Moisture sorption isotherm and thermodynamic properties of Centella asiatica L. (CAL) powder

Abstract

Centella asiatica L. (CAL) powder contains many medicinal properties that could be subjected to rapid deterioration due to presence of water. Moisture sorption isotherm has been widely used to predict shelf-life and storage stability as well as understanding the sorption mechanism between a material and water molecules. Aims of this study were to analyze the moisture sorption isotherm of CAL powder and its thermodynamic properties (net isosteric heat of sorption, differential entropy, and enthalpy-entropy compensation theory). A study was conducted at three different temperatures (30, 40, and 50 °C) with a range of water activities (0.06–0.92) using static gravimetric method. Results showed that CAL powder demonstrated Type-III moisture sorption isotherm behavior. Peleg isotherms relationship was found to be the most suitable model to describe the sorption isotherm curves followed by BET and Halsey. Furthermore, the net isosteric heat of sorption was determined using Peleg’s sorption isotherm model and the results showed exponentially increased as the equilibrium moisture content decreased. Enthalpy-entropy compensation theory was satisfied based on a linear relationship between differential enthalpy and entropy data obtained. Ccompensation theory also indicated that the process is an enthalpy-driven and undergoes a non-spontaneous reaction.

Keywords:

• Centella asiatica L.
• herbs
• isosteric heat
• medicine
• moisture sorption isotherm
• thermodynamic properties

Acknowledgements

The authors would like to thank Universiti Teknologi MARA (UiTM) for the financial support under FRGS/1/2018/TK02/UITM/02/12 and the Faculty of Chemical Engineering, UiTM Shah Alam for the facilities provided to conduct this work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Nomenclature
ΔG	=	Gibbs’ free energy (kJ/mol)
ΔS	=	Differential entropy (kJ/mol K)
aw	=	Water activity
A, B, C, K1, K2, n1, n2	=	Constant
BET	=	Brunauer, Emmett, and Teller model
db	=	dry basis
MRE	=	Mean relative percentage error (%)
N	=	Number of experimental data
n	=	Number of isotherm
qst	=	Net isosteric heat of sorption (kJ/mol)
R	=	Universal gas constant (kJ/mol K)
R2	=	Coefficient of correlation
SEE	=	Standard error of estimate
T	=	Temperature (Kelvin)
Tβ	=	Isokinetic temperature (Kelvin)
Thm	=	Harmonic temperature (Kelvin)
Ti	=	Experimental temperature (Kelvin)
Wi	=	Weight initial (g)
Wf	=	Weight final (g)
Xe	=	Equilibrium moisture content (% dry basis)
Xm	=	Monolayer equilibrium moisture content (% dry basis)
${\tilde{X}}_{e }$	=	Mean experimental equilibrium moisture content (% dry basis)
Xei, exp	=	Experimental equilibrium moisture content (% dry basis)
Xei,model	=	Model equilibrium moisture content (% dry basis)

Additional information

Funding

The financial support was provided by Universiti Teknologi MARA (UiTM) under FRGS/1/2018/TK02/UITM/02/12