In the vinegar fermentation, the rate at which ethanol is oxidized to acetic acid depends on the presence and availability of oxygen and the surface area represented by the air-liquid interface. In other words, the ability of acetic acid bacteria to perform the acetic acid fermentation is limited primarily by the diffusion or transport of oxygen from the atmosphere to the cell surface. It is possible to significantly accelerate the oxidation of ethanol by increasing the surface area to which oxygen is exposed. This observation, which was originally described nearly 300 years ago by the famous Dutch physician-chemist Hermann Boerhaave, forms the basis for the trickling generator processes, which are sometimes referred to as simply quick vinegar processes.
The earliest examples of trickling generators used on an industrial scale were the Schutzenbach and Ham processes, introduced in 1823 and 1824, respectively. In these systems, which are still widely used, ethanolic substrates are circulated or trickled through cylindrical fermentation vessels or vats containing inert packing materials, such as curled wood shaving, wood staves, or corn cobs (Figure 11-6). Many other materials can be used, provided they do not contribute flavor or are degraded. The main requirement is that they have a large surface area.
As the inoculated substrate or feedstock passes from the top to the bottom of the vessel, the total surface area of the liquid material increases as it moves around and between the particulate packing material. Growth of acetic acid bacteria will then occur at the air-liquid interface, such that the ethanol concentration decreases and the acetic acid concentration increases during the transit of substrate from top to bottom. Holes can be drilled into the side of the vessel to ensure that aeration is adequate. When the substrate reaches the bottom section (the collection chamber), it may be returned to the top until the effluent is sufficiently acidic to be called vinegar.Alternatively, a second tank can be used to increase the oxidation rate and product throughput.The inoculated mash is fermented first in Tank 1, then
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