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Mass Transfer Properties of Osmotic Solutions. I. Water Activity And This article was downloaded by: [b-on: Biblioteca do conhecimento online UP] On: 24 October 2011, At: 16:02 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK International Journal of Food Properties Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ljfp20 Mass transfer properties of osmotic solutions. I. Water activity and osmotic pressure Vassilis Gekas a , Chelo Gonzalez b , Alberto Sereno c , Amparo Chiralt b & Pedro Fito b a Food Engineering, Lund University, Lund, Sweden b Universidad Politecnica de Valencia, Valencia, Spain c Escola Superior de Biotecnologia, Oporto, Portugal Available online: 02 Sep 2009 To cite this article: Vassilis Gekas, Chelo Gonzalez, Alberto Sereno, Amparo Chiralt & Pedro Fito (1998): Mass transfer properties of osmotic solutions. I. Water activity and osmotic pressure, International Journal of Food Properties, 1:2, 95-112 To link to this article: http://dx.doi.org/10.1080/10942919809524570 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and- conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, 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. INTERNATIONAL JOURNAL OF FOOD PROPERTIES, 1(2), 95-112 (1998) MASS TRANSFER PROPERTIES OF OSMOTIC SOLUTIONS. I. WATER ACTIVITY AND OSMOTIC PRESSURE Vassilis Gekas1'*, Chelo Gonzalez2, Alberto Sereno3, Amparo Chiralt2, and Pedro Fito2 1Food Engineering, Lund University, Lund, Sweden 2Universidad Politecnica de Valencia, Valencia, Spain 3Escola Superior de Biotecnologia, Oporto, Portugal. *Corresponding author ABSTRACT In this review paper data on water activity, solute activity and osmotic pressure of" binary and multi-component osmotic solutions are provided. The Characteristics of the osmotic solutions are needed for the optimization of mass transfer during osmotic process, and for the improvement of final product quality. The vant Hoff equation and Gibbs Duhem theorem are commonly used to estimate osmotic pressure and solute activity. Water activities can be easily estimated through experimental determination of the freezing point depression. The possibilities of the group contribution models such as the Analytical Solution of Groups (ASOG) approach are also explored. The future needs especially in the case of multicomponent solutions consisting of electrolyte and non-electrolyte mixtures are pointed out. INTRODUCTION A number of food processing unit operations imply immersion of the food in a high osmotic pressure medium containing sugars, such as sucrose, glucose, fructose, syrups Downloaded by [b-on: Biblioteca do conhecimento online UP] at 16:02 24 October 2011 and salts, such as sodium chloride or their mixtures. Foods that are treated this way are fruits and vegetables or also meat and fish (Fito et al., 1994; Lazarides, 1994; Lenart and Flink 1984a, 1984b; Lenart, 1994; Lerici et al., 1985). The aims of the osmotic process are: partial dehydration before the final treatment such as drying or freezing, impregnation of solute to improve quality (i.e., cryoprotectant), osmo-freezing or thawing directly in an osmotic medium, and direct formulation of food products. 95 Copyright © 1998 by Marcel Dekker, Inc. 96 GEKAS ET AL. Research on the above topics so far has shown that the performance of the osmotic unit operations depends on the properties of the osmotic solutions used. From the mass transfer point of view the most important osmotic solution parameter is its water activity lowering capacity in terms of water activity or osmotic pressure- this is an important property for the purpose of dehydration. Due to the simultaneous mass transfer, i.e. water transport from the food to the osmotic medium and solute transport from the osmotic medium to the food, additional information on the solute size and solute activities are also important. A literature review has shown that so far the properties of the osmotic solution considered in order to interprete the unit operations results were solute concentration and only in a few cases there has been reported solution water activity data and to the authors' knowledge there is absent of osmotic pressure data or solute activity data. It is also known that concentrated solutions used in osmosis are real solutions which might deviate strongly from the ideal situations, thus activities in addition to concentrations should provide a more sound theoretical basis for the characterization of the osmotic solutions and better interpretation of the osmotic process. Commonly used osmotic solutions, based on FSTA database 1969-1996 are presented in Table 1. As it is shown, a common osmotic medium used especially for fruits is the sucrose solution or syrup of a concentration range of 40-70 Brix and most frequently used one is 60 Brix. Other sugars such as glucose, fructose, lactose have also been used. Various Dextrose Equivalent (DE) corn syrups have been used for fruits and vegetables whereas for potato, fish and meat, salt solutions (NaCl 15% are being the most common among them) were the preferred media. In a few cases a combination between a sugar(s) and a salt was used. The objective of this paper is to review data of water activity and their prediction models for binary and multi-component osmotic solutions commonly used for osmotic dehydration of fruits. A. PUBLISED EXPERIMENTAL DATA ON WATER ACTIVITIES. In Table 2 there are shown values of freezing point depression for various osmotic solutions obtained at our laboratory of Lund University. For comparison litterature values are also presented. In Table 3 the values of water activity of the same solutions as in Table 2 are presented along with the litterature values for comparison. Table 4 shows water activity values of glycerol solutions along with refractive index values of this solutions (Rizvi, 1995). Table 5 contains water activity values of NaCl from Chirife and Resnik (1984). Tables 6 and 7 provide literature data of osmosities and Downloaded by [b-on: Biblioteca do conhecimento online UP] at 16:02 24 October 2011 water activity of sugar and electrolyte solutions. B. METHODS OF MEASUREMENT Freezing Point Depression Different methods of water activity measurement are reviewed by Labuza (1984), Rizvi (1995), and Rahman (1995). It is common and simple to measure water activity (or osmotic pressure) of two-component and three-component osmotic solution using OSMOTIC SOLUTIONS. I 97 Table 1. Osmotic solutions commonly used in osmosis Solution Type Concentration & temperature Types of foods BINARY Sucrose 40-70B, 30-70°C Apple, pineapple, carrot, kiwi, grapes, mushroom, papaya, coconut. Glucose 40-60 B, 25-40°C Strawberries, plum, pineapple, apple, pear, cherry, apricot, carrot Glyserole 10/25%, 5°C Strawberries NaCl 8-25% , 8-40°C Potato, okra, pepper, carrot, aubergine, green beans, meat, fish MULTICOMPONENT Sucrose + NaCl 45 % - 15% Potato, apple, pineapple or 50%-10% 20-40°C Sucrose +Xylitol 30% + 70% Vegetables Corn syrup solids 34-70%, Papaya, apple, some vegetables DE10-40 35-55°C Corn Syrup / Sucrose/Water 5/3/1, 70B Cherries Table 2. Freezing point depression of sugars (Gonzalez et al., 1995) Solution type Measured values Litterature values in the authors's laboratory Mean Fructose 30% -4.84 -4.75 -4.79 -4.70 Sucrose 50% -7.24 Downloaded by [b-on: Biblioteca do conhecimento online UP] at 16:02 24 October 2011 -7.64 -7.54 -7.61 -7.64 Sucrose 52% -8.97 -8.87 -8.92 -8.40 Sucrose 60% -12.30 -12.90 -12.70 -12.45 -12.90 Sucrose 60% + NaCl 10% -28.54 -28.14 -28.34 98 GEKAS ET AL. Table 3. Water activity measured in the authors' laboratory and the litterature values Solution From measured values From litterature values Fructose 30% 0.955 0.954 (a) 0.961 (b) Sucrose 50% 0.930 0.929 (a) Sucrose 52% 0.918 0.922 (a) Sucrose 60% 0.882 0.874 (a) Sucrose 60% + NaCl 10% 0.757 0.751 (c) (a) From Ferro-Fontan-Chirife Equation (b) Measured by electric hygrometer (c) From Caurie model Table 4. Water activity of glycerol solutions (Rizvi, 1995) Concentration Refractive Index Water Activity (kg/L) 1.3463 0.98 1.3560 0.96 0.2315 1.3602 0.95 0.3789 1.3773 0.90 0.4973 1.3905 0.85 0.5923 1.4015 0.80 0.6751 1.4109 0.75 0.7474 1.4191 0.70 0.8139 1.4264 0.65 Downloaded by [b-on: Biblioteca do conhecimento online UP] at 16:02 24 October 2011 0.9285 1.4387 0.55 0.9760 1.4440 0.50 1.4529 0.40 OSMOTIC SOLUTIONS. I 99 Table 5. Water activity of NaCl solutions1"2 Concentration Water Activity Concentration Water Activity (%, w/w) (%, w/w) 0.5 0.997 10 0.935 1.0 0.994 11 0.927 1.5 0.991 12 0.919 2.0 0.989 13 0.911 2.5 0.986 14 0.902 3.0 0.983 15 0.892 3.5 0.980 16 0.883 4.0 0.977 17 0.873 4.5 0.973 18 0.862 5.0 0.970 19 0.851 5.5 0.967 20 0.839 6.0 0.964 21 0.827 6.5 0.960 22 0.815 7.0 0.957 23 0.802 7.5 0.954 24 0.788 8.0 0.950 25 0.774 9.0 0.943 26 0.759" 1 In the temperature range 15-5O°C 2 Data source Chirife and Resnik (1984) a Saturation point freezing point depression method.
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