Design and experiment of vacuum-steam pulsed blancher for fruits and vegetables

Abstract: Blanching is an essential thermal processing using hot water or steam to treat fruits and vegetables, which is often carried out prior to the preservation process like drying, canning and freezing. Blanching can effectively inactivate enzymes in products such as polyphenol oxidase (PPO) and peroxidase (POD) enzymes, which cause deterioration reactions, off-flavor and undesirable changes in color. Under the condition of blanching, fruits and vegetables can keep their original colors, flavors and nutritional ingredients. Hot water and steam are by far the most widely used blanching methods. However, the main disadvantage of hot water blanching is that it causes nutritional substances especially sugar, proteins, carbohydrates, vitamins and minerals lost into water because of leaching and diffusion. What's more, hot water blanching can also cause environmental pollution due to the release of waste water containing considerable amount of nutrients. On the other hand, the main problems of steam blanching are blanching uniformity and small load. It is reported that solid foods are surrounded by thin-layer air and water, and steam cannot pass through these barriers of air and water, which act as insulation against the steam. The nonuniform blanching is possibly due to the existing of thermal resistance between material and steam. The thermal resistance prevents steam from transferring heat to the material. In addition, the steam will be condensed due to the air of low temperature. Considering those reasons, the pulsed vacuum-steam blanching machine is designed. This machine consists of vacuum system, steam system, automatic control system and blanching body system. The cold air and water around the material and useless steam can be wiped out by vacuum system in time. Therefore, thermal resistance is removed and heat can be transferred to material directly. In order to ensure the uniformity of the internal flow field, the flow field of inner blanching body was simulated by computational fluid dynamic (CFD) software Fluent, and the steam jet structure was optimally designed by adding interceptor in the bottom of the blanching body. The result showed that the velocity magnitude seemed to be equivalent and flow filed presented anti-clockwise in the xoz plane. Automatic control system used LCD12864 to show real-time temperature, pressure and working condition in the blanching body and switch the working condition between vacuum and steam blanching regularly through controlling the electromagnetic valve. The median filtering was applied in the control system to eliminate accidental factors which influenced temperature and pressure signal. Lily was adopted to test the performance of pulsed vacuum-steam blanching equipment. One group of experiment was that lily slice was blanched to validate its uniformity and another was single factor experiment designed by pulsed vacuum-steam blanching. In the first group, the drying time tended to be equivalent. It could be concluded that the thermal resistance was removed after the vacuum processing and steam could contact with every piece of lily sufficiently. Additionally, the strategy of adding interceptor in the bottom of the blanching body was feasible. The comparison of color values of dried lily slices also showed the uniformity of pulsed vacuum-steam blanching machine. The single factor experiment results showed that drying time would decrease and then increase as the increment of blanching time and cycle times. Besides, drying time showed a decrease tendency with the decreasing of vacuum degree. In this paper, the designed vacuum-steam pulsed blancher for fruits and vegetables has improved blanching loading capacity and uniformity, which has provided theoretical foundation and technical support for its popularization and application.