Fractional conversion modeling of color changes in apple during simultaneous dry-blanching and dehydration process using intermittent infrared irradiation

Color is the most important feature of food appearance, since it strongly affects consumer acceptance. The abnormal color is closely related to degradation of food quality or food spoilage. Various factors affecting color of the product include the composition and surface properties of the foodstuff, process conditions (temperature and time), and the type of pretreatment processes. Therefore, for the design of a new process, kinetic modeling is essential for extracting basic kinetic information in a system to predict changes. Intermittent irradiation is the novel processing method in food industry which the surface temperature of product is kept constant. Darkening process occur in the fruit slices during drying, resulting in undesirable color changes. Undesirable color variations can be attributed to Millard's browning reactions between sugar and amine compounds or ascorbic acid oxidation. Thermal degradation kinetic models define maximum maintenance conditions for qualitative factors to produce a safe food. Color descriptive models are known as zero order, first order, and fractional conversion model. The fractional conversion model represents the response rate required at a given time to complete a phenomenon. This modeling is necessary for designing online quality control systems for thermal processing in the food industry and preserving the apparent quality of the product by optimizing the most important drying parameters. In this study, the kinetic of color changes in apple slices was investigated, in order to maintain appearance quality of product during simultaneous infrared dry-blanching and dehydration with intermittent irradiation.
Apple slices (Golden Delicious Variety) were dried in three thicknesses of 5, 9 and 13 mm using three constant surface temperatures of 70, 75 and 80 ° C. Image acquisition of apple slices was made use of a flatbed scanner with an interval of 15 minutes during processing. The treated samples were placed on the scanner and then a black box was utilized so as to prevent the interferences of the peripheral lights. The images featured a 300 dpi quality and were saved in TIFF-24 bit format. The color analysis of the images was carried out in color space of L*a*b* by the use ImageJ software, version 1.6.0. The lightness parameter (L), redness (a), yellowness (b), the intensity of the color changes (∆E), chroma (Cr) and browning index (BI) were described during product processing using fractional conversion model. Equilibrium color parameters (Cf) were also used as an indicator to compare different process conditions. The fitting of the model was done using the curve fitting toolbox in the 2009 version of the MATLAB software with 95% confidence level (P
The results showed that the control of surface temperature of product and its thickness during irradiation can be effective to prevent the darkening of the product during blanching and dehydration. In fact, the desired color in product (more light, less red and more yellow) was observed with decreasing temperature and increasing in thickness. However, the thickness of the product showed a slight and opposite effect on yellowness (62.583 and 57.491 for thickness of 5 and 13 mm at 70 ° C, respectively). The maintenance of the yellowness parameter in lower thickness (rapid removal of moisture and reduced overall heating time) and lower temperature (lower energy intakes) can be caused by reducing the destructive reactions of natural pigmentation agents of color in the apple (carotenoid). It is observed that in the beginning of the process changing in color parameters was faster, because of under the influence of drying the surface fibers of the product will increase the overall color variation and create the conditions for the production of pigment. The rate of color darkening was strongly dependent on product thickness. The reducing of surface temperature caused lower browning index, but the decrease was not significant (104.761, 108.482 and 162.99 for thickness of 5 mm at temperature of 70, 75 and 80°C, respectively). This fact indicates that the dry blanching and dehydration with maintained product surface temperature, prevents the occurrence of undesirable darkening reactions of color. In general to reduce undesirable color changes, the use of infrared dry blanching and dehydration largely depends on the thickness of apple, and low surface temperatures during processing increased further yellowing and reduced the darkness of the apple slice. It can be said that in the case of low-thickness cuttings, the use of low temperatures is very important in maintaining the desirability of color. Finally, the results showed that the application of the fractional conversion model could appropriately describe the color variations.