The major use of lactic acid is in food and food-related applications, which, in the U. S., accounts for approximately 85% of the demand. The rest (-15%) of the uses are for nonfood industrial applications. As a food acidulant, lactic acid has a mild acidic taste in contrast to other food acids. Lactic acid is nonvolatile, odorless, and is classified as GRAS (generally recognized as safe) for use as a general purpose food additive by the FDA in the U. S. and other regulatory agencies elsewhere. It is a very good preservative and pickling agent for sauerkraut, olives, and pickled vegetables. It is used as acidulant/flavoring/pH buffering agent or inhibitor of bacterial spoilage in a wide variety of processed foods, such as candy, breads and bakery products, soft drinks, soups, sherbets, dairy products, beer, jams and jellies, mayonnaise, and processed eggs — often in conjunction with other acidulants (6). An emeiging new use for lactic acid or its salts is in the disinfection and packaging of carcasses, particularly those of poultry and fish, where the addition of aqueous solutions of lactic acid and its salts during the processing increased shelf life and reduced the growth of anaerobic spoilage organisms such as Clostridium botulinum (7-8).

A large fraction (> 50%) of the lactic acid for food-related uses goes to produce emulsifying agents used in foods — particularly for bakery goods. These emulsifying agents are esters of lactate salts with longer chain fatty acids, and the four important products are calcium and sodium, stearoyl-2-lactylate, glyceryl lactostearate, and glyceryl lactopalmitate. Of the stearoyl lactylates, the calcium salt is a very good dough conditioner, and the sodium salt is both a conditioner and an emulsifier for yeast-leavened bakery products. The glycerates and palmitates are used in prepared cake mixes, other bakery products, and in liquid shortenings. In prepared cake mixes, the palmitate improves cake texture, whereas the stearate increases cake volume and permits mixing tolerances (6). The manufacture of these emulsifiers requires heat-stable lactic acid — hence, only the synthetic or the heat-stable fermentation grades are used for this application.

Technical-grade lactic acid has long been in use in the leather tanning industry as an acidulant for deliming hides and in vegetable tanning. In various textile finishing operations and acid dying of wool, technical-grade lactic acid was used extensively. Cheaper inorganic acids are now more commonly used in these applications. The future availability of lower cost lactic acid and the increasing environmental restrictions on waste salt disposal may reopen these markets for lactic acid.

Lactic acid is currently used in a wide variety of small-scale, specialized industrial applications where the functional specialty of the molecule is desirable. Some examples are pH adjustment of hardening baths for cellophane that is used in food packaging, terminating agent for phenol-formaldehyde resins, alkyd resin modifier, solder flux, lithographic and textile printing developers, adhesive formulations, electroplating and electropolishing baths, detergent builders (with maleic anhydride to form carboxymethoxysuccinic acid-type compounds). Because of the current high cost and low volume of production, these applications account for only 5-10% of the consumption of lactic acid (6, 9).

Lactic acid and ethyl lactate have long been used in pharmaceutical and cosmetic applications and formulations, particularly in topical ointments, lotions, parenteral solutions, and biodegradable polymers for medical applications (such as surgical sutures, controlled-release drugs, and prostheses). A substantial part of pharmaceutical lactic acid is used as the sodium salt for parenteral and dialysis applications. The calcium salt is widely used for calcium-deficiency therapy and as

an effective anti-caries agent. As humectants in cosmetic applications, the lactates are often superior to natural products and more effective than polyols (6, 9). Ethyl lactate is the active ingredient in many anti-acne preparations. The use of the chirality of lactic acid for synthesis of drugs and agrichemicals is an opportunity for new applications for optically active lactic acid or its esters. The chiral synthesis routes to R (+) phenoxypropionic acid and its derivatives using S (-) lactate ester as a chiral synthon has been described (6). These compounds are used in herbicide production. Another use as an optically active liquid crystal whereby lactic acid is used as a chiral synthon has been recently described (10). These advances could open new small-volume specialty chemical opportunities for optically active lactic acid and its derivatives.

Lactic acid can be manufactured by either (1) chemical synthesis or (2) carbohydrate fermentation — both are used for commercial production. In the U. S., lactic acid is manufactured synthetically by means of the lactonitrile route by Sterling Chemicals, Inc. In Japan, Musashino Chemical Co. used this technology for all of Japan’s production. CCA Biochemical b. v. of the Netherlands uses carbohydrate fermentation technology in plants in Europe and Brazil and markets worldwide. Prior to 1991, the annual U. S. consumption of lactic acid was estimated at 18,500 metric tonnes, with domestic production of approximately 8,600 tonnes, by Sterling Chemical and the rest imported from Europe and Brazil. The worldwide consumption was estimated at approximately 40,000 tonnes/yr.

(1) Chemical Synthesis. The chemical-synthesis routes produce only the racemic lactic acid. The commercial process is based on lactonitrile, which used to be a by-product from acrylonitrile synthesis. It involves base catalyzed addition of hydrogen cyanide to acetaldehyde to produce lactonitrile. This is a liquid-phase reaction and occurs at atmospheric pressures. The crude lactonitrile is then recovered and purified by distillation and is hydrolyzed to lactic acid by using either concentrated hydrochloric or sulfuric acid, producing the corresponding ammonium salt as a by-product. This crude lactic acid is esterified with methanol, producing (1) methyl lactate, which is recovered and purified by distillation and hydrolyzed by water under acid catalysts to produce lactic acid, which is further concentrated, purified, and shipped under different product classifications, and (2) methanol, which is recycled (equations 1-3).

СНз CHO + HCN catalyst > CH3 CHO HCN (1)

CH3 CHO HCN + H20 + і H2 S04 ->• CH3 CHOH COOH + і (NH4>2 SO4 (2) CH3 CHOH COOH + CH3 OH -> CH3 CHOH COOCH31 + h2o

CH3 CHOH COO CH3 + H2O -> CH3 CHOH COOH + CH3OH t (3)

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 chloropropionic acid (prepared by chlorination of propionic acid), and nitric acid oxidation of propylene, among others. None of these routes have led to technically and economically viable processes (9, 11).

(2) Carbohydrate Fermentation. The fermentation technology can make a desired stereoisomer of lactic acid. The existing commercial production processes use homolactic organisms, such as Lactobacillus delbrueckii, L. bulgaricus,

L. leichmanii. A wide variety of carbohydrate sources can be used (molasses, com syrup, whey, dextrose, cane, or beet sugar). The use of a specific carbohydrate feedstock depends on its price, availability, and purity. Proteinaceous and other complex nutrients required by the organisms are provided by com steep liquor, yeast extract, and soy hydrolysate, for example. Excess calcium carbonate is added to the fermenters to neutralize the acid produced and produce a calcium salt of the acid in the broth. The fermentation is conducted as a batch process, requiring 4 to 6 days to complete. Lactate yields of approximately 90% (w/w) from a dextrose equivalent of carbohydrate are obtained. Keeping the calcium lactate in solution is desirable so that it can be easily separated from the cell biomass and other insolubles, and this limits the concentration of carbohydrates that can be fed in the fermentation and the concentration of lactate in the fermentation broth, which is usually around 10% (w/v). The broth containing calcium lactate is filtered to remove cells, carbon treated, evaporated, and acidified with sulfuric acid to convert the salt into lactic acid and insoluble calcium sulfate, which is removed by filtration. The filtrate is further purified by carbon columns and ion exchange and evaporated to produce technical and food-grade lactic acid, but not a heat-stable product, which is required for the stearoyl lactylates, polymers, and other value-added applications. The technical-grade lactic acid can be esterified with methanol or ethanol, and the ester is recovered by distillation, hydrolyzed by water, evaporated, and the alcohol is recycled. This separation process produces a highly pure product, which, like the synthetic product, is water white and heat stable (equations 4-7).

C6 Hi2 Об + Ca(OH)2 Fermentation > (2 . CH3 CHOH COO) CA++ + 2H20 (4)

(2 • CH3 CHOH COO) Ca++ + H2S04—— > 2 CH3 CHOH COOH + CaS04 і (5)

CH3 CHOH COOH + CH3 OH—— > CH3 CHOH COO CH3 t + H20 (6)

CH3 CHOH COOCH3 + H20——- > CH3 CHOH COOH + CH3 OH T (7)

Some of the major economic hurdles and process cost centers of this conventional carbohydrate fermentation process are in the complex separation steps that are needed to recover and purify the product from the crude fermentation broths. Furthermore, approximately one ton of gypsum by-product is produced and needs to be disposed of for every ton of lactic acid produced by the conventional fermentation and recovery process. These factors had made large-scale production by this conventional route economically and ecologically unattractive.