Effect of ultrasound on mass transfer during vacuum impregnation and selected quality parameters of products: A case study of carrots

Many unit operations in the food industry are diffusional driven. These processes are usually very slow and difficult to handle for specific groups of raw materials. Vacuum impregnation (VI) is one example. Impregnating low-porous or densely-structured materials is problematic and often requires low pressure, which can negatively affect product quality and be expensive in energy consumption. This research aimed to evaluate ultrasound (US) as a factor in intensifying mass transfer and enhancing its effectiveness in the VI process. Experiments on impregnation enhanced with ultrasound applied at different stages of the process were carried out. Carrot, a difficult-to-process raw material, was impregnated with ascorbic acid as a mass transfer marker. The process's effectiveness and selected quality parameters were then analyzed. Ultrasound was found to have a positive influence on mass transfer during VI. The effects of ultrasound enhancement were different for particular processes, and depended on the stage of the application and duration of US exposure. The greatest increase in the tissue's ascorbic acid content (60% compared to the non-ultrasound-assisted process) was observed when ultrasound was applied continuously throughout the process. Applying ultrasound only during the relaxation (at atmospheric pressure) or aeration periods resulted in a similar effect - c.a. 20% increase in the marker's content. The smallest increase (10%) was observed when ultrasound was applied only during the vacuum period. Applying US did not result in any unfavorable color change. In most cases, pH decreased, which is favorable for the semi-product's stability. The carotenoid and phenolic compounds' content did not decrease. The results unequivocally indicate that ultrasound has great potential for use as a mass transfer accelerator in the VI process for low porosity materials. The effectiveness of the US is influenced not only by pressure but also by exposure duration. The synergistic effect observed using ultrasound-enhanced impregnation throughout the process confirmed this hypothesis.