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Latin American applied research

Print version ISSN 0327-0793On-line version ISSN 1851-8796

Lat. Am. appl. res. vol.43 no.2 Bahía Blanca Apr. 2013

 

Effect of ultrasonic pre-treatment on water absorption characteristics of chickpeas (Cicer arietinum)

A. Ranjbari, M. Kashaninejad, M. Aalami, M. Khomeiri and M. Gharekhani

Department of Food Science & Technology, Gorgan University of Agricultural Sciences and Natural Resources, Beheshti Ave., Gorgan, 49138-15739, Iran
Abbasranjbari@yahoo.com, Kashaninejad@yahoo.com, Mehranalami@gau.ac.ir, M.gharekhani@yahoo.com, Mkhomeiri@yahoo.com

Abstract— In this study, the chickpea seeds were exposed to ultrasonic irradiation at vibration amplitude setting 40%, 70% and 100% of nominal power for 3 and 10 min at 24 kHz. Water absorption characteristics, moisture content, leaching loss and soaking water conductivity of chickpea at five different soaking temperatures (25, 30, 40, 50 and 60°C) and various soaking times up to 10 h were studied. The soaking rates of treated samples were compared with untreated seeds. Peleg's model was used to describe the hydration kinetics of chickpea seeds during soaking process at different temperatures by weight gain method. Ultrasonic treatment was very effective in water uptake of seeds and was observed that ultrasonic pre-treatment could decrease the soaking time by up to 4 h. The Peleg constant K1 decreased from 10.8×10-3 to 0.99×10-3 h %-1 with increasing temperature from 25 to 60°C and constant K2 increased linearly from 6.72×10-3 %-1 to 9.9×10-3 %-1with increasing the temperature. The leaching loss and conductivity value also increased significantly when the soaking temperature increased. In soaking process, leaching loss changed from 1.66/100 g to 12.27/100 g and conductivity changed from 367 to 1867 µS/cm as soaking temperature increased from 25 to 60°C.

Keywords— Ultrasonic Pre-treatment; Water Absorption; Peleg Model; Soaking; Chickpea.

I. INTRODUCTION

Chickpea (Cicer arietinum) is one of the most important legume crops in Iran, Indian, Australia, Turkey and other countries. Soaking is an integral part of a number of treatments, such as cooking, canning, germination, and fermentation. Soaking is a slow process controlled by the diffusion of water in the grain (Kashaninejad et al., 2007). The rate of water absorption has significance in the formulation of foods. It consists of hydration of the seeds in water, usually till they reach maximum weight, with or without discarding of the soaking medium, and obtained results depend on factors such as legume genus, species and variety, process duration, temperature, pH, salinity of the soaking media, and also the storage conditions undergone before processing. The absorption of moisture into the grain during soaking is influenced by the water temperature among other factors. The effect of soaking temperature on the hydration rate of legumes is pronounced and well documented (Hung et al., 1993; Singh and Kulshrestha, 1987; Sopade and Obekpa, 1990).

Warm-water soaking is a common method to shorten the soaking time. Increasing temperature increases hydration rate but the long times required for legumes soaking process is one of the drawbacks of food industry. One of the constant challenges that encounter food scientists is the development of new food processing technologies and new food products with specific functionalities. Ultrasound technology has a wide range of current and future applications in the food industry. Ultrasonic is a rapidly growing field of research and development in the food industry. This study has investigated the use of ultrasound as a pre-treatment prior to chickpea soaking. This process involves the immersion of the chickpeas in water to which ultrasound is applied. The advantage of ultrasound is that the process can be carried out at ambient temperature and no heating is required, reducing the probability of food degradation (Mason, 1998). The influence of time of ultrasound irradiation on the chickpea water diffusivity was examined. The ultrasonic pre-treatment was used successfully to decrease soaking time. The application of ultrasound in this study is important because it has effect on cell wall and texture and it can decrease the water absorption time of chickpea during soaking. Also, ultrasonic reduces treatment time and expenditures.

Mathematical modeling of hydration processes is known to be important for the design and optimization of food process operations (Turhan et al., 2002, Sopade et al., 2007). The Peleg's equation is a popular empirical non-exponential model and some of its parameters are of immense practical significance in hydration kinetics applied to weight gain during rehydration (Peleg, 1988, Singh and Kulshrestha, 1987, Turhan et al., 2002, Sopade et al., 2007). Peleg (1988) proposed a two-parameter sorption equation and tested its prediction accuracy during water absorption of food products. The original form of the Peleg model is as in Eq. (1), which can be rearranged to yield Eq. (2):

(1)
(1)

It follows that the absorption rate at the beginning of soaking process is expressed subsequently as showing that K1 is linked to water absorption rate (Peleg 1988).

(3)

The Peleg capacity constant, K2, relates to maximum attainable moisture content. As t → ∞, Equation (4) gives the relation between equilibrium moisture content (Me) and K2.:

(4)

The applicability of Peleg's equation has been demonstrated for some food products. Sopade and Obekpa (1990) reported that Peleg's equation could adequately describe the absorption behavior of some grain legumes such as soybean, cowpea and peanut in the temperature range 2°C to 42°C. Also Hung et al. (1993) have recorded that Peleg's model could be used to predict water absorption in chickpea and field pea. The objective of this study was to examine the capability of Peleg's equation in modeling the water absorption behavior of chickpea upon the application of a pre-treatment with ultrasound to characterize the equation constants. As the main properties of Peleg constants, K1 and K2, have identified well in previous studies (Sopade and Obekpa), a second aim of this study was to try to re-write Peleg's equation by substituting empirical relationships for K1 and K2.

II. MATERIAL AND METHODS

A. Preparation of samples

Chickpea (Cicer arietinum L.), Arman variety was obtained from the Gorgan filed crops Research Institute, Golestan, Iran. Sample was hand-selected to remove foreign material and broken, cracked and damaged grains. The chickpeas had an initial moisture content of 8% (wet basis). Chemical analyses of the samples were done according to AOAC (2006) using analytical grade reagents. Average chemical composition of the chickpea is summarized in Table 1.

Table 1: Average physico-chemical analysis of chickpea (g/100 g dry weight)

a Nitrogen × 6:25. b By difference from 100%. c Standard deviation.

B. Pre-treatment with ultrasound

Chickpeas (100 g) were soaked in distilled water (250 ml) at 20°C. Ultrasonic irradiation was performed by UP 200 H ultrasonic processor horn type in 40%,70%,100% waves amplitude (P1, P2, P3) for 3 and 10 minutes (t1, t2), (24 kHz, , and maximum power density of 600 W). Ultrasound designed by Dr. Hielscher GmbH (Treptow, Germany) and had a radial sonotrode S14 (14 mm diameter, maximum immersion depth of 90 mm). At the end of the specified time, the soaked chickpeas treated with ultrasound were removed from the water and placed onto two layers of paper towels. The difference between the sample weight after ultrasound treatment and the original weight was taken to calculate the moisture content on % d.b (Dry basis). In order to prevent absorption of moisture, they were stored in a dry place at 20 °C before using.

C. Determination of the kinetics of water absorption

Water uptake of the chickpeas was determined by soaking of around 5g of each treated sample in 50 ml of distilled water at six different soaking temperatures of 25,30, 40, 50 and 60°C (T1, T2, T3, T4, T5) for 600 min. The samples along with the water soaking were kept in a water bath (WNB 14, Memmert GmbH Co., Germany) with a temperature control accuracy of ±0.5°C. About of 15 beakers were prepared for each sample. The chickpea samples were soaked at each temperature for several times and soaked samples were taken out at different time intervals. After reaching the required soaking time, the sample was drained on a paper and the excess water eliminated with adsorbent paper, and the soaked chickpea were weighed with an electronically balance (Sartorius, TE3135, 0.001 g. Canada) to determine moisture content. All samples were studied in triplicate and the average result noted as percent moisture on dry basis (% d.b). The weight uptake was calculated as;

(5)

D. Leaching loss

Leaching loss was determined after each of hydration experiment. The water in the filtrate was allowed to evaporate by keeping in an oven maintained at 105°C followed by weighing the dry residue. The leaching loss was calculated as the weight of solids present in the filtrate divided by the initial weight and expressed as mg solids per g initial sample.

E. Conductivity of soak water

Conductivity of soak water which is a measure of electrolyte leakage was measured by a Conductimeter (Cond-model 720, Inolab Research, Germany). Sufficient amount of distilled water was added to the whole filtrate (not aliquot) to make up to a total volume 50 ml and then, the measurement was carried out. The result was expressed as µS/cm.

F. Analysis of soaking data and kinetic model for the soaking time

All determinations were in triplicate and reagents were analytical grades. To prevent absorption of moisture, they were stored in a dry place at 20°C until required. The effects of soaking time and temperature on water uptake and equilibrium moisture content of wheat kernels were determined using the analysis of variance (ANOVA) method and significant differences of means was compared using the Duncan's test at 95% significant level. Mathematical modeling of hydration processes is known to be important for the design and optimization of food process operations. In the analysis of water absorption data of chickpeas, the moisture ratio (MR) is essential to describe Peleg's model:

(6)

After calculation the moisture ratio, the Peleg model was fitted to the soaking data. There are several criteria such as coefficient of determination (R2), root mean square error (RMSE), and residual plotting to evaluate the fitting of a model to experimental data.

Non-linear regression procedure was performed on all soaking runs to estimate the parameters associated with Peleg's model from the experimental data using SPSS software (version 16.5, 2005).

III. RESULTS AND DISCUSSION

A. Effect of ultrasound on increasing the initial moisture content

Chickpea soaking procedures utilized in the food industry depend mainly on soaking the chickpeas for lengthy periods such as 12-16 h in order to ensure full hydration (Turhan et al., 2002). Apart from increasing processing time, the long exposure of the chickpeas to the hydration medium often resulted in bacterial growth. The efficacy of ultrasound power on the increase of samples initial weight after 3 (t1) and 10 (t2) min of irradiation was investigated using cavitation levels between 40 (P1) and 100% power setting of the device (Table 2). Maximum water absorption in pre-treatments was achieved with 100% (P3) power setting for 10 min. Increasing ultrasound power and irradiation time improved water absorption and showed an increase from only 7.75% (w.b.)(Wet basis) initial moisture of non-sonicated chickpeas to 19.75% (w.b.) for chickpeas treated for 3 min at 40% (P1) of power setting or 32.87% (w.b.) for treated 10 min at 100% (P3) power setting. The interactive effect between time and power of ultrasonic pre-treatment was significant at the 5% level (P>0.05).

Table 2: Interaction between power of Ultrasonic waves (40, 70 and 100%) and Irradiation time (3 and 10 min) on sample moisture content (w.b%)

As shown, the sample absorbed water at high power faster than at low power. Equilibrium power was dependent on time in water absorption. The results indicated that treated sample with ultrasound absorbed water faster than the control. In this study, pre-treatment with ultrasound was found to have a significant effect on increasing the initial hydration rate of chickpeas, and thus equilibrium condition were attained in much shorter times compared to un-treated chickpeas. The most probable mechanism for ultrasonic enhancement of water absorption is the intensification on mass transfer and easier access of the water to the interior of the cell wall structure. The collapse of cavitation bubbles near cell wall would be expected to produce cell disruption together with good penetration of water into the cells, through the ultrasonic jet. Also, other reason may be rupture of large molecular structure that induce more water absorption points and consequently, formation of hydrogen bonding between molecular water and the structure lead to more water absorption in sample treated with ultrasound. (Toma et al., 2001; Mason, 1998; Hebling and Dasilva, 1995; Yaldagard et al., 2008). Moreover, Chickpea sonication prior to soaking has the advantage of enhancing the plasticity of the seed coat as well as eliminating the presence of hard shell chickpeas which decrease water absorption during soaking. The enhanced plasticity of the beans was demonstrated in the formation of rubbery seed coats and nearly a similar high hydration rate for the five soaking temperatures studied. Consequently, the ultrasonic pre-treatment was successful in increase of initial moisture content.

B. Water absorption in soaking process

Initial moisture content of chickpeas was very different of the absorption curves but they followed a similar pattern. The pattern is characteristic of the diffusive processes at temperatures below the gelatinization temperature of the product. Initial water absorption has rapid rate which is followed by a slower rate in the later stages and comes to equilibrium or saturation moisture content.

For moisture uptake in soaking process, effects between power of ultrasound (P), time of irradiation (t) and temperature (T) of soaking were significant at the 5% level (P>0.05) (Table 3). Effects of soaking time and temperature on water uptake of untreated chickpeas by ultrasound are shown in Fig 1. Figure 2 shows the effect of soaking time on water absorption of chickpea at 70% power of ultrasound and irradiation time of 10 min at different temperatures. It can be seen that the initial moisture content in raw chickpeas is low (m0= 7.5% on wet basis), but treated chickpeas by ultrasound had higher initial moisture content (19-33%, on wet basis). Water absorption in treated chickpea took place in shorter times than un-treated chickpea during of soaking process. These results demonstrated that water absorption of chickpea seed was improved by sonication. The decrease in average of water absorption time with sonication may be due to mechanical effect of ultrasound and cavitation cell disruption. However, the moisture content of the raw sample is similar to that of the treated samples after 250-300 min of soaking time depending to the temperature soaking.

Table 3: Effect of different soaking treatments on moisture content, solid loss and conductivity.

* P (P0, P1, P2 and P3), t (t0, t1 and t2) and T (T1, T2, T3, T4 and T5) show Power of Ultrasonic waves (0, 40, 70 and 100%), Irradiation times (0, 3 and 10 min) and soaking temperatures (25, 30, 40, 50 and 60°C), respectively.


Fig 1. Effects of soaking time and different temperature on water uptake of chickpeas.


Fig 2. Effects of soaking time at 70% power of ultrasound and 10 minute irradiation on water uptake of chickpeas

Although the moisture content increased continuously with time, a closer examination of data revealed some interesting patterns, valid for all temperatures. Analysis of variance revealed that time and temperature had significant differences. The samples exhibited the characteristics moisture absorption behavior. During the first 30 min, the moisture content increased sharply. Between 30 and 300 min, depending on temperature, the rate of increase in moisture content was, more or less, uniform (Figs. 3 and 4). A rapid initial water uptake during soaking of chickpea is probably due to the filling of capillaries and cracks on the surface of the kernel. These cracks and capillaries allowed to enter water rapidly, the rate of which increases as the temperature increases (Tagawa et al., 2003).


Fig 3. Effects of different soaking temperature at 40% power of ultrasound and 3 minute irradiation on water uptake of chickpeas


Fig 4. Effects of different soaking temperature at 100% power of ultrasound and 10 minute irradiation on water uptake of chickpeas.

Chickpeas soaked at higher temperatures absorbed more water than samples soaked at lower temperatures (P<0.05). A regular increase in water absorption was observed as temperature increase. From 25°C to 60°C, Chickpeas soaked at higher temperatures absorbed more water than grains soaked at lower temperatures because of an increased water diffusion rate. The obtained results about of temperature effect is in agreement with published studies (Hsu, 1983; Sopade and Obekpa, 1990; Lu et al., 1994); Temperature-induced softening has been reported for soybean (Singh and Kulshrestha, 1987), chickpea (Hung et al., 1993), kidney bean (Abu-Ghannam and McKenna, 1997), white rice (Kashaninejad et al., 2007), wheat (Maskan, 2001), rough rice (paddy) and milled rice (Miah et al., 2002). Soaking temperature closer to the gelatinization of starch resulted in greater water uptake.

Effect between soaking temperature and soaking time was significant (P>0.05) on moisture uptake. The amount of water absorbed per unit weight increased with time. Equilibrium time in water absorption was temperature depended. Maximum water absorption varied non-significantly, but soaking at elevated temperatures increased the rate of water absorption and decreased the time required for maximum absorption. Generally, the rate of water absorption is initially rapid and then slows down as equilibrium approaches and increasing extraction rate of soluble solids from the grains. This asymptotic behavior is related to the decrease of the driving force for water transfer as hydration progresses and system is close to equilibrium. The extraction of soluble solids in the reverse direction to the water movement offers more resistance to the water transfer (Abu-Ghannam and McKenna, 1997). The influence of initial moisture content and physico-chemical characteristics on absorption capacity has been studied by several research groups (Hsu, 1983). Hsu (1983) reported negative correlation between absorption capacity and kernel size (and specific volume). But it appears that the effect of chemical composition of food materials on absorption capacity is still uncertain. Jambunathan et al. (1981) reported that main structure for controlling the rate of water absorption during legume soaking is the seed coat. The ratios of coat thickness to seed size and to some extent, seed coat structure were the only obvious differences between the Desi and Kabuli chickpea types (Jambunathan et al., 1981).

C. Leaching loss and conductivity of soak water

The amount of solids and electrolytes in soaking water increased with hydration time and temperature (Table 3). Furthermore, as hydration time progressed chickpeas began to split and some parts disintegrated into the soak water due to the softened grain structure. Similar results were found by Singh and Kulshrestha (1987). The leached solids in chickpea and other similar products have been reported to be phytic acid, non-protein nitrogenous compounds, sugars, minerals (Fe, Cu, Zn, Mn, P, Ca, Mg), water soluble vitamins such as thiamine, riboflavin and niacin (Koksel et al., 1999). For leaching loss the interactive effect between power of ultrasound, time of irradiation and temperatures of soaking was significant at the 5% level (P>0.05). With the increase of temperature from 25 to 60°C, the leaching loss changed from 1.66 g/100 g related to sonication at 100% (P3), 10 (t2) min and 25°C treatment, increase to 12.27 g/100 g related to sonication at 40% (P2), 3 (t1) min and 60°C. This amount for control sample was 2.56 g/100 g at 25°C and 13.49 g/100 g at 60°C. These result showed that use of ultrasound technology can be useful in decrease the leaching loss. Effect between power of ultrasound and soaking temperature was significant (P>0.05) for conductivity values of soak water. Also, conductivity values of soaked water increased from 367 to 1867 µS/cm with increasing of temperature from 25 to 60°C. For control sample conductivity changed from 540 µS/cm at 25°C to 2238 µS/cm at 60°C. A linear relationship was obtained between leaching loss and conductivity values (Fig. 5). Similar results reported by Maskan (2001). It has been noted that the electrolytes found in the leached solids cause the conductivity of the solution to increase and determine the structure of soaked product (Resio et al., 2006). Since most of canned food is drained before eating, the knowledge of leaching loss and electrolyte concentration in the soak water is essential.


Fig 5. Linear relationship between leaching loss and conductivity values.

The power of ultrasound, time of sonication treatment and temperatures of soaking should be optimized in order to maximize nutrient retention. These hydration processes could bring improvements in preparation of semi ready-to-eat foods and the commercial practice of canning of legumes. The leaching loss and electrolyte concentration in soak water during hydration could be used to maximize nutrient retention and predict the size of processing and handling equipments (Maskan, 2001). To keep intact grain and soaking losses at minimum, hydration of chickpeas at temperatures lower than 60°C would be most suitable.

D. Modeling and statistical analysis

Plot of t/(Mt -Mo) versus soaking time (t) allows the characteristics of the constants to be studied. The Peleg's rate constant K1 and capacity constant K2, coefficient of determination (R2) and root mean square error (RMSE) calculated. The Peleg model evaluated based on coefficient of determination (R2), root mean square error (RMSE) and mean relative deviation modulus (P). The details of the statistical analysis for chickpea, determined at each of temperatures were presented in Table 4.

Table 4: Peleg's model parameters and Statistical results obtained at different soaking conditions of chickpeas.

* P (P0, P1, P2 and P3), t (t0, t1 and t2) and T (T1, T2, T3, T4 and T5) show Power of Ultrasonic waves (0, 40, 70 and 100%), Irradiation times (0, 3 and 10 min) and soaking temperatures (25, 30, 40, 50 and 60°C), respectively.

The coefficient of variation (R2) values varied from 0.988 to 0.999. This confirms the adequacy of the equation for describing the water absorption kinetics of chickpea within the temperature range investigated. In this study, K1 values were inversely related to temperature. This indicated increased water absorption rate at higher temperature. The Peleg constant K1 decreased from 10.8×10-3 to 0.99×10-3 (h/%d.b.) while the soaking temperature increased from 25 to 60°C. Increasing water absorption rate, namely decreasing K1, with increasing temperature, is an expected sorption behavior. Note that at a given temperature, the lower K1, the greater the amount of water absorbed. This observation is in agreement with previous studies for different beans, peas and cereal grains (Sopade and Obekpa, 1990; Hung et al., 1993; Turhan et al., 2002; Resio et al., 2006).

It is observed that the Peleg constant K2 defined the equilibrium moisture content for chickpea and was function of temperature (Table 3). Similar trends have been observed for chickpea (Sayar et al., 2001, Turhan et al., 2002), pigeon pea (Singh and Kulshrestha, 1987) and red kidney beans (Abu-Ghannam and McKenna, 1997). Study of Water absorption by using the Peleg model showed inverse results about of effect of temperature on K2 with previous reports of other workers (Sopade and Obekpa., 1990, Hung et al., 1993, Maharaj and Sankat, 2000).

Table 4 showed that the constant K2increased with increasing of soaking temperature. K2 values were in range of 6.72×10-3 %-1 to 9.9×10-3 (h/ % d.b.) for different treatments. This is due to decreasing of water absorption capacity of chickpea with increasing of temperature. Sopade and Obekpa (1990) reported that the Peleg constant K2 is inversely related to the absorption ability of food. This statement is in agreement with our results that with increasing soaking temperature, equilibrium moisture content of chickpea increased (Table 3). Effect of temperature on water absorption capacity of food materials, namely on K2, is mixed and depends on the type of material and if soluble solids loss during soaking is considered in the calculation of moisture content of samples (Sayar et al., 2001, Turhan et al., 2002). Soluble solid loss was due to the enhanced plasticity of grain cells at high temperatures during soaking. Therefore, the grain absorbed more water at high temperatures. Although it was not constant in all published investigations on soaking, an average value of K2 has been used in some resulting equations (Hung et al., 1993 for some chickpea cultivars; Abu-Ghannam and McKenna, 1997 for unblanched beans).

IV. CONCLUSION

This study showed that using ultrasound as a pre-treatment can decrease the water absorption time of chickpea during soaking. Ultrasonic pre-treatment increased initial moisture content of chickpea significantly. Also, ultrasound decreased the hydration time in soaking process. The Ultrasonic pre-treatment was most effective for soaking at temperature below 60°C. Water absorption was associated with leaching loss and conductivity of soak water. In all cases, the rate of water uptake, leaching loss and conductivity of soak water increased with increasing temperature and soaking time. Peleg's equation represented water absorption behavior of chickpea during soaking process at different temperatures, successfully and could be used to estimate the moisture content at a given soaking time and temperature within experimental conditions. The Peleg K1 is related of temperature for chickpea and decreased with increasing of soaking temperatures but K2 increased with increasing of temperature.

MOMENCLATURA

K1

Peleg rate constant , (h / % d.b.)

K2

Peleg rate constant , (h / % d.b.)

Mt

Moisture content, at time t

M0

Initial moisture content (% d.b.)

Me

Equilibrium moisture content (% d.b.)

MR

Moisture ratio

N

Soaking constant

P

Mean relative deviation modulus

W

Mass of wet wheat kernel (g)

W0

Initial mass of wheat kernel (g)

P

Power of waves amplitude (%)

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Received: January 10, 2011
Accepted: July 30, 2012
Recommended by Subject Editor: María Luján Ferreira

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