MOISTURE SORPTION ISOTHERM CHARACTERISTICS OF SMOKE- DRIED

1. MOISTURE SORPTION ISOTHERM CHARACTERISTICS OF SMOKE- DRIED CATFISH (Clarius gariepinus) AND MACKEREL (Trachurus trachurus) OCHIMANA, T. O., NKAMA, I. EZE, J. I. AND NGODY, P. O. DEPARTMENT OF FOOD SCIENCE AND TECHNOLOGY UNIVERSITY OF NIGERIA, NSUKKA. AT ANNUAL NIFST CONFERENCE OCTOBER, 2015

2. Introduction • Fish are vertebrates living in fresh or salt water bodies. They are members of paraphyletic group of organisms that consist of all gill bearing aquatic animals that lacks limbs. • Fish is an important source of food for humans, enriched with nutrients. • It is cheap and highly acceptable with little or no religious bias which gives it advantage over pork or beef.

3. Introduction cont. • It is usual for fish to be preserved under frozen condition but the current irregular supply of electricity is a major challenge. • Smoked fish is a premium product due to its delicacy but the knowledge of its moisture sorption isotherm which is imperative to ensure a longer shelf life is limited.

4. Introduction cont. • Moisture sorption isotherm (MSI) is a curve describing the amount of water adsorbed or desorbed by a • substance at a particular constant temperature as the function of the equilibrium vapor pressure, water activity (aw) or relative humidity. • Type III BET isotherm was reported for African catfish (Olajide, et al., 2002), while Labuza et al. (1985) observed the Type 11 BET isotherm for fish flour.

5. Introduction cont. • Since different food products possess diverse sorption behaviors, • it is imperative to develop appropriate technique for processing and storage of each food instead of extrapolating from existing ones. • The aim of this work was to determine the sorption characteristics of smoke-dried catfish (Clarius gariepinus) and mackerel(Trachurustrachurus) at 400C within water activity range of 0.1 and 0.9.

6. Materials and methods • The two species of Fish (Catfish and Mackerel) were bought from Nsukka main market in Enugu State, Nigeria, cleaned, filleted and smoked in a modified smoking kiln. • The samples were conditioned to constant weight and the initial moisture contents were determined.

7. Materials and methods cont. • The EMC was determined using integral gravimetric method at six different water activities ranging between 0.147 and 0.981 created with sulphuric acid at the required 400C temperature. • The equilibration of the samples were monitored for about two weeks and the equilibrium moisture content was calculated by material balance using the formula: • M /100 (WI) + (W3-W2) = EMC/100 (WI+ (W3-W2).

8. Materials and methods cont. • Where M = initial moisture content of the sample, W1= Weight of sample used during sorption experiment, W2= initial weight of sample and crown cork, W3= final weight of sample and crown cork at equilibrium, and EMC = Equilibrium moisture content. • Completely randomized experimental design was used, and results of each determination were unified by calculating the means • while the mean equilibrium moisture content values were used for plotting sorption isotherms.

9. Results and discussion • The moisture adsorption and desorption isotherms of smoke dried catfish and mackerel showed type III BET isotherm shape with hysteresis effect at 40 0C (figure 1a and 1b). • Similar results were obtained at 20 0C and 30 0C but results not shown.

10. Results and discussion cont. • The type III isotherm exhibited by smoke dried catfish and mackerel is as would be expected since food materials rich in protein have been found to show the BET type III shapes (Olajideet al., 2002). • The high value of EMC by protein rich material is the function of numerous sorption sites exhibited by proteins in which each water molecules forms hydrogen bonds with up to four neighboring water molecules.

11. Results and discussion cont. • The gap created by the difference between the adsorption and desorption curve at the constant temperature (40 0C) and water activity is referred to as hysteresis. • The higher moisture content in desorption path (showing hysteresis) agreed with postulation that • during desorption, solutes hold more water as there are more closed pores in dried products than during adsorption.

12. Results and discussion cont. • Therefore, as the food go through desorption, the capillaries undergoes shrinkage, trapping much molecules of water. • The magnitude of hysteresisis is determined primary by the composition of the material, the temperature and pre-treatments given to food materials. • The more the available water in the environment, the higher the water vapor gradient, hence the higher will be the equilibrium moisture content (EMC).

14. Legend • EMC ads = Equilibrium moisture content for adsorption pathway • EMC des = Equilibrium moisture content for desorption pathway • aw = water activity

15. References • Labuza, T. P. Kaanane, A. and Chen, J. Y. (1985). Effect of temperature on moisture sorption isotherms and water activity shift of two dehydrated foods. Journal Food Science. 41:910-917. • Olajide, S., Ibironke, P. and Loreto, M. (2012). Experimental and mathematical description of sorption isotherms and thermodynamics Properties of salted and dried Africa catfish (Clariasgariepinus). International Journal of Food Science and Technology, 25 : 350-359.

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