O-15
Dissolved oxygen and fobbing performance of in-tank aeration devices for
yeast propagation. The successful dissolution of oxygen into growth media, without excessive
fobbing, is an essential requirement of brewery yeast propagations. In order to
achieve this, several designs of in-tank aeration devices are commercially
available. This study considered the comparative aeration performance, fobbing
potential and surface disruption of four such devices in water and wort without
the presence of yeast. The four devices considered were: a lance; a spray ball;
a serrated disc; and a sintered candle. These were separately installed in the
base of a 5-hl cylindrical tank and supplied with air. The tank was filled with
water or 15.5 degree P wort as required, and a suspended dissolved oxygen probe
with a data-logging facility was used to monitor DO levels in the tank. A
full-height sight glass in the tank wall allowed bubble patterns to be observed and
recorded using a stills digital camera. Under fixed air flowrate conditions in
initially de-aerated water the sintered candle aerated the water quickest, as
expected, with the other three devices all producing much slower but similar
dissolution rates. This was surprising as the spray ball and serrated disc
seemingly had more complex designs lending themselves to bubble disengagement.
However, closer examination of the bubble patterns revealed substantial amounts
of bubble coalescence in these three devices. This contrasted with the sintered
candle, which showed good bubble disengagement together with a distinct stream
of individual bubbles with little discernible coalescence. Aeration in wort
using the spray ball and sintered candle, at the same air flowrate showed the
sinter to produce a more rapid rate of fob formation and once the air flowrate
had been stopped, a slower rate of collapse. Using differing air flowrates to
produce matched dissolution rates, the rate of fob formations were similar.
However, the sinter's rate of collapse was still much slower. This is clearly
indicative of a potential operational problem.
Dr. Bob Stafford is currently Engineering Development Consultant for South
African Breweries based in SAB's Corporate Technical Centre, Sandton,
Johannesburg. He joined SAB at the beginning of 2001. However, prior to that he
lectured in Chemical Engineering at Heriot-Watt University in Scotland where he
was heavily involved with the development of the distance learning p.g.dip/M.Sc
in Malting & Brewing. In the preceding years his work concentrated on two-phase
flow and heat transfer in process plant conducted within academia and the
scientific civil service. He currently holds a Visiting Lectureship at
Heriot-Watt University.
Manoj Gosai, Danielle Bekker, and ROBERT A. STAFFORD. The South African
Breweries Ltd., PO Box 782178, Sandton, 2146, South Africa.