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A process can be run in batch or continuous mode. In continuous mode, there is a constant flow of fermented sugar out of the reactor that is equal to a continuous flow of fermentation medium into the reactor. During batch fermentation, there can be an inflow of medium, but there is no outflow [58]. Batch fermentation needs to be inoculated with a starter culture every time, whereas this is not needed in a continuous fermentation setup. However, in case of problems, the continuous fermentation needs to be restarted, so an infrastructure for starter cultures is needed anyway. A high volumetric production rate can be achieved when combining continuous fermentation with biomass retention, leading to smaller fermenter size [59]. It must be stated that the lactic acid concentration is lower compared with batch culture [58]. The concentration of lactic acid influences the water balance in the production plant.

FIGURE 1.5 Simplified block scheme of traditional lactic acid production process.

In all scenarios, microorganisms produce an aqueous lactic acid solution, comprising mainly lactate and counterions from the base, impurities from raw materials or fermentation by‐products, residual sugars and polysaccharides, and the microorganism itself.