Document Type : Research Article
Authors
Department of Food Science and Technology, Ferdowsi University of Mashhad, Mashhad, Iran.
Abstract
Introduction: Kilka Fish is one of the most popular seafood products available for aquaculture development. It is a rich source of proteins, vitamins and minerals such as calcium and phosphor. The highly perishable nature of Kilka fish has strongly attracted researchers to find new methods to extend its shelf life. Kilka fish powder can be used in many foods, such as soups, sauces, snacks, etc. Foam-mat drying is a new effective technique has recently gained much attention because it does not suffer from major problems associated with traditional dehydration methods, such as poor rehydration characteristics, unfavorable sensory profile, and long drying time. The main objectives of this study were: (1) to optimize the effective parameters (egg white powder and xanthan gum concentration, and whipping time) on foaming properties of Kilka fish; (2) to study the effects of drying temperature and foam thickness on the drying characteristics and select a suitable model for thin-layer drying of foam; (3) to compute effective moisture diffusivity and activation energy of foam during drying; and (4) to study the effects of drying conditions on moisture content, water activity, hygroscopicity and color of powders and microstructure of dried Kilka fish foams.
Materials and method: Fresh Kilka fish (C. delicatula) was obtained from the Pak Samar Miroud Company (Mazandaran, Iran). Xanthan gum and egg white powders were purchased from Sigma Chemical Company and Gol Powder Company (Golestan Province, Iran) respectively. The Kilka fish skin removed and cleaned, and then crushed using a kitchen blender to obtain a homogeneous and uniform mixture. Based on preliminary tests, xanthan gum solution was prepared by dissolving a suitable amount of the selected gum powder in distilled water and mixed with a magnetic stirrer to obtain a uniform solution. The resulted solution was cooled at 4°C for 18–24h to complete hydration. According to the experimental design, to prepare 100 g of samples, appropriate amount of fish, egg white powder, xanthan gum solution and distilled water were mixed a 250-mL beaker. The mixture was then whipped with a mixer (Gosonic, model No. GHM- 818, 250W, China) with maximum speed at ambient temperature during given time, which was recommended by Design-Expert software version 6.02 (Stat-Ease, Inc., Minneapolis, MN). Foaming conditions, namely amount of egg white powder, xanthan gum and whipping time, optimized using response surface methodology (RSM) for minimizing foam density (FD) and drainage volume (DV). To evaluate drying behavior of the optimized foam, drying was carried out in a batch-type thin-layer dryer at three drying temperatures (45, 60 and 75°C) on 3- and 5-mm thicknesses. Ten thin-layer drying models were evaluated in the kinetics research. The higher values of R2 and lower values of χ2 and RMSE were selected as the basis for goodness of fit. The effective moisture diffusivity was calculated using method of slopes. A Fick’s diffusion model with slab geometry was used to describe the transport of moisture during drying inside a single Kilka fish foam mat. Activation energy was calculated by a simple Arrhenius-type relationship, by plotting the ln (Deff) against the reciprocal of absolute temperature (1/T). The microstructure of the dried fish foam-mats was analyzed by a scanning electron microscope. Finally, the effects of drying conditions on physicochemical properties of fish powder including moisture content, water activity, hygroscopicity and color were investigated.
Results and Discussion: The optimum values of variables for best product quality in terms of minimum FD and DV corresponded to egg white powder 3.67 % (w/w), xanthan gum 0.28% (w/w) and whipping time 8.93 min. The amount of FD and DV for foam at these optimum conditions were 0.92 g/cm3 and 0 ml, respectively. As expected, the increase in drying temperature and decrease in foam load led to acceleration of the dehydration of Kilka fish foam. Based on the statistical tests performed, Weibull distribution model can describe drying behavior of foams for all drying processes. Moreover, Fick’s second law was employed to calculate the effective moisture diffusivity that varied from 7.266×10-8 to 1.31×10-7 m2/s and from 7.404×10-8 to 1.693×10-7 m2/s with activation energy values of 18.201 and 25.615 kJ/mol for foam thicknesses of 3 and 5 mm, respectively. It was noted that the effective moisture diffusivity was increased when the drying temperature increased. Results showed that at the higher thickness, the internal moisture migration occurs along a longer distance rather than lower thickness. Therefore, an increase in moisture diffusivity occurs both with increase in drying temperature and sample thickness. The analysis of scanning electron microscopy (SEM) micrographs showed that there is wide porous structure of dried foams at higher speeds drying. With increasing drying temperature (due to reduction of drying times), combination of adjacent bubbles and therefor, collapse of foam structure occur less. Higher temperature reduced moisture content and aw , and thickness rise increased moisture content and aw under the same thickness and drying temperature, respectively. This is due to the fact that at higher temperatures, the rate of heat transfer to the sample would increase, therefore, it provides greater driving force for moisture evaporation which results the dried foams with reduced moisture content and aw. Moreover, with increasing foam thickness due to increased drying times and probably collapsed structure of foams and therefor weak pore structure, drying was performed difficultly which causes to retain more water in dried foams. Drying conditions had significant effect on hygroscopicity of powders. Results of color investigation showed that in both thicknesses, powder produced at 60°C have higher L* and lower a*.
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