Alternative techniques to thermal processing such as intense light pulses, high hydrostatic pressure processing (HPP) and high pressure carbon dioxide have been intensively investigated in the last decades to produce high quality foods that are microbiologically safe and stable, and in the first instalment of a four part series looking at shelf life we assess recent work by a University of Ghent team on HP CO2.
Professor Frank Devlieghere, who heads up the Department of Food Safety and Food Quality at Ghent University, said that, while there is currently very low take up, HP carbon dioxide has garnered a lot of interest as it involves low temperature compared to thermal methods and needs 10 times lower pressure than high hydrostatic pressure (HPP).
Moreover, carbon dioxide is a non-toxic and inexpensive gas, so the technique can be considered environmentally safe, he added.
Fluid functionality
And his team, who were evaluating the treatment in relation to both gram positive and gram negative pathogens, determined that HP carbon dioxide treatment is most effective when used with heat sensitive fluids such as liquid eggs, a key ingredient in many processed foods, and that it can also render fruit juices safe.
“We found a good level of inactivation of microbes in liquid eggs using this method coupled with acceptable shelf life and due to the low temperatures employed typical processing problems associated with heating this product such as blocking of pipes from coagulation of the egg and reduced nutritional value are avoided,” he told FoodProductionDaily.com
The limitations around HP CO2 though are that in can be used in batch processing only, stressed Devlieghere, as he explained that continuous process would require higher cost of installation due to the high complexity of machinery at those pressures when made continuous.
And a further disadvantage is the difficulty of using the treatment with solid foods and viscous products as their density prevents diffusion of carbon dioxide to all parts to allow total inactivation of bacteria, continued Devlieghere.
Spore resistance
Importantly, the team said that, as this is a treatment on the ‘edge of inactivation’, they found increased resistance of certain microbes after repetitive cycles of treatments such as acid resistant microorganisms - lactic acid bacteria and yeast - with some strains of E. coli, he added, also showing better survival rates than others.
“Reprocessing of HP CO2 treated products is not recommended as it optimises the conditions for the survival of certain bacteria, which are in a sub-lethally injured cell state.
Furthermore, unlike the HPP technique, where food is treated in package, thorough disinfectation of equipment is required in between cycles to render resistant microbes harmless,” said the professor.
Stress inducers
His team, however, identified appropriate combinations of inhibiting factors to control the microbial safety of foods and liquids produced using mild processing methods that minimize the risk of reinvigoration of pathogenic strains with increased stress resistance.
“We found that mild heat treatments such as HP CO2 can improve food quality and extend shelf life when an approach is employed involving the inclusion of an antimicrobial gas such as carbon dioxide in the packaging coupled with a slight lowering of the product’s pH from 7 to 6.5,” concluded Devlieghere.
He said that whereas the post packaging technique HPP is relatively well established in many countries, with 38 installations in Europe and 80 in the US, HP carbon dioxide has yet to take off with processors needing to stress the benefits of the technique for the industry to equipment manufacturers in order to accelerate its commercialisation and adoption.