What Is Lactic Acid Biology Essay

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Lactic acid has been produced on a industrial scale since the end of the nineteenth century mainly to support the needs of numerous application were made from lactic acid such as in pharmaceutical, food, leather and textile industries (Averous, 2008). The demand for lactic acid tends to increase due to increasing application in preparation of biodegradable polymers, medical sutures and green solvents (Datta et al., 1995; Litchfield, 1996). Lactic acid (α-hydroxypropionic acid) used as a substrate in the synthesis of poly lactic acid (PLA), belongs to the hydroxyacids and it is characterised by greater acidity than acid devoid of hydroxy groups since the presence of hydroxy considerably facilitates the dissociation of carbonyl groups (Dutkiewicz et al.,2003). Chemical such as acrylic acid, propylene glycol, acetaldehyde and 2-3 pentanedone were converted from lactic acid due to both hydroxyl and carboxyl groups that have high reactivity (Hurok et al., 2004).

2.1.2 History of Lactic Acid

Historically, lactic acid was discovered in 1780 as component of sour milk  by Swedish chemist, Carl Wilhelm Scheele (Jim et al., 2010). The French scientist Frémy produced lactic acid by fermentation and this gave rise to industrial production in 1881. The lactic acid production was around 5,000 kg per year and in 1982, it was approximately 24,000 to 28,000 metric tonnes per year (Naveena, 2004). By 1990, the worldwide production volume of lactic acid had grown to approximately 40,000 metric tonnes per year with two significant producers, CCA Biochem in The Netherlands, with subsidiaries in Brazil and Spain, and Sterling Chemicals in Texas City, TX, USA, as the primary manufacturers (Rojan et al., 2007).

Furthermore, Cargill joined forces with Dow Chemical and established a Cargill-Dow polylactic acid (PLA) polymer venture based on carbohydrate fermentation technology in late 1997 and brought out Dow from this joint venture and established NatureWorks LLC as a wholly owned subsidiary in early 2005 (Wee et al., 2006). NatureWorks LLC has recently constructed a major lactic acid plant in Blair, NE, USA, with a nameplate capacity of 300 million pounds per year for the production of lactic acid and PLA, and it began operating in late 2002 (Wee et al., 2006; Datta and Henry 2006). For product commercialization, they have partnered with many potential end-users and polymer processing equipment manufacturers.

2.1.3 Properties of Lactic Acid

Lactic acid is a carboxylic acid containing three carbon organic acid : one terminal carbon atom is part of an acid or carboxyl group; the other terminal carbon atom is part of a methyl or hydrocarbon group; and a central carbon atom having an alcohol carbon group (Narayanan et al., 2004). According to Narayanan (2004), it is colourless to yellow liquid in which exists in two optically active isomeric lactic acid (or it esters or amides) may undergo are xanthation with carbon bisulphide, esterification with organic acids and dehydrogenation or oxygenation to form pyruvic acid or its derivatives. Lactic acid is soluble in water and water miscible organic solvents but insoluble in other organic solvents. It exhibits low volatility.

(b)

Figure 2.1: (a) L (+) Lactic acid and (b) D (-) Lactic acid ( Narayanan et al., 2004)

Table 2.1: physical properties of lactic acid ( Chemicalbook. Inc, 2008 and Narayanan et al., 2004).

Molecular weight

90.08 g/mol

Melting point

16.8 oC

Boiling point

82 oC at 5mmHg

122 oC at 14mmHg

Dissociation constant, Ka at 25 oC

1.37 X 10-4

Heat of combustion, ∆HC

1361 KJ/ mole

Specific Heat, Cp at 20 oC

190 J/mole/oC

2.2 Synthesis of PLA

The synthesis of PLA is a multistep process which starts from the production of lactic acid. There are two methods of synthesis which are the polycondensation of L(+) lactic acid and ring-polymerization (ROP) of the dimer form of lactic acid, lactide which carried out in bulk or in solution (Dutkiewicz et al., 2003).According to Jim (2010), polymerization of PLA has been conducted since 1932 and both of the process (polycondensation or ROP) rely on highly purified polymer-grade lactic acid or lactide to produce PLA with a good quality, high molecular weight and high yield. However, the production process, the yield and the characteristic of PLA are influenced by the crude lactic acid with impurities whether it is chemical or optical purity (Garlotta, 2001; Mehta et al., 2005; Henton et al., 2005).Thus, purification of lactic acid from fermentation is decisive importance.

2.2.1 Polycondensation

Polycondensation or also known as azeotropic condensation is a well studied polymerization route conducted since 1994. The outstanding achievements in this field belong to Japanese scientist (Ajioka et al., 1995). Basically,the polycondensation of lactic acid used azeotropic distillation in a refluxing, aprotic solvent results in PLA with low weight-average molecular weights about 300,000 (Enomoto et al., 1994; Kashima et al.,1995; Ohta et al.,1995). It is difficult to obtain high molecular weights in a solvent free system, but it has advantage that the polymer has a low cost.

Furthermore, the proper selection of catalyst in polycondensation is very important. It is because it activates the dehydrating reaction, deactivated the formation of lactide and increases the molecular weight (Orozco et al., 2007). Orozco (2007) proposed that the catalyst activity is essential since it can change the polarity of the polycondensation system. Initially, lactic acid and its primary condensates have high polarity since they are all consist both carboxyl and hydroxyl groups in a high ratio, while the final PLA contained of less polar ester group, leading to great decrease of polarity, then it is very crucial to add the catalyst after dehydration of lactic acid and the formation of oligo(lactic acid) (Moon et.al., 2000).

2.2.2 Ring-polymerization method

The ring-polymerization of the dimer form of lactic acid is a synthetic pathway that eliminates the use of solvents. The process starts with a continuous condensation reaction of lactic acid to produce low molecular weight PLA pre-polymer, which is then converted to a mixture of lactide stereoisomers using tin catalysts to enhance the rate and selectivity of intramolecular cyclization reaction (Henton et al., 2005). The lactide is purified by vacuum distillation and ring-open polymerized in the melt with a tin catalyst. Upon completion of polymerization, the unreacted lactide is removed by vacuum and recycled to the beginning of the process (Avérous, 2008)

Figure 2.2 : Synthesis methods for high-molecular-weight PLA (Garlotta, 2001)

2.3 Polylactic Acid (PLA)

Nowadays, most plastics are derived from non renewable resources such from crude oil which is not environmental friendly. This has lead to the research on producing biodegradable polylactic acid from renewable energy such as plant (Garlotta, 2001). The other term of polylactic acid is known as polylactide, a biodegradable polymer with good properties such as mechanical strength, transparency, compostability, environmental safety and biocompatibility. PLA has various applications in industry which include the composite manufacturing industry. The total consumption of biodegradable polymers such PLA are 14000 to 85000 m ton from 1996 to 2005 and it is projected that the consumption will be double by 2010 (Auras et al., 2004).

2.3.1 Properties of PLA

Polylactic acid (PLA) belongs to the family of aliphatic polyesther commonly made from α-hydroxy acids. It is one of few polymers in which the stereochemical structure can be easily modified by polymerizing a controlled mixture ofland D-isomers (Avérous, 2008). Lim (2008) claimed that polylactic acid can replaced polyolefin group in production of many materials due to its good physical properties if compared to polystyrene. This is because polylactic acid obtained high modulus and strength but low toughness. In addition, polylactic acid able exhibit crystallinity behaviour which depends on its monomer stereochemistry; isotactic poly(S-lactide) is crystalline thermoplastic with transition temperature (Tg) about 60oC and melting temperature (Tm) 170oC to 180oC while atactic polylactic acid has shown amorphous state behaviour (Lim et.al.,2008).

According to the figure 2.2, comparison of Tg and Tm value for polylactic acid with other thermoplastic materials were made and polylactic acid tend to have relatively high Tg and low Tm.

Figure 2.3: Comparison of glass transition and melting temperatures of PLA with other thermoplastics. ( Lim et al.,2008)

PLA is a thermoplastic, high strength, high modulus polymer that have undergoes thermal degradation at temperature above 200 oC by hydrolysis, lactide reformation, oxidative main chain scission and inter-or intramoleculat transesterification reactions (Garlotta, 2001). Its molecular weight is adjusted either through polycondensation polymerization or ring polymerization method. High molecular weight of PLA is normally between 15,000 to 500,000 g/mol.

 

PLA

PET

PS

HIPS

PP

Tg ( C)

55

75

105

 

-10

Density (g/cc)

1.24

1.33

1.05

 

0.9

Heat capacity (BTU/lb- F)

0.39

0.44

0.54

 

0.3

Thermal Conductivity (BTU/hr.lb.F)

0.075

0.105

0.121

 

0.068

Notched Izoc (ft-lb/in)

0.24

2.6

0.5

2.3

2

Gardner Impact (in-lb)

0.5

2.8

4.5

100

7

Tensile Strenght @Break (psi)

7700

7900

6500

3300

4500

Tensile Modulus (kpsi)

500

400

420

300

130

% Elongation

6

130

7

45

120Table 2.2: Comparison of PLA properties to several petroleum based resins (Dorgan et al., 2001).

2.4 Kenaf fibre as raw material

Several raw materials have been evaluated as potential inexpensive substrates for lactic acid production. It is frequently derived from feed stocks such as corn, beet sugar, molasses, whey, and barley malt (Narayanan et al., 2004 and Maas et al., 2008). However these raw materials draw less economical for long term since it has competing food value. Due to this issue, lignocellulosic biomass such kenaf fibre is chosen. Indeed, it is inexpensive and widely available renewable carbon source, natural, and organic with various potential applications.

2.4.1 Introduction

Kenaf (Hibiscus cannabinus L) is a warm season annual fibre and non food crop .It is a member of family Malvaceae and found to be the third largest fibre crop of economic importance after cotton and jute (Starr and Page 1990; Adegbite et al., 2005). Kenaf offers remarkable for both the production of industrial raw materials and as bio-fuel. Due to global environmental issues and inadequate raw fibre resources, scientists worldwide have realise and begun to explore the full potential of kenaf and its diverse uses ( Keshk et al., 2006). Kenaf in Malaysia was officially introduced in 2008 by former prime minister of Malaysia, Tun Abdullah Ahmad Badawi. During that time, he announced and launched the East Coast Economic Regions (ECER) Kenaf centre for collection, processing packaging and distribution in Bachok, Kelantan due to huge scale production and potential use of kenaf for the industry. This wonderful crop also identified by National Tobacco Board, Malaysia to be an excellent supplement to tobacco but not to replace it (Intropa, 2008).

2.4.2 History

Historically, kenaf is believed to have originated from Africa since it is grown as food crop in several African nations. According to Adegbite (2005), it is most likely originated from Sudan and commonly cultivated for both food and fibre in West Africa. However, Kobayashi (1991) and Keshk (2006) claimed that it is believed to have had its origin in ancient Africa (Western Sudan) and have been cultivated in Egypt as early as 4000 BC. Furthermore, India has produced and used kenaf for the last 200 years, while Russia started producing kenaf in 1902 and introduced the crop to China in 1935 (Dempsey, 1975; Weeber and Bledsoe 2002). United States begun to show their interest on kenaf in 1941's after World War II when the import of jute was shut off (Rymsa, 1999). Starting from that time, research and production of kenaf has begun due to its potential to substitute jute. The United States Department of Agriculture (USDA) had determined kenaf as a promising "new" crop. As a result of the New Crops, New Uses program started in the 1950's and early 1960's by the President Dwight David Eisenhower.

Kenaf also was determined to be the excellent cellulose fibre source for a large range of paper products such as newsprint and bond paper (Weeber and Bledsoe, 2002). Many research and development were done in 1990's which demonstrated kenaf suitability for use in building materials, adsorbents, textiles, livestock feed, and fibres in new and recycled plastics (Bledsoe and Webber ,2001; Webber and Bledsoe, 2002 ).

2.4.3 Kenaf's description and its application

Kenaf is mostly unbranched and rapidly reaches maturity, in only 4 to 5 months the plants can grow to 2 to 5 metres tall. Leaves are individually stalked and lobed to some degree. Flowers are yellow or white with a red centre and can be up to 10cm in diameter. Fruits are fleshy; producing seed capsules 1cm long containing many seeds. Seeds are brown and wedge shaped, 5mm long with a 1000-grain weight of 25g (Weeber and Bledsoe, 2002; Mache 2002).

The traditional uses of kenaf focus on its production for fibre used in making rope, sacks, canvas and carpets (Kaldor and Verwest, 1990). The leaves may be edible to both animals and humans where they can be used as herbs in some dishes (Mache, 2002). Due to global environmental issues and inadequate of raw fibres resources, scientists explored the new diverse use of kenaf. In such case, kenaf fibre in both retted and raw forms is used in the manufacture cordage and newsprint. Leaves and small branches have high digestibility when ground and can be used as source of roughage and protein for livestock (Wildeus et al., 1995). Other new applications of kenaf that had been discovered were in the pulp and paper industry, for oil absorption, as a potting medium, in the manufacture of broadcloth, in filtration, and as an additive in animal feed (Sellers and Reichert, 1999 ; Keshk et. al., 2006).

A new use for kenaf was tested in July 1987, when paper made from kenaf fibre was used to print 83,000 copies of the Bakersfield Californian newspaper (Robinson, 1988). Reports indicate that the paper has excellent ink-retention characteristics and its high strength is well suited to new high-speed presses. KP Products Inc. dba Vision Paper has been producing and marketing paper products made from 100% kenaf fibre and from blends of kenaf fibre and recycled paper since 1992 (Rymsza, 1998). Rymsza (1999) claimed that about 80% bast and 20% core of kenaf used in pulping as the whole stalk pulping of kenaf appears to offer the best economics for the papermaker. Then this pulping process was continued with a single-stage hydrogen peroxide (H2O2) bleach process. As a result, stronger, brighter, and cleaner pages papers were produced. (Sanadi et al.,1995).

Among the several applications of kenaf products, at the present it is an interest as biomass crop for energy production, since yield can reach 22.75 t per ha of fresh material (Danalatos and Archontoulis, 2005). Furthermore, kenaf is also utilized in composite industry. It becomes natural fibre composite material in which it is able to substitute fibreglass and reinforce plastics as to manufacture medium density fibre board and particleboard. Due to that extend, Panasonic Malaysia has taken the opportunity to manufacture kenaf fibre boards and then export them to Japan. The initial aim is to achieve 1000 tons exported per month but this will expect to double once shipments to Japan begin (Graupner et al., 2009).

2.4.4 Properties of kenaf

Kenaf's properties basically identified based on its component which includes stalks, leaves, flowers, seeds, bast and core. For example, Kenaf stalk yields normally in the range from 11 to 18 tonnes t per ha, oven dry weight while its leaves produced with serrated edges on the main stalk (stem) and along the branches. The position of these leaves alternate from side to side on the stalk and branches. As the kenaf plant matures and additional leaves are produced, the newer leaves start to differentiate into the leaf shape characteristic (Weeber and Bledsoe, 2002).

According to Mache (2002), kenaf is best suited to the tropics or subtropics where the mean daily temperature during the growing season is more than 20°C; it is also sensitive to photoperiod. The length of the growing season, the average day and night temperatures, and adequate soil moisture are considered the key elements affecting kenaf yields. Furthermore, kenaf contain less lignin; approximate 9 % less than pine, about 10 % of hemicellulose and about 30% cellulose.

Furthermore, kenaf's production can be increased on widely varying soil types. It can be also adapted to a wide climatic range even though the crop is frost tender. However, its production is therefore limited to warm temperate zones through to the equator and this situation is not encouraged north of southern Europe. Optimum temperatures for growth of kenaf are 15oC to 27°C. Nevertheless, its mean daily temperatures which are above 20°C are favourable throughout the growing season (Lapenta et al., 1993; Mache, 2002).

The advantages of kenaf as a source of pulp and papermaking include: 1) it has a short growing cycle of 120 to 130 days as compared to 13 to 16 years for trees; 2) the possibility of growing two crops per year the under certain conditions; 3) it has less lignin than soft or hard woods; 4) good growth or yield with irrigation water in warm dry areas; 5) its production costs that are half that of pulpwood and 6) the uses of kenaf in the newsprint industry will discourage the depletion of forests and importation of wood pulp from other countries to Nigeria (Adegbite et al., 2005).

2.5 Lignocellulose

Lignocellulose has great potential as a renewable feedstock or raw materials for production of high value biodegradable polymers and chemical via biological fermentation. The examples of lignocellulosic materials are wood agricultural crops, like jute or kenaf; agricultural residues, such as bagasse or corn stalks; grasses; and other plant substances (English et al., 1994). It represents the largest reservoir of organic carbon fixed by green plant and forms structural framework of the plant cell wall itself. The major component of lignocelluloses materials are polysaccharides (cellulose and hemicelluloses) and lignin.

Cellulose is the most abundant constituent and renewable biopolymers found in plant as micro fibrils (2 to 20 nm diameter and 100 to 40,000 nm long) such as wood (30-40%), paper, linen and cotton (over 90%) (Walford, 2008 and Nishiyama, 2009). According to Teter (2006) and Walford (2008), it is homo polysaccharide composed entirely of β-1,4-glucosidic linked glucose monomers and hydroglucopyronose polymer (six-carbon sugars). The molecular weight of different cellulose can range from 200 to 2000 kDa where the number of glucose residue can exceed 15, 000 per polymer molecule. Cellulose has such linear structure which enables the formation of extensive hydrogen bonding due to the aggregation of overlapping, staggered glucose flat sheets into water-insoluble crystalline fibrils which is highly intractable (Ding and Hemmel, 2006 ; Teter et al. 2006 ; Walford, 2008). Only agents that can attack the glycosidic linkages between glucose residues or which can disrupt the hydrogen bonding can solubilise cellulose. This is because of the combination of structure and hydrogen bonding give the cellulose a high tensile strength other than makes it resistance against microbial attack and insoluble in most solvent (Teter et al.,2006 and Walford 2008).

Figure 2.4: Partial structure of cellulose (Lee, 2003)

Next, hemicelluloses which are plant cell wall heteropolymeric sugars and sugar acids with a backbone of 1,4 linked β- D pyranosyls in which O4 is in equatorial orientation ( Teter et al., 2006). Hemicelluloses are usually shorter than cellulose typically containing a number of different sugars including both hexose (glucose, mannose and galactose) and pentoses (xylose and arabinose) since its major component is xyloglucan, a beta-(1→4) linked polymer of xylose with mono-, di-, or triglycosyl side chains, via O6 composed of variety substituent such as acetyl, arabinosyl or glucuronosyl units (Saha, 2003). Other than that, Teter ( 2006) pointed out that it is usually consist of fewer than 200 1,4 linkage, low degree of polymerization (typically below 200) , highly branched and easily hydrolyzed by acid or base. Furthermore, hemicellulose serves as the interface between cellulose and lignin in plant cell walls and may form covalent and non covalent linkages with other cell wall constituents such as pectin, glucans and proteins. It is classified according to the main sugar in polymer backbone; for example xylan and mannan (Sun et al., 2004 and Walford 2008).

Furthermore, lignin is a complex three dimensional polymer formed by carbon-carbon or ether bonds between phenylpropane units (Walford, 2008). It is a highly complex, amorphous and heterogenous comprising syringyl, guaiacyl and p- hydroxyphenol component which is embedded in the hemicellose and cellulose. This polymers is highly resistant to enzymatic, chemical, and microbial hydrolysis because of its extensive cross linking (Teter et al., 2006 and Walford 2008).

2.6 Pre-treatment

Lignocellulosic materials such as kenaf fibre contain many different components which include polysaccharides, protein, lignin, lipids and minerals. The major components are polysaccharides in the forms of cellulose (40 to 50%) and hemicellulose (25 to 30%) and, lignin (25 to 30%) (Teter et al., 2006). However, cellulose tends to form fibrils which are embedded in macromolecules of hemicelluloses and lignin thereby make it naturally resistant to breakdown to its structural sugars (Zahedifa ,1996 and Zhou, 1997).

Cellulase enzyme is commonly used to catalyze the lignocellulosic materials. However, the enzyme degradation rate of lignocelluloses materials which is low because of the resistant crystalline structure of cellulose and the physical barrier formed by lignin that surrounds the cellulose (Mc Millan ,1994; Mtui et al, 2009). Mtui (2009) claimed that pre-treatment process such hydrolysis is an essential prerequisite to disrupt the lignin seal and thereby enhancing the susceptibility of lignocellulosic materials to enzyme action. Efficient pre-treatment reduces the lignin content, cellulose crystallinity and increase the surface area for enzymatic reactions (Millett et al., 1975 and Mtui 2000).

The goal of the pre-treatment process is to remove lignin and hemicellulose, reduce the crystallinity of cellulose, and increase the porosity of the lignocellulosic materials. Pre-treatment must meet the following requirements:

(1) Improve the formation of sugars or the ability to subsequently form sugars by hydrolysis

(2) Avoid the degradation or loss of carbohydrate

(3) Avoid the formation of by products that are inhibitory to the subsequent hydrolysis and fermentation processes, and

(4) Be cost-effective

According to Kumar (2009), pre-treatment methods can be roughly divided into different categories: physical (milling and grinding), physicochemical (steam pre-treatment or auto hydrolysis, hydrothermolysis, and wet oxidation), chemical (alkali, dilute acid, concentrated acid, oxidizing agents, and organic solvents), biological, electrical, or a combination of these. The following pre-treatment technologies have promise for cost-effective pre-treatment of lignocellulosic biomass for biological conversion to chemical such lactic acid.

2.6.1 Physical pre-treatment

Physical pre-treatment can increase the accessible surface area and size of pores, and decrease the crystallinity and degrees of polymerization of cellulose. Different types of physical processes such as milling (example: ball milling, two-roll milling, hammer milling, colloid milling, and vibro energy milling) and irradiation (example: by gamma rays, electron beam or microwaves) can be used to improve the enzymatic hydrolysis or biodegradability of lignocellulosic waste materials. However, this method is far too expensive which involved the higher consumption of energy for size reduction of lignocellulosic materials (Kumar et al., 2009)

2.6.2 Physicochemical pre-treatment

Common physicochemical pre-treatment is steam explosion. The chipped biomass (reduced size of biomass usually 10 to 30 mm after chipping and 0.2 to 2 mm after milling or grinding) is treated with high-pressure saturated steam and then the pressure is rapidly reduced, which makes the materials to undergo an explosive decompression. Steam explosion (auto hydrolysis) is initiated at a temperature of 160 to 260 °C (corresponding pressure 0.69 to 4.83 MPa) for several seconds to a few minutes before the material is exposed to atmospheric pressure (Sun and Cheng, 2002). The factors that affect steam-explosion pre-treatment are residence time, temperature, chip size, and moisture content (Duff and Murray, 1996; Wright, 1998). Optimal hemicellulose solubilization and hydrolysis can be achieved by either high temperature and short residence time (270 °C, 1 min) or lower temperature and longer residence time (190 °C, 10 min) ( Duff and Murray 1996;Kumar et al., 2009). The advantages of steam-explosion pre-treatment include the low energy requirement compared to mechanical comminution and no recycling or environmental costs.

2.6.3 Chemical pre-treatment

a) Acid hydrolysis

Sulphuric acid (H2SO4) is commonly utilized as the chemical for lignocellulosic biomass hydrolysis such kenaf fibre. Based on the concentration of acid used, two types of hydrolysis, dilute and concentrated hydrolysis are used in laboratory research and pilot scale study. Kumar (2009) stated that dilute acid hydrolysis is conducted with acid concentrations of less than 2% at temperature between 160 to 25oC; for reaction time less than one hour. However, Wyman (1999) came out with a new idea to conduct dilute acid hydrolysis in 160oC for 20 min with 0.49% concentration of sulphuric acid when dealing with the corn stover yield pre-treatment. It is found that the dilute-acid hydrolysis process uses high temperatures (160 to 230 °C) and pressures (10 atm) (Broder et al., 1995; Patrick et al., 1997).

Taherzadeh and Karimi (2008) found that the dilute H2SO4 pre-treatment can achieve high reaction rates and significantly improve the cellulose hydrolysis. In addition, dilute acid effectively removes and recovers most of the hemicellulose as dissolved sugars, and glucose yields from cellulose increase with hemicellulose removal to almost 100% for complete hemicellulose hydrolysis (Chieffalo and Lightsey,1996). Hemicellulose is removed when H2SO4 is added and this enhances digestibility of cellulose in the residual solids (Mosier et al., 2005). High temperature in the dilute-acid treatment is favourable for cellulose hydrolysis (McMillan, 1994 and Kumar et al.,2009).

Concentrated acid hydrolysis is conducted with 35 to 60% H2SO4 in temperature between 20 to 100 oC for reaction times of 10 minutes to 6 hours (Zhou, 1997). Due to large quantities of acid used, a substantial fraction of these acid used must be recovered to achieve economic operation. Indeed, acid hydrolysis technologies lead to high operating costs and various environmental and corrosion problems.

b) Alkaline hydrolysis

Saponification of intermolecular ester bonds cross linking hemicellulose and other components is believed to be the mechanism of alkaline pre-treatment (Sun and Cheng, 2002; Wang et al., 2008). Main effect of alkaline pre-treatment is delignification- removal of structural polymer lignin of lignocellulosic biomass, thus enhancing the reactivity of the remaining carbohydrates. Alkaline pre-treatments also remove acetyl and different kinds of uronic acid substitutions on hemicellulose, which lowers the extent of enzymatic hydrolysis of cellulose and hemicellulose (Chang, 2000). Studies have been carried out using sodium hydroxide (NaOH) such as pre-treatment of raw jute fabric at 100 oC for 30 minutes (Mahalanabis et al., 2006), coastal bermudagrass at 121 oC for 90 minutes (Wang et al., 2008), dried ground cattail at room temperature for 24 hours (Zhang et al., 2010) and corn corb at room temperature for 2 hours (Ojumu et al., 2003). From the studies, sodium hydroxide is found effectively enhances lignocellulose digestibility by increasing internal surface area, decreasing the degree of polymerization and the crystallinity of cellulose, and separating structural linkages between lignin and carbohydrates (Fan et al., 1987). The digestibility of NaOH-treated hardwood increased with the decrease of lignin content (Millet et.al., 1976, and Bjerre et al., 1996). Dilute sodium hydroxide is usually used for alkali treatment (Fan et al., 1987).

2.6.4 Biological pre-treatment

Biological treatment involves the use of whole organisms or enzymes in pre-treatment of lignocellulosic materials. Commonly used microorganisms are fungi and bacteria. Fungal pre-treatment of agricultural residues is a new method for improvement of digestibility (Taniguchi et al., 2005). White-, brown- and soft-rot fungi are used to degrade lignin and hemicellulose in waste materials whereby brown rots mainly attack cellulose, while white and soft rots attack both cellulose and lignin. White-rot fungi are the most effective basidiomycetes for biological pre-treatment of lignocellulosic materials (Sun and Cheng ,2002 ; Mtui 2009). Phanerochaete chrysosporium, a species of white rot fungi produces both lignin peroxidases and manganese-dependent peroxidases for lignin degradation (Waldner, 1988 and Boominathan, 1992). Polyphenol oxidases, laccases, H2O2 producing enzymes and quinosine-reducing enzymes also degrade lignin (Blanchette, 1991). Biological treatment requires low energy and normal environmental conditions but the hydrolysis yield is low and requires long treatment times.

2.6.5 Enzymatic hydrolysis

Enzymatic hydrolysis is commonly done in order to enhance the chemical pre-treatment using acid or alkaline. The enzymatic approach in which hydrolyzing cellulose to glucose is promising because enzymes can achieve high yields and do not catalyze glucose degradation reactions common to dilute acid process (Schell et al., 1992). However, the cellulose must be accessible to enzymatic attack, which depends on the severity of the pre-treatment process. A greater degree of hemicelluloses and or lignin removal during pre-treatment increases the accessibility of cellulose, thus the efficacy of enzymatic cellulose hydrolysis also increases (Taherzadeh and Karimi, 2008). The reducing sugars are the products of the enzymatic hydrolysis that is conducted at mild conditions (pH 4.8 and temperature 45 to 50 °C) and due to that condition it does not caused a corrosion problem (Duff and Murray, 1996).

2.7 Optimization of process condition

Since lignocellulosic materials such kenaf fibre is complex in structure. It needs to undergo the pre-treatment process in order to liberate the sugar (glucose) before the fermentation to lactic acid can be done. The optimization is the crucial part in the experiment. This is because the more liberated sugar that results from hydrolysis process, the more lactic acid can be produced. Several aspects need to be considered in order to optimize the hydrolysis processes which are time, temperature and the mass of raw material.

Temperature

To optimize on lignocellulosic materials, temperature is needed to take into consideration. From known pre-treatment temperature, the suitable optimization condition can be determined. Generally, the temperature used to conduct the pre-treatment of lignocelluloses materials is approximately from the range of 120 to 240 o C. However, many studies of pre-treatment is carried out in the temperature 180 to 210 o C (Torget et al., 2000; Ahring et al., 2003). O'Connor (2009) argued that if the temperature is above 220 C, the side reactions (regardless pH) of the pretreatment become so fast and overall process is difficult to control. Examples of side reactions are lignin polymerization and precipitation, degradation of sugar, complex formation between lignin and other component in the solution.

2.7.2 Time

O'Connor (2009) stated in his patent document (US20090176286) that the suitable time for pre-treatment of lignocelluloses material is between about 1 minute and about 24 hours, preferably between about 5 minutes and about 2 hours and more preferably between about 10 minutes and about 1 hour. Generally, the time and temperature is inversely proportion. The increase the temperature of pre-treatment, the decrease or less time needed.

Mass of raw material

Mass of the raw material also needed to take into account for the optimization of pre-treatment process' purposes. According to Gao (2008) and Lu ( 2009), the ratio of raw material to solvent used is 3:10 (w/w). However, some researcher claimed that they used 30g/L of raw materials for the pretreatment process ((Wee and Ryu, 2009). Some of them used 250g/L of raw materials for the process (Mahalanabis et al.,2006) . the raw material used is not specific into one condition. It can be varied depends on the time and temperature of the process.

2.8 Fermentation process

Lactic acid is naturally occurring organic acid that can be produced chemical synthesis and biological fermentation. The ultimate objective of lactic acid production is to produce it in a process that is more effective and economical ( Rojan et al., 2007). Between both processes, biological conversion has an important role in waste utilization, and it is likely that various foods processing waste may contain useful substrates which can be used for lactic acid production. Other than that, biological fermentation offer low cost of substrates, low production temperature, and low energy consumption (Pandey et al., 2001)

2.8.1 Chemical synthesis

Chemical synthesis of lactic acid is mainly based on the lactonitrile by strong acids which provide only racemic mixture of D- and L- lactic acid ( Rojan et al., 2007). Li and Cui (2010) explained, for chemical synthesis, acetaldehyde and hydrogen cyanide are reacted in the presence of base under high pressure to produce lactonitirile. They also claimed that purification of crude lactonitrile is done using distillation. The purified lactonitrile is then hydrolyzed with sulphuric acid to produce lactic acid. A byproduct of ammonium salt is also produced (Narayanan et al., 2004; Wee et al., 2006; Li and Cui, 2010). Other possible chemical synthesis routes for lactic acid include base-catalyzed degradation of sugars, oxidation of propylene glycol, reaction of acetaldehyde, carbon monoxide and water at elevated temperatures and pressures, hydrolysis of chloropionic acid and nitric acid oxidation of propylene among others (Datta et al., 1995). There is no chemical synthesis route of lactic acid lead to feasibility of economy.

2.8.2 Biological fermentation

Biological fermentation or known as microbial fermentation is a process which involved the catalyse of chemical reaction by microorganism to break simple sugars or amino acids into lower molecular weight materials such as organic acids and neutral solvents. Lactic acid was first produced commercially via fermentation in the United States in 1881 (Zhou, 1997 and Jim et al., 2010). Usually, three forms of lactic acid, L (+), D(-) and inactive racemic D L mixtures are produced by different microorganisms . Microorganisms contain enzyme(s) L (+)-lactate dehydrogenase (EC 1.1.1.27) , D(-)-lactate dehydrogenase (EC 1.1.1.28) or racemase produced different isomers of lactic acid conversion from pyruvate (Jim et al., 2010) . In order to discover the new application of lactic acid for the biodegradable polymers which is polylactic Acid (PLA) industry, optically pure lactid acid (L(+)) is highly preferred optical isomer (Zhou, 1997).

Figure 2.5 : Overview of the two manufacturing methods of lactic acid; chemical synthesis (a) and microbial fermentation (b).(Yong et al., 2007).

2.8.3 Microbial sources for lactic acid

Lactic acid can be produced by an enormous variety of bacteria, yeasts, and fungi. On the basis of the nature of fermentation, lactic acid microorganism which is commonly bacteria is classified into (1) homofermentative and (2) heterofermentative. Homofermentative lactic acid bacteria produce lactic acid as a sole end product- single product, whereas the hetero fermentative lactic acid bacteria produce other products such as ethanol, diacetyl, formate, acetoin or acetic acid and carbon dioxide along with lactic acid (Rojan et al., 2007; Zhou, 1997; Li and Cui, 2010; Jim et al., 2010). The desirable characteristics of industrial microorganisms are their ability to rapidly and completely ferment cheap raw materials, requiring minimal amount of nitrogenous substances, providing high yields of preferred stereo specific lactic acid under conditions of low pH and high temperature, production of low amounts of cell mass and negligible amounts of other byproducts (Narayanan et al.,2004). The choice of an organism primarily depends on the carbohydrate to be fermented.

Most of microbial sources for lactic acid are anaerobic such as Lactobacillus, Leuconostoc, Pediococcus and Bifidobacterium, utilized pyruvate which is the end product of Embden-Meyerhof pathway. Narayanan (2004) said that Lactobacillus is found to be the most important commercial species for lactic acid production by fermentation. It is gram positive facultative anaerobic and microphilic bacteria which has complex nutritional requirements, as they are those groups of microorganisms that have lost their ability to synthesize their own growth factors (Beasley, 2004). They cannot grow solely on carbon source and inorganic nitrogen salts. Lactobacillus is very heterogeneous genus, encompassing species with a large variety of phenotypic, biochemical, and physiological properties ( Todar, 2009).

Most species of Lactobacilli are homofermentative, but some are heterofermentative. The genus has been divided into three major subgroups and over 70 species are recognized (Hans et. al., 2002). Group I Lactobacilli are obligate homo fermentative and produce lactic acid as a major end product (>85%) from glucose. They are represented by L. delbrueckii and L. acidophilus. They grow at 45oC but not at 15oC. Group II, also homofermentative, grow at 15oC and show variable growth at 45oC. Represented by L. casei and L. plantarum, they can produce more oxidized fermentations (e.g. acetate) if O2 is present. Group III Lactobacilli are heterofermentative. They produce lactic acid from glucose, along with CO2 and ethanol (Hammes and Whiley, 1993).

Furthermore, the sources for production of lactic acid can also be aerobic microorganisms such fungi - Rhizopus and Mucor (Naveena, 2004). When fungus such as Rhizopus is used, the aerobic fermentation requires significant agitation and aeration with high energy cost and long fermentation time due to its slow growth and production rates (Jim et al., 2010). Even though, Rhizopus promised a colourless and relatively high purity of lactic acid but still it has not been used commercially for lactic acid production (Zhou, 1997). However, about 90% of the literature of lactic acid production is carried out using anaerobic bacteria since it offered a robust, fast growing, low pH, high yield strain with low cost nutrient requirement and low consumption of energy for the fermentation.

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