Mathematical Simulation of Drying Process and Sorption Isotherms of Three Varieties of Olive-Waste Cake, a By-Product of the Olive Oil Industry

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1 Mathematical Simulation of Drying Process and Sorption Isotherms of Three Varieties of Olive-Waste Cake, a By-Product of the Olive Oil Industry Elsa Uribe 1, Roberto Lemus 1, Luis Puente 2, Kong Ah-Hen 3, Antonio Vega 1 (1) Dpto. de Ingeniería en Alimentos, Universidad de La Serena, Av. Raúl Bitran s/n, La Serena. (2) Dpto. de Ciencia de los Alimentos y Tecnología Química, Universidad de Chile, Av. Vicuña Mackenna, Santiago. (3) Instituto de Ciencia y Tecnología de los Alimentos, Universidad Austral de Chile, Av. Julio Sarrazín s/n, Valdivia (Chile) (euribe@userena.cl) ABSTRACT Olive cake is an important agro industrial by-product with the dried cake being the input material of many applications areas. In this research, the drying kinetics of three olive cake varieties during convective dehydration at five temperatures (40, 50, 60, 70, 80 and 90 C) was investigated. A physicochemical analysis was determinated (moisture, protein, fibre, ash, carbohydrate and lipid). The three varieties showed a high fiber content (<30%) and a low protein content (>8%). Air temperature showed a significant effect on drying curves and drying rates. Effective moisture diffusivities of olive cake varieties Arbequina, Picual and Frantoio were in the range of x10-9 m 2 /s, x10-9 m 2 /s and x10-9 m 2 /s, respectively. Based on the statistical tests results (r 2 and %E), the diffusion equation is a suitable model to describe the experimental drying curves. INTRODUCTION Favored by appropriate climatic conditions in its central regions, Chile has steadily increased its olive oil production. Along with mechanical olive oil extraction, the olive industry has to face an important issue with accumulation of solid and liquid residues. With increasing emphasis on cost reduction of industrial processes and value addition to agro-industrial residues, oil cakes are seen as an ideal source of proteinaceous nutrients and could also find use as a support matrix for various biotechnological processes (Ramachandran et al. 2007). The use of such wastes as nutrient sources for animals may further enhance the efficiency of industrial vegetal and animal production, consequently increasing profitability, considering that this waste product represents a cheap raw material with a high potential to be converted in valuable products after proper treatment. Some of these residues may also be used as a natural fertilizer or as a dye biosorbing material. In addition, it has also a great potential for bio-energy exploitation in many parts of the world since its energy content is over 15 MJ/kg dry matter and can therefore be used for direct burning after drying. However, olive cakes are composed of olive pulp, skin, stone, a remaining quantity of oil and a high water content, where the cake needs to be dried or concentrated prior to further processing. Convective dehydration is a widely used technology for the production of stabilized materials from a quality point of view. Besides, this type of dehydration is usually applied for by-products from industrial processes (Doymaz et al., 2004). To control the process itself, mathematical modelling of

2 the process is required. Modelling allows the choice of suitable operating conditions for minimizing drying time and changes in the quality of products when exposed to high temperatures. Nowadays, there is an increasing demand for additional supplementary products in pure forms and for commercial applications that exploit the broad-spectrum bioactivity of these compounds. Agricultural wastes represent a largely ignored source of high-value phytochemicals and valueadded industrial products that could contribute to sustainability of objectives with considerable economic benefits (Özbek and Dadali, 2007). The aims of this study were to evaluate the effect of drying temperature on drying kinetics and effective moisture diffusivity of the waste olive cake (var. Frantoio). Furthermore, simulation of experimental drying curves and mathematical modelling are also presented. MATERIALS AND METHODS The olive cake was supplied by an agro-food company (Agronoble S.A.) from the city of Ovalle, Chile. The waste resulted from a continuous cold process of olive oil production. The olive varieties used in this process were Frantoio, Arbequina and Picual, harvested at optimum ripeness and pressed without delay. The samples used for analysis were packed in polyethylene bags and kept in a freezer at 20 C. Before drying, olive cake samples were thawed during 24 h under refrigeration conditions at 5 C. Drying process was carried out at five different temperatures (40, 50, 60, 70, 80 and 90 C) in a convective dryer, at a constant air flow rate of 2.0±0.2 m/s. The dehydrated samples were packed and kept in the dark in polyethylene bags until analysis. All experiments were done in triplicate. The desorption isotherm of olive waste was determined at 60 C following the methodology recommended by project COST 90. The equation for modeling the isotherm was Guggenheim, Anderson and de Boer, commonly known as GAB. Proximal composition was carried out on protein content (Kjeldahl method x6.25), lipid content (Soxhlet extraction with petroleum ether), crude fibre content (Weende method by an acid/ alkaline hydrolysis), crude ash content (incineration at 550 C), available carbohydrate (by difference) and moisture content (AOAC ). All measurements were done in triplicate. Then, a statistical analyses by program Statgraphics Plus 5.1 (Statistical Graphics Corp., Herndon, USA) was used to performed a one-way analysis of variance (ANOVA) in order to determine significant differences among samples (α = 0.05). In addition, the multiple range test (MRT) was used to demonstrate the existence of homogeneous groups. The drying process was carried out in a convective tray dryer designed and built by the Department of Food Engineering of Universidad de La Serena. The dryer has a control unit to set the air velocity and temperature, which is heated through electrical resistances (Figure 1). Six temperatures were used in the study of the drying kinetics, including 30, 40, 50, 60, 70 and 80 ºC, and each experiment was made in triplicate. Drying air velocity was held constant at 2.0 m/s measured with an omnidirectional anemometer (Extech Instrument Inc., , Waltham, MA, USA). The inlet relative humidity was of 68.0±6.8%, measured by an ambient digital hygrothermometer (Extech Instrument Inc., , Waltham, MA, USA). Samples of 20±0.5 g were arranged as a thin layer in a stainless steel basket. This mass was measured on an analytical balance (Ohaus, SP402, USA) with an accuracy of ±0.01 g at time intervals defined, connected by

3 a system interface (Ohaus, RS232, USA) to a PC, which recorded and stored the weight decrease data. The experiments were finished at the point of reaching constant weight. In this model the dependent variable is the moisture ratio (MR) which relates the gradient of the sample moisture content in real time to both initial and equilibrium moisture content (Eq. 2) (Babalis & Belessiotis, 2004). MR t e (2) o e Where, t is real time moisture content (g water/g d.m.); o is initial moisture content (g water/g d.m.); e is equilibrium moisture content (g water/g d.m.); and t is time (minute). In order to study the mass transfer phenomena during dehydration of olive cake samples, Fick s second law of diffusion was used. MR 2 6 D we t exp 2 4L t e 2 o e Where: t : is diffusion coefficient (m 2 /s) and L is sample thickness (m). RESULTS AND DISCUSSION Table 1 shows the proximate analysis of the three varieties of olive cake samples. Factors such as proportion of different physical components (stone, skin, pulp and water), residual oil extraction, harvest year, geographic origin of olives and contamination with soil would cause great variability in chemical composition (Chuah et al. 2008). The differences in chemical composition may also be due to the oil extraction process and degree of extraction. The oil extraction method and extent of de-stoning also affect both the nutritional value and the preservation characteristics of the olive cake. Average values obtained for the GAB parameters when estimating olive cake desorption behavior were m = 0.032±0.002 g water/g d.m., C = 23.74±1.22 and K = 1.00 ±0.02. The observed sigmoid shape of the isotherm, type III according to Brunauer, is typical of food systems. This isotherm temperature was an average temperature of air-drying outlet for all drying experiments. When applying Diffusion equation for each set of experimental drying data at different air-drying temperatures and the three varieties, the values of effective moisture diffusivity were found to vary in the range of 1.97x10-9 to 13.77x10-9 m 2 /s in the range of 40 to 90 C. Table 2 shows the average values of the diffusion coefficient D we of three varieties of olive cake at different working temperatures. These values were closed to those reported by Doymaz et al. (2004). Statistical analysis of the data indicated that some of the D we found in the three varieties of dehydrated olive cakes have significant differences in their values (p<0.05). The profiles of experimental moisture ratio as function of time during drying of olive cake samples at different air-drying temperatures are shown in Figure 1. It can be seen that moisture ratio decreases continuously with temperature. In addition, drying rate is a function of air-drying temperature since high temperature (e.g., 90 C) leads to lower process time to reach final water content. Similar effects of temperature on drying

4 kinetics were reported during drying of olive cake by Akgun and Doymaz (2005) and Doymaz et al. (2004). Table 1. Proximate analysis of three varieties of fresh olive cakes (g/100g d.m.) Parameters Picual Frontoio Arbequina Water content* 2,09 ± 4,17 a 1,91 ± 1,21 b 2,20± 4,87 c Protein 6,48 ± 0,16 a 6,83 ± 0,52 a 6,49 ± 1,81 a Lipid 8,70 ± 0,26 a 4,80 ± 0,10 b 8,91 ± 0,59 a Fibre 34,14 ± 4,77 a 43,88 ± 1,66 a 41,82 ± 2,15 a Ash 5,15 ± 0,12 a 5,02 ± 0,33 a 5,03 ± 0,29 a Carbohydrates 12,8 ± 0,76 a 17,72 ± 0,16 b 13,11 ± 4,59 ab Table 2. Average values of the diffusion coefficient D we (m 2 /s) of three varieties of olive cake at different working temperatures. Variety\T C 40 C 50 C 60 C 70 C 80 C 90 C Arbequina 2.66±0.13 a 3.26±0.28 b 3.99±0.31 b 6.43±0.22 c 8.94±0.45 d 13.77±1.23 e Picual 1.97±0.09 a 2.53±0.12 b 3.99±0.48 c 4.32±0.25 c 5.38±0.10 d 6.05±0.28 e Frontoio 2.49±0.28 a 2.93±0.27 a 4.09±0.22 b 4.36±0.19 b 5.91±0.20 c 8.08±0.11 d * D we, x10-9 (m 2 /s) CONCLUSION The reported results revealed that this by-product is an important material which can be used for the food and cosmetic industries because it is a very promising source of value-added substances as well as a relief to environmental issues of the olive oil processing industry. In the light of the reported results, drying between 60 and 70 C may give a dried olive cake of high quality at a reasonable energy input. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided by FONDECYT Regular n Project for publication of this research.

5 Water content (wet basis) Water content (wet basis) Water content (wet basis) Picual Frontoio Time (min) Time (min) Arbequina Time (min) Figure 1. Effect of air-drying temperature on drying kinetics. REFERENCES Babalis, S.J., Belessiotis, V.G.; Influence of the drying conditions on the drying constants and moisture diffusivity during the thin-layer drying of figs. Journal of Food Engineering: 65, (2004). Doymaz, I., Tugrul, N., Pala, M.; Drying characteristics of dill and parsley leaves. Journal of Food Engineering: 77, (2006). Özbek, B., Dadali, G.; Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment. Journal of Food Engineering: 83, (2007).

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