Figure 11-6. Trickling generator system.
passed, with aeration, into Tank 2, and back and forth until all of the ethanol is converted to vinegar.About three days are required to convert a 12% (v/v) ethanol solution to a vinegar containing 10% to 12% acetic acid.
Many modern trickling generator-type systems have been developed over the past seventy years based on the same principle as described above, but with more control and operational features. The Frings generator, introduced in the 1930s, and still widely used today in various modified formats, provides a means for incorporating air, in a counter-current direction, directly into the fermentation vessel. These systems can also accommodate a heat-exchange step, so that a constant temperature can be maintained. In addition, the feedstock is ordinarily dispensed via a sparging arm that breaks up the liquid into smaller droplets that are more easily distributed within the interior matrix.
Advances in biotechnology, and in particular, the fermentation industry, have led to the development of modern industrial fermentors capable of rapid, high throughput bioconversion processes. Although submerged fermentation systems are now widely used for many industrial fermentation processes, they were actually developed by the vinegar industry (and the Heinrich Frings Company, in particular) more than fifty years ago. The Acetator (the Frings fermentor), Cavitator, Bubble fermentor, and other similar units are constructed of stainless steel, can be easily cleaned and sanitized, and can operate in batch, semi-continuous, or continuous mode. Most of the vinegar produced worldwide is now made using submerged fermentation systems.
The most important feature of the submerged fermentation systems is their ability to provide rapid and efficient aeration. For example, the Frings Acetator is equipped with turbines that mix the liquid with air or oxygen and deliver the aerated mixture at very high rates inside the fermentor. The aeration system's ability to break up air bubbles and facilitate transfer of oxygen molecules from the gas phase to the liquid phase is essential, since the success of submerged fermentation systems relies on the transfer of oxygen from the medium to the bacteria.The viability of Aceto-bacter cells can be quickly compromised if aeration is even momentarily lost.
In addition to the aeration system, process controls for propeller speed (up to 1,750 rpm), nutrient feed, foam-handling, and temperature further provide for consistent, semi-continuous operation. Temperature control may be of particular concern in large-scale fermentations because ethanol oxidation is an exothermic reaction and heat is released. The medium temperature, if not controlled, can increase from an optimum of 28°C to 30°C to as high as 33°C. Even this modest increase in temperature can result in a serious decrease in fermentation productivity.
The size and design of submerged fermentation systems vary considerably. Production capacities of 25 liters per day of 10% acetic acid for very small-scale fermentors, up to 30,000 liters per day for large-scale reactors, are possible. In general, the starting material contains both ethanol and acetic acid (from a prior fermentation). After a single cycle of sixteen to twenty-four hours, the ethanol concentration will be reduced from about 5% to less than 0.5%, and the acetic acid will increase from around 7% to 12%. Up to half of the vinegar is then removed and replaced with fresh ethanol feedstock. Cell growth during a typical fermentation cycle is modest, with only a single doubling of the initial population.
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