Sorption Isotherms Behaviour of Some Egyptian Date Fruit Varieties

Transcription

1 World Journal of Dairy & Food Sciences 8 (1): 74-81, 01 ISSN X IDOSI Publications, 01 DOI: /idosi.jdfs Sorption Isotherms Behaviour of Some Egyptian Date Fruit Varieties 1 Seham F. Abdel Moitte and Y.A. Heikal 1 Department of Food Science, Desert Research Center, El-Mataryia, Cairo, Egypt Department of Food Science, Faculty of Agriculture, Ain Shams University, Cairo Egypt Abstract: Moisture content of the date fruits is the limiting factor influencing their storage stability and shelf life. Therefore, the aim of this ork as to determine the equilibrium moisture contents of Egyptian Sei and Amry date fruits at different ater activity values and to establish sorption isotherms curves for these fruits. Fresh date fruits ere dried in a solar drier and subjected to different relative humidity atmospheres to establish their equilibrium moisture contents values. The static method, ith standard saturated salt solutions, as applied to obtain equilibrium moisture contents at constant temperature (5 C±) in the range of a = 0.11 to The obtained sorption isotherms curves ere subjected to modeling by several isotherms equations such as Henderson, Halsey, Iglesias & Chirife and GAB equations. The results shoed that Amry variety contained higher sugar content (69.4%) compared ith Sei variety (56.4%). The initial moisture contents for the dry Sei and Amry date varieties ere 15.78% and.04% respectively. At all ater activity levels, equilibrium moisture contents of Amry date variety ere higher than those of Sei date variety. The optimum ater activity level for their safe storage as 0.54 and 0.65, respectively. Sorption isotherms curves of the to date varieties took the sigmoid, "j" type typical for sugar foods. Evaluation of the different sorption isotherms models as carried out statistically according to their values for correlation coefficient (R - value), residual sum of squares (SSR), error sum of squares (SSE) and mean relative percentage deviation modulus (E%). The GAB model as found to be the most suitable equation to represent the sorption data of the date fruits, due to its high R - values (0.89 to 0.967) and to the loest E%- values (6.77% and 5.01%), respectively. The monolayer moisture content (M o) as in the range of 0.05 to 0.059g HO/g DM ith Amry variety being have higher Mo value than Sei variety. Key ords: Date fruits Amry Sei Equilibrium moisture content Sorption isotherms Sorption models INTRODUCTION that in humid atmosphere, dates of lo moisture content ill absorb moisture from the air. Extend of such change Egypt is the leading country in production of date can be predicted ith the help of an equilibrium moisture fruits ith an annual production of 1.1 million tons, from curve []. Such a curve shos graphically the relation hich about 894 tons ere processed for local market beteen the relative humidity in the air and the moisture and for export [1]. The mature date fruits could be content of dates and it can be used for the determination categorized into three groups according to their moisture of safe moisture content and relative humidity for storage content as folloing: The et group (moisture content as ell as for the requirements of dehydration. Several higher than 50%), semi dry group (moisture content <50% studies have been carried out to establish equilibrium and higher than 4%) and dry group (ith moisture moisture curves for some date varieties. Data on content less than 4%) [1]. Moisture content of the fruits equilibrium moisture content of Libyan dates measured at is the limiting factor influencing their storage ability and 0 C and 5 C as reported by Barreveld [], hile Shukr shelf life. Wet verities of date fruits could be preserved by [] studied the moisture equilibrium curves of some Iraqi cooling and freezing, hile semi dry fruits, such as Sei date varieties and found out the active constituents that variety, hich represent about 17% of total production in may accentuate hygroscopicity and established Egypt, should be preserved by drying. It is ell knon possible means to overcome the problem. Furthermore, Corresponding Author: Yehia A. Heikal, Department of Food Science, Faculty of Agriculture, Ain Shams University, Cairo Egypt. 74

2 World J. Dairy & Food Sci., 8 (1): 74-81, 01 Myhara et al. [4] studied the ater sorption isotherms of some Omani date varieties and found that such moisture equilibrium curves could be mathematically best described by the Guggenheim- Anderson -de Boer (GAB) equation. Pectin-sugar gels (as simulation for seet fruits) for their sorption isotherms data as tested by Tsami et al. [5], ho found that the obtained equilibrium curves comply ith the GAB model. On the other hand, Achour et al. [6], Kechaou and Maalej [7], Nabill et al. [8] and Bellagha et al. [9] examined some Tunisian date varieties for their equilibrium moisture contents and they found that the date fruits experienced dehydration at relative humidities inferior to 50%, hile hydration increases as relative humidity rises above 50%. Pastes of three Saudi date cultivars ere reported to loss moisture at relative humidities loer than 55% [10]. Further study, Belarbi et al. [11] reported that ater sorption isotherms of eleven date varieties collected from Saudi- Arabia and Tunisia could correctly fitted to the GAB and Brunnauer- Emmett -Teller (BET) models. Recently, Baloch et al. [1] and Chaku [1] pointed out that shelf stability of Pakistani Dhakki and Omani Khalas date varieties as influenced by ater activity and head space atmosphere. There are no published data on the equilibrium moisture behavior of Egyptian date varieties. Therefore, the purpose of this paper as to examine the sorption isotherms behavior of Egyptian Sei and Amry date varieties, hich represent about 0% of the date production in Egypt. MATERIALS AND METHODS Materials: Sei dates ere obtained from Abo Raash, Giza, hile Amry dates ere obtained from Sinai Peninsula, Egypt. The date fruits ere collected at the beginning of Tamr maturity stage. The fruit samples ere dried in a solar drier. The dry dates ere packed in poly ethylene bags under vacuum. The moisture content of the fresh date as 45.% for Sei and 54.% for Amry on fresh eight basis. Methods: Major Chemical Composition: Moisture content of the fresh and dried date fruits as measured according to AOAC [14] under vacuum at 70 C for 7h. Total sugars, reducing and non-reducing sugars ere determined according to the method of AOACO [14].Total nitrogen as determined by the micro- kjeldahl method using the conversion factor of 5.95 to calculate the crude protein content. AOAC [14] methods ere applied for fat and ash Table 1: Saturated salt solution and their relative humidity Solubility (g) Solubility (g) in 100 ml in 100 ml Salt % RH (HO at 5 C) Salt % RH (HO at 5 C) Li Cl Br(No ) CHCook Na NO MgCl Na Cl KCO K Cl determination. Crude fiber as determined according to Rangana [15].Ash content as determined after ignition in a muffle oven at 550 C to a constant eight. Total acidity as determined as citric acid according to the method described by Rangana [15]. Determination of Equilibrium Moisture Contents: The static method ith standard saturated salt solutions to maintain constant relative humidity values as applied to obtain equilibrium moisture content at constant temperature as described by Spiess and Wolf [16]. Eight saturated salt solutions (Table 1) ere prepared to cover a ater activity range from a = 0.11 to a = 0.85 [17].Glass desiccators containing the salt solutions and maintaining the eight different relative humidity (ater activity) levels ere kept at room temperatures (5 C±). All experiments ere carried out in duplicate. Duplicate samples of halved date fruits of about 10 g each ere carefully eighted inside Petri dishes and placed over perforated plates above the saturated salt solutions in each desiccator. According to Barreveld [], halved dates ere used to avoid the impermeability of the skin in hole dates and to accelerate the achieving of equilibrium moisture content. The samples ere eighted periodically until eight constant using analytical balance. The final moisture contents ere then defined as the equilibrium moisture content. According to Barreveld [], the influence of temperature on equilibrium moisture content of date fruits as small in the range of 0 to 0 C. Therefore the present experiment as carried out at room temperature. Fitting Sorption Data to Isotherms Models: According to Alhamdan and Hassan [10], as ell as Nabill et al. [8], the sorption isotherms curves of sugar-rich foods could be mathematically represented by several sorption isotherms models as folloing: Henderson Equation: Log [-In (1- a )] = B. log M+ log AT (1) 75

3 Halsey Equation: World J. Dairy & Food Sci., 8 (1): 74-81, 01 SSR S R = N-P (8) LnM= LnA- Ln Ln B B a () SSE S E = M-N (9) Iglesias and Chirife Equation: 1/ L n [M+ (M + M 0.5) ] =B. a + A () GAB (Guggenheim- Anderson - De Boer) Equation: M o. C. K. a M= (1-K.a ) (1- K. a + C.K.a ) ( ) BET( Brunauer, Emett and Teller ) Equation [18]: a 1 C 1 = +.a o o M 1-a M M Where: a = Water activity (-). M = Equilibrium moisture content (g H O/g DM). M o = The Monolayer moisture content (g HO/g DM). T = Absolute temperature. K = A and B, are constants. C = Constant related to heat of sorption of monolayer K = Constant related to total heat of sorption. The BET equation provides the value of monolayer moisture content, hich is an important parameter in food deterioration studies and it hold for ater activities up to about 0.5. The parameters ere estimated by fitting the mathematical model to the experimental data by non- linear regression (least- squares, estimates) using SAS - package for analysis of variance (SAS). Experimental values of equilibrium moisture content ere taken as dependent variables. The goodness of fit of each model as evaluating by the obtained results of statistical analysis as follos: Residual Sum of Squares (SSR) SSR = ESS = ( y y ) N / i= i Error sum of Squares (ESS): ( y y ) N / i= i (4) (5) (6) (7) R - value = correlation coefficient. Mean Relative Percentage Deviation Modules, E: as defined by equation: 100 E%= N Se < 1 N i= i / Mc M c Mc According to F-test, the model is acceptable hen: SR F N-P,M-N Where: ( ) (10) (11) y = The experimental value of the dependent variable according to the applied isotherms model. / y = the predicted value of the dependant variable. y = The mean value of the experimental dependent variable. N = The total number of experiments. P = The number of constants according to the applied model. M = The total number of replicates in each experiment (=16) Mc = The experimental value of equilibrium moisture content. / Mc = The predicted value of equilibrium moisture content. M 0.5 = The moisture content at the ater activity of 0.5 S R = he mean square value of residual sum of squares. S E = The mean square value of error sum of squares. F 1- = F- distribution value for (N-P) and (M-N) degrees of freedom at significant level (1- ). For model evaluation the significance level of (1-0.95) as considered. Statistical Analysis: All experiments ere carried out in triplicates and the collected data ere subjected to regression analysis using SAS [19] 76

4 World J. Dairy & Food Sci., 8 (1): 74-81, 01 RESULTS AND DISCUSSION Major Chemical Composition: Table shos the major chemical composition of Egyptian Sei and Amry dates. The moisture content of the dried-fruits as 15.78% and.04% for Sei and Amry varieties, respectively. The fresh fruits ere at the beginning of the rutab maturity stage. Hoever, on dry eight basis, the Amry variety contained higher sugar content (69.4%) compared ith the Sei variety (56.4%). Total sugar of both varieties composed mainly of reduced sugars (75%), hile the non- reducing sugars ere 5% of the total sugars. The to tested date varieties contained considerable amounts of fibers (4.76% and 5.78%) for Sei and Amry varieties, respectively. Sei variety contained relatively higher protein and fat content compared ith those of Amry variety, hile the Amry variety sho relatively higher acidity (0.%) compared ith that of Sei variety (0.18%). Hoever, both varieties shoed a very high ratio of sugar: acid ranging beteen 47.1 and 440.6, hich express the high seetness of the date fruits. The high moisture content of the fresh date fruits makes them susceptible to microbial damages by yeasts and moulds, since the ater activity values (a-values) of the fresh fruits varies beteen (0.89 and 0.9) for Sei and Amry varieties, respectively, hich makes the establishment of equilibrium moisture contents for the fruits in relation to the surrounding atmosphere a necessity in order to ensure the suitable conditions for fruit safety and stability during storage. The obtained results are in agreement ith those reported by FAO [1], Achour et al. [6], Salem and Hegazi [0], Ahmed et al. [1] and Myhara et al. [] Equilibrium Moisture Content of Date Fruits: Fig. 1 shos the change in moisture content versus time for Sei (a) and Amry (b) date varieties at different ater activity levels, ranging from a = 0.11 to a = 0.85 at room temperature (5 C). By definition, the equilibrium moisture content of each sample is the final moisture content at the particular environment. The initial moisture content for the dry Sei and Amry date varieties ere 15.78% and.04%, respectively. As shon in Fig. 1, both varieties required about 90 days to reach the corresponding equilibrium moisture content. Samples stored at ater activity level of a = 0.44 and less have achieved their equilibrium moisture content through moisture loss, hile those stored at a = 0.54 and higher have achieved their equilibrium moisture content by moisture gain. At all ater activity levels, equilibrium moisture contents of Moisture content ( % on dry eight basis ) Moisture content ( % on dry eight basis ) Time (days) Time (days) Sei RH(11.%) RH(.8%) RH(%) RH(44%) RH(54%) RH(65%) RH(77.5%) RH(85%) Amry RH(11.%) RH(.8%) RH(%) RH(44%) RH(54%) RH(65%) RH(77.5%) RH(85%) Fig. 1: Change in moisture content versus time for Sei (a) and Amry (b) at different ater activity levels Table : Chemical composition of dried Egyptian Sei and Amry date. Component Sei Amry Moisture content (%) Total sugar (g/100g DM) Reduced (g/100g DM) Non reduced (g/100g DM) Acidity, (as citric acid) sugar/acid ratio Protein.1.19 Fat Fibers Ash.6.4 Amry date variety ere higher than their corresponding values of the Sei date variety (Table ). At a - value = 0.85, the equilibrium moisture contents of both varieties ere 47.78% and 54.9, respectively, hich as almost equal to the moisture content of the fresh fruits. The optimum ater activity levels for the storage of Sei variety as a = 0.54, hile that of Amry variety as a = 0.65, because at this a-value, the dried date fruits maintained alays their initial moisture content ithout moisture loss or gain. The difference in the values of equilibrium moisture content as ell as in the optimum ater activity level beteen the to varieties for a safe storage could be referred to the differences in their total sugar content and 77

5 World J. Dairy & Food Sci., 8 (1): 74-81, 01 Table : Equilibrium moisture content (g HO/100g DM) and rate of moisture desorption and adsorption (K-value) of Sei and Amry date Equilibrium moisture % Change in moisture content 1 K-value (g HO/g DMd ) a Sei Amry Sei Amry Sei Amry *10-6.7* * * * * * * * * * * * * * *10 hence, their hygroscopisity for moisture adsorption and desorption. Bolin [] reported an optimum a- ranges of 0.51 to 0.6 as a safe a - level for raisins, hich agree ith the safe a - level for date varieties of the present ork. As shon in Fig. 1, the course of the equilibrium curves as exponential ith different intensity according to the applied ater activity level. According to Lim et al. [4], such course of these curves could be described according to the folloing equation: M=M.e o KT (1) here: M o = Initial moisture content. M = Moisture content at time (t) during the sorption experiment. T = Time of contact in days. K = The rate of moisture sorption (9 HO/g DM.d) Table shos the calculated k-values for both date varieties (Sei and Amry). Negative k-values means that the sample has achieved the equilibrium moisture content by moisture loss (desorption), hile positive k-values means moisture gain (adsorption). For Sei date variety, the moisture desorption lasted until a ater activity value of a = 0.54, hile for Amry variety this phenomenon lasted until a = From these results, it could be concluded that the optimum relative humidity values for safe storage of Sei and Amry date varieties ill be a = 0.54 and a = 0.65, respectively. These results are in agreement ith those obtained by Alhamdan and Hassan [10] and Belarbi et al. [11], here the sorption behavior of Egyptian Sei date variety corresponds to that of Ruziz and Barhi date cultivars, hile the sorption behavior of Egyptian Amry date variety corresponds to that of Deglet-Tunisian Nour date variety reported by Nabill et al. [8]. The largest amount of moisture adsorbed at higher relative humidities (RH 75% and 85%) could be referred to the dominant Fig : Sorption isotherms curve of Sei (a) and Amry (b) ater sorption of sugars present in the date fruits. According to Shukr [], plant sugars are naturally amorphous ith a consequently greater ability to adsorb moisture. Sorption Isotherms Behavior: Fig. shos the sorption isotherms curves of Egyptian Sei and Amry date varieties. Both curves take the sigmoid "j" type, hich is typical for high sugar foods. By examining both curves, it can be noted that Sei variety absorbed loer moisture than that of Amry variety, due to the composition of both varieties, especially the sugar content. Table 4 shos the results of the obtained regression analysis as ell as the values of constants of each applied mathematical models 78

6 World J. Dairy & Food Sci., 8 (1): 74-81, 01 Table 4: Estimated parameters for GAB, Henderson, Halsey, Iglesias & Cherifi and BET equations for dried date fruits at 0 C. GAB Henderson Halsey Iglesias and Chirife BET Calculated parameter Sei Amry Sei Amry Sei Amry Sei Amry Sei Amry C K A B Mo SSR SSE S R S E R S R/ S E E% F-value = 0.45 Fig : Plot of residual fit of GAB, Henderson, Halsey as ell as Iglesias and Chirifi models to sorption data of dried Sei(1) and Amry () date fruits. (equations 1,,, 4 and 11). The GAB- model as the Both varieties shoed similar K-values most suitable equation to represent the sorption data of (1.011 and 0.94), but the heat constant (C- value) for the the date fruits, since it has shoed reasonable R -values monolayer moisture content of Amry variety (107.8) (0.89 and 0.967) as ell as the loest mean relative as much higher than that of Sei variety (14.65). percentage deviation (E-value ) of 6.77% and 5.01% for This difference reflects the high hygroscopicity of the Sei and Amry date varieties, respectively. Amry date fruit, due to its higher sugar content (Table 1). 79

7 World J. Dairy & Food Sci., 8 (1): 74-81, 01 The Halsey-equation provided reasonable mean relative. Shukr, M.M., Hygroscopicity of certain dried percentage deviation (E-value) of 7.08% and 10.0% as ell as high R -values (0.981 and 0.946), but it could not be accepted due to the higher values of S R/S E-ratios, hich exceeded the tabulated F (P<0.5) - Value. Henderson as ell as Iglesias and Chirife-equations proved to be unsuitable for representation of sorption data of tested date fruits, due to the loer R - values and the higher mean relative percentage deviation (E-values), although their S R/S E ratios ere not much higher than the tabulated F-value. To justify the suitability of GAB- model to represent the sorption data of the tested date fruits, an analysis of the residual (mean relative deviation E% of experimental and calculated equilibrium moisture contents) as carried out and represented in Fig.. According to Durakova and Menkov [5] a model could be accepted, if the residuals ere randomly distributed. As shon in Fig., only GAB-equation can meet these requirements, hile the other tested equations provided abnormality from randomness. The monolayer moisture content (M o) as calculated using both GAB and BET- equations and the results are included in Table 4. The obtained M -values ere in the range of o and g H O/g DM ith Amry variety being had higher M -value than did Sei- variety due to their o difference in sugar content. Values of Mo-obtained by BET equation ere relatively higher than those obtained by GAB-equation. The obtained results agree ith those reported by Kechaou and Maalej [7], Nabill et al. [8], Bellagha et al. [9], Alhamdan and Hassan [10], Belarbi et al. [11] and Chaku [1].They stated that experimental sorption data of date fruits and pastes ere fitted ell ith GAB equation suitable for high sugar foods. Hoever, for calculation of Mo-value, Belarbi et al. [11] suggested the use of BET model, since the values of its parameters ere more realistic. The monolayer moisture content for the to varieties (Sei and Amry) ere recommended as the least moisture content required for stability of dried date fruits during storage. REFERENCES 1. FAO 00. Date palm cultivation FAO plant production and protection paper 156 (Rev.1) Food and Agricultural organization of the United Nations, Rome.. Barreveld, W.H., 196. Equilibrium moisture content curve. In: Doson, V.H.W. and A. Aten, 196. Date handling, processing and packaging, FAO (Food and Agriculture Organization of the United Nations, Rom, Pendix 4: 96-99). date products. Date palm J., 4(): Myhara, R.M., M.S. Taylor, B.A. Slomins Ki and I. Al-Bulushi, Moisture sorption isotherms and composition of Omani dates. Journal of Food Engineering, 7: Tsami, E., K. Krokida and A.E. Drouzas, Effect of drying method on the sorption characters of model fruit poders. J. of Food Engineering, 8(4): Achour, M., S. Hamdi and L. Benzarti, Determination des isothermes de sorption de dates Deglet Nour interposes in Tunisia. Fruits, 54(): Kechaou, N. and M. Maalej, 000. Desorption isotherms of Tunisia Deglet Nour dates application of the theory. Brazilian Journal of Food Technology, : Nabill, A., H. Bouabidl, M. Trifi and M. Marrakchi, 005. Water vapor desorption of Deglet noor and Alig-Dates. Fruits. 05: 1-16 September 005, Montpellier France: pp: Bellagha, S., A. Sahli and A. Farhat, 008. Desorption isotherms and isosteric heat of three Tunisian date cultivars. Food and Bioprocess Technology, 1(): Alhamdan, A.M. and H.H. Hassan, Water sorption isotherms of date pastes as influenced by date cultivar and storage temperature. Journal of Food Engineering, 9: Belarbi, A., C. Aymard, J.M. Meot, A. Themelin and M. Reynes, 000. Water desorption isotherms for eleven varieties of dates. Journal Food Engineering, 4: Baloch, A.K., W.A. Baloch, M.K. Baloch and S.A. Saleem, 007. Shelf stability of dhakki dates as influenced by ater activity and headspace atmosphere. ISHS Acta Horticulture. Proceedings of III international date palm conference, ISHS Acta Horticulture, Abu Dhabi, UAE, 19-1, Chaku, O., 010. Moisture - sorption study of dried date fruits. AU J. T., 1(): Association Official Analytical Chemists (AOAC), th Official Methods of Analysis (AOAC). 15 Ed. Washington DC, USA. 15. Rangana, S., Handbook of Analysis of and Quality Control for Fruits and Vegetable Products. Tata McGra- Hill publishing Company, India. 80

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