Kinetics of tea infusion. Part 2: The effect of tea-bag material on the rate and temperature dependence of caffeine extraction from black Assam tea

Abstract

Extraction experiments of tea inside tea-bag material were carried out at different temperatures. The rates of extraction were measured for caffeine from black orthodox Assam tea into distilled water. The leaf size used was 1.18–1.40 mm and the results obtained were compared with those of loose tea. The first order rate constants for the tea-bag tea were found to be 29% smaller than those of loose tea. The activation energies were found to be 45±2 and 43±2 kJ mol−1 for loose tea and tea-bag tea, respectively. These results show that the tea-bag material slows the infusion.

Full text

Kinetics of tea infusion. Part 2: the eect of tea-bag material
on the rate and temperature dependence of caeine extraction
from black Assam tea
Deogratius Jaganyi *, Siphumelele Mdletshe
Department of Chemistry, University of Natal Private Bag X01, Scottsville, 3209, Pietermaritzburg, South Africa
Received 5 May 1999; accepted 25 October 1999
Abstract
Extraction experiments of tea inside tea-bag material were carried out at dierent temperatures. The rates of extraction were
measured for caeine from black orthodox Assam tea into distilled water. The leaf size used was 1.18±1.40 mm and the results
obtained were compared with those of loose tea. The ®rst order rate constants for the tea-bag tea were found to be 29% smaller
than those of loose tea. The activation energies were found to be 452 and 432 kJ mol
ÿ1
for loose tea and tea-bag tea, respec-
tively. These results show that the tea-bag material slows the infusion. #2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The series on kinetics and equilibria of tea infusion by
Spiro and co-workers, has concentrated on the rate of
extraction of caeine and mineral ions from both black
and green teas. Investigations on how the rate is aected
by the leaf size, manufacturing method (Jaganyi &
Price, 1999; Price & Spiro, 1985a), composition of the
aqueous extracting medium (Spiro & Price, 1985b), and
temperature dependence (Spiro, Jaganyi & Broom,
1992) have been carried out. Despite the fact that most
tea is brewed using tea-bags, little has been published on
their eect on the rate of extraction. The only result
which has been reported involving the eect of the tea-
bag material on tea brewing is that it does not aect the
formation of tea scum in hard water (Spiro & Jaganyi,
1994). It is well known that the tea-bag material does
slow down the infusion process, but no data have
appeared in the public domain. In practice, smaller tea
leaves, which infuse faster, are used inside tea-bags so
that the overall infusion rate is increased over that with
larger-sized loose tea leaf. Also tea-bags have many
practical advantages: the tea is easier to handle and
simpler and less messy to dispose of. The present paper
looks at the hindrance eect of the tea-bag material on
the rate of extraction of caeine from black tea.
2. Material and methods
The tea used was black orthodox Assam, which was
sieved into dierent particle sizes using a set of stainless
steel Endecotts sieves. The leaf size range chosen for this
investigation was 1.18±1.40 mm. A plastic ¯ask contain-
ing 200 ml of distilled water was allowed to equilibrate to
the temperature of the water bath before adding 4.0 g of
tea leaves. The addition was performed with the help of a
glass funnel having a wide spout, a modi®cation of Spiro
and Siddique (1981). The mixture was stirred by an
underwater magnetic stirrer. A total of 12 samples (1 ml)
were withdrawn at 30 s intervals for the ®rst 3 min and
later at longer time intervals. The equilibrium sample
was taken after 60 min. These samples were transferred
into vials containing 9 ml of distilled water. This was to
avoid evaporation of the sample and to prevent cream
formation. The sampling was done with the help of a 5
ml disposable syringe ®tted to a thin plastic tube as
described by Jaganyi, Vanmare and Clark (1997).
The tea-bag was made from the material manu-
factured by Dexter Nonwoven. This was cut into equal
pieces which were folded into two, making sure that the
heat-seal was on the inside. Two sides were then sealed
using a hot iron. Using the remaining side, 4 g of the
loose tea were transferred into the bag and the side
sealed. The ®nal size of the tea bag was 7 cm by 8 cm.
To ensure that the tea bag was not going to interfere
0308-8146/00/$ - see front matter #2000 Elsevier Science Ltd. All rights reserved.
PII: S0308-8146(99)00262-9
Food Chemistry 70 (2000) 163±165
www.elsevier.com/locate/foodchem
* Corresponding author.

with the movement of the stirrer bar inside the ¯ask, a
stainless steel wire gauze was cut into a circle and the
side was bent by 1 cm. It was then squeezed into a
round-bottom ¯ask and ¯attened to form a ¯at base in
the bottom of the ¯ask with the stirrer bar below it. To
ensure that the tea bag remained ¯at on the wire mesh
and immersed in water at all times, two small stainless
steel bolts were attached to the opposite sides of the tea-
bag. It was ensured that these were far apart when the
tea-bag was added into the infusion ¯ask. The tea-bag
was added into the ¯ask in such a way that one end was
in the water and the opposite side allowed air to escape
as it was dipping into the water. Through this process
no air was trapped in the bag. The sampling procedure
was similar to that for loose tea.
The samples were then analysed for caeine using
high performance liquid chromatography; the type of
instrument as well as the column and the mobile phase
used were similar to those reported in the literature
(Jaganyi & Price, 1999). The concentrations were cor-
rected for evaporation and sampling (Jaganyi, 1992;
Spiro & Jago, 1982).
3. Results and discussion
3.1. Rate constants
It has been shown (Spiro & Jago, 1982) that the rate
of infusion of caeine from loose tea leaves is a ®rst-
order process, dependent upon the rate of diusion
within the leaves themselves. With tea leaves inside tea-
bags there will be a further ®rst-order process of caeine
¯ow through the tea-bag membrane (Jaganyi, 1992; Spiro
& Jaganyi, submitted for publication). The results have,
therefore, been analysed using the overall ®rst-order rate
law employed previously:
ln c1
c1ÿc

kobsta1
where cis the corrected concentration at time t, while
k
obs
is the observed rate constant and ais a semi-
empirical intercept. The corrected concentrations were
plotted against time. Typical plots showing the extrac-
tion of caeine with time from loose tea as well as from
tea-bag tea are shown in Fig. 1. The curves show that,
even in the presence of a tea-bag, there is an initial rapid
increase in the concentration of caeine with time. This
tails o towards an equilibrium value c1; these values
are tabulated in Table 1. The values for loose tea and
for tea-bag tea were found to be of a similar magnitude.
It is clear from Fig. 1 that the c1value for tea-bag tea is
attained after a long time when compared with that for
the loose tea. The equilibrium concentrations obtained
were independent of temperature and the values agree
well with those reported in the literature (Spiro et al.,
1992; Spiro & Lam, 1995).
The data were found to ®t very well into Eq. (1) as
can be seen in Fig. 2 showing the least square plots,
representing the extraction of caeine at 80C through
the loose tea and tea-bag tea. Similar plots were
obtained for the other temperatures. Table 1 sum-
marises the rate constants calculated from the slopes of
these ®rst-order plots at various temperatures plus the
ratios of kTBT
=kLT
. Included in Table 1 is the time
taken for the concentration of caeine to reach a value
of half its equilibrium concentration, the half-life t1=2,
which was calculated from the equation
t1=2ln 2ÿa=kobs 2
The intercepts, a, from Eq. (1) were very small in all
cases and some of the plots for the tea-bag tea passed
Fig. 1. Plots of concentration versus time for the extraction of caeine
from loose tea and tea-bag tea.
Table 1
Kinetic and equilibrium data for caeine infusion from loose and tea-
bag tea over a temperature range
Temperature
C
Type of
tea
kobs/
10
ÿ3
s
ÿ1
Ratio
kTBP
=kLT
t1=2ac
1/
mM
60 LT
a
2.43 277 0.02 4.62
TBT
b
1.70 0.70 425 ÿ0.03 4.66
65 LT 2.94 232 0.01 4.74
TBT 2.27 0.77 305 0.00 4.73
70 LT 3.79 159 0.09 4.72
TBT 2.74 0.72 253 0.00 4.68
75 LT 4.79 132 0.06 4.73
TBT 3.37 0.70 206 0.00 4.68
80 LT 6.04 110 0.03 4.71
TBT 4.17 0.69 161 0.02 4.69
a
LT, loose tea.
b
TBT, tea-bag tea.
164 D. Jaganyi, S. Mdletshe / Food Chemistry 70 (2000) 163±165

through the origin. All the kinetic data are the averages
of at least three independent runs.
The observed rate constants for the tea-bag tea were
29% smaller than those of loose tea over the tempera-
ture range.
3.2. Determination of activation energy
The Arrhenius equation
dlnkobs
d1=T
Ea
R3
where Ris the gas constant and Tthe temperature, was
used to determine the activation energies, Ea, for the
diusion processes. The least square plots of ln kobs
against 1=Tare shown in Fig. 3. The product of the
slope and the gas constant produced an activation
energy of 452 kJ mol
ÿ1
for the loose tea and 432 kJ
mol
ÿ1
for the tea in the bag. One would have expected
the activation energy to be bigger in the case of the
infusion through the tea-bag due to the additional
resistance from the bag material. The results obtained
are in agreement with earlier ®ndings (Spiro et al.,
1992).
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D. Jaganyi, S. Mdletshe / Food Chemistry 70 (2000) 163±165 165