Lactic Acid Production from Porphyra umbilicalis Through Sequential Saccharification and Fermentation with a Lactobacilli Consortium

Lactic acid is a compound that is consistently in high demand due to its wide range of applications. Aiming at the use of an alternative third-generation substrate for the microbial production of this organic acid, the fermentation of Porphyra umbilicalis with lactobacilli was studied. This seaweed revealed a total carbohydrate content of 51.6 ± 1.7 g/100 g biomass dry weight (DW), thus showing great potential for fermentation purposes. Thermal-acidic (at 121 °C for 30 min) hydrolysis of 100 g/L P. umbilicalis with sulfuric acid (H2SO4 5% w/v) led to the release of 37.9 ± 1.1% of the total sugars in the seaweed substrate, producing a hydrolysate with 14.7 ± 0.4, 1.1 ± 0.04 and 0.9 ± 0.04 g/L of galactose, glucose and 5-hydroxymethylfurfural (HMF), respectively. After optimization of the oxygen supply conditions, fed-batch fermentation of the hydrolysate by a consortium (4LAB) of Levilactobacillus brevis, Lactiplantibacillus plantarum, Lacticaseibacillus rhamnosus, and Lacticaseibacillus casei in a 2 L bioreactor produced up to 65 g/L of lactic acid with a yield of 0.58 g/g of consumed carbon sources. The 4LAB consortium was not inhibited by up to 1 g/L HMF in the medium and also showed the capacity to convert up to 88.5% of the initial HMF titer during fed-batch fermentation in the bioreactor.