Adsorption Isotherm and Thermodynamic Characteristics Of Soy- Melon Enriched “Gari” Semolina From Cassava

M. O. Oluwamukomi, I. A. Adeyemi, O. O. Odeyemi


The adsorption equilibrium moisture contents of soy-melon and control gari semolina samples
from cassava were determined using the static gravimetric method. Determinations were made
within a range of water activities of 0.20-0.92 at three different temperatures of 20o, 30o and
40oC. Results showed that the moisture sorption curves were sigmoidal in shape, which
conformed to type II classification characteristic of most biological tissues. A thermodynamic
approach was used to interpret the experimental adsorption isotherm data. The net isosteric heat
of sorption was determined from the equilibrium adsorption data, using the Clausius–Clapeyron
equation in the temperature range used in the experiment. Results indicated that the slopes of the
sorption isosteres decreased as water content increased indicating a decrease in the binding
energy for water molecules. The values of the isosteric heat of sorption for the two gari semolina
samples were consistently higher at lower moisture content and decreased exponentially as

moisture increased. Isosteric heat of sorption of soy-melon gari decreased from a value of 42
kJ/mol as the moisture content increased from 4.0% (db) while for the control (unsupplemented)
gari, the heat of sorption decreased from a value of 57.5kJ/mole at a moisture content of
10%(db). At moisture content above 18% for the Control gari and 12% for the Soy-melon gari,
the isosteric heats of sorption reached minimum values and remained practically constant. The
net isosteric heat of sorption of contol gari was generally higher than that of soy-melon gari
while control gari existed at higher moisture contents at the water activities used for this study
(0.23-0.91). This further explains the ability of control gari to absorb more moisture than soymelon
gari at those water activity levels. The lower values of the thermodynamic functions of
soy-melon gari were attributable to the higher amount of oil the soy-melon flour used for
supplementing the gari semolina while its higher value than other similar products was due to the
gelatinization of the starch in gari semolina as a result of its roasting.

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