Abstract
Lactic acid is a building block for polylactic acid, which is one of the most promising polymers based on renewable resources and is used mainly in packaging industry. This bio-based polymer is biodegradable and provides an ecological and economical alternative to petrochemical plastics. The largest cost blocks of biotechnological lactic acid production, accounting for up to 38% of the total costs, are substrate and nutrient sources, such as peptone, meat, and yeast extract. Based on a systematic analysis of nutritional requirements, the substitution of yeast extract by low-cost protein-rich agricultural hydrolysates was estimated for the production of l-lactic acid with Lactobacillus casei. Cultivations in 24-well microtiter plates enabled analysis of nutrient requirements and the usage of various hydrolysates with a high parallel throughput and repeated sampling. Rapeseed meal (RM) and distillers’ dried grains with solubles (DDGS) were tested as low-cost protein-rich agricultural residues. By using chemically or enzymatically hydrolyzed rapeseed meal or DDGS, 70% of the nutrient sources was replaced in the fermentation process at identical productivity and product yields. All in all, the total costs of l-lactic acid production with Lactobacillus casei could potentially be reduced by up to 23%.
Highlights
Bio-based building blocks for the polymer and packaging industries compete with low-priced petrochemical plastics
The combination of 5 g·L−1 yeast extract, 10 g·L−1 peptone, and 10 g·L−1 meat extract was defined as first reference cultivation and resulted in a final titer of 106.3 g·L−1 l-lactic acid, and a maximum productivity of 4.63 g·L−1 ·h−1 was obtained
Comparable results of final titer (>100 g·L−1 l-lactic acid) and maximum productivity (>4.3 g·L−1 ·h−1 ) were achieved with a combination of 5 g·L−1 yeast extract and 5 g·L−1 meat extract with and without addition of peptone, whereas the total nitrogen concentration was reduced by 42% and 67% compared to the reference
Summary
Bio-based building blocks for the polymer and packaging industries compete with low-priced petrochemical plastics. Polylactic acid (PLA), a bio-based and biodegradable polyester derived from lactic acid, is one of the most promising alternatives to petrochemical plastics [2]. Ninety percent of lactic acid is produced by bacterial fermentation [3], and the nutritional requirements of lactic acid bacteria are high because of their auxotrophy towards amino acids and vitamins [4,5,6,7,8]. Optical pure l- or d-lactic acids can be produced by the appropriate lactic acid bacteria, which is advantageous over chemical production wherein a racemic mix of dl-lactic acid is formed [2]. There is a need for optically pure l- or d-lactic acids, since the properties of PLA depend on the enantiomeric ratio of l- to d-lactic acid. Pure l-lactic acid is used in the pharmaceutical and food industries [10]
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