CTI Switzerland (Commission of Technology and Innovation)
March 2005 – February 2009
Pierre Schaller, Alcosuisse, Bern, Switzerland
This project aims to determine and optimize the most suitable method for producing bioethanol from whey in Switzerland. The first part of the project will be to determine the costs of a production plant and to define the limitations of the existing knowledge.
Whey permeate being a very unstable substrate, it has a shelf life of less than 3 days when stored at 4°C. A major limitation to the successful commercial production of bioethanol will be to solve the problem of raw feedstock stability and storage.
An important part of this work will be to obtain a range of commercial alcohol tolerant yeasts and to determine their growth and production kinetics on hydrolyzed whey or glucose and to determine which would be most suitable for whey fermentation.
In parallel to the use of alcohol tolerant yeasts another part of the study will involve the application of in situ product recovery techniques to continuously remove the alcohol as it is formed to maintain a non-inhibitory concentration in the bioreactor.
Direct and indirect fermentations provided comparable results in the simulations at the beginning of the project. The software of simulation Aspen Plus allowed identifying the critical parameters which mainly consist in: transport, refrigeration for preservation means and fermentation parameters.
Whey permeate instability is mainly due to the presence of lactic acid bacteria which grow and consume lactose during subsequent storage and transport. In order to overcome this, seven compounds were tested for their ability to stabilize ultrafiltrated- and reverse osmosed-treated whey permeates over a period of 21 days. Of these compounds, formic acid was selected due to the high level of growth inhibition of lactic acid bacteria and could easily be transformed in a non toxic compound for yeasts. Ethanol production yield of 0.67 C-mol/C-mol and final ethanol concentration of 19.1 g/L with 91% of the initial lactose concentration consumed within 30 hours.
The influence of temperature, type of sugar, culture mode, initial biomass concentration, and initial sugar concentration on ethanol production from whey permeate was tested with the two yeasts K. marxianus CBS 5795 and CBS 397. The assessment was focused on the impact on the lag phase. By suitably setting these parameters the lag phase could be reduced from 154 to 20 minutes.
Direct and indirect ethanol fermentation from whey provides several major differences such as ethanol tolerance and ethanol production yield. It was thus necessary to select, characterize and compare efficient ethanol producing yeasts. Eight yeast strains were then compared for five working conditions. K. marxianus CBS 5795 was found to be the most efficient organism.
Various ISPR techniques were tested for ethanol extraction from an aqueous medium. One method involved the use of liquid-core microcapsules in which an organic phase (oil) is surrounded by a hydrogel membrane. Since the microcapsules are less dense than water, they may be readily recovered by flotation. Oleylalcohol and Laurinaldehyde were both tested in ethanol fermentation experiments and showed to increase the global efficiency of the system.
Finally a mixed culture containing one bacterium and two complementary yeasts showed to be very efficient. Global fermentation time could be reduced by a factor of two with raw whey permeate compared to previous studies when starting with the same biomass concentration. An equivalent ethanol production yield was in addition observed.