<Insert Table 1: Chemical composition of essential
oils extracted by hydrodistillation (HD) and Microwave assisted
hydrodistillation (MAHD) from Amomum subulatum seeds>
The compounds were identified as α-pinene, β-pinene, 1, 8-cineol,
terpinen-4-ol, and α- terpineol. Results show that there is the
dominance of oxygenated compound over the monoterpene hydrocarbons in
essential oil extracted by both i.e. MAHD and HD methods. Presence of
oxygenated compound in the essential oil is good as those are
odoriferous and are mainly responsible for the characteristic aroma of
the essential oil[27][28]. Essential oil obtained by MAHD is
more concentrated in oxygenated monoterpenes - 1, 8-cineol (89.62),
terpene-4-ol (1.42) and α-terpineol (3.10) - as compared to essential
oil by HD where the percentage of oxygenated monoterpenes are 88.75 (1,
8-cineol), 1.21 (terpene-4-ol) and 1.50 (α-terpineol). The composition
of extracted essential oil by MAHD and HD methods reveals that there is
a decrease in the percentage of monoterpene hydrocarbon and increase in
oxygenated monoterpene. Increase in yield of the essential oil and
greater proportion of oxygenated monoterpenes is possibly due to less
thermal and hydrolytic impact as heating is very fast in case of MAHD as
compared to HD and lesser use of water in MAHD than in
HD[26][16]. Another reason may be as polar compound adsorbs
microwave irradiation more radially than others[29][27]. In
connection with above theory here we observed that 1, 8-cineol,
terpene-4-ol, and α-terpineol are more polar as compared to α and
β-pinene.
Optimization of Parameters
3.3.1. Effect of microwave power
In the extraction of Callistemon citrinus skeels by MAHD microwave power
is a key factor that influences the efficiency due to the interaction of
microwave with the extraction material. Microwave powers of different
wattage (160-800 W) level were studied for MAHD extraction of essential
oils by keeping time 80 min and temperature 1000c
constant. Each experiment was done twice, and the average was taken.
< Insert Figure 2 Influence of microwave power on essential
oil yield >
The outcomes of the trials are presented in fig. 2. The result
illustrates that there is an increase in the yield of essential oil as
we increase the microwave power from 160 to 800 W from 2.1 to 3.3 %
respectively. The increase in the yield with the increase in the
microwave power is due to the rapid generation of heat in the sample
material (mostly polar material in oil) with the absorption of microwave
(Thakker, Parikh, & Desai, 2016). We get the highest yield of 3.3 % at
the 800 W microwave power.
3.3.2. Effect of extraction time
Fig.3 shows that yield of extracted essential oil at different time
interval where the microwave power 800W and the temperature
1000c are kept constant. The rate of extraction at an
initial stage of 50 min to 80 min increases rapidly from 2.5 to 3.3 %
and then no rises as we increase the time up to 90 min yield is 3.3 %.
This shows that more will be extraction time more is extraction yield.
As we go from 80 min to 90 there no increase in this means that after
some certain period the extraction yield was constant, which is well
accordance with the previous studies (M.-T. Golmakani & Rezaei, 2008;
Man, Hamzah, Jamaludin, & Abidin, 2012; Ranitha et al., 2014).
< Insert Figure 3 Influence of extraction time on essential
oil yield >
3.3.3. Effect of temperature
Fig. 4 shows the yield of extraction at different temperatures having
the time and the microwave power was kept constant. The graph shows that
the rate of extraction from 700c to
800c there is little increase in yield. As we go
beyond 800c the rate of increase in yield is more up
to 1000c, as we further increase the temperature there
is no further improvement in the yield of extracted oil. Higher
extracting temperature is gainful for extraction owed to the improved
solubility. The system we used here is closed one and in a closed
microwave system the temperature of the water in flask could be
increased above the boiling point. The solubility of the essential oil
is increased because of increase in temperature and the efficiency of
extraction in enhance. higher temperature causes intermolecular
interactions and have higher molecular motion. The higher temperature
favours opening of the cell matrix and increases the availability of
essential oil.
< Insert Figure 4 Influence of temperature on essential oil
yield >
Antibacterial Study
In vitro antibacterial study of essential oil extracted by both the
methods was done against gram negative and gram-positive bacteria.
Microbial growth inhibited by amomum subulatum seed oil extracted by
MAHD and HD methods are shown in fig.5.
<Insert Figure 5 Antibacterial activities of a: HD essential
oil against E-coli b: HD essential oil against S. aureus c: MAHD
essential oil against E-coli and d: MAHD essential oil against S.
aureus>
Fig. 6 reveals that oils obtained by both methods have good
antibacterial activity against Staphylococcus aureus and Escherichia
coli. Zones of inhibition for microwave essential oil and hydrodistilled
oil were respectively 1.5 ± 0.2mm and 1.1 ± 0.2mm in the case of
Staphylococcus aureus. Similarly, in case of Escherichia coli were 1.2 ±
0.2mm and 1± 0.2mm respectively. In case of both gram-positive and
gram-negative bacteria zone of inhibition of MAHD extracted oil was more
as compared to that of HD extracted oil. The higher antibacterial
activity of MAHD extracted essential oil may be due to the higher amount
of oxygenated compounds present[26].
<Insert Figure 6: Zone of inhibition of MAHD and HD extracted
essential oil against Escherichia coli and Staphylococcus aureus
bacteria>
Antioxidant activity
In this study scavenging activity of both the essential oil was studied
against DPPH. DPPH is a stable radical which shows higher absorption at
515nm[30][31].
\begin{equation}
DPPH\bullet\ +AH\ \rightarrow DPPH-H+A\bullet\nonumber \\
\end{equation}Above reaction demonstrate that DPPH radially undergoes reduction by an
antioxidant (AH)[30]. DPPH assay is a rapid, trustworthy and
reproducible parameters for showing in vitro antioxidant activity of
plant extracts because of the ease and convenience of
reaction[25][30]. The studied essential oils were able to reduce
the stable radical DPPH from deep violet to the light yellow color
indicating their antioxidant activity[24][32].