Online publication date: 27 May 2010

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1 This article was downloaded by: [TÜBİTAK EKUAL] On: 31 May 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Drying Technology Publication details, including instructions for authors and subscription information: Moisture Sorption Isotherms and Storage Stability of Spray-Dried Yogurt Powder Banu Koç a ; Melike Sakin Yilmazer a ; Pınar Balkır b ; Figen Kaymak Ertekin a a Ege University, Faculty of Engineering, Department of Food Engineering, Bornova, Izmir, Turkey b Ege University, Ege Technical and Business College, Bornova, Izmir, Turkey Online publication date: 27 May 2010 To cite this Article Koç, Banu, Yilmazer, Melike Sakin, Balkır, Pınar and Ertekin, Figen Kaymak(2010) 'Moisture Sorption Isotherms and Storage Stability of Spray-Dried Yogurt Powder', Drying Technology, 28: 6, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Drying Technology, 28: , 2010 Copyright # 2010 Taylor & Francis Group, LLC ISSN: print= online DOI: / Moisture Sorption Isotherms and Storage Stability of Spray-Dried Yogurt Powder Banu Koç, 1 Melike Sakin Yilmazer, 1 Pınar Balkır, 2 and Figen Kaymak Ertekin 1 1 Ege University, Faculty of Engineering, Department of Food Engineering, Bornova, Izmir, Turkey 2 Ege University, Ege Technical and Business College, Bornova, Izmir, Turkey Moisture adsorption isotherms of spray-dried yogurt powder were determined at 10, 25, and 40 C using the standard, static-gravimetric method. Experimental data were fitted to five mathematical models (Guggenheim-Anderson-de Boer [GAB], Brunauer-Emmett-Teller [BET], Halsey, modified Oswin, and modified Henderson). A nonlinear regression analysis method was used to evaluate the constants of the sorption equations. The shelf-life of yogurt powder was predicted based on the relationship between permeability coefficient of the packaging material and moisture adsorbed by the powder determined by the GAB equation. Furthermore, the storage stability of spray-dried yogurt powder in terms of quality parameters including moisture content, water activity, color change, and thiobarbituric acid (TBA) value was studied in aluminum laminated polyethylene (ALPE) pouches under storage conditions of 25 C and 50% relative humidity (RH). Keywords Moisture adsorption isotherm; Sorption model; Spray drying; Storage stability; Yogurt powder INTRODUCTION Yogurt powder, produced by drying of fresh yogurt, can be used as an ingredient for the manufacturing of many food products such as confectioneries, bakery foods, yogurt drink mixes with fruits or vegetables, instant drink mixes, soup bases, dips, and sauces as well as for direct consumption after reconstitution. Dried yogurt, with a long-term ambient shelflife, can reduce transportation and storage costs as well. [1] Common effects of improper storage conditions on yogurt powder quality are browning in color or an unpleasant stale flavor as a result of generation of insoluble compounds from the Maillard reaction. [2] The oxidative stability of powder is influenced by the heat treatment, storage temperature, packaging material, moisture content, and water activity. [3] As an indication of lipid oxidation, some increases were noted in peroxide and thiobarbutyric acid (TBA) values during storage of powder products. Lipid peroxidation is responsible for the changes in taste Correspondence: Prof. Dr. Figen Kaymak Ertekin, Ege University, Faculty of Engineering, Department of Food Engineering, 35100, Bornova, Izmir, Turkey; figen.ertekin@ ege.edu.tr and odors of food products such as milk powders and yogurt powders through the development of off-flavors, which are caused by the formation of secondary reaction products (alkanes, alkenes, aldehydes, and ketones). [4] For a low-moisture food product, shelf-life prediction by theoretical calculations is practical because actual storage testing, using a long-term stability study, is time consuming and costly because of regular analysis at each time interval during storage. The theoretical shelf life calculation methods are based on the relationship between moisture content of food product, storage environment, and barrier property of packaging material. [5] Moisture sorption isotherms of foods provide critical information that can be used in predicting shelf-life by theoretical calculation. Moisture sorption isotherms can also be used to investigate structural features such as specific surface area, pore volume, pore size distribution, and crystallinity of food products. [6] Many research studies on the sorption isotherms of certain foods, temperature dependence of isotherms, determination of heat of sorption, and mathematical models to represent sorption isotherms have been reported in the literature. [7 15] However, there are very few reports on moisture sorption isotherms of yogurt powder. [16 19] Because the sorption behavior and the reconstitution properties of yogurt powder are affected by the drying method, particle size, and raw material properties, determination of the adsorption isotherms of yogurt powder is required. The objectives of this study were to measure the water sorption behavior of the spray-dried yogurt powder (produced by spray drying at optimum conditions) to predict storage stability. Furthermore, the storage stability of yogurt powder was investigated in terms of quality parameters (moisture content, water activity, color, TBA value) after packaging in aluminium-laminated polyethylene (ALPE; 12 mm PET, 8mm Al, 58mm PE) pouches and stored at 25 C, 50% relative humidity (RH) for 90 days. The shelf-life of spray-dried yogurt powder was predicted by theoretical calculation on the basis of critical water activity value of powder. The prediction of the quality parameters other than water activity and moisture content are also necessary for real shelf-life estimation of yogurt powder. 816

3 SPRAY-DRIED YOGURT POWDER 817 MATERIAL AND METHODS Materials Yogurt powders used in adsorption and storage experiments were obtained by drying in a pilot-scale spray dryer (Niro-Atomizer Mobile Minor Unit, Soeborg, Denmark) at the optimum conditions previously determined. [20] The spray-drying conditions were outlet air temperature of 60.5 C, inlet air temperature of 171 C, and feed temperature of 15 C. The composition and properties of spraydried yogurt powder were determined, experimentally. Its moisture content (%, wet basis) is 6.92; titratable acidity (lactic acid %) is 1.12; ph is 4.17; total lactic acid bacteria count (cfu=g dry matter) is 4.59 * 10 8 ; and it contains no yeast and mold. For storage experiments, the ALPE film was supplied from Sesa Inc. (Izmir, Turkey). Determination of Sorption Characteristics of Yogurt Powder The adsorption isotherms of yogurt powder were determined with the standard static-gravimetric method. [21] Eight saturated salt solutions selected to give different relative humidity in the range of were used. The salt solutions used and their corresponding relative humidities at different temperatures were taken from data reported by Greenspan [22] and given in Table 1. Three replications of the same experiment were carried out. The glass sorption jars were placed in a temperature-controlled cabinet, with an accuracy of 1 C at the selected temperatures 10, 25, and 40 C. Equilibrium was reached when the sample weight difference between two successive measurements was less than the balance accuracy g. The equilibrium moisture content of the samples was determined by drying in an oven at 102 C for 2 h. [23] To establish moisture sorption isotherms, the equilibrium moisture content values were plotted against water TABLE 1 Water activity values of the saturated salt solutions at three temperatures used in the experiment Salt Temperature ( C) LiCl CH 3 COOK MgCl K 2 CO Mg(NO 3 ) NaBr SrCl KCl Adapted from Greenspan. [22] activity (a w ). Five mathematical models (Guggenheim- Anderson-de Boer [GAB], Brunauer-Emmett-Teller [BET], Halsey, modified Oswin, and modified Henderson) were used describing the relationship between the equilibrium moisture content and water activity. The sorption equations are shown in Table 2. The parameters of the sorption models were estimated from the experimental results using nonlinear regression analysis (SPSS 13.0 for Windows), which minimizes the residual sum of squares. Best-fitting equations were evaluated with the mean relative percentage deviation (P) value and the percentage root mean squares error (RMSE) value. P value is defined as P ¼ 100 X N M exp M cal N M i¼1 exp and RMSE value defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 Mcal M exp Mexp RMSE ¼ 100 N where M exp andm cal are experimental and predicted moisture content values, respectively, and N is the number of experimental data. A model was considered acceptable if the P values and RMSE values were below 10% and R 2 values were higher than TABLE 2 Equations describing the sorption equilibrium isotherms Name of the equation BET [30] GAB [31] M ¼ M ¼ Equation ð1þ ð2þ M 0 caw ½ð1 a w Þþðc 1Þð1 a w Þa w Š ð3þ M 0 CKa w ½ð1 Ka w Þð1 Ka w þ CKa w ÞŠ ð4þ C ¼ C 0 exp ðdh c =RTÞ K ¼ K 0 exp ðdh k =RTÞ ð5þ ð6þ Halsey [32] a w ¼ exp ½ k=m n Š ð7þ Modified Henderson [33] a w ¼ 1 ½expð a ðt þ cþm b ÞŠ ð8þ Modified Oswin [34] a w M ¼ k þ c TÞð Þ n ð9þ 1 a w

4 818 KOÇ ET AL. Prediction of Shelf-Life and Assessment of Quality Changes in Yogurt Powder during Storage ALPE pouches (120 mm 150 mm) were filled with 100 g of yogurt powder samples and were closed by heat sealing, avoiding any air space but without vacuum application. The water vapor permeability, k (g * m 2 * day 1 * Pa 1 ), of the packaging material (ALPE) was measured using a water vapor permeability analyzer (Mocon Permetran-W, 3=33 G, Mocon Company, Minneapolis, MN, USA). ALPE sealed pouch samples were placed in desiccators containing a saturated solution of magnesium nitrate giving desirable RH at 25 C and the desiccator was placed in an incubator maintained thermostatically at 25 C in the dark. Storage was carried out at 25 C and 50% RH for 90 days and was replicated at the same conditions. Pouches were placed in a vertical pouch holder to ensure that they should not contact each other and that pouches are exposed to the same environment. One ALPE pouch was taken out from the desiccator at regular intervals of 15 days during 90 days storage and analyzed for quality parameters like moisture content, water activity, color, and TBA value. Sampling was repeated following the same procedure as explained above at each analysis date. The shelf life was calculated using the following equation [5] : Z t ¼ w d k A Z mc m i dm ðp 0 p i Þ ð10þ where t is the storage time (day), w d is the dry weight of yogurt powder (g), m is the moisture content of yogurt powder on % dry basis, m i is the initial moisture content (%, db) of the sample, m c is the critical moisture content (%, db), k is the water vapor permeability coefficient (g * m 2 * day 1 * Pa 1 ), A is the area of the package (m 2 ), p 0 is the partial pressure of water vapor (Pa) of storage environment, and p i is the partial pressure of water vapor (Pa) inside the package headspace related to the moisture content of the product determined by GAB model. The moisture content was determined gravimetrically by oven drying at 102 Cfor2h. [23] The water activity (a w )of yogurt powder was measured using a water activity measurement device (Testo AG400, Lenzkirch, Germany) with sensitivity. The color values of powder samples were measured as CIE parameters L, a, and b with a colorimeter (Hunter Lab, Color Flex, Reston, VA). The L, a,andb values of the fresh as well as stored powder were measured and the net (total) color difference (DE) was calculated using Eq. (11). DE ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DL 2 þ Da 2 þ Db 2 ð11þ The TBA was measured according to the method modified by King [24] and the results were expressed as optical density measured at 532 nm in a spectrophotometer (Novaspec II (Pharmacia Biotech, Pharmacia LKB, Montreal, Canada). RESULTS AND DISCUSSION Fitting of Sorption Models to Experimental Data The adsorption isotherms of spray-dried yogurt powder at 10, 25, and 40 C are shown in Fig. 1, which shows temperature dependence of the isotherms. The equilibrium moisture content at each aw represents the mean value of three replications. The standard deviation of each experimental point (kg=kg dry solids) was within the range of to The adsorption isotherms of spray-dried yogurt powder showed type II behavior according to the BET classification, typical of the many sorption isotherms of food. [8,9] The equilibrium moisture content increases with decreasing temperature at constant a w. This phenomenon can be explained by the fact that the proteins and carbohydrates are known to have more water-binding capacity at low temperatures compared to high temperatures. [8] This indicates that the powder became less hygroscopic at higher temperatures. Similar results have been reported in the literature for the sorption curve of yogurt powder. [17,18] The results of nonlinear regression analysis of fitting the sorption equations to the experimental data are presented in Tables 3, 4, and 5. The mean relative percentage deviation (P), RMSE, and correlation coefficient (R 2 ) values are also given in Tables 3, 4, and 5. It can be seen that the GAB equation (Table 3) gave the best fit to the experimental adsorption data of yogurt powder for a wide range of water activity ( ). This observation is consistent with the results obtained by Kim and Bhowmik [16] and Kumar and Mishra. [17] In order to confirm the GAB equation s ability in predicting the equilibrium moisture content data for yogurt powder, P (%) values at three temperatures were calculated and the results were found to be 6.097% at 10 C, 8.687% at 25 C, and 8.520% at 40 C. FIG. 1. Adsorption isotherms of yogurt powder samples at different temperatures.

5 SPRAY-DRIED YOGURT POWDER 819 TABLE 3 Estimated parameters for GAB model of adsorption isotherms of spray-dried yogurt powder M 0 (kg=kg dry solids) C 0 K 0 DH c (kj=mol) DHk (kj=mol) P (%) RMSE (%) R TABLE 4 Estimated parameters, P (%), RMSE (%), and R 2 values of the sorption equations that account for temperature effect Model Coefficients RMSE k c n b P (%) (%) R 2 Modified Oswin Modified Henderson Constant K values for spray-dried yogurt powder in the GAB equation varied between 0.85 and 0.91 at different temperatures. The value of K provides a measure of the interactions between the molecules in multilayers with the adsorbent and tends to fall between energy value of the molecules in the monolayer and that of liquid water. If K value is equal to 1, the multilayers have properties of liquid water and the sorption behavior could be modeled by the BET equation. [25] The K and C values decreased with increasing temperatures as observed by other researchers. [16,17] The monolayer moisture content (M 0 ) helps to define physical and chemical stability of foods because it has a direct influence on lipid oxidation, enzyme activity, nonenzymatic browning, flavor preservation, and product structure. The BET and GAB monolayer moisture contents for yogurt powder were kg=kg dry solid and TABLE 5 Estimated parameters, P (%), RMSE (%), and R 2 values of the sorption equations that are invariant with temperature Model Parameters Temperature ( C) Halsey k n P (%) RMSE (%) R BET a M c P (%) RMSE (%) R a a w Range for BET equation was ; only the first four data points are used kg=kg dry solid, respectively (Tables 3 and 5). It should be noted from Table 5 that monolayer moisture content values (M 0 ) obtained by the BET model decreased with increasing temperature. Another point to note is that the M 0 value obtained by the GAB equation was higher than that obtained by the BET model. Basu et al. [26] also reported that the M 0 value given by the BET isotherms is always lower than the monolayer value derived from GAB and, reversibly, the BET value of energy constant is always higher than the GAB value. High M 0 values may be due to considerably high sorption capacity through the swelling of colloidal material increasing more polar sites. Kumar and Mishra [17] reported that M 0 values estimated from the GAB model ( % dry basis) for adsorption of convectively dried yogurt powder were lower than our findings. Moreover, the modified Oswin and modified Henderson equations (Table 4) gave a satisfactory prediction of the adsorption equilibrium moisture content of yogurt powder. As seen in Table 5, the BET equation gives the lowest P % and RMSE % value in the water activity range Furthermore, the Halsey equation representing multilayer adsorption gave the best fit to the experimental data for yogurt powder studied at different temperatures within a wide water activity. It should be noted that the goodness of fit of any sorption model to the experimental data shows only a mathematical quality and not the nature of the sorption process. [8] The experimental adsorption data and the predicted values obtained from different sorption models at 25 C are shown in Fig. 2. Similar plots were obtained for the other temperatures. Shelf-Life of Yogurt Powder Experimental shelf-life value of the spray-dried yogurt powder in ALPE packages stored at 25 C and 50% RH

6 820 KOÇ ET AL. FIG. 2. Comparison of experimental and predicted adsorption equilibrium moisture contents of spray-dried yogurt powder at 25. for 90 days was determined. On the other hand, the simulation model (Eq. (10)) was used to predict the shelf-life of the packaged yogurt powder using the relationship between moisture content of the product determined by the GAB equation and permeability coefficient of the packaging material. The rate of moisture uptake is governed by the environmental conditions and the water activity inside the package headspace as well as package permeability. The initial moisture content of yogurt powder was 7.35% (db). The final moisture content (8.66% db) was decided on the basis of the free-flowing ability of the powder and its water activity. This moisture content was considered as critical moisture content and the corresponding water activity was calculated using the GAB parameters. The values of the GAB parameters M 0, C, and K were 8.37% (db), , and , respectively, at 25 C. The water vapor permeability coefficient of ALPE was * 10 7 g * m 2 * day 1 * Pa 1. The area A of pouches ALPE was m 2. By putting the values in Eq. (10), the shelf-life of the spray-dried yogurt powder was calculated as 140 days in ALPE pouches under these storage conditions. Packaged yogurt powder at a certain storage period is subject to moisture gain depending on a number of factors: the storage temperature and relative humidity; the sorption properties of the product; the water activity gradients relative to the storage atmosphere; and the water vapor permeability of the packaging materials. [5] Quality Changes in Yogurt Powder during Storage Change in moisture content (in dry basis) of yogurt powder during storage is shown in Fig. 3. There was a slight increase in moisture content of samples. Reh et al. [27] stated that the moisture content had a strong influence on the physicochemical stability of milk powder during storage and distribution and the oxidative stability of milk powder varied with the moisture content. FIG. 3. Changes in moisture content (% dry basis) of spray-dried yogurt powder packaged in ALPE packages during storage at 25 C and 50% RH. The a w value of yogurt powders was steady at 0.14 at the end of the storage, as shown in Fig. 4. After 90 days storage, however, the a w values of yogurt powders were also considered in acceptable ranges, which corresponded to the monolayer moisture content for the survival of lactic acid bacteria. Packaging material must ensure minimal penetration of moisture into the product, and the food material should have the lowest possible water activity to minimize water migration. [28] One remarkable and undesirable effect of high relative humidity is caking, a process that results from particle bridging and is favored by high moisture contents. Color change with storage time is shown in Fig. 5. Color change was steady up to 75 days storage but then began to increase with storage time. Nielsen et al. [29] also reported that the powdered milk gradually turned darker and yellower with increasing storage time. The thiobarbutyric acid value is a measure of the formation of secondary lipid oxidation products; for example, carbonyls, which are responsible for the sensory impact of lipid oxidation. The optical density of spray-dried yogurt FIG. 4. Changes in water activity of spray-dried yogurt powder packaged in ALPE packages during storage at 25 C and 50% RH.

7 SPRAY-DRIED YOGURT POWDER 821 FIG. 5. Changes in color (DE) of spray-dried yogurt powder packaged in ALPE packages during storage at 25 C and 50% RH. FIG. 6. Changes in TBA values of of spray-dried yogurt powder packaged in ALPE packages during storage at 25 C and 50% RH. powder increased to by 90 days of storage for ALPE packaged samples from the initial optical density of yogurt powder of 0.09 (Fig. 6). Storage time significantly affected the TBA value. In yogurt powder the amounts of phospholipids decreased on storage whereas the contents of monoglycerides and 1,2- and 1,3-diglycerides and free fatty acids (measured as peroxide and TBA) increased. CONCLUSIONS The equilibrium moisture content of yogurt powder increased with decreasing temperature at constant water activity. Among the sorption models tested, the GAB and modified Oswin equations were recommended for characterizing the adsorption behavior of spray-dried yogurt powder in the temperature range of C and water activity range of The adsorption data were also expressed by the Halsey equations in a wide range of water activity and by the BET equation in the range of water activity The shelf-life of yogurt powder in ALPE pouches at 25 C and 50% RH storage conditions was determined as 140 days based on the equilibrium moisture content of the powder, described by the GAB equation and the package permeability. During storage at 25 C and 50% RH for 90 days, a slight increase in moisture content and a steady a w value of yogurt powder was observed. The a w value was within the acceptable ranges, which corresponded to the monolayer moisture content for the survival of lactic acid bacteria. Yogurt powder color was nearly steady up to 75 days of storage but then began to increase with storage time. In addition, storage time significantly resulted in an increase in TBA value of the powder product, which gave a sensory impact of lipid oxidation. Because moisture content is probably not the only parameter influencing the shelf-life of the packaged yogurt powder, other quality parameters such as color and TBA value of powder should be taken into account in shelf-life estimation. NOMENCLATURE A Area of the package (m 2 ) a CIE parameters a w Water activity b CIE parameters b, c, k, n Empirical parameters db Dry basis C, K Empirical GAB parameters in Eq. (6) C 0, K 0 Entropic accommodation factors in Eqs. (7) and (8) DH c, DH k Functions of the heat of sorption of water (kj=mol); DH c ¼ H m H n ; DH k ¼ H H n H Heat of condensation of water vapor (kj=mol) H m, H n Heats of sorption of mono- and multilayers of water (kj=mol) DHvap Heat of vaporization (kj=mol water) k Water vapor permeability coefficient (g * m 2 * day 1 * Pa 1 ) L CIE parameters M Equilibrium moisture content (kg=kg dry solids); M 0 the monolayer; M 0.5 at the water activity of 0.5; M cal the predicted; M exp the experimental m c Critical moisture content (% db) m i Initial moisture content (% db) N Number of experimental data P Mean relative percentage deviation p i Partial pressure of water vapor (mm Hg) inside the package headspace related to the moisture content of the product determined by the GAB model p 0 Partial pressure of water vapor (Pa) of storage environment R Universal gas constant (kj=mol. K) R 2 Correlation coefficient RMSE Percentage root mean square error T Absolute temperature (K) t Storage time (day) Weight of the dry solids (g) w d

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