Ammonia assimilation yeasts

In 1980, Miflin and Lea pointed out that much of the information on biochemical pathways has arisen from the use of mutants of bacteria and yeast larking key enzymes. They complained at the time that no mutants of higher plants were available in the ammonia assimilation pathway. A glance at Table I clearly indicates that a number of such mutants are now available. In addition mutants of maize with low (but not zero) levels of glutamate dehydrogenase have also been studied (Rhodes et ai, 1989). 

When grapes are fully ripe, mineral nitrogen represents less than 10% of total nitrogen, or a few tens of mg/1 expressed in ammonia. Ammonia, or more exactly the NHj ammonium cation, is the form most directly assimilable by yeasts. Its concentration affects the rapidity with which must starts to ferment as well as its potential... 

The general mechanism of nitrosation of secondary amines is shown in Figure 12.26. Nitrosation reagents are formed in acidic solutions from nitrous acid and nitrites, respectively, by a sequence of reactions indicated in Figure 12.27. Nitrites are present in foods as additives or contaminants. In foods and beverages obtained by fermentation, nitrites may arise by reduction of nitrates by microbial reductases. In beer, for example, wild yeasts partly assimilate nitrates to give ammonia, and the activity of nitrate reductase... 

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