There are a number of techniques for acetification which differ in the
means by which the three interacting components, ethanol, bacteria and
oxygen, are brought together. Surface culture techniques, where the
bacteria form a surface film at the interface between the acetifying
medium and air, are the simplest but can be applied with varying levels
of sophistication. In the Orleans process, vinegar stock in partially filled
casks drilled with air holes (Figure 9.15) is left to acetify until the acidity
reaches the appropriate level determined by the initial GK value. At this
point a proportion, typically one-third to two-thirds, is drawn off
through the tap, replaced with fresh stock and the process restarted.
The vinegar stock is usually addedviaa pipe passing through the top of
the barrel and resting on the bottom. In this way the surface film of
bacteria is not disturbed and the delays and losses that result from having
to reform the film are avoided. Usually the time taken to complete one
acetification cycle is of the order of 14 days.
Only a small proportion of the world’s vinegar is produced by surface
culture today, although it is claimed to produce the finest quality vinegar.
More elaborate surface culture techniques based on series of trays have
been described but these have received only very limited application.
The quick vinegar process derives its name from the faster rates of
acetification achieved by increasing the area of active bacterial film and
improving oxygen transfer to the acetifying stock. The acetic acid
bacteria grow as a surface film on an inert support material packed into
Figure 9.14 Oxidation of ethanol by acetic acid bacteria
358 Fermented and Microbial Foods