moisture adsorption behavior of the banana flours (musa

Transcription

División Ciencias de la Vida
Campus Irapuato-Salamanca
XII CONGRESO NACIONAL DE CIENCIA
Y TECNOLOGIA DE ALIMENTOS
Jueves 27 y Viernes 28 de Mayo de 2010
Guanajuato, Gto.
MOISTURE ADSORPTION BEHAVIOR OF THE BANANA FLOURS (MUSA
PARADISIACA) UNMODIFIED AND MODIFIED BY ACID-TREATMENT
Andrés Aguirre-Cruza*, Roselis Carmona-Garcíab and Luis A. Bello-Pérezc
a
Universidad del Papaloapan. Instituto de Biotecnología. Circuito Central #200, colonia
Parque Industrial, C. P. 68301, Tuxtepec, Oax., México. México. Tel: + 52 287 8759240;
Ext. 220 fax: 52 287 87 59240 ext 230. E-mail: aaguirre@unpa.edu.mx
b
Instituto Tecnológico de Tuxtepec. División de estudios de posgrado e InvestigaciónDepartamento de Ingeniería Química y Bioquímica. Calzada Dr. Víctor Bravo Ahuja S/N
Col. 5 de Mayo 27, C.P. 68360. Tuxtepec, Oax., México.
c
Centro de Desarrollo de Productos Bióticos del IPN. Km 8.5 Carr. Yautepec-Jojutla,
Colonia San Isidro, Apartado Postal 24, 62731 Yautepec, Morelos, México.
RESUMEN:
Las isotermas de adsorción de la harina de plátano sin tratamiento (UBF) y harinas de plátano con
tratamiento ácido (ATBF) se determinaron mediante el método estático gravimétrico de soluciones
salinas saturadas a una temperatura de 30 ºC. Se evalúo la actividad de agua (aw) en un rango de
0,14 y 0,97. El equilibrio de humedad de los datos se ajustaron a cuatro modelos de sorción
ecuación Brunauer-Emmett-Teller (BET), Guggenheim, Anderson y de Boer (GAB), Smith y Chirife
Iglesias. El contenido de humedad en la monocapa (X0) para UBF y ATBFs fue de 4.06-5.47
(modelo BET) y 3.87-5.88 (modelo de GAB). El modelo de GAB fue el que mas se ajuto a los datos
experimentales, modelo más adecuado para describir las isotérmas de sorción de agua de la UBF
y ATBF en los intervalos propuesta de aw. Los valores X0 de ambos modelos (BET y GAB)
aumentan con el aumento de aw. La harina de platano tratadas durante 11 días (ATBF3) presenta
el mayor valor de X0 en comparación con todas las muestras. Este resultado sugiere que el
mecanismo de adsorción de agua y la estructura molecular en ATBFS se vio afectado, lo cual, se
atribuye a cambios en la morfología y la cristalinidad de las muestras sometidas a tratamiento
ácido.
ABSTRACT:
The moisture sorption isotherms of untreated banana flour (UBF) and acid treated banana flours
(ATBFs) were determined using the static gravimetric method of saturated salt solutions at
temperatures of 30 ºC. The range of water activities (aw) were calculated to be in the range of 0.14
and 0.97. The equilibrium moisture content absorption data were fitted to four sorption models
Brunauer-Emmett-Teller equation (BET), Guggenheim, Anderson and de Boer (GAB), Smith and
Iglesias-Chirife. Monolayer moisture content (X0) for UBF and ATBFs were found in the range of
4.06-5.47 (BET model) and 3.87-5.88 (GAB model). The GAB model was found to be the most
suitable model to describe the isothermal water sorption of UBF and ATBFs in the intervals
proposed of aw. The X0 values of both models (BET and GAB) increase with increasing aw. The
Banana flour treated for 11 days (ATBF3) presents the highest value of X0 compared with all
samples. This result suggests that mechanism of adsorption of water and molecular structure in
ATBFS was affected, attributed to changes in morphology and crystalinity of the samples with
treatment.
KEYWORDS: Banana flour, acid treatment, water sorption isotherm
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INTRODUCTION
Starch is the main reserve carbohydrate synthesized by superior plants that constitutes an
essential source of energy to many living organisms, principally the man (1).
This represents the majority component of a large number of agriculture products like the
cereals (corn, wheat and rice) and some fruits, unripe banana, which can have starch
content from 70 to 80% (Bello-Pérez et al, 1999). Banana is one of the fruits that grow well
in the tropical and sub-tropical regions of the world and are mainly transported to urban
areas, where they would be eaten as fruit. However, unavoidable delay in transport, poor
post harvest technology and fluctuating market demand result in overripe and senescence
of fruits prior to market delivery. For this reason, large quantities of fruits are lost during
commercialization as a consequence of deficient postharvest handling. Therefore,
nowadays, new economical strategies of banana uses are now considered, such as the
production of unripe banana flour (BF) due to its high starch content (2, 3).
On the other hand, controlling the moisture content during the processing of foods is an
ancient method of preservation. This is achieved by either removing water, or binding it
such that food becomes stable to both microbial and chemical deterioration (4). Water
activity is an important property for food quality, stability and safety. It is widely utilized in
the food industry for quality assurance and is an integral part of the FDA’s definition of
potentially hazardous foods. Both water activity and moisture sorption isotherms are
important for new product development, ingredient research, shelf-life estimation, and to
fully understanding the moisture within a product (5, 6). The relationship between the total
moisture content and water activity of the food, over a range of values, at a constant
temperature and under equilibrium conditions, yields a moisture sorption isotherm when
expressed graphically (Al-Muhtaseb et al., 2004). Besides, thermodynamics of water
sorption has been used to explain the behavior and the structure of water at the surface
and inside the foods (7).
Nowadays, the new product development area in those industries is interesting in
searching for starches with improved functional products such as viscosity, solubility, low
retrogradation and syneresis tendency, etc. Since some years ago, the tendency is looking
for alternative sources to obtain starches exhibiting better physicochemical and
functional characteristics. The native starches present limitations that reduce their use at
the industrial level. Therefore, in recent years, it has been studied the acid modified
starch, because their potential application. The modification with acid is widely used in the
starch industry to produce thin boiling starches for use in food, paper, textile and other
industries (8). Acid modification treatments change the morphological, crystalline,
gelatinization (transition temperatures and gelatinization enthalpy) and viscoelastic
properties of starch. Beside acid modification of starch could be very helpful to understand
the inner structure of starch granules (9, 10). Accordingly, this study evaluates the water
sorption behavior of untreated banana flour (UTBF) and acid-treated banana flour (ATBF),
as a function of reaction time. Sorption isotherms were fitted to four models and their result
were correlated to structural modification of the starch granules.
MATERIALS AND METHODS
PREPARATION OF BANANA FLOUR (BF)
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Commercial hard green (unripe) preclimateric banana (Musa paradisiaca L.) fruits were
purchased from the local market in Cuautla, Morelos State, Mexico. Fruits were cut into 1
cm slices and were immediately rinsed in citric acid solution (0.3% w/ v). The slices were
dried at 50 ºC, ground with a commercial grinder (Mapisa Internacional Sociedad Anonima
de Capital Variable, México, Distrito Federal) to pass a US 50 sieve, and stored at 25 ºC in
sealed plastic containers until further analyses to be carried out.
CHEMICAL COMPOSITION OF BANANA flOUR (BF)
The dietary fiber contents were analyzed according to AACC methods 32.05 (AACC,
2000).
ACID MODIFICATION OF BANANA FLOUR (BF)
Acid hydrolysis of samples was done by reacting 100 g of BF in 400 mL of 1.6 M HCl
at 38 °C for different reaction time (1-20 days) with a stirrer operating at 200 rpm. After the
reaction, the blend was neutralized with NaOH at the same concentration of HCl used; the
pH was adjusted to 7.0. Thereafter, the wet powder was washed with distilled water. The
residue was dried in an oven at 50 ºC for 24 h.
DETERMINATION OF SORPTION ISOTHERMS
Water sorption isotherms were obtained gravimetrically by exposing the sample at different
humidity contents in sealed containers by the aid of different salt solution (ASTM E10402). Three grams of the samples were placed in weighed sample holder dishes and
dehydrated and vacuum oven at 70 °C for 8 h (11). The samples by triplicate 3 g dry basis,
of untreated banana flour (UBF) and acid-treated banana flour (ATBF) were put inside
desiccators containing saturated solutions, prepared as specified in Table 2 (11). The
samples were allowed to equilibrate until there was no discernible weight change, as
evidenced by constant weight values (±0.001 g), during a period of approximately 7 to 8
days. After that time sample were weighed and dry mass was determined gravimetrically
by using a thermo balance OHAUS MB 45 at 130 ºC for 15 min.
DATA ANALYSIS BY SORPTION MODELS
Sorption model that are used in the in the present communication are briefly explained:
BET : X m = X0 CB aw /(1− aw )+(CB −1)(1− aw ) aw
GAB : Xm = X0 CG aw /(1− k aw )(1− k awCG k aw )
Smith : Xm = B+Alog(1− aw)
Iglesias −Chirife : Xm = B1(aw /1− aw )+B2
The BET model (Brunauer-Emmett-Teller) represents a fundamental milestone in the
interpretation of multilayer sorption isotherms, particularly Types II and III (Timmermann,
1989). Many researchers have modified the BET model and modified equation to give a
good fit up to water activity (12). The GAB model (Guggenheim-Anderson-de Boer) is one of
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the most widely accepted models for sorption isotherms (11, 13). The Smith model (1947) is
useful in describing the sorption isotherm of biological materials, such as, starch and
cellulose, similarly the semi-empirical model (14), is used for understanding the equilibrium
moisture content of cereal grains.
Where X0 is the equilibrium moisture content, g water/g dry matter, X0 is monolayer
moisture content, g water/g dry matter, aw is the water activity, CB constant of GAB model,
CG and k constants of BET model, B and A constants of Smith model and finally B1 and B2
constants related to heat of sorption of monolayer in the Iglesias-Chirife model. To
evaluate the fitness of each model, the mean relative percentage deviation modulus (%E)
was used, which is calculated by:
%E =
100
∑
mi − mpi
mi
i =1
Where mi is the experimental value, mpi is the predicted value, and N is the number of
experimental data. The mean relative percentage deviation modulus (%E) is widely
adopted throughout the literature. If it is obtained a modulus value below 10% indicates of
a good fit for practical purposes (15).
X-RAY DIFFRACTION ANALYSIS
X-ray diffraction allows the determination of crystallinity and composition of crystalline
phases in starch samples. The samples were stored at room temperature before analysis.
They were scanned in the angular range 3-37.8 (2θ) with an Advance D8 MCA Bruker
AXS (Coventry, UK) at 35 kV with Cu Ka radiation (1.542 Å).
SCANNING ELECTRON MICROSCOPY (SEM)
Samples were fixed on aluminum stubs with the aid of a conductive double glued tape of
copper in order to allow surface and cross-section of samples be visualized. All samples
were examined using an accelerating voltage of 5 kV with a JEOL JSMP 100 (Japan)
Scanning Electron Microscope.
RESULTS AND DISCUSSION
EQUILIBRIUM MOISTURE CONTENT
The adsorption isotherms for UBF and ATBF at 30 °C are shown in Figure 1. These curves
represent a typical sigmoidal shape, reflecting a B.E.T. Type II isotherm, which are the
most frequent ones in foods, such as fruits and vegetables (Brunauer et al., 1940;
Martínez et al., 1998). This typical physical adsorption behavior has been seen for potato
starch (Lagoudaki, et al, 1993) and grapes (Rouquerol and Sing, 1999).
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Fig. 1 The adsorption moisture isotherms of untreated banana flour (UBF) and acidtreated banana flour (ATBF) observed data at 30 °C; ATBF1: modified for 1 day, ATBF2;
modified for 5 days, modified for 11 days and ATBF; modified for 20 days.
The equilibrium moisture content at each water activity represents the mean value of three
samples. The sample data showed an increase in equilibrium moisture content with
increasing water activity (0.3-0.97), at constant temperature (30 °C). The starch sorption
isotherm is mainly attributable to hydrogen-bonding of water molecules to the available
hydroxyl groups of amylose and amylopectin chains, those in the amorphous regions and
on the surfaces of the crystallites (Urquhart, 1959). The crystalline regions exhibit
resistance to solvent penetration. Hence, water affects the structure acting as a plasticizer
of the amorphous regions. At low aw<0.3 the plasticizing affect is very small and the
mobility of the amorphous regions is restricted. However, as the water activity increases
(0.3-0.97), the absorbed moisture causes a subsequent swelling of the granules of starch,
the degree of crystallinity decreases, and there is an increasing availability of the polar
groups to the water molecules (16).
Constants obtained from each equation (BET, GAB, Smith and Iglesias-Chirife) and the
mean relative percentage deviations (% E) for adsorption are presented in Table 3. The
values of the molecular adsorbed moisture in a monolayer (X0) showed in table 3, for UBF
and ATBF calculated by BET model varied of 4.06-5.47 and using the GAB model this
value varied of 3.87-5.88. These values were higher than values obtained from BET
model; however, they present the same trend. Chirife and Iglesias (14) and Yanniotis (17),
reported average values of a monolayer (X0) of 7.36 for starchy foods at a temperature of
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30°C, using the BET model. Tsami et al., (18) reported values of X0 in the range of 9.7 of
17.3 in dried fruits (raisins, currants, figs, prunes and apricots) by using the GAB model.
More recently Erbas et al., (19), conducted a study on the behavior of the absorption
isotherms in semolina (from hard wheat) and farina (from soft wheat), reporting value X0 of
4.32-4.39 (by BET model) and 6.45-6.01 (by GAB model) at temperature 35 °C.
In the same way the values (20.29-75.82 to semolina y 8.77-31.95 to farina) of the
constant CB y CG, respectively, are within the ranges calculated in this communication.
Menkov and Durakova (2007) reported a value of X0 of sesame flour of 5.2 (using GAB
model). It can be seen that X0 values using the model of BET and GAB are the most
consistent with the results reported by other researchers (20).
The differences between UBF and acid-treatment banana flours (ATBF1, ATBF2, ATBF3
and ATBF4) suggest that the mechanism of water adsorption by chain starch involves
other factors than the hydration affinity of the characteristic groups, such as, the availability
of the polar groups in the molecule, the distribution and arrangement of these groups in
the starch granule, conformation, the degree of crystallinity and packing of the chains (21).
The water adsorption isotherms of the samples changes as a function of reaction time.
Water uptake has small increments as the acid reaction time increases in comparison with
UBF.
Table 1. Estimated values of coefficients and mean relative percentage deviation moduli
obtained for adsorption models applied to experimental adsorption data for UBF and
ATBF.
Models
BET
Samples
aw
0.1-0.7
%E
UBF
6.2
ATBF1
4.5
ATBF2
7.5
ATBF3
3.2
ATBF4
6.0
X0
4.8239
5.2966
5.4725
4.47
4.060
CB
82.92
94.4
16.6090
48.608
49.26
0.9921
0.9921
0.9932
99.45
0.9885
Constants
2
R
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GAB
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0.1-0.97
%E
6.9
4.3
5.9
6.2
4.0
X0
5.0032
5.4011
5.8791
4.7363
3.8764
CG
18.0305
18.6570
19.3363
17.8507
16.604
k
0.6520
0.6528
0.6515
0.650
0.6701
R
0.989
0.9767
0.97
0.989
0.974
%E
9.2
8.9
10.2
11.6
10.5
A
-16.92
-17.549
-18.396
-16.59
-15.873
B
5.4916
5.8909
6.3113
5.1601
5.1601
2
Smith
0.1-0.92
2
IglesiasChirife
0.1-0.7
R
5.4916
0.9735
0.9294
%E
9.9
B1
10.3
10.9
4.7318
5.2432
5.7762
5.2432
3.8888
B2
5.3367
5.4622
5.635
5.4622
5.0116
2
0.9884
0.9891
0.9894
0.9935
0.985
R
0.9735
9.7
0.9664
11.3
ATBF1 (one day treatment) is has practically the same water adsorption isotherm curve as
UBF. Once reaction time is increased, ATBF2 (five days treatment), ATBF3 (eleven days
treatment), isotherm curve presents a shift toward upper values of adsorbed water. This
tendency does not repeat with ATBF4 (twenty days treatment), on the contrary, it present
opposite behavior, adsorbed water are smaller than UBF. This apparently unusual
behavior can be fully explained in terms of changes in crystallinity.
The higher the reaction time the higher the crystallinity of the treated banana flour
(obtained by X-Ray diffraction), except for ATBF4, in which, crystallinity decreases.
Therefore there exists a correlation between crystallinity of the samples with water
adsorption isotherms, because in this ATBF crystallinity is smaller than the rest of the
samples and it is precisely when tendency changes, the curve is located underneath that
the one corresponding to UBF. In summary, adsorbed water in Banana Flour was found to
be a function of crystallinity. The values of the constants, B1 and B2, for the model of
Iglesias-Chirife for UBF and ATBF were in the range of 4.7-5-3 y 5.6-5.77, respectively.
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These values are higher than those reported by Erbas et al., (19) in semolina (from hard
wheat) and farina (from soft wheat). Chirife and Iglesias (14), and Ayranci et al., (21) applied
their model to evaluate the data sorption in nine foods with high sugar (mostly fresh fruits)
content. In this works, they reported that the model has a good fitness for foods with high
sugar content.
X-RAY DIFFRACTION ANALYSIS
The results of crystallinity percentage of ATBF and their UBF counterpart obtained by Xray diffraction were shown in Table 2.The percent of crystallinity for UBF was 19.3% and
ATBF1 of 18.9%. This corresponds perfectly with SEM micrographs because no obvious
external structure difference between UBF and ATBF1 for one day of acid treatment, was
observed (compare figure 2a with 2b). This is perhaps due to hydrolysis time was very
short to detect changes. However, when time of hydrolysis are increased (5 to 11 days)
the percentage of crystallinity raise up to 20.5% (ATBF2) to 22.2% (ATBF3). This is due to
the fact that acid treatment selectively removed amorphous regions of starch granules
(amylopectin) and, thus, yielded materials enriched in amylopectin crystallites, as was
demonstrated in previous results (22-25).
Table 2. Shows the values of crystallinity of the different samples
Sample
UBF
ATBF1
ATBF2
ATBF3
ATBF4
Crystallinity (%)
18.9
19.3
20.5
22.2
17.0
Usually the acid treatments of starches seem to produce partial rupture of the branching
points in the amylopectin molecule, increasing amylose (increasing crystallinity) content
such as Aparicio-Saguilán et al., (24) reported. Therefore, starches with the highest content
of amylose have the highest rate of retrogradation (22). One beneficial effect of
retrogradation would be the formation of this aid degradation structures (crystalline
structures organized best) which are highly resistant to enzymatic attack, forming resistant
starch. This is an interesting finding because this type of starch has been associated with
the prevention of diseases such as colon cancer, reduction of cholesterol and glucose in
blood, as well as problems of obesity (26, 24).
SCANNING ELECTRON MICROSCOPY (SEM)
SEM micrographs of UBF and ATBFS (Acid Traded Banana Flours) are displayed in
Fig. 2.The micrographs of banana starch and UBF are very similar and show the presence
of large oval and small quasi spherical granules (Figure 2a and 2b). The surface of the
granule appears to be smooth at that magnification, with no evidence of any fissures in
both samples. During the first day of reaction slight erosion and fissures of granular
surface was observed (Figure 2c). These fissures are formed due to the surface attack of
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the starch granule by the acid (27). It can be noted that the acid hydrolysis does not occur
uniformly because some granules are more intact than others.
Figure 2. Scanning electron microscopy of banana starch (a), untreated banana flour (b),
acid-traded banana flour 1 day (c), acid-traded banana flour 5 days (d), acid-trated banana
flour 11 days (e) and acid-treated banana flour 20 days (f).
Jiping et al., (28) propose that the acid acts by first attacking the surface and forming the
cracks on the surface, causing surface alternations and degrade the external part of the
granule by erosion. It is clear that, the more time granules are in the reaction vessel, the
more degradation they have. It is clear that as the reaction time increases, zones of
complete granules coalition appeared. These results are similar to those reported for other
researchers (29, 30). In that reports, it was showed that the acid preferentially attacked the
exterior of starch granules and then the interior, to generate this coalition.
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After 5 days of hydrolysis (Figure 2d), some granule starch show in the surface erosion
and destruction, due to extensive hydrolysis. This effect becomes more evident at the 11
and 20 days of the hydrolysis. The starch granules show fractures and some holes start to
appear (Figure 2e). Shujun et al., (30) found similar structures in yam starch when subjected
to 16 days of hydrolysis. After 20 days of acid hydrolysis, any intact starch granules could
not be found and all the fragments were conglomerate together due to the heavy acid
erosion (Figure 2f). This is due to all the starch granules were completely destroyed. This
phenomenon demonstrated that the hydrolysis time is a key factor in the process of acid
modification of starch generating different morphological structural and functional
properties.
CONCLUSIONS
Equilibrium moisture contents were found to increase with increasing water activity in acid
treated and non-treated banana flour.The behaviour of water adsorption for acid treated
and non-treated banana flour is best fitted with the BET model in comparison with GAB,
Smith, and Iglesias-Chirife model. Calculations of the monolayer moisture values obtained
from the models used in the present communication were comparable with other values
reported in the literature for various cereal grains and starches. Starch degraded by acid
treatment presents morphological and crystalline changes corroborated by X-ray
Diffraction and SEM techniques. Besides, adsorbed water in Banana Flour was found to
be a function of crystallinity.
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moisture adsorption behavior of the banana flours (musa

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