Effects of neutralizing agents on lactic acid production by Rhizopus oryzae using sweet potato starch

Metabolic engineering of Zymomonas mobilis for co-production of D-lactic acid and ethanol using waste feedstocks of molasses and corncob residue hydrolysate

Lactate is the precursor for polylactide. In this study, a lactate producer of Z. mobilis was constructed by replacing ZMO0038 with LmldhA gene driven by a strong promoter PadhB, replacing ZMO1650 with native pdc gene driven by Ptet, and replacing native pdc with another copy of LmldhA driven by PadhB to divert carbon from ethanol to D-lactate. The resultant strain ZML-pdc-ldh produced 13.8 ± 0.2 g/L lactate and 16.9 ± 0.3 g/L ethanol using 48 g/L glucose. Lactate production of ZML-pdc-ldh was further investigated after fermentation optimization in pH-controlled fermenters. ZML-pdc-ldh produced 24.2 ± 0.6 g/L lactate and 12.9 ± 0.8 g/L ethanol as well as 36.2 ± 1.0 g/L lactate and 40.3 ± 0.3 g/L ethanol, resulting in total carbon conversion rate of 98.3% ± 2.5% and 96.2% ± 0.1% with final product productivity of 1.9 ± 0.0 g/L/h and 2.2 ± 0.0 g/L/h in RMG5 and RMG12, respectively. Moreover, ZML-pdc-ldh produced 32.9 ± 0.1 g/L D-lactate and 27.7 ± 0.2 g/L ethanol as well as 42.8 ± 0.0 g/L D-lactate and 53.1 ± 0.7 g/L ethanol with 97.1% ± 0.0% and 99.1% ± 0.8% carbon conversion rate using 20% molasses or corncob residue hydrolysate, respectively. Our study thus demonstrated that it is effective for lactate production by fermentation condition optimization and metabolic engineering to strengthen heterologous ldh expression while reducing the native ethanol production pathway. The capability of recombinant lactate-producer of Z. mobilis for efficient waste feedstock conversion makes it a promising biorefinery platform for carbon-neutral biochemical production.

The cell wall and the response and tolerance to stresses of biotechnological relevance in yeasts

In industrial settings and processes, yeasts may face multiple adverse environmental conditions. These include exposure to non-optimal temperatures or pH, osmotic stress, and deleterious concentrations of diverse inhibitory compounds. These toxic chemicals may result from the desired accumulation of added-value bio-products, yeast metabolism, or be present or derive from the pre-treatment of feedstocks, as in lignocellulosic biomass hydrolysates. Adaptation and tolerance to industrially relevant stress factors involve highly complex and coordinated molecular mechanisms occurring in the yeast cell with repercussions on the performance and economy of bioprocesses, or on the microbiological stability and conservation of foods, beverages, and other goods. To sense, survive, and adapt to different stresses, yeasts rely on a network of signaling pathways to modulate the global transcriptional response and elicit coordinated changes in the cell. These pathways cooperate and tightly regulate the composition, organization and biophysical properties of the cell wall. The intricacy of the underlying regulatory networks reflects the major role of the cell wall as the first line of defense against a wide range of environmental stresses. However, the involvement of cell wall in the adaptation and tolerance of yeasts to multiple stresses of biotechnological relevance has not received the deserved attention. This article provides an overview of the molecular mechanisms involved in fine-tuning cell wall physicochemical properties during the stress response of Saccharomyces cerevisiae and their implication in stress tolerance. The available information for non-conventional yeast species is also included. These non-Saccharomyces species have recently been on the focus of very active research to better explore or control their biotechnological potential envisaging the transition to a sustainable circular bioeconomy.

Production of nisin and lactic acid from the starch of sweet potato by simultaneous saccharification and fermentation with two stage pH adjustment

Nisin is an antimicrobial peptide which is widely used as preservative, while lactic acid is a natural organic acid applied in the food industry. The aim of this work was to study the process for nisin and lactic acid production from starch of sweet potato with simultaneous saccharification and fermentation (SSF) by Lactococcus lactis subsp. Lactis with two stage pH adjustment. The factors impacting the nisin and lactic acid production including starch concentration, glucosidase concentration, CaCO₃ and Tween-80 were studied. The nisin titre reached a high of 2516.41 IU/mL, while the lactic acid reached a high of 37.06 g/L when the optimal conditions were 40 g/L starch, 100 U glucosidase/g starch, 2.5% CaCO₃ and 1 mL/L Tween-80. The lactic acid and nisin were separated by a two stage pH adjustment at last. The SSF of starch from sweet potato coupled with a two stage pH adjustment is a promising method to produce nisin and lactic acid.

Sensory profile, functional properties and molecular weight distribution of fermented pea protein isolate

Pea protein isolate (PPI, from Pisum sativum L.) was fermented with six different lactic acid bacteria strains for 24 ​h and 48 ​h. The fermented samples were analyzed regarding their retronasal aroma and taste, their protein solubility, emulsifying and foaming capacity. Changes in the molecular weight distribution were analyzed to monitor potential effects of fermentation on the main allergenic protein fractions of PPI. After 24-h fermentation, PPI's characteristic aroma attributes and bitter taste decreased for all fermented PPI. However, after 48-h fermentation, cheesy aroma, and acid and salty tastes were increased. The PPI fermented with L. plantarum showed the most neutral taste and the panel's highest preference; instead, fermentation with L. fermentum led to a fecal aroma and was the least preferred. The protein solubility and emulsifying capacity decreased after PPI fermentation, while foaming capacity remained constant in comparison to the untreated PPI. The electrophoretic results showed a reduction in the intensity of the allergenic protein fractions; however, these changes might be attributed to the reduced protein solubility rather than to a high proteolytic effect of the strains. Fermentation of PPI for 24 ​h and 48 ​h might not be a suitable method for the production of highly functional pea proteins. Further modification methods have to be investigated in the future.

Effect of nitrogen sources and neutralizing agents on D-lactic acid production from Kodo millet bran hydrolysate: comparative study and kinetic analysis

D-lactic acid (DLA) serves as a key monomer enhancing both the mechanical and thermal properties of Poly(lactic) acid films and coatings, extensively used in the food packaging industry. Economically viable production of optically pure DLA by Lactobacillus delbrueckii NBRC3202 was achieved using a low-cost carbon source, Kodo millet bran residue hydrolysate (KMBRH) and nitrogen source (casein enzyme hydrolysate (CEH) resulting in a high DLA yield of 0.99 g g-1 and KMBRH conversion to final product (95.3%). The optimum values for kinetic parameters viz., specific growth rate (0.11 h-1), yield coefficient of biomass on KMBRH (0.10 g g-1) and DLA productivity (0.45 g L-1 h-1) were achieved at 5 g L-1 of CEH dosage under controlled pH environment. A comparative study and kinetic analysis of different neutralizing agents (NaOH, NH3, CaCO3 and NaHCO3) under pH controlled environment for KMBRH based DLA production was addressed effectively through bioreactor scale experiments. Maximum cell concentration (1.29 g L-1) and DLA titer (45.08 g L-1) were observed with NH3 as a neutralizing agent. Kinetic analysis of DLA production under different neutralization agents demonstrated that the logistic derived model predicted biomass growth, KMBRH consumption and DLA production efficiently (R 2 > 0.92).

High-titer and productivity of l-(+)-lactic acid using exponential fed-batch fermentation with Bacillus coagulans arr4, a new thermotolerant bacterial strain

Bacillus coagulans arr4 is a thermotolerant microorganism with great biotechnological potential for l-(+)-lactic acid production from granulated sugar and yeast extract. The highest l-(+)-lactic acid production was obtained with Ca(OH)₂. The maximum production of l-(+)-lactic acid (206.81 g/L) was observed in exponential feeding using granulated sugar solution (900 g/L) and yeast extract (1%) at 50 °C, pH 6.5, and initial granulated sugar concentration of 100 g/L at 39 h. 5.3 g/L h productivity and 97% yield were observed, and no sugar remained. Comparing the simple batch with exponential fed-batch fermentation, the l(+) lactic acid production was improved in 133.22% and dry cell weight was improved in 83.29%, using granulated sugar and yeast extract. This study presents the highest productivity of lactic acid ever observed in the literature, on the fermentation of thermotolerant Bacillus sp. as well as an innovative and high-efficiency purification technology, using low-cost substances as Celite and charcoal. The recovery of lactic acid was 86%, with 100% protein removal, and the fermentation medium (brown color) became a colorless solution.

Environmentally Friendly Production of D(-) Lactic Acid by Sporolactobacillus nakayamae: Investigation of Fermentation Parameters and Fed-Batch Strategies

The interest in the production of lactic acid has increased due to its wide range of applications. In the present study, the variables that affect fermentative D(−) lactic acid production were investigated: neutralizing agents, pH, temperature, inoculum percentage, agitation, and concentration of the medium components. An experimental design was applied to determine the optimal concentrations of the medium components and fermentation was studied using different feeding strategies. High production (122.41 g/L) and productivity (3.65 g/L·h) were efficiently achieved by Sporolactobacillus nakayamae in 54 h using a multipulse fed-batch technique with an initial medium containing 35 g/L of yeast extract (byproduct of alcohol production), 60 g/L of crystallized sugar, and 7.5 mL/L of salts. The fermentation process was conducted at 35°C and pH 6.0 controlled by NaOH with a 20% volume of inoculum and agitation at 125 rpm. The production of a high optically pure concentration of D(−) lactic acid combined with an environmentally friendly NaOH-based process demonstrates that S. nakayamae is a promising strain for D(−) lactic acid production.