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Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures
Accepted Manuscript Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures M.S. Rosalizan, K. Nurul Afza, O. Hensel, B. Sturm PII: DOI: Reference:
S2451-9049(17)30475-4 https://doi.org/10.1016/j.tsep.2018.07.007 TSEP 201
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
3 December 2017 21 March 2018 13 July 2018
Please cite this article as: M.S. Rosalizan, K.N. Afza, O. Hensel, B. Sturm, Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures, Thermal Science and Engineering Progress (2018), doi: https://doi.org/10.1016/j.tsep.2018.07.007
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Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures M.S. Rosalizan,1,2 K. Nurul Afza2, O. Hensel11 and B.Sturm1 1
Postharvest Technologies and Processing Group, Department of Agricultural and Biosystem
Engineering, University of Kassel, Nordbahnhofstrasse. 1a, 37213 Witzenhausen, Germany 2
Plant and Soil Science Research Centre, Malaysian Agriculture Research &, Development Institute, 43400 Serdang, Malaysia Tel.: +49-15-258-753-624, Fax: +49-561 981-520, E-mail address: [email protected]
Abstract Purple flesh sweet potato or Ipomea batatas is one of the most important crops in Malaysia. Currently the crop is exclusively consumed and marketed in a dried form as traditional snacks and chips. So, understanding the moisture sorption isotherms of this crop is imperative for determining its storability and optimal shelf life of the dried product. Thus, the objective of this study was to determine the adsorption isotherms of purple flesh sweet potato experimentally and the data obtained were used to model the process at different temperatures by employing several mathematical models using non-linear regression analysis. Several mathematical models (GAB, BET, Oswin, Chung and Pfost, Peleg, Henderson, Langmuir, Caurie and Halsey) were tested and compared with the experimental data. The best fitting mathematical model for all temperatures was Caurie’s equation with highest R2 value within the range of 0.9919 and 0.9949 and lowest chi square (λ2) values of 0.0000151 to 0.0000229 . The monolayer moisture content for optimum moisture level for safe storage can be obtained from BET, GAB and Caurie equations. The monolayer moisture content from the GAB equation were found to be 0.1314, 0.1253 and 0.1182 kg
1
w
/ kg
DM
and from Caurie’s
equation, the values were 0.071, 0.0707 and 0.0692 kg w/kg DM. Both at 25°C, 50°C and 60°C respectively. Keywords: Adsorption isotherm; mathematical modelling; purple flesh sweet potato Nomenclatures Xe & Xeq
Equilibrium moisture content
Xm
Monolayer moisture content
aw
Water activity
T
Temperature
n
Number of constants
A,B,C,D,E,K,C,n1,n2,k1,k2
Constants
1. Introduction Sweet potato is a very important source of carbohydrates for large parts of the population in Asia countries. The crop is the fifth most important food crop in developing countries [1] and it has been identified as a household food security crop for many people and as such it contributes significantly towards their livelihoods [2]. In Malaysia, sweet potato is a popular cash crop grown by small farmers and the demand of sweet potato is likely to increase not only for direct fresh consumption, but potentially as a raw ingredient in food processing [3]. The crop is rich in dietary fibers, minerals, vitamins and antioxidants such as phenolics acids, anthocyanin and beta-carotene [4]. Sweet potato has been identified as a potential crop to substitute wheat flour in Malaysia [3] [5]. Currently, the import value of wheat flour into Malaysia is around 25 million USD annually and it is estimated to be increased with time due to high demands in consumption of wheat based product [3]. Recently, purple flesh sweet potato has been given a research priority in Malaysia by the 2
government under the 11th Malaysia’s Plan of National Agricultural Policy especially on product development in order to develop a national strategy for reducing the import of wheat flour as well as to promote the health benefits of this crop to the consumer [3]. Current research has shown that the Malaysian variety of purple sweet potato can be processed into flour which then can be used as an ingredient for a wide range of bakery product, pasta and extruded snacks [3] [6]. The majority of the purple flesh sweet potato crop in Malaysia is consumed and marketed in dried form of traditional chips and snacks called kerepek. Most farmers prefer to store the crop in dried form because this variety is highly perishable. Storage stability of the dried products is greatly depends on its moisture sorption isotherm [7]. Furthermore, the sorption isotherms of this variety has never been established before. Knowledge of the moisture adsorption isotherm of food is of great importance in new formulation [8] and food processing particularly in drying and packaging [9]. It is also important for product development, ingredient research, shelf life estimation, and also it is necessary to fully understand the moisture within the product for product formulation [10]. Moisture sorption isotherms describe the relationship between water activity and moisture content at a constant temperature. The nature of this relationship depends on the interaction between water and other components in the food system. The amount of water vapour that can be absorbed by a product depends on its chemical composition, physical-chemical state, and physical structure [11]. Thus, it is utmost importance to establish the adsorption isotherm characteristics of this crop in order to predict its optimal shelf life and storability after drying. An adsorption isotherm study is necessary to evaluate the stability of a product at different water activities after drying since dried products will be exposed to different humidities and temperature fluctuations during storage and distribution. Dehydrated product becomes unstable at high water activity within the range of 0.6 to 0.85. At this stage, the 3
product is prone to quality deterioration and, thus, the shelf life is reduced [12]. Most quality deterioration in dried product is due to microbial spoilage such as bacteria, yeast and moulds that could be hazardous to human consumption [12]. So, controlling the water activity in dried product is necessary for food safety. From a drying standpoint, this information is required in order to define the end point of the drying process to a pre-defined moisture level at which the product is safe for storage and shelf life extension [13]. The moisture sorption isotherm has been used to explain the behaviour and the structure of water at the surface and inside the foods [14] [15]. Many mathematical models were proposed in this context [16]. Most of them are empirical and semi empirical [17]. Due to the complex nature of different food stuffs, no single model is precise enough to represent all the sorption isotherms of all products. The most common models used in this context are BET, GAB, Oswin, Halsey and others as presented in Table 1. The GAB equation has been applied widely to explain the moisture sorption isotherms and the equation is fitted well for most fruits and vegetables [16]. The parameters of interest to the drying operation and storage such as monolayer moisture content (Xm) can be determined from the material's moisture adsorption behaviour. The monolayer moisture content which can be obtained from the Brunauer-Emmett-Teller (BET), GAB, Langmuir, Halsey and Caurie’s equations is the point at which all polar and ionic surface sites are occupied by a water molecule, and it has been described to appreciably limit the rate of most water-dependent reactions [18] [16]. Monolayer moisture content is the measure of bound water in the materials which cannot participate as a solvent for chemical reaction that can cause food spoilage. Thus, monolayer moisture content indicates the level of moisture content for maximum stability for storage of dehydrated product [ [16] [19] [20] ]. Many researchers also reported the effect of temperature on moisture sorption isotherms for different type of food such as fruits [21] [22], vegetables [23] and medicinal plant and herbs 4
[24] [25]. A limited numbers of studies reported on different varieties of sweet potato such as New Zealand and Philippine varieties [26], Taiwan’s variety of sweet potato [27] and North Carolina variety [28]. However, there is no available data documented on the adsorption isotherms of Malaysian variety of purple flesh sweet potato. By comparing the previous studies on adsorption isotherm of sweet potato, it was observed that the moisture adsorption characteristics varied among species or varieties [27]. The aim of this study was to determine and describe the moisture sorption isotherms of purple flesh sweet potato (var. kedudut) at different temperatures experimentally by employing static gravimetric method in order to provide useful data for estimating the stability and storage life of the dehydrated product. Table 1 Mathematical models used to describe the sorption isotherms of foods Model
Equation
BET
Reference
Crops
[8]
Cocoa beans; Berries, mushroom Potato
GAB
[16]
Tomato,
corn
flour, banana, mango,
pear,
walnut,
yam,
passion fruit. Caurie
[46]
Cocoa powder, maize, legumes, cereals,
5
cowpeas, orange, groundnut, hazelnut 1/n
Halsey
[7]
Chicken, corn, nutmeg, thyme,
fish,
wheat
flour,
paranut Henderson
[16]
Passion
fruit,
pineapple, yam, walnut Langmuir
[16]
Pear,
mint
leaves Oswin
B
[16] [17]
Chillean papaya,
Red
bell pepper Peleg
[16]
Yam,
dried
potato, walnut, green
tea,
pistachio nut Chung &
[45]
Pfost
Rough yoghurt powder, mushroom
6
rice,
2. Materials and methods 2.1. Sample preparation Freshly harvested purple flesh sweet potato was obtained from MARDI’s experimental plot in Serdang. Thirty marketable size tubers with the weight range from 120 to 150 gram were chosen for this experiment. The crop was sliced at 3mm thickness and dried directly after harvest in an oven (model ULM 800, Mermert, GmbH, Germany) at 50°C until the moisture content reached approximately 7% (w.b.) or 0.08 (d.b) which is the level of moisture typically used for dried sweet potato products. Moisture content was determined by placing the sample in an oven at 105°C for 24 hours (model ULM 400, Mermert, GmbH, Germany) [29]. The dried tubers were grounded and about 5g of the samples were placed in a desiccators containing saturated salt solutions at a relative humidity ranging from 11 to 93% as stated below (Table 2). The method used was adopted from static gravitation method recommended by the COST 90 Project [30]. The adsorption temperatures used in this experiment were 25°C, 50°C and 60C°. The selected temperatures are based on the average ambient and room temperature in Malaysia for indoor storage. Furthermore, testing the materials at higher temperatures of 50°C and 60C° are necessary since irreversible quality changes can occur at high temperatures, so experimental measurements at these temperatures are required [31]. The experiments were carried out in triplicate and the weight of the sample was recorded using micro balance ( model BM22, AND, Tokyo, Japan) at 2 days interval. All the samples at 25°C, 50°C and 60°C reached constant weight at 23, 5 and 4 days respectively. The final equilibrium moisture content (EMC) of the samples was determined by the oven method at 105° for 24 hours. The water activity of the salt solution was obtained by dividing the equilibrium relative humidity (ERH) of saturated salt solution (in percentage) with 100 [8].
7
Table 2 Relative humidity over saturated salt solution at 25°C Salt solution
Relative humidity (%)
Lithium chloride
11.301
Magnesium chloride
32.780
Potassium carbonate
43.161
Magnesium nitrate
52.893
Potassium iodide
68.860
Sodium chloride
75.252
Potassium chloride
84.344
Potassium nitrate
93.581
2.2. Modelling equations For the purpose of this work, several equations as indicated in Table 1 were chosen to fit the experimental sorption data. Non-linear regression analysis using Excel Solver for Windows 10.0 (Microsoft®) was used to estimate the model coefficient simultaneously including the monolayer moisture content from the experimental sorption data [17] [32] . The most suitable equations were selected based on highest R2 value and lowest chi square ( 2) value as given in equation (1) and (2) [19].
Where, EMCexp is the equilibrium moisture content obtained from the experiment, EMCpre is 8
the predicted value from the models, N is the number of experimental data on relative humidity and n is the number of constants in each model.
3. Results and discussion 3.1. Equilibrium Moisture Content The experimental curves for the equilibrium moisture contents of purple flesh sweet potato for each water activity at 25 °C, 50 °C and 60°C are presented in Figure 1. The curve is sigmoidal type which is typical for most agricultural products [33]. The equilibrium moisture content (EMC) was increased as the water activity increased. The increase in equilibrium moisture content is due to an increase in relative humidity as a results of increased vapour pressure [33]. It was observed that, the EMC for each water activity was decreased as the temperature increased at a constant water activity. This phenomenon has been explained by Mclaughlin and Magee [34]. It is related to excitation states of molecules. When temperatures rise, water molecules are in an elevated state of excitation, thus, increasing their distance and decreasing the attractive forces between them will leads to a decrease in the degree of water sorption at a given relative humidity with increasing temperature. This also implies that increasing temperatures will cause the product to be less hygroscopic as reported by Ocheme et al. [35]. The same observation was made by Ariahu et al. [36], who stated that at constant moisture content, the increase in water activity at higher temperature indicates that the product is more susceptible to microbial spoilage as the values of the water activity are above the critical level for safe storage. That implies that the rate of deterioration is faster at higher temperatures. The sorption isotherm curves show three different regions which corresponds to different modes of water attachment to the product. The first region is at low water activity of 0.1 to 0.3 in which the water is strongly linked by hydrogen bonds as explained by Mabrouk and Mariem [33]. The water is almost impossible to 9
remove due to increase in adsorption energy and, thus, is not available to react as a solvent or reagent in any chemical reaction. The second zone is an intermediate zone water activity at 0.3 to 0.8. The water molecules within this range will be absorbed on the monolayer of the surface and will form a multilayer. During this time, the surface of the product is saturated with loosely linked water and the water is moderately active and readily available as a solvent or reagent for any chemical reaction. The third zone presents a water activity higher than 0.8. In this area, the water present in a liquid state and free water fraction which allows the growth of microorganisms and enzymatic reactions that could damage the product and cause food spoilage. At this stage, the water is very reactive as solvent or reagent for any chemical reactions [37] [38]. 3.2. Mathematical modelling and fitting of sorption isotherms The experimental results for the equilibrium moisture contents of purple flesh sweet potato at each water activity at 25 °C, 50 °C and 60°C are presented in Figure 1. 0.3 EMC at 25°C EMC at 50°C EMC at 60°C
EMC (kg w / kg DM)
0.25
EMC model at 25°C EMC model at 50°C EMC model at 60°C
0.2
0.15
0.1
0.05
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Water activity, aw
Fig. 1. Comparison of equilibrium moisture content (EMC) of purple flesh sweet potato at different temperatures obtained from experiment and Caurie’s equation. 10
1
The value of constants at the three investigated temperature of 25°C, 50°C and 60°C for all models are shown in Table 3. The Caurie’s equation was the best fitted model to describe the experimental data for all temperatures studied with a highest R2 of 0.992, 0.995 and 0.993 and lowest chi square (λ2) value (Table 3) followed by GAB and Chung & Pfost equations. The value for monolayer moisture content (Xm) were obtained from GAB, BET, Langmuir and Caurie’s equations. The monolayer moisture content was found decreased as the temperature increased which is similar with other agricultural crops as reported by other authors [39] [40]. The modelling results showed that the monolayer moisture content which is based on GAB’s equation is between 0.11 to 0.13 kg w /kg DM while for the Caurie equation, the value is between 0.069 to 0.071 kg
w
/ kg
DM.
According to Labuza and Altunakar [41], monolayer
moisture content is normally at water activity in the range between 0.2 and 0.3. The reason for a decrease in monolayer moisture content with increasing temperature might be due to the reduction in the number of sites available for water binding due to the changes in physical and chemical properties as well as structural changes in the starch polymers due to a reduction in the degree of hydrogen bonding at elevated temperatures [28][42]. The value of monolayer moisture content based on Caurie’s equation from this study is slightly lower as compared with different variety of sweet potato which grown in North Carolina, USA as reported by Fasina [28]. It can be observed that, monolayer moisture content of sweet potato varies between varieties. However, the monolayer moisture content calculated from the BET equation was much lower at 0.020, 0.018 and 0.013 at 25°C, 50°C and 60°C respectively as compared with GAB and Caurie’s equations with lower R2 values at 0.909, 0.867 and 0.843 and also higher chi square (λ2 ) at all measured temperatures. The results indicate that the BET model was not fit enough for this product. Palipane and Driscoll [43] suggested that at increased temperature some water molecules are activated to energy levels that allow them to 11
break away from their sorption sites, thus decreasing the equilibrium moisture content. The value for the constant C in GAB and Caurie equations were decreased with increased in temperature but the value of the constant K in the GAB equation was found increased with temperature. It was observed that some parameters for certain models are affected by temperature. Similar results were also reported by Vega-Galvez [44] on Chillean papaya. However, few studies in the literature reported on the effect of temperature on value of constants for moisture adsorption isotherm in agriculture crops. Table 3 Isotherm model parameters for purple flesh sweet potato at 25°C, 50 °C and 60°C Model
Constant
25oC
500C
60OC
BET
C
49.76652
29.71426
95.18277
Xm
0.020251
0.017764
0.012588
R2
0.909249
0.866652
0.843006
0.0037702
0.003838
0.02464
Xm
0.131443
0.125373
0.118157
K
0.919082
0.997818
1.144358
C
0.128661
0.057178
-0.00349
R2
0.988461
0.982963
0.979886
0.000828339
0.000196
0.000133
Xm
0.110031
0.091611
0.075788
A
0.010031
0.041611
0.008092
2
GAB
2
Halsey 1/n
12
R2
0.897993
0.854351
0.816778
0.004146327
0.003978
0.002608
K
0.183627
0.207385
0.284746
C
0.099954
0.100123
0.100232
R2
0.982875
0.969357
0.937624
0.000600733
0.969357
0.000692
Xm
30.72493
10.8722
1.152109
C
0.006783
0.017022
0.128591
R2
0.954445
0.967189
0.968523
0.000483306
0.000268
0.000133
A
0.02161
0.018854
0.021555
B
0.35834
0.331461
0.25512
R2
0.88532
0.840279
0.808155
0.005141768
0.004837
0.005719
k1
0.107882
0.096381
0.064822
k2
0.107882
0.096383
0.064818
n1
1.162557
1.174165
0.870203
n2
1.162558
1.174159
0.870206
R2
0.96245
0.974056
0.972304
2
Henderson
2
Langmuir
2
Oswin B
2
Peleg
13
2
Chung &Pfost
0.000966124
0.000551
0.000269
E
0.571377
0.339721
0.229241
D
0.053543
0.046367
0.029725
C
-103.975
-0.00361
0.299987
R2
0.983364
0.986545
0.993356
0.000206596
0.000193
0.0000391
Xm
0.0710042
0.0706826
0.0692192
C
0.127218
0.092593
0.086695
R2
0.991905168
0.994918
0.992996
0.0000673
0.0000151
0.0000230
2
Caurie
2
4. Conclusions Moisture adsorption isotherm of Malaysian variety of dried purple flesh sweet potato at different temperature has been established experimentally at various water activities. As expected, the equilibrium moisture content (EMC) was increased as the water activity increased for all temperature studied. The best fitting mathematical model for all temperatures is Caurie equation with highest R2 and lowest chi square (λ2) values and followed by GAB and Chung & Pfost equations. The less significant results were obtained from BET, Oswin, Peleg, Henderson, Langmuir and Halsey equation. The monolayer moisture content of purple flesh sweet potato which is based on the most fitting curve of Caurie equation is between 0.069 to 0.071 kg w/kg
DM
and this value indicates the most stable moisture content for safe
storage of purple flesh sweet potato grown in Malaysia. The established data from this experiment can be applied to predict storage stability and shelf life estimation of dried product 14
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[36] C.C. Ariahu, S.A. Kaze and C.D.Achem, Moisture sorption characteristics of tropical fresh water crayfish (Procambarus clarkii), Journal of Food Engineering, 75(2006) 355-363. [37] J.C. Cheftel, and H. Cheftel, Introduction a la biochimie et a la technologie des aliments Techniques et Documentation-Lavoisier, Paris,1(1977) 371. [38] P. Mafart, Les procedes physiques de conservation, In : Genie industrielalimentaire Technique et Documentation Lavoisier, Paris, 1(1991) 246-251. [39] R.Martinez- Las Heras, A.Heridia, M.L.Castello and A.Andres, Moisture sorption isotherms and isosteric heat of sorption of dry persimmon leaves. Food Bioscience, 7 (2014) 88–94. [40] M.Erbas, M.F.Ertugay and M.Certel, Moisture adsorption behaviour of Samolina and Farina. Journal of Food Engineering, 69(2)(2005) 191-198. [41] T.P. Labuza and B. Altunakar, Water activity in foods: Fundamental and applications. Blackwell publishing, 2007. Diffusion and sorption kinetics of water in food, 215-237. [42] P.Westgate, J.Y. Lee and M.R. Ladisch, Modelling of equilibrium sorption of water vapour on starch materials. Trans. A.S.A.E., 35(l)(1992) 213-219. [43] K.B. Palipane and R.H. Driscoll, Moisture sorption characteristics of in-shell macadamia nuts. Journal of Food Engineering,18(1992) 63-76. [44] A.Vega-Galvez, M.Palacios and R. Lemus-Mondaca, Moisture sorption isotherms and isosteric heat determination in chilean papaya (Vasconcelleapubescens). Quim. Nova, 31 (6) (2008) 1417-1421. [45] E. Timmerman, Water sorption isotherms of foods and foodstuffs: BET or GAB parameters. Journal of Food Engineering, 48(2001) 19-31.
19
[46] M. Caurie, A new model equation for predicting safe storage moisture levels for optimum stability of dehydrated foods. Journal of Food Technology, 5 (1970) 301307.
20
Highlights
Moisture adsorption isotherms of Malaysian variety of purple flesh sweet potato has been established for storage stability and shelf life estimation at different temperatures. The best fitting mathematical models for all temperatures is Caurie’s equation with highest R2 and lowest chi square values. The equilibrium moisture content (EMC) was increased as the water activity increased for all temperature studied and it is decreased with increased in temperature at constant water activity. Monolayer moisture content of Malaysian variety of purple flesh sweet potato obtained from Caurie’s equation at 25°C, 50°C and 60°C were found to be 0.071, 0.0707 and 0.0692 kgw/kg DM respectively which indicates the optimum moisture levels for safe storage.
21
S2451-9049(17)30475-4 https://doi.org/10.1016/j.tsep.2018.07.007 TSEP 201
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
3 December 2017 21 March 2018 13 July 2018
Please cite this article as: M.S. Rosalizan, K.N. Afza, O. Hensel, B. Sturm, Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures, Thermal Science and Engineering Progress (2018), doi: https://doi.org/10.1016/j.tsep.2018.07.007
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Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures M.S. Rosalizan,1,2 K. Nurul Afza2, O. Hensel11 and B.Sturm1 1
Postharvest Technologies and Processing Group, Department of Agricultural and Biosystem
Engineering, University of Kassel, Nordbahnhofstrasse. 1a, 37213 Witzenhausen, Germany 2
Plant and Soil Science Research Centre, Malaysian Agriculture Research &, Development Institute, 43400 Serdang, Malaysia Tel.: +49-15-258-753-624, Fax: +49-561 981-520, E-mail address: [email protected]
Abstract Purple flesh sweet potato or Ipomea batatas is one of the most important crops in Malaysia. Currently the crop is exclusively consumed and marketed in a dried form as traditional snacks and chips. So, understanding the moisture sorption isotherms of this crop is imperative for determining its storability and optimal shelf life of the dried product. Thus, the objective of this study was to determine the adsorption isotherms of purple flesh sweet potato experimentally and the data obtained were used to model the process at different temperatures by employing several mathematical models using non-linear regression analysis. Several mathematical models (GAB, BET, Oswin, Chung and Pfost, Peleg, Henderson, Langmuir, Caurie and Halsey) were tested and compared with the experimental data. The best fitting mathematical model for all temperatures was Caurie’s equation with highest R2 value within the range of 0.9919 and 0.9949 and lowest chi square (λ2) values of 0.0000151 to 0.0000229 . The monolayer moisture content for optimum moisture level for safe storage can be obtained from BET, GAB and Caurie equations. The monolayer moisture content from the GAB equation were found to be 0.1314, 0.1253 and 0.1182 kg
1
w
/ kg
DM
and from Caurie’s
equation, the values were 0.071, 0.0707 and 0.0692 kg w/kg DM. Both at 25°C, 50°C and 60°C respectively. Keywords: Adsorption isotherm; mathematical modelling; purple flesh sweet potato Nomenclatures Xe & Xeq
Equilibrium moisture content
Xm
Monolayer moisture content
aw
Water activity
T
Temperature
n
Number of constants
A,B,C,D,E,K,C,n1,n2,k1,k2
Constants
1. Introduction Sweet potato is a very important source of carbohydrates for large parts of the population in Asia countries. The crop is the fifth most important food crop in developing countries [1] and it has been identified as a household food security crop for many people and as such it contributes significantly towards their livelihoods [2]. In Malaysia, sweet potato is a popular cash crop grown by small farmers and the demand of sweet potato is likely to increase not only for direct fresh consumption, but potentially as a raw ingredient in food processing [3]. The crop is rich in dietary fibers, minerals, vitamins and antioxidants such as phenolics acids, anthocyanin and beta-carotene [4]. Sweet potato has been identified as a potential crop to substitute wheat flour in Malaysia [3] [5]. Currently, the import value of wheat flour into Malaysia is around 25 million USD annually and it is estimated to be increased with time due to high demands in consumption of wheat based product [3]. Recently, purple flesh sweet potato has been given a research priority in Malaysia by the 2
government under the 11th Malaysia’s Plan of National Agricultural Policy especially on product development in order to develop a national strategy for reducing the import of wheat flour as well as to promote the health benefits of this crop to the consumer [3]. Current research has shown that the Malaysian variety of purple sweet potato can be processed into flour which then can be used as an ingredient for a wide range of bakery product, pasta and extruded snacks [3] [6]. The majority of the purple flesh sweet potato crop in Malaysia is consumed and marketed in dried form of traditional chips and snacks called kerepek. Most farmers prefer to store the crop in dried form because this variety is highly perishable. Storage stability of the dried products is greatly depends on its moisture sorption isotherm [7]. Furthermore, the sorption isotherms of this variety has never been established before. Knowledge of the moisture adsorption isotherm of food is of great importance in new formulation [8] and food processing particularly in drying and packaging [9]. It is also important for product development, ingredient research, shelf life estimation, and also it is necessary to fully understand the moisture within the product for product formulation [10]. Moisture sorption isotherms describe the relationship between water activity and moisture content at a constant temperature. The nature of this relationship depends on the interaction between water and other components in the food system. The amount of water vapour that can be absorbed by a product depends on its chemical composition, physical-chemical state, and physical structure [11]. Thus, it is utmost importance to establish the adsorption isotherm characteristics of this crop in order to predict its optimal shelf life and storability after drying. An adsorption isotherm study is necessary to evaluate the stability of a product at different water activities after drying since dried products will be exposed to different humidities and temperature fluctuations during storage and distribution. Dehydrated product becomes unstable at high water activity within the range of 0.6 to 0.85. At this stage, the 3
product is prone to quality deterioration and, thus, the shelf life is reduced [12]. Most quality deterioration in dried product is due to microbial spoilage such as bacteria, yeast and moulds that could be hazardous to human consumption [12]. So, controlling the water activity in dried product is necessary for food safety. From a drying standpoint, this information is required in order to define the end point of the drying process to a pre-defined moisture level at which the product is safe for storage and shelf life extension [13]. The moisture sorption isotherm has been used to explain the behaviour and the structure of water at the surface and inside the foods [14] [15]. Many mathematical models were proposed in this context [16]. Most of them are empirical and semi empirical [17]. Due to the complex nature of different food stuffs, no single model is precise enough to represent all the sorption isotherms of all products. The most common models used in this context are BET, GAB, Oswin, Halsey and others as presented in Table 1. The GAB equation has been applied widely to explain the moisture sorption isotherms and the equation is fitted well for most fruits and vegetables [16]. The parameters of interest to the drying operation and storage such as monolayer moisture content (Xm) can be determined from the material's moisture adsorption behaviour. The monolayer moisture content which can be obtained from the Brunauer-Emmett-Teller (BET), GAB, Langmuir, Halsey and Caurie’s equations is the point at which all polar and ionic surface sites are occupied by a water molecule, and it has been described to appreciably limit the rate of most water-dependent reactions [18] [16]. Monolayer moisture content is the measure of bound water in the materials which cannot participate as a solvent for chemical reaction that can cause food spoilage. Thus, monolayer moisture content indicates the level of moisture content for maximum stability for storage of dehydrated product [ [16] [19] [20] ]. Many researchers also reported the effect of temperature on moisture sorption isotherms for different type of food such as fruits [21] [22], vegetables [23] and medicinal plant and herbs 4
[24] [25]. A limited numbers of studies reported on different varieties of sweet potato such as New Zealand and Philippine varieties [26], Taiwan’s variety of sweet potato [27] and North Carolina variety [28]. However, there is no available data documented on the adsorption isotherms of Malaysian variety of purple flesh sweet potato. By comparing the previous studies on adsorption isotherm of sweet potato, it was observed that the moisture adsorption characteristics varied among species or varieties [27]. The aim of this study was to determine and describe the moisture sorption isotherms of purple flesh sweet potato (var. kedudut) at different temperatures experimentally by employing static gravimetric method in order to provide useful data for estimating the stability and storage life of the dehydrated product. Table 1 Mathematical models used to describe the sorption isotherms of foods Model
Equation
BET
Reference
Crops
[8]
Cocoa beans; Berries, mushroom Potato
GAB
[16]
Tomato,
corn
flour, banana, mango,
pear,
walnut,
yam,
passion fruit. Caurie
[46]
Cocoa powder, maize, legumes, cereals,
5
cowpeas, orange, groundnut, hazelnut 1/n
Halsey
[7]
Chicken, corn, nutmeg, thyme,
fish,
wheat
flour,
paranut Henderson
[16]
Passion
fruit,
pineapple, yam, walnut Langmuir
[16]
Pear,
mint
leaves Oswin
B
[16] [17]
Chillean papaya,
Red
bell pepper Peleg
[16]
Yam,
dried
potato, walnut, green
tea,
pistachio nut Chung &
[45]
Pfost
Rough yoghurt powder, mushroom
6
rice,
2. Materials and methods 2.1. Sample preparation Freshly harvested purple flesh sweet potato was obtained from MARDI’s experimental plot in Serdang. Thirty marketable size tubers with the weight range from 120 to 150 gram were chosen for this experiment. The crop was sliced at 3mm thickness and dried directly after harvest in an oven (model ULM 800, Mermert, GmbH, Germany) at 50°C until the moisture content reached approximately 7% (w.b.) or 0.08 (d.b) which is the level of moisture typically used for dried sweet potato products. Moisture content was determined by placing the sample in an oven at 105°C for 24 hours (model ULM 400, Mermert, GmbH, Germany) [29]. The dried tubers were grounded and about 5g of the samples were placed in a desiccators containing saturated salt solutions at a relative humidity ranging from 11 to 93% as stated below (Table 2). The method used was adopted from static gravitation method recommended by the COST 90 Project [30]. The adsorption temperatures used in this experiment were 25°C, 50°C and 60C°. The selected temperatures are based on the average ambient and room temperature in Malaysia for indoor storage. Furthermore, testing the materials at higher temperatures of 50°C and 60C° are necessary since irreversible quality changes can occur at high temperatures, so experimental measurements at these temperatures are required [31]. The experiments were carried out in triplicate and the weight of the sample was recorded using micro balance ( model BM22, AND, Tokyo, Japan) at 2 days interval. All the samples at 25°C, 50°C and 60°C reached constant weight at 23, 5 and 4 days respectively. The final equilibrium moisture content (EMC) of the samples was determined by the oven method at 105° for 24 hours. The water activity of the salt solution was obtained by dividing the equilibrium relative humidity (ERH) of saturated salt solution (in percentage) with 100 [8].
7
Table 2 Relative humidity over saturated salt solution at 25°C Salt solution
Relative humidity (%)
Lithium chloride
11.301
Magnesium chloride
32.780
Potassium carbonate
43.161
Magnesium nitrate
52.893
Potassium iodide
68.860
Sodium chloride
75.252
Potassium chloride
84.344
Potassium nitrate
93.581
2.2. Modelling equations For the purpose of this work, several equations as indicated in Table 1 were chosen to fit the experimental sorption data. Non-linear regression analysis using Excel Solver for Windows 10.0 (Microsoft®) was used to estimate the model coefficient simultaneously including the monolayer moisture content from the experimental sorption data [17] [32] . The most suitable equations were selected based on highest R2 value and lowest chi square ( 2) value as given in equation (1) and (2) [19].
Where, EMCexp is the equilibrium moisture content obtained from the experiment, EMCpre is 8
the predicted value from the models, N is the number of experimental data on relative humidity and n is the number of constants in each model.
3. Results and discussion 3.1. Equilibrium Moisture Content The experimental curves for the equilibrium moisture contents of purple flesh sweet potato for each water activity at 25 °C, 50 °C and 60°C are presented in Figure 1. The curve is sigmoidal type which is typical for most agricultural products [33]. The equilibrium moisture content (EMC) was increased as the water activity increased. The increase in equilibrium moisture content is due to an increase in relative humidity as a results of increased vapour pressure [33]. It was observed that, the EMC for each water activity was decreased as the temperature increased at a constant water activity. This phenomenon has been explained by Mclaughlin and Magee [34]. It is related to excitation states of molecules. When temperatures rise, water molecules are in an elevated state of excitation, thus, increasing their distance and decreasing the attractive forces between them will leads to a decrease in the degree of water sorption at a given relative humidity with increasing temperature. This also implies that increasing temperatures will cause the product to be less hygroscopic as reported by Ocheme et al. [35]. The same observation was made by Ariahu et al. [36], who stated that at constant moisture content, the increase in water activity at higher temperature indicates that the product is more susceptible to microbial spoilage as the values of the water activity are above the critical level for safe storage. That implies that the rate of deterioration is faster at higher temperatures. The sorption isotherm curves show three different regions which corresponds to different modes of water attachment to the product. The first region is at low water activity of 0.1 to 0.3 in which the water is strongly linked by hydrogen bonds as explained by Mabrouk and Mariem [33]. The water is almost impossible to 9
remove due to increase in adsorption energy and, thus, is not available to react as a solvent or reagent in any chemical reaction. The second zone is an intermediate zone water activity at 0.3 to 0.8. The water molecules within this range will be absorbed on the monolayer of the surface and will form a multilayer. During this time, the surface of the product is saturated with loosely linked water and the water is moderately active and readily available as a solvent or reagent for any chemical reaction. The third zone presents a water activity higher than 0.8. In this area, the water present in a liquid state and free water fraction which allows the growth of microorganisms and enzymatic reactions that could damage the product and cause food spoilage. At this stage, the water is very reactive as solvent or reagent for any chemical reactions [37] [38]. 3.2. Mathematical modelling and fitting of sorption isotherms The experimental results for the equilibrium moisture contents of purple flesh sweet potato at each water activity at 25 °C, 50 °C and 60°C are presented in Figure 1. 0.3 EMC at 25°C EMC at 50°C EMC at 60°C
EMC (kg w / kg DM)
0.25
EMC model at 25°C EMC model at 50°C EMC model at 60°C
0.2
0.15
0.1
0.05
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Water activity, aw
Fig. 1. Comparison of equilibrium moisture content (EMC) of purple flesh sweet potato at different temperatures obtained from experiment and Caurie’s equation. 10
1
The value of constants at the three investigated temperature of 25°C, 50°C and 60°C for all models are shown in Table 3. The Caurie’s equation was the best fitted model to describe the experimental data for all temperatures studied with a highest R2 of 0.992, 0.995 and 0.993 and lowest chi square (λ2) value (Table 3) followed by GAB and Chung & Pfost equations. The value for monolayer moisture content (Xm) were obtained from GAB, BET, Langmuir and Caurie’s equations. The monolayer moisture content was found decreased as the temperature increased which is similar with other agricultural crops as reported by other authors [39] [40]. The modelling results showed that the monolayer moisture content which is based on GAB’s equation is between 0.11 to 0.13 kg w /kg DM while for the Caurie equation, the value is between 0.069 to 0.071 kg
w
/ kg
DM.
According to Labuza and Altunakar [41], monolayer
moisture content is normally at water activity in the range between 0.2 and 0.3. The reason for a decrease in monolayer moisture content with increasing temperature might be due to the reduction in the number of sites available for water binding due to the changes in physical and chemical properties as well as structural changes in the starch polymers due to a reduction in the degree of hydrogen bonding at elevated temperatures [28][42]. The value of monolayer moisture content based on Caurie’s equation from this study is slightly lower as compared with different variety of sweet potato which grown in North Carolina, USA as reported by Fasina [28]. It can be observed that, monolayer moisture content of sweet potato varies between varieties. However, the monolayer moisture content calculated from the BET equation was much lower at 0.020, 0.018 and 0.013 at 25°C, 50°C and 60°C respectively as compared with GAB and Caurie’s equations with lower R2 values at 0.909, 0.867 and 0.843 and also higher chi square (λ2 ) at all measured temperatures. The results indicate that the BET model was not fit enough for this product. Palipane and Driscoll [43] suggested that at increased temperature some water molecules are activated to energy levels that allow them to 11
break away from their sorption sites, thus decreasing the equilibrium moisture content. The value for the constant C in GAB and Caurie equations were decreased with increased in temperature but the value of the constant K in the GAB equation was found increased with temperature. It was observed that some parameters for certain models are affected by temperature. Similar results were also reported by Vega-Galvez [44] on Chillean papaya. However, few studies in the literature reported on the effect of temperature on value of constants for moisture adsorption isotherm in agriculture crops. Table 3 Isotherm model parameters for purple flesh sweet potato at 25°C, 50 °C and 60°C Model
Constant
25oC
500C
60OC
BET
C
49.76652
29.71426
95.18277
Xm
0.020251
0.017764
0.012588
R2
0.909249
0.866652
0.843006
0.0037702
0.003838
0.02464
Xm
0.131443
0.125373
0.118157
K
0.919082
0.997818
1.144358
C
0.128661
0.057178
-0.00349
R2
0.988461
0.982963
0.979886
0.000828339
0.000196
0.000133
Xm
0.110031
0.091611
0.075788
A
0.010031
0.041611
0.008092
2
GAB
2
Halsey 1/n
12
R2
0.897993
0.854351
0.816778
0.004146327
0.003978
0.002608
K
0.183627
0.207385
0.284746
C
0.099954
0.100123
0.100232
R2
0.982875
0.969357
0.937624
0.000600733
0.969357
0.000692
Xm
30.72493
10.8722
1.152109
C
0.006783
0.017022
0.128591
R2
0.954445
0.967189
0.968523
0.000483306
0.000268
0.000133
A
0.02161
0.018854
0.021555
B
0.35834
0.331461
0.25512
R2
0.88532
0.840279
0.808155
0.005141768
0.004837
0.005719
k1
0.107882
0.096381
0.064822
k2
0.107882
0.096383
0.064818
n1
1.162557
1.174165
0.870203
n2
1.162558
1.174159
0.870206
R2
0.96245
0.974056
0.972304
2
Henderson
2
Langmuir
2
Oswin B
2
Peleg
13
2
Chung &Pfost
0.000966124
0.000551
0.000269
E
0.571377
0.339721
0.229241
D
0.053543
0.046367
0.029725
C
-103.975
-0.00361
0.299987
R2
0.983364
0.986545
0.993356
0.000206596
0.000193
0.0000391
Xm
0.0710042
0.0706826
0.0692192
C
0.127218
0.092593
0.086695
R2
0.991905168
0.994918
0.992996
0.0000673
0.0000151
0.0000230
2
Caurie
2
4. Conclusions Moisture adsorption isotherm of Malaysian variety of dried purple flesh sweet potato at different temperature has been established experimentally at various water activities. As expected, the equilibrium moisture content (EMC) was increased as the water activity increased for all temperature studied. The best fitting mathematical model for all temperatures is Caurie equation with highest R2 and lowest chi square (λ2) values and followed by GAB and Chung & Pfost equations. The less significant results were obtained from BET, Oswin, Peleg, Henderson, Langmuir and Halsey equation. The monolayer moisture content of purple flesh sweet potato which is based on the most fitting curve of Caurie equation is between 0.069 to 0.071 kg w/kg
DM
and this value indicates the most stable moisture content for safe
storage of purple flesh sweet potato grown in Malaysia. The established data from this experiment can be applied to predict storage stability and shelf life estimation of dried product 14
from Malaysian variety of purple flesh sweet potato. The results also can be used as a reference and guideline in process design, product development, ingredients mixing and formulation derived from Malaysian variety of purple flesh sweet potato in the future. Further research need to be conducted on stability of relevant phytochemicals in the dried product at different water activities since it will greatly influences the quality and safety of the dehydrated products. Acknowledgments The authors gratefully acknowledge Malaysian Agricultural Research & Development Institute (MARDI) for the financial support and technical assistance in this study. References [1] R.G.O. Rumbaoa, D.F. Cornag. and I.M. Geronima, Phenolic content and antioxi-dant capacity of Philippin sweet potato (Ipomoea batatas) varieties. Food Chemistry, 113(2009) 1133-1138. [2] S. Tumwegamile, R. Kapinga, D. Zhang, C. Chrissman and S. Agili, Opportunities for promoting orange fleshed sweet potato as a mechanism for combating vitamin A deficiency in Sub-Saharan Africa. African Crop Science Journal, 12(3)(2004) 380384. [3] S.L.Tan, Sweet potato – Ipompea batatas– a great health food. UTAR Agricultural Science Journal, 1(3)(2015) 15-27. [4] S.Kammona, R.Othman, I.Jaswir and P.Jamal, Identification of total carotenoids and Beta carotene content in different flesh tuber colours of local sweet potato (Ipomoea batatas) for pharmaceutical industry. Australian Journal of Basic and Applied Sciences, 9(17) (2015) 60-63. [5] S.L. Tan, A.M. Abdul Aziz and Z.Ariffin, Selection of sweetpotato clones for flour production. J.Trop.Agric. and Fd. Sc. 35(2)(2007) 205 – 212. 15
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Highlights
Moisture adsorption isotherms of Malaysian variety of purple flesh sweet potato has been established for storage stability and shelf life estimation at different temperatures. The best fitting mathematical models for all temperatures is Caurie’s equation with highest R2 and lowest chi square values. The equilibrium moisture content (EMC) was increased as the water activity increased for all temperature studied and it is decreased with increased in temperature at constant water activity. Monolayer moisture content of Malaysian variety of purple flesh sweet potato obtained from Caurie’s equation at 25°C, 50°C and 60°C were found to be 0.071, 0.0707 and 0.0692 kgw/kg DM respectively which indicates the optimum moisture levels for safe storage.
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