Yukihiro Tashiro

Recent advances in lactic acid production by microbial fermentation processes

Fermentative production of optically pure lactic acid has roused interest among researchers in recent years due to its high potential for applications in a wide range of fields. More specifically, the sharp increase in manufacturing of biodegradable polylactic acid (PLA) materials, green alternatives to petroleum-derived plastics, has significantly increased the global interest in lactic acid production. However, higher production costs have hindered the large-scale application of PLA because of the high price of lactic acid. Therefore, reduction of lactic acid production cost through utilization of inexpensive substrates and improvement of lactic acid production and productivity has become an important goal. Various methods have been employed for enhanced lactic acid production, including several bioprocess techniques facilitated by wild-type and/or engineered microbes. In this review, we will discuss lactic acid producers with relation to their fermentation characteristics and metabolism. Inexpensive fermentative substrates, such as dairy products, food and agro-industrial wastes, glycerol, and algal biomass alternatives to costly pure sugars and food crops are introduced. The operational modes and fermentation methods that have been recently reported to improve lactic acid production in terms of concentrations, yields, and productivities are summarized and compared. High cell density fermentation through immobilization and cell-recycling techniques are also addressed. Finally, advances in recovery processes and concluding remarks on the future outlook of lactic acid production are presented.

High butanol production by Clostridium saccharoperbutylacetonicum N1-4 in fed-batch culture with pH-Stat continuous butyric acid and glucose feeding method.

A pH-stat fed-batch culture by feeding butyric acid and glucose has been studied in an acetone-butanol-ethanol (ABE) fermentation using Clostridium saccharoperbutylacetonicum N1-4. The specific butanol production rate increased from 0.10 g-butanol/g-cells/h with no feeding of butyric acid to 0.42 g-butanol/g-cells/h with 5.0 g/l butyric acid. The pH value in broth decreases with butyric acid production during acidogenesis, and then butyric acid reutilization and butanol production result in a pH increase during solventogensis. The pH-stat fed-batch culture was performed to maintain a constant pH and butyric acid concentration in the culture broth, but feeding only butyric acid could not support butyric acid utilization and butanol production. Subsequently, when a mixture of butyric acid and glucose was fed, butyric acid was utilized and butanol was produced. To investigate the effect of the feeding ratio of butyric acid to glucose (B/G ratio), several B/G ratio solutions were fed. The maximum butanol production was 16 g/l and the residual glucose concentration in broth was very low at a B/G ratio of 1.4. Moreover, yields of butanol in relation to cell mass and glucose utilization were 54% and 72% higher in pH-stat fed-batch culture with butyric acid than that of conventional batch culture, respectively.

Fermentative production of lactic acid from renewable materials: Recent achievements, prospects, and limits

The development and implementation of renewable materials for the production of versatile chemical resources have gained considerable attention recently, as this offers an alternative to the environmental problems caused by the petroleum industry and the limited supply of fossil resources. Therefore, the concept of utilizing biomass or wastes from agricultural and industrial residues to produce useful chemical products has been widely accepted. Lactic acid plays an important role due to its versatile application in the food, medical, and cosmetics industries and as a potential raw material for the manufacture of biodegradable plastics. Currently, the fermentative production of optically pure lactic acid has increased because of the prospects of environmental friendliness and cost-effectiveness. In order to produce lactic acid with high yield and optical purity, many studies focus on wild microorganisms and metabolically engineered strains. This article reviews the most recent advances in the biotechnological production of lactic acid mainly by lactic acid bacteria, and discusses the feasibility and potential of various processes.

Identification and production of a bacteriocin from Enterococcus mundtii QU 2 isolated from soybean.

Aims:  Identification of the bacteriocin produced by Enterococcus mundtii QU 2 newly isolated from soybean and fermentative production of the bacteriocin.

Recent advances to improve fermentative butanol production: Genetic engineering and fermentation technology

Butanol has recently attracted attention as an alternative biofuel because of its various advantages over other biofuels. Many researchers have focused on butanol fermentation with renewable and sustainable resources, especially lignocellulosic materials, which has provided significant progress in butanol fermentation. However, there are still some drawbacks in butanol fermentation in terms of low butanol concentration and productivity, high cost of feedstock and product inhibition, which makes butanol fermentation less competitive than the production of other biofuels. These hurdles are being resolved in several ways. Genetic engineering is now available for improving butanol yield and butanol ratio through overexpression, knock out/down, and insertion of genes encoding key enzymes in the metabolic pathway of butanol fermentation. In addition, there are also many strategies to improve fermentation technology, such as multi-stage continuous fermentation, continuous fermentation integrated with immobilization and cell recycling, and the inclusion of additional organic acids or electron carriers to change metabolic flux. This review focuses on the most recent advances in butanol fermentation especially from the perspectives of genetic engineering and fermentation technology.

Efficient Homofermentative l-(+)-Lactic Acid Production from Xylose by a Novel Lactic Acid Bacterium, Enterococcus mundtii QU 25

ABSTRACT Enterococcus mundtii QU 25, a newly isolated lactic acid bacterium, efficiently metabolized xylose into l-lactate. In batch fermentations, the strain produced 964 mM l-(+)-lactate from 691 mM xylose, with a yield of 1.41 mol/mol xylose consumed and an extremely high optical purity of ≥99.9% without acetate production.

Continuous butanol fermentation from xylose with high cell density by cell recycling system

A continuous butanol production system with high-density Clostridium saccharoperbutylacetonicum N1-4 generated by cell recycling was established to examine the characteristics of butanol fermentation from xylose. In continuous culture without cell recycling, cell washout was avoided by maintaining pH>5.6 at a dilution rate of 0.26 h(-1), indicating pH control was critical to this experiment. Subsequently, continuous culture with cell recycling increased cell concentration to 17.4 g L(-1), which increased butanol productivity to 1.20 g L(-1) h(-1) at a dilution rate of 0.26 h(-1) from 0.529 g L(-1) h(-1) without cell recycling. The effect of dilution rates on butanol production was also investigated in continuous culture with cell recycling. Maximum butanol productivity (3.32 g L(-1) h(-1)) was observed at a dilution rate of 0.78 h(-1), approximately 6-fold higher than observed in continuous culture without cell recycling (0.529 g L(-1) h(-1)).

Isolation and characterisation of lactic acid bacterium for effective fermentation of cellobiose into optically pure homo l-(+)-lactic acid

Effective utilisation of cellulosic biomasses for economical lactic acid production requires a microorganism with potential ability to utilise efficiently its major components, glucose and cellobiose. Amongst 631 strains isolated from different environmental samples, strain QU 25 produced high yields of l-(+)-lactic acid of high optical purity from cellobiose. The QU 25 strain was identified as Enterococcus mundtii based on its sugar fermentation pattern and 16S rDNA sequence. The production of lactate by fermentation was optimised for the E. mundtii QU25 strain. The optimal pH and temperature for batch culturing were found to be 7.0°C and 43°C, respectively. E. mundtii QU 25 was able to metabolise a mixture of glucose and cellobiose simultaneously without apparent carbon catabolite repression. Moreover, under the optimised culture conditions, production of optically pure l-lactic acid (99.9%) increased with increasing cellobiose concentrations. This indicates that E. mundtii QU 25 is a potential candidate for effective lactic acid production from cellulosic hydrolysate materials.

Recent advances and future prospects for increased butanol production by acetone‐butanol‐ethanol fermentation

Presently, several researchers are increasingly focusing on producing butanol as the next‐generation fuel by acetone‐butanol‐ethanol (ABE) fermentation. Butanol has many superior characteristics compared to other biofuels, such as ethanol. However, its production by ABE fermentation faces the challenges of low productivity and yield because of product inhibition and heterofermentation, respectively, and thereby, high costs. Until date, molecular biological techniques and fermentation engineering methods have been applied for high butanol production. Although glucose remains the substrate of choice since traditional research, it is now necessary to substitute glucose derived from edible starch to other substrates from low‐cost feedstock, such as agricultural residue. In addition, ABE‐producing clostridia cannot directly produce butanol from lignocelluloses. Therefore, recent research is focusing on pretreatment and enzymatic saccharification of the complex molecules derived from agricultural residue for use as feedstock in butanol production. This article reviews traditional research, including the metabolism patterns and characteristics of ABE‐producing clostridia. Furthermore, this article describes developments in ABE fermentation with respect to the establishment of highly efficient butanol production processes, such as batch, fed‐batch, and continuous cultures, with the introduction of butanol removal, as well as butanol production from lignocellulosic biomasses or alternative substrates to sugars.

Development of high-speed and highly efficient butanol production systems from butyric acid with high density of living cells of Clostridium saccharoperbutylacetonicum

Living cells are alive and have the butanol-producing ability but not much proliferation under nitrogen source-limited condition. We investigated various butanol production systems with high density of living cells of Clostridium saccharoperbutylacetonicum N1-4 supplemented with methyl viologen (MV) as an electron carrier and nutrient dosing for activity regeneration. In continuous butanol production with high density of living cells, butanol yield was drastically increased from 0.365 C-mol/C-mol with growing cells to 0.528 C-mol/C-mol at a dilution rate of 0.85 h⁻¹, being increased with the butanol to total solvent ratio. This yield was increased to 0.591 C-mol/C-mol by adding 0.01 mM MV. MV addition increased not only butanol yield but also butanol concentration and productivity as compared to those without MV addition. However, living cells lost their activity with incubation time, which lowered the operational stability of the system. Therefore, to maintain constant stability, activity regeneration was carried out with high density of living cells and MV. This system produced butanol at high concentration (9.40 g l⁻¹) and productivity (7.99 g l⁻¹ h⁻¹) for approximately 100 h with maintenance of considerably high yield of butanol (0.686 C-mol/C-mol). Thus, we established a high-speed and highly efficient butanol production system.