Industrial biotechnology is a continuously expanding field focused on the application of microorganisms to produce chemicals using renewable sources as substrates. Currently, an increasing interest in new versatile processes, able to utilize a variety of substrates to obtain diverse products, can be observed. A robust microbial strain is critical in the creation of such processes. Lactic acid bacteria (LAB) are used to produce a wide variety of chemicals with high commercial interest. Lactic acid (LA) is the most predominant industrial product obtained from LAB fermentations, and its production is forecasted to rise as the result of the increasing demand of polylactic acid. Hence, the creation of new ways to revalorize LA production processes is of high interest and could further enhance its economic value. Therefore, this review explores some co-products of LA fermentations, derived from LAB, with special focus on bacteriocins, lipoteichoic acid, and probiotics. Finally, a multi-product process involving LA and the other compounds of interest is proposed.Fermentation 2020, 6, 23 2 of 21 by-products, were proposed to improve industrial single-product biotechnological processes [2]. One example is the use of cellulose and hemicellulose from sugarcane bagasse (a residue obtained during the bioethanol production and usually used for energy generation) for the production of value-added chemicals [3]. Likewise, glycerol, a by-product of the biodiesel industry with low value in the market, is targeted as a molecule of interest in fermentation processes. Such multi-product processes, based on the conversion of renewable materials into biobased products, are known as biorefineries [4].The study, development, and application of robust microbial strains, able to utilize a variety of substrates and to produce a wide range of products, is considered a milestone in the development of biorefineries [4]. Lactic acid bacteria (LAB) are a diverse group with recognized potential for the development of integrated biorefineries [5]. LAB are non-sporulating, non-motile, acid-tolerant, non-respiring but aerotolerant, catalase-negative, Gram-positive cocci or rods. They are characterized by the production of lactic acid (LA) as the major end metabolic product of carbohydrate fermentation [6][7][8][9]. Given the lack of a functional respiratory system, LAB obtain energy through substrate-level phosphorylation following two metabolic pathways for hexose fermentation, i.e., homofermentative and heterofermentative. As shown in Figure 1, the first pathway is based on glycolysis with the production of mainly LA, whereas the second one, known as the pentose phosphate pathway, is characterized for the production of CO 2 and ethanol, or acetate in addition to LA [6].

Biotechnological production of lactic acid (LA) is based on the so-called first generation feedstocks, meaning sugars derived from food and feed crops such as corn, sugarcane and cassava. The aim of this study was to exploit the potential of a second generation resource: Common reed (Phragmites australis) is a powerfully reproducing sweet grass which grows in wetlands and creates vast monocultural populations. This lignocellulose biomass bears the possibility to be refined to value-added products, without competing with agro industrial land. Besides utilizing reed as a renewable and inexpensive substrate, low-cost nutritional supplementation was analyzed for the fermentation of thermophilic Bacillus coagulans. Various nutritional sources such as baker’s and brewer’s yeast, lucerne green juice and tryptone were investigated for the replacement of yeast extract. The structure of the lignocellulosic material was tackled by chemical treatment (1% NaOH) and enzymatic hydrolysis (Cellic® CTec2). B. coagulans DSM ID 14-300 was employed for the homofermentative conversion of the released hexose and pentose sugars to polymerizable L‑(+)-LA of over 99.5% optical purity. The addition of autolyzed baker’s yeast led to the best results of fermentation, enabling an LA titer of 28.3 g L−1 and a yield of 91.6%.

A new biorefinery concept is proposed that integrates the novel LX-Pretreatment with the fermentative production of L-(+)-lactic acid. Lignocellulose was chosen as a substrate that does not compete with the provision of food or feed. Furthermore, it contains lignin, a promising new chemical building material which is the largest renewable source for aromatic compounds. Two substrates were investigated: rye straw (RS) as a residue from agriculture, as well as the fibrous digestate of an anaerobic biogas plant operated with energy corn (DCS). Besides the prior production of biogas from energy corn, chemically exploitable LX-Lignin was produced from both sources, creating a product with a low carbohydrate and ash content (90.3% and 88.2% of acid insoluble lignin). Regarding the cellulose fraction of the biomass, enzymatic hydrolysis and fermentation experiments were conducted, comparing a separate (SHF), simultaneous (SSF) and prehydrolyzed simultaneous saccharification and fermentation (PSSF) approach. For this purpose, thermophilic B. coagulans 14-300 was utilized, reaching 38.0 g L−1 LA in 32 h SSF from pretreated RS and 18.3 g L−1 LA in 30 h PSSF from pretreated DCS with optical purities of 99%.

Investigating alternatives for petrobased substrates, lignocellulose is an interesting yet complex feedstock that offers various possibilities for the design of new and sustainable chemical routes. The novel energy-saving LX-pretreatment was combined with thermophilic Bacillus coagulans. By this, corn straw was used in an innovative cascade obtaining biogas, lignin as well as polymerisable L-(+)-lactic acid of over 99 percents optical purity.

This study demonstrated the bioconversion of lignocellulosic by-product corn stover (CS) to the value-added fermentative product L-lactic acid using the furfural tolerant Enterococcus mundtii WX1 and Lactobacillus rhamnosus SCJ9. The efficacy of dilute acid pretreatment by sulfuric and formic acids varying from 1% to 4% (v/v) concentration was compared. CS pretreated with 1% (v/v) sulfuric acid was selected for L-LA fermentation regarding the highest efficacy of fermentable sugar release when combined with the enzymatic hydrolysis process. Optimal conditions achieved a highest sugar release of 24.5 g/L glucose and 11.2 g/L of xylose from 100 g/L pretreated CS with 1% (v/v) sulfuric acid at 121 °C for 30 min, followed by enzymatic hydrolysis with Cellic CTec2 30 FPU/g pretreated CS at 50 °C for 48 h. The maximum L-LA titer, yield, and average productivity reached 31.4 g/L, 0.90 g/g, and 1.73 g/L/h, respectively. Moreover, addition of a hemicellulose-degrading enzyme complex combined with Cellic CTec2 led to an increase in xylose release, which resulted in a higher L-LA titer of 36.7 g/L at 48 h fermentation. Moreover, the purification of LA from culture broth by a process of electrodialysis with 331 g/L of LA and purity of 99.7% (w/w), was successful, with an optically pure L-LA of 99.9%. This study not only presents a feasible process for L-LA production from lignocellulose hydrolysate derived from abundant corn stover; this study also showed an alternative approach for solving the problem of haze air pollution caused by inappropriate management of corn production residuals.