Study of some factors affecting water absorption by amaranth grain

Journal of Food Engineering 60 (2003) 391–396
Study of some factors affecting water absorption
by amaranth grain during soaking
Andrea N. Calzetta Resio, Roberto J. Aguerre, Constantino Su
Departamento de Industrias Tecnologia de Alimentos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,
Ciudad Universitaria, Nunez, Buenos Aires 1428, Argentina
Received 23 September 2002; accepted 8 February 2003
Water absorption by amaranth grain in plain water was determined at 30, 40, 50 and 60 °C by recording the weight increase in
grain with respect to time. Water absorption kinetics at these temperatures was described by the FickÕs second law solution for
diffusion out of sphere. Effective diffusion coefficients varied between 2.63 and 8.25 1012 m2 /s for the range investigated. The
activation energy for diffusion obtained was 32.1 kJ mol1 . The rates of absorption in SO2 aqueous solution (0.01%, 0.02% and
0.03%, v/v) were slightly higher than in plain water at 40 and 60 °C. However, the increase in SO2 concentration did not seem to
increase significantly water absorption. Amaranth grain soaked in 0.02% (v/v) SO2 aqueous solution and variable lactic acid
concentrations (0.0025% and 0.0050%, v/v) was performed at 40 °C. Absorption rates for lactic acid concentrations were 9.1%
higher than those steeped only in SO2 aqueous solution; lactic acid concentration had no effect on the absorption rate. The amount
of total solids leached in sulfur dioxide solution with lactic acid (200 ppm SO2 + 0.005% lactic acid) was the highest when compared
with the other steeping media investigated.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Diffusion; Moisture; Saturation; Sulfur dioxide; Lactic acid; Solids
1. Introduction
Different factors have contributed to increase the interest of grain amaranth as food and feed source. Among
them, the composition and properties of this crop have
received special attention (Teutonico & Knorr, 1985).
The main component of the grain is starch, about 48–
60% (Uriyapongson & Rayas-Duarte, 1994), which is
considered to have promising applications in food and
non-food areas. Another components of the grain are
lysine rich protein, 12–18%, and 5–8% fat that contain
high level of squalene (Myers & Fox, 1994).
In the past two decades substantial investigation was
performed on the functional and physicochemical
properties of the main components of grain amaranth,
starch and protein (Calzetta Resio, Tolaba, & Suarez,
2000; Paredes-L
opez, Schevenin, Hern
opez, &
Carabez-Trejo, 1989; Yanez, Messinger, Walker, &
Rupnow, 1986; Zhao & Whistler, 1994). Despite the
Corresponding author. Tel./fax: +54-11-4576-3366.
E-mail address: [email protected] (C. Suarez).
0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
substantial interest on both products, separation of
amaranth starch from protein is somewhat difficult because of the close bounding between the two components. Currently, there is no commercial process for
recovering starch from amaranth (Myers & Fox, 1994).
Conventional methods such as pearling (Betschart,
Irving, Shephard, & Saunders, 1981) and stone-milling
(Becker, Irving, & Saunders, 1986) resulted effective
in separating amaranth protein and starch fractions.
Myers and Fox (1994) used a modified laboratory
method for the wet milling of amaranth to recover
starch from it. However, such as pointed out by Lehmann (1996), if the ring embryo conveys novel benefits
in amaranth nutrition, then further wet milling techniques need to be explored.
In the wet-milling process, grains are almost invariably steeped in water at temperatures below gelatinization temperatures in the presence of chemical agents. In
the wet-milling of corn, the grains are steeped in aqueous solution of SO2 to facilitate the isolation of starch.
The presence of lactic acid in the steeping solution was
investigated by Du, Li, Lopez-Filho, Daniels, and
Eckhoff (1996), who found an increase of starch yield in
A.N. Calzetta Resio et al. / Journal of Food Engineering 60 (2003) 391–396
comparison with the conventional corn wet-milling
procedure. According to Haros and Suarez (1999) the
presence of lactic acid in steep water, not only increases
the rate of water absorption by corn kernels, reducing so
the effective steeping time, but also improves starchprotein separation.
As we are interested to determine whether a wetmilling process with separation procedures similar to
those used in the wet-milling of corn could be adapted
for amaranth, a preliminary study was conducted to
investigate the kinetics of water absorption by amaranth
grain. As it is well recognized in various investigations
concerning with the wet-milling of grains, hydration is
one the most important factors in producing goodquality products (Haros, Viollaz, & Suarez, 1995; Norris
& Rooney, 1970; Yuan, Chung, Seib, & Wang, 1998).
Many works studied the quantitative analysis of water
absorption for cereal grains (Becker, 1960; Fan, Chun,
& Shellenberger, 1961; Tolaba, Viollaz, & Suarez, 1990),
among others. In those investigations, FickÕs law of
diffusion with constant diffusion coefficient was used
without introducing too much error in the results. Other
investigations, however, have shown that the diffusion
equation with constant diffusivity is inadequate in describing the water absorption curve (Hsu, 1983).
Based on the present considerations the absorption
characteristics of amaranth grain in plain water and
various temperatures were investigated. Also, the effect
of certain chemical agents, SO2 and lactic acid, on the
water absorption was investigated as well as a means to
predict, quantitatively, the absorption kinetics in amaranth grain.
2. Materials and methods
2.1. Materials
Mature grains of Amaranthus cruentus were harvested
in the experimental cultivars of Facultad de Agronomıa,
Santa Rosa, and La Pampa. After harvesting, the grains
were cleaned from foreign materials by screening and
stored at room temperature in sealed containers until
used. The characteristics of the grain investigated are
summarized in Table 1. Moisture content, fat, ash and
protein were determined following standard AOAC
methods 943.01, 920.39. 923.03 and 976.05, respectively
Table 1
Characteristics of Amaranthus cruentus
Moisture content (% db)
Protein, (N 6.25)%
Fat (%)
Starch (%)
Ash (%)
Average diameter (mm)
(AOAC, 1995). Starch content was assessed by AACC
method 76–11 (AACC, 1995). The average diameter of
grains was evaluated from a size distribution analysis
obtained by using three metal sieves with pore diameters
of 870, 920 and 1000 lm. The weighed average diameter,
d, was calculated from the expression:
ðdi qi Þ=q
where di is the average screen opening, qi is the weight
between (i 1)th
P and ith sieve starting from the largest
one, and q ¼ i qi is the total sample weight.
2.2. Experimental procedure
2.2.1. Effect of temperature
Water absorption data were obtained by placing
10 0.5 g of amaranth grains in 150 ml screw-cap flasks
containing distilled water. The flasks were placed in
constant-temperature water bath controlled within 0.5
°C of the testing temperature. At regular intervals the
flasks were removed from the bath for moisture content
determination. For this purpose, the grains were rapidly
removed from the flasks and superficially dried on a
large filter paper to eliminate the surface water. The
grains were then weighed to determine the moisture
uptake. Experiments were conducted at 30, 40, 50 and
60 °C and for immersion periods from several minutes
to about 3 h.
2.2.2. Effect of sulfur dioxide on soaking
To study the effect of sulfur dioxide on water absorption three solutions containing 0.01%, 0.02% and
0.03% of the chemical, by volume, were prepared by
dissolving the appropriate amounts of NaHSO3 in distilled water. The grain samples were placed in screw-cap
flasks to prevent escape of SO2 gas. The flasks were
immersed in a thermostatic water bath at 40 and 60 °C
for water absorption determinations.
2.2.3. Effect of sulfur dioxide and lactic acid on soaking
Soaking solutions containing 0.02%, v/v, SO2 and
variable concentration of lactic acid: 0.0025% and
0.005% by volume were prepared to evaluate the effect
of both agents on water absorption. The soaking temperature tested for such purpose was 40 °C.
2.3. Total solids determination
The mass of solids leached during water steeping in
plain water, in aqueous solutions of 0.02%, v/v, SO2 and
0.02% SO2 + 0.0025% lactic acid, by volume, was determined by the method of Steinke and Johnson (1991).
A 10 ml sample of the steeping water was placed in a
weighed aluminum flask and air dried in an oven at 65 °C
for 24 h. After that, the sample was fully dried at the
A.N. Calzetta Resio et al. / Journal of Food Engineering 60 (2003) 391–396
same temperature in a vacuum oven until constant
weight, in presence of P2 O5 as desiccant. The percent of
total solids was referred to the weight of fully dried
3. Results and discussion
In order to predict water absorption during soaking,
the second FickÕs law solution for diffusion out of sphere
was tested. For this purpose the following assumptions
were made: (i) the effective diffusion coefficient, De , is
independent of moisture concentration; (ii) the volume
of the grain does not change during absorption, (iii) the
surface of the grain reaches moisture saturation instantaneously upon immersion in absorption media. Based
on these considerations, the solution of FickÕs second
law (Cussler, 1984) was used to calculate diffusivity
w w1
6 X
De n2 p2 t
¼ 2
w0 w1 p n¼1 n2
Where w is the moisture content at a given time t, w0 the
initial moisture content, w1 the saturation moisture
concentration, and R the average radius of amaranth
grain. To evaluate w1 the following method was used;
when time becomes large, the limiting form of Eq. (2)
w w1
De p2 t
exp 2
w0 w1 p2
From this equation it results that for any set of three
moisture contents taken at equally spaced time intervals
of duration j, the following expression for w1 results:
w1 ¼
wi wiþ2j w2iþj
wi þ wiþ2j 2wiþj
uration moisture concentration calculated from Eq. (4).
The maximum relative error in saturation values was
2.5% while the corresponding to De was almost 4.2%.
The predicted water absorption curves of amaranth in
plain water are shown in Fig. 1 together with the experimental data for the four soaking temperatures tested. The moisture content of amaranth grains increased
rapidly during the initial stages of hydration. This may
be due to characteristics of structure of the outermost
layers; i.e. the seed coat and pericarp, which being relatively porous appear to quickly attain equilibrium with
the soaking water by capillary imbibition (Becker, 1960).
Another significant factor that contributes to the rapid
initial water absorption may be the moisture absorbed
in the void space between the husk and the kernel of the
grain. The major discrepancies between experimental
and predicted curves are evident at the highest moisture
concentrations, because hydration occurs at much
higher rate than predicted by Eq. (2). Such inadequacy,
also observed by some investigators during water absorption process in grains and seeds (Abdel Kader,
1995; Hsu, 1983; Sopade & Obepka, 1990), might be
attributed to certain dependence of the diffusion coefficient on moisture concentration.The comparison of the
diffusion coefficients for water steeping obtained in this
work for amaranth grain with those reported in the literature for other grains is shown in Table 3. It can be
concluded that amaranth grain had diffusion coefficients
only comparable to that of maize grain.
In order to evaluate the goodness of the fit of Eq. (2)
to the experimental water absorption, the root mean
square error, RMSE, defined as:
N u1 X
we wp
RMSE ð%Þ ¼ 100 t
N 1
wi and wj being the moisture concentrations at time i
and j. Eq. (2) was used to estimate the effective diffusion
coefficient during water absorption of amaranth grain in
plain water by programming this equation on a digital
computer. A non-linear regression procedure was used
to calculate De ; diffusivity determinations were replicated three times. The average value of these replications
is given in Table 2 together with the values of the sat-
was calculated where we and wp are, respectively, the
experimental and predicted moisture concentrations and
N the number of experimental data. The values of
RMSE (%) are reported in Table 2. If values of RMSE
(%) less than 5% are generally considered as a good fit
(Lomauro, Bakshi, & Labuza, 1985), the relative high
value of this parameter indicates some degree of inadequacy of the model to describe the absorption process as
a whole.
Table 2
Saturation moisture contents, w1 , effective diffusion coefficient, De , and RMSE (%) values for water absorption of amaranth grain in plain water
Temperature (°C)
w1 (kg water/kg dry solid)a
De 1012 (m2 /s)b
RMSE (%)
0.715 0.015
0.810 0.013
0.889 0.017
0.96 0.020
2.63 0.11
3.84 0.08
5.81 0.21
8.25 0.10
Mean values standard deviation (n ¼ 5). Significant at P < 0:05.
Mean values standard deviation (n ¼ 22). Significant at P < 0:05.
A.N. Calzetta Resio et al. / Journal of Food Engineering 60 (2003) 391–396
for the different grains. For example, Becker (1960)
found for wheat that the saturation values resulted
practically independent of the soaking temperature and
equal to a moisture concentration of 75% in dry basis. A
similar trend was found by Haros et al. (1995) during the
steeping of maize in aqueous solution of sulfur dioxide,
with a saturation value of 79% for soaking temperatures
ranging from 45 to 65 °C.
At this point of the analysis it may be interesting to
compare the activation energy resulting from the variation of the saturation moisture concentration with
temperature, with the value of Ea obtained from the
diffusive process. The activation energy for variation of
the saturation moisture content with temperature was
calculated to be 6.71 kJ mol1 . The comparison with the
value of the diffusive process (32.1 kJ mol1 ) reveals a
substantial difference between these values. This difference might suggest that soaking at higher steeping
temperatures will affect the diffusion coefficient in the
endosperm somewhat differently than in the seed coat.
Considering that the seed coat is moisture saturated
since the beginning of the absorption process, less
variation can be expected for the value of the diffusion
coefficient as temperature increases.
Fig. 1. Water absorption rates for amaranth grain in plain water. (M)
30 °C; (}) 40 °C; () 50 °C; (j) 60 °C; Eq. (2), full line.
The diffusion coefficients reported in Table 2 were
correlated with the reciprocal of the absolute temperature, according to the Arrhenius type equation
De ¼ D0 expðEa =RT Þ
3.1. Effect of SO2 on the absorption rate
The effect of SO2 on the rate of water absorption
compared with plain water is illustrated in Fig. 2 for the
soaking temperature of 40 °C (similar effects not depicted in this figure were found at 60 °C). As can be seen
from this figure, the difference in moisture increase between amaranth grains soaked in plain water and
aqueous SO2 solution was not very marked, though
significant. The effective diffusion coefficient of water
into the kernel obtained for absorption in plain water at
40 °C was 3.84 1012 m2 s1 , while for SO2 (0.01%, v/v)
was of 4.30 1012 m2 s1 for the same temperature. On
the contrary, the variation of SO2 concentration in the
soaking media did not cause a significant variation on
the absorption rate: the diffusion coefficients at 0.01%
and 0.03%, v/v, SO2 were, respectively, 4.30 1012 and
4.19 1012 m2 /s.
where Ea is the activation energy, R the gas constant and
T the absolute temperature. The value of Ea was calculated by linear regression of ln De versus 1=T being its
value 32.1 kJ mol1 . This value is comparable to those
reported in the literature for water absorption in some
grains. So, Engels et al. (1986) found Ea values ranging
from 22.5 to 64.51 kJ mol1 , depending on the moisture
concentration of rice grain, while Haros et al. (1995)
reported an energy value of 39.41 kJ mol1 for water
absorption in corn activation.
It is also observed from Table 2 that the saturation
moisture content increases as the soaking temperature is
also increased. Such behavior is not apparently the same
Table 3
Diffusion coefficients for steeping in plain water of amaranth grain compared with other grains
Rough rice
Temperature range (°C)
De (m2 /s)
4.86 10
3.63 1011
6.67 109
9.03 109
1.08 1010
2.00 1010
3.84 1012
8.25 1012
Haros et al. (1995)
Becker (1960)
Engels, Hendrickx, De Samblanx,
De Gryze, and Tobback (1986)
Hsu (1983)
This work
A.N. Calzetta Resio et al. / Journal of Food Engineering 60 (2003) 391–396
corn steeping and attributed to the reduction of disulfide
bonds of corn proteins by SO2 .
3.2. Effect of lactic acid on water absorption
Fig. 2. Water absorption rates for amaranth grain in plain water (})
and SO2 solutions: (þ) 0.01% (v/v) and () 0.03% (v/v) at 40 °C.
The effect of SO2 on the variation of total solids in the
steeping water, with time is shown in Fig. 3. It can be
seen that the amount of solids released in the SO2 solution after a short induction period was considerably
higher than in plain water, being this effect more marked
with the increase of soaking time. Such difference could
be due to the soluble action of SO2 on the endosperm
proteins of amaranth grain. A similar behavior was
observed by Neuman, Wall, and Walker (1984) during
The water absorption curves of amaranth in aqueous
solution of 0.02%, v/v, SO2 and variable concentrations
of lactic acid: 0.0025% and 0.005% (v/v) are shown in
Fig. 4. The beneficial effect of lactic acid on water absorption rate became significant at concentrations of
0.0025% (v/v) being the diffusion coefficient 4.69 1012
m2 s1 , for the soaking temperature of 40 °C. However,
this effect did not seem to increase when the concentration of lactic acid is twofold. The estimated diffusion
coefficient at 40 °C for SO2 0.02% (v/v), combined with
lactic acid, 0.005% (v/v), was 4.73 1012 m2 s1 .
The effect of lactic acid on grain steeping is not well
understood and the results available in the literature on
water absorption rates are scarce and contradictory. For
corn, Shandera, Packhurst, and Jackson (1995) found
that the water absorption rate increased when lactic acid
was added to the steeping media, whereas Haros and
Suarez (1999) observed that the incorporation of lactic
acid into the steeping solution slightly affected the rate
of water absorption of that grain.
The variation of total solids leached in steep water
with time is illustrated in Fig. 3. A marked increase is
observed in the amount of solids leached on adding
lactic acid, compared to that found during soaking
in plain water or in SO2 solution. This may be due to
the softening action of lactic acid, which increases the
porosity of cellular membranes, enhancing in turn, the
total solids (g/g dry grain)
time (min)
Fig. 3. Variation with time of total solids leached at 40 °C in: plain
water (M); 0.02% v/v, SO2 solution (); 0.02% v/v, SO2 solution + 0.005% v/v, lactic acid ().
Fig. 4. Water absorption curves at 40 °C of amaranth grain in 0.02%
v/v, SO2 with 0.0025% v/v, lactic acid (M) and 0.005% v/v, lactic acid
A.N. Calzetta Resio et al. / Journal of Food Engineering 60 (2003) 391–396
dissolving action of SO2 on the proteinaceous matrix
that which binds the starch granules.
We acknowledge the financial support from Consejo
Nacional de Investigaciones Cientificas y Tecnicas
(CONICET) and Universidad de Buenos Aires. Special
thanks to Ing Rosa Troiani and Facultad de Agronomia, Universidad Nacional de La Pampa, for providing
grain amaranth samples.
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