American Journal of Analytical Chemistry, 2013, 4, 1-7
http://dx.doi.org/10.4236/ajac.2013.410A3001 Published Online October 2013 (http://www.scirp.org/journal/ajac)
Hygroscopic Behavior of Lyophilized Powder of
Grugru Palm (Acrocomia aculeata)
Dalany Menezes Oliveira1*, Edmar Clemente2, Marcos Rodrigues A morim Afonso3,
José Maria Correia da Costa3
1Center of Agricultural Sciences, State University of Maringá, Maringá, Brazil
2Department of Chemistry, State University of Maringá, Maringá, Brazil
3Department of Food Technology, Federal University of Ceará, Fortaleza, Brazil
Email: *firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
Received June 23, 2013; revised July 24, 2013; accepted August 19, 2013
Copyright © 2013 Dalany Menezes Oliveira et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The objective of this study was to determine adsorption isotherms and hygroscopic behavior of lyophilized powder
from grugru palm. The powders of grugru palm were obtained by lyophilization process without maltodextrin (T1) and
with 8% matodextrin (T2). The experimental data were obtained through the static gravimetric method at temperatures
(25˚C, 30˚C, 35˚C and 40˚C), with different saturated solutions of salts. The models of GAB, BET, Henderson and Os-
win were fitted to experimental data. The values obtained for hygroscopicity were 7.68% and 6.86% and the degrees of
caking were 0.33% and 0.09% for T1 and T2, respectively. Mathematical models of adsorption isotherms for grugru
palm powders can be classified as Type III. The GAB and Oswin models represented better the behavior of isotherms
for T1 and T2. Grugru palm powder showed an increase in the humidity of the monolayer Xm along with increasing
temperature. The grugru palm powder demonstrated to be a non-hygroscopic product, non-caking features.
Keywords: Acrocomia aculeate; Adsorption Isotherms; Hygroscopicity; Degree of Caking
Grugru palm (Acrocomia aculeata (Jacq.) Lodd. ex-Mart.)
is native to the grasslands, savannas and open forests of
Tropical America and occurs in dense populations in
some localities .
It presents a fresh pulp, which is consumed fresh or
used in the production of flour that can be utilized in dif-
ferent foods; it has a sweet taste and is mucilaginous.
This fruit can be eaten fresh or cooked, and used in soft
drinks and ice creams .
Ramos et al.  demonstrated the potential of grugru
palm pulp as a nourishing food, able to contribute to the
enrichment of the regional diet in supplemental feeding
programs, and a natural source of β-carotene, vitamin A
and minerals as copper, potassium and zinc.
Dehydration of fruits and food is used as a method of
preservation and storage, providing a new product on the
market, which has motivated the investments in agricul-
tural production and processing [4,5].
According to Toneli et al. , foods in storage period
are exposed to various relative humidities and to differ-
ent conditions of temperature; so it is necessary to adjust
the moisture in order to reach equilibrium with the envi-
ronment. These authors observed physical changes (cak-
ing) in inulin powder when there is humidity variation.
Mathlouthi and Rogé  stated that to explain the be-
havior of different insoluble food products, several ma-
thematical models were proposed for sorption isotherms.
Studies by Kaymak-Ertekin and Gedik  for sorption
isotherms in apples and potatoes showed an increase in
temperature, resulting in decrease of equilibrium for
The understanding of moisture sorption isotherms in
food has great importance in food science and technol-
ogy for many uses, such as design and optimization of
industrial processes, e.g., drying, assessing the problems
of packaging, modeling of moisture changes that occur
during drying, forecasting the stability of shelf life and
predicting mixtures of ingredients .
Obtaining grugru palm powder is still handmade, and
studies are necessary to show the best techniques for ob-
taining and storing this product. Given the above, the
objective of this work was to evaluate the hygroscopic
opyright © 2013 SciRes. AJAC
D. M. OLIVEIRA ET AL.
behavior and the application of mathematical models in
predicting the adsorption isotherms of lyophilized pow-
ders from grugru palm.
2. Material and Methods
2.1. Raw Material
Grugru palm fruits were harvested in Araripe Plateau in
the Cariri Region, Ceará State, Brazil, and taken to the
Laboratory of Food Quality Control and Drying (UFCE),
which were selected and sanitized and subsequently
peeled for pulp separation. The pulp was stored at −20˚C
until the lyophilization process.
Two lyophilized powders of grugru palm were used:
one treatment without addition of drying adjuvant and
other with maltodextrin of dextrose equivalent (DE) 20.
Control treatment (T1)—powder without maltodextrin
Treatment with maltodextrin (T2)—powder with addition
of maltodextrin 8%.
The pulp was unfrozen and triturated; after this process
the pulp of T1 and T2 were spread on stainless steel trays
with diameter of 15 cm. The trays were placed in ul-
tra-freezer (CL 90 - 40 V, Terroni) at −40˚C for 24 hours;
after this period the pulp was taken to a bench lyophilizer
(LS 3000, Terroni) for 25 hours. Right after the lyophili-
zation, the dried product was taken to a rotating-knives
mill (MA 048, Marconi) in order to get a powder with
2.3. Modeling of Adsorption Isotherms
In determining the moisture adsorption isotherms, a static
gravimetric method described by Wolf et al.  was
applied using saturated solutions of salts according to
Greenspan , at room temperature of 25˚C ± 2˚C. The
solutions of salts were prepared and put in containers of
tempered glass closed with silicone.
Measurements of adsorption isotherms were per-
formed in triplicate. For each cell about 0.20 g of each
treatment were weighed in pre-librated aluminum cruci-
bles. Afterward, the crucibles were taken to cells, which
contained the saturated solutions of salts.
The process was monitored by weighing of samples on
analytical balance every 24 hours to obtain equilibrium.
After balance the water activity of the samples was de-
termined at temperatures of 25˚C, 30˚C, 35˚C and 40˚C,
using aw meter, model AQUALab 4TEV. Then they were
taken to a lab chamber with air circulation at 105˚C for
determination of moisture content.
The equilibrium moisture (Xeq, Equation (1)) was cal-
culated as the difference between the mass of balanced
sample and the dried mass:
where: Xeq = equilibrium moisture (db); meq = mass of
balanced sample (g); ms
= mass of dried sample (g).
For adjusting of the experimental data from adsorption
isotherms of grugru palm powder, mathematical models
of GAB (Equation (2)), BET (Equation (3)), Henderson
(Equation (4)) and Oswin (Equation (5)) were used, rep-
resented respectively by the equations in Table 1. The
calculations of parameters of each model were performed
using the software Statistic version 7.0 .
The quality of adjusting different models was evalua-
ted through the best values of the determination coeffi-
cient (R2) and the relative mean deviation (E%, Equation
(6)), defined by Iglesias and Chirife :
where: E% = relative mean error; Xeqe = experimental
Table 1. Mathematical models used to adjust the experimental data of adsorption isotherms.
GAB (Guggenheim-Andersen-de Boer) 
Oswin  1
*aw = water activity; Xm = moisture content in the molecular monolayer (g H2O·g−1 dry basis); Xeq = equilibrium moisture content (g H2O·g−1 dry basis); C =
constant related to heat of sorption of the molecular layer; a, b, K = adjusting parameters.
Copyright © 2013 SciRes. AJAC
D. M. OLIVEIRA ET AL. 3
values; Xeqp = values predicted by the model; n = num-
ber of experimental data.
The analysis was performed using the methodology A14a,
described by GEA Niro Research Laboratory , con-
sisting in powder exposure to air relative humidity (RH)
of 79.5%, which was adsorbed through the powder sam-
ple until a constant weight is reached. The calculation of
hygroscopicity is given by Equation (7). The parameters
used to characterize the hygroscopicity of grugru palm
powder were obtained according GEA Niro Research
% Hygroscopicity100 %
where: %FW = % free water;
b ac b ; a = weight of plate (g); b
= weight of plate + powder (g); c = weight of plate +
powder in equilibrium (g).
2.5. Degree of Caking
The analysis was performed through the methodology
A15a, described by GEA Niro Research Laboratory ,
which consists of exposing the powder to absorb mois-
ture from the air (79.5% relative humidity) until reaching
equilibrium. Afterward, the powder was dried and sieved
under standard conditions (sieve mesh modified to 1200
μm). What was left in the sieve was expressed as degree
of caking. The calculation of the degree of caking is
given by Equation (8). The parameters used to character-
ize the degree of caking of grugru palm powder were
obtained according to GEA Niro Research Laboratory
%Degree ofCakingba100 (8)
where: a = grams of powder used; b = grams of powder
retained in the sieve.
2.6. Statistical Analysis
Data were analyzed statistically through analysis of va-
riance (ANOVA), and differences between the averages
for hygroscopicity and degree of caking were determined
by Tukey test at 5% probability using the software Sta-
tistic version 7.0 .
3. Results and Discussion
3.1. Adsorption Isotherms
The adsorption isotherms of lyophilized powders from
grugru palm fruits showed a behavior characteristic to
Type III, according to Brunauer classification . It is
can be observed in Figures 1 and 2 that the values of the
experimental data for equilibrium moisture (Xeq) of ly-
Figure 1. Adsorption isotherms at 25˚C of lyophilized pow-
der of grugru palm without addition of maltodextrin (T1).
Figure 2. Adsorption isotherms at 25˚C of lyophilized pow-
der of grugru palm with addition of 8% maltodextrin (T2).
ophilized grugru palm powders raised in accordance to
increasing water activity, under conditions of constant
temperature, e.g. at 25˚C shown in Figures cited.
The parameters of the models applied to experimental
data of adsorption isotherms of lyophilized powders of
grugru palm, the determination coefficients (R2) and the
relative average deviations (E%) used as evaluation cri-
teria for the representation of isotherms are shown in
According to the results presented in Table 2, one
notes that among the models studied (GAB, BET, Hen-
derson and Oswin) all can represent the behavior of gru-
gru palm powders because they showed representative R2
e E%, respectively, from 0.9843% to 0.9996% and
0.23% to 1.57%, values below the criteria recommended
by Aguerre et al. , where E% less than 10% indicates
reasonable representative models, and by Labuza et al.
, in which the representation of isotherms is consid-
ered extremely good for E% less than 5%. However, the
GAB model (triparametric) represents more accurately
the T1 treatment and the Oswin model (bi-parametric)
Copyright © 2013 SciRes. AJAC
D. M. OLIVEIRA ET AL.
Table 2. Adjustment results of experimental data of adsorption isotherms at 25˚C, 30˚C, 35˚C and 40˚C for lyophilized pow-
ders of grugru palm.
25˚C 30˚C 35˚C 40˚C Models Parametersc
25˚C 30˚C 35˚C 40˚C
Xm 0.07050.0709 0.08250.1027Xm 0.1122 0.1166 0.11560.0981
C 1.2663 1.2072 0.96230.6949C 0.5982 0.5626 0.55960.7402
K 0.9791 0.9925 0.98860.9796K 0.9278 0.9369 0.94590.9822
R2 0.9996 0.9993 0.99920.9996R2 0.9897 0.9900 0.99040.9965
E% 0.23 0.31 0.40 0.29
E% 1.13 1.24 1.16 0.69
Xm 0.07550.2301 0.06900.1579Xm 0.2241 0.1259 0.13870.1524
C 1.4186 0.2770 1.32860.4451C 0.3314 0.6404 0.54780.4968
n 1.3371-0.6423 1.0522 1.1523n -0.7423 1.3501 1.3105 1.2144
R2 0.9988 0.9916 0.99910.9994R2 0.9859 0.9961 0.99540.9976
E% 0.39 1.39 0.40 0.34
E% 1.39 0.76 0.79 0.58
a 0.58360.5591 0.55330.5450a 0.6155 0.6047 0.59420.5873
b 3.36853.1554 3.00702.8519b 3.5360 3.3701 3.26533.0546
R2 0.9951 0.9940 0.99500.9966R2 0.9843 0.9833 0.98280.9883
E% 1.01 1.08 0.93 0.78
E% 1.45 1.62 1.57 1.29
a 0.07730.0766 0.08010.0827a 0.0756 0.0775 0.07800.0837
b 0.88590.9421 0.96871.0099b 0.9120 0.9417 0.96470.9962
R2 0.9995 0.9993 0.99910.9993R2 0.9961 0.9958 0.99540.9979
E% 0.26 0.32 0.40 0.31
E% 0.63 0.79 0.79 0.54
aT1: lyophilized powder without addition of maltodextrin; bT2: lyophilized powder with addition of 8% maltodextrin. cAbbreviations: R2 = detemination coeffi-
cient. E% = relative average error. Xm = moisture content in the monolayer (g H2O·g−1 dry basis). C = constant related to the heat of sorption in the molecular
layer. K = GAB constant related to multi-layers. n = BET constant related to multi-layers. a e b = adjusting parameters of the Henderson and Oswin models.
the T2. Oliveira et al.  also obtained better adjust-
ments of GAB model and Oswin model for lyophilized
The GAB model is reported by several researchers in
forecasting adsorption isotherms for various dehydrated
foods, as Ferreira and Pena  in obtaining adjustment
of the model for peach palm powder; Silva et al. , for
yellow mombin powder; Kaymak-Ertekin and Gedik 
for dehydrated grapes and potatoes; and Goula et al. ,
who obtained the best prediction for tomato powder.
The Oswin model did not fit to mathematic modeling
in predicting the behavior of foods studied by Goula et al.
 and Al-Muataseb et al. . This report does not fit
to that observed in T2 in this study, which showed R2 =
0.9961 to 0.9979 and E% = 0.63 to 0.79. Kaymak-Er-
tekin and Gedik  also observed that the Oswin model
presented data well correlated for apples and potatoes.
Lomouro et al. (1985), quoted by Al-Muataseb et al. ,
reported that the Oswin model accounted for only 57% of
the isotherms described for foods.
The values of moisture in the monolayer (Xm) in the
GAB model for T1, between the temperatures of 25 and
40˚C, has raised with increasing temperature (Table 2).
Similar behavior was observed in the determination of
adsorption isotherms of flour of peach palm (Bactris
gasipaes) at 15˚C and 35˚C (23) for Surinam cherry
powder at temperatures of 20˚C and 40˚C .
The values of water content in the monolayer with in-
creasing temperature of T1 varied from 0.07 to 0.10 g
water g−1 dry basis, and in T2 it was 0.07 to 0.08 g water
g−1 dry basis; values of T2 has the same trend observed
in the range presented by Rahman (1995), quoted by
Cova et al. , which is from 0.05 to 0.08 g water g−1
dry basis for water content in the monolayer of starch
rich products. The addition of maltodextrin in T2 may be
the explanation for this treatment because it stands in the
profile of starch rich products. However, Talla et al. 
found values for banana, mango, pineapple between
0.080 and 0.185 g water g−1 dry basis, values higher than
The values of K in this study were inferior to 1.0 at all
temperatures, presenting little variation between 0.97 and
0.99. Vieira et al. , quoting Fernandez (1995), re-
ported that values of K lower than 1.0 show a character-
istic indicating that the isotherm for food products tends
to an asymptote when activity equals to 1.0.
One can observe that increasing temperature from
25˚C to 40˚C leads a reduction in values of the constant
C; a compatible behavior was observed by Gabas et al.
 for pineapple powder with addition of maltodextrin,
Copyright © 2013 SciRes. AJAC
D. M. OLIVEIRA ET AL. 5
which demonstrates temperature is dependent to the con-
stant C. These same authors reported that strong adsor-
bate-adsorbent interactions are favored by lower tem-
peratures, reflecting on C increase and showing that this
variation along with temperature may be the result of a
mathematic compensation between C and K.
In Figures 3 and 4, there is a graphical representation
of the adsorption isotherms of moisture of grugru palm
powder, for all temperatures studied, adjusted by GAB
and Oswin model for T1 and T2, respectively.
There is little influence of temperature on equilibrium
moisture content of grugru palm powder for water activ-
ity below 0.55 in T1, being observed from this point that
for the same aw there is an increase in the equilibrium
moisture content along with rising temperatures (Figure
3). In T2, this effect was observed from water activity
above 0.50 (Figure 4). In studies carried out with Suri-
nam cherry powder, it was observed an influence of
temperature on the equilibrium moisture content from aw
above 0.30 ; this shows that grugru palm powder
presents a hygroscopicity below that observed in Suri-
nam cherry powder.
As may be observed, the curves of Figures 3 and 4 do
not intersect themselves with the increasing temperature,
a fact also reported by Kaymak-Ertekin and Gedik  for
apples with high content of pectin and sugar, and for po-
tatoes with high content of starch. This similar behavior
of grugru palm powder to data of the previously cited
authors is due to its sweet fruit with approximately 35%
carbohydrates , being 13.3% glucose .
3.2. Hygroscopicity and Degree of Caking
Table 3 shows the values of hygroscopicity and degree
of caking for T1 and T2. Treatment T1 presented greater
caking in relation to T2. This fact may be related to the
Figure 3. Best model (GAB) for adsorption isotherms at
different temperatures of lyophilized grugru palm powder
without maltodextrin (T1).
addition of drying adjuvant, maltodextrin, in which its
use entails a reduction in the powder hygroscopicity; this
fact is also evidenced in adsorption isotherms of this
Powder caking is an undesirable phenomenon, which
initially consists of transformation of powder into solid
and sticky material, resulting in decreased functionality
and fluidity, so quality loss, and the main cause of ag-
glomeration is the presence of water induced by plastici-
zation of the surface of particles .
In accordance with GEA Niro Research Laboratory
, in which is characterized the hygroscopic behavior
and the degree of caking, respectively, the grugru palm
powder is classified as a non-hygroscopic and non-cak-
ing product, Oliveira et al.  showed similar response
about this classification (hygroscopicity 5.17% and 6.39%;
degree of caking 0.03% and 3.11%) for powder grugru
palm obtained by oven dryer.
In studies carried out with sucrose, Mathlouth and
Rogé  reported the formation of agglomerates is re-
lated to the solid bridges created during the process,
which turns out sucrose more hygroscopic. The same can
be observed in T1, where the powder formed the greatest
caking and consequently making the grugru palm powder
Costa et al.  stated that different variations in the
hygroscopic behavior of food powders are also attributed
to their size, as finer particles have higher contact surface
and therefore higher number of active sites. This report
may explain the non-hygroscopicity behavior of grugru
palm powder when compared to that of fruits found in
the literature, which showed high hygroscopicity, as the
particle size of grugru palm powder is 1200 μm and the
particle size of most fruit powders is in the range from
500 to 600 μm.
Figure 4. Best model (Oswin) for adsorption isotherm at
different temperatures of lyophilized grugru palm powder
with 8% maltodextrin (T2).
Copyright © 2013 SciRes. AJAC
D. M. OLIVEIRA ET AL.
Table 3. Hygroscopicity and degree of caking of lyophilized
powders from grugru palm.
Treatmenta Hygroscopicity (%) Degree of caking (%)
T1 7.68 a* 0.33 a
T2 6.86 a 0.09 b
aT1 = lyophilized powder without maltodextrin; T2 = lyophilized powder
with 8% maltodextrin. *Equal lower-case letters in the same column do not
differ by Tukey test at 5% probability.
Mathematical models of adsorption isotherms for grugru
palm powders can be classified as Type III, according to
All models studied adjust themselves to lyophilized
powder of grugru palm, and GAB and Oswin models
represent best the behavior of the adsorption isotherms
for T1 and T2, respectively.
Grugru palm powder showed an increase in the hu-
midity of the monolayer X
m along with increasing tem-
perature. The values of C were reduced along with in-
creasing temperature. The constant K suffered small var-
iations in function to temperature.
Lyophilized powders of grugru palm were characte-
rized as non-hygroscopic and non-caking product.
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