Experiment and dynamics of dehydration and inactivation of enzyme of potato slices by simultaneous infrared dry-blanching

Abstract: A simultaneous infrared dry-blanching and dehydration (SIRDBD) method utilizes infrared dry heat to blanch fruits and vegetables and simultaneously remove a certain amount of moisture. This study investigated the effects of three processing parameters, e.g. distance between infrared emitter and material (9.5, 12.5 and 15.5 cm), thickness (0.8, 1 and 1.3 mm, and the diameter was 6 cm) and processing time which was determined based on the complete inactivation of polyphenol oxidase (PPO), on blanching and dehydration of potato slices. The measured performance parameters were temperature, moisture ratio (MR), drying rate, relative residual PPO and surface color change (ΔE). In the test, the surface temperatures of all samples increased to predetermined temperatures within 1 min and then were kept at those temperatures for the rest of processing. In general, the tests showed that the radiation distance decreased from 15.5cm to 9.5 cm and the thickness of samples decreased from 1.3mm to 0.8 mm, which would lead to an increase of the maximum dehydrating rates of potato slices from 0.24 g/(g·s) to 0.59 g/(g·s), a decrease of the reduction time of PPO inactivation from 364 s to 152 s, and a reduction of the color difference value ΔE from 5.26 to 1.66. Therefore, the test results indicated that the thin slices and narrow distance between the infrared emitter and sample were desirable for the quick moisture reduction, fast inactivation of enzymes and less color change. The residual PPO activities after application of various processing conditions were between 0-8.51%, which corresponded to a moisture reduction from 37.96% to 55.42% (w.b.), and a processing time from 150 s to 420 s. This indicated that infrared heating could blanch fruits and vegetables and simultaneously remove a certain amount of moisture. In addition, when the thickness of potato slices increased, the activation energies of potato slices decreased, but the effective moisture diffusivities increased. The reduced distance between infrared emitter and sample improved effective moisture diffusivities of potato slices. The effective moisture diffusivities of potato slices under different processing conditions were between 3.37×10-10 m2/s and 6.36×10-10 m2/s (R2=0.974-0.983), and the activation energies of potato slices ranged from 20.34 kJ/mol to 32.59 kJ/mol (R2=0.951-0.996). First-order kinetics fitted well for PPO inactivation (R2 was from 0.926 to 0.988), and the predicted kinetic parameters provided useful information for the blanching processing. The rate constants of PPO inactivation under different infrared radiation conditions were between 0.0063 s-1 and 0.0151 s-1, and the reduction times ranged from 152 s to 364 s. The best processing parameters for the infrared heating mode were 9.5 cm radiation distance, 0.8 and 1 mm potato slice thickness and 150 and 180 s corresponding processing time, which resulted in 8.51% and 2.44% residual of PPO, 55.44% and 61.49% moisture content (w.b.), 1.66 and 2.24 ΔE, respectively. It has been concluded that SIRDBD method can be used as an alternative to manufacture blanched and partially dehydrated fruits and vegetables. For infrared dry-blanching, a narrow distance between infrared emitter and material with high temperature should is recommended.