International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 1||Issue|| 10 ||Pages|| 22-36 || November 2018|| ISSN: 2581-4540
Moisture Adsorption Isotherms Characteristics of a New Moisture Adsorbers 1,
Yué Bi Yao Clément, 2,Bouatene Djakalia, 3,Tano Kablan
Department of Food Science and Technology, Nangui Abrogoua University, 02 BP 801 Abidjan 02 (Côte d’Ivoire) Department of Food Science and Technology, Nangui Abrogoua University, 02 BP 801 Abidjan 02 (Côte d’Ivoire) Department of Food Science and Technology, Nangui Abrogoua University, 02 BP 801 Abidjan 02 (Côte d’Ivoire)
----------------------------------------------------ABSTRACT----------------------------------------------------The purpose of the study was to analyze the effect of the replacement rate of the most hygroscopic moisture adsorber on the adsorption properties of a new moisture adsorbers formulated. Moisture adsorption isotherms of this new moisture adsorbers formulated were determined at 4 ± 1 °C, using the standard static gravimetric method. All moisture adsorption isotherms of new moisture adsorbers were type II following Brunauer’s classification. The moisture isotherms were sigmoid shaped and showed a clear dependence on the substitution rates of the most hygroscopic moisture adsorber (ads 3). Of the eighteen new moisture adsorbers formulated, only AB (40:60) and AL (60:40) adsorbers had the characteristic appearance of better moisture adsorbeers in modified atmosphere packaging of fruits and vegetables (tomato and mushroom). Thus, their incorporation into the packaging could avoid the condensation of water vapor by maintaining an optimal relative humidity. Based on statistical parameters, additive isotherm approach of Labuza (1968) could not correctly describe the adsorption characteristics of all this new moisture adsorbers formulated. However, this model gave better prediction with the new AB moisture adsorbers than with the new AL moisture adsorbers with sums of relative mean errors of 165.89% and 293.71%, respectively.
KEYWORDS - adsorption isotherms, moisture adsorber, modeling. ------------------------------------------------------------------------------------------------------------------------------------------Date of Submission: Date, 09 November 2018 Date of Accepted: 13 November 2018 -------------------------------------------------------------------------------------------------------------------------------------------
I.
INTRODUCTION
Polymeric films used in fresh fruit and vegetable packaging have lower rates of water vapor transmission than fresh products. As a result, high moisture conditions prevail in the packages, resulting in condensation of moisture, microbial growth and product degradation. One possible solution for controlling humidity is to use moisture adsorbers. However, existing moisture adsorbers have low adsorptive capacity and / or adsorb moisture quickly, making them unfit for fresh fruits and vegetables. Previous studies have already made it possible to mix fast-adsorbing moisture adsorbers such as sorbitol with a slow-adsorbing desiccant, such as silica gel, in different proportions [1, 2]. However, these studies have not been able to develop a new generation of moisture adsorbers with high retention capacity and slower moisture adsorption rates for fresh fruits and vegetables such as mushrooms and tomatoes. Moisture adsorption isotherms illustrate the steady-state amount of water held by the food solids as a function of aw at constant temperature [3]. Thus, the moisture sorption isotherm is an extremely valuable tool for food scientists since it can be used to predict which reactions will decrease stability at a given moisture; it allows for ingredient selection to change the aw to increase stability and can be used to predict moisture gain or loss in a package with known moisture permeability [4]. Moisture sorption isotherms in foods are of special interest in many aspects of food storage, drying, preservation by dehydration, especially for the prediction of the shelf life of a dried product in a packaging material or the prediction of drying times of foodstuffs [5]. Moisture adsorption isotherm moisture adsorbers describes the relationship between equilibrium moisture content and relative humidity at constant temperature [6]. It gives an insight into the moisture binding characteristics, sorption mechanisms, and interaction of food biomaterial with water.
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Moisture Adsorption Isotherms Characteristics‌ The moisture sorption properties of moisture adsorbers have been shown to be influenced by their composition, processing treatment, temperature, and relative humidity [5]. In addition, it is also extremely important for predicting storage life and appropriate packaging material [7]. Many of the useful properties of different moisture adsorbers are related to their adsorption capacity of water. The moisture adsorption behavior of the various moisture adsorbers is important to assess their stability, to understand how the water migrates in them and to determine how they are gaining or losing water. The influence of moisture on solid state stability has been the subject of numerous studies to understand and anticipate the behavior of an adsorber exposed to various environmental conditions. This is necessary to recommend appropriate storage conditions to maintain the quality of a moisture adsorber [8]. The interaction between a powdered moisture adsorbers and water may affect its chemical stability and its physical and mechanical properties. Understanding the moisture adsorption isotherms of the adsorber is also very important in food science and technology. It makes it possible to predict the hygroscopic behavior of new mixtures of two moisture adsorbers. This makes it possible to design and optimize the stability predictions of these new moisture adsorbers in modified atmosphere packaging of fresh fruits and vegetables, guaranteeing the safety, quality and stability of these foods [4]. As each moisture adsorber powder has a single isotherm, the mixture of two moisture adsorbers will also have unique isotherms depending on their level of substitution and the state in which they are in the formulated adsorbers. As indicated previously, it would be useful to predict the isotherms of these new moisture adsorbers formulated to predict the understanding of the moisture relationship of these new moisture adsorbers formulated with the environment during their formulation. Thus, the objective of this work was to provide experimental data for the adsorption characteristic of a new moisture adsorbers formulated to model the adsorption isotherms using additive isotherm approach of Labuza [3].
II.
MATERIAL AND METHODS
Materials: Sodium bicarbonate, "beta" lactose and four polymers of polyacrylate acids or water super adsorbent polymers (ASAP 02942 = water adsorbent 1; NALCO 93QS028 = water adsorbent 2; NALCO 1181 = water adsorbent 3 and LIQSO = water adsorbent 4), in the form of granular powders were used in this study. Sodium bicarbonate, "beta" lactose and the various salts that are: LiCl, CH3COOK, MgCl2, K2CO3, Ca (NO3)2, NaNO2, NaCl, KCl, KNO3, K2SO4 came from Polychimie (Abidjan, CĂ´te d'Ivoire) When polymers of super absorbents of water, they were from Nalco Chemical Company (Naperville, Illinois, USA). Composition of new moisture adsorbers : Two new types of moisture adsorbers (AB and AL) have been formulated manually. Moisture adsorbers (AB) consisted of mixtures of sodium bicarbonate and the most hygroscopic of the four water super absorbent polymers. Those of the type AL were obtained by the mixtures of "beta" lactose and this same water super absorbent polymers. These different mixtures were made at masscontrolled ratios corresponding to 10, 20, 30, 40, 50, 60, 70, 80 and 90% most hygroscopic water super adsorbent polymer (Table 1 and 2). Table 1: Composition of new formulated moisture adsorbers (AB): mixtures of most hygroscopic water super adsorbent polymers and sodium bicarbonate new moisture adsorbers formulated (AB) AB (10:90) AB (20:80) AB (30:70) AB (40:60) AB (50:50) AB (60:40) AB (70:30) AB (80:20) AB (90:10)
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% most hygroscopic water super adsorbent polymers 10 20 30 40 50 60 70 80 90
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% sodium bicarbonate 90 80 70 60 50 40 30 20 10
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Moisture Adsorption Isotherms Characteristics… Table 2: Composition of new formulated moisture adsorbers (AL): mixtures of most hygroscopic water super adsorbent polymers and “beta” lactose new moisture adsorbers formulated (AL) AL (10:90) AL (20:80) AL (30:70) AL (40:60) AL (50:50) AL (60:40) AL (70:30) AL (80:20) AL (90:10)
% most hygroscopic water super adsorbent polymers 10 20 30 40 50 60 70 80 90
% “beta" lactose 90 80 70 60 50 40 30 20 10
Moisture adsorption isotherms : The equilibrium moisture content was determined by standard gravimetric method by exposing the moisture adsorbers to constant relative humidity environment created by saturated solution of salt at room temperature. Ten different salts viz., LiCl, CH3COOK, MgCl2, K2CO3, Ca (NO3)2, NaNO2, NaCl, KCl, KNO3, K2SO4 were used to maintain respective water activity (aw) inside separate vacuum desiccators in the range of 0.11 to 0.98. Moisture adsorbers samples (2±0.001 g) were placed in previously weighed aluminum dishes and paced in sealed glass jars. A small quantity of toluene was placed in each sorption jar to prevent fungal activity [9]. The glass sorption jars were placed in a temperature controlled (refrigerator), with an accuracy of ± 1°C at the selected temperature 4°C. Equilibrium was reached when the sample weight difference between two successive measurements was less than the balance accuracy 0.001 g. The equilibrium moisture content of the samples was determined by drying in a vacuum oven at at 105°C for 24 hours. Each set of experiment was repeated thrice, and mean values were recorded. Modelling of adsorption isotherms : Several methods have been investigated in the literature for predicting isotherms for multicomponent powder. The most common method has been using an additive isotherm approach. Labuza [3] suggested that it can be assumed that the amount of water adsorbed at any water activity can be predicted by the mass weighted addition of the moisture that the components would sorb alone, assuming no interactions between components occur. This approach, shown by equation 1, has been used by several authors [10, 11, 12, 13, 14] with satisfactory results being found in some case [10, 12, 13]. This additive isotherm approach model of Labuza [3] was used to predict experimental adsorption isotherms of a new moisture adsorbers formulated (AB and AL). The choice of this model was due to its ease of application and its wide use in the literature [15]. With the knowledge of the experimental adsorption isotherm of each moisture adsorbers, the search for the prediction of the moisture isotherms of the different mixtures of moisture adsorbers formulated seems more than necessary. So, when two components A and B are mixed or contacted with water contents of MAi and MBi, respectively at the same water activity, the diffusion of water will occur from the most hygroscopic material at least hygroscopic with water until thermodynamic equilibrium. The moisture adsorption isotherms of the mixture can be predicted by averaging the water contents according to the Labuza equation [3]: Mi = fA ×MAi + fB × MBi (1) Where: Mi = the total water content of the mixture at a given water activity; MAi = water content of component A at the same activity of water; MBi = water content of component B at the same water activity, fA = dry mass fraction of A (g of solid of A) / (g of total solid); fB = dry mass fraction of B (g of B solid) / (g of total solid). The parameters of the adsorption model were estimated from the experimental results using the nonlinear regression analysis (SPSS 9.0 for Windows) which minimizes the residual sum of squares. Best fitting equations were evaluated with the mean relative percentage deviation (E%) value. E% value is defined as: E% =
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,
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Moisture Adsorption Isotherms Characteristics… Where Mei and Mci are experimental and predicted moisture content values, respectively, and N is the number of experimental data. A model is considered acceptable if the E values are below 10% [16, 17].
III. RESULTS 3Comparison of the experimental moisture adsorption isotherms of four water super adsorbent polymers, sodium bicarbonate and “beta” lactose : The flattest moisture adsorption isotherms were observed for sodium bicarbonate and beta lactose (Fig. 1). It was observed that “beta” lactose was the least susceptible to environmental water and only above the water activity of 0.96 was a very intensive increase in moisture content observed in that material. This tendency was demonstrated for powder and agglomerate. The same was true for sodium bicarbonate which exhibited an intensive increase in moisture content was observed above aw = 0.96. Maximum adsorbed water in the environment with aw = 1 by lactose was determined at the level of approximately 24.09 g H2O/100 g solids and this value was four times less than that of sodium bicarbonate (107.31 g H2O/100 g solids). According to the classification of Brunauer et al. (1940), all adsorption isotherms obtained exhibited Type III behavior, in which a little water was adsorbed at below water activity and a larger amount was adsorbed at higher water activity, and once the bulk moisture point has been reached, the powder rapidly adsorbed high amounts of water vapor, causing it to deliquesce and leading to a steep rise in the third part of the curve, corresponding to the formation of hydrate [18]. The linear shape at the first part of the isotherms was caused by water adsorption on to the biopolymers and the sharp increase in water content at high water activity was due to the gradual dissolution of solutes such as salts and sugars [19]. These results suggested that the sodium bicarbonate and beta lactose were characterized by high hygroscopicity because of high solute content (salt and sugar) mostly in the amorphous state, which promotes undesirable effects (e.g. caking). Al-Muhtaseb et al. [19] reported that foods rich in soluble components show isotherms with Type III behavior owing to the solubility of the components in water. Similar isotherm behavior has been found in crushed chilies [18], pistachio powder [20] and model fruit powder [21]. The shape of those moisture isotherms may be characteristic for crystalline objects [22, 23]. It should be noted that sodium bicarbonate is significantly more hygroscopic than beta lactose. The four water super adsorbent polymers (ads 1, ads 2, ads 3 and ads 4) showed sigmoid sorption isotherms (Figure 1). According to the BET model, they may be classified as type II, characteristic for most food products [23)). They were significantly more hygroscopic (ads 1 = 145.2, ads2 = 192, ads 3 = 228.6 and ads 4 = 172 g H2O/100 g solids) than sodium bicarbonate (107.31 g H2O/100 g solids) and “beta” lactose (24.09 g H2O/100 g solids). Like most food products, these four water super adsorbent polymers had a low water adsorption capacity at aw ≤ 0.4 and a high-water adsorption capacity at aw = 1. Similar curves were reported for several raw and processed starch containing food materials such as potatoes [24], rice flour [25, 26]. of the six moisture adsorbers characterized hygroscopically, the water super absorbent polymers abs 3 was the most hygroscopic. The mixture of abs 3 with one of the least hygroscopic two moisture adsorbers that were sodium bicarbonate and “beta” lactose, would give new moisture adsorbers to intermediate adsorption capacity.
Fig. 1: Comparison of moisture adsorption isotherms of four water super adsorbent polymers, sodium bicarbonate and "beta" lactose
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Moisture Adsorption Isotherms Characteristics‌ Comparison of experimental adsorption isotherms of new formulated moisture absorbers : Analysis of the moisture sorption isotherms of components is important for the possible evaluation of an effect of the change in composition of the raw material on the sorption capacities of the water shown by their mixtures. The experimental adsorption isotherm of the new formulated moisture adsorbers (AB and AL) had distinct sigmoid patterns of type II. The moisture contents of these formulated moisture adsorbers increased with the increase of the amount of ads3 incorporated in the mixture (Fig. 2 and 3). This resulted in new moisture adsorbers with water contents ranging from 107.31 to 197.5 g H2O/100 g solids for AB mixtures and from 48.09 to 216.96 g H2O/100 g solids for AL mixtures. Dominating abs 3 content determined the course and shape of the isotherm of the formulated moisture adsorbers. Partial replacement of abs 3 in mixtures (AB and AL) had a significant effect on moisture adsorption isotherms. Thus, mixtures AB (90:10) and AL (90:10) had the highest moisture contents (197.5 and 216.96 g H2O/100 g solids, respectively) at higher water activity (≼0.99). The sorption isotherms of the different mixtures had a very similar evolution (Fig 2 and 3). This was associated with the water adsorption properties of abs 3. A powerful effect of this compound on the plot of curves and a significant increase in the amount of adsorbed water are visible in the case of mixtures. The water contents of these formulated mixtures were significantly higher than those of the least hygroscopic base absorbent. The analysis of the sorption isotherms of these mixtures clearly showed the most dominant effect of the addition of abs 3. Presence of this compound in the composition was reflected by very high amounts of adsorbed water, especially for values over aw = 0.56. Increasing moisture content as a function of the amount of abs 3 incorporated in the formulated moisture adsorbers was due to an increase in the number of active sites due to chemical and physical changes induced by abs 3. The magnitude of this increase was dependent on the composition food [23]. According to Pedro et al. [27], although the isotherms of pure passion fruit pulp powder (PP) and additives have shown the same qualitative behavior, considerable differences in the values of equilibrium moisture content for the studied samples was noted. It was observed that the isotherms of the PP samples showed higher values of equilibrium moisture content than samples containing maltodextrin and gum arabic. These results showed that these additives cause changes in the hygroscopic behavior of dehydrated passion fruit pulp. The decrease in equilibrium moisture content in juices spray dried with additives reflected a reduction in the number of active sites able to link with water, that are ordinary in juices, as hydroxyl groups of small carbohydrates. These groups were linked with the carrier or were immobilized by the carrier.
Fig. 2: Effect of change of ads 3 incorporation rate on moisture adsorption isotherms of new moisture adsorbers formulated (AB)
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Moisture Adsorption Isotherms Characteristics‌
Fig. 3: Effect of change of ads 3 incorporation rate on moisture adsorption isotherms of new moisture adsorbers formulated (AL) Prediction of experimental adsorption isotherms of new absorbents (AB and AL) : The experimental and theoretical moisture adsorption isotherms of the new moisture adsorbers (AB and AL) were close to the experimental adsorption isotherms of the component having the highest incorporation rate in the adsorbers formulated (Figure 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i and 5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h, 5i). The experimental adsorption isotherms of the mixture AB (50:50) and AL (50:50) were equidistant between the experimental isotherms of their respective two basic components. The new moisture adsorbers AB whose incorporation rates of abs3 were less than or equal to 50% presented experimental adsorption isotherms and theoretical very close. At abs 3 substitution rates greater than 50%, the theoretical adsorption isotherms were generally superior to the experimental (Figure 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h and 4i). (a)
(b)
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Moisture Adsorption Isotherms Characteristics… (c)
(d)
(e)
(f)
(g)
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(h)
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Moisture Adsorption Isotherms Characteristics‌ (i)
Fig. 4: Agreement between the experimental and predicted moisture adsorption isotherms of new moisture adsorbers formulated (AB): (a (10:90), b (20:80), c (30:70), d (40:60) e (50:50), f (60:40), g (70:30), h (80:20), i (90:10)). It is the same for the moisture adsorbers AL, when the substitution rate of abs 3 is higher than 20%, the theoretical adsorption isotherms were superior to the experimental adsorption at aw of 0 to 0.75 (5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h and 5i). There was a difference between the predicted and experimental moisture content at these water activities. At higher water activity (aw ≼0.75), experimental and predicted values were close. It should be noted that the new moisture adsorbers AL (10:90) and AL (90:10) had experimental adsorption isotherms close to the theoretical values, compared to the other substitution rates (Figure 5a and 5i). (a) (b)
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Moisture Adsorption Isotherms Characteristics… (c)
(d)
(e)
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(f)
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Moisture Adsorption Isotherms Characteristics‌ (g)
(h)
(i)
Fig. 5: Agreement between the experimental and predicted moisture adsorption isotherms of new moisture adsorbers formulated (AL): (a (10:90), b (20:80), c (30:70), d (40:60) e (50:50), f (60:40), g (70:30), h (80:20), i (90:10)). Depending on the ads 3 amount used in the formulation of the new moisture adsorbers (AB and AL), the mean relative percentage deviation E (%) made it possible to distinguish three types of approximation between the experimental moisture content and those predicted (table 3 and 4). For AB mixtures, when the proportion of abs3 in the mixture was 50% or less, the predicted values were significantly (p <0.05) different from those measured. The sum of the relative average errors was 94.17% for the four mixtures (AB 10:90, AB 20:80, AB 30:70 and AB 40:60). At the 50% substitution rate, the coincidence between the predicted values and the measured values was better. Because this mixture AB (50:50) had the value of E (%) (12.07%) the weakest. For substitution rates greater than 50%, the experimental and theoretical values showed a smaller difference compared to the substitution rates of less than 50%. This was explained by the sum of the relative mean errors, which was 59.65% for the four moisture adsorbers (AB 60:40, AB 70:30, AB 80:20 and AB 90:10). This sum of
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Moisture Adsorption Isotherms Characteristicsâ&#x20AC;Ś the relative average errors was lower than the substitution rates lower than 50% (94.17%). Four mixtures AB (40:60), AB (50:50), AB (80:20) and AB (90:10) had their different mean relative percentages deviation E (%) less than 15% (Table 3). This means that the experimental water contents of these four mixtures are like the theoretical water contents according to Vega-Galvez et al. [28]. Table 3: Mean relative percentage deviation (E%) of new generation of moisture adsorbers AB new moisture adsorbers formulated AB AB (10:90)
mean relative percentage deviation (E %) 39.32
AB (20:80)
17.83
AB (30:70)
23.68
AB (40:60)
13.34
AB (50:50)
12.07
AB (60:40)
15.79
AB (70:30)
16.71
AB (80:20)
14.91
AB (90:10)
12.24
For moisture adsorbers AL, ads3 incorporation rates of less than 50% showed a better match between theoretical and experimental values than that of the above 50% rate. This resulted in the sum of the relative average errors of 109.97% for the incorporation rate below 50%. This sum was lower than the substitution rate of over 50%, which was 135.71%. At the substitution rate of abs 3 of 50%, the difference between the theoretical and experimental values was greater. This incorporation rate had the value of the highest E (%) (48.03%). It must be pointed out that the 90% substitution rate in the AL mixture had the best coincidence between the theoretical and experimental values for this type of mixture. Its value of the relative average error E (%) was the smallest (17.4%) (table 4). Table 4: Mean relative percentage deviation (E%) of new generation of moisture adsorbers AL new moisture adsorbers formulated AL AL (10:90)
mean relative percentage deviation (E %) 21.98
AL (20:80)
27.97
AL (30:70)
28.67
AL (40:60)
31.35
AL (50:50)
48.03
AL (60:40)
45.07
AL (70:30)
43.30
AL (80:20)
29.94
AL (90:10)
17.40
IV. DISCUSSION Sorption properties of powders are among the most important parameters affecting quality of a product, its suitability for food production and storage stability [29]. A materialâ&#x20AC;&#x2122;s ability to absorb water is determined by its chemical composition and structure. Sorption isotherms, reflecting the relationship between water content in a product and water activity, constitute a source of valuable data regarding water status in the material. Knowledge of the course of sorption isotherms allows, among other things, determination of the character of factors influencing food spoilage, determination of water content and activity optimal for a material, and design of both food processing processes and composition of a complex matrix of food products. Sorption isotherms also allow theoretical interpretation of physical phenomena occurring at the interface of a product [30, 31].
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Moisture Adsorption Isotherms Characteristicsâ&#x20AC;Ś In general, the moisture adsorption isotherms of the new moisture adsorbers had shown a drop in the water content with the decrease of the incorporation rate of ads3. The results showed that ads3 is very hygroscopic compared to the other two adsorbers that were sodium bicarbonate and "beta" lactose. The proportion of ads3 incorporated in the formulation of the new moisture adsorbers influenced their adsorption isotherms. This result confirms the work of Falade et al., [32] on the effect of the substitution of soybean by Chinese cowpea (Vigna unguiculata L. Walp) on the sorption isotherms of Akbomoso akara (food of the Nigeria). This work has shown that the water content increases with the amount of the most hygroscopic component, which is the Chinese cowpea. The large amount of water adsorbed by the new moisture adsorbers reflected the greater surface area of ads3 and its interlayer swelling characteristics. The hygroscopicity of new moisture adsorbers was a function of the incorporation rate of ads3. Hygroscopicity can be attributed to the number of active polar sites for the binding of the water molecule. Therefore, decreasing abs 3 amount decreased the number of active polar sites, resulting in loss of hygroscopicity [5]. Similar results have been reported by Likos and Lu [33] on the sorption behavior of the mixture of red kaolin and white kaolin. The decrease in the hygroscopicity of the moisture adsorbers may also be due to the decrease in the number of their proteins and carbohydrates, as was the case of the substitution of soybeans by the Chinese cowpea (Vigna unguiculata L. Walp) in the manufacture of Akara of Ogbomoso (food from Nigeria). Increasing proportion soy reduced the hygroscopicity of Akara because Chinese cowpea was richer in protein and carbohydrates than soy. Proteins and carbohydrates adsorbed a significant amount of water [32]. The general shape of the moisture adsorption isotherms indicated that adsorption occurred incrementally as molecular monolayers until the adsorbed film grows to a thickness such that it is no longer strongly influenced by the particle surface. The inflection occurring near 85% RH may indicated the point where continued sorption occurs as capillary condensation in the interparticle pore-space [34]. Moisture adsorption isotherms of new moisture adsorbers represented the integrated hygroscopic properties of their different components. Therefore, any modification in the composition of new moisture adsorbers influenced its sorption properties independently of any structural modification of each component such as pore size [5]. The moisture adsorption depends on the nature and the hygroscopic state of the moisture adsorbers. They had a sigmoidal appearance, such as those commonly presented by food products, such as lentils [35] and powdered peppers [36]. Two of the new moisture adsorbers formulated (AB 40: 60 and AL 60: 40) had low water adsorption capacities at relatively low relative humidities and high-water adsorption capacities at higher relative humidities. This characteristic hygroscopic behavior of an ideal water absorbent will allow their use as a moisture adsorber in modified atmosphere packaging of fruit and vegetable. Change in the composition of the new moisture adsorbers formulated had a significant effect on the course of their adsorption isotherms. Similar claims have been obtained for mixtures containing hygroscopic materials. Gondek and Jakubczyk [37] have studied the effect of the addition of maltodextrin on the course of sorption isotherms of apple powder. They found that adsorption isotherms of dried apples containing maltodextrin, from the water activity at 0.225, was significantly different from that of the others. Addition of 15% maltodextrin caused the isotherm to move to lower water content values for substantially all its course. This means that at the same water activity in the environment, the water content of a product containing maltodextrin was lower. Similar relationships have been observed by Gabas et al. [29], who added maltodextrin and gum arabic to pineapple and determined isosteric sorption heat. The authors demonstrated that the powder without the addition of maltodextrin was characterized by a higher number of polar sites, which resulted in higher water vapor absorption. At higher water activity (0.907), dried pineapple containing maltodextrin absorbed nearly half the volume of water absorbed by the material without any additions. Moisture adsorption isotherms of the new moisture adsorbers formulated were predicted by the additive isothermâ&#x20AC;&#x2122;s method of Labuza [3], considering that the amount of water adsorbed to any water activity was deducted from the mass percentage that each component would adsorb alone. The predicted adsorption isotherms were generally higher than those measured. This is consistent with the work of Iglesias and Chirife [14] on the prediction of food adsorption isotherms from the knowledge of the sorption isotherms of their components. This work had shown that when there was a considerable difference between the measured and predicted sorption curves, the predicted sorption isotherms were generally higher than those measured. The predicted moisture content was almost always higher than that measured in the 0 to 0.75 water activity range. This would be due to the polymer-water interactions that would occur during the formulation of these moisture adsorbers.
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Moisture Adsorption Isotherms Characteristics… Between the components, the interactions had to change the sorption properties of each component, which resulted in considerable differences between the predicted and measured moisture contents. Throughout the water activity range (aw from 0 to 1), the predicted moisture content was slightly higher than that measured. This would indicate that the interactions were to decrease the moisture sorption. It must be said that the hydrogen bonds between the components would compete with the hydrogen bonds between the components and the water. This situation would contribute to a decrease in water sorption [14]. There was no reconciliation between the measured adsorption isotherms and those predicted for water activities below 0.75 for the new moisture adsorbers AL. This would be due to the crystallization of lactose. This crystallization of lactose occurs at activities in the water of between 0.25 and 0.75. During the crystallization of lactose, the crystal releases water to the surface. This would generate a transition point of the isothermal sorption curve [10]. The Labuza’s equation [3] was perfectly applicable to the four new moisture adsorbers AB (40:60); AB (50:50); AB (80:20) and AB (90:10) whose E (%) were less than 15% [28]. This result is in line with the work of Berlin et al. [10] concerning the comparison of the sorption of milk powder components. Berlin et al. [10] compared the isotherm of powdered milk with the combined isotherms of lactose, salt (Jenness-Koops anhydrous salt mixture), centrifuged casein and β-lactoglobulin. There was a good agreement, the predicted isotherm being slightly lower than the measured isotherm, up to the breaking point of the isotherm resulting from the crystallization of lactose. It has also been reported that the comparison of the relative masses adsorbed by the components with different water activities suggests that the sorption of water vapor by the milk powder occurs through a sequence of sorption sites determined by the activity of water. Casein preferentially absorbs moisture when water activities are low. As water activity increases, lactose binds water more strongly, and crystallization occurs when enough moisture has been adsorbed. When the activities in the water are greater than 0.5, the salts bind to the water, which can allow the destabilization of the proteins. Similar observations have been noted by Iglesias et al. [13] in the sugar beet root. The measured isotherm was compared to that of the components (sucrose and water). Good agreement was found up to the breaking point of the measured isotherm, which occurred at about 0.5 aw for the isotherms measured at 35 and 47 ° C, due to the crystallization of amorphous sucrose. Iglesias and Chirife [14] also compared the predicted and measured isotherms for starch gels. There was a good deal with some isotherms and unacceptable differences with other isotherms. The predicted isotherms were generally higher than those measured, indicating that interactions between the components occur, resulting in decreased sorption of water. It has been stated that these interactions probably consist of polymer-polymer hydrogen bonds, which compete with polymer-water hydrogen bonds, thereby reducing water sorption. All these works cited gave satisfaction to the application of Labuza's equation [3]. This equation also allowed Brondlund and Paterson [38] to predict the sorption isotherm of a crystalline lactose powder containing 9% amorphous lactose until crystallization of lactose. It should be noted that this equation of Labuza [3] could not correctly predict fourteen of the eighteen new water adsorbents formulated. This statement resembles that of Palnitkar and Heldman [11] on the sorption characterization of freeze-dried powder mixtures, stating that this equation did not correctly predict their sorption isotherms. In this study, Labuza equation [3] was more applicable to the new moisture adsorbers formulated AB than to AL absorbers. This would be due to the interaction between the lactose "beta" and the abs3 superabsorbent water polymer. This interaction would be the result of the competition between lactose and water for hydrogen bonding with the abs3 water absorbent [15]. The prediction of the sorption isotherms of compound products by the Labuza equation [3] is not always perfect [10, 11, 12, 13]. Although in many other studies, the agreement between the predicted and measured adsorption isotherms can be considered "acceptable" [10, 13]. Despite this, the simple prediction method based on the weighted addition of the components isotherms appears to be a good method for predicting the isotherm for a multicomponent powder. The accuracy of the predicted isotherm is strongly affected by the accuracy of the component’s isotherms, hence the effort put into obtaining accurate isotherm prediction equations for the components.
V. CONCLUSION Ads 3 was the most hygroscopic of four water super adsorbent polymers. Also, the four water super adsorbent polymers were significantly more hygroscopic than sodium bicarbonate and beta-lactose. The hygroscopicity of new formulated moisture adsorbers was a function of the incorporation rate of abs 3. Labuza model [3] correctly predicted the hygroscopic behavior of four new formulated moisture adsorbers (AB 40:60, AB 50:50, AB 80:20 and AB 90:10). It was therefore unable to predict the adsorption isotherms of most of the new formulated moisture adsorbers, especially those of AL. Of the new moisture adsorbers formulated, only AB (40:60) and AL (60:40) had adsorption isotherms characteristic of ideal moisture adsorbers, for modified atmosphere packaging
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Moisture Adsorption Isotherms Characteristics… of fruits and vegetables (mushrooms and tomatoes). These two new moisture adsorbers could be used as hygroscopic materials in these packages to maintain optimum relative humidity.
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