Polylactic acid is aliphatic polyester made from lactic acid monomers ; it was foremost produced in 1932 by Carothers Lunt, 1998. The production of lactic acid from renewable natural stuffs by microbic agitation has made the production of PLA really economic and made PLA an of import member of biodegradable plastics. A mixture of PLA and polyglycolic acid was used in suturas, this was foremost reported commercial usage of PLA. These suturas were available under the trade name name Vicryl in the American market in 1974 ( Jamshidian et. al. , 2010 ) . Along with biodegradability PLA is besides biocompatible in human organic structure and hence has been widely used in production of implants, suturas and besides used for drug bringing. PLA is first biopolymer that is produced on a commercial graduated table and it can be biologically degraded to the full. PLA is produced in two stairss ; foremost lactic acid is produced by microbic agitation of saccharide rich agricultural by merchandises followed by polymerisation. PLA is considered as the best and most competitory bio-plastic produced from renewable resources. Nature Works LLC is the market leader in PLA production and its works in Blair, USA has a production capacity of 300 million lbs per twelvemonth ( Nampoothiri et. al. , 2010 ) . PURAC Biomaterials is another major manufacturer of PLA and industries resorbable gay and hetero polymers ( Nampoothiri et. al. , 2010 ) .
Plastics produced from petrochemicals are man-made polymers and neglect to degrade of course. About 140 million dozenss of man-made polymers are manufactured per twelvemonth across the Earth. The ground for man-made plastics being used widely can be attributed to low cost of production as they have been used for a long clip and has established production line. Man-made plastics are really lasting and strong mechanical belongingss. Degradability is a serious job for man-made plastics ( Jamshidian et. al. , 2010 ) . Man-made plastics have a singular consequence on the eco system doing accretion of these a serious menace to the environment ( Nampoothiri et. al. , 2010 ) . The cost of recycling and direction of man-made plastics is a major 1. As consequence of this there is demand for economical and biodegradable replacement for man-made plastics. Along with the environmental factor, there are concerns with fluctuating crude oil monetary value which adds to increasing demand for biodegradable plastics form easy available natural stuff.
The construction of the polymer and the natural stuff used for its production, imparts the belongings of biodegradability to the polymer. Biopolymers are made from assortment of natural resources and are categorised in figure 1. Polyesters are widely used as biodegradable plastics as they can be hydrolysed easy. Today many biodegradable polyesters are available and many are in developmental procedure, these are: PLA ( polylactic acid ) , PHB ( polyhydroxybutyrate ) , PCL ( polycaprolactonate ) , PHA ( polyhydroxyalkanoates ) , PET ( polyethylene terephthalate ) , PHH ( polyhydroxyhexanoate ) , PHV ( polyhydroxyvalerate ) , PHB ( polyhydroxybutyrate ) , AAC ( Aliphatic Aromatic copolyesters ) , polybutylene adipate and polymethylene adipate ( Nampoothiri et. al. , 2010 ) . PLA could be considered as a replacement for crude oil based plastics as it can be produced on a big graduated table by microbic agitation utilizing saccharide rich substances as natural stuffs.
Figure 1: beginnings of biopolymers
Figure 2: Lactic acid isomers. Top: L ( + ) lactic acid and underside: D ( – ) lactic acid ( Beginning: Polylactic acid )
Lactic acid ( PLA monomer ) production
Lactic acid was discovered by Carl Wilhelm Scheele in 1780 and was considered as a milk constituent boulder clay 1857, when Louis Pasteur found lactic acid is produced as a consequence of agitation by micro-organisms ( Wee et. al. , 2006 ) . Lactic acid is used widely in many industries like nutrient, cosmetics, drink, pharmaceutical, chemical, electronic etc. There are two optical isomeric signifiers of lactic acid: L ( + ) lactic acid and D ( – ) lactic acid ( Figure 2 ) . These enantiomorphs are produced by different enzymes known as Lactate dehydrogenases. L ( + ) lactic acid is normally produced by bugs, D ( – ) may be besides present with L ( + ) ( Jem et. al. , 2010 ) .
Figure 3: ( adapted and modified from Wee et. al. , 2006 ) Different fabricating methods of lactic acid. SSF: coincident saccharification and agitation.
Recovery & A ; purification
Hydrolysis ( H2SO4 )
HCN and accelerator additionLactic Acid can be manufactured either by chemical synthesis or by microbic agitation ( Figure 3 ) . Fermentative production of lactic acid is by and large method pick for production of lactic acid over chemical synthesis. Petrochemicals are get downing stuff for chemical synthesis and there is monetary value fluctuation every bit good as handiness issues with petrochemicals. Merely racemic mixture of DL-lactic acid can be produced by chemical synthesis. From the top 15 lactic acid manufacturers in the universe merely Musashino, Japan uses chemical synthesis method for bring forthing DL-lactic acid ( Jem et. al. , 2010 ) . Microbial agitation has many advantages over chemical synthesis: low cost of natural stuffs, procedure tallies at low temperature and lower force per unit area than chemical synthesis, less energy ingestion and it is environmental friendly ( Nampoothiri et. al. , 2010 ) . Optically pure L ( + ) or D ( – ) lactic acid is produced by microbic agitation. Optical pureness is really indispensable for leaving physical belongingss of PLA. Bacteria and Fungis are the two classs of bugs used in production of lactic acid. The type of enantiomorph produced depends on the being used for agitation. The agitation procedure can be classified on the type of bacteriums used as Homo-fermentative and hetero-fermentative. In homo-fermentative method 2 molecules of lactic acid are produced per molecule of glucose, it has higher output and less by merchandises ; so
is widely used in industrial production of lactic acid. The hetero-fermentation produces lower degree of lactic acid along with other metabolites like acetic acid, ethyl alcohol, C dioxide etc. ( Hartman, 1998 ) . Agitation is carried out at acidic PH ( about 6 ) , about 40 degree temperature and anaerobic or really low O degree. Racemization may happen at high temperature, therefore it is critical to avoid high temperature to obtain optically pure lactic acid ( Jem et. al. , 2010 ) .
Typically, batch, fed batch or uninterrupted agitations are used for bring forthing lactic acid. Continuous civilizations provide high productiveness whereas batch and fed batch assure higher concentrations ( Hofvendahl et. al. , 2000 ) . Kwon et. Al. demonstrated a novel and a successful effort to bring forth lactic acid utilizing uninterrupted agitation utilizing Lactobacillus rhamnosus. They produced 92 g/L of lactic acid and productiveness of 57 g/ ( L.h ) , by utilizing membrane cell rhythm bioreactors connected in series ( Kwon et. al. , 2001 ) . Wee et. Al. have reviewed different agitation attacks used for lactic acid production in greater item. End merchandise suppression is an major issue during lactic acid production and therefore lactic acid must be continuously removed from agitation stock. Downstreaming procedure for merchandise recovery involves biomass separation by filtration, centrifugation and decantation ; followed by chromatography and esterification / distillment, to divide the drosss ( Jem et. al. , 2010 ) .
Lactic acid bring forthing bugs
An ideal micro-organism suited for industrial or commercial production of lactic acid should hold the undermentioned features: 1. should be capable to utilize natural stuffs to the full and quickly 2. Should use minimal sum for nitrogen-bearing substances and growing factors 3. Should give maximal sum for required lactic acid stereo type 4.should be able to ferment at low PH and low temperature 5. Production of biomass and byproducts should be really low ( Narayanan et. al. , 2004 ) . Lactic acid bring forthing micro-organisms can be categorised as bacteriums and Fungis ( Wee et. al. , 2006 ) . Lactic acid bring forthing bacteriums have been widely research than filiform Fungis as fungous agitation has lower merchandise output and it produces by merchandises like ethyl alcohol and fumaric acid ( Tay et. al. , 2002 ) .
Rhizopus, are filiform Fungis capable of bring forthing lactic acid through aerophilic agitation of glucose. Rhizopus oryzae and Rhizopus arrhizus have the ability to change over amylum straight to L ( + ) lactic acid extinguishing the demand for pre-treatment of natural stuff due to its amlolytic enzyme activity ( Oda et. al. , 2002 ) . The medium demands for fungous agitation are comparatively simple but it requires extended aeration which may increase cost of production. Many research workers have attempted to maximise productiveness and output of lactic acid utilizing fungous agitation, but no important consequences were obtained. Tay and Yang used hempen bed immobilized Rhizopus oryzae to heighten production of lactic acid form amylum ( Tay et. al. , 2002 ) . Park et. Al. used Rhizopus oryzae NRRL 395 strain in an air lift bioreactor and obtained lactic acerb output of 104.6 g/L. They besides suggested that mycelial floc formation improved lactic acid formation ( Park et. al. , 2008 ) . Despite of all efforts made to better production of lactic acid ; bacterial fermentative production of lactic acid has an upper manus and is used to bring forth lactic acid on an industrial graduated table. Porro et. Al. studied production of L ( + ) lactic acid from, Saccharomyces cerevisiae and Kluyveromyces lactis, for their desirable belongings to digest high concentration of H ions ( refer page 171, ( Narayanan et. al. , 2004 ) .
As mentioned earlier lactic acid bring forthing bacteriums are divided into homo-fermentative and hetero-fermentative. Homo-fermentative lactic acid bacteriums give better output of lactic acid per molecule of glucose, which is more than 0.90 g/g and have no major by merchandises associated. For this ground homo-fermentative bacteriums are used for commercial production of lactic acid ( Wee et. al. , 2006 ) . As different lactic acid bacteriums have specificity for different substrates therefore an being should be selected based on saccharide to be fermented ( Narayanan et. al. , 2004 ) . Table 1 lists lactic acid bring forthing bacteriums and the saccharide they can use. In add-on to C and N beginnings lactic acid bacteriums besides require growing factors as they can non synthesise amino acids, vitamin B and minerals ( Wee et. al. , 2006 ) .
Table 1: Carbohydrate use by lactic acid bacteriums ( made from reappraisal by Narayanan et. Al. )
Lactic acid bacteriums
Lactose and galactose
Sulphite waste spirits
Commercially used strains of micro-organisms are proprietary and much information is non available on them. Most of the current probes involve usage of Lactobacillus for production of lactic acid. By utilizing the strain Lactobacillus delbrueckii NCIMB 8130 Kotzanmanidis et. Al. produced lactic acid from Beta vulgaris molasses, they produced 90 g/L of lactic acid with output of 0.97 g/g and productiveness of 3.8 g/ ( L.h ) ( Kotzanmanidis et. al. , 2002 ) . Robel et. Al. performed coculturing of Lactococcus lactis ssp. lactis cells and Aspergillus awamori to bring forth lactic acid from cassava amylum. They produced 90 g/L of lactic acid with output of 0.76 g/g and with somewhat low productiveness of 1.6 g/ ( L.h ) ( Robel et. al. , 2003 ) . Similarly, Hujanen and Linko obtained high of 0.91 g/g with 82 g/L of lactic acid production. They used Lactobacillus casei NRRL B-441 strain in tried effects of different temperature and N beginnings ( Hujanen and Linko, 1996 ) . Assorted different surveies have been carried out to analyze lactic acid production utilizing Lactobacillus rhamnosus, Lactobacillus helveticus, Lactobacillus bulgaricus, Lactobacillus plantaru and Lactobacillus pentosus ( Wee et. al. , 2006 ) .
2.2 Genetically Engineered Microbes
Many research workers have genetically modified to better or bring on production of L ( + ) lactic acid and D ( – ) lactic acid.
Lactobacillus helveticus ( Kyla-Nikkila et. al. , 2000 )
Kyla-Nikkila et. Al. used Lactobacillus helveticus CNRZ32 strain and attempted to increase production on L ( + ) lactic acid. They constructed two strains negative for ldhD ( lactate dehydrogenase D cistron ) look. In one strain ldhD was replaced by ldhL and in the 2nd strain the booster for ldhD was deleted therefore barricading the written text. Their consequences show that there was 2 fold addition in production of L ( + ) lactic acid and it was tantamount to the entire lactic acid produced by wild type strain.
Lactobacillus plantarum ( Ferain et. al. , 1994 )
They cloned the lactate dehydrogenase L cistron signifier Lactobacillus plantarum and cloned it into E.coli. After sequencing this cistron Ferain et. Al. strains of Lactobacillus plantarum were constructed either over showing or missing look of ldhL cistron. This resulted in 13 fold addition in look of cistron but had negligible consequence on L ( + ) lactic acid production.
Chang et. Al. produced recombinant strain of E.coli by presenting ldhL cistron from Lactobacillus casei into E.coli mutant lacking phosphotransacetylase and ldhD cistron. They besides induced IdhD production by presenting ldhD cistron. They were successful in demoing that E.coli can be genetically modified to bring forth optically pure L ( + ) or D ( – ) Lactic acid. Dien et. Al. genetically modified E.coli to bring forth L ( + ) lactic acid from hexose sugar and besides pentose sugar ( Narayanan et. al. , 2004 and Wee et. al. , 2006 ) . Jung et. Al. constructed a metabolically engineered strain of E.coli with improved propionate CoA-transferase PHA synthase activity. They successfully produces PLA and P ( 3HB-co-LA ) utilizing glucose in individual measure without usage of any external inducer. The strain of E.coli developed in this survey was E. coli JLXF5 ( Jung et. al. , 2011 ) .
A mutation with lower limit ( 5 % ) intoxicant dehydrogenase activity produced lactic acid from pyruvate in O restricting status. Normally the fungus grows merely for short period in absence of O due to presence intoxicant dehydrogenase ( Narayanan et. al. , 2004 ) .
Sweet sorghum, Wheat, Corn, Cassava, Potato, Rice, Rye, barley
Corn hazelnut & A ; stover, Waste paper, Wood, Alfalfa fiber, Wheat barn & A ; straw
Whey and Molasses
Figure 4: Categorization of natural stuffs used for lactic acid production ( made from reappraisal by Wee et. al. , 2006 )
2.3 Natural stuffs
Different natural stuffs used in production of lactic acid have been classified in figure 4. By and large starchy stuffs, cellulosic stuffs and industrial waste merchandises are used as natural stuffs for production of lactic acid by microbic agitations. Starchy and cellulosic stuffs have intrigued research workers late as they are low-cost, present in big measures in the environment and most significantly they are easy renewable. These features are critical for commercial manufacturers of lactic acid and PLA.
Production of lactic acid from starchy stuffs requires a hydrolysis measure to change over them into fermentable sugars as glucose is linked by I±- ( 1,4 ) and I±- ( 1,6 ) bonds because all bugs can non hydrolyze these bonds. Lactobacillus amylophylus, Lactobacillus amylovirus, Rhizopus oryzae and Rhizopus arrhizus, have amlolytic belongingss hence they can straight change over starchy stuffs into lactic acid ( Wee et. al. , 2006 ) . Similar to starchy stuffs cellulosic stuff need a pre-treatment to change over lignocellulose to cellulose. During the pre-treatment measure several compounds are generated which inhibit the hydrolysis of lignocellulose ; these are acetic acid, furfural etc. Wee et. Al. suggested that this consequence can be reduced to an certain extent by direct version of lactic acid bacteriums to wood hydrolysate medium ( Wee et. al. , 2006 ) . Industrial waste merchandises like molasses and whey contain sugars which act as substrate for lactic acid bacteriums. Table 2 lists natural stuffs and micro-organisms used to bring forth lactic acid.
3 Polylactic acid production
Lactic acid is polymerized to bring forth PLA. The PLA industry requires lactic acid which has high optical pureness with L ( + ) lactic acid concentration more than 98 to 99 % . There are two widely used ways to bring forth PLA from lactic acid. First one is direct polycondensation of lactic acid and the other being via formation of lactide ( cyclic dimer of lactic acid ) followed by pealing opening reaction of lactide. Both these procedures are commercially used to bring forth PLA ( Jem et. al. , 2010 ) .
Table 2: Production of lactic acid form inexpensive natural stuffs ( Narayanan et. al. , 2004 and Wee et. al. , 2006 )
Lactic acid output ( g/L )
Wheat and rice barn
Lactobacillus casei NRRL B-441
Lactobacillus paracasei No. 8
84.5 – 81.5
Wood and pretreated wood
48 – 108
Rhizopus sp. MK-96-1196
24 – 90
Rhizopus oryzae NRRL 395
Industrial waste stuff
Lactobacillus delbrueckii NCIMB 8130
Lactobacillus helveticus R211
The direct polycondensation procedure involves formation of oligomers by desiccating lactic acid which is so followed by polymerisation of the oligomers to obtain high molecular weight PLA by coincident desiccation. The major drawback of this method is removal of wet from the molten syrupy polymer. The caparison of H2O molecule can ensue in low molecular weight PLA. Hence this procedure is non used widely. Mitsui Toatsu chemicals is one of the few companies that uses direct condensation and organic dissolver based azeotropic distillment to take H2O continuously ( Ajioka et. al. , 1995 ) .
The most normally used method to obtain high molecular weight PLA is known as pealing opening polymerisation ( ROP ) of lactide. The first measure in this procedure is polycondensation of lactic acid to organize oligomers. This is followed by depolymeriztion to take H2O, which consequences in formation cyclic dimer of lactic acid called lactide ( Narayanan et. al. , 2004 ) . The lactide in so polymerised my pealing opening polymerisation into polyester. The ROP procedure is performed at temperature more than runing temperature of lactide but below debasement temperature ( Jem et. al. , 2010 ) . ROP procedure is catalysed by stannic octoate accelerator. Cargill Dow LLC has patented procedures of ROP in which synthesis of PLA is performed in thaw and ROP is catalysed by Sn accelerator which is environment friendly ( Nampoothiri et. al. , 2010 ) . Both these methods for polylactic acid production are reviewed in deepness by Nampoothiri et. Al. and Narayanan et. Al.
3. Applications of Polylactic acid
PLA is used in many industries like nutrient, medical, fabric, electronics, agribusiness etc. and has assortment of applications. Nature works markets PLA plastics and PLA fibers under the trade name name Ingeo. They 11 different classs of merchandises made from PLA viz. apparels, bottles cards, packaging, lasting goods, lactides, packing stuffs, movies etc. ( www.natureworksllc.com ) . Leading companies like Mc Donalds, Wallmart, Panasonic, Sony, Fila, Cargill dow, Fujitsu, Toyota etc. , in the universe usage PLA in their merchandises. The list of applications of PLA is eternal. In this reappraisal some of the of import applications of PLA will be discussed.
PLA is turning in popularity as a “ Green ” packaging stuff for nutrient. Today PLA is chiefly used in packaging industry ; approximately 70 % of PLA produced is used as packing stuff ( Jamshidian et. al. , 2010 ) . The wadding industry chiefly uses PLA in signifier of thermoplastic containers and movies. Biodegradability is non the lone ground for its usage in packaging industry but PLA is besides used for its mechanical belongingss and low-cost monetary value. PLA has potential to be used as antimicrobic packaging for nutrient to better shelf life by matching it with bacteriocins such as nicin and pediocin ( Jamshidian et. al. , 2010 ) . PLA can be used in doing compost bags, disposable tabular array ware, loose fill packing etc. ( Nampoothiri et. al. , 2010 ) . PLA is besides used in garment and fabric fabrication.
PLA is GRAS ( by and large regarded as safe ) . It has belongingss biocompatibility, bioabsobability and biodegradability ; PLA has applications in biomedical and drug bringing. PLA and copolymers can be used to present drugs like rapamycin, Amoxil, gentamiycin etc. ( Jamshidian et. al. , 2010 ) . PLA has been used in industry of suturas since 70 ‘s. It is aslo used in bone arrested development stuff ( prison guards for mortise joints, articulatio genus ) and microsphere for drug bringing. Copolymer of PLA, PLGA ( Poly lactic-acid co-glycolic acid ) is chiefly used for drug bringing. PLA-PEG copolymers bringing system is really utile as inside human organic structure as it can drive proteins and therefore stops interaction between polymer and cells. This is due to the hydrophilic belongings of PEG and PLA is hydrophobic ( Nampoothiri et. al. , 2010 ) . Uhrich et. Al. demonstrated that PLA-PEG nanoparticles stay in blood watercourse in vivo than PLA entirely ( Uhrich et. al. , 1999 ) .
PLA besides has application in agribusiness. Presently PLA is used in production of mulching movies by Novamont and Cargill Dow ( Jamshidian et. al. , 2010 ) . Large sum of plastics are used in fabrication of weedkillers, PLA can be used as a replacement in this country. PLA can be used as matrix for scattering of weedkillers which can cut down green consequence on environment and aid to increase output of the harvest ( Nampoothiri et. al. , 2010 ) . PLA is besides usage in doing Cadmium, parts of computing machines like keyboard keys, Walkman and cell phone shell, wheel cap for autos. In general possibilities of utilizing biodegradable polymers are eternal.
Recent fluctuation in monetary value and supply in fuel monetary values, above all major concern of accretion of man-made plastic in environment ; have shifted attending towards more environmental friendly biodegradable plastics. PLA has high ranking in environmental plastics due to its competitory cost and versatile belongingss. Man-made plastics are still been used in big measures due to their low cost and established production apparatus. Use of renewable natural resources and industrial for production of PLA by microbic agitation has helped in cut downing the cost of PLA late. Assortments of genetically modified micro-organisms are being developed to increase output of PLA which will farther assist in cost decrease of PLA. Besides usage of PLA is increasing steadily in many industries and expects to increase more by 2020. Although there are a few challenges to be surpassed like improved stuff belongingss of PLA, usage of nanotechnology will assist to get the better of these barriers. In close hereafter it PLA can be expected to replace usage of petrochemical based polymers in pharmaceutical and packaging industry.