Lactic acid production from renewable resources

FIGURE 13.3 Schematic illustration of metabolic pathway of glucose in lactic acid bacteria. Homofermentative pathway (a), heterofermentative pathway (b). 

Although most investigations of LA production were carried out with LAB, filamentous fungi such as Rhizopus have also been utilized for LA production. Fungal fermentation has some advantages in that Rhizopus oryzae requires only a simple medium and can produce l-LA from starchy materials with the aid of its own amylolytic enzyme activity, but it also requires vigorous aeration (Yin et al., 1997 Table 13.1). 

Detailed information concerning conventional LA fermentation was excellently reviewed by Hofvendahl and Hahn-Hagerdal (2000) and Wee et al. (2006). 

5 LACTIC ACID PRODUCTION FROM RENEWABLE RESOURCES 

There have been many attempts to produce LA from cheap raw materials such as starchy and cellulosic materials, whey, and molasses. Among these, starchy and ligno-cellulosic materials are currently receiving a great deal of attention, because they are cheap, abundant, and renewable (Wee et al., 2006). One bottleneck for LA production from starchy and lignocellulosic materials is the cost of pretreatment of raw materials. 

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Hofvendahl, K. and Hahn-Hagerdal, B. 2000. Factors Affecting the Fermentative Lactic Acid Production from Renewable Resources. Enzyme Microb. Technol., 26, 87-107. 

Lactic Acid Production From Renewable Resources... 

Hofvendahl K, Hahn-Hagerdal B. (2000). Factors affecting the fermentative lactic acid production from renewable resources. Enzyme Microb Technol, 26, 87-107. 

In order to decrease human consumption of petroleum, chemists have investigated methods for producing polymers from renewable resources such as biomass. Nature Works polylactic acid (PLA) is a polymer of naturally occurring lactic acid (LA), and LA can be produced from the fermentation of corn. The goal is to eventually manufacture this polymer from waste biomass. Another advantage of PLA is that, unlike most synthetic polymers which litter the landscape and pack landfills, it is biodegradable. PLA can also be easily recycled by conversion back into LA. It can replace many petroleum-based polymers in products such as carpets, bags, cups, and textile fibers. 

The blending of different polymers is a frequently used technique in industrial polymer production to optimize the material s properties. The biodegradable polymer poly(3-hydroxybutyrate) (PHB) [45, 46], for example, which can be produced by bacteria from renewable resources, has the disadvantage of being stiff and brittle. The mechanical properties of PHB, however, can be readily enhanced by blending with another biopolymer, poly(lactic acid) (PLA) [47]. In order to prepare the optimum blend, it must be noted that the miscibility of different polymers depends on their concentration, the temperature, and their structural characteristics [48]. 

Polymers derived from renewable resources (biopolymers) are broadly classified according to the method of production (1) Polymers directly extracted/ removed from natural materials (mainly plants) (e.g. polysaccharides such as starch and cellulose and proteins such as casein and wheat gluten), (2) polymers produced by "classical" chemical synthesis from renewable bio-derived monomers [e.g. poly(lactic acid), poly(glycolic acid) and their biopolyesters polymerized from lactic/glycolic acid monomers, which are produced by fermentation of carbohydrate feedstock] and (3) polymers produced by microorganisms or genetically transformed bacteria [e.g. the polyhydroxyalkanoates, mainly poly(hydroxybutyrates) and copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV)] [4]. 

Okano K, Tanaka T, Ogino C, Fukuda H, Kondo A (2010) Biotechnological production of enantiomeric pure lactic acid from renewable resources recent achievements, perspectives, and limits. Appl Microbiol Biotechnol 85 413 23... 

Biodegradable polymers can be mainly classified as agro-polymers (starch, protein, etc.) and biodegradable polyesters (polyhydroxyalkanoates, poly(lactic acid), etc.). These latter, also called biopolyesters, can be synthesized from fossil resources but main productions can be obtained from renewable resources (Bordes et al. 2009). However for certain applications, biopolyesters cannot be fully competitive with conventional thermoplastics since some of their properties are too weak. Therefore, to extend their applications, these biopolymers have been formulated and associated with nano-sized fillers, which could bring a large range of improved properties (stiffness, permeability, crystallinity, thermal stability). The resulting nano-biocomposites have been the subject of many recent publications. Bordes etal. (2009) analyzed this novel class of materials based on clays, which are nowadays the main nanoflllers used in nanocomposite systems. 

Production of lactic acid from renewable resources. 

In mid-1970, due to the petroleum crisis, the production of plastics from renewable resources became economically attractive (Lenz and Marchessault 2005). The price of an oil barrel increased at very high values, and the same occurred to all petroleum products. So, at that time, an extensive search of materials that could replace synthetic polymers took place. Many polymers were proposed and investigated regarding to its biodegradability and its possibility of industrial application, such as cellulose, starch, polycaprolactone, poly(lactic acid), and PHA. Among these polymers, PHA are of particular interest due to their biodegradability, biocompatibility, and mainly because of their similarity to conventional thermoplastics (Zinn et al. 2001). 

Biomass products from agro-resources (agro-polymers) These bioplastics are either synthesized naturally from plants and animals, or entirely synthesized from renewable resources. This class includes starch, cellulose, proteins, lignin, chitosan, poly lactic acid (PLA) and polyhydroxy-alkanoates/polyhydroxybutyrates. A recent breakthrough in this class of bioplastics is the development of technology to synthesize polymers like polyethylene, polypropylene and nylon from biological resources ... 

Due to abundantly available feedstock and low cost, poly lactic acid (PLA) is one of the most promising bio-based polymers. PLA is obtained by the controlled polymerization of lactic acid monomers which in turn are obtained from renewable resources such as sugar feedstock, wheat, maize, com, and waste products from food or agriculture industry by fermentation (Siracusa et al., 2008). Properties of PLA vary according to the Z - to - D lactylenantiomeric ratio. Table 8 lists some important properties of PLA. 

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