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Journal of Bioscience and BioengineeringVOL. 119 No. 1, 10e18, 2015

REVIEW

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

Ying Wang,1 Yukihiro Tashiro,2,3 and Kenji Sonomoto1,4,*

Laboratory of Microbial Technology, Division of Applied Molecular Microbiology and Biomass Chemistry, Department of Bioscience and Biotechnology, Faculty of Agriculture,Graduate School, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan,1 Institute of Advanced Study, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka812-8581, Japan,2 Laboratory of Soil Microbiology, Division of Applied Molecular Microbiology and Biomass Chemistry, Department of Bioscience and Biotechnology, Faculty ofAgriculture, Graduate School, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan,3 and Laboratory of Functional Food Design, Department of Functional

Metabolic Design, Bio-Architecture Centre, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan4

Received 16 April 2014; accepted 3 June 2014Available online 27 July 2014

* CorrespondMolecular MicBiotechnology,Hakozaki, Higa

E-mail add

1389-1723/$http://dx.doi

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

� 2014, The Society for Biotechnology, Japan. All rights reserved.

[Key words: Lactic acid; Renewable materials; Microbial fermentation; Lactic acid bacteria; Metabolic pathways]

Lactic acid (2-hydroxypropanoic acid) is a naturally occurringhydroxycarboxylic acid, first refined from sour milk by the Swedishchemist Scheele in 1780 (1). Subsequently, due to its versatile ap-plications as an acidulant, flavour enhancer, and preservative, lacticacid has occupied a prime position in the food, pharmaceutical,cosmetic and other chemical industries (2,3). Recently, new uses forthis compound are emerging. Lactic acid production has received asignificant amount of interest because it can be used as a feedstockfor the production of poly-lactic acid (PLA), a polymer present inmedical applications and environmentally friendly biodegradableplastics, which can be substituted for synthetic plastics derivedfrom petroleum resources (2e4). In nature, lactic acid occurs in twooptical isomers, D (�)- and L (þ)-lactic acids. L (þ)-Lactic acid is thepreferred isomer in the food and drug industries, because only thisform is adapted to be assimilated by the human body. Therefore, theforms of lactic acid in pure isomers are more valuable for differentspecific applications (4,5). Copolymerization of the D (�)- and L (þ)-isomers results in amorphous materials, whereas homopolymersform regular structures and are in a crystalline phase (6).

Lactic acid can be manufactured commercially by either chem-ical synthesis or biotechnological production by lactic acid

ing author at: Laboratory of Microbial Technology, Division of Appliedrobiology and Biomass Chemistry, Department of Bioscience andFaculty of Agriculture, Graduate School, Kyushu University, 6-10-1shi-ku, Fukuoka 812-8581, Japan. Tel./fax: þ81 (0)92 642 3019.ress: sonomoto@agr.kyushu-u.ac.jp (K. Sonomoto).

e see front matter � 2014, The Society for Biotechnology, Japan..org/10.1016/j.jbiosc.2014.06.003

fermentation. Themost commonmethod to synthesize lactic acid isbased on the hydrolysis of lactonitrile. However, chemical synthesisalways yields a racemic mixture of DL-lactic acid from petroleumresources. On the other hand, an optically pure L (þ)- or D (�)-lacticacid can be obtained by the microbial fermentative method.Currently, approximate 90% of lactic acid is produced by the mi-crobial fermentation. With the development of industrial biocon-version, microbial fermentation by the appropriate microorganismhas become the dominant method of lactic acid production due toenvironmental concerns, low production temperature, low energyrequirements, and high purity (7).

In recent times, the consumption of lactic acid as a feedstock forthe production of PLA has increased considerably. However, theamount of PLA production (450 million kilograms per year) is stillmuch lower than the total amount of plastics production (200billion kilograms per year) (8). PLA production is restricted by highproduction costs, although the annual industrial investment isseveral million dollars (9). It has been reported that the cost of rawmaterials for the fermentative production of lactic acid usuallyaccounts for greater than 34% of the total manufacturing cost (10).Thus, the efficiency and economics of lactic acid fermentation is stilla problem frommany points of view, and the substrate plays a vitalrole in the improvement of such a process. There have been variousattempts to produce lactic acid efficiently from economic resources,such as rice bran (11), paper sludge (12), and green microalga (13).Nowadays, renewable materials such as lignocellulose and starchfrom agricultural residues and forestry resources are generally

All rights reserved.

VOL. 119, 2015 LACTIC ACID PRODUCTION FROM RENEWABLE MATERIALS 11

considered to represent an attractive substrate as feedstock for theproduction of lactic acid due to their abundance (5). However, onebottleneck for lactic acid production utilizing renewable materialsis the cost of pretreatment. Most renewable materials are notdirectly available for lactic acid fermentation without pretreatmentdue to their intimate association with lignin and the lack of hy-drolytic enzyme production by lactic acid-producing strains (5).Another limiting factor is the recovery and purification of lactic acidfrom the fermentation broth, because complex media consisting ofvarious nutrients hampers not only the separation but also thepurification of lactic acid. Therefore, there are many challenges inthe industrial bioconversion of renewable materials to lactic acid.

This article presents a review of recent advances in thebiotechnological production of lactic acid using renewable mate-rials from the aspects of metabolic and enzymatic mechanisms, andmetabolic engineering associated with lactic acid production. Themajor production processes, renewable materials, bioreactor sys-tems, and fermentation modes are reviewed. We also describerecent achievements and limitations in simultaneous saccharifica-tion and fermentation (SSF) and molecular genetic approaches inthe production of lactic acid from renewable materials.

MICROORGANISMS INVOLVED IN BIOTECHNOLOGICALPRODUCTION OF LACTIC ACID

Wild type of LAB Lactic acid bacteria (LAB) are defined asfacultative anaerobic or micro-aerophilic organisms and charac-terized by the following aspects: (i) can grow at temperatures aslow as 5�C or as high as 45�C, (ii) can grow at pH 4.0e4.5, some alsoproliferate at pH 3.2 or 9.6, and (iii) generally require complex ni-trogens, vitamins, and minerals for growth and lactic acid pro-duction (14). Studies on LAB constitute approximately 90% of theliterature on lactic acid production because they can producelactic acid with high yield and high productivity. The mostcommon LAB species belong to the genera Lactobacillus,Lactococcus, Pediococcus, Aerococcus, Carnobacterium, Oenococcus,Tetragenococcus, Vagococcus, Weisella, Leuconostoc, Streptococcus,and Enterococcus. Among them, optically pure L (þ)-lactic acid isproduced by several species such as Enterococcus mundtii (15e17)and Lactococcus lactis (18), while D (�)-lactic acid can beproduced by Lactobacillus delbrueckii (19).

Wild-type LAB, isolated and screened from various sources, arealways the most powerful source for obtaining fermentable andgenetically stable strains, which are widely utilized in lactic acidproduction. Two Lactobacillus strains of OND 32T and YAM 1 wereisolated from sour cassava starch fermentation, and can directlyferment starchy biomass to lactic acid (20). Abdel-Rahman et al.(16,17) presented a novel wild-type strain of E. mundtii QU 25,which is a very attractive candidate for efficiently metabolizinglignocellulose-derived sugars into optically pure homo L (þ)-lacticacid. The strain can produce lactic acid from a high concentration ofxylose via the pentose phosphate pathway (16) as described indetail in next section, through which 3 mol of xylose yield 5 molof lactic acid. This means that the theoretical yield is close to100%, and few by-products are formed through fermentation bythis strain (16).

Wild type of Bacillus genus and fungi Although LAB arewidely used in lactic acid production, some other strains such asthose of the genus Bacillus, as well as fungi, also produce lactic acid.Patel et al. (21) isolated Bacillus sp. strains 17C5 and 36D1 from soiland geysers, and proved their ability to produce L (þ)-lactic acidfrom sugarcane bagasse with a maximum productivity of6.7 mmol L�1 h�1 in SSF. Another group focused on thebioconversion of paper sludge to lactic acid by the thermophilicBacillus coagulans strain P4-102B (12).

The best-known fungal source as a lactic acid producer isRhizopus oryzae. In general, R. oryzae has relatively lower nutri-tional demands but the mycelial morphology and oxygen supplyare considered to influence lactic acid productivity. The first reporton efficient D (�)-lactic acid fermentation by R. oryzae was in 1936(22). R. oryzae NRRL 395 has been recognized as one of the mostsuitable fungi for lactic acid fermentation (23). Guo et al. (24)described a fermentation process involving the simultaneous uti-lization of hemicellulose and cellulose in corncobs by a newlyisolated R. oryzae.

Engineered microorganisms by mutagenesis and metabolicengineering Studies also focus on engineered microorganismsin order to meet the commercial requirements including improvedoptical purity of the product, reduction of nutritional supply,improved yield and productivity, a broader substrate specificity,and the elimination of plasmids and antibiotic markers. The initialefforts of genetic modifications were mainly on improving LAB bytraditional approaches, which involve exposing the bacterium tomutagens such as ethylmethyl sulphonate, N-methyl-N0-nitro-N-nitrosoguanidine, and ultraviolet radiations (25,26).

On the other hand, efficient genetic tools targeting several mi-croorganisms have been developed over the past few decades.Amongst these, metabolic engineering is an important tool for in-dustrial biotechnology (27). According to the manipulation ofenzyme functions, transcription, and the regulatory system in themicroorganisms, metabolic engineering redirects metabolic flux,changes cellular protein levels, and regulates gene expression inseveral hosts such as Saccharomyces cerevisiae (28), Escherichia coli(29), Corynebacterium glutamicum (30), R. oryzae (31), and Lacto-coccus lactis (32). Table 1 shows a summary of recent studies onengineering approaches for lactic acid production.

STUDIES ON METABOLIC PATHWAYS WITH LAB

LAB ferment sugars such as hexose and pentose via differentmetabolic pathways that lead to homo- or hetero-fermentation(Fig. 1). Homo-fermentation produces virtually only lactic acid asthe end product via the EmbdeneMeyerhofeParnas (EMP)pathway and the pentose phosphate (PP)/glycolic pathway fromhexose and pentose, respectively. The theoretical yield of lactic acidfrom glucose is 1.0 g g�1 (2.0 mol mol�1) via the EMP pathwaywhile pentose exhibits a theoretical yield of 1.0 g g�1

(1.67 mol mol�1) of lactic acid via the PP/glycolic pathway (43,44).In the EMP pathway, the first steps of glycolysis are the phos-phorylation of glucose to fructose 1,6-diphosphate (FDP) and itssubsequent cleavage into dihydroxyacetone phosphate (DHAP) andglyceraldehyde 3-phosphate (GAP). The GAP is then converted topyruvate via a route that includes 2 substrate-level phosphoryla-tion steps. Finally, pyruvate is reduced to lactic acid by L-LDH or D-LDH with the oxidation of NADH to NADþ for the redox balancing.In the PP/glycolic pathway, 3 mol of xylulose 5-phosphate (xylulose5-P; 5 carbons), generated by the phosphorylation of pentosesugars such as xylose and arabinose, is converted to 5 mol of GAP(3 carbons) via 2 key enzymes: transketolase and transaldolase. Theresulting GAP is converted to pyruvate and then to lactic acid(3 carbons) as the final product, thereby providing a theoreticalyield of lactic acid from pentose of 1.0 g g�1 (1.67 mol mol�1).

On the other hand, in hetero-fermentation, equimolar amountsof lactic acid, carbon dioxide, and ethanol (or acetate) are formedvia the phosphoketolase (PK) pathway, in which glucose 6-phos-phate (6 carbons) is initially converted to ribulose 5-phosphate(5 carbons) and carbon dioxide (1 carbon) in a reaction catalyzed byseveral enzymes (45). Then, the resulting xylulose 5-P from ribu-lose 5-phosphate is cleaved to an equimolar amount of GAP andacetyl phosphate (acetyl-P). The acetyl-P is reduced to ethanol

TABLE

1.Metab

olic

enginee

ringap

proaches

forlactic

acid

productionby

seve

ralmicroorga

nisms.

Metab

olically

enginee

redmicroorga

nism

Gen

us

Approach

Outcom

eSu

bstrate

Lactic

acid

g(g

L�1)

Yield

Y(g

g�1)

Referen

ce

Cand

idautilisNBRC09

88cu

pdc1

D4-ldh2

Yea

stcu

pdc1

deletion,L-ldh

2ex

pression

L-Lactic

acid

production

Gluco

se10

3.3

0.95

33Kluyv

erom

yces

lactisCBS2

359pEP

L2Yea

stL-ldhL

expression

Improve

dyieldof

lactic

acid

Gluco

se10

90.60

34Pich

iastipitisCBS6

054VTT

-C-045

90Yea

stL-ldhL

expression

Red

ucedby

-product

Xylose/Gluco

se58

/41

0.58

/0.44

35Sa

ccha

romyces

cerevisiae

CEN

.PK18

2RW

B85

0-2

Yea

stpd

cdeletion

Hom

oferm

entative

lactateproduction

Gluco

se12

20.83

�0.02

28S.

cerevisiae

OC2YILM-2B

Yea

stpd

c1deletion

D-Lacticacid

production

Gluco

se61

.50.61

36Ba

cillu

scoag

ulan

sP4

e10

2BQZ1

9Bacteria

D-ldh

andalsdeletion

D-Lactate

production

Gluco

se90

e37

Coryne

bacterium

glutam

icum

X5C

1Bacteria

xylA,x

ylB,b

glF3

17Aan

dbg

lAex

pression

Broad

ensu

garutiliz

ationrange

Cellobiose/Gluco

se/X

ylose

z40

.5e

38C.

glutam

icum

RDldhA

/pCR

B204

Bacteria

D-ldh

Aex

pression

Improve

dproductivityof

D-lacticacid

Gluco

se12

00.87

29Esch

erichiacoliATC

C70

0926

ECOM3

Bacteria

cydA

B,cyo

ABC

D,a

ndcbdA

Bdeletion

Lactateproductionunder

oxic

andan

oxic

grow

thco

nditions

Gluco

see

0.8

39

E.coliATC

C11

303TG

114

Bacteria

msgAdeletion

Improve

dlactateproductivityan

dcellyield

Gluco

se11

80.98

40E.

coliATC

C70

0926

LA02

Ddld

Bacteria

pta,

adhE

,frdA,d

lddeletionan

dglpK

-glpD

expression

D-Lacticacid

production

Glycerol

320.85

41

E.coliW

3110

SZ40

Bacteria

pflBmutation

Improve

dyieldof

D-lacticacid

Gluco

se51

0.97

e0.99

29Lactococcu

slactisIL

1403

ptk::tkt

Bacteria

xylRABex

pression

L-Lactic

acid

ferm

entation

from

xylose

Xylose

50.1

0.79

32Lactob

acillus

plan

tarum

NCIM

B88

26DldhL

1-xp

k1::tkt

Bacteria

D-ldh

L1an

dxp

k1deletion,tkt

expression

D-Lacticacid

ferm

entation

from

arab

inose

Arabinose

38.6

0.82

42

Rhizopu

soryzae

NRRL39

5pLd

hA71

XFu

ngi

L-ldhA

expression

Increa

seLD

Hactivity

Gluco

se77

.5e

31

e,n

otdetermined

.ldh

,lactate

deh

ydroge

nasege

ne;

pdc,pyruva

tedecarbo

xylase

gene;

cupd

c1,p

yruva

tedecarbo

xylase

gene;

pfl,p

yruva

teform

atelyasege

ne;

msg,m

ethylglyo

xalsyn

thasege

ne;

cydA

B,cyoA

BCD,cbd

AB,

cytoch

rome

oxidases

genes;g

lpK-glpD,g

lycerolk

inasean

dglyc

erol-3-phosphatedeh

ydroge

nasege

ne;

dld,

D-lactate

deh

ydroge

nasege

ne;

xylA,xylRAB,x

yloseisom

erasege

nes;x

ylB,x

ylulokinasege

ne;

bgl,b-gluco

sidasege

ne;

als,acetolactate

synthasege

ne;

xpk,

phosphok

etolasege

ne;

pta,

phosphateacetyltran

sferasege

ne;

adhE

,alcoh

ol/acetaldeh

ydedeh

ydroge

nasege

ne;

frdA

,fumaratereductasege

ne;

tkt,tran

sketolasege

ne;

LDH,lactate

deh

ydroge

nase.

12 WANG ET AL. J. BIOSCI. BIOENG.,

(2 carbons) via acetyl-CoA and acetaldehyde intermediates, orconverted to acetate via acetate kinase, while GAP further enters toEMP pathway to form lactic acid (3 carbons) (5). As the result, thetheoretical yield of lactic acid from glucose reaches only 0.5 g g�1

(1.0 mol mol�1) with hetero-fermentative LAB. The ratio of ethanolto acetate is dependent on the redox potential in the cells. In termsof the metabolism of pentose sugars such as xylose and arabinose,xylulose 5-P (5 carbons) is the common intermediate, which is thencleaved to GAP and acetyl-P via the PK pathway. The resultingacetyl-P is metabolized to synthesize acetic acid or ethanol (both 2carbons), whereas the GAP is converted to pyruvic acid and then tolactic acid (3 carbons) as the final product, with a theoretical yieldof lactic acid from pentose sugars of 0.6 g g�1 or 1.0 mol mol�1 viathe PK pathway (16,21). Therefore, the EMP and PP pathways aremore effective than the PK pathway in improving lactic acid yieldand sugar utilization (44). A few wild-type and metabolicallyengineered strains showed the ability to metabolize pentose byhomo-lactic acid fermentation, such as E. mundtii QU 25 (16,17) andLactobacillus plantarum NCIMB 8826 DldhL1-xpk1::tkt (42,46). Inaddition, the concentration of pentose sugars also affected themetabolic fluxwith LAB. It was reported that when xylosewas usedas the sole carbon source, a higher yield of lactic acid was obtainedwith a high concentration of initial xylose than with a low con-centration (16,43).

EFFECTS OF SEVERAL FACTORS ON LACTIC ACID FERMENTATIONBY LAB

Effects of mixed sugars as carbon source Co-fermentationby mixed sugars has been recognized as effective for lactic acidfermentation. In particular, simultaneous and efficient fermenta-tion using mixed sugars of hydrolyzates derived from renewablematerials including pentoses and hexoses is a significant hurdle(47). Various refined sugars such as glucose, xylose, sucrose,cellobiose, and lactose have been used in the process of lacticacid production (48e51). In general, glucose is the most favouredsugar for most lactic acid-producing strains, which results inmany studies reporting lactic acid fermentation from glucose asthe sole carbon source (30,39,52). However, glucose has beenshown to limit the ability to effectively utilize other sugars toproduce lactic acid, a phenomenon known as carbon cataboliterepression (49). To date, it has been reported that several studieshave aimed at investigating efficient lactic acid fermentationusing mixed sugars in the presence of glucose by wild-type ormetabolically engineered strains.

Abdel-Rahman et al. (5) isolated a wild-type LAB, E. mundtii QU25, which was able to produce optically pure L-lactic acid fromglucose, cellobiose, and xylose. E. mundtii QU 25 has been shown tometabolize glucose and cellobiose simultaneously with a high yieldof lactic acid (>0.9 g g�1) without by-products.

Yoshida et al. (51) reported co-fermentation with glucose,xylose, and arabinose for homo-D-lactic acid production. By intro-ducing xylA (xylose isomerase) and xylB (xylulokinase) into thegenome of Lactobacillus plantarum NCIMB 8826, replacing the xpk1gene (PK) with the tkt gene (TK), and by deleting the ldhL1 (L-LDH)and xpk2 (PK) genes, Lactobacillus plantarum DldhL1::PxylAB-xpk1::tkt-Dxpk2::PxylAB was rendered capable of utilizing xyloseand arabinose in the presence of glucose. This is the first report thataccomplished homo-D-lactic acid fermentation from mixed sugarsincluding hexose and pentose, without carbon catabolite repressionby metabolically engineered LAB.

Yun and Ryu (49) studied lactic acid production using Entero-coccus faecalis RKY1 in co-fermentations with different sugarmixtures of glucose/fructose, glucose/maltose, and fructose/maltose as the carbon sources. The strain grown on a mixtureof glucose/fructose simultaneously consumed these sugars to

Mannose 6-P Fructose 6-P

Glucose 6-P

GAP

Fructose1,6-P

GlucoseRibulose 5-P

Xylulose 5-P

Cellobiose

Mannose

Glucose1-P Galactose1-P

Galactose Arabinose Xylose

Ribulose Xylulose

Dihydroxy- acetone

phophate

Pyruvate

Lactic acid

Xylulose 5-PRibose 5-P

GAP Sedoheptulose 7-P

Erythrose 4-P

UDP-galactose UDP-glucoseATP

ADP

6-PhosphogluconateNAD(P)+

NAD(P)H

NAD(P)+NAD(P)H

Xylulose 5-P

GAP Acetyl-P

Pyruvate

NAD+

NADH

ADP

ATP

H20

NADH

NAD+

Lactic acid

CoA

Pi

Acetyl-CoA

Acetaldehyde

NADH

NAD+

NADH

NAD+

Ethanol

ADP

ATP

Acetic acid

NADH

NAD+

ADP

H20

ATP

NAD+

NADH

ATP

ADP

ATP

ADP ATP

ADP

ATP

ADP

PP/Glycolytic Pathway(Homo-lactic acid metabolism)

PK Pathway(Hetero-lactic acid metabolism)

β-glucosidase

ATP

ADP

CO2

PEP

Pyr

(1) (2) (3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(9)(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)(24)

(25)

(26)

(14)

CellooligosaccharidesExo-β-(1→4)-glucanase

FIG. 1. Pathways for lactic acid production from renewable materials-derived sugars (glucose, xylose, cellobiose, arabinose, mannose, and galactose) by lactic acid bacteria (4,7,16,19).GAP, glyceraldehyde 3-phosphate; PEP, phosphoenolpyruvate; Pyr, pyruvate. Enzymes: 1, galactokinase; 2, arabinose isomerase; 3, xylose isomerase; 4, mannose phospho-transferase system; 5, hexokinase; 6, glucose-6-phosphate dehydrogenase; 7, 6-phosphogluconate dehydrogenase; 8, ribulose-5-phosphate 3-epimerase; 9, transketolase;10, transaldolase; 11, 6-phosphofructokinase; 12, fructose-bisphosphate aldolase; 13, triosephosphate isomerase; 14, lactate dehydrogenase; 15, phosphomannose isomerase;16, phosphoglucose isomerase; 17, phosphoglucomutase; 18, galactose-1-phosphate uridyl transferase; 19, glucosyltransferase; 20, ribulokinase; 21, xylulokinase; 22, phospho-ketolase; 23, acetate kinase; 24, phosphotransacetylase; 25, aldehyde dehydrogenase; 26, alcohol dehydrogenase.

VOL. 119, 2015 LACTIC ACID PRODUCTION FROM RENEWABLE MATERIALS 13

produce lactic acid without carbon catabolite repression, whereasutilization of maltose was repressed in the presence of glucose orfructose. Therefore, carbon catabolite repression of maltose meta-bolism was suggested to be caused by preferentially metabolizedsugars.

Taniguchi et al. (50) described the production of optically pure L-lactic acid with a yield of 0.63 g g�1 from a model lignocellulosehydrolyzate including xylose and glucose by a mixed culture ofEnterococcus casseliflavus IFO 12256 and Lactobacillus casei 2218.Although the mixed culture exhibited carbon catabolite repression,the complete consumption of 50 g L�1 xylose and 100 g L�1 glucosewas achieved.

Effects of nutrients on lactic acid fermentation by LAB LABgenerally have complex nutritional requirements due to theirlimited ability to synthesize elements for their own growth. While

carbon sources are used to generate energy for proliferation,LAB require other nutrients such as nitrogen sources, vitamins,and minerals for maintenance, cellular growth, and lactic acidsecretion.

The ratio of carbon source to nitrogen source (C/N ratio) is amajor factor that affects the lactic acid conversion process.Generally, a proper C/N ratio, achieved by the addition of complexnitrogen sources such as yeast extract, peptone, and meat extract,has a positive effect on lactic acid production (53). In particular,yeast extract is an efficient nutrient for high lactic acid productionby LAB, although the utilization of yeast extract results in a highcost of lactic acid production. Therefore, the replacement of yeastextract with an economic alternative nitrogen source wouldreduce the cost of nutrients. To date, various cheap nitrogensources have been investigated for use as an alternative to yeastextract, such as hydrolyzate of fish waste (54), corn steep liquor

14 WANG ET AL. J. BIOSCI. BIOENG.,

(55), wheat bran extract (56), and silkworm larvae (57). By usingcorn steep liquor as the cheap nitrogen sources, the lactic acidproductivity increased with decreasing the C/N ratio (mol/mol)from 37:1 to 19:1, while the further decrease to 12:1 had no pos-itive impact on lactic acid production by Lactobacillus sp. MKT-LC878 (58).

In addition to carbon sources and nitrogen sources, vitamins andminerals have significant effects on lactic acid fermentation by LAB.For example, pyridoxine stimulated the growth of some LAB, whileneither 2,4,5-trimethyl-3-hydroxypyridine nor 2,4-dimethyl-3-hydroxy-5-hydroxymethyl pyridine showed the same effect (59).Chauhan et al. (60) screened media components for high lactic acidproduction with Lactobacillus sp. KCP01. They found thatMgSO4$7H2O, KH2PO4, sodium citrate, NaCl, and sodium succinatehad less of an effect on lactic acid production, and that sodiumsulphate, sodium acetate, K2HPO4, MnSO4$4H2O and FeSO4$7H2Owere found to be significant.

Effects of fermentation condition on lactic acidfermentation by LAB In the field of fermentation technology,fermentation conditions such as pH, temperature, and inoculumsize were considered important factors for cell growth, lactic acidconcentration, lactic acid productivity, and lactic acid yield.

The initial or controlled optimal pH value for lactic acid pro-duction varies between 5.0 and 7.0, depending on the used LAB.Without pH control, the pH of the fermentation broth decreaseswith increasing lactic acid production, resulting in the inhibition ofcell growth and its production. Idris and Suzana (61) reported theeffect of initial pH from 4.5 to 8.5 on lactic acid production duringbatch fermentation by Lactobacillus delbrueckii ATCC 9646. Aninitial pH of 6.5 caused the early induction of sugar consumption,maximal rate of sugar consumption, and maximal lactic acid con-centration. Guyot et al. (62) studied lactic acid production in batchfermentation with pH control by Lactobacillus manihotivorans LMG18010T at different pH values (�4.5). Compared to non-pH-controlled batch fermentation, pH-controlled batch fermentationat 6.0 reduced the fermentation period by half, and increased thelactic acid concentration to 12.6 g L�1 and the OD600 value to 5.1from 7.6 g L�1 and 1.65, respectively.

Since LAB are mesophilic bacteria, LAB can grow and producelactic acid at a maximum temperature of approximately 50�C. Inaddition, the growth rates of the LAB used differed based on thefermentation temperature, and the optimum temperature for lacticacid concentration did not correspond to lactic acid yield or lacticacid productivity. For Lactobacillus amylophilus ATCC 49845,Yumoto and Ikeda (63) reported optimal temperatures of 25�C and35�C for maximum lactic acid productivity and yield, respectively.Abdel-Rahman et al. (17) examined the effect of temperature on theproduction of lactic acid from 20 g L�1 cellobiose by E. mundtii QU25. Both the maximum lactic acid concentration and the highestyield were obtained at 30e43�C.

An inoculum size of 5e10% (v v�1) is desirable to arrest hetero-lactic fermentation and to reduce the duration of the lag phase (64).Because an inoculum size of more than 5% is costly and constrainsthe operation, the inoculum size should be decreased. To investi-gate the influence of inoculum size on lactic acid production,different amounts (1e5% v v�1) were inoculated into the fermen-tation medium. The maximum lactic acid concentration of33.72 g L�1 by Lactobacillus casei NBIMCC 1013 was observed withan inoculum size of 2e4% (v v�1), whereas a low lactic acid pro-duction was attributed to a low density of starter culture (1% v v�1)(65). Qi and Yao (66) studied the effect of inoculum size from 3% to8% on lactic acid fermentation from rice straw with Lactobacilluscasei GIM 1.159. An inoculum size of 6% was found to be optimal forthe production of lactic acid. Lower inoculum sizes resulted ininsufficient biomass for lactic acid conversion, whereas higher

inoculum sizes caused excessive depletion of the nutrients neces-sary for cell growth.

DIFFICULTIES AND PROBLEMS IN LACTIC ACID FERMENTATIONBY LAB

Although approximately 90% of worldwide lactic acid is pro-duced bymicrobial fermentation, there are still many challenges formore effective and economical production. One of the obstacles inthe process of co-culture using mixed sugars is carbon cataboliterepression when using renewable materials containing variouscomponents (16,67). A fewwild-type andmetabolically engineeredstrains have been shown to simultaneously consume differentsugars such as glucose/cellobiose (17), xylose/arabinose/glucose(68), and cellobiose/glucose/xylose (38). Moreover, the strategy ofmixed cultures with different microorganisms has been studied forimproving product yield and sugar consumption (50,69).

Another obstacle is end-product inhibition in the lactic acidproduction process by LAB. As fermentation progresses along withsubstrate utilization, the concentration of lactic acid gradually in-creases, causing acidification of fermentation broth and leading toslowing of the fermentation process, including cell growth, sub-strate utilization, and lactic acid production. Thus, to alleviate theend-product inhibition effect caused by lactic acid during fermen-tation, lactic acid should be removed selectively in situ from thefermentation broth by a separation method such as electrodialysis(70), or lactic acid-resistant LAB should be selected (7).

By-products such as ethanol and acetic acid are usually formed inhetero-fermentation using pentoses, which results in a theoreticalyield of only 0.60 g g�1 of lactic acid from pentose (16,42,43). Cost ofseparation and purification also increases with the lowered purity ofthe lactic acid product. Some potential pentose-utilizing strains arereported to overcome theproblem, e.g., thewild-typeofE.mundtiiQU25 can consume xylose homofermenatively with little or no acetateproduction and with high yield (16). A metabolically engineeredLactobacillus plantarum DldhL1-xpk1::tkt was also reportedly able toconvert xylose (46) and arabinose (42) to lactic acid in homo-fermentation.

The most serious obstacle for lactic acid production commer-cially is the availability and cost of feedstock for lactic acidfermentation (4,7). Particularly, carbon sources are significant,should be consumed by the LAB, and are divided into 2 groups:purified sugars such as glucose, xylose, sucrose, and sugar-con-taining materials. However, the use of purified sugars as feedstockfor lactic acid production is very expensive. From the perspective ofthe economics of the feedstock in terms of its geographic avail-ability, domestic renewablematerials such as wastes and unutilizedresources from the agro-industry and forestry are preferred due totheir availability and low cost.

LACTIC ACID PRODUCTION FROM RENEWABLE MATERIALS

In most cases, glucose is the preferred carbon source for lacticacid fermentation by LAB. However, as cheap and widely existingrenewable materials, starch (e.g., wheat starch, corn starch, sagostarch, potato starch) and lignocellulose (e.g., woodymaterials, cropresidues) are recognized to meet the requirements for the biotech-nological production of lactic acid economically and efficiently.

Starch from various plant products is a potentially renewablematerial in terms of cost and availability. As a type of polysaccharideof glucose, starch mainly exists in tubers such as potatoes andcassava, and seeds of grains including wheat, corn, and rice. On theother hand, lignocellulose, another carbohydrate source, is availablein large quantities, with widespread distribution and comparablylower cost (6,71). The major constituents of lignocellulose are

TABLE 2. Recent investigations on lactic acid production from renewable materials by different fermentation modes and methods.

Microorganism Substrate Fermentation mode Lactic acidg (g L�1)

YieldY (g g�1)

ProductivityP (g L�1 h�1)

Reference

Bacillus sp. strain XZL9 Corncob molasses Fed-batch 74.7 0.50 0.38 68Enterococcus faecium No. 78 Sago starch Batch 16.6 � 0.8 0.93 1.105 � 0.06 15

Continuous 10.4 e 1.56Continuous (with high cell density) 11.7 0.76 3.04

E. faecalis RKY1 Wood hydrolyzate Batch 93.0 0.93 1.7 81Lactobacillus bifermentans DSM

20003TWheat bran hydrolyzate Batch 62.8 0.83 1.2 82

L. brevis S3F4 Corncob Batch 39.1 0.70 0.81 83L. casei NCIMB 3254 Cassava bagasse SSF 83.8 0.96 1.40 79L. casei G-02 Jerusalem artichoke tubers Fed-batch, SSF 141.5 0.936 4.7 84L. casei LA-04-1 Soybean meal hydrolyzate Fed-batch (fed with CSL) 162.5 0.897 1.69 85

Fed-batch (fed with YE) 180 0.903 2.14L. defbrneckii NRRL B-445 Alfalfa fibers SSF 35.4 0.35 0.75 72L. delbrueckii mutant Uc-3 Cellobiose and cellotriose Batch 90.0 0.90 2.3 86L. delbrueckii ZU-S2 Corncob residue Batch 48.7 0.95 1.01 87

Continuous 44.2 0.92 5.7L. delbrueckii mutant Uc-3 Molasses Batch 166 0.87 4.2 88L. delbrueckii IFO 3202 Rice bran SSF 28.0 0.28 0.78 89L. paracasei LA104 Green microalga SSF 37.11 0.46 1.03 13L. pentosus CECT-4023T Trimming vine shoots Batch 24.0 0.76 0.51 90L. plantarum 14431 Alfalfa fibers SSF 46.4 0.46 0.64 72L. rhamnosus CECT-288 Apple pomace Batch 32.5 0.88 5.4 91L. rhamnosus HG 09 Hydrolyzed acorn starch Batch 57.61 � 1.37 0.46 1.60 � 0.12 53Lactobacillus sp. MKT-878 NCAIM

B02375Wheat starch SHF 118 0.94 1.71 92

SSF 97 0.89 1.74Lactobacillus sp. RKY2 Rice and wheat bran Batch 129 0.95 3.1 93

Lignocellulosic hydrolyzates Cell-recycle 27.0 0.90 6.7 94Lactococcus lactis IO-1 Sugarcane bagasse Batch 10.9 0.36 0.17 18

SSF, simultaneous saccharification and fermentation; SHF, separate hydrolysis and fermentation; CSL, corn steep liquor; YE, yeast extract; e, not determined.

VOL. 119, 2015 LACTIC ACID PRODUCTION FROM RENEWABLE MATERIALS 15

cellulose (linear b-D-glucan), hemicellulose (hetero-poly-saccharides including xylose, glucose, mannose, galactose, andarabinose), and lignin (a polymer of three closely-related phenyl-propane moieties). Basically, cellulose forms a skeleton, which issurrounded by hemicellulose and lignin (6).

Pretreatment of raw renewable materials Enzymatic hy-drolysis of raw renewable materials easily exposes the fermentativesugars consumed by the used strain, and the hydrolyzates can then beused in SSF, or direct fermentation (72). However, enzymatichydrolysis of lignocellulosic biomass without pretreatment is usuallynot very effective because of the high stability caused by theassociation of cellulose and hemicellulose with lignin (73). Therefore,pretreatment of renewable materials is performed to remove lignin,separate cellulose and hemicellulose, increase the accessible surfacearea, partially depolymerise cellulose, and increase the porosity ofthe material to aid in the subsequent access of the hydrolyticenzymes. Pretreatment methods include physical (mechanical),physico-chemical (steam pretreatment, hydrothermolysis, wetoxidation, ammonia fiber expansion), chemical (dilute acid, alkaline,ionic liquids), and biological methods (microorganisms, enzymes)(5,73). As the result of pretreatment, the size of the material isreduced and its physical structure is opened. On the other hand,when starchy materials are used as the substrate for lactic acidproduction, the bioconversion requires a pretreatment processincluding the gelatinization and liquefaction of starch, which iscarried out at a temperature between 90�C and 130�C for 15min (74).

Acid pretreatment is extensively applied in the hydrolysis ofstarchy and lignocellulose materials (6). Pretreatment with diluteacid at high temperature, or strong acid (H2SO4, SO2, H3PO4) at lowpH, usually results in the hydrolysis of hemicellulose to monomericsugars (e.g., xylose, arabinose) and minimizes the need for hemi-cellulases (75). However, the utilization of various chemicals in thepretreatment procedures is a major drawback and affects the totalcost of the fermentation. Physical or physico-chemical methods ofpretreatment such as milling, steam explosion, and irradiationreduce the particle size, thereby increasing the available surface area

for enzymatic attack. Sasaki et al. (76) pretreated sugarcane bagasseusing steam explosion, prior to the processes of enzymatic hydro-lysis. After steam explosion at 20 atm, enzymatic saccharification ofpretreated raw bagasse and thewater-insoluble residue provided thehighest recovery rates of glucose (73.3% and 94.9%, respectively).Interestingly, washing of the residue with water after steam explo-sion removed inhibitors in the hydrolyzate of steam-explodedbagasse. Biological delignification is another pretreatment methodthat can enhance enzymatic hydrolysis. Kurakake et al. (77) evalu-ated the biological pretreatment of office paper for enzymatic hy-drolysis by Sphingomonas paucimobilis and Bacillus circulans.Pretreatment with the combination of these 2 strains improved theefficiency of enzymatic hydrolysis of office paper, and exhibited 94%sugar recovery under optimal conditions.

Enzymatic hydrolysis The bioconversion of renewable ma-terials to lactic acid requires fermentable components as the carbonsources by the LAB strains used, and enzymatic hydrolysis canimprove the efficiency of the subsequent fermentation processdrastically (78). The purpose of enzymatic hydrolysis is to breakdown polysaccharides into easily fermentable sugars, even in thewater-insoluble solid fraction that remains after pretreatment (5).

a-Amylase (EC 3.2.1.1), b-amylase (EC 3.2.1.2), and glucoamylase(EC 3.2.1.3) are well-known amylolytic enzymes that catalyze thehydrolysis of a-glucosidic bonds in starch and related saccharides.In the process of enzymatic hydrolysis of cassava bagasse for L-lacticacid production, a-amylase and glucoamylase were used in SSF(79). A lactic acid yield of 96% was obtained from starch using15.5% w v�1 of cassava bagasse as the substrate.

Enzymatic hydrolysis of cellulose is generally carried out by amixture of several cellulases. At least 3 major groups of cellulasesare involved in this process: (i) endo-b-1,4-glucanases (EG, EC3.2.1.3), which attack regions of low crystallinity in the cellulosefiber, creating a new reducing and non-reducing chain end pair; (ii)exo-b-1,4-glucanases (or cellobiohydrolases) (CBH, EC 3.2.1.91),which degrade the molecule further by cleaving cellobiose unitsfrom the free chain-ends; and (iii) b-glucosidases (b-G, EC 3.2.1.21),

16 WANG ET AL. J. BIOSCI. BIOENG.,

which hydrolyze cellobiose into 2 glucose molecules (5). Unlikecellulose, hemicelluloses are not chemically homogeneous. Xylansare the most abundant among hemicelluloses. Besides xylose, xy-lans contain arabinose, glucuronic acid or its 4-o-methyl ether,acetic acid, ferulic acid, and p-coumaric acid. There are a number ofenzymes that attack hemicellulose, such as glucuronidase, xyla-nase, b-xylosidase, galactomannanase, and glucomannanase. Berlinet al. (80) optimized enzyme complexes for the efficient enzymatichydrolysis of cellulose and xylan components of lignocellulose bysupplying cellulase with crude enzyme preparations enriched inxylanase, pectinase, and b-glucosidase.

Fermentation modes and methods Several fermentationmodes were investigated for lactic acid production by various lacticacid-producing strains using purified sugars and renewable mate-rials (Table 2) as the substrates, including batch fermentation, fed-batch fermentation, semi-continuous/repeated batch fermentation,continuous fermentation, SSF, and separate hydrolysis andfermentation (SHF). In general, batch fermentation exhibitedhigher lactic acid concentration and yield, but lower lactic acidproductivity than did continuous fermentation (15,81,88). In batchfermentation, most of the substrate in the fermentor is consumed,whereas the residual substrate would become the effluent incontinuous fermentation, resulting in a higher yield andconcentration of lactic acid in the former. On the other hand, end-product inhibition of lactic acid is repressed by diluting thefermentation broth in continuous fermentation; the reducinginhibition effect leads to higher productivity (7). By integrating acell recycling system with continuous fermentation, higher celldensity than seen in conventional continuous fermentation can beachieved, using modules such as microfiltration and ultrafiltration,which would drastically increase lactic acid productivity. Tashiroet al. (3) first reported a continuous fermentation system forD-lactic acid production using high cell density by cell recycling, inwhich they obtained a high productivity of 18.0 g L�1 h�1 at adilution rate of 0.87 h�1.

SHF from renewable materials was also investigated, in whichthe processes of enzymatic hydrolysis and fermentation wereconducted separately; each process could thereby be carried outunder optimal conditions (5). In the enzymatic hydrolysis process,however, sugars yielded from renewable materials may stronglyinhibit hydrolytic enzyme activity (feedback inhibition), leading toa requirement for more enzyme loads or a longer total period. Onthe other hand, SSF is an effective method by which enzymatichydrolysis and fermentation are conducted in the same reactorunder the same conditions. The SSF process has several advantagesover SHF such as the usage of a single reactor for both steps, rapidprocessing time, reduced feedback inhibition by the generatedsugars, and increased productivity (72). Nevertheless, the require-ment for different optimal temperatures and pHs for saccharifica-tion and fermentation is the main limiting factor for SSF (5). Theoptimal conditions for enzymatic hydrolysis are a temperature ofw50�C and a pH below 5.0, whereas the optimum conditions forlactic acid fermentation are a temperature of 37e43�C and a pHvalue of 5.0e7.0. Some compromises between the conditions forenzymatic hydrolysis and fermentation are necessary in order toobtain high overall lactic acid concentration, yield, and productivityusing SSF (5,19).

Separation and purification of lactic acid produced in lacticacid fermentation Efficient separation and purification tech-nologies are very important steps because they have a significantinfluence on the final quality and cost of lactic acid. For the purifi-cation of lactic acid, calcium carbonate is usually added to thefermentation broth, and the pH is adjusted to approximately 10,followed by heating and filtering. The procedure converts all of thelactic acid to calcium lactate, coagulates protein in themedium, kills

cells, removesexcesscalciumcarbonate, andhelps todecomposeanyresidual sugar in the medium (4). Lactic acid is further recovered byhydrolysis, esterification, and distillation. The solvent is removed byevaporation, and then the salt is decomposed to yield free lactic acid.Meanwhile, other alternative methods for lactic acid purificationwithout using calcium carbonate such as electrodialysis (95), theuse of a membrane bioreactor (96), liquid surfactant membraneextraction (97), and adsorption (98) exhibited good potential andhad the advantage of simultaneous separation and concentrationof lactic acid. The choice of separation process is based on theefficient and economical usage of these extractants.

Conclusions and future scope Fermentative production oflactic acid has generated a significant amount of interest because italso offers solutions to the environmental pollution caused by thepetrochemical industry and the limited supply of petrochemicalresources. Two studies have been conducted to achieve efficientfermentations for valuable substances from renewable materials.First, most of the recent studies select the targeted renewablematerials initially, and then acquires an excellent strain with highabilities of degrading and converting them to valuable substancesefficiently by screening, mutagenesis, and molecular breedingmethods. Furthermore, an establishment of efficient productionprocess has been performed by the obtained strain.

On the other hand, we have recently proposed another study,termed as designed biomass study (5). Designed biomass refers tocompetent substances that can be designed for the correspondingfermentation processes. In this type of study, all the technologiespreviously developed in the recent studies (i.e., excellent strains orefficient processes) can be applied; thereafter, the researcherswould identify or modify the renewable materials to be suitable forthe existing technologies. Of course, our concept involves not onlysingle sugar but also mixed sugars derived from various renewablematerials such as lignocellulosic biomasses after pretreatments andenzymatic hydrolysis. One of the goals in designed biomass study isto construct an adaptive production process by using the excellentstrain, efficient process, and designed biomass. Based on ourconcept, we recently reported that the designed mixed sugars withthe ratio of glucose/cellobiose/xylose at 1:8:4 could achievesimultaneous utilization of sugars by an excellent LAB of E. mundtiiQU 25 without carbon catabolite repression (99), which has beenserious drawback using mixed sugars in lactic acid fermentation asdescribed in the section effects of several factors on lactic acidfermentation by LAB. In addition, we established the adaptive lacticacid production process in the fed-batch fermentation using thedesigned mixed sugars and E. mundtii QU 25 with much higherlactic acid concentration (99) than those using the respective solesugar of glucose (48), cellobiose (17), and xylose (16).

In this review article, we have summarized many recent studieson lactic acid fermentation from various renewable materials andthe production processes developed by the microbiologists in thebroad fields including fermentation technology, molecular micro-biology, and metabolic engineering, and so on, and have corrobo-rated the designed biomass study. Recently, genetically engineeredlignocellulosic biomasses such as rice straw have been reported bythe researchers in the field of plant breeding, being improved inenzymatic hydrolysis efficiency (100) and being modified in com-positions of cellulose and hemicellulose (101). Therefore, the activecollaboration with researchers between microbiology and plantbreeding should motivate the designed biomass study further andwould contribute to effective and economical lactic acid productionfrom renewable materials.

References

1. Benninga, H.: A history of lactic acid making pp. 1e59. Kluwer AcademicPublishers, Dordrecht, Netherlands (1990).

VOL. 119, 2015 LACTIC ACID PRODUCTION FROM RENEWABLE MATERIALS 17

2. Abdel-Rahman, M. A., Tashiro, Y., and Sonomoto, K.: Recent advances inlactic acid production by microbial fermentation processes, Biotechnol. Adv.,31, 877e902 (2013).

3. Tashiro, Y., Kaneko, W., Sun, Y. Q., Shibata, K., Inokuma, K., Zendo, T., andSonomoto, K.: Continuous D-lactic acid production by a novel thermotolerantLactobacillus delbrueckii subsp. lactis QU 41, Appl. Microbiol. Biotechnol., 89,1741e1750 (2011).

4. Vijayakumar, J., Aravindan, R., and Viruthagiric, T.: Recent trends in theproduction, purification and application of lactic acid, Chem. Biochem. Eng. Q.,22, 245e264 (2008).

5. Abdel-Rahman, M. A., Tashiro, Y., and Sonomoto, K.: Lactic acid productionfrom lignocellulose-derived sugars using lactic acid bacteria: overview andlimits, J. Biotechnol., 156, 286e301 (2011).

6. Neureiter, M., Danner, H., Madzingaidzo, L., Miyafuji, H., Thomasser, C.,Bvochora, J., Bamusi, S., and Braun, R.: Lignocellulose feedstocks for theproduction of lactic acid, Chem. Biochem. Eng. Q., 18, 55e63 (2004).

7. Wee, Y. J., Kim, J. N., and Ryu, H. W.: Biotechnological production of lactic acidand its recent applications, Food Technol. Biotechnol., 44, 163e172 (2006).

8. Christensen, C.H., Rass-Hansen, J.,Marsden, C. C., Tarning,E., andEgeblad,K.:The renewable chemicals industry, ChemSusChem, 1, 283e289 (2008).

9. Pacheco, A., Talaia, G., Sa-Pessoa, G., Bessa, D., Gonçalves, M. J., Moreira, R.,Paiva, S., Casal, M., and Queiros, O.: Lactic acid production in Saccharomycescerevisiae is modulated by expression of the monocarboxylate transportersJen1 and Ady2, FEMS Yeast Res., 12, 375e381 (2012).

10. Åkerberg, C. and Zacchi, G.: An economic evaluation of the fermentativeproduction of lactic acid from wheat flour, Bioresour. Technol., 75, 119e126(2000).

11. Watanabe, M., Makino, M., Kaku, N., Koyama, M., Nakamura, K., andSasano, K.: Fermentative L-(þ)-lactic acid production from non-sterilized ricewashing drainage containing rice bran by a newly isolated lactic acid bacteriawithout any additions of nutrients, J. Biosci. Bioeng., 115, 449e452 (2013).

12. Budhavaram, N. K. and Fan, Z. L.: Production of lactic acid from paper sludgeusing acid-tolerant, thermophilic Bacillus coagulans strains, Bioresour. Tech-nol., 100, 5966e5972 (2009).

13. Nguyen, C. M., Kim, J. S., Hwang, H. J., Park, M. S., Choi, G. J., Choi, Y. H.,Jang, K. S., and Kim, J. C.: Production of l-lactic acid from a green microalga,Hydrodictyon reticulum, by Lactobacillus paracasei LA104 isolated from thetraditional Korean food, makgeolli, Bioresour. Technol., 110, 552e559 (2012).

14. Caplice, E. and Fitzgerald, G. F.: Food fermentations: role of microorganisms infood production and preservation, Int. J. Food Microbiol., 50, 131e149 (1999).

15. Shibata, K., Flores, D. M., Kobayashi, G., and Sonomoto, K.: Direct L-lacticacid fermentation with sago starch by a novel amylolytic lactic acid bacte-rium, Enterococcus faecium, Enzyme Microb. Technol., 41, 149e155 (2007).

16. Abdel-Rahman, M. A., Tashiro, Y., Zendo, T., Hanada, K., Shibata, K., andSonomoto, K.: Efficient homofermentative L-(þ)-lactic acid production fromxylose by a novel lactic acid bacterium, Enterococcus mundtii QU 25, Appl.Environ. Microbiol., 77, 1892e1895 (2011).

17. Abdel-Rahman, M. A., Tashiro, Y., Zendo, T., Shibata, K., and Sonomoto, K.:Isolation and characterization of lactic acid bacterium for effective fermen-tation of cellobiose into optically pure homo L-(þ)-lactic acid, Appl. Microbiol.Biotechnol., 89, 1039e1049 (2011).

18. Laopaiboon, P., Thani, A., Leelavatcharamas, V., and Laopaiboon, L.: Acidhydrolysis of sugarcane bagasse for lactic acid production, Bioresour. Technol.,101, 1036e1043 (2010).

19. Hofvendahl, K. and Hahn-Hägerdal, B.: Factors affecting the fermentativelactic acid production from renewable resources, Enzyme Microb. Technol.,26, 87e107 (2000).

20. Morlon-Guyot, J., Guyot, J. P., Pot, B., Haut, I. J. D., and Raimbault, M.:Lactobacillus manihotivorans sp. nov., a new starch-hydrolysing lactic acidbacterium isolated from cassava sour starch fermentation, Int. J. Syst. Bac-teriol., 48, 1101e1109 (1998).

21. Patel, M. A., Ou, M. S., Harbrucker, R., Aldrich, H. C., Buszko, M. L.,Ingram, L. O., and Shanmugam, K. T.: Isolation and characterization of acid-tolerant, thermophilic bacteria for effective fermentation of biomass-derivedsugars to lactic acid, Appl. Environ. Microbiol., 72, 3228e3235 (2006).

22. Lockwood, L. B., Ward, G. E., and May, O. E.: The physiology of Rhizopusoryzae, J. Agric. Res., 53, 849e857 (1936).

23. Ruengruglikit, C. and Hang, Y. D.: L(þ)-Lactic acid production from corncobsby Rhizopus oryzae NRRL-395, Lebensm. Wiss. Technol., 36, 573e575 (2003).

24. Guo, Y., Yan, Q., Jiang, Z., Teng, C., and Wang, X.: Efficient production oflactic acid from sucrose and corncob hydrolysate by a newly isolated Rhizopusoryzae GY18, J. Ind. Microbiol. Biotechnol., 37, 1137e1143 (2010).

25. Yu, L., Pei, X. L., Lei, T., Wang, Y. H., and Feng, Y.: Genome shuffling enhancedL-lactic acid production by improving glucose tolerance of lactobacillusrhamnosus, J. Biotechnol., 134, 154e159 (2008).

26. Singhvi, M., Joshi, D., Adsul, M., Varma, A., and Gokhale, D.: D-(-)-Lactic acidproduction from cellobiose and cellulose by Lactobacillus lactis mutant RM2-24, Green Chem., 12, 1106e1109 (2010).

27. Bron, P. A. and Kleerebezem, M.: Engineering lactic acid bacteria forincreased industrial functionality, Bioeng. Bugs, 2, 80e87 (2011).

28. van Maris, A. J. A., Winkler, A. A., Porro, D., van Dijken, J. P., and Pronk, J. T.:Homofermentative lactate production cannot sustain anaerobic growth ofengineered Saccharomyces cerevisiae: possible consequence of energy-dependent lactate export, Appl. Environ. Microbiol., 70, 2898e2905 (2004).

29. Zhou, S., Causey, T. B., Hasona, A., Shanmugam, K. T., and Ingram, L. O.:Production of optically pure D-lactic acid in mineral salts medium by meta-bolically engineered Escherichia coli W3110, Appl. Environ. Microbiol., 69,399e407 (2003).

30. Okino, S., Suda, M., Fujikura, K., Inui, M., and Yukawa, H.: Production ofD-lactic acid by Corynebacterium glutamicum under oxygen deprivation, Appl.Microbiol. Biotechnol., 78, 449e454 (2008).

31. Skory, C. D.: Lactic acid production by Rhizopus oryzae transformants withmodified lactate dehydrogenase activity, Appl. Microbiol. Biotechnol., 64,237e242 (2008).

32. Shinkawa, S., Okano, K., Yoshida, S., Tanaka, T., Ogino, C., Fukuda, H., andKondo, A.: Improved homo L-lactic acid fermentation from xylose by abol-ishment of the phosphoketolase pathway and enhancement of the pentosephosphate pathway in genetically modified xylose-assimilating Lactococcuslactis, Appl. Microbiol. Biotechnol., 91, 1537e1544 (2011).

33. Ikushima, S., Fujii, T., Kobayashi, O., Yoshida, S., and Yoshida, A.: Geneticengineering of Candida utilis yeast for efficient production of L-lactic acid,Biosci. Biotechnol. Biochem., 73, 1818e1824 (2009).

34. Porro, D., Bianchi, M. M., Brambilla, L., Menghini, R., Bolzani, D.,Carrera, V., Lievense, J., Liu, C. L., Ranzi, B. M., Frontali, L., andAlberghina, L.: Replacement of a metabolic pathway for large-scale produc-tion of lactic acid from engineered yeasts, Appl. Environ. Microbiol., 65,4211e4215 (1999).

35. Ilmén, M., Koivuranta, K., Ruohonen, L., Suominen, P., and Penttila, M.:Efficient production of L-lactic acid from xylose by Pichia stipites, Appl. Envi-ron. Microbiol., 73, 117e123 (2007).

36. Ishida, N., Suzuki, T., Tokuhiro, K., Nagamori, E., Onishi, T., Saitoh, S.,Kitamoto, K., and Takahashi, H.: D-Lactic acid production by metabolicallyengineered Saccharomyces cerevisiae, J. Biosci. Bioeng., 101, 172e177 (2006).

37. Wang, Q., Ingram, L. O., and Shanmugam, K. T.: Evolution of D-lactate de-hydrogenase activity from glycerol dehydrogenase and its utility for D-lactateproduction from lignocellulose, Proc. Natl. Acad. Sci. USA, 108, 18920e18925(2011).

38. Sasaki, M., Jojima, T., Inui, M., and Yukawa, H.: Simultaneous utilization ofD-cellobiose, D-glucose, and D-xylose by recombinant Corynebacterium gluta-micum under oxygen-deprived conditions, Appl. Microbiol. Biotechnol., 81,691e699 (2008).

39. Portnoy, V. A., Herrgard, M. J., and Palsson, B. O.: Aerobic fermentation ofD-glucose by an evolved cytochrome oxidase-deficient Escherichia coli strain,Appl. Environ. Microbiol., 74, 7561e7569 (2008).

40. Grabar, T. B., Zhou, S., Shanmugam, K. T., Yomano, L. P., and Ingram, L. O.:Methylglyoxal bypass identified as source of chiral contamination in L(þ) andD(�)-lactate fermentations by recombinant Escherichia coli, Biotechnol. Lett.,28, 1527e1535 (2006).

41. Mazumdar, S., Clomburg, J. M., and Gonzalez, R.: Escherichia coli strainsengineered for homofermentative production of d-lactic acid from glycerol,Appl. Environ. Microbiol., 76, 4327e4336 (2010).

42. Okano, K., Yoshida, S., Tanaka, T., Fukuda, H., and Kondo, A.: Homo d-lacticacid fermentation from arabinose by redirection of phosphoketolase pathwayto pentose phosphate pathway in L-lactate dehydrogenase gene-deficientLactobacillus plantarum, Appl. Environ. Microbiol., 75, 5175e5178 (2009).

43. Tanaka, K., Komiyama, A., Sonomoto, K., Ishizaki, A., Hall, S. J., andStanbury, P. F.: Two different pathways for D-xylose metabolism and theeffect of xylose concentration on the yield coefficient of L-lactate in mixed-acid fermentation by the lactic acid bacterium Lactococcus lactis IO-1, Appl.Microbiol. Biotechnol., 60, 160e167 (2002).

44. Oshiro, M., Shinto, H., Tashiro, Y., Miwa, N., Sekiguchi, T., Okamoto, M.,Ishizaki, A., and Sonomoto, K.: Kinetic modeling and sensitivity analysis ofxylosemetabolism in Lactococcus lactis IO-1, Biosci. Bioeng.,108, 376e384 (2009).

45. Reddy, G., Altaf, M. D., Naveena, B. J., Venkateshwar, M., and Kumar, E. V.:Amylolytic bacterial lactic acid fermentationea review, Biotechnol. Adv., 26,22e34 (2008).

46. Okano, K., Yoshida, S., Yamda, R., Tanaka, T., Ogino, C., Fukuda, H., andKondo, A.: Improved production of homo-D-lactic acid via xylose fermenta-tion by introduction of xylose assimilation genes and redirection of thephosphoketolase pathway to pentose phosphate pathway in L-lactate dehy-drogenase gene-deficient Lactobacillus plantarum, Appl. Environ. Microbiol.,75, 7858e7861 (2009).

47. Eiteman, M. A., Lee, S. A., and Altman, E.: A co-fermentation strategy toconsume sugar mixtures effectively, J. Biol. Eng., 2, 3 (2008).

48. Abdel-Rahman, M. A., Tashiro, Y., Zendo, T., and Sonomoto, K.: Improvedlactic acid productivity by an open repeated batch fermentation system usingEnterococcus mundtii QU 25, RSC Adv., 3, 8437e8445 (2013).

49. Yun, J. S. and Ryu, H. W.: Lactic acid production and carbon cataboliterepression from single and mixed sugars using Enterococcus faecalis RKY1,Process Biochem., 37, 235e240 (2001).

18 WANG ET AL. J. BIOSCI. BIOENG.,

50. Taniguchi, M., Tokunaga, T., Horiuchi, K., Hoshino, K., Sakai, K., andTanaka, T.: Production of L-lactic acid from a mixture of xylose and glucose byco-cultivation of lactic acid bacteria, Appl. Microbiol. Biotechnol., 66, 160e165(2004).

51. Yoshida, S., Okano, K., Tanaka, T., Ogino, C., and Kondo, A.: Homo-D-lacticacid production from mixed sugars using xylose-assimilating operon-inte-grated Lactobacillus plantarum, Appl. Microbiol. Biotechnol., 92, 67e76 (2011).

52. Moon, S. K., Wee, Y. J., and Choi, G. W.: A novel lactic acid bacterium for theproduction of high purity L-lactic acid, Lactobacillus paracasei subsp. paracaseiCHB2121, J. Biosci. Bioeng., 114, 155e159 (2012).

53. Lu, Z. H., He, F., Shi, Y., Lu, M. B., and Yu, L. J.: Fermentative production ofL(þ)-lactic acid using hydrolysed acorn starch, persimmon juice and wheatbran hydrolysate as nutrients, Bioresour. Technol., 101, 3642e3648 (2010).

54. Gao, M. T., Hirata, M., Toorisaka, E., and Hano, T.: Acid-hydrolysis of fishwastes for lactic acid fermentation, Bioresour. Technol., 97, 2414e2420(2006).

55. Dailey, O. D. J., Dowd, M. K., and Mayorga, J. C.: Influence of lactic acid on thesolubilization of protein during corn steeping, J. Agric. Food Chem., 48,1352e1357 (2000).

56. Kotzamanidis, C., Roukas, T., and Skaracis, G.: Optimization of lactic acidproduction from beet molasses by Lactobacillus delbrueckii NCIMB 8130,World J. Microbiol. Biotechnol., 18, 441e448 (2002).

57. Timbuntam, W., Sriroth, K., and Tokiwa, Y.: Lactic acid production fromsugar-cane juice by a newly isolated Lactobacillus sp., Biotechnol. Lett., 28,811e814 (2006).

58. Hetényi, K., Németh, Á., and Sevella, B.: Examination of medium supple-mentation for lactic acid fermentation, Hungar. J. Ind. Chem., 36, 49e53 (2008).

59. El-Sabaeny, A. H.: Influence of medium composition on lactic acid productionfrom dried whey by Lactobacillus delbrueckii, Microbiologia, 6, 12e20 (1996).

60. Chauhan, K., Trivedi, U., and Patel, K. C.: Statistical screening of mediumcomponents by PlacketteBurman design for lactic acid production by Lacto-bacillus sp. KCP01 using date juice, Bioresour. Technol., 98, 98e103 (2007).

61. Idris, A. and Suzana, W.: Effect of sodium alginate concentration, beaddiameter, initial pH and temperature on lactic acid production from pineapplewaste using immobilized Lactobacillus delbrueckii, Process Biochem., 41,1117e1123 (2006).

62. Guyot, J. P., Calderon, M., and Morlon-Guyot, J.: Effect of pH control on lacticacid fermentation of starch by Lactobacillus manihotivorans LMG 18010,J. Appl. Microbiol., 88, 176e182 (2000).

63. Yumoto, I. and Ikeda, K.: Direct fermentation of starch to L-(þ)-lactic acidusing Lactobacillus amylophilus, Biotechnol. Lett., 17, 543e546 (1995).

64. Salminen, S., Deighton, M., and Gorbach, S.: Lactic acid bacteria in healthand disease, pp. 199e225, in: Salminen, S. and Wright, A. V. (Eds.), Lactic acidbacteria. Marcel Dekker, Inc., New York (1993).

65. Panesar, P. S., Kennedy, J. F., Knill, C. J., and Kosseva, M.: Production of L(þ)lactic acid using Lactobacillus casei from whey, Braz. Arch. Biol. Technol., 53,219e226 (2010).

66. Qi, B. Q. and Yao, R. S.: L-Lactic acid production from Lactobacilluscasei by solid state fermentation using rice straw, BioResources, 2, 419e429(2007).

67. Görke, B. and Stülke, G.: Carbon catabolite repression in bacteria: manyways to make the most out of nutrients, Nat. Rev. Microbiol., 6, 613e624(2008).

68. Wang, L., Zhao, B., Liu, B., Yu, B., Ma, C., Su, F., Hua, D. L., Li, Q. G., Ma, Y. H.,and Xu, P.: Efficient production of L-lactic acid from corncob molasses, awaste by-product in xylitol production, by a newly isolated xylose utilizingBacillus sp. strain, Bioresour. Technol., 101, 7908e7915 (2010).

69. Cui, F. J., Li, Y. B., and Wan, C. X.: Lactic acid production from corn stoverusing mixed cultures of Lactobacillus rhamnosus and Lactobacillus brevis,Bioresour. Technol., 102, 1831e1836 (2011).

70. Hongo, M., Nomura, Y., and Iwahara, M.: Novel method of lactic acid pro-duction by electrodialysis fermentation, Appl. Environ. Microbiol., 52,314e319 (1986).

71. Vidal, B. C., Dien, B. S., Ting, K. C., and Singh, V.: Influence of feedstockparticle size on lignocellulose conversionea review, Appl. Biochem. Bio-technol., 164, 1405e1421 (2011).

72. Sreenath, H. K., Moldes, A. B., Koegel, R. G., and Straub, R. J.: Lactic acidproduction by simultaneous saccharification and fermentation of alfalfa fiber,J. Biosci. Bioeng., 92, 518e523 (2001).

73. Taherzadeh, M. J. and Karimi, K.: Pretreatment of lignocellulosic wastes toimprove ethanol and biogas production: a review, Int. J. Mol. Sci., 9,1621e1651 (2008).

74. Idrees, M., Adnan, A., and Qureshi, F. A.: Optimization of sulfide/sulfitepretreatment of lignocellulosic biomass for lactic acid production, Biomed.Res. Int., 2013, 934171 (2013).

75. Bungay, H. R.: Product opportunities for biomass refining, Enzyme Microb.Technol., 14, 501e507 (1992).

76. Sasaki, C., Okumura, R., Asakawa, A., Asaba, C., and Nakamura, Y.: Pro-duction of D-lactic acid from sugarcane bagasse using steam-explosion, J. Phys.Conf. Ser., 352, 12054e12063 (2012).

77. Kurakake, M., Ide, N., and Komaki, T.: Biological pretreatment with twobacterial strainsfor enzymatic hydrolysis of office paper, Curr. Microbiol., 54,424e428 (2007).

78. Watanabe, M., Ichinose, K., Sasano, K., Ozaki, Y., Tsuiki, T., Hidaka, H., andKanemoto, S.: Effect of enzymatic treatment on sedimentation and floccu-lation abilities of solid particles in rice washing drainage and its relationshipwith protein profiles, J. Biosci. Bioeng., 112, 67e70 (2011).

79. John, R. P., Nampoothiri, K. M., and Pandey, A.: Simultaneous saccharifica-tion and fermentation of cassava bagasse for L-(þ)-lactic acid production us-ing Lactobacilli, Appl. Biochem. Biotechnol., 134, 263e272 (2006).

80. Berlin, A., Maximenko, V., Gilkes, N., and Saddler, J.: Optimization ofenzyme complexes for lignocellulose hydrolysis, Biotechnol. Bioeng., 97,287e296 (2007).

81. Wee, Y. J., Yun, J. S., Park, D. H., and Ryu, H. W.: Biotechnological productionof L(þ)-lactic acid from wood hydrolyzate by batch fermentation of Entero-coccus faecalis, Biotechnol. Lett., 26, 71e74 (2004).

82. Givry, S., Prevot, V., and Duchiron, F.: Lactic acid production from hemi-cellulosic hydrolyzate by cells of Lactobacillus bifermentans immobilized in Ca-alginate using response surface methodology, World J. Microbiol. Biotechnol.,24, 745e752 (2008).

83. Guo, W., Jia, W., Li, Y., and Chen, S.: Performances of Lactobacillus brevis forproducing lactic acid from hydrolysate of lignocellulosics, Appl. Biochem.Biotechnol., 161, 124e136 (2010).

84. Ge, X. Y., Qian, H., and Zhang, W. G.: Enhancement of L-lactic acid productionin Lactobacillus casei from Jerusalem artichoke tubers by kinetic optimizationand citrate metabolism, J. Microbiol. Biotechnol., 20, 101e109 (2010).

85. Li, Z., Ding, S. F., Li, Z. P., and Tan, T. W.: L-Lactic acid production by Lacto-bacillus casei fermentation with corn steep liquor-supplemented acid-hy-drolysate of soybean meal, Biotechnol. J., 1, 1453e1458 (2006).

86. Adsul, M., Khire, J., Bastawde, K., and Gokhale, D.: Production of lactic acidfrom cellobiose and cellotriose by Lactobacillus delbrueckii mutant Uc-3, Appl.Environ. Microbiol., 73, 5055e5057 (2007).

87. Shen, X. L. and Xia, L. M.: Lactic acid production from cellulosic material bysynergetic hydrolysis and fermentation, Appl. Biochem. Biotechnol., 133,251e262 (2006).

88. Dumbrepatil, A., Adsul, M., Chaudhari, S., Khire, J., and Gokhale, D.: Utili-zation of molasses sugar for lactic acid production by Lactobacillus delbrueckiisubsp. delbrueckii mutant Uc-3 in batch fermentation, Appl. Environ. Micro-biol., 74, 333e335 (2008).

89. Tanaka, T., Hoshina, M., Tanabe, S., Sakai, K., Ohtsubo, S., andTaniguchi, M.: Production of D-lactic acid from defatted rice bran by simul-taneous saccharification and fermentation, Bioresour. Technol., 97, 211e217(2006).

90. Moldes, A. B., Torrado, A., Converti, A., and Dominguez, J. M.: Completebioconversion of hemicellulosic sugars from agricultural residues into lacticacid by Lactobacillus pentosus, Appl. Biochem. Biotechnol., 135, 219e227(2006).

91. Gullon, B., Yanez, R., Alonso, J. L., and Parajo, J. C.: L-Lactic acid productionfrom apple pomace by sequential hydrolysis and fermentation, Bioresour.Technol., 99, 308e319 (2008).

92. Hetényi, K., Németh, Á., and Sevella, B.: Investigation and modeling of lacticacid fermentation on wheat starch via SSF, CHF and SHF technology, Per. Pol.Chem. Eng., 55, 11e16 (2011).

93. Yun, J. S., Wee, Y. J., Kim, J. N., and Ryu, H. W.: Fermentative production of DL-lactic acid from amylase treated rice and wheat brans hydrolyzate by a novellactic acid bacterium, Lactobacillus sp., Biotechnol. Lett., 26, 1613e1616 (2004).

94. Wee, Y. J. and Ryu, H. W.: Lactic acid production by Lactobacillus sp. RKY2 in acellerecycle continuous fermentation using lignocellulosic hydrolyzates asinexpensive raw materials, Bioresour. Technol., 100, 4262e4270 (2009).

95. Wee, Y. J., Yun, J. S., Lee, Y. Y., Zeng, A. P., and Ryu, H. W.: Recovery of lacticacid by repeated batch electrodialysis and lactic acid production using elec-trodialysis wastewater, J. Biosci. Bioeng., 99, 104e108 (2005).

96. Moueddeb, H., Sanchez, J., Bardot, C., and Fick, M.: Membrane bioreactor forlactic acid production, J. Membr. Sci., 114, 59e71 (1996).

97. Sirman, T., Pyle, D. L., and Grandison, A. S.: Extraction of organic acids usinga supported liquid membrane, Biochem. Soc. Trans., 19, 274e279 (1991).

98. Aljundi, I. H., Belovich, J. M., and Talu, O.: Adsorption of lactic acid fromfermentation broth and aqueous solutions on Zeolite molecular sieves, Chem.Eng. Sci., 60, 5004e5009 (2005).

99. Wang, Y., Abdel-Rahman, M. A., Tashiro, Y., Xiao, Y. T., Zendo, T., Sakai, K.,and Sonomoto, K.: L-(þ)-Lactic acid production by co-fermentation of cello-biose and xylose without carbon catabolite repression using Enterococcusmundtii QU 25, RSC Adv., 4, 22013 (2014).

100. Nigorikawa, M., Watanabe, A., Furukawa, K., Sonoki, T., and Ito, Y.:Enhanced saccharification of rice straw by overexpression of rice exo-gluca-nase, Rice, 5, 14 (2012).

101. Sumiyoshi, M., Nakamura, A., Nakamura, H., Hakata, M., Ichikawa, H.,Hirochika, H., Ishii, T., Satoh, S., and Iwai, H.: Increase in cellulose accu-mulation and improvement of saccharification by overexpression of arabi-nofuranosidase in rice, PLoS One, 8, e78269 (2013).