Whey is a by-product in milk processing, the world production amounts to about 82 million metric tons, and especially the acid whey is seen as a waste product. After separation of the valuable whey proteins a residue is remaining, which is costly in disposal due to its high COD (chemical oxygen demand). Besides inorganic salts the main component is milk sugar (lactose), which is only of little importance in food production because of its poor sweetening power. But lactose can be converted by lactic acid bacteria to lactic acid (lactate), which is used as a preservative and acidulant in food industry and as a raw material in chemical industry – e.g. in the production of polylactides, biologically degradable polymers.
Today, lactic acid is mainly produced from molasses from sugar production. The fermentative conversion is achieved in industrial scale in stirred tank bioreactors with low productivities. The splitting of lactic acid from the salt is performed by addition of sulfuric acid and subsequent separation of the emerging gypsum. The gypsum has no further use and is problematically in disposal.
The objective of the project was therefore, to develop an environmentally sound process for the manufacturing of lactic acid from acid whey, in order to combine waste treatment with the production of valuable materials.
Biological production of lactic acid and the use of membrane technology for downstream processing
The combination and integration of recovery methods like microfiltration and electrodialysis with an efficient biological system (organism, cultivation medium, bioreactor) are leading to a high-performance process for the economic production of lactic acid and an extensive reduction of the organic load of the remaining sewage.
Flow diagram of the integrated overall process.
In the first process step, the valuable whey proteins are separated from the acid whey. The lactose of the remaining whey permeate is converted in an unaerated bioreactor by a special strain of lactic acid bacteria, which do not need any further supplements, to lactate (salt of lactic acid). High concentrations of biomass and thereby high and economic lactic acid productivities are achieved by cell recycle by an integrated cross-flow-filtration unit. The product recovery is carried out by bipolar electrodialysis, converting lactate directly into the free acid.
Advantages of the process
COD reduction of waste water by over 90 % with simultaneous recovery of useful products (L-lactic acid, proteins, biomass)
Minimal amounts of supplements and additional chemicals
High lactic acid productivities by cell recycle
Production of the free lactic acid and recovery of the neutralizing agent used in fermentation
Economic production of lactic acid by the application of environmentally sound technologies
Up to now the process was operated in laboratory and technical scale. Scale-up calculations have shown an economic operation of the process in industrial scale. 4,200 metric tons can be produced form 100,000 tons of acid whey assuming production costs of 0.80 Euro/ kg of lactic acid. The current market price is around 1.20 Euro.