The Effect of Brewing Temperature on Coffee Concentration

Experiment #1: The Effect of Brewing Temperature on Coffee Concentration



















Stephanie Farrell, Robert Hesketh, and Jeffrey Steingarten
August 30, 2002

















Submitted to: Dr. Mariano Savelski
Principles of Food Engineering
0901-501

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Abstract

This report describes an investigation of the effect of grinding time on the concentration
of Starbuck’s French Roast Coffee produced in an Automatic Drip Coffee Maker. Coffee
concentration was determined using a spectrophotometer. The concentration of coffee in the
brewed product demonstrated an exponential dependence on the grinding time of the roasted
beans.

Introduction

The manufacturers of coffee and coffee making equipment advise that choosing the
appropriate grind of coffee for the brewing method used is of critical importance in obtaining a
good quality product. Using the wrong grind can quickly destroy the flavor of the very best
gourmet beans. If the grind is too fine, the coffee will be bitter and over-extracted. If the grind is
too coarse, the coffee will taste weak and sour1.
The grind of coffee is determined by the length of time the roasted beans are ground.
One of the primary factors affecting the taste of coffee is the strength, or concentration, of the
coffee. The purpose of this study is to determine the effect of the grinding time on the strength
of Starbuck’s French Roast Coffee, brewed in an Automatic Drip Coffee Maker. The results will
be compared to the model developed by Farrell and Hesketh2.
Theory

Leaching is the process by which a substance is removed from a solid by a liquid
extraction medium. Important factors affecting leaching are the temperature, contact time,
contact area, and solvent selection3. The contact area per unit mass of a solid particle increases
with decreasing particle size.
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In the preparation of coffee, the particle size is determined by the time for which the
roasted beans are ground. Thus the grinding time is expected to have an effect on the
concentration of the brewed coffee.
Farrell and Hesketh2 have shown previously that coffee strength exhibits the following
dependence on grinding time:

2
C = k t + k t + k (1)
1
2
3
Where C is the coffee concentration (g/L), t is the grinding time (s), k1 is a constant with units of
(g/L.s2) and k2 is a constant with units of (g/Ls) and k3 is a constant with units of (g/L). This
relationship was developed for La Colombe Breakfast Blend Coffee produced in an automatic
coffee maker with a water temperature of 97 oC.
Materials and Methods
French Roast Coffee beans were obtained from Starbuck’s Coffee (Seattle, WA) and
stored in an airtight container at room temperature. A Krups (Nummener, Germany) 203-42 Fast
Touch coffee grinder was used to grind the beans. A Capresso (Switzerland) Team Luxe
Automatic Drip Coffee maker was used to brew the coffee. Distilled water was used in the
brewing. The temperature of hot water was monitored using a type T thermocouple and data
acquisition. A Spectronic 21 Spectrophotometer was used for concentration determination.
Experimental Procedure
Approximately 30 g of coffee beans were placed in the coffee grinder and ground for a
specified amount of time (6-30 s). 25 g of ground coffee was removed and placed in the coffee
maker’s gold tone filter. The reservoir was filled with 0.5 L distilled water. After turning on the
coffee maker, the temperature of the hot water was monitored to ensure that a constant 97 oC was
maintained.
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Upon completion of the brewing, a 10 ml sample was removed and stored in a sealed
glass sample vial for analysis, which was performed after the sample cooled to room
temperature.
Coffee concentration was determined using a spectrophotomer with a wavelength of 640
nm. A calibration curve was prepared by plotting known concentrations as a function of
Absorbance at 640 nm. The concentration of the calibration samples was determined by drying
and weighing the residue.
Results
The calibration curve for the determination of coffee concentration by spectrophotometer is
shown in Figure 1. The absorbance, A, increases linearly with concentration according to the
equation
?
g ?

C = ?
81
.
16
?(A) (2)
?
L ?
14
Data
)
L 12
Calibration Curve: C = (16.81g/L) A
10
tion (g/
tra
n
8
6
Conce
e
f
e
4
Cof
k
2
Bul
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Absorbance

Figure 1. Calibration curve for coffee concentration determination using a spectrophotometer at 640 nm

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The coffee concentration exhibited an increase with increased grinding time, as shown in
Figure 2. Since the particle size of the ground coffee decreases with increased grinding time, the
results are consistent with leaching theory which states that concentration increases with
increasing surface area (smaller particle size). The solid line shown in Figure 2 shows the best
fit of the data to the polynomial model developed by Farrell and Hesketh2. The close agreement
between the data and the model shows that the concentration-grinding time dependence of
Starbuck’s French Roast Coffee brewed in an automatic drip coffee maker can be described by
the Farrell and Hesketh2 leaching model, with k1=-0.007 (g/ls2), k2= 0.3975 (g/Ls) and k3 =
(0.5368 g/L). The Laboratory Data Sheet for this experiment is provided in Appendix 1.
7
6
5
/
L
)
4
t
r
a
t
i
o
n (g
y = -0.007x2 + 0.3975x + 0.5368
en 3
o
nc
C 2
1
0
0
5
10
15
20
25
30
35
Time (s)

Figure 2. Concentration as a function of grinding time for Starbuck’s French Roast Coffee brewed in an
Automatic Drip Coffee Maker.

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Conclusions

The concentration of Starbuck’s French Roast Coffee brewed in an automatic drip coffee
maker increases with the grinding time of the bean. The concentration follows a polynomial
dependence on grinding time which is consistent with previous results for La Colombe Breakfast
Blend Coffee reported by Farrell and Hesketh2.

Future studies should include the determination of coffee particle size as a function of
grinding time. This would allow a direct correlation between coffee concentration and particle
size (or area).

References

1 Caffeine, M.J. and I Colombian,. Factors affecting coffee quality, Int. J. Coffee, 35(2) 2001,
23-31.
2 Farrell, S. and R. Hesketh, Factors affecting coffee strength, J. Coffee Qual., 47(3) 2002, 45-41.
3 Perry, R. and D.W. Green, , Perry’s Chemical Engineers’ Handbook, 7th ed., McGraw Hill,
New York, 1997.
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Document Outline

• Materials and Methods

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