Lactic acid bacteria diacetyl

Acetaldehyde and acetoin can be excreted in substantial amounts by lactic acid bacteria, in addition to diacetyl and methylglyoxal. For example, acetoin excretion has been studied in Streptococcus mutans (Hillman et al., 1987). Acetaldehyde (Nordbo, 1971) and methylglyoxal (Armstrong, 1964) stain teeth in vitro. 

Hydroxy-2-butanone (acetoin) is a characteristic constituent of butter flavour used for flavouring margarine and can be obtained as a by-product of molasses-based and lactic acid fermentations [49, 71]. The closely related 2,3-butanedione (diacetyl) has a much lower organoleptic threshold than acetoin and is an important strongly butter-like flavour compound in butter and other dairy products [72] in buttermilk, for instance, the diacetyl concentration is only about 2-4 mg [73]. a-Acetolactate (a-AL) is an intermediate of lactic acid bacteria mainly produced from pyruvate by a-acetolactate synthase. In most lactic acid bacteria, a-AL is decarboxylated to the metabolic end product acetoin by a-AL decarboxylase (ALDB) [71] (Scheme 23.5). 

Scheme 23.5 Metabolic pathways of lactic acid bacteria leading from pyruvate to a-acetolactate and acetoin and chemical diacetyl formation. ALS a-acetolactate synthase, ALDB a-acetolactate decarboxylase, DDH diacetyl dehydrogenase. (Adapted from [72])...

Cultured buttermilk is manufactured by fermenting whole milk, reconstituted nonfat dry milk, partly skimmed milk, or skim milk with lactic acid bacteria. Most commercial cultured buttermilk is made from skim milk. Mixed strains of lactic streptococci are used to produce lactic acid and leuconostocs for development of the characteristic diacetyl flavor and aroma. Buttermilk is similar to skim milk in composition, except that it contains about 0.9% total acid expressed as lactic acid. The percentage of lactose normally found in skim milk is reduced in proportion to the percentage of lactic acid in the buttermilk. According to White (1978), the fat content of buttermilk usually varies from 1 to 1.8%, sometimes in the form of small flakes or granules to simulate churned buttermilk, the by-product of butter churning. Usually 0.1% salt is added. 

Lactic acid bacteria and bifidobacteria are preferred as protective and probiotic cultures, and have been used since the beginning of history as starter cultures. They have a long history of being safely used and consumed. LAB are widely used for fermentation of milk, meat, and vegetable foods. In fermentation of dairy products, lactose is metabolized to lactic acid. Other metabolic products, hydrogen peroxide, diacetyl, and bacteriocins may also play inhibitory roles and contribute to improving the organoleptic attributes of these foods, as well as their preservation (Messens and De Vuyst, 2002). 

The cream used for butter may be fresh ( pH 6.6) or ripened (fermented pH 4.6), yielding sweet-cream and ripened cream (lactic) butter, respectively. Sweet-cream butter is most common in English-speaking countries but ripened cream butter is more popular elsewhere. Traditionally, the cream for ripened cream butter was fermented by the natural microflora, which was variable. Product quality and consistency were improved by the introduction in the 1880s of cultures (starters) of selected lactic acid bacteria, which produce lactic acid from lactose and diacetyl (the principal flavour component in ripened cream butter) from citric acid, A flavour concentrate, containing lactic acid and diacetyl, is now frequently used in the manufacture of ripened cream butter, to facilitate production schedules and improve consistency. 

The most important off-flavour and aroma associated with the lactic acid bacteria is the sweet, butterscotch or honey note provided by diacetyl and related vicinal diketones. It can be discerned readily in lager beers at concentrations as low as 0 5 (xg/ml. The defect was formerly called Sarcina sickness after Sarcina, the outdated generic name for brewery gram-positive bacteria. 

A second group of wort bacteria include Bacillus and Clostridium species, spo-rogenic species with high heat-resistant spores that may enter into beer processes via raw materials (malt and hops). These are associated with the production of butyric acid and sulfur compounds (Back, 2005). A third group includes the genera Lactobacillus, Enterococcus, Lactococcus, and Pediococcus many producing lactic acid and diacetyl (Back, 2005). 

CTobials, e.g. acetic acid, propionic acid, diacetyl, CO2 and bacteriocins, may be produced by some lactic acid bacteria. 

Diacetyl. In many dairy products, citrate-utilizing lactic acid bacteria play an important role as they are responsible for the formation of the flavouring compound diacetyl. Much of the literature describing the antimicrobial activity of diacetyl has been reported by Jay (1982). It is active against a broad spectrum of microorganisms including Bacillus,... 

A Sensory properties. Lactic acid bacteria also produce volatile substances, for example, diacetyl and acetaldehyde, that contribute to the typical flavour of cultured buttermilk and yoghurt. Starter cultures also possess some proteolytic and lipolytic activity, which, especially during the maturation of cheeses, contributes to their characteristic flavour. 

The main role of propionic acid bacteria in cheese ripening consists in the utilization of lactate produced by lactic acid bacteria as an end product of lactose fermentation. Lactate is then transformed into propionic and acetic acids and CO2. The volatile acids provide a specific sharp taste and help preserve a milk protein, casein. Hydrolysis of lipids with the formation of fatty acids is essential for the taste qualities of cheese. The release of proline and other amino acids and such volatile compounds as acetoin, diacetyl, dimethylsulfide, acetaldehyde is important for the formation of cheese aroma. Carbon dioxide released in the processes of propionic acid fermentation and decarboxylation of amino acids (mainly) forms eyes, or holes. Propionic acid bacteria also produce vitamins, first of all, vitamin At the same time, an important condition is to keep propionibacteria from growing and producing CO2 at low temperatures, since this would cause cracks and fissures in cheese. 

One of the most important odor active compounds produced by lactic acid bacteria is 2,3-butandione, or diacetyl (Fornachon and Lloyd, 1965 Collins, 1972 El-Gendy et al., 1983 Rodriguez et al., 1990 Martineau and... 

Diacetyl may be synthesized by either homolactic or heterolactic pathways of sugar metabolism as well as by utilization of citric acid (Fig. 2.9). Citric acid is hrst converted to acetic acid and oxaloacetate the latter is then decarboxylated to pyruvate. Although earlier reports indicated that diacetyl synthesis by lactic acid bacteria does not proceed via a-acetolactate (Gottschalk, 1986), more recent evidence suggests that this pathway is active in lactic acid bacteria (Ramos et al., 1995). Here, pyruvate undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldehyde. This compound then reacts with another molecule of pyruvate to yield a-acetolactate, which, in... 

Figure 2.9. Biochemical formation of diacetyl, acetoin, and 2,3-butanediol by lactic acid bacteria. Adapted from Ramos et al. (1995), Bartowsky and Henschke (2004b), and Ribereau-Gayon et al. (2000). TPP refers to thiamine pyrophosphate.

During growth, malic and citric acid utilization by lactic acid bacteria may occur concomitantly, although utilization of citric acid proceeds at a much slower rate (Pimentel et al., 1994). Thus, complete conversion of citric acid does not necessarily coincide with completion of MLF, and levels of citric acid remaining in the wine post-MLF may be sufficient to stimulate bacterial formation of diacetyl and acetic acid. 

Table 2.3. Factors that affect diacetyl synthesis by lactic acid bacteria.

El-Gendy, S.M., H. Abdel-Galil, Y. Shahin, and F.Z. Hegazi. 1983. Acetoin and diacetyl production hy homo- and heterofermentative lactic acid bacteria. J. Food Prot. 46 420-425. 

Metabolic products of lactic acid bacteria, such as diacetyl, ethanal, dimethylsulfide, acetic acid and lactic acid contribute to this aroma. Carbon dioxide also appears to be inqtortant. In good... 

The so-called starter distillates used by the dairy industry are now produced on a commercial scale from lactic acid cultures. These distillates in which 70% of the substrate is converted to diacetyl have been patented(67) and are used to impart a buttery taste to edible oils. They are manufactured by the steam distillation of cultures of bacteria grown on a medium of skim milk fortified with 0.1% citric acid. Organisms used are Streptococcus lactis. S. cremoris. S. lactis subsp. diacetvlactis. Leuconostoc citrovorum and L dextranicum. Diacetyl comprises 80-90% of the flavor compounds in the aqueous distillate but is present at only 10-100 ppm. 

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