Diacetyl, lactic acid bacteria production

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). 

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. 

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,... 

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. 

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... 

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). 

---