Author : Anuthida Phaiphan

Summary : This research was conducted to evaluate antioxidant activity and identification of
bioactive compounds of S. siamea leaf extracted using solvent and ultrasound-assisted
extractions (UAB). A randomized complete block design with three replicates and
central composite design (CCD) as a response surface methodology (RSM) were used
to design and optimize the experiments. Effects of different solvents and locations on
the bioactive compounds and antioxidant activities of S. siamea leaf were investigated.
The results showed that the various solvent extractions had different amount of
antioxidant compounds and activities. Methanol extract presented the highest
efficiency values of yield, TPC, tannin content, DPPH, FRAP, and TEAC values,
followed by extracts of water, ethanol, and ethyl acetate, respectively. Water extracts
had the highest concentration of ascorbic acid, varying from 6.59-9.43 mg/g dw.
Meanwhile, Beta-carotene content of ethyl acetate extracts was the highest (28.20
mg/g dw) with significant (p<0.05) differences among locations. The correlation
coefficients of antioxidant compounds with three of the antioxidant activity assays
(%DPPH, FRAP, and TEAC) revealed that TPC, tannin content, and ascorbic acid of
the extracts correlated strongly with antioxidant activities. The average of R2 of TPC,
tannin content, and ascorbic acid was 0.976, 0.955, and 0.833, respectively. The
optimization of solvent extraction and UAB of antioxidant compounds and antioxidant
activities from S. siamea leaf was investigated. RSM and CCD were used to design
and optimize experimental conditions. The result showed that the optimum conditions
for solvent extraction were 49% of ethanol concentration, temperature at 51°C, and
22.5 min extraction time. Ethanol concentration, temperature, and time were the most
important variables that had effect on the responses. For UAB, the optimum conditions
were 40%ethanol concentration, temperature at 37°C, and time of 7 min. Ethanol
concentration was the most important variable that had an effect on the responses,
followed by time and temperature. The UAE extracts exhibited higher efficiency,
yield, TPC, %DPPH, and FRAP values than solvent extracts. The yield, TPC,
%DPPH, and FRAP values increased 10.6%, 20.7%, 9.4%, and 29.6%, respectively
from the solvent extraction. These results from the RSM clearly revealed that UAE
has potential and is efficient to be used to extract TPC and antioxidant activities in higher yields compared with solvent extractions. However, more identification studies
are required to identify the bioactive compounds in the most desirable extract from S.
siamea leaf. The individual phenolic acids and flavonoids were carried out using high
performance liquid chromatography. The results showed that bioactive compounds in
soluble extracts are significantly (p<0.05) higher than cell wall-bound extracts. It was
further revealed that soluble phenolic acids are the major phenolic acids in the leaf
Caffeic acid was found as the most abundant phenolic acid in the leaf extracts,
followed by gallic acid, and protocatechuic acid at 42.12, 25.27, and 21.65 mg/g dw,
respectively, mainly in its soluble form. On the other hand, protocatechuic acid was
the main phenolic acid found in cell wall-bound extracts at 19.13 mg/g dw. The
flavonoid contents were in the range of 0.38-137.14 mg/g dw in the extracts.
Epicatechin and catechin contents were the main flavonoids present in soluble
extracts, which were 137.14 and 27.02 mg/g dw and also were the only flavonoids
detected in cell wall-bound extracts, which were 10.19 and 16 mg/g dw, respectively.
The results exhibited that the main portions of bioactive compounds in the leaf were
present in soluble extracts. It indicated that S. siamea leaf may be regarded as a
valuable source of antioxidant-rich nutraceuticals. The potential applications of S.
siamea leaf antioxidants could be a value-added to the cosmetic or processed foods
industries.