Growth of Kluyveromyces marxianus and formation of ethyl acetate depending on temperature

Abstract

Conversion of lactose into ethyl acetate by Kluyveromyces marxianus allows economic reuse of whey-borne sugar. The high volatility of ethyl acetate enables its process-integrated recovery by stripping. This stripping is governed by both the aeration rate and the partition coefficient, K EA,L/G. Cultivation at elevated temperatures should decrease the K EA,L/G value and thus favor stripping. K. marxianus DSM 5422 as a potent producer of ethyl acetate was cultivated aerobically in whey-borne media for studying temperature-dependent growth and ester formation. Shake flask cultivation proved thermal tolerance of this yeast growing from 7 to 47 °C with a maximum rate of 0.75 h(-1) at 40 °C. The biomass yield was 0.41 g/g at moderate temperatures while low and high temperatures caused distinct drops. The observed μ-T and Y X/S-T dependencies were described by mathematical models. Further cultivations were done in an 1-L stirred reactor for exploring the effect of temperature on ester synthesis. Cultivation at 32 °C caused significant ester formation (Y EA/S = 0.197 g/g) while cultivation at 42 °C suppressed ester synthesis (Y EA/S = 0.002 g/g). The high temperature affected metal dissolution from the bioreactor delivering iron for yeast growth and preventing ester synthesis. Cultivation at 32 °C with a switch to 42 °C at the onset of ester synthesis allowed quick and efficient ester production (Y EA/S = 0.289 g/g). The high temperature lowered the K EA,L/G value from 78 to 44 L/L which heightened the gas-phase ester concentration (favoring ester recovery) without increasing the liquid-phase concentration (avoiding product inhibition).