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Lactic acid is a hydroxycarboxylic acid, which is widely used in food, pharmaceutical, leather, cosmetic and textile industries. It can be polymerized to biodegradable and biocompatible plastic, i.e. polylactic acid, which has environment-friendly and great potential for replacing petrochemical plastic. Industrially, it can be produced by either chemical synthesis or microbial fermentation. Presently, approximately 90% of lactic acid was produced by lactic acid bacteria fermentation. Fermentative production has the advantage that by choosing a strain of LA bacteria producing only one of the isomer and optically pure product. The widely used substrates for lactic acid production are pure sugar, which are expensive. It is also possible to use lignocellulosic biomass, especially by product or waste materials from agriculture and industrial waste as substrates for fermentation. Therefore, the use of alternative substrates, low-cost and raw materials become thus of special interest for lactic acid production. The present study the utilization of industrial wastes and lignocellulosic as an alternative substrate for lactic acid production.
Lactic acid (2-hydroxy propionic acid) is a chemical compound that a role in several biochemical processes. It is a carboxylic acid with a chemical formula of C3H6O3. Lactic acid is chiral and has two optical isomers as shown in figure 1.
Figure 1 L-lactic acid and D-lactic acid
Source: Nexant (2008)
Lactic acid is organic solvent that can dissolve in water but insoluble in other organic solvents. Other properties of lactic acid are presented in Table 1.
Table 1. Physical properties of lactic acid.
L : 53 °C
D : 53 °C
D/L : 16.8 °C
82 °C at 0.5 mm Hg
122 °C at 14 mm Hg
Dissociation constant, Ka at 25 ° C
1.37 x 10-4
Heat of combustion, Hc
Specific Heat, Cp at 20 ° C
190 J/mole/ °C
Source: Vickroy (1985)
Lactic acid can be produced by chemical synthesis or microbial fermentation. The chemical synthesis produced a racemic mixture of D, L lactic acid. Presently, about 90% of lactic acid made by LAB fermentation and the remainder is produced synthetically by the hydrolysis of lactonitrile. The advantage of fermentation technologies is possible to use renewable resources as substrates, such as starch and cellulose in fermentative production. In figure 2 described the diagram of commercial uses and applications of lactic acid. Food applications reported for approximately 85% of the total lactic acid, while the nonfood industrial applications reported for only 15% of the demand.
Figure 2 Diagram of commercial uses and applications of lactic acid
Source: Wee et al. (2006)
Alternative substrate for lactic acid production
In fermentation process, lactic acid bacteria (LAB) need carbon source, essentially simple sugars and nitrogen source, as vital nutrients for their growths. The widely used substrates for lactic acid production are refined sugar, which are expensive. Therefore, attention has turned towards lignocellulosic biomass and industrial wastes to provide a source of carbohydrate for lactic acid production.
Lignocellulosic resources are generally considered to represent an interesting and inexpensive raw material for microbial fermentation of lactic acid production, as they are renewable and cheap. A schematic diagram of the procedures for the preparation of lignocellulosic hydrolyzates is provided in figure 3.
Figure 3 Schematic diagrams of the procedure for the preparation of lignocellulosic hydrolyzates.
Source: Wee and Ryu (2009)
Wee and Ryu (2009) reported the production of lactic acid from lignocellulosic, glucose and lignocellulosic hydrolyzates were used as the carbon source. The concentration of lactic acid decreases with increases in the dilution rate. Generally, the cell concentration obtained from lignocellulosic hydrolyzates media was approximately 10-15% lower than observed with glucose media. The lactic acid yields were provided at more than 0.90 gg-1 the result are shown in table 2.
Table 2 Lactic acid yield and substrate conversion at different initial substrate concentration and dilution rate
Source: Wee and Ryu (2009)
The lignocellulosic hydrolyzates have to be detoxified in order to reduce these inhibitory effects prior to fermentation, as some of the by-products released during the pretreatment (Mussatto and Roberto, 2004). Ruengruglikit and Hang (2003), reported the lactic acid production from lignocellulosic materials by R. oryzae. After an enzymatic hydrolysis and 48-h fermentation, lactic acid yields of 0.3 gg-1.
Food wastes, which are supplied at lower costs, high in moisture and rich in carbohydrates. They could be suitable alternative substrate for lactic acid production. The food wastes contain polysaccharides as well as various oligosaccharides.
Ohkouchi and Inoue (2005) reported the production of lactic acid from food wastes. The composition of food wastes is shown in Table 3. The optimum pH for L (+)-lactic acid production by L. manihotivorans LMG 18011 was 5.0 and under these condition the L (+)-lactic acid was 19.5 g produced from 200 g food wastes
Table 3 The composition of food wastes
Source: Ohkouchi and Inoue (2005)
Kim et al. (2003) determined lactic acid production from food wastes by simultaneous saccharification fermentation technical. The effect of food wastes concentration on lactic acid production as shown in figure 4.
Figure 4 Effect of food wastes concentration on lactic acid production yield in the SSF
Source: Kim et al. (2003)
In figure 4, the highest yield was obtained from 65 g/L of food waste with a final lactic acid concentration of 44.3 g/L, while the highest lactic acid concentration of 79.7 g/L was obtained from 145 g/L of food waste.
Most of the work using starch, beet molasses and sugar cane as the fermentation media for lactic acid production. Recently, Uno (2003) used grape invertase to attractive the production of lactic acid utilizing pineapple syrup as substrate. The efficiency of lactic acid production has been an affect from various aspect and ability to use fruit waste.
Idris and Suzana (2005) reported the liquid pineapple waste, it is feasible to use produce lactic acid by immobilized L. delbrueckii. The maximum lactic acid can be obtained when parameters initial pH of 6.5, temperature of 37 ° C and sodium alginate concentration at 2% with a bead diameter of 1 mm in diameter. The highest values of kinetic parameters are obtained at 37 ° C and initial pH 6.5 (Table 4 and Table 5).
Table 4 Effect of pH on kinetic parameter
Source: Idris and Suzana (2005)
Table 5 Effect of temperature on kinetic parameter
Source: Idris and Suzana (2005)
Recycled paper sludge
Recycled paper sludge is an industrial waste has high polysaccharides (mainly cellulose) content. The conversion of polysaccharides on sludge to be broken down into the monomers and released sugars to be fermented to lactic acid. The methods for conversion of a polysaccharide into the monomer consist of enzymatic and acid hydrolysis.
Marques et al. (2008) studied the utilization of recycled paper sludge as an alternative substrate for lactic acid product. The maximum production of lactic acid was produced 73 g/L of lactic acid, maximum productivity of 2.9 g/L/h, with 0.97 g LA per g of carbohydrates on initial substrate. The fermentative parameters concerned the lactic acid production of all the cultivations in this work are presented in table 6.
Table 6 Final product concentration, productivity and yields obtained for lactic acid production in the different experiment
Source: Marques et al. (2008)
The widely used substrates for lactic acid production are refined sugar, which are expensive. It is also possible to use lignocellulosic biomass, especially by product or waste materials from agriculture and industrial waste as substrates for fermentation. Therefore, the use of alternative, low-cost and raw materials become thus of special interest for lactic acid production. The ability to utilize this industrial wastes and lignocellulosic as alternative carbon sources for lactic acid production will help reduce of environmental pollution problem and also reduce production costs.
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