The Lactic Acid BacteriaThis group is composed of 13 genera of Gram-positive bacteria at this time:Carnobacterium Oenococcus ,Enterococcus Pediococcus, Lactococcus ParalactobacillusLactobacillus Streptococcus, Lactosphaera Tetragenococcus, Leuconostoc VagococcusWeissellaWith the enterococci and lactococci having been removed from the genus Streptococcus, the member of this genus of most importance in foods is S. salivarius subsp. thermophilus. S. diacetilactis has been reclassified as a citrate-utilizing strain of Lactococcus lactis subsp. lactis. Related to the lactic acid bacteria but not considered to fit the group are genera such as Aerococcus, Microbacterium, and Propionibacterium, among others. The last genus has been reduced by the transfer of some of its species to the new genus Propioniferax, which produces propionic acid as its principal carboxylic acid from glucose.80 The history of our knowledge of the lactic streptococci and their ecology has been reviewed by Sandine et al.63 These authors believe that plant matter is the natural habitat of this group, but theynote the lack of proof of a plant origin for Lactococcus cremoris. It has been suggested that plant streptococci may be the ancestral pool from which other species and strains developed.47 Although the lactic acid group is loosely defined with no precise boundaries, all members share the property of producing lactic acid from hexoses. As fermenting organisms, they lack functional heme-linked electron transport systems or cytochromes, and they obtain their energy by substrate-level phosphorylation while oxidizing carbohydrates; they do not have a functional Krebs cycle. Kluyver divided the lactic acid bacteria into two groups based on end products of glucose metabolism. Those that produce lactic acid as the major or sole product of glucose fermentation are designated homofermentative The homolactics are able to extract about twice as much energy from a given quantity of glucose as are the heterolactics. The homofermentative pattern is observed when glucose is metabolized but not necessarily when pentoses are metabolized, for some homolactics produce acetic and lactic acids when utilizing pentoses. Also the homofermentative character of homolactics may be shifted for some strains by altering growth conditions such as glucose concentration, pH, and nutrient limitation.8,42 Those lactics that produce equal molar amounts of lactate, carbon dioxide, and ethanol from hexoses are designated heterofermentative (Figure 7–1(B)). All members of the genera Pediococcus, Streptococcus,Lactococcus, and Vagococcus are homofermenters, along with some of the lactobacilli. Heterofermenters consist of Leuconostoc, Oenococcus, Weissella, Carnobacterium, Lactosphaera, and some lactobacilli (Table 7–1). The heterolactics are more important than the homolactics in producing flavor and aroma components such as acetylaldehyde and diacetyl The genus Lactobacillus was subdivided historically into three subgenera: Betabacterium, Streptobacterium, and Thermobacterium. All of the heterolactic lactobacilli in Table 7–1 are betabacteria. The streptobacteria (for example, L. casei and. plantarum) produce up to 1.5% lactic acid with an optimal growth temperature of 30◦C, whereas the thermobacteria (such as L. acidophilus and L. Delbrueckii subsp. bulgaricus) can produce up to 3% lactic acid and have an optimal temperature of 40◦C.43 More recently, the genus Lactobacillus has been arranged into three groups based primarily on fermentative features.70 Group 1 includes obligate homofermentative species (L. acidophilus, L. Delbrueckii subsp. bulgaricus, etc.). These are the thermobacteria, and they do not ferment pentoses. Group 2 consists of facultative heterofermentative species (L. casei, L. plantarum, L. sakei; etc.). Members of this group ferment pentoses. Group 3 consists of the obligate heterofermentative species, and it includes L. fermentum, L. brevis, L. reuteri, L. sanfranciscensis, and others. They produce CO2 from glucose. The lactobacilli can produce

a pHof 4.0 in foods that contain a fermentable carbohydrate, and they can grow up to a pH of about 7.1.70 In terms of their growth requirements, the lactic acid bacteria require preformed amino acids, B vitamins, and purine and pyrimidine bases—hence their use in microbiological assays for these compounds. Although they are mesophilic, some can grow below 5◦C and some as high as 45◦C.With respect to growth pH, some can grow as low as 3.2, some as high as 9.6, and most grow in the pH range 4.0–4.5. The lactic acid bacteria are only weakly proteolytic and lipolytic.69 The cell mucopeptides of lactics and other bacteria have been reviewed by Schleifer and Kandler.64 Although there appear to be wide variations within most of the lactic acid genera, the homofermentative lactobacilli of the subgenus Thermobacterium appear to be the most homogeneous in this regard in having l-lysine in the peptidoglycan peptide chain and d-aspartic acid as the interbridge peptide. Thelactococci have similar wall mucopeptides.

Uses of lactic acid(i) It is used in the baking industry. Originally fermentation lactic acid was produces to replace tartarates in baking powder with calcium lactate. Later it was used toproduce calcium stearyl 2- lactylate, a bread additive.(ii) In medicine it is sometimes used to introduce calcium in to the body in the form ofcalcium lactate, in diseases of calcium deficiency.(iii) Esters of lactic acid are also used in the food industry as emulsifiers.(iv) Lactic acid is used in the manufacture of rye bread.(v) It is used in the manufacture of plastics.(vi) Lactic acid is used as acidulant/ flavoring/ pH buffering agent or inhibitor of

bacterial spoilage in a wide variety of processed foods. It has the advantage, incontrast to other food acids in having a mild acidic taste.(vii) It is non-volatile odorless and is classified as GRAS (generally regarded as safe) bythe FDA.(viii) It is a very good preservative and pickling agent. Addition of lactic acid aqueoussolution to the packaging of poultry and fish increases their shelf life.(ix) The esters of lactic acid are used as emulsifying agents in baking foods (stearoyl-2-lactylate, glyceryl lactostearate, glyceryl lactopalmitate). The manufacture of theseemulsifiers requires heat stable lactic acid, hence only the synthetic or the heatstable fermentation grades can be used for this application.(x) Lactic acid has many pharmaceutical and cosmetic applications and formulationsin topical ointments, lotions, anti acne solutions, humectants, parenteral solutionsand dialysis applications, for anti carries agent.(xi) Calcium lactate can be used for calcium deficiency therapy and as anti cariesagent.(xii) Its biodegradable polymer has medical applications as sutures, orthopaedicimplants, controlled drug release, etc.(xiii) Polymers of lactic acids are biodegradable thermoplastics. These polymers aretransparent and their degradation can be controlled by adjusting the composition,and the molecular weight. Their properties approach those of petroleum derivedplastics.(xiv) Lactic acid esters like ethyl/butyl lactate can be used as environment-friendlysolvents. They are high boiling, non-toxic and degradable components.(xv) Poly L-lactic acid with low degree of polymerization can help in controlled releaseor degradable mulch films for large-scale agricultural applications.Physical properties of lactic acidAppearance Yellow to colorless crystals or syrupy 50% liquidMelting point 16.8°CRelative density 1.249 at 15°CBoiling point 122° @ 15 millimeterFlash point 110°CSolubility Soluble in water, alcohol, furfurolSlightly soluble in etherInsoluble in chloroform, petroleum ether, and carbonDisulfide