiology, 142, 1322-l 329. Stensvold, I., Tverdal, A. and Foss, 0. P. (1989) The effect of coffee on blood lipids and blood pressure. Results from a Norwegian cross-sectional study, men and women 4w2 years. J. Clin. Epiakmiol, 42, 877-884. Tverdal, A., Stensvold, I., Solvoll, K., Foss, 0. P., Lund-Larsen, P. and Bjartveit, K. (1990) Coffee consumption and death from coronary heart disease in middle aged Norwegian men and women. British Medical Journal, 300, 566 569. USDA Nutrient Data Base for Standard Reference, Release 1993. [On-line]. Available:http://www.nal.usda.gov/ 10. fnic/foodcomp/Data/SRlO/ah8-1Cdat. USDA-ARS Nutrient Data Laboratory, Riverdale, MD. Weusten-Van der Wouw, M. P. M. E., Katan, M. B., Viani, R., Huggett, A. C., Liardon, R., Lund-Larsen, P. G., Thelle, D. S., Ahola, I., Aro, A., Meyboom, S. and Beynen, A. C. (1994) Identity of the cholesterol-raising factor from boiled coffee and its effects on liver function enzymes. J. Lipid Res., 35, 721-733. van Dusseldorp, M., Katan, M. B., van Vliet, T., Demacker, P. N. M. and Stalenhoef, A. F. H. (1991) Cholesterol-raising factor from boiled coffee does not pass a paper filter. Arterioscler. Thromb., 11, 586-593.
(Received 26 September 1996; accepted 30 November 1996)
0963-9969/96 $15.00 + 0.00
Caffeine content in coffee as influenced by grinding and brewing techniques Leonard N. Bell,* Clinton R. Wetzel & Alexandra N. Grand Department of Nutrition and Food Science, Auburn University, AL 36849, USA
The caffeine content of coffee as influenced by various coffee preparation methods was investigated. The variables studied included the coffee solids to water
volume ratio, the volume of coffee prepared, home versus store grinding, and drip/filtered versus boiling. Caffeine contents per 177ml (6 oz) of coffee ranged from 50 to 143mg, depending upon the mode of preparation. As expected, more coffee solids and larger extents of grinding led to significantly higher caffeine contents in filtered coffee. Larger volumes of coffee prepared at a constant coffee solids to water ratio also yielded significantly higher caffeine contents. Homegrinding yielded caffeine contents similar to that of store-ground coffee. Boiled coffee had caffeine contents equal to or greater than filtered coffee, depending upon the length of boiling time. The variable caffeine contents in coffee resulting from the mode of preparation should be recognized and addressed by both food composition data bases and epidemiologists. (Q 1997 Published by Eisevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology Keywords: caffeine, coffee, brewing.
for coffee prepared by drip methods neglected to specify the amount of grounds and the ground size. Similarly, the USDA Nutrient Data Base for Standard Reference, an electronic version of Agriculture Handbook 8, lists brewed coffee as containing, on average, 58 mg caffeine/ 100 g coffee ( x 103 mg/ 177 ml), which may vary significantly depending upon how the coffee is prepared. To more accurately estimate caffeine consumption, more details about the coffee preparation need to be provided. Several differences exist in the preparation of coffee which may influence caffeine consumption. The coffee grounds to water ratio will influence the caffeine content of coffee. In addition, the volume of coffee prepared, at a constant coffee grounds to water ratio, may influence the caffeine content of the beverage and this has not been previously investigated. Recently, home coffee grinders have enabled consumers to purchase whole beans and grind them freshly at home. It is estimated that 15-20% of the coffee is now purchased as whole beans (McCue, 1995; Moukheiber, 1995). The effect of home-grinding, as compared to store-grinding, on the caffeine content of coffee has also not been evaluated. Many researchers (Bak & Grobbee, 1989; Pietinen et al., 1990; van Dusseldorp et al., 1991; Lindahl et al., 1991) have studied the effect of boiled coffee on heart disease,
INTRODUCTION Caffeinated beverages are consumed largely for the stimulatory effect of caffeine. While some research has linked caffeine to various health problems (Minton ef al. 1979; Bergman et al., 1990; Stanton & Gray, 1995), conclusive evidence is lacking, and caffeine remains generally recognized as safe by the US Food and Drug Administration. Part of these inconclusive findings may arise from the way epidemiological studies are performed. Epidemiological studies frequently equate caffeine with cups of coffee. Schreiber et al. (1988) discussed the errors of estimating caffeine consumption using the amount of coffee consumed, citing coffee preparation techniques as one factor which influences the amount of caffeine in the beverage. Studies demonstrating the influence of coffee preparation methods on its caffeine content are lacking. While Schreiber et al. (1988) point out how caffeine content varied with instant, percolated, and drip coffees, the value of 138 mg caffeine/6 oz cup *To whom correspondence should be addressed, at 328 Spidle Hall, Nutrition and Food Science, Auburn University, AL 36849, voice (334) 844-3272, fax (334) 844-3268. [email protected]. 785
786
L. N. Bell, C. R. Wetzel, A. N. Grand
but have failed to determine its caffeine content and compare it to that of drip/filtered coffee. Thus, the objective of this project was to determine how variations in the preparation of coffee influence its caffeine content. The variables evaluated included the coffee grind to water volume ratio, the volume of coffee prepared, home- and store-grinding, and boiling of the grounds. The caffeine contents from the various preparation techniques were determined and compared.
MATERIALS AND METHODS Sample preparation Whole coffee beans (Original Eight O’clock Bean Coffee, Compass Foods, Montvale, NJ) were purchased at a local grocery store. This coffee is a medium roasted 100% Arabica blend grown in Mexico and Brazil. Three bags were mixed to avoid variations between bags. Personal preference dictates how strongly coffee is prepared. In this study, coffee was prepared by using 1 tablespoon (T) to brew a 177 ml (6 oz) cup of coffee, which is similar to that recommended by most coffee brands (e.g. Maxwell House, Folgers, JFG), but less than the frequently used amount of 2T per 177ml water. Using a Braun model KSM2 home grinder (Lynnfield, MA), 2T of coffee beans and 8T of beans were ground for 8 and 18 s, respectively. The grounds were transferred into a coffee filter (Mr. Coffee, Bedford Heights, OH) and weighed. The 2 T of grounds weighed approximately 8 g while the 8T grounds weighed approximately 32g. Coffee was brewed using a Mr. Coffee automatic drip coffee maker (Bedford Heights, OH) with 355ml(12oz) or 1420ml(48 oz) of water per 2 or 8T coffee, respectively. Immediately after the brewing was completed, a 7 ml sample was removed for caffeine and total solids analysis. Coffee was ground and brewed in triplicate. Approximately one pint of whole beans was ground using a large store-type grinder (Grindmaster Corp., Louisville, KY) set on espresso (fine grind), auto-drip (medium grind), and percolator (coarse grind). Before each batch was ground, a small portion of beans was ground at the next setting to eliminate any of the previously ground coffee lodged in the grinder. To be consistent with the home ground coffee, 8 or 32g of coffee grounds were weighed into a filter. Coffee was prepared via the drip/filter method, at least in triplicate, as discussed above. Aliquots were removed for analysis. Samples of the finely ground store-ground coffee were also prepared in triplicate using 3 T (12 g) and 4 T (16 g) coffee grounds per 355 ml (12 oz) water. Caffeine contents were determined for these beverages. An 8 g portion of the coarsely ground coffee was also boiled. The water (355 ml) was brought to a boil, the
coffee was added, and boiling continued for either 1 or 2 min. The coffee was immediately filtered through cheesecloth. A 7ml aliquot of the resulting beverage was removed for analysis. Caffeine analysis Each of the triplicate coffee brews was diluted 1:lO in duplicate prior to caffeine analysis by high performance liquid chromatography (HPLC). The HPLC method was a variation of that used by Blauch and Tarka (1983). Similar HPLC methodologies have been used in our previous caffeine research (Grand & Bell, 1996; Hicks et al., 1996). A mobile phase consisting of 15% acetonitrile and 85% water (v/v) acidified to pH 3 with phosphoric acid was run isocratically at 1 ml/min through a 250 x 4.6 mm C-18 column having a 5 pm particle size and a 20% carbon load (Ultracarb 5 ODS 20, Phenomenex, Torrance, CA). Caffeine eluted around 7 min and was detected at 254nm. The filtered coffee was analyzed without further filtration of solids while the boiled coffee was passed through a 0.2pm syringe filter. Filtration of a caffeine standard through the syringe filter indicated caffeine did not bind to the filter. Caffeine (Sigma, St. Louis, MO) was dissolved in purified water to form standard solutions. These solutions were prepared in duplicate and serially diluted to prepare the standard curve. The standard curve had a linear correlation of 0.999. Total solids Approximately 5ml of each triplicate brew were weighed into glass vials which were placed in a drying oven set at 80°C for three to four days. The total solids (dissolved and suspended) were determined based on the amount of dry coffee residue. Particle size estimation Home-ground and store-ground coffee powders were sieved to estimate the particle size distribution. A portion of each powder was passed through a sieve having 25 holes per inch (approximately 0.6 mm pore size). The portion that did not go through was classified as “coarse” while that which did pass through was sieved through a smaller orifice (45 holes/inch or approximately 0.3mm pore size). The powder that passed through the 0.3 mm sieve was classified as “fine” while that which did not pass through was classified as “medium.” Sifting was performed in triplicate. Statistics The means with standard deviations were calculated for all analyses. Differences between means were tested
Cafeine content in coffee using a paired t-test with the level of confidence set at 95%.
RESULTS
m
8g(2T)/3!Xml
II
32 g (8 TM ,420 ml
AND DISCUSSION
Filtered coffee Figure 1 shows the mean caffeine contents of storeground coffee. When coffee was prepared using 8 T coffee grounds/l420ml water, the extent of grinding did not significantly (p 2 0.05) affect the caffeine content. All the caffeine contents, regardless of grinding, were approximately 70 mg/ 177 ml (6 oz cup). However, when only 2T of coffee grounds were brewed in 355ml of water, the finely ground coffee yielded a significantly (p < 0.05) higher caffeine content than the other grinds while the coarsely ground coffee yielded significantly @<0.05) lower caffeine values. This difference in caffeine content between finely and coarsely ground coffee is expected because more finely ground coffee would have a larger surface area, allowing for greater caffeine extraction. The larger the extent of grinding also led to more total solids being extracted from the coffee grounds (Fig. 2). Despite the ratio of coffee grounds to water being constant, the volume of coffee produced significantly (p < 0.05) influenced the caffeine contents of the medium and coarsely ground coffees (Fig. 1). Smaller volumes of coffee yielded less caffeine than larger volumes. Coffee prepared as 2T grounds/355ml water took approximately 3 min to brew while 8 T grounds/l420 ml water took over 10 min to brew. Lee et al. (1992) showed the extraction of caffeine was dependent upon the time of brewing. While consumers at home do not have the ability to control the flow rate of water in their coffee makers, using larger volumes of water accomplishes similar results. The longer brew time implies longer 90 m I
f
8o
E
70
warse
medium
fine
Extent of Grinding
Fig. 2. Total extracted solids of drip-prepared coffee as influenced by extent of store-grinding and volume of coffee prepared (n= 3, vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
contact time between the water and coffee grounds leading to more complete caffeine extraction. While caffeine content depended on both the extent of grinding and the volume of coffee prepared, the total extracted solids appears to depend to a greater degree on grinding. Figure 3 shows the mean caffeine contents in coffee as influenced by home-grinding. While the trend is less clear, the highest caffeine content (around 70 mg/ 177 ml) occurred in coffee prepared by brewing 8 T/1420 ml water while the lowest caffeine content (59mg/177ml) occurred in coffee prepared by brewing 2T/355ml water. The total extracted solids as a function of grinding in Fig. 4 resembles the pattern of caffeine in Fig. 3. Both trends are similar to that of the store-ground coffee. The lesser degree of significance in the home-ground coffees (Figs 3 and 4) could be due to less distinctive grinding patterns as shown by the particle size distribution (Table 1). As the extent of store-grinding increased, 80
8 g (2 T)/355 ml 32 g (8 T)/l,420 ml
b
P p0 9 550 0 .z 40 c LYI 20 coarse
medium
fine
Extent of Grinding
8g@T)
8g(2T)
8sec
18 sac
32 g (8 T) 8sac
32g(8T) 18 set
Extent of Grinding
Fig. 1. Caffeine content of drip-prepared by extent of store-grinding and volume (n = 3-5; vertical bars represent standard letters represent differences at
coffee as inthreneed of coffee prepared deviations; differing p < 0.05)
Fig. 3. Caffeine content of drip-prepared coffee as influenced by home-grinding (n = 3-5; vertical bars represent standard deviations; differing letters represent differences at p < 0.05)
L. N. Bell, C. R. Wetzel, A. N. Grand
788
b
10 8gt2T) 8 set
8gt2T) 18 set
32 g (8 9 8 set
32 g (8 T) 18 set
Extent of Grinding
Fig. 4. Total extracted solids of drip-prepared
coffee as influ-
enced by home-grinding (n= 3; vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
the percent of fine and medium grinds increased while the amount of coarse grinds decreased (Table 1). This distinctive grinding pattern was not evident in the home-ground coffees. Nonetheless, longer grinding times and coffee brewed in larger volumes resulted in higher caffeine contents in coffee prepared from homeground beans as was the case for store-ground beans. Increasing the amount of coffee grounds per given amount of water gave the expected increase in the caffeine content of the beverage. The caffeine content of coffee prepared from 2T of finely ground coffee per 355 ml water was around 70mg/177ml. This increased to 109mg and 143mg/177ml for coffee prepared using 3 T and 4 T grounds, respectively. These values translate into 17.4 to 18.1 mg of caffeine being extracted per gram of coffee powder, which is comparable to the value of 17.3 mg caffeine/g grounds derived from data presented by Bunker and McWilliams (1979). Using the data from Blauch and Tarka (1983), drip-prepared coffee contained approximately 15.9 mg caffeine/g grounds. Based on the previous discussion, more coarsely ground coffee brewed in small volumes would yield less than the 1718 mg caffeine/g coffee grounds due to less complete extraction.
Boiled coffee
Figure 5 shows the caffeine contents of boiled coffee as compared to filtered coffee. The caffeine content of coffee boiled for 1 min was not significantly different from that of filtered coffee prepared using 2 T/355 ml water. Boiling for 2 min yielded a significant @ < 0.05) increase in caffeine as compared to the filtered coffee prepared from 2T grounds. A different coarsely ground coffee was boiled for 3 min (the same time it takes 12 oz of filtered drip coffee to brew). The coffee boiled for 3 min contained over 40% more caffeine than a similar filtered coffee. However, the highest caffeine content remained that prepared by the drip method using a filter and 8T coffee grounds/l420 ml water. The interest in boiled coffee arises from its link to heart disease. Boiled coffee elevates serum cholesterol (Aro et al., 1987; Stensvold et al., 1989; Tverdal et al., 1990), and people who consumed filtered coffee had lower cholesterol values than those consuming boiled coffee (Bak & Grobbee, 1989; Pietinen et al., 1990; van Dusseldorp et al., 1991; Lindahl et al., 1991). The studies on boiled coffee did not address the possible differences in caffeine contents between boiled and filtered coffee. Results from the current study suggest that caffeine contents can be higher in boiled coffee than in filtered coffee, depending upon the length of boiling. However, the caffeine contents from boiled coffee are often less than filtered coffee prepared as large volumes (i.e. 1420ml), suggesting that the caffeine content of boiled coffee is not responsible for the beverage’s cholesterol-elevating effect, which is consistent with the findings of Weusten-Van der Wouw et al. (1994) who determined that the lipids cafestol and kahweol were responsible for the cholesterol-elevating effect of boiled coffee.
80 ,
Table 1. Average particle size distribution of coffee grounds (n=3)
Fine (%)
Medium (%)
Coarse (%)
Store (fine)
26
Store (medium) Store (coarse) Home (2 T, 8 s)
13 6 22
41 19 11 30
33 61 83 48
Home (2 (8 T, 818s)s) Home (8 T, 18 s)
46 14 32
39 23 40
:: 28
Grinding Method
filtered 8g
boiled 1 min
boiled 2 min
filtered 32 g
Extent of Grinding
Fig. 5. Caffeine content of boiled coffee as compared to drip/ filtered coffee (n = 3; vertical bars represent standard deviations; differing letters represent differences at p < 0.05).
Caffeine content in coflee
CONCLUSION The use of more coffee grounds, larger extents of grinding, and larger volumes of coffee prepared yielded higher caffeine contents in filtered coffee. Home-grinding, which yields less distinctive grinding patterns, had caffeine contents similar to that of store-ground coffee. Boiled coffee had similar or higher caffeine contents than filtered coffee, depending upon the length of boiling time. Consumers desiring to moderate their caffeine intake, researchers performing epidemiological studies, and food composition data bases should recognize how the coffee preparation variables influence the caffeine content of the beverage.
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(Received 26 September 1996; accepted 30 November 1996)