Defence Jeremy Bolt: "Preservation of cut flowers using supercritical carbon dioxide"

Promotors: 1^st promotor: prof. Gert Jan Euverink, 2^nd promotor: prof. Francesco Picchioni

Abstract: The structure and rigidity of fresh flowers is provided by the turgor pressure, where accumulation of water inside the cell causes the cell membrane to swell; pushing against the cell wall [1]. This hydrostatic pressure is maintained as long as the flower has access to minerals and sugars, and energy. When the stem is cut, its food supply is cut short. This switches the system from a hypotonic state to the hypertonic state, resulting in drying, wilting and shrinkage of the flower.

Preserved flowers could immortalize important botanical findings and personal events in life, facilitate their use as semi-permanent decoration and broaden their geographical market, while also saving in logistic costs. The general concept revolves around the inactivation of water, either by physical removal of water or by incorporating compounds that bind to water. Common side-effects however, are shrinkage, brittleness and loss of-, or change in colour and colour intensity [2]. More importantly, the procedures often consist of lengthy processes utilizing costly, potentially hazardous, organic solvents and fossil-based compounds.

Longbloom uses supercritical carbon dioxide (scCO2) to finalize the process [3, 4]. A supercritical fluid is defined as the region in the phase diagram where the liquid and gas phase have equivalent physicochemical properties, making them virtually indistinguishable, while combining the properties of both the liquid- and gas phase [5]. Air drying results in shrinkage as a result of capillary forces, destroying the structure of the flower. scCO2 has high diffusivity and no surface tension, thus can extract compounds from the substrate without adverse effects.

The main question is whether this process can be simplified into a single-step process by removing or integrating pre- and post-treatment steps, while maintaining the natural properties and ideally while making the process and products ‘green’. The multi-step procedure has been translated into a stirred batch vessel to simplify the procedure. The reactor supports injection and removal of solvents and a continuous scCO2 flow. This batch procedure gives good results for several flower species, provided the correct procedure, conditions and recipe are applied. The aim is to reduce the quantity of solvents and plasticizers as much as possible, while optimizing the product properties by bleaching and dyeing during the process.

Dissertation