Improvement of dense phase carbon dioxide microbial inactivation through mechanical effects: from elucidation of mechanisms to feasibility

Abstract
This study presents a novel process for the microbial inactivation in liquids that can be used instead of pasteurization of fruit and vegetable juices. Instead of using high temperatures, high-pressure inactivation with carbon dioxide was improved by additional shear stress in turbulent flow with multiphase carbon dioxide caused by pressure drop and rapid degassing. The underlying mechanical effects, which are crucial for the efficiency of microbial inactivation and thus preserve the valuable ingredients of juices, were investigated.
The development of efficient, economical and sustainable approaches for the gentle microbial inactivation of sugary liquids is a highly relevant topic in food science. This is dictated by the population's increasing interest in direct juices and fresh-like juices.
Microbial inactivation with dense phase carbon dioxide (DPCD) and high-pressure carbon dioxide (HPCD) have been reported to be viable methods. However, the major challenge was to develop a more efficient and sustainable method that requires lower temperatures, less energy, and less time.
In this work an enhanced DPCD and HPCD approach has been investigated that utilizes the application of mechanical forces due to the sudden release of carbon dioxide and a concomitant pressure drop in a mini tube with a diameter of less than one millimeter. In order to improve the microbial inactivation efficiency of the new „Multi-Phase-Pressure-Drop“ (MPPD) method, different parameters like the initial concentration of Escherichia coli (E. coli) to be inactivated, the initial inactivation pressure, the pressure drop, and the operating temperature have been varied.
Performance tests were positive, as data from bacterial viability test and microscopic morphology studies revealed evidence of bacterial damage and disintegration. These observations suggest that strong mechanical forces were indeed acting on the cell walls, thus disrupting the integrity of the bacteria.
The demonstration of improved MPPD mediated inactivation of bacteria confirms its potential as a promising, effective, and energy-efficient method as an alternative for liquid pasteurization.