JFAE(Food & Health-PartA) Vol2-3&4 (right2004)

WFL Publisher Science and Technology

Journal of Food, Agriculture & Environment Vol.2 (3&4) : 34-39. 2004

www.world-food.net

Water sorption isotherms and heat of sorption of osmotically pretreated and oven-dried papaya varieties K.O. Falade* and P. I. Uzo-Peters Department of Food Technology, University of Ibadan, Ibadan, Nigeria, P. O. Box 9508 U I Post Office Ibadan, Nigeria. *e-mail: [email protected], [email protected] Received 6 August 2004, accepted 18 October 2004.

Abstract Adsorption isotherms of osmotically pretreated and subsequently oven-dried papaya (Carica papaya) varieties (Sunrise Solo, Homestead Green, Homestead Purple) were investigated using gravimetric static method. Ripe papaya varieties were cut into 10 mm x 10 mm x 50 mm slabs and immersed into 52, 60 and 68°B sucrose solutions maintained at 25°C in water bath. A fruit:solution ratio of 1:20 was maintained for 12 hours. Osmosed papaya slabs were subsequently oven-dried at 60°C for 72 hours. Adsorption isotherms of osmotically pretreated and oven-dried papaya varieties followed the type III (J-shaped) isotherms. Depression of water activity occurred in preosmosed oven-dried papaya slabs pretreated at higher sucrose solution concentrations. Guggenheim-Anderson-deBoer (GAB) model was used to fit the experimental data and to calculate monolayer moisture (Mm) content. Mm increased with an increase in pretreatment sucrose solution concentration. However, Mm decreased with increased equilibrium temperature. Isosteric heat of sorption increased with decreased moisture contents and pretreatment sucrose solution concentration. Heat of sorption of preosmosed oven-dried papaya slices was in the following order: ‘Homestead Purple’ > ‘Homestead Green’ > ‘Sunrise Solo’. Key words: Adsorption isotherm, heat of sorption, osmotic dehydration, oven-drying, papaya.

Introduction Papaya (Carica papaya) originated in tropical America but is now common in the tropics worldwide. Papaya fruit is an important source of vitamins and minerals, especially vitamin A and C. High demand for fresh fruit and availability of high yielding and quick-maturing varieties with little or no labour input have produced new papaya plantations in many parts of Nigeria1. However, during the papaya season, there is usually a glut, resulting in over 50% postharvest losses due to inadequate handling and storage facilities and spoilage. Moreover, in spite of the great potential of papaya fruits, the utilization is very low. Consumption is therefore limited mostly to the production area and in the fresh form. Processing of papaya fruits into intermediate moisture (IM) fruit products using minimal inputs and low cost of production is needed. Osmotic dehydration or dewatering and impregnation soaking process (DISP) can be used as a pretreatment before any complementary processing and may lead to energy savings and quality improvements 2. Complementary treatments such as hot air or vacuum drying may be applied to previously osmosed fruit to produce intermediate moisture fruit product 3-5. Partial dehydration by immersion of slices of fruits or vegetables in concentrated solution of sugar or salt creates basically two simultaneous mass transfers: water out-flow from product to solution and solute transfer from solution to product. Thus, partial dewatering and direct formulation of food pieces can be obtained by immersion in concentrated solution. Moisture–solid transfer during osmotic pretreatment alters product composition and thus, influences drying, sorption and storage characteristics of the final product. Moisture sorption isotherms are useful thermodynamic tools 34

for determining interaction of water and food substance and for providing information to assess food processing operations such as drying, mixing, packaging and storage 6. Sorption data can also be used to select appropriate drying and storage conditions and packaging systems that optimize or maximize the retention of aroma, colour, texture and nutrients and biological stability. In order to better control the quality of preosmosed oven-dried papaya during production and storage, data are needed on physical properties including equilibrium moisture content (EMC)/ water activity relationships, which may be expressed as moisture sorption isotherms. Heat of sorption of water in dehydrated and intermediate moisture foods is essential for modeling of various food processes and storage 7, 8. It can be used to estimate the energy requirements of food dehydration, and it provides information on the state of water in food products 9. The objectives of this work were to evaluate the effect of osmotic pretreatment on the adsorption isotherms of preosmosed oven-dried papaya varieties and to fit the experimental data into Guggenheim-Anderson-deBoer equation and also to estimate the isosteric heat of sorption of the products. Materials and Methods Three varieties of papaya, Sunrise Solo, Homestead Green and Homestead Purple, were obtained from the orchard of National Institute of Horticultural Research and Training (NIHORT), Ibadan, Nigeria. Mature fruits were harvested and kept at 25°C until they attained a ripe (95% yellow surface colour) but firm stage. Sucrose (table sugar) was used in preparing the osmotic solutions of 52, 60 and 68°B. The sucrose solutions were maintained at 25°C.

Journal of Food, Agriculture & Environment, Vol.2 (3&4), August&December 2004

Papaya fruits were washed, peeled and cut into halves to facilitate the removal of the reeds and rags. Papaya pulps were cut manually into slabs of 10 mm x 10 mm x 50 mm using sharp stainless steel knives. The fruit slabs were packed into perforated wire meshes and immersed in the sucrose solutions. A fruit:solution ratio of 1:20 was maintained to prevent changes in solution concentrations. After 12 hours of immersion, papaya slabs were removed, drained free of syrup, arranged on stainless steel trays and oven-dried at 60°C in a Gallenkamp (Model OV-160) forced raught oven for 72 hours. Determination of moisture sorption isotherm of osmotically pretreated and oven-dried papaya slices: Adsorption isotherms of osmotically pretreated and oven-dried papaya slabs were determined using gravimetric-static method. Solutions of sulphuric acid were prepared to give different constant relative humidities of 10-90% at 20 and 40°C 10. Preosmosed oven-dried papaya slabs were weighed on plates and placed in desiccators. The desiccators were kept in a constant temperature cabinet incubators (illuminated cooled Griffin incubator). Samples were weighed regularly until equilibrium was reached after about 18 days. Moisture contents of equilibrated slices were determined according to the method of Sankat et al.3. Averages of the triplicate equilibrium moisture contents determinations were computed for each sample and at selected temperatures. Adsorption isotherms were obtained by plotting the equilibrium moisture content (EMC) against water activity (equilibrium relative humidity/100). Determination of isosteric heat of sorption: The net isosteric heat of sorption, ∆Hst, was determined from the following expression derived from Clausius-Clapeyron equation 11, 12 applied to food and pure water.

· §. ¨ w l a ¸ R¨ n w ¸ ¨ w1 ¸ T ¹ ©

ǻHst =

Results and Discussion It is expected that moisture contents of dried materials increase with increased water activity (aw). Adsorption isotherms of osmosed and oven-dried papaya slabs followed the expected trend (Figs 1-3). Generally, preosmosed oven-dried papaya slabs sorbed low moisture at low and intermediate aw (0.1-0.6) range. However, moisture contents of products increased rapidly in the high aw range (0.6-0.9). Adsorption isotherms of preosmosed oven-dried papaya slices followed the type I (J-shaped) classification scheme of Brunauer et al.13. Such isotherms are usual for sugar foods 8, 11, 14. The low moisture sorbed at low and intermediate aw range by the preosmosed oven-dried papaya slabs indicated physical sorption on the strongly active sites of the biopolymers 8, 11, 15. The rapid increase in moisture sorbed at high aw range (0.60-0.90) is due to dissolution of the sugars in the preosmosed oven-dried papaya slabs. Equilibrium moisture content of a food product increases considerably when placed in an atmosphere with water activity above the vapour pressure of saturated solution of its soluble solids 14, 16. Moreover the sugars become amorphous and extend numerous sorptive sites to bind excessive amounts of water 17. Generally, papaya slabs preosmosed in higher sucrose solution concentration prior to oven-drying sorbed more moisture than those pretreated in lower sucrose solution concentration (Figs 1-3). Higher osmotic solution concentration increased solids infusion into fruit pieces during osmotic pretreatment18. Total solids content of papaya slices increased with an increase in pretreatment sucrose solution. Higher solids in the preosmosed oven-dried papaya resulted in lowering of aw due to quantitatively different composition and high sugar content. Bolin 6 reported similar results for raisins prepared from grapes with different solids content. Depression of water activity by dissolved solutes is one well-known factor in foods19. In fact, sugars (humectants) are used to bind water and to allow food products to maintain a soft palatable texture 20.

where ∆Hst is the net isosteric heat of sorption at constant moisture content (kJ mol-1 H2O), aw is the water activity, T is the absolute temperature (K) and R is the gas constant (8.314 J mol-1K-1). The water activity of a food material is equal to the equilibrium relative humidity fraction of the surrounding space: aw = P/Pw , where P and Pw are respectively the partial pressure of water vapour and the saturated water vapour pressure at the system temperature. Assuming that the net isosteric heat of sorption does not change significantly with temperature, equation can be integrated to yield: aw = αo exp (-∆Hst / RT), where αo is a constant. The study of isotherms at least two temperatures provides thermodynamic data on isosteric heat of sorption through use of the integrated form of the above equation at fixed equilibrium moisture content:

§a l n ¨¨ w1 © a w2

· ¸¸ ¹

Qst § 1 1 · ¨ ¸ R ¨© T1 T2 ¸¹

where aw1 and aw2 are water activities at temperatures T1 and T2, respectively. Figure 1. Effect of sucrose pretreatment solution concentration on adsorption isotherm of oven-dried ‘Sunrise Solo’ papaya slices at 40°C.


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Figure 2. Effect of sucrose pretreatment solution concentration on adsorption isotherm of oven-dried ‘Homestead Purple’ papaya slices at 40°C.

Figure 5. Effect of temperature on adsorption isotherm of oven-dried ‘Homestead Purple’papaya slices preosmosed in 52°B sucrose solution.

Figure 4. Effect of temperature on adsorption isotherm of oven-dried ‘Sunrise Solo’ papaya slices preosmosed in 52°B sucrose solution.

In general, moisture content is expected to decrease with increasing temperature at a given water activity. Adsorption isotherms of preosmosed oven-dried papaya varieties followed this same trend (Figs 4-6). Thus, preosmosed oven-dried papaya slabs became less hygroscopic at higher temperature, as expected on the basis of the fundamental thermodynamic equation21. Pääkkönen and Mattila22 reported a similar trend for freeze-dried strawberries. 36

Figure 3. Effect of sucrose pretreatment solution concentration on adsorption isotherm of oven-dried ‘Homestead Green’ papaya slices at 40°C.

Monolayer moisture (M m ) content: Tables 1-3 show the Guggenheim-Anderson-deBoer (GAB) model parameters and correlation coefficients (r2) for the preosmosed oven-dried papaya varieties. Generally, monolayer moisture (Mm) content increased with an increase in pretreatment sucrose solution concentration. Moreover, M m decreased with an increase in equilibrium temperature. The temperature dependence of the monolayer value has been linked 23 to a reduction in sorption active sites as a result of physico-chemical changes induced by temperature 24.


Table 1. GAB parameters of osmotically pretreated and oven-dried ‘Sunrise Solo’ papaya slices. Equilibrium temperature(°C)

Pretreatment solution concentration (°B)

k

C

20 20 20 40 40 40

52 60 68 52 60 68

0.94939 0.95447 0.95101 0.92337 0.91271 0.93324

20.3019 22.6702 20.9734 13.2544 11.1728 15.0838

Mm (g H20/g d.s.)

Correlation coefficient (r2)

0.0527 0.0542 0.0547 0.0405 0.0438 0.0487

0.8925 0.8923 0.9236 0.8969 0.8811 0.910

k and C are GAB model constants

Table 2. GAB parameters of osmotically pretreated and oven-dried ‘Homestead Purple’ papaya slices. Equilibrium temperature(°C) 20 20 20 40 40 40

Pretreatment solution concentration (°B)

k

C

52 60 68 52 60 68

0.95047 0.94395 0.93753 0.94103 0.93784 0.94015

19.3372 17.1323 15.2808 16.9372 16.2891 17.0461

Mm (g H20/g d.s.)

0.0640 0.0659 0.0681 0.0432 0.0458 0.0483


0.9308 0.91451 0.9242 0.9462 0.93952 0.9511


Table 3. GAB parameters of osmotically pretreated and oven-dried ‘Homestead Green’ papaya slices. Equilibrium temperature(°C) 20 20 20 40 40 40

Pretreatment solution concentration (°B) 52 60 68 52 60 68

k

C

Mm (g H20/g d.s.)


0.94619 0.96260 0.94048 0.9431 0.93952 0.95110

18.3091 16.1620 16.960 16.9520 17.6586 15.9833

0.0528 0.0535 0.0536 0.0442 0.0457 0.0504

0.9552 0.9829 0.9329 0.9653 0.9338 0.9445


Figure 6. Effect of temperature on adsorption isotherm of oven-dried ‘Homestead Green’ papaya slices preosmosed in 52°B sucrose solution.

Generally, the Mm was in the range of 0.0405–0.0547, 0.0432–0.0681 and 0.0442-0.0536 g water/g dry solids for preosmosed oven-dried ‘Sunrise Solo’, ‘Homestead Green’ and ‘Homestead Purple’ respectively. Mm content corresponds to the optimal moisture content for minimizing deteriorative reactions during storage 25. Heat of sorption of preosmosed oven-dried papaya varieties: Evaluation of the net isosteric heat of sorption, Qst, as a function of the equilibrium moisture content was conducted between 20

Figure 7. Isosteric heat of sorption of oven-dried (60°C) ‘Sunrise Solo’ papaya slices preosmosedin sucrose solutions.

and 40°C. Generally, net isosteric heat of sorption increased with decreased moisture content (Figs 7-9). The values of Qst increased sharply as the moisture contents of the preosmosed oven-dried ‘Homestead Purple’ and ‘Homestead Green’ were reduced from 0.2 to 0.1 g water/g d.s., but showed slight change above 0.2. This indicates the presence of water is strongly bound to the food by existence of polar groups on the surface of the product7. However, preosmosed oven-dried ‘Sunrise Solo’ showed a gradual increase in heat of sorption as the moisture content was


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dehydration caused a depression of aw during moisture sorption determination. Calculated Mm content was in the range of 0.405-0.055, 0.0432-0.0681 and 0.0442-0.0536 g water/g d.s, for preosmosed oven-dried ‘Sunrise Solo’, ‘Homestead Purple’ and ‘Homestead Green’, respectively. The analysis of isosteric heat of sorption showed a rapid increase in Qst at low moisture (0.1-0.2 g water/g d.s) for both preosmosed oven-dried ‘Homestead Purple’ and ‘Homestead Green’ papaya varieties. However, preosmosed oven-dried ‘Sunrise Solo’ showed a gradual increase in isosteric heat of sorption from 0.5 to 0.1 g water/g d.s. Isosteric heat of sorption of preosmosed oven-dried papaya varieties was in the order: ‘Homestead Purple’ > ‘Homestead Green’ > ‘Sunrise Solo’.

References Okoli, E.C. and Ezenweke, L.O. 1990. Formulation and shelf-life of a bottled pawpaw juice beverage. International Journal of Food Science and Technology 25: 706–710. 2 Raoult-Wack, A.L. 1994. Recent advances in the osmotic dehydration of foods. Trends in Food Science and Technology 8(5): 255. 3 Sankat, C.K., Castaigne, F. and Maharaj, R. 1996. The air drying behaviour of fresh and osmotically dehydrated banana slices. International Journal Food Science and Technology 31:123–135. 4 Dixon, G.M., Jen, J.J. and Paynter, V.P. 1976. Tasty apple result from combined osmotic dehydration and vacuum-drying process. Food Product Development 10(7): 60. 5 Dixon, G.M. and Jen, J.J. 1977. Changes of sugars and acids of osmovac-dried apple slices. Journal of Food Science 42: 1126. 6 Bolin, H.R. 1980. Relation of moisture to water activity in prunes and raisins. Journal of Food Science 45: 1190. 7 Tsami, E., Marinos-Kouris, D. and Maroulis, Z.B. 1990a. Water sorption isotherms of raisins, currants, figs, prunes and apricots. Journal of Food Science 55(6): 1594 8 Tsami, E., Marinos-Kouris, D. and Maroulis, Z.B. 1990b. Heat of sorption of water in dried fruits. International Journal Food Science and Technology 25: 350–362. 9 Rizvi, S.S.H. 1986. Thermodynamic properties of foods in dehydration. In Rao, M.A. and Rizvi, S.S.H. (eds). Engineering properties of foods. Marcel Dekker. New York. pp. 133–214. 10 Perry, R.H., Green, D.W. and Maloney, J.O. (eds) 1984. Perry’s chemical engineers’ handbook. 6th edn. McGraw-Hill, New York, USA. 11 Ayranci, E., Ayranci, G. and Dogantan, Z. 1990. Moisture sorption isotherms of dried apricot, fig and raisin at 20oC and 36oC. Journal of Food Science 55: 1591–1593, 1625. 12 Lim, L.T., Tang, J. and He, J. 1995. Moisture sorption characteristic of freeze-dried blueberries. Journal of Food Science 60: 810–813. 13 Brunauer, S., Emmett, P.H. and Teller, E. 1938. Adsorption of gases in multi-molecular layers. Journal of American Chemical Society 60: 309. 14 Singh, T. 1994. Malt concentrates and their mixtures with sweetened condensed milk : Moisture sorption isotherms. Journal of Food Science 59(4): 1100–1103. 15 Weisser, H. 1985. Influence of temperature on sorption equilibria. In Simatos, D. and Multon, J.L. (eds). Properties of water in foods. Martinus Nijhoff Publ. Dordrecht. pp. 95–118. 16 Saravacos, G.D. and Stinchfield, R.M. 1965. Effects of temperature and pressure on the sorption of water vapour by freeze-dried whey powders. Journal of Food Science 30: 779. 17 Saltmarch, M. and Labuza, T.P. 1980. Influence of relative humidity on the physicochemical state of lactose in spray-dried whey powders. Journal of Food Science 45: 1231. 18 Beristain, C.I., Azuara, E., Cortes, R. and Garcia, H.S. 1990. Mass transfer during osmotic dehydration of pineapple rings. International Journal of Food Science and Technology 25: 576–582. 1

Figure 8. Isosteric heat of sorption of oven-dried (60°C) ‘Homestead Purple’ papaya slices preosmosed in sucrose solutions.

Figure 9. Isosteric heat of sorption of oven-dried (60°C) ‘Homestead Green’ papaya slices preosmosed in sucrose solutions.

reduced from 0.5 to 0.1 g water/g d.s. This trend is similar to that reported by Lim et al.12. High net heat of sorption of water at low moisture contents is an indication of strong water-food component interactions in the dried fruit. As the moisture content increases, the available sites for sorption of water diminish resulting in lower values of ∆Hst7, 8. Generally, the magnitude of the heat of sorption of the preosmosed oven-dried papaya was in the order: ‘Homestead Purple’ > ‘Homestead Green’ > ‘Sunrise Solo’. Heat of sorption curves of preosmosed oven-dried papaya varieties showed no negative heat of sorption as was observed in some dried fruits by Falade et al.26 and Ayranci et al.11. No inversion of isotherms occurred as observed by Tsami et al. 7. Negative ∆Hst results from the effects of cross-over in the moisture sorption isotherms12. Conclusions Adsorption isotherm of preosmosed oven-dried papaya slabs followed type I (J-shaped) isotherms, according to BET classification. Solute infusion into papaya slabs during osmotic 38


Barbosa-Canova, G.V. and Vega-Mercado, H. 1996. Dehydration of foods. International Thompson Publ., New York. 20 Lindsay, R.C. 1985. Food additives. In Fennema, O.R. (ed.). Food chemistry. 2nd Edition, Marcel Dekker, New York. 21 Rizvi, S.S.H. 1995. Thermodynamic properties of food in dehydration. In Rao, M.A. and Rizvi, S.S.H. (eds). Engineering properties of foods. Marcel Dekker, Inc., New York. pp. 223–307. 22 Pääkkönen, K. and Mattila, M. 1991. Processing, packaging and storage effects on quality of freeze-dried strawberries. Journal of Food Science 56: 1388. 23 Iglesias, H.A., Chirife, J. and Lombardi, J.L. 1975. An equation for correlating moisture content in foods. Journal of Food Technology 19: 589–602. 24 Sopade, P.A., Ajisegiri, E.S. and Abass, A.B. 1996. Moisture sorption isotherms of dawadawa, a fermented African locust bean (Parkia biglobosa Jacq. Benth). Food Control 7(3): 153–156. 25 Moreira, R., Vazquez, G. and Chenlo, F. 2002. Influence of the temperature on sorption isotherms of chickpea: Evaluation of isosteric heat of sorption. Electronic Journal of Environmental, Agricultural and Food Chemistry 1: 1. 26 Falade, K. O., Adetunji, A. I. and Aworh, O. C. 2003. Adsorption isotherms and heat of sorption of fresh- and osmo-oven dried plantain slices. European Food Research and Technology 217(3): 230-234. 19


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