Lactic acid bacteria homofermentative

Mannitol production by homofermentative lactic acid bacteria 392... 

MANNITOL PRODUCTION BY HOMOFERMENTATIVE LACTIC ACID BACTERIA... 

The fermentative production of lactic acid from carbohydrates has repeatedly been reviewed recently [36, 41, 42]. Two classes of lactic acid producers are discerned the homofermentative lactic acid bacteria, which produce lactic acid as the sole product, and the heterofermentative ones, which also produce ethanol, acetic acid etc. [43]. Recently, the focus has been on (S)-L-lactic acid producing, homofermentative Lactobacillus ddbrueckii subspecies [42]. 

The reaction in Figure 1.3 takes place in the so-called homofermentative lactic acid bacteria (LAB). Homofermen-tative bacteria have almost exclusively lactic acid as a... 

Lactose is only synthesised in the mammary glands of mammals. Cows milk contains 4-5% of lactose, human milk 5.5-7% of lactose. In addition to lactose, milk contains smaller amounts of D-glucose and a wide variety of free oligosaccharides. Lactose is also present naturally in all milk-containing products (e.g. milk chocolates and ice creams). Its content in the products prepared using homofermentative lactic acid bacteria (such as yoghurt, acidophilic milk and kefir) is lower than in fresh milk, generally about 1% or less. 

Homofermentative lactic acid bacteria (such as Lactococcus lactis and Streptococcus lactis) produce (-l-)-L-lactic acid (e.g. in sour cream). Both isomers, (-l-)-L-lactic acid and (-)-D-lactic acid (8-65), are formed during milk fermentation by heterofermen-tative bacteria (lactic acid bacteria are mostly heterofermentative bacteria) and lactic acid thus also occurs as a racemate in sauerkraut, pickled cucumbers, olives and silage. For example, bacteria of the genus Leuconostoc produce d-lactic acid, while bacteria Pediococcus acidilactici and other bacteria produce racemic lactic acid. The content of lactic acid in dairy products is 0.5-1.0%. L-Lactic acid in yoghurt represents about 54% and in sour cream 96% of the total lactic acid content. The total lactic acid content in sauerkraut is 1.5 2.5%, in fermented cucumbers it ranges from 0.5 to 1.5% and fermented green olives contain 0.8 to 1.2% lactic acid. 

Figure 8.41 Mechanism of homofermentative lactic acid bacteria fermentation (P = phosphate residue).

Some cells lack certain enzymatic steps and therefore lack certain pathways. Studies of the homo- and heterofermentative organisms are instructive in this regard. In the homofermentative lactic acid bacteria, the utilization of C Mabeled glucose to form lactate and the distribution of in the lactate suggest that the basic degradative procedure is that of the Embden-Meyerhof scheme. An observed slight randomization of... 

Driehuis F, Oude Elferink SJWH, Van Wikselaar PG (2001) Fermentation characteristics and aerobic stability of grass silage inoculated with Lactobacillus buchneri, with or without homofermentative lactic acid bacteria. Grass Forage Sci 56(4) 330-343. doi 10.1046/j.l365-2494.2001.00282.x... 

The primary function of cheese starter cultures is to produce lactic acid at a predictable and dependable rate. The metabolism of lactose is summarized in Figure 10.12. Most cheese starters are homofermentative, i.e. produce only lactic acid, usually the L-isomer Leuconostoc species are heterofermentative. The products of lactic acid bacteria are summarized in Table 10.4. 

Many names have been assigned to the lactic acid bacteria associated with brewing. It is probable however that most rod-shaped isolates may be classified as the heterofermentative species Lactobacillus brevis, the homo-fermentative species . casei and L, plantarum, and the homofermentative thermophilic species L. delbrueckii [14]. Cocci are also encountered, notably the homofermentative Pediococcus damnosus. (Less common because they are more sensitive to hop resins are P. pentosaceuslacidilactici. Streptococcus saprophyticus, S. epidermis and Micrococcus varians.) Micrococcus kristinae is however resistant to hop resins and low pH, but requires oxygen for growth [15]. An American report states that many breweries encounter L. brevis, L. plantarum and P. damnosus. When the primary fermentation is complete, Pediococcus continues to grow at the bottom of the fermenter in the deposited yeast [16]. 

The biochemistry of the lactic acid bacteria has received attention [4, 17-20]. Homofermentative strains such as the Pediococci use the glycolytic pathway for the dissimilation of carbohydrates, such as glucose, to yield pyruvic acid. Pyruvic acid acts as a hydrogen acceptor and is converted to lactic acid by means of an NADH-dependent lactic dehydrogenase. It is believed that the homofermentative strains use in addition the hexose monophosphate pathway and possibly a phosphoketolase pathway (Fig. 21.2) when pentoses are degraded. The heterofermentative strains on the other hand lack both aldolase and hexose isomerase, essential for the operation of the glycolytic pathway, while pyruvic acid will not readily function as a... 

FIGURE 13.3 Schematic illustration of metabolic pathway of glucose in lactic acid bacteria. Homofermentative pathway (a), heterofermentative pathway (b). 

Lactic acid bacteria are often described as being either heterofermentative or homofermentative microorganisms. Physiologically, this describes the... 

Mannitol formation is used as a laboratory diagnostic test for the separation ofheterofermentative from homofermentative bacteria (Section 15.4.10). Although primarily a property ofheterofermentative lactic acid bacteria, a few homofermentative strains can also produce small amounts of the sugar alcohol (Wisselink et al., 2002). 

Table 4.3 lists the lactic acid bacteria most often encountered in grape must and wine. Oenococcus oeni is known for ensuring malolactic fermentation in the great majority of cases. So far, the strictly homofermentative lactobacilli of Group I have not been isolated in must or wine. The species are therefore divided into facultative and strict heterofermenters for lactobacilli and into homofermenters (Pediococcus) and heterofermenters (Leuconostoc) for cocci. It is likely that this classification will be modified—on one hand due to anticipated progress in the identification of new species in wine, and on the other hand dne to eventual reclassifications of lactobacilli in the gronps described above. 

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