Mold and yeast, growth

Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth (Labuza 1980). This information is summarized in Table 1-9. Enzyme activity is virtually nonexistent in the monolayer water (aw between 0 and 0.2). Not surprisingly, growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely... 

A related antibacterial substance is nata-mycin, identical to pimaricin. Natamycin is effective in controlling the growth of fungi but has no effect on bacteria or viruses. In fermentation industries, natamycin can be used to control mold or yeast growth. It has a low solubility and therefore can be used as a surface treatment on foods. Natamycin is used in the production of many varieties of cheese. 

The thermal death point for molds and yeasts is heating to 60°C (140°F) for about 30 min (84). Bacteria cannot grow in the absence of moisture (82, 83). Thus, they will not grow in dry oils and fats or in any other form of dry material. Free water, even to the extent of one fourth of 1%, however, is sufficient to permit the growth. 

Effect of Different Brands of Aiabfiob on the Growth of Molds and Yeasts. 

Features Inhibits growth of gram-negative and gram-positive bacteria, fungi, mold and yeast... 

Sodium or potassium benzoate at a concentration of 0.05% is a universally used preservative agent active against yeast and mold. At higher concentrations benzoate is also effective against bacteria. It is most effective at a pH between 2.0 and 4.0. Sodium or potassium sorbate inhibits the growth of yeast and mold and is most effective below pH 6.5. 

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

The strength of cell walls differs among bacteria, yeasts, and molds. The strength also varies with the species and the growth conditions, and must Be determined experimentally. Beads of 0.5 mm are typically used for yeast and bacteria. Recommended bead charge is 85 percent for 0.5 mm beads, and 80 percent for 1 mm beads [Schuette et al.. Enzyme Microbial Technology, 5, 143 (1983)]. 

---