Vol.:(0123456789) 1 3
Journal of Food Measurement and Characterization
https://doi.org/10.1007/s11694-018-9879-0
ORIGINAL PAPER
Investigation of water adsorption and thermodynamic properties
of stevia powder
Nadia Hidar
1
· Mourad Ouhammou
1
· Ali Idlimam
2
· Abderrahim Jaouad
1
· Mohamed Bouchdoug
3
·
Abdelkader Lamharrar
2
· Mohammed Kouhila
2
· Mostafa Mahrouz
1
Received: 28 April 2018 / Accepted: 6 July 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Stevia rebaudiana is a natural non-caloric substitute to conventional sugar. Moisture adsorption isotherms of stevia powder,
a sweet plant, were investigated at three diferent temperatures (30, 40 and 50 °C) using a gravimetric technique. The sorp-
tion isotherms were found to be typical type II sigmoid with the sorption capacity decreasing with increasing temperature.
Experimental data were ftted using GAB model, the monolayer moisture content tended to decrease as temperature increased.
Moreover, these experimental data curves allow us to calculate the value of the optimal water activity for the conservation
and to determine the surface area of powder studied. The isosteric heat of sorption, sorption entropy and spreading pressure
were determined as a function of moisture content. The net isosteric heat of adsorption and diferential entropy decreased
with increasing moisture contents. A plot of diferential heat versus entropy satisfed the enthalpy–entropy compensation
theory. The spreading pressures increased with increasing water activity but decreased with increasing temperature.
Keywords Adsorption isotherms · GAB equation modelling · Stevia powder · Surface area · Thermodynamic properties ·
Water activity
List of symbols
EMC Xeq equilibrium moisture content (% d.b)
Am Area of a water molecule (1.06 × 10
−19
m
2
)
N
A
Avogadro’s number (6 × 10
26
molecules mol
−1
)
K
B
Boltzmann constant (1.38 × 10
−23
J K
−1
)
MRE Mean relative error (%)
Xeq
i,exp
Experimental equilibrium moisture content (%
d.b)
Xeq
i,pred
Predicted equilibrium moisture content (% d.b)
GAB Guggenheim Anderson de Boer
X
m
Monolayer moisture content
P
0
Vapour pressure of pure water at the same
temperature (atm)
ΔH
d
Isosteric heat of sorption (kJ mol
−1
)
Δh
d
Net isosteric heat of sorption (kJ mol
−1
)
ΔS
d
Entropy of sorption (J mol
−1
K
−1
)
R Universal gas constant (8.3145 J mol
−1
K
−1
)
ΔG
β
Free enthalpy at the isokinetic temperature
(J mol
−1
)
M
f
Mass of dry matter (g)
M
i
Mass of wet matter (g)
a
w
Water activity
Temperature (°C)
d.b Dry weight basis
R
2
Coefcient of determination
N Number of data points
T
β
Isokinetic temperature (K)
Eq Equation
Ads Adsorption
P Partial pressure of water in the food (atm)
Ei Residual of estimate
T
hm
Harmonic temperature (K)
φ Spreading pressure (J m
−2
)
a
wop
Water activity optimal
C, K, X
m
GAB model coefcients
* Ali Idlimam
aidlimam@gmail.com
1
ERIDDECV (Research Team of Innovation and Sustainable
Development & Expertise in Green Chemistry), Department
of Chemistry, Faculty of Sciences Semlalia, Cadi Ayyad
University, B.P. 2390, 40000 Marrakesh, Morocco
2
LESPAM Laboratory of Solar Energy and Medicinal Plants,
Teacher’s Training College, Cadi Ayyad University, BP 2400,
Marrakech, Morocco
3
REMATOP (Research Laboratory on Materials Reactivity
and Process Optimization), Department of Chemistry,
Faculty of Sciences Semlalia, Cadi Ayyad University, BP
2390, Marrakech, Morocco