Processing and quality characteristics of apple slices under simultaneous infrared dry-blanching and dehydration (SIRDBD)

High quality processed fruits and vegetables are always needed to promote public health and enhance product accessibility to consumers. Blanched and partially dehydrated or dehydrofrozen fruits and vegetables can be used in the baking industry. Blanching and dehydration are two essential processes for producing such products. However, the conventional methods (steam blanching and hot air drying) have many drawbacks such as low energy efficiency, long processing time, quality deterioration and environmental problems. A newer method, simultaneous infrared dry-blanching and dehydration (SIRDBD), utilizes efficient catalytic infrared heating to combine blanching and dehydration into a one step process which is simpler and more energy efficient.;Two heating modes of SIRDBD were studied. The first mode was continuous heating which maintained constant radiation intensity of the emitters by continuously supplying natural gas. The other mode was intermittent heating which kept the surface temperature of apple slice stable. A three-factor factorial design was used in experiments for each heating mode. The four processing parameters included slice thickness (5, 9, and 13 mm), processing time (2 to 40 min), and radiation intensity (3000, 4000, and 5000 W/m2) for continuous heating or product surface temperature (70, 75, and 80 °C) for intermittent heating. The studied processing characteristics were product surface and center temperatures, moisture reduction, heating rate and drying rate. The product quality was evaluated as residual polyphenol oxidase (PPO) and peroxidase (POD) activities and overall surface color change (DeltaE).;The continuous heating mode resulted in shorter processing time, lower moisture reduction and similar overall surface color change of apple slices compared to intermittent heating mode when achieving 90% POD inactivation. The advantage of intermittent heating mode was that the surface color could be preserved better when prolonged heating and high moisture reduction were necessary. For the continuous heating mode, higher radiation intensity and/or thinner slice resulted in faster increase of the surface and center temperatures, more moisture reduction and quicker inactivation of enzymes of the product than lower radiation intensity and/or thicker slice. For the intermittent heating mode, the slice thickness had a greater effect on the product quality and processing characteristics than the surface temperature. The thinner slice led to faster inactivation of enzymes and quicker moisture removal than the thicker slice. The higher surface temperature usually resulted in higher center temperature, faster inactivation of enzymes and less surface color change than the lower surface temperature. For both heating modes, the page model performed well in fitting the drying curves.;POD was more heat resistant than PPO. PPO inactivation curves fitted a first order kinetics model very well for both the continuous heating mode and the intermittent heating mode. On the contrary, a fractional conversion model and biphasic model showed better performance than the first order kinetics model in describing POD inactivation for the continuous heating mode and intermittent heating mode, respectively. During continuous heating, the surface color change of apple slices was mainly due to the decrease of L value and increase of a value, which corresponded to enzymatic browning during the process. On the other hand, during prolonged heating under the intermittent heating mode, the significant increase of b value contributed to the overall surface color change and indicated non-enzymatic browning.;Apple slices were selected as the model fruit to investigate the effects of SIRDBD process parameters on the product quality and processing characteristics. The specific objectives of this study were to: (1) determine the differences in processing characteristics and product quality between continuous and intermittent heating modes of SIRDBD; (2) investigate the processing parameters on processing characteristics and product quality; (3) establish mathematical relationships between processing parameters and product quality by using empirical modeling methods; (4) recommend appropriate processing parameters to achieve desirable product quality.;To achieve 1 log reduction of POD, the continuous heating mode resulted in moisture reductions in the range of 15.35% to 49.29% and overall surface color change (DeltaE) between 2.030 and 5.518 while the intermittent heating mode led to moisture reduction of 19.78% to 59.48% and Delta E between 2.274 and 5.5880. The indicated range of Delta E values stand for acceptable surface color change by visual observation. The recommended processing conditions are 5 mm thick slices treated with 4000 W/m2 for 4.6 min or at 75 °C for 7.5 min for continuous and intermittent heating mode, respectively.;In summary, SIRDBD with both the continuous and the intermittent heating modes showed promise as an alternative blanching and partial dehydration method for producing blanched and partially dehydrated fruits and vegetables.