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The Lactic Acid Bacteria

This group is composed of 13 genera of Gram-positive bacteria at this time:

Carnobacterium Oenococcus ,Enterococcus Pediococcus, Lactococcus Paralactobacillus

Lactobacillus Streptococcus, Lactosphaera Tetragenococcus, Leuconostoc Vagococcus

Weissella

With 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

asAerococcus, 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 they

note 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 strainsdeveloped.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 homolacticsproduce 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 71(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 71). 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 71 are betabacteria. The streptobacteria (for example, L. casei and. plantarum)

produce up to 1.5% lactic acid with an optimal growth temperature of 30C, whereas the

thermobacteria (such as L. acidophilus and L. Delbrueckiisubsp. bulgaricus) can produce up

to 3% lactic acid and have an optimal temperature of 40C.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.

Delbrueckiisubsp. bulgaricus, etc.). These are the thermobacteria, and they do not ferment

pentoses. Group 2 consists offacultative 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

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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 baseshence their use in

microbiological assays for these compounds. Although they are mesophilic, some can grow

below 5C and some as high as 45C.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 4.04.5. The lactic acid bacteriaare 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. The

lactococci 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 to

produce calcium stearyl 2- lactylate, a bread additive.

(ii) In medicine it is sometimes used to introduce calcium in to the body in the form of

calcium 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

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bacterial spoilage in a wide variety of processed foods. It has the advantage, in

contrast 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) by

the FDA.

(viii) It is a very good preservative and pickling agent. Addition of lactic acid aqueous

solution 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 these

emulsifiers requires heat stable lactic acid, hence only the synthetic or the heat

stable fermentation grades can be used for this application.

(x) Lactic acid has many pharmaceutical and cosmetic applications and formulations

in topical ointments, lotions, anti acne solutions, humectants, parenteral solutions

and dialysis applications, for anti carries agent.

(xi) Calcium lactate can be used for calcium deficiency therapy and as anti caries

agent.

(xii) Its biodegradable polymer has medical applications as sutures, orthopaedicimplants, controlled drug release, etc.

(xiii) Polymers of lactic acids are biodegradable thermoplastics. These polymers are

transparent and their degradation can be controlled by adjusting the composition,

and the molecular weight. Their properties approach those of petroleum derived

plastics.

(xiv) Lactic acid esters like ethyl/butyl lactate can be used as environment-friendly

solvents. They are high boiling, non-toxic and degradable components.

(xv) Poly L-lactic acid with low degree of polymerization can help in controlled release

or degradable mulch films for large-scale agricultural applications.

Physical properties of lactic acidAppearance Yellow to colorless crystals or syrupy 50% liquid

Melting point 16.8C

Relative density 1.249 at 15C

Boiling point 122 @ 15 millimeter

Flash point 110C

Solubility Soluble in water, alcohol, furfurol

Slightly soluble in ether

Insoluble in chloroform, petroleum ether, and carbon

Disulfide

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