12 Feb The thousand bubbles of the micro-oxygenation: diffusers and microbubbles in the distribution of oxygen
In a system of micro-oxygenation, the precision and reliability of the dosage are indispensable but insufficient conditions. Oxygen, supplied in a continuous and constant mode, should be distributed to the wine in the most opportune of means until all of the supplied gas is effectively dissolved in the wine via the most homogeneous means possible.
It is here that the diffuser comes into play. Immersed a few centimetres from the base of the tank, the diffuser generates a column of fine oxygen bubbles that climb upwards, transferring the desired dose to the wine, thanks to a phenomenon of gas-liquid diffusion,.
The diffusers utilised in micro-oxygenation are of a varying nature and quality, differing in form, dimensions and materials. Generally (but not always, as we shall see further on), one deals with spargers being of a more or less porous ceramic material or of sintered steel.
There are bubbles and bubbles
The fundamental thing to bear in mind is that not all diffusers are the same. The dimensions of the microbubbles impact upon the velocity of ascent and the passage of the oxygen from within the wine in dissolved form. The key is that all the oxygen supplied by the diffuser is dissolved into the wine before the bubbles reach the surface of the liquid. In the case to the contrary, there is a risk of creating a build-up of oxygen on the surface of the wine, with the possible consequent proliferation of aerobic and filmogenic microorganisms as well as the insurgence of uncontrolled oxidation phenomena.
Some will recall how, throughout the first years after the introduction of the micro-oxygenation technique into cellars, limitations were given for the height of the tanks (generally of around 2.5 metres) within which it was possible to micro-oxygenate.
A group of researchers from the University of Bordeaux, among them Audrey Devatine and Igor Chiciuc, have studied in-depth the physics of the gas bubbles in micro-oxygenation and the dynamics of oxygen transfer to the wines.
In their research, they characterised above all the dimensions of the microbubbles produced by diffusers with differing characteristics. In accordance with the differentiating microporosity and material, the various diffusers are able to produce microbubbles with diverse dimensions and greater or lesser homogeneousness (fig. 1).
In the study also published in “Oxygen et vin” (Parsec ed.), the same researchers evidenced the existent relationship between the bubble dimensions and the characteristics of the means and the dose of micro-oxygenation. The dimensions of the bubbles increase together with increments in the dose (range), just as occurs with the increase of the alcohol concentration, whilst not exerting a significant influence neither on the acidity nor the content of polyphenolic substances in the wine.
In a study published in 2009 on Chemical Engineering Science from the same Audrie Devatine and Martine Mietton-Peuchot, it was finally clarified that there exists a relationship between the column height along which the bubbles rise back up and the diameter of the pores of the diffuser. The authors went on to express this relationship in a mathematical model that considers all the factors at play, such as the characteristics of the wine like the alcohol content, the dissolved CO2 content and the progressive oxygen depletion of the bubble during its ascent towards the surface.
Simplifying the concept: the more one utilises a dosage system capable of dispensing low or very low doses as well as diffusers and materials able to produce fine bubbles, the more slowly these shall rise back up and it will be possible to apply micro-oxygenation to containers with heights less than the two and a half metres indicated by the first constructors.
One diffuser for each tank and for each application
“Experiments have explained that, which even just with our own experiences, we had verified,” explained Giuseppe Floridia, CEO of Parsec s.r.l. “The minimum height of the vessel depends on the dosage applied, the dimensions of the bubbles produced by the type of diffuser utilised, the pressure of the oxygen outflow, as well as the continuity and homogeneity of the gaseous flow. Today, Parsec offers not only a precise and reliable dosage system, but also a vast range of spargers, from which it is possible to select those best suited to the geometry of each tank and to apply micro-oxygenation also to containers with a very low drum level such as in the barriques.”
But what spargers are most suited to each application? The dimension of the bubbles depends upon the material and the adhesion force with which the bubbles in their formation are retained. Ceramic material is characterised by a lower adhesion force than steel, meaning that with an equal pore size, the bubbles produced by ceramic diffusers are finer and more homogeneous in respect of those spreading from diffusers in sintered steel. Besides the material, also the form of the sparger has an influence on the dimensions and the distribution of micro-bubbles.
“Generally, with the same characteristics of the material, the ceramic diffusers are capable of producing finer and more homogeneous bubbles,” Floridia further elaborates, “and as such, they are more suitable for the micro-oxygenation of small- and medium-sized tanks, whilst those in sintered steel can be utilised in the macro-oxygenation and micro-oxygentation of vats with greater dimensions.”
“Nonetheless, the form is also important. When the bubbles meet other bubbles along their route, they create the so-called coalescence phenomena, causing an increase in the average dimensions of the bubbles and a diminution in their homogeneity,” continues Floridia. “This occurs frequently in cylindrical spargers in which diffusion is via a vertical surface placed parallel to the pathway of the bubbles. To remedy this inconvenience, Parsec has invented flat diffusers, having a disk-like shape, whose surface is positioned horizontally. The bubbles distributed by these diffusers are homogeneous and extremely fine, being released in a column of liquid having a greater surface, further reducing the drum of liquid necessary for the application of micro-oxygenation.”
For more information:
Biondi Bartolini A., Cavini F., De Basquiat M., OXYGENE ET VIN: Du role de l’oxygene al la technique de micro-oxygenation. Firenze, Italy: Parsec S.r.l. 2009.
Audrey Devatine, MartineMietton-Peuchot, A mathematical approach for oxygenation using microbubbles application to the micro-oxygenation of wine. Chemical Engineering Science 64 (2009) 1909 — 1917