acid production process.
In all scenarios, microorganisms produce an aqueous lactic acid solution, comprising mainly lactate and counterions from the base, impurities from raw materials or fermentation by‐products, residual sugars and polysaccharides, and the microorganism itself.
1.2.5 Downstream Processing/Purificationof Lactic Acid
When Scheele discovered lactic acid, he recovered and purified the lactic acid from sour whey by saturation with lime, filtering off the crude calcium lactate, acidifying the crystal mass with “acid of sugar” (oxalic acid), filtering off the calcium oxalate, and evaporating to obtain a crude viscous lactic acid [12, 13]. Basically, this process with a calcium‐based neutralized fermentation and sulfuric acid instead of oxalic acid is the same process used in industry today for the production of crude lactic acid. Drawbacks are the continuously rising costs of lime/chalk, sulfuric acid, and other chemicals and the disposal of large quantities of gypsum (CaSO4·2H2O), as an unavoidable side product of this technology.
In such a process also the first downstream processing (DSP) step, biomass removal by filtration, can be accomplished relatively easily in a (mild) liming step, in essence quite similar to the traditional liming step to remove protein in sugar beet or sugarcane processing in sugar mills. A simplified block scheme of the traditional lactic acid production process including fermentation is shown in Figure 1.5.
TABLE 1.2 Summary of Lactic Acid Purification Methods
| Lactic Acid Purification Method | Advantages | Disadvantages |
|---|---|---|
| Crystallization [27, 28] | Highly pure lactic acid product | Amount of mother liquor by‐product, scalability |
| Esterification/distillation [52] | Highly pure acid, scale‐up | Relatively high utility cost, amount of residue as by‐product |
| Lactic acid distillation [27, 28, 53] | Good splitting for heavy compounds | Amount of residue as by‐product |
| Extraction [54, 55] | Potentially high yield | Complex (e.g., for emulsion, entrainment issues), extractant cost |
1.2.5.1 Purification Methods for Lactic Acid
Crude lactic acid, which may be upgraded by simple active carbon treatment and/or ion exchange to remove impurities and salts, can be directly used in a large number of food applications. Traditionally, taste, smell, and heat stability for color formation have been used to express lactic acid quality. The presence of acids (e.g., acetic acid and pyruvic acid), alcohols (e.g., methanol and ethanol), and esters can directly influence taste and smell [4]. The presence of residual sugar and nitrogen compounds greatly influences heated color, that is, browning of the liquid upon heating. The formation of color upon heating prohibits the use of crude acid in foods that need to undergo pasteurization/sterilization. Over the decades, the demand for purer lactic acid with improved color stability upon heating has increased, as exemplified by the need for ultrapure lactic acid as a sodium lactate base in pharmaceutical infusion products. At present, a chemical engineer can choose from a number of mature industrial methods to purify lactic acid. Table 1.2 lists their relative advantages and disadvantages.
Choices in an overall process are governed by raw material costs, utility costs, and, last but not least, outlets for by‐products.
The purification methods described above each involve considerable technological know‐how:
Esterification/Saponification. Esterification of lactic acid with methanol/ethanol yields systems with good separation characteristics to separate many impurities with different boiling points [60]. However, the energy demand of a full reaction/distillation route from crude acid to pure acid is high.
Crystallization. Crystallization can yield an excellent lactic acid grade, but the yield is low.
Lactic Acid Distillation. Industrial equipment is available to distill lactic acid at low vacuum. Higher‐molecular‐weight components such as sugar and protein will leave the system as a residue. Heat‐stable lactic acid is obtained as the top product. In the stages of dewatering the crude lactic acid prior to distillation, the formation of oligomers will limit an overall high distillation yield.
Extraction. An extraction/back‐extraction process, for example, with the well‐described tertiary amine systems, is a suitable way to purify lactic acid [61, 62]. The possible combination of extraction with low‐pH fermentation yields an elegant concept to arrive at a gypsum‐free process.
For future large‐scale, low‐cost lactide/PLA production, lactic acid DSP will need to meet new challenges:
Use of Low‐Cost and Nonedible Substrates. Whereas production of lactic acid from sucrose or glucose syrup is well established, crude sources (starches, sugars, or future lignocellulose hydrolysates) will form the next hurdle as they contain much more impurities and possible fermentation inhibitors.
Gypsum‐Free Processing. For large‐scale, sustainable PLA production, a fermentation process that does not coproduce a mineral salt is a must.
1.2.5.2 Gypsum‐Free Lactic Acid Production
Gypsum‐free lactic acid production can be briefly categorized as follows:
Low‐pH Fermentations Coupled to In Situ Product Removal. As discussed in Section 1.2.4, fermentations can be carried out without neutralization at pH 2–3 with genetically modified yeast or at pH 4 with LAB with partial neutralization [50]. When a separation method to recover the undissociated acid is integrated with fermentation, a process route can be designed in which no gypsum is produced. In the literature, a number of separation methods are described with an emphasis on extraction [63]. Cost efficiency in the fermentation (e.g., nutrients, yield) and the practical processing of large dilute streams need breakthroughs for economical processing.
Electrochemical Splitting of a Neutral Lactate Salt. Numerous articles have described the splitting of a lactate salt, notably sodium lactate, into lactic acid and the original base [64]. With this principle, a gypsum‐free process can be designed, with electrodialysis separate from or integrated with fermentation. The use of electrodialysis with new bipolar membranes is straightforward, but a large‐scale commercial breakthrough as in the 1980s and 1990s with monopolar membranes for the chloro‐alkali process is still pending. Electrodialysis involves relatively high electricity costs and a huge membrane area, but these costs may be managed in biorefinery concepts with integrated energy production.
Chemical Salt Splitting of a Lactate Salt. Lactate salts can be split with the help of auxiliary chemicals and the regeneration of these chemicals. A patent by Baniel et al., for example, describes a method in which a sodium lactate solution is
