Pre-drying of fermented black tea leaves using far-infrared

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4th International Conference of Agricultural Sciences II

AgInsight-2022
4th International Conference of Agricultural Sciences XII
Track: EA-2-Soil science
Session Chair: Snr. Prof. Lal P. Vidhana Arachchi
Session Rapporteur: Ms. Ovini Dissanayake
Session Coordinator: Ms. W.G. Madurangi
7
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plantations replacing native ecosystems: A case from the low and mid
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24-27
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of mustard in-vitro
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Investigation of underlying causes for sudden tea bush death and
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51-54

AgInsight-2022
4th International Conference of Agricultural Sciences XIII
15
Pre-drying of fermented black tea leaves using far-infrared
H.M.N.P. Kumara, G. Abhiram, K. Raveendran, K.G. Prematilake
55-57
16
Recent applications of UAVs in sustainable horticulture: A review
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B. Shiyamala, T.H. Seran, H.F.L. Upendri
83-86

AgInsight-2022
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Pre-drying of fermented black tea leaves using far-infrared
H.M.N.P. Kumara1, G. Abhiram1, 2*, K. Raveendran3, K.G. Prematilake1
1 Department of Export Agriculture, Faculty of Animal Science and Export Agriculture Uva Wellassa University,
Badulla, Sri Lanka.
2 School of Agriculture & Environment, Massey University Private Bag 11 222, Palmerston North 4442, New
Zealand.
3 Process Technology Division, Tea Research Institute, Thalawakelle, Sri Lanka.
* gabhiram004@gmail.com
1. Introduction
Orthodox broken types of tea are produced in about 130 tea factories in Sri Lanka. Fluid bed
tea driers (FBD) are the preferred option to dry the types of tea. The FBDs are available in
different sizes and their output varies between 220 and 360 kg dried-tea hr -1. During tea drying,
chemical reactions are arrested and its moisture content is reduced to 3%. The main drawback
of the FBD method is using firewood as the main source for producing hot air for drying tea.
This releases harmful gases (carbon mono and dioxides) and it is difficult to find dried firewood
during the wet season. To overcome these limitations, far-infrared (FIR) could be used as an
alternative source of tea drying.
Far-infrared (FIR) has been used to dry many food commodities (Abhiram & Amaratunga,
2014). It does not release harmful toxic substances. This drying method increases energy
efficiency, decreases the processing time, provides uniform heating and has a high heat transfer
coefficient (Sakare et al., 2020). Only limited knowledge is available for the application of FIR
for tea drying. For example, Subankan et al. (2016) developed a batch-type FIR tea dryer and
found that the optimum drying conditions were 185 °C and 2.5 minutes of exposure time. In
this study, FIR was used for pre-drying of tea leaves since pre-drying could decrease the
firewood consumption for FBD drying and arrested the fermentation quickly. The pre-dried tea
samples were analysed for (theaflavin (TF): thearubigin (TR) ratio and organoleptic properties,
and compared with the factory-made tea samples.
2. Materials and Methods
A prototype tea dryer was developed with four FIR emitters and a thin layer of fermented tea
was pre-dried by changing the combinations of temperatures (350, 300, 250, 200 and 150 °C)
and exposure times (2.2 and 3.2 mins) (Table 1). The pre-dried tea was dried using a laboratory-
scale fluid bed drier. Sensory evaluation was conducted on dried tea by three professional tea
tasters and chemical analysis (theaflavin (TF) and thearubigin (TR)) was conducted for quality
parameters. Samples of tea dried using conventional fluid bed drier, collected from a typical tea
factory (factory samples), were included along with the test teas for the analysis.
Table 01. The sample numbers of the tea dried under different combinations of
temperature and exposure time
Time
Temperature
350 °C
300 °C
250 °C
200 °C
150 °C
2.2 minutes
S1
S3
S5
S7
S9
3.2 minutes
S2
S4
S6
S8
S10

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3. Results and Discussion
Results of the 5-point hedonic scale sensory evaluation on dried tea with the estimated median
value for liquor characteristics (liquor colour, flavour and aroma), dried tea leave characteristics
(appearance and colour) and overall acceptability is presented in a radar graph (Figure 1). The
overall acceptability of dried tea samples with exposed temperature of 250 °C & exposed time
of 2.2 minutes (S5) and exposed temperature of 250 °C & exposed time of 3.2 minutes (S6) in
the FIR pre-drier were superior to most of the other dried tea as well as to factory dried tea
sample (Figure 1). Dry leaf colour and appearance are mainly attributed to the results.
Figure 1. Radar graph for 5-point hedonic scale sensory evaluation of dried tea leaves and
tea liquor
The results of the chemical analysis showed a high percentage of theaflavin (TF) and a moderate
percentage of thearubigin (TR) in the dried tea samples, S5 & S6. Lowest TR: TF ratio, further
attributed to the overall acceptability. Higher percentages of TR in dried teas exposed to 2.2
minutes compared to that with 3.2 minutes at exposure temperatures of 350, 300 & 250 °C
indicate post-fermentation reaction. Longer exposure time under the FIR emitter led to arresting
of fermentation reactions. Longer exposure time at exposure temperature of 200 & 150 °C
facilitated post-fermentation during pre-drying with elevated temperature.
Table 02. The theaflavin (TF), thearubigin (TR) and TF:TR ratio of the samples dried
under FIR
Sample
TF%
TR%
TR: TF ratio
S1
0.557
8.838
15.858
S2
0.476
7.519
15.810
S3
0.507
9.124
18.012
S4
0.456
8.456
18.534
S5
0.553
8.724
15.780
S6
0.564
7.750
13.736
S7
0.410
8.838
21.582
S8
0.422
9.057
21.458
S9
0.404
8.541
21.147
S10
0.424
9.236
21.800

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4. Conclusions
Fermented tea types were pre-dried under FIR emitter at different exposure time and
temperature and finally dried using a laboratory-scale fluid bed drier. Sensory evaluation and
chemical analysis were conducted for dried tea samples and factory tea sample. When the
exposure time increased from 2.2 to 3.2 minutes, enzymatic reactions appears to be arrested at
high temperatures in the range of 250 to 350 °C. The sample dried under 250°C temperature
and 2.2 minutes exposure time showed overall better characteristics. These results showed the
potential use of FIR emitter for pre-drying of tea before fluidized bed drying.
5. References
Abhiram, G., & Amaratunga, K. S. P. (2014). The performance analysis of far infrared emission
on rice flour gelatinization. International Research Symposium on Postharvest
Technology, Institute of Postharvest Technology, Sri Lanka. 114-117.
Sakare, P., Prasad, N., Thombare, N., Singh, R., & Sharma, S. C. (2020). Infrared drying of
food materials: recent advances. Food Engineering Reviews, 12(3), 381-398.
Subankan, K., Withanage, N. S. & Abhiram, G. (2016). Performance analysis of prototype far-
infrared tea dryer in black tea manufacturing. International Research Symposium, Uva
Wellassa University, Sri Lanka. 169.