Moisture desorption isotherms of fresh Jamun (Syzygium cumini) fruit

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1 Indian J. Agric. Res., 51 ( : Print ISSN: / Online ISSN: X AGRICULTURAL RESEARCH COMMUNICATION CENTRE.arccjournals.com/.ijarjournal.com Moisture desorption isotherms of fresh Jamun (Syzygium cumini fruit S. Bisal 1 *, M. Mohapatra, M.K. Panda and S.K. Dash College of Agricultural Engineering and Technology, Orissa University of Agriculture and Technology, Bhubanesar , Odisha, India. Received: Accepted: DOI: /ijare.v51i ABSTRACT Moisture desorption isotherms of fresh Jamun fruit, ere investigated using the methods of AOAC. Equilibrium moisture content of the samples ere measured at three temperature levels viz., 5, 10 and 25 C and nine relative humidity conditions in the range of 10-90% approximately using MSI jar. The sorption isotherms of jamun fruit folloed type III (J-shaped isotherms, characteristic of high sugar products. The reverse temperature effect occurred at ater activity, >0.75, resulting in a crossing over of the sorption curves for the adsorption isotherms only. The application of the GAB model to the experimental data, using direct nonlinear regression analysis, provided agreement beteen the experimental and predicted values. The GAB model parameters monolayer moisture content (Mo and heat of sorption for monolayer (C ere found to decrease ith increasing temperature. Key ords: Equilibrium moisture content, GAB model, Jamun, Monolayer, Sorption isotherms. INTRODUCTION Jamun or Indian blackberry (Syzygium cumini is a minor and highly perishable fruit. It is enriched ith flavonoids, essential oils, anthocyanins, phenolic compounds and other antioxidants (Rai et al Water activity ( has long been considered as one of the most important quality factors, especially for long-term storage. All chemical and microbial deterioration reactions are directly affected by changing ater activity; therefore, the determination of the relationship beteen ater activity and moisture content is significant. There are lots of very important applications of ater sorption isotherms in food science and technology. They are mainly used to control deterioration during food storage. Also, enthalpy of adsorption and desorption of ater are used in drying operations. Sorption isotherm is the plot of equilibrium moisture content of a material subjected to different relative humidities at a given temperature in closed environment. Depending on the direction of ater transferred the process is named as adsorption or desorption. Moisture sorption isotherms are either measured during desorption (starting from the et state or during adsorption (starting from the dry state. Several researchers have determined ater sorption isotherms for various products and temperatures and at selected ranges of ater activity (Labuza, 1984; Maroulis et al., 1988; McLaughlin & Magee, 1998; Gabas et al., Empirical and semi-empirical equations have been proposed to correlate the EMC ith the ater activity of food products. The GAB (Guggenheim-Anderson-deBoer equation has been suggested as being the most versatile sorption model available in the *Corresponding author s sonalicaet@gmail.com literature and has been applied successfully to various dehydrated foods (Kiranoudis et al., It is based on the BET (Brunauer Emmett-Teller theory (Brunauer et al and involves three coefficients hich have physical significance, to of them being functions of temperature (Maroulis et al., Moreover, the GAB equation has been recommended by the European project group COST90 on the physical properties of foods (Wolf et al., 1985 as the fundamental equation for the characterisation of ater sorption of food materials. MATERIALS AND METHODS Ra material : Fresh Jamun as purchased from local vendors from Bhubanesar for the study during the month of June, The fruits ere cleaned and graded manually to remove the damaged, bruished and infested fruits. Uniform sized fruits having average eight of 3.35 ± 0.2 g and diameter of 2.5±0.3 cm ere selected for the experiment. Desorption isotherm : Average initial moisture content of the sample used as %( /. Equilibrium moisture content of the samples ere measured at three temperatures viz., 5, 10 and 25 C and nine relative humidity conditions in the range of 10-90% approximately using MSI jar (desiccator. Constant relative humidity as sustained by using saturated salt solutions (Leis Greenspan, The percent relative humidities observed for the saturated salt solutions are given at each operating temperature and for the set-up used. A portion of each saturated salt solution as transferred into separate glass jars, so that a depth of 1.5 cm covered the jar bottom. To avoid direct contact ith the salt solution, the sieves ere placed on tripods in the jars hich ere then tightly closed. It as observed that constant relative

2 268 INDIAN JOURNAL OF AGRICULTURAL RESEARCH humidity inside the chamber as maintained, at maximum, in 30 minutes. To obtain the desorption isotherm; the saturated solutions ere used for the same sample, but in this case in decreasing relative humidity order. Once equilibrium is reached (after 14 days, the equilibrium moisture content of samples as measured. The sample as eighed intermittently until no change in eight as observed, as measured to the second decimal place (Zurith et.al., After the desorption procedure as completed, final moisture content for each replicate as determined by oven drying method at 105 C for 24 hrs. Upon completing a set of desorption data, the temperature as re-adjusted to another experimental value and the entire procedure, as explained above, as repeated for all of the temperatures. Experiments ere carried out in replications of three sample in a dessicator for each temperature (5, 10 and 25 o C. As a result, equilibrium moisture contents ere calculated as a mean value of each temperature and diagrams of sorption isotherms for each temperature ere dran for moisture content versus ater activity (. Special care as taken about microbial groth in all ater-activities. Nevertheless, only the analyzed samples at ater activity higher than 0.7 presented evidence of fungal groth during storage upon examination of visual analysis. Mathematical models of sorption isotherms curves : Thus a mathematical model for prediction of their sorption behavior as a function of ater activity as suggested for temperatures 5, 10 and 25 o C. In order to determine the mathematical model for temperatures, non-linear regression analysis method as used. Aiming to mathematically express the relation beteen the ater activity of food and its moisture content, diverse models have been developed such as non-linear, linear, regressional models, constituted in their parameters by to, three, four and six partial regression coefficients, hich explain each one of the three zones that the isotherm of sorption of humidity conforms. The most common equations that ere selected for modelling sorption isotherm curves are used for describing sorption in food products are the BET equation, the Osin model, the Smith model, the GAB model as shon in equations 1, 2 and 3. KCa GAB Model : M = (1 (1-K (1-K +CK a Osin Model : M = 1-a (2 Smith model : M= c - b 1n (1- ( (3 Where, M= moisture content % (db = the moisture content monolayer (moisture corresponding at hydration limit on the surface of the matrix % (db C, C BET and K are constants related to the energies of interaction beteen the first and further molecules at the individual sorption sites. = ater activity, a,b,c, n are model parameters These values are obtained from the regression curves using statistical tools using Microsoft Excel 2007 Solver tool. Development of the GAB mathematical model for sorption isotherms : The EMC datere processed using Microsoft Excel (2007 softare and analysed using the polynomial regression method on transformed binomial equations for the GAB parameters.the GAB equation is given by the folloing equation (Rizvi, 1986; M = MoKC (1-K (1-K +CK Where: M = equilibrium moisture content, %, d.b. Mo = GAB monolayer moisture content % db C = constant related to the monolayer heat of sorption K = factor related to the heat of sorption of the multilayer =Water activity, decimals The three GAB parameters ere determined folloing the method of the transformed form of the GAB isotherm (Timmermann et al., The GAB model can be rearranged to a polynomial equation (4 as follos: a Ka 2 + (c - 2 ( 1 /c-1 M C + 1 = (4 o CK M If the three GAB parameters are given the values: K (C-2 1,, Mo( 1 /c-1 C CK then the GAB equation takes the form as shon in equation M = a (5 A polynomial direct nonlinear regression method of /M versus as carried out using Microsoft Excel (2007 in order to determine the values of the coefficient of the quadratic term α, the linear term coefficient β and the constant ε (Equation 6, 7 and 8. Then the GAB parameters ere calculated as follos: K= f -2-2z (6 1 M O = (7 C = 2+ K (8 Where: f = -4

3 The suitability of model fit as determined in terms of mean relative percentage deviation (P% value given in equation 9 (Boquet et al., 1978; Lomauro et al., A model is considered to be suitable if P is < 10% (Lomarou et al., 1985; Wang and Brennan, 1991 and R 2 value >0.90. Volume 51 Issue 3 (June P(% = N i=1 N M exp - M pred (9 M Development of the Smith mathematical model for sorption isotherms : Smith considered the sorbed ater to and outer surface of sorbent. The parameter c represents moisture bound to the surface and parameter b represents the moisture in a uni-molecular layer of normally condensed moisture. The smith model is given by the folloing equation: M = c - b 1n(1 - The Smith parameters ere obtained by plotting lnm verses ln(1- hich makes out to be a logarithmic plot. Development of the Osin mathematical model for sorption isotherms: The Osin equation is also one of the idley used equation in modeling experimental sorption data of fruits. The equation can be altered to logarithmic form hich is more suitable regression analysis.the Osin model is given by, a M = a( 1 - a Where, M is the equilibrium moisture content (g ater/g dry matter, the ater activity, M is monolayer moisture content. a and n are parameters hich are the intercept and slope of the graph. Osin plots dran by ln M versus ln ( /1- at three different temperatures and the osin model parameters are determined by microsoft excel solver Data analysis : Different parameters ere used to evaluate the models correlated to experimental kinetics, namely correlation coefficient (R 2, mean relative percentage deviation (P %. For a model to fit best R 2 value should be as close to 1 and P% value < 10 %. RESULTS AND DISCUSSION The desorption isotherms of fresh jamun at three different temperatures viz., 5, 10 and 25 C are shon in Fig.1. Data are plotted as equilibrium moisture content values, on dry basis, at different ater activities ( corresponding to relative humidity ranging from 10 to 90 % for each temperature. Moisture desorption isotherms of jamun fruit exhibited sigmoid shape behaviour of type III. In the ater activity range beteen 0.1 to 0.6 the variation in EMC values at different temperature are very less, EMC values increases ith a decrease in temperature. Hoever, ater activity above 0.6, decrease of Fig. 1: Experimental desorption isotherms of fresh jamun at three different temperature temperature from 25 to 5 ºC sharply increases the EMC values. It means at lo ater activities the products desorbed relatively small amounts of ater, and as ater activity increased the desorption increases considerably. At lo the desorption is mainly a superficial phenomenon due to the biopolymers and crystalline sugars, hereas at high a the increase in desorption is mainly due to the dissolution of sugars (López-Malo et al., A similar behaviour has been reported by other authors for different fruits straberry (Moraga et al., 2004, guava, mango and pineapple (Hubinger et al., As reported by Kaya et al. (2007 this can be explained by the excitation states of molecules hich could be due to temperature augmentation, increase their distance apart and hence attractive forces beteen them decreases. Subsequently, increasing temperature lessens the ater sorption degree. This finding is consistent ith the results of various researchers. As ell, for constant value of moisture content rising temperature leads to ater content increase. In general, the equilibrium moisture content is expected to decrease ith increasing temperature at a given ater activity. This trend has also occurred in jamun ith a high ater activity ( >0.75 for desorption also. Determination of empirical parameters ith different MSI Models: The data obtained ere treated according to GAB (Guggenheim-Anderson-De Boer, Smith, Osin isotherm models. It as tried to correlate the data using a relevant model of MSI and to find the best fitting model ith the experimental values. The values hich ere found out from experiments ere fitted ith GAB model using microsoft excel At different temperatures the different values of the GAB parameters are obtained for three different temperatures hich are given in the Table 1. The monolayer moisture content ( value of the GAB model decreases ith increasing temperature. This as because increased temperatures raise activation energy of a number of ater molecules to high levels that facilitate their taking apart from the sorption sites. These results agree ell ith those of Kumar and Mishra (2006; Labuza (1968; Hossain et al. (2001 and Rahman and Labuza (1999. Also the decrease in values may be due to a reduction in the total number of active sites for ater binding as a result of physical and/or chemical changes in the product induced by temperature (Iglesias and Chirife, 1976; Mazza and Le Maguer,

4 270 INDIAN JOURNAL OF AGRICULTURAL RESEARCH Table 1: Model parameters obtained for the sorption isotherms of jamun fruit Models Temp(ºC C K Mo(%db c b a n P(% R 2 GAB Smith Osin Fig.2: EMCs predicted by the GAB model compared ith the experimental EMCs at 5, 10 & 25ºC The values for the constant K in the GAB model varied beteen 0.90 and These results are in agreement ith Kumar and Mishra (2006. The moisture desorption experimental values at three different temperatures and ater activity ranging from 0.1 to 0.9 are plotted against the GAB fitted model and shon in Fig.2. The Smith model parameters ere determined (Table 1. and the c value decreased ith decrease in the temperature. The c parameter estimates the moisture bound to the surface. At lo temperature desorption is less, therefore c values are also less, hich are in accordance ith the desired phenomena. The b parameter increased ith decrease in the temperature. The R 2 as Fig.3: EMCs predicted by the Smith model compared ith the experimental EMCs at 5, 10 & 25ºC Fig. 4: EMCs predicted by the Osin model compared ith the experimental EMCs for desorption data at 5, 10 & 25ºC above 0.90 but as less than GAB model as ell as the P% value as higher than GAB model. Fig.3 shos the experimental and predicted values of EMCs folloed ith Smith model. It shos that this model can be used for a lo temperature range of 5-10 C and ater activity of for determining the moisture sorption and shelf life prediction. The parameter a of Osin model decreased ith increase in temperature along ith the n parameter. The values of parameter a remains in the range of The predicted and experimental EMC values ere plotted and shon in Fig. 4. The R 2 values remains much higher than 0.90, but the P% remained more than the values in GAB model. CONCLUSIONS The GAB model fitted best over the hole range of ater activity, giving P (% varied beteen 1.04 and 1.38 for three temperatures. This result is <10% and much less as compared to the Smith or Osin model. Therefore, it is an indication of good fit according to both Wang and Brennan (1991 and Lomauro et. al (1985. It can also be observed that the GAB model gives a very close estimate of the experimental EMC for all the temperatures, hereas Smith model fits better for 10 C and Osin model for 25 C only. Therefore, the GAB model can be recommended for characterising the moisture desorption characteristics of jamun in the temperature range of 5-25 C and ater activity range of to 0.91.

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