Mass transfer kinetics and mathematical modeling of the osmotic dehydration of carrot cubes in glucose syrup and salt solutions

The osmotically dehydrated carrots can be added directly into soups, stews or can be used in a broad range of food formulations including instant soups, snack seasoning and etc. Osmotic dehydration is a suitable way to produce the shelf-stable products or partially dehydrated foods ready to place in other complementary processes such as air-drying, freezing and others. Modeling can certainly make differences in the food industry, leading to reduced costs and increased profitability. In food technology, at the simplest level, there are equations that determine the relationship between two or more variable. Simulation models in operation units and food preservation systems have attracted much attention in the past four decades. The mathematical equations describing mass transfer during osmotic dehydration, allow a better understanding of the composition of the material and operating parameters during dewatering. In this regard, many experimental and theoretical models have been reported in the literature but experimental models have more popularity because of easier applications. Regarding the classification of modeling in food processes, kinetic models are classified among theoretical models. It was ideal if we could use kinetic models based on fundamental scientific theories for the purposes of prediction and controlling the changes that occur in real food systems. But the complexity of the food makes the direct application of basic models impossible. The alternative is the direct study of kinetics on real food. As a result, the obtained models would be experimental or semi-experimental. Kinetics has developed as a powerful tool in modeling food quality features and in other words the modeling of food quality estimation is almost equivalent to the modeling of reaction kinetics in foods. The present study aimed to evaluate kinetics of osmotic dehydration of carrot cubes in terms of solid gain and water loss, which was studied at three glucose syrup concentration levels (30, 40 and 50% w/w), three salt concentration levels (5, 10 and 15% w/w) and three temperature levels of osmotic solution (30, 40 and 50°C) for 240 min. The experimental data were fitted to different semi-empirical kinetic models including Magee, Peleg and Page.
Fresh well graded carrots were washed and peeled manually. A vegetable dicer was used to prepare carrot cubes of dimensions 1 cm× 1cm× 1 cm. The cubes were washed with fresh water to remove the carrot fines adhered to the surface of the fruit. The initial moisture content of the fresh carrot cubes varied from 86% to 90% (wet basis). Considering the greater effectiveness of a mixture of solutes over a single solute, a binary solution of salt and glucose syrup was used as the osmotic solution. The samples were excluded from the osmotic solution after 15, 30, 60, 120, 180 and 240 minutes. Carrot cubes were then washed with deionized distilled water, and were dried using a paper towel. Evaluation of mass exchange between the solution and sample during osmotic dehydration were made by using water loss and solid gain parameters. The experimental data were then fitted to different semi-empirical kinetic models including Magee, Peleg and Page which are widely used in biologic fields and the parameters of the models were determined. Data fitting was conducted using Microsoft Excel spreadsheet (Microsoft Office, 2010) using SOLVER add-in. Coefficient of determination (R2), chi-squared (χ2) and root mean square error (RMSE) were used to determine the best suitable model. An analysis of variance was conducted to determine the significant effects of process variables on solid gain and water loss.
At the beginning of the osmotic dehydration process, because of the high osmotic driving force between the concentrated solution and the fresh sample, the rate of water removal and solid gain was relatively high. Although water loss reached nearly the equilibrium conditions towards the late processing times, solid gain kept increasing. This increase in solid gain blocks the surface layers of the product, which reduces the concentration gradient between the product and osmotic solution, posing an additional resistance to mass exchange and lowering the rates of water loss at further processing times. It was also observed that while increasing the salt concentration, the solid gain in most of the samples significantly (p