. Hence, enhancement of kLa can be achieved by increasing
‘kL’ and/or ‘a’ [10]. Various factors including diffusivity of gases,
viscosity and density of liquid medium, and gas–liquid affinity
have been known to affect kL values [10]. For instance, addition of
(nano)particles in the media have been demonstrated to enhance
kL values by facilitating the transport of gas molecules into the bulk
liquid [12,13]. However, additives with potential toxicity in the
fermentation media could lead to cell damage, possibly interrupting
the cultivation of microorganism [14].
Gas–liquid interfacial area (‘a’) could relatively easily be
increased by decreasing the bubble size [11]. Certain additives
such as electrolytes have been known to inhibit bubble
coalescence, leading to smaller bubble size and enhanced gas–
liquid interfacial area [15]. However, high salt concentration
could hinder growth of microorganisms [14,16].