The slow Orleans process is employed for the manufacture of high-quality vinegars (Fig. 9.1). The starting liquor is held in large casks containing wood shavings or grape stalks that represent a large surface area on which the microbes can thrive. Acetification commences and after 8 days, the liquid is withdrawn and transferred to barrels so as to become half to two-thirds full. Fresh vinegar stock is introduced into the main cask to replace that which has been removed. Acidity reaches a maximum after approximately 3 months. On a weekly basis, one-quarter to two-thirds of the contents are removed from the base of each barrel to be replaced from the main cask.
Other processes aim at closer contact of liquid and organism, presenting the highest possible surface area so as to facilitate access of oxygen, thereby reducing the time for acetification. Tanks of wood or steel incorporate cooling coils (temperature maintained at 27-30° C) and are vented to allow circulation of air. They feature false bottoms to support wood shavings (preferably beech) or grape stalks. There is a spray mechanism to further facilitate rousing (Fig. 9.2) and distribution. The liquid trickles over the support and is pumped back to a header tank. Acetification will be complete after approximately
1 week. A proportion of the vinegar is removed from the base of the tanks and replaced with an equal volume of fresh feedstock. Some 20% evaporative loss occurs and the shavings must be replaced annually.
The submerged process, which is now the main approach, does not employ wood shavings and depends on carefully selected cultures of Acetobacter growing in aerated deep culture. It is conducted in tanks of stainless steel or polypropylene reinforced with fibreglass and with capacities of up to 120 hL. The vessel incorporates systems to ensure continuous flow of air and also coils to maintain a temperature of around 30°C. Oxidation starts slowly and air is introduced hourly to permeate completely. Acetification is complete when 0.2-1.5% (w/v) alcohol survives. It is a very rapid process. About half of the vinegar is bled off, with the remainder acting as the 'mother' for the next batch. Yields are high (90-95%) due to much less loss by evaporation than in the other approaches. However, the vinegar tends to be more cloudy and less aromatic, as there is less opportunity for flavour development to occur, for example that catalysed by the esterases.
Finally the vinegar is filtered and perhaps loaded into wooden casks to allow ageing. Vinegar is customarily matured in sealed, completely filled vats of stainless steel or wood for up to 1 year to allow flavour refinement and settling of insolubles. Bentonite is the most common clarification agent employed.
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