Production of Lactic Acid from Barley: Strain Selection, Phenotypic and Medium Optimization

Originally, lactic acid was produced from pure starch or from glucose. Increasingly, however, agricultural feedstocks such as grains and green biomass are also being used as raw materials for the production of microbial lactic acid. A high productivity lactic acid bacterium strain was selected, process parameters were optimized for the batch fermentation on a laboratory scale, and its lactate productivity for cultivation on a barley hydrolyzate medium was examined. The parameter optimization showed that the strain Lactobacillus paracasei 168 can achieve a maximum productivity of 11.8 g/(L h), in batch cultivation on a nutrient enriched MRS medium, at a temperature of 39 °C and a pH value of 6.3. However, this parameter optimum does not correspond to the parameter optimum of lactate yield (96.8 %, T  = 30 °C, pH 5.75). In order to define working points for the large‐scale production of lactic acid, parameter combinations which guarantee high values of both lactate productivities (≥ 8 g/(L h)) and lactate yields (≥ 92 %) in a short process duration (≤ 35 h) were predicted by means of three‐dimensional model equations. As a consequence, productivities of 8 g/(L h)–11 g/(L h) with lactate yields of 93–95 % can be achieved with the strain L. paracasei 168. Cultivation on MRS barley hydrolyzate medium resulted in production data with about the same range of values as those obtained with the MRS medium. Under these conditions, this strain accumulated more than 100 g of lactate/L in the medium. Medium optimization experiments showed that the main components of the nitrogen containing nutrients in the MRS medium (peptone and yeast extract) can be replaced by protein extracts from green biomass (lucerne green juice). This positive result for the utilization of different renewable resources in lactic acid fermentation has to be confirmed and optimized for the continuous mode and pilot scale, respectively.