Conversion of agricultural crop residues and agro-industrial waste streams into lactic acid is a process with dual beneficial effects. On one hand agricultural waste poses an environmental threat and on the other hand represents an abundant renewable raw material for several value-added products e.g. bio-gas, ethanol, fibers and different organic acids such as lactic acid. Lactic acid is an important chemical used in a variety of industrial and food applications, but its main potential is as precursor for manufacture of the biodegradable polymer, poly(lactic acid) (PLA). PLA is a bio-based and biodegradable polyester that can replace traditional plastics synthesized from fossil hydrocarbon resources in many industrial applications including packaging of foods, and hence, solve many waste problems related to packaging materials.
The art of producing lactic acid by fermentation has received much attention in the past decade and judging by the number of resent patents and scientific articles research has been especially tense in the past 3-5 years. There are several vital factors affecting large scale fermentative production of lactic acid. The choice of substrate, lactic acid bacteria species, operating conditions and separation processes are a few main factors of importance for commercial production.
Following fermentation, lactic acid must be recovered and purified. One major hurdle in fermentative production of lactic acid from agricultural waste is finding an economical competitive recovery process that honors the demand for high product purity.
4.1.2 Lactic acid
2-hydroxypropanoic acid better known as lactic acid is a chiral molecule with the two optically active forms, L(+)-lactic acid and D(-)-lactic acid, as shown in Figure 4.1. Lactic acid is the simplest and most widely occurring hydroxyl-carboxylic acid and is found in most living organisms from anaerobic prokaryotes to humans (Datta and Glassner 1990).
OH H O
H
CH3 O
L(+)-laclic acid
OH OH H
CH3 O
D(-)-laclic acid
Figure 4.1 The two optically active forms of lactic acid.
In pure anhydrous form lactic acid is a white crystalline solid with a melting point of 52.7-52.8°C for either pure isomer and 17-33°C for a racemic mixture. The pure form is quite difficult to obtain and for industrial purposes, lactic acid is handled in concentrated aqueous solutions. The consistence is syrupy and depending on how it is produced, it can be colorless or slightly yellow.
Lactic acid is a mono-carboxylic acid with a pKa-value of 3.88 at 25°C, resulting in a dissociation curve as shown on Figure 4.2, where the degree of dissociation is plotted as function of pH. It can be seen that below pH 2 virtually all of the acid is undissociated and above pH 6, lactic acid is almost totally dissociated.
0,10 0,2 0,3 0,40,5 0,6 0,7 0,80,91
0 1 2 3 4 5 6 7
pH
Degree of dissociation
Figure 4.2 Degree of dissociation of lactic acid as function of pH.
As a bulk chemical lactic acid is available in technical, food and pharmaceutical grade at concentrations of typically 50, 80 or 88%. Pharmaceutical grade lactic acid satisfies United States Pharmacopoeia (USP) or European Pharmacopoeia (Ph. Eur.), which dictate the strictest requirements regarding the purity. The food grade must live up to the requirements of the E270 number in Europe, but no particular restrictions apply to technical grade lactic acid, other than those set by buyers.
4.1.3 Lactic acid fermentation
There are two main routes for producing lactic acid; synthetically, by the hydrolysis of lactonitrile or through fermentation of carbohydrates by action of lactic acid bacteria (LAB). The predominant part of the lactic acid production is achieved by carbohydrate fermentation, which is also the preferred technology when it comes to meeting future demands for lactic acid.
The conventional method for producing lactic acid by fermentation consists of 4-6 days batch fermentation with pH-control by addition of calcium carbonate. Lactic acid concentration reaches around 10 wt% (Datta and Glassner 1990). The broth is then filtered to remove biomass, treated in carbon columns, evaporated and acidified using sulfuric acid to convert the calcium lactate to lactic acid and insoluble calcium sulfate (gypsum). There are several drawbacks to this conventional process. In the carbon columns waste are generated and active carbon must frequently be replaced.
More importantly, every ton of lactic acid produced also generates one ton of gypsum is as waste.
value. Furthermore, the batch process can be subject to both strong product and substrate inhibition that limits performance.
The above mentioned shortcomings can be circumvented by working with a continuous system (Ohashi et al. 1999;Tejayadi and Cheryan 1995;Zayed and Winter 1995) providing means for continuous removal of lactic acid and recycling or immobilization of cells, hereby increasing productivity.
4.1.4 Organic acid extraction and purification
Extracting and purifying lactic acid from fermentation broth include separating the acid from the LAB cells, the many proteins and amino acids, the inorganic ions, the residual, unconverted carbohydrates (sugars) and other organic acids produced during the fermentation. Several other research groups worldwide have studied this field since Hongo (Hongo et al. 1986) in 1986 was among the first to suggest electrodialysis as a possibility for a continuous recovery of organic acids from fermentation broth. Numerous problems associated with this recovery method have been pointed out during the last couple of decades, and numerous solutions have been suggested to overcome these problems.
Figure 4.3 Principle design of continuous fermentation with constant organic acid (product) removal and pH-control.
One of the most significant obstacles when trying to extract organic acids directly from a fermentation broth through electrodialysis is membrane fouling. The biological material from the fermentation broth tends to adsorb on to ion-exchange membranes since most of the biological proteins and cells have some kind of locational charged groups, by which they are attracted to the surface of oppositely charged membranes.
Another problem that arises when purifying organic acids through electrodialysis with bipolar membranes is scaling. Divalent cations like calcium and magnesium that passes through cation-exchange membranes into an alkaline solution, immediately precipitates on the membrane surface as hydroxide salts (Ca(OH)2, Mg(OH)2). This scaling affects the process fast by reducing effective membrane area, and increasing electrical resistance.
Several pretreatments to extraction of organic acids from fermentation broth have been suggested, investigated and patented:
Microfiltration is commonly used for filtration of fermentation broth (Borgardts et al.
1998b;Boyaval et al. 1996;Kulozik 1998). Microfiltration retains cells but allows proteins to
permeate through the membrane along with the organic acids, which presents a fouling problem for a subsequent electrodialysis processing.
Ultrafiltration have received much attention because this process retain most fouling bio-matter, including cells and proteins, allowing only small molecules like inorganic salts, organic acids, peptides and amino acids to pass. Ultrafiltration is also subject to fouling, but module designs employing back-flushing or back-shock techniques or where the flow creates high surface shear reduce the impact of bio-matter build-up considerably. Ultrafiltration of fermentation broth is well known (Van Nispen et al. 1991) and reduces bio-matter content sufficiently for subsequent electrodialysis processing. Unfortunately, ultrafiltration allows inorganic cations like calcium and magnesium to pass, and these ions present severe risk for membrane scaling in later steps involving electrodialysis.
Nanofiltration and reverse osmosis has received attention as this filtration method is able to retain most calcium and magnesium. Shortcomings of nanofiltration include fouling and low fluxes for this operation.
The type of unit operations needed for further purification strongly dependents on which kind of impurities the initial feedstock contains, but many processes including electrodialysis (Boniardi et al. 1996;Boniardi et al. 1997a;Boniardi et al. 1997b;Borgardts et al. 1998a;Borgardts et al.
1998b;Czytko et al. 1987;de Raucourt et al. 1989;Hongo et al. 1986;Lee et al. 1998;Mani and Hadden 1998;Miao 1997;Nomura et al. 1987;Van Nispen et al. 1991;Vonktaveesuk et al. 1994) and reverse osmosis (Liew et al. 1995;Timmer et al. 1993;Timmer et al. 1994), nanofiltration (Jeantet et al. 1996;Mani and Hadden 1998;Miao 1997;Timmer et al. 1993;Timmer et al. 1994), diffusion dialysis (Portner and Markl 2001), Donnan dialysis (Zheleznov et al. 1998), ion exchange (Beschkov et al. 1995;Mani and Hadden 1998;Miao 1997;Rincon et al. 1997), carbon adsorption, extraction (Kleerebezem et al. 2000;Scholler et al. 1993;Siebold et al. 1995), evaporation and recently esterification combined with pervaporation/distillation and hydrolysis of the ester (Eyal et al. 1998a) has been suggested.
A common factor for the separation processes mentioned above is the fact that no single process is able to meet all the demands listed previously. By combining several of the processes, it is possible to meet requirements but each additional process adds to the operation costs. For commercial production of lactic acid as food additive or poly(lactic acid), it is necessary to keep production costs at a competitive level. This can only be achieved by keeping the separation steps at a minimum, and each necessary step must be operated at high efficiency.
Several process combinations have been patented (Borgardts et al. 1998a;Cockrem and Johnson Pride 1993;Datta and Glassner 1990;Datta and Tsai 1998;Eyal et al. 1998a;Soine 1998) and a few have been tested commercially, but none have reached a commercial break-through yet.