Yeasts lactose-fermenting

Cheese whey soHds contain 70—75% lactose, which can serve as the carbon source for lactose fermenting yeasts such as Klujveromjcesfragilis. The total volume produced is considerably smaller than for the other yeasts described. 

The production of fermented milks no longer depends on acid production by the indigenous microflora. Instead, the milk is inoculated with a carefully selected culture of LAB and for some products with LAB plus lactose-fermenting yeasts (Table 10.12). The principal function of LAB is to produce acid at an appropriate rate via the pathways summarized in Figure 10.12. The yoghurt fermentation is essentially homofermentative but the characteristic flavour of cultured buttermilk is due mainly to diacetyl which is produced from citrate by Lactococccus lactis ssp. lactis biovar diacetylactis, which is included in the culture for this product (Figure 10.31). 

Kefir and Koumiss contain about 1 and 6% ethanol, respectively, which is produced by lactose-fermenting yeasts, usually Kluyveromyces marxianus. The ethanol modifies the flavour of the products and the C02 produced in the fermentation affects both their flavour and texture. Koumiss, which is produced traditionally from mares milk, mainly in Russia and surrounding areas of Asia, is not in fact coagulated. 

In Russia, a popular milk drink traditionally made from unpasteurized mare s milk is known as kumiss. The fermentation is caused principally by L. bulgaricus, lactose-fermenting Torula yeasts, and Lactobacillus leichmannii. 

Lactose-fermenting yeasts have been known for some time, but their use to produce ethyl alcohol and yeast from whey received serious attention only much later. Certain Torula species yield more alcohol than might have been expected from statements in the literature. Four kefir yeasts, two Torula species, one of Torulopsis and one additional yeast species, produced alcohol yielding 68 to 80% of the theoretical quantity. A maximal yield, 80.3%, based on a theoretical yield of 4 moles of alcohol per mole of lactose fermented, was obtained with a strain of Torula cremoris in a 21.7-hr fermentation at 30 to 32°C (Marth 1974). 

Lactose-fermenting yeasts contain the following vitamins in milligrams percent on a dry basis vitamin A, traces Bi, 12.8 B2, 4.4 nicotinic acid, 8.3 ascorbic acid, 7.8 and provitamin A, 40.5 (Springer... 

In the presence of yeast, lactose is capable of alcoholic fermentation, which takes place slowly, and, as it appears, without previous transformation of the lactose into either glucose or galactose. On contact with putrefying buminoids it enters into lactic fermentation. 

Whey wine is an experimental product being test marketed. After deprotein-izing with UF, the lactose is fermented for about a week by special lactose-fermenting yeast. The finished product is a pale-yellow, tart, dry wine with a subdued aroma and bouquet. 

Fluid milk contains about 5 % of lactose. Lactose has low solubility and low sweetness, and a significant part of the world s population does not tolerate lactose in the diet. For these reasons the conversion of lactose to the component sugars, glucose and galactose would be worthwhile. Commercial lactases have been developed from lactose-fermenting yeasts and shown to have utility in the hydrolysis of lactose in a variety... 

Rogosa, M. Synthesis of riboflavin by lactose-fermenting yeasts. J. 

Lactate. Hard rennet cheeses represent a selective habitat for propionic acid bacteria, since they contain lactate formed as the end product of lactose fermentation by lactic acid bacteria. Unlike many other bacteria, propioni-bacteria can utilize lactate efficiently, which is the reason why propioni-bacteria are so abundant in hard, ripened cheeses. Propionic acid bacteria use lactate best in the presence of yeast extract (Antila, 1954), but even higher stimulatory effect is exerted by cell-free extracts of lactic acid bacteria. Streptococcus thermophilus and Lactobacillus spp. (Hietaranta and Antila, 1953). Lactate as a carbon source supports higher growth rates of propionic acid bacteria than lactose (El-Hagarawy et al., 1954). 

Yeast Fermenting in Dough. When yeast is in a bread dough the traces of sugars present can be fermented directly. As yeast contains the enzyme invertase, any sucrose present can be inverted into dextrose and fructose which can then be fermented. If any dextrose from a high DE glucose syrup is present then it can be directly fermented. If there is any lactose present it can not be fermented at all. Similarly, any polyols such as sorbitol can not be fermented. 

Lactose is a disaccharide reducing sugar. Unlike the other sugars mentioned, lactose is not particularly soluble. A property that has some use in yeast-containing products is that lactose is not fermented by baker s yeast. 

Fischer then examined the lactose yeast in the same manner as he did the Frohberg yeast and found it to contain both an invertin-like enzyme and a lactose-cleaving enzyme, which he termed lactase. From these results he concluded that the first step in the fermentation of lactose, as for the fermentation of sucrose and maltose, is the hydrolysis of the disaccharide to mono-sacharide. From this observation, he drew the landmark conclusion that he considered it most unlikely that any polysaccharide (the term included di-saccharides) can be fermented without first being hydrolyzed to hexose (31). 

Lactose is readily fermented by lactic acid bacteria, especially Lactococcus spp. and Lactobacillus spp., to lactic acid, and by some species of yeast, e.g. Kluyveromyces spp., to ethanol (Figure 2.27). Lactic acid may be used as a food acidulant, as a component in the manufacture of plastics, or converted to ammonium lactate as a source of nitrogen for animal nutrition. It can be converted to propionic acid, which has many food applications, by Propionibacterium spp. Potable ethanol is being produced commercially from lactose in whey or UF permeate. The ethanol may also be used for industrial purposes or as a fuel but is probably not cost-competitive with ethanol produced by fermentation of sucrose or chemically. The ethanol may also be oxidized to acetic acid. The mother liquor remaining from the production of lactic acid or ethanol may be subjected to anaerobic digestion with the production of methane (CH4) for use as a fuel several such plants are in commercial use. 

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