Bhat et al.

principle was that the bacteria were free of substrate and air was forced

intoȱtheȱmediumȱbyȱaȱstirrerȱatȱbottomȱofȱtank.ȱTheȱfermentorȱwasȱequippedȱ

withȱaȱmechanicalȱagitationȱwhichȱgeneratedȱfineȱbubblingȱinȱsystem.ȱAtȱ

theȱ topȱ ofȱ aȱ fermentorȱ thereȱ wasȱ anȱ airȱ outflow,ȱ thermometer,ȱ coolingȱ

coils and a system to control and remove the build-up of foam. Studies

revealed that system was very sensitive, and since bacteria were dispersed

inȱmedium,ȱevenȱaȱshortȱinterruptionȱofȱairȱinflowȱandȱstirringȱcouldȱresultȱ

in cell death (Mazza and Murooka 2009).

Submerged process could be used for production of vinegar in either a

discontinuous, semi-continuous or continuous system. In discontinuous

system, vinegar was produced in batches where a volume of substrate

wasȱloadedȱandȱinoculatedȱwithȱbacteriaȱandȱafterȱacetificationȱvolumeȱ

was completely unloaded from the fermentor. However semi-continuous

system was most commonly used and required a start-up period.

Fermentorȱwasȱloadedȱandȱinoculated.ȱOnceȱacetificationȱhadȱproceededȱ

to desired level, about 40–50% of volume was unloaded, while the vinegar

left behind was used as inoculum for next cycle. Advantage of this system

was that there was a shortened bacterial lag time for growth, resulting in

moreȱefficientȱproduction.ȱTheȱcontinuousȱsystemȱwasȱbasedȱonȱaȱconstantȱ

composition of medium at a state where bacteria were in exponential

growth phase and therefore have their highest growth rate, and produced

a conversion of 8-9% acetic acid in 24-48 hours (Garcia-Garcia et al., 2009).

Typical operation mode in industrial submerged culture was semi-

continuous system which consisted of development of successive

discontinuousȱcyclesȱofȱacetification.ȱResultsȱindicatedȱthatȱatȱendȱofȱeveryȱ

cycle,ȱaȱgivenȱvolumeȱofȱaceticȱacidȱwasȱdischargedȱandȱrefilledȱwithȱmash.ȱ

Best temperature for industrial production of 11 to 12% vinegar was kept

upto86°F (30°C). In addition, the concentration of acetic acid produced

wasȱaffectedȱbyȱbacterialȱconditionsȱ(DeȱOryȱ et al., 2004).

Most important feature of submerged fermentation systems was their

abilityȱtoȱprovideȱrapidȱandȱefficientȱaeration.ȱAerationȱsystem’sȱabilityȱwasȱ

to break air bubbles and thus facilitated transfer of oxygen from medium

to bacteria. It was found to be an essential step in order to prevent the

Acetobacter cell death. However, even traditional open vat method could be

run in a semi-continuous fashion and had been used in many parts of the

world (Janssens and Swings 2002).

148