Abstract MENKOV, N. D., K. DINKOV, A. DURAKOVA and N. TOSHKOV, 2009. Sorption characteristics of buckwheat grain. Bulg. J. Agric. Sci., Moisture equilibrium data (adsorption and desorption) of buckwheat grain were determined using the static gravimetric method of saturated salt solutions at three temperatures 10, 25 and 40°C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with increase in storage temperature at constant water activity. A suitable model was selected to describe the water sorption isotherms. The monolayer moisture content of the grain was estimated and the optimal storage water activity was proposed

A Durakova

K Dinkov

N D Menkov

N Toshkov

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281
Bulgarian Journal of Agricultural Science, 15 (No 4) 2009, 281-285
Agricultural Academy
SORPTION CHARACTERISTICS OF BUCKWHEAT GRAIN
N. D. MENKOV, K. DINKOV, A. DURAKOVA and N. TOSHKOV
University of Food Technologies, Department of Process Engineering, BG - 4000 Plovdiv, Bulgaria
Abstract
MENKOV, N. D., K. DINKOV, A. DURAKOVA and N. TOSHKOV, 2009. Sorption characteristics
of buckwheat grain. Bulg. J. Agric. Sci., 15: 281-285
Moisture equilibrium data (adsorption and desorption) of buckwheat grain were determined using the static
gravimetric method of saturated salt solutions at three temperatures 10, 25 and 40°C. The range of water activi-
ties for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with increase in
storage temperature at constant water activity. A suitable model was selected to describe the water sorption
isotherms. The monolayer moisture content of the grain was estimated and the optimal storage water activity was
proposed.
Key words: buckwheat, sorption, isotherms, monolayer moisture, modeling, water activity
E-mail: nimenkov@yahoo.com
Introduction
With recent consumer interest in functional food,
buckwheat grain (Fagopyrum esculentum Moench)
is increasingly being used as an ingredient and a func-
tional modifier in many foods (Li and Zhang, 2001;
Otles and Cagindi, 2006; Radovic et al., 1996;
Wijngaard and Arendt, 2006).
Sorption properties of foods (equilibrium moisture
content and monolayer moisture) are essential for the
design and optimization of many processes such as
drying, packaging and storage (Al-Muhtaseb et al.,
2002). The water activity of the grains as a hygro-
scopic material exerts a strong influence on its quality
and technological properties (Abdullah et al., 2000).
Mazza (1993) investigated the influence of storage,
relative humidity, and temperature on lipid, and pro-
tein content, color and flavor characteristics of buck-
wheat and developed recommendations for long-term
storage of buckwheat at a low temperature at a water
activity below 0.45.
Tagawa et al. (1993) obtained the sorption iso-
therms of buckwheat without reading the hysteresis
effect. Koloor Tabatabaee et al. (2006) and Mazza
(1993) built the sorption isotherms using solely five
experimental points, as between them only one is at
water activity under 0.5.
A number of models have been suggested in the
literature for the dependence between the equilibrium
moisture content (EMC) and the water activity (aW)
(Van der Berg and Bruin, 1981). The modified Chung-
Pfost, modified Henderson, modified Halsey, modi-
fied Oswin and Guggenheim-Anderson-de Boer
(GAB) equations which incorporate the temperature
effect have been adopted as standard equations by
the American Association of Agricultural and Biologi-
cal Engineers for the description of sorption isotherms
(ASABE, 2007).
Modified Chung-Pfost ⎥⎦
⎤
⎢⎣
⎡ −
+
−
= )exp(
t
exp CM
B
A
Wa  (1) 
 
Modified Halsey ⎥⎦
⎤
⎢⎣
⎡ +−= CM
BtA )exp(expWa  (2) 
 
Modified Oswin ( )
C
a
aBtAM ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
+=
W
W
1
(3) 
 
Modified Henderson ])(exp[(1 W
CMBtAa +−=−  (4) 
 
GAB  ( )( )WWW
W
11 aCBaBaB
aCBA
M
′′+′−′−
′′
=  (5) 
 ′ =
⎛
⎝
⎜
⎞
⎠
⎟B
h
RT
Bexp 1 (6) 
  ′
⎛
⎝
⎜
⎞
⎠
⎟C C
h
RT
= exp 2 ,(7) 
282
The monolayer moisture content value and the
corresponding water activity (aWm) are important pa-
rameters in food storage and deterioration. The higher
values of aW led to the faster reaction rate because of
the greater solubility and increased mobility. If a prod-
uct is susceptible to oxidation of unsaturated lipids,
the rate of shelf life loss increases as aW decreases
below aWm (Bell and Labuza, 2000).
The objectives of this work are: 1. to obtain ex-
perimental equilibrium sorption isotherms of buck-
wheat grain at 10, 25, and 40°C; 2. to find out the
suitable model describing the isotherms; 3. to calcu-
late the monolayer moisture content.
Materials and Methods
Material
Buckwheat grain variety “Dialog” produced in
Bulgaria was used in this study. AOAC standard pro-
cedures (AOAC, 1990) were used for the determi-
nation of the proximate chemical composition (on wet
basis): moisture – 14.9 %, crude fat – 3.5 %, protein
– 13.5 %, carbohydrate – 64.6 %, fiber -3.5%.
Procedure
The EMC of buckwheat grain was determined at
10, 25, and 40 °C. The static gravimetric method was
applied (Bell and Labuza, 2000; Wolf et al., 1985).
For the adsorption process, grain was dehydrated in
a dessicator with P2O5 at a room temperature for 20
days prior to the beginning of the experiment. The
desorption isotherms were determined on samples
hydrated in a glass jar over distilled water at a room
temperature to moisture content approximately 23 %
dry basis (d.b.). Samples of 1±0.02 g were weighed
in weighing bottles. The weighing bottles were then
put in hygrostats with eight saturated salt solutions
(LiCl, MgCl2, CH3COOK, K2CO3, Mg(NO3)2,
NaBr, NaCl, KCl), used to obtain constant water
activities environments (Greenspan, 1977). All used
salts were of reagent grade. At high water activities
(aW > 0.70) crystalline thymol was placed in the
hygrostats to prevent the microbial spoilage of the grain
(Bell and Labuza, 2000). The hygrostats were kept
in thermostats at 10, 25, and 40 +/- 0.2oC. Samples
were weighed in balance, sensitivity +/- 0.0001 g ev-
ery three days. Equilibrium was reached when three
consecutive weight measurements showed a differ-
ence of less than 0.001 g. The moisture content of
each sample was determined by the oven method by
means of triplicate measurements.
Analysis of data
The description of the sorption isotherms was veri-
fied according to the following five models proposed
in ASABE Standard D245.6 (ASABE, 2007):
where M is the moisture content (% d.b.), aW is
the water activity (fraction), A, B, C, h1 and h2 are
coefficients; t is the temperature (oC), T is the tem-
perature (K); R is the universal gas constant (J.mol-
1.K-1).
A nonlinear, least squares regression program was
used to fit the five models to the experimental data (all
replications). The suitability of the equations was evalu-
ated and compared using the mean relative error (%),
standard error of moisture and randomness of residuals
(Chen and Morey, 1989):
where Mi and Mi are experimentally observed and
predicted by the model values of the EMC, respec-
tively, N is the number of data points, and df is the
N. D. Menkov, K. Dinkov, A. Durakova and N. Toshkov
BET 
)1)(1( WWW
W
Caaa
CaM
M e
+−−
=  (11) 
M e a n   
r e l a t i v e   
e r r o r  ∑ −=
i
ii
ˆ1 0 0
M
MM
N
P    ( 8 )  
 
S t a n d a r d  e r r o r   
o f  m o i s t u r e
( )
f
2
ii ˆ-
d
MM
S E M ∑=    ( 9 )  
R e s i d u a l   ii M̂Me −= ,        ( 1 0 )  
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
Water activity
M
oi
stu
re
 c
on
te
nt
, %
d.
b.
desorption
adsorption
0
5
10
15
20
25
30
0 0.2 0.4 0.6 0.8 1
Water activity
M
oi
st
ur
e 
co
nt
en
t, 
%
d.
b.
10 ºC
25 ºC
40 ºC
Mod. Ch-Pf model
283
degree of freedom (number of data points minus num-
ber of constants in the model).
The monolayer moisture contents (Me) for each
temperature were calculated using the Brunauer-
Emmett-Teller (BET) equation with the experimental
data for water activities up to 0.45 (Bell and  Labuza,
2000; Brunauer et al., 1938):
Results and Discussion
The obtained adsorption and desorption isotherms
at 25oC are presented in Figure 1. The hysteresis ef-
fect is statistical insignificant in the most experimental
points. Similar hysteresis lack is observed for the iso-
therms at 10oC and 40oC, which confirms the results
obtained from Tagawa et al. (1993).
The unified (adsorption and desorption) mean val-
ues of EMC at the respective water activities and tem-
peratures are presented in Figure 2. The sorption iso-
therms have an S-shape profile. The EMC values
decreased with an increase in the temperature at con-
stant aW. Similar trends for many foods have been
reported in the literature Al-Muhtaseb et al. (2002).
The comparison with the published data for buck-
wheat grown in Asia (Tagawa et al., 1993) and in
America (Mazza, 1993; Koloor Tabatabaee et al.,
2006) shows that the buckwheat grown in Europe is
with higher sorption capacity independently of the simi-
lar chemical composition.
The coefficients of the models, P and SEM values
are presented in Table 1. The P and SEM values ob-
tained by the Modified Chung-Pfost model were
lower. The comparison between measured and pre-
dicted values with this model of EMC in Figure 2,
confirm the goodness of fit.  Figure 3 shows randomly
distribution of the residuals of Modified Chung-Pfost
model. Similar results were obtained in the literature
for another grain – medium rough rice (Basunia and
Abe, 2001) and amaranth (Pagano and Mascheroni,
2005).
The values of BET monolayer obtained with the
standard procedure are present in figure 4. The val-
Fig. 1. Equilibrium sorption isotherms
of buckwheat grain (adsorption and
desorption) at 25oC
Fig. 2. Equilibrium sorption isotherms
(measured and predicted with Chung-Pfost
model) at several temperatures
Sorption Characteristics of Buckwheat Grain
M o d if ie d  B E T  
( )
( )( ) =+−−
−
=
WWW
W
11 C aaa
C aB tA
M  
( )
( )( )WWW
W
8 7 9 35 611
8 7 9 35 60 6 1 3 304 9 8 99
a.aa
a.t..
+−−
−  ( 1 2 )  
M o n o l a y e r  m o is tu r e  
t..B tAM e 0 6 1 3 304 9 8 99 −=−=  ( 1 3 )  
R2=0.9988
0
2
4
6
8
10
0 10 20 30 40 50
Temperature ( oC)
standard procedure
modified BET
-1.5
-1
-0.5
0
0.5
1
1.5
0 10 20 30
Moisture content, %d/b.
R
es
id
ua
ls
, %
d.
b.
284
Model A B C h 1 h 2 P SEM
Mod. Chung-Pfost 638.7255 38.86128 0.190578 3.58 0.58
Mod. Oswin 15.91225 -0.08017 0.254796 5.44 0.88
Mod. Halsey 7.718706 -0.01722 2.94587 6.37 0.93
Mod. Henderson 0.000211 18.69474 1.72488 19.34 2.75
GAB 11.34283 0.059308 18.39309 5555973 657157 4.75 0.78
Table 1
Coefficients (A , B , C, h 1, h 2), mean relative error (P , %) 
and standard error of moisture (SEM ) of the models
Fig. 3. Distribution of residuals for the
Modified Chung-Pfost model
Fig. 4. Effect of temperature on the
monolayer moisture content
ues decreased with an increase in temperature. Menkov
et al. (1999) established a linear dependence between
monolayer moisture content and temperature for a
large number of biological products and proposed a
three-parameter modification of BET equation incor-
porating the temperature effect. The parameters of
modified BET equation were obtained with a pro-
gram for nonlinear regression with all experimental data
for aW < 0.45:
Figure 4 compare the BET monolayer moisture
obtained with the standard procedure and modified
BET equation and shows a good agreement.
The water activity value (aWm) corresponding to
the monolayer moisture content was calculated from
equation (12) by substituting M = Me (Iglesias and
Chirife, 1976) as:
We recommend this value (aWm = 0117) for buck-
wheat as optimal for grain storage. The value of aWm
N. D. Menkov, K. Dinkov, A. Durakova and N. Toshkov
1170
1879356
1879356
1
1 .
.
.
C
Ca =
−
−
=
−
−
=Wm  (14) 
285
is inversely proportional to the value of model con-
stant C and is independent of temperature in the dia-
pason from 10oC to 40oC.
Conclusions
The sorption capacity and monolayer moisture
content of buckwheat grain decreased with an increase
in temperature at constant water activity. The modi-
fied Chung-Pfost and GAB models are suitable for
describing the relationship between the equilibrium
moisture content, the water activity, and the tempera-
ture for the buckwheat grain. The modified BET model
is suitable for calculating of monolayer moisture for
several temperatures and water activity correspond-
ing to the monolayer moisture.
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Received April, 12, 2009; accepted for printing June, 23, 2009.
Sorption Characteristics of Buckwheat Grain


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