S. L. J. Tea Sci .58 (2), 87-91, 1989 Printed in Sri Lanka. 

WATER ACTIVITY IN RELATION TO THE 

SIZE OF BLACK TEA PARTICLES 

A. Thevathasan and D. S. A. Samaraweera 
(Tea Research Institute of Sri Lanka, Talawakele, Sri Lanka) 

Determinations of moisture content and water activity made on eight different size 
groups of tea particles (0.50 to 1.68 mm) indicate that water activity is Independent 
of the particle size. It is concluded that the shelf life of tea grades depends only 
on moisture content and water activity and hot on particle size. 

INTRODUCTION 

In most homogeneous food materials the water activity Is independent of the 
particle size, even though particle size does play a considerable part in the rate of 
equilibration (Taylor, 1961). In dried black tea dhools the particles are polydispersed 
(de Silva, 1972). The behaviour of water activity in relation to particle size 
of teas is not known. It is however noted that some physical and chemical charac­
teristics of black tea vary with particle size and therefore variation in water activity 
may as well be expected. 

In this investigation ungraded tea dhools (exclusive of Big Bulk) were separated 
into different particle size groups and the behaviour of water activity in relation 
to the moisture content was studied. 

MATERIALS AND METHODS 

The laboratory instrument used to measure the equilibrium relative humidity 
in this investigation was the Rotronic - Hydroskop DT. The instrument and the 
method adopted for measurement of water activity are similar to those described 
by Thevathasan and Samaraweera (1985). 

Partially dried tea dhool samples were taken from the Fluid Bed Drier (FBD) 
and separated into eight different particle size groups by the method adopted by 
de Silva (1972) using a sieve shaker fitted with a set of nine British Standards test 
sieves (BS 410, 1962). The particle sizes of the groups as characterized by the sieve 
diameters through which they were extracted (Snowsill, 1985)were 1.68, 1.40, 1,20, 
1.00, 0.85, 0.71, 0.60 and 0.50 mm. The moisture content of the partially dried tea 

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dhools collected from the FBD was around 10 per cent on a dry basis. This 
was to avoid clogging of the test sieves during separation of the dhools with 
high moisture. The separated tea dhools were oven dried at a low temperature of 
60°C. Samples with moisture contents varying between 0 to 10 per cent were 
taken from the oven at different intervals. Nine such samples from each of the 
eight groups were collected. Each sample was then divided into two portions and 
moisture content in one and water activity measurement in the other were made. 
Both measurements were made on duplicate samples. The moisture measurement 
was done by the oven method (Thevathasan and Samaraweera, 1985) and the tem­
perature of the samples was kept constant around 22.5°C for the water activity 
measurements. Though it was more desirable to first dry the samples in the oven 
and then sieve them, this step was not taken because of the considerable absorption 
of moisture during the sieving operation. 

RESULTS AND DISCUSSION 

The per cent moisture content oh a dry basis (M) and the corresponding 
water activity (A ) for different particle size groups are given in Tables la and b. 

w 

TABLE la—Moisture content (M) and water activity (A ) of tea dhools. Particle 
sizes-168, 1.40, 1.20 and 1.00 mm. w 

Particle Size (mm) 

1.68 1.40 1.20 1.00 

M A M A M A M A 
w w w w 

6.8 0.334 6.8 0.340 7.1 0.328 7.2 0.321 

6.0 0.312 6.2 0.329 5.8 0.294 6.2 0.317 

5.8 0.281 5.9 0.263 5.6 0.250 5.5 0.240 

3.1 0.062 5.3 0.260 5.2 0.207 4.6 0.109 

1.7 0.042 4.6 0.177 4.6 0.152 4.2 0.107 

1.4 0.036 4.4 0.173 2.8 0.018 4.0 0.077 

1.0 0.014 3.7 0.052 1.0 0.014 1.9 0.017 

0.6 0.008 1.3 0.026 0.6 0.006 0.7 0.005 

0.4 0.006 0.5 0.008 0.4 0.003 0.5 0.003 

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TABLE lb —Moisture content (M) and water activity (A ) of tea dhools 
Particle sizes-0.85, 0.71, 0.60 and 0.50 mm. w 

0.85 0.71 
Particle size (mm) 

0.60 0.50 

M A M A M A M A 
W W W W 

9.3 0.402 9.3 0.434 9.4 0.554 8.2 0.511 
6.6 0.389 6.6 0.354 7.3 0.467 7.6 0.455 
4.7 0.108 4.2 0.098 6.2 0.263 7.3 0.368 
3.2 0.047 3.4 0.070 3.4 0.033 6.5 0.267 
I.I 0.018 2.2 0.016 2.2 0.021 3.8 0.051 
0.7 0.15 1.0 0.012 I.I 0.010 2.2 0.025 
0.5 0.009 0.6 0.009 1.0 0.009 1.5 0.007 
0.3 0.008 0.4 0.005 0.5 0.008 0.7 0.005 
0.2 0.005 0.3 0.003 0.4 0.005 0.5 0.004 

A relationship between water activity and moisture content was proposed 
by Henderson (1952) and Is given by : 

log (log (l/(l-A ) = n log M + log K + log T 
e e w e e e 

Where, A = Water activity, 
w 

M = Moisture content (dry basis), 

T = Absolute temperature of the sample and 

K and n = Constants for the material. 
The equation can be re-written in the form Y = b * X + a 

Where, Y = log (log (l/(l-A ), 
e e w 

X = log M 
e 

b = n, 
and a = log K + log T. 

e e 

An analysis of variance for regression was carried out on X and Y as shown 
above for the sets of data of different particle size groups. The parameters of 
the fitted lines and their correlations along with their particle sizes are given 
in Table 2. 

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TABLE 2 —The parameters of the fitted lines, particle sizes of the groups and 
correlations. 

Particle Slope (b) Intercept (a) Correlation(r) 
size (mm) 

1.68 1.55 -3.98 0.9874 
1.40 1.53 -3.98 0.9631 
1.20 1.69 -4.40 0.9855 
1.00 1.20 -3.70 0.9711 
0.85 1.86 -4.75 0.9643 
0.71 1.47 -4.21 0.9661 
0.60 1.62 -4.37 0.9434 
0.50 1.95 -4.64 0.9682 

A suitable statistical anal/sis (Davies and Goldsmith, 1972) was carried out 
on these eight lines to find out whether they could be represented by a single line 
or whether they belong to a set of separate parallel lines or to independent 
skew lines. Statistically ( P > 0.5) it was found that the data were best represented 
by a single line given by: 

Y = 1.57 X —4.23 

The constants K and n for the tea dhools were calculated from the common 
equation and given by : 

K = 4.92 x 10-s ( 'K-i ) and 

n = 1.57 

These values are in agreement with the values for tea grades given earlier 
Thevathasan and Samaraweera, (1985). 

CONCLUSION 
The fact that the relationship between water activity and moisture content of 

black teas of the different size groups investigated in this study could be ex­
pressed by a single equation ofthe same form as suggested by Henderson (1952) 
implies that the water activity is related to the moisture content but this relationship 
Is Independent of the particle size. 

As in many other food products it Is known that loss of useful character in 
made teas is mainly due to proliferation of micro-organisms during storage 
which in turn is due to the water activity of the product (Labuza, 1980 ; 
Schwimmer, 1980; Taylor and Christian, 1978; Trailer, 1980). Although it is 
commonly believed that the small leaf grades (for example B.O.P.F. and Dust) are 
spoiled faster than the larger size leaf grades (for example B.O.P.) this study 
shows that this is unlikely because the water activity is independent of the particle 
size. Faster spoilage in smaller size teas can however be due to faster absorption 
of moisture in view of their larger specific surface area. As a consequence it is also 
likely to result in higher moisture levels in smaller size teas at any given stage in 
post drying operations. 

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R E F E R E N C E S 

OE SILVA, W. C. A. (1972). Unfolding particle size distribution of tea grades. I. High 
grown Broken Orange Pekoe and Broken Orange Pekoe Fannings. Tea Q. 43, 21-35. 

HENDERSON, S. M. (1952). A basic concept of equilibrium moisture. Agric. Eng. 33(4), 29-32 

LABUZA, T. P (1980). Effect of water activity on reaction kinetics of food deterioration, 
Food Technol. 34(4), 36. 

DAVIES, O. L. and GOLDSMITH, P. L. (1972). Statistical Methods in Research and Production. 
ICI, Edinburgh. 

SCHWIMMER, S. (1980). Influence of water activity on enzyme reactivity and stability. Food 
Techno/. 34(5), 64. 

SNOWSILL, W. L. (1985). Particle sizing, instrument technology - 'Mechanical Measurement'— 
Butterworth, U.K. 

TAYLOR, A. A. (1961). Determination of moisture equilibrium In dehydrated foods. Food 
Techno/. 15, 536-540. 

TAYLOR, A. A., and CHRISTIAN, J. H. B. (1978). Water activity and food. Academic Press 
New York. 

TROLLER, J. A. (1980). Influence of water activity on micro-organisms in foods, food Techno/ 
34(5), 76-82. 

THEVATHASAN, A. and SAMARAWEERA, D. S. A. (1935). Water activity in tea. S.L.J. 
Tea Sci. 54, 78-83. 

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