Yeast water activity level

Enzymes as different as yeast alcohol oxidase, mushroom polyphenol oxidase, and horse-liver alcohol dehydrogenase demonstrated vastly increased enzymatic activity in several different solvents upon an increase in the water content, which always remained below the solubility limit (Zaks, 1988b). While much less water was required for maximal activity in hydrophobic than in hydrophilic solvents, relative saturation seems to be most relevant to determining the level of catalytic activity. Correspondingly, miscibility of a solvent with water is not a decisive criterion upon transition from a monophasic to a biphasic solvent system, no significant change in activity level was observed (Narayan, 1993). Therefore, the level of water essential for activity depends more on the solvent than on the enzyme. 

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... 

The storage condition of raw materials, particularly hygroscopic substances, is important, and as a minimum water activity (Aw) of 0.70 is required for osmophilic yeasts, 0.80 for most spoilage moulds and 0.91 for most spoilage bacteria, precautions should be taken to ensure that dry materials are held below these levels. Some packaging used for raw materials, such as unlined paper sacks, may absorb moisture and may itself be subject to microbial deterioration and so contaminate the contents. For this reason polythene-lined sacks are preferable. Some liquid or semi-solid raw materials contain preservatives, but others such as syrups depend upon osmotic pressure to prevent the growth of os-mophiles, which are often present. With this type of material it is important that they are held at a con-... 

Yeast, bacteria, and molds all require minimal levels of moisture for growth. In general, most wine-borne yeast and bacteria need more moisture than molds. During solidification of cultivation media, agar binds water and lowers free moisture (a, ) or water activity. As such, it is important to adhere closely to the supplier s recommendations regarding the amounts of agar to include in formulations because a could be reduced too far to support growth. 

Wild apricot (Prunus armenica L.) grows naturally in hilly areas of northern India. It is highly acidic, fibrous, and low in TSS, and, thus, not utilized commercially. Preparation and evaluation of a vermouth from its fruit was undertaken (Abrol, 2009). Vermouths at different sugar (8,10, and 12 °Brix), alcohol (15%, 17%, and 19%), and spices levels (2.5% and 5%) were prepared. Those used in extract preparation are shown in Plate 8.1. The base wine was prepared from crushed fruit, adjusted to 24 °Brix, and diluted in a 1 2 ratio with water. To this mixture was added 200 ppm sulfur dioxide, 0.1% diammonium hydrogen phosphate (DAHP), and 0.5% pectinase enzyme. A 24-h active yeast culture initiated fermentation. The procedure is illustrated in Fig. 8.4. A maturation period of 6 months improved the quality of the vermouth. 

In this chapter, two subjects of our study were described. One was concerned with the catalysis by enzymes entrapped in water pools and photomerization at the level of a biomembrane model in vivo. Based on the study of the activity of yeast HK in the water pools, the activity of HK can be seen in noncharged polyoxyethylene mantles with relatively low micropolarity in which almost all the water molecules are bound up with EO chains. This suggests that yeast HK can work more actively in the vicinity of mitochondrial membranes in vivo. The photomerization of cysteine in the water pool with UV irradiation shows that cysteine is easily converted into cystine with lower Wg. This suggests that active oxygen is generated at the interface of the biomembrane rather than in bulk aqueous solution in vivo and SH groups of proteins in the cell membrane are oxidized similarly with UV irradiation. 

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