Microbial Cells

Microbial cells may be regarded as a very natural form of immobilized enzymes that provide self-regeneration of catalytic activities and of all cofactors, active transport sys tems for substrates, and an optimal spatial arrangement of enzyme chains. Empirical biotechnologies rely on Schi-zophyta and Protoascomycotina to improve keeping quality, digestibility, and, as a welcome side effect, flavor of foods and beverages.

Food Strains

Fatty acids and simple carbonyl compounds are the predominant volatile compounds produced by Streptococci and Lactobacilli (54). Penicillium strains used in the manufacture of surface-molded cheese and raw sausages add meth-ylketones and, if the cells were injured by mechanical stress, Cg-compounds to the spectrum. Typical yeast volatiles are esters, thio-compounds, and reduced carbonyls. The fermentation of food strains on classical or on chemically defined substrates usually resulted in the production of these common flavor molecules. More interesting compounds were rarely formed or formed in low amounts (<mg L_1) only.

However, some recent applications demonstrate that industry is willing to exploit the capabilities and the operational advantages of microbial cells, despite their obvious genetic limitations. The examples selected (Table 5) reflect some of the actual trends: new substrates and old strains, for example, for the malolactic fermentation of fruit and vegetable juices; better defined, often continuously operated systems for better understanding of biochemical regulation phenomena; cell recycle and immobilized systems for reuse of the biocatalyst (55,56). It has been observed that both quality and quantity of volatiles produced by mutants or mixed cultures with enhanced or complementary metabolic abilities were superior to products fermented with a pure standard strain. The cell counts in mixed cultures often run through transient maxima for the single strains resulting in fluctuations of formation of volatiles and their precursors. Sometimes the success of the entire process is jeopardized. Genetic engineers are now in the position to step forward from combining strains to recombinant strains with a more stable and complex physiology.

The reported generation of Furaneol (2,5-dimethyl-4-hydroxy-3(2if)-furanone; "Enhansol" in the United States) in an aerated culture of Lactobacillus helveticus deserves special attention (57). This heterocyclic character impact compound with an intense fruity/pineapple to caramel/ burnt note (depending on its actual concentration) belongs to the most potent flavor compounds, but it is rather unstable in aqueous fermentation media. Special precautions

Table 4. Cofactor Coupled Reactions Generating Volatile Flavor

System components Product Ref.

Geraniol/Acetaldehyde/NAD+ /ADH Geranial 48 CinnamylOH/Octanal/NAD+/ADH/

Lip-DH/DCIP Cinnamaldehyde 49 L-Leucine/EtOH/NAD+/

Enzyme mixture 3-Methylbutanal/ol 50

Oxo compound/NADH Chiral alcohol 51

Table 5. Food Strains and Flavor Formation

Substrate

Catalyst

Flavor

Ref.

Citrate

Streptococcus lactis

Diacetyl, etc

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