Modern Industrial Microbiology and Biotechnology

286 Modern Industrial Microbiology and BiotechnologyThe finished acidity of the vinegar is about 12%; when it is higher, production dropsoff. In order not to exceed this level of acidity, when drawing off vinegar, the amount ofalcohol in the replacement should be such that the total amount of alcohol is less than 5%.14.5.3 Submerged GeneratorsWith knowledge in submerged fermentation gained from the antibiotics and yeastindustry it is not surprising that vinegar production was soon produced by the method.Several submerged growth vinegar generators have been described or are in operation.The common feature in all submerged vinegar production is that the aeration must bevery vigorous as shortage of oxygen because of the highly acid conditions of submergedproduction, would result in the death of the bacteria within 30 seconds. Furthermore,because a lot of heat is released (over 30,000 calories are released per gallon of ethanol) anefficient cooling system must be provided. All submerged vinegar is turbid because of thehigh bacterial content and have to be filtered. Some submerged generators will bediscussed below.14.5.3.1 Frings acetatorFirst publicized in 1949, most of the world’s vinegar is now produced with thisfermentor. It consists of a stainless steel tank fitted with internal cooling coils and a highspeedagitator fitted through the bottom. Air is sucked in through an air-meter located atthe top. It is then finely dispersed by the agitator and distributed throughout the liquid.Temperature is maintained at 30°C, although some strains can grow at a highertemperature. Foaming is interrupted with an automatic foam breaker. Essentially it isshaped like the typical aerated stirred tank fermentor described in Chapter 9).It is operated batchwise and the cycle time for producing 12% vinegar is about35 hours. Details of the parts of the Frings acetator are shown in Fig. 14.2. It is selfaspirating,no compressed air being needed. The hollow rotor is installed on the shaft ofa motor mounted under the fermentor, connected to an air suction pipe and surroundedby a stator. It pumps liquid that enters the rotor from above outward through the channelsof the stator that are formed by the wedges, thereby sucking air through the openings ofthe rotor and creating an air–liquid emulsion that is ejected outward at a given speed.This speed must be chosen adequately so that the turbulence of the stream causes auniform distribution of the air over the whole cross section of the fermentor.The Frings alkograph automatically monitors the alcohol content and signals the endof the batch when the alcohol content falls to 0.2% (v/v). At this stage about one third ofthe product is pumped out and fresh feed pumped to the original level. The aeration mustcontinue throughout the period of the unloading and loading. The Frings Alkograph isan automatic instrument for measuring the amount of ethanol in the fermenting liquid.Small amounts of liquid flow through the analyzer continuously, at first through aheating vessel and then through three boiling vessels. The boiling temperature of theincoming liquid is measured in the first boiling vessel. While alcohol is distilled offcontinuously from the second and the third boiling vessel, the higher boiling point of theliquid from which ethanol has been removed is measured in the third boiling vessel. Thedifference in temperature corresponds to the ethanol content and is recorded