Renewable materials, lactic acid production

Biobased polymers from renewable materials have received increased attention recently. Lactate is a building block for bio-based polymers. In the United States, production of lactic acid is greater than 50,000 metric tons/yr and projected to increase exponentially to replace petroleum-based polymers. Domestic lactate is currently manufactured from corn starch using the filamentous fungus Rhizopus oryzae and selected species of lactic acid bacteria. The produced lactic acid can then be polymerized into polylactic acid (PLA) which has many applications (Hatti-Kaul et al., 2007). However, so far, no facility is built to use biomass derived sugars for lactic acid production. More research needs to be done to develop microbes using biomass derived sugars for lactate production. 

Polylactic acid (PLA) is a biodegradable polymer derived from lactic acid. It is a highly versatile material and is made from 100% renewable resources like corn, sugar beet, wheat and other starch-rich products. Polylactic acid exhibits many properties that are equivalent to or better than many petroleum-based plastics, which makes it suitable for a variety of applications. 

For the last 70 years or so the chemical industry has been based on crude oil (petroleum) and natural gas as basic raw materials, hence the name petrochemicals. This may not be so for much longer, however. The chemical industry is currently on the brink of a new revolution, based on the switch from fossil resources to renewable agriculture-based raw materials. From a distance the production facility of Cargill in Blair, Nebraska looks very much like a small oil refinery or medium-sized petrochemicals plant. However, closer inspection reveals that it is a corn-processing plant a biorefinery producing, inter alia, high-fruc-tose corn syrup, ethanol and lactic acid. As James R. Stoppert, a senior executive of Cargill pointed out, the chemical industry is based on carbon and it does not matter if the carbon was fixed 2 million years ago or 6 months ago [1]. 

Renewable feedstocks can also be used as the raw materials for the synthesis of green, biodegradable polymers. A pertinent example is polylactate, derived from lactic acid which is produced by fermentation (see earlier). Another recent example is the production of polycarbonates by reaction of C02 with (R)-limonene oxide in the presence of a zinc catalyst (Fig. 8.47) [221]. 

Polylactic acid is a biodegradable polymer. It can be made from lactic acid, which can be produced by fermentation of glucose. Because it is biodegradable and can be manufactured from agricultural products, polylactic acid is potentially a renewable material. US 6,326,458 assigned to Cargill Inc. describes a process for making polylactic acid from lactic acid. Estimate the cost of production of the purified polymer. 

Lactic acid is a major end product from fermentation of a carbohydrate by lactic acid bacteria (Tormo and Izco, 2004). However, lactic acid can be produced commercially by either chemical synthesis or fermentation. The chemical synthesis results in a racemic mixture of the two isomers whereas during fermentation an optically pure form of lactic acid is produced. However, this may depend on the microorganisms, fermentation substrates, and fermentation conditions. Lactic acid can be produced from renewable materials by various species of the fungus Rhizopus. This has many advantages as opposed to bacterial production because of amylolytic characteristics, low nutrient requirements, and the fungal biomass, which is a valuable fermentation by-product (Zhan, Jin, and Kelly, 2007). 

Zhang, Z.Y., Jin, B., and Kelly, J.M. 2007. Production of lactic acid from renewable materials by Rhizopus fungi. Biochemical Engineering Journal 35 251-263. 

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]. 

Wang, Y., Tashiro, Y, and Sonomoto, K. (2015) Fermentative production of lactic acid from renewable materials recent achievements, prospects, and limits. J. Biosci. Bioeng., 119, 10—18. 

Sucrose is the most available low-molecular-weight carbohydrate but until now was mainly used for nutrition purposes and for the fermentation industry -above all for the production of bioethanol, citric acid and lactic acid - and only to a smaller extent as chemical feedstock. Though the use of renewable resources in biotechnological processes has increased in the last years, there is a strong price-driven competition between sucrose and starch as raw materials for fermentative processes being influenced by regional differences. 

A comparison of the properties of materials is made to indicate a potential replacement of existing materials with emerging, commercially available renewable, compostable alternatives. The analysis is limited to products that are commercially available, e.g., starch, cellulose, whole crop polymers, and polyesters such as poly(lactic acid) (PLA), poly(hydroxyalkanoate), and others (5). Commodity plastics used in food packaging are summarized in Table 7.1. 

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