Yeasts: osmophilic

Erythritol is a starch-derived product. The starch is enzymatically hydrolyzed into glucose which is turned into erythritol via a fermentation process, using osmophilic yeasts or fungi (e.g. Moniliella, Trigonopsis, or Torulopsis) ... 

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-... 

Osmophilic yeasts Saccharomyces rouxii) few molds Aspergillus echinulatus, Monascus bisporus) Dried fmits containing 15-20% moisture some toffees and caramels honey. 

There are, however, several complications. The terms halophilic and osmophilic already suggest that different substances have different effects. Halophilic bacteria can tolerate a high concentration of salts, but not of sugars, and it is the other way round for osmophilic yeasts. Table 8.1 gives some examples of the lowest aw tolerated, when caused by various components. It is seen that the variation is considerable, by a factor of 5 in (1 — aw). This implies that the components have specific effects. Ethanol and glycerol cannot be kept out of the cell by most organisms, and thus do not cause an osmotic pressure difference over the cell membrane. 

The water content of honey should be less than 20%. Honey with higher water content is readily susceptible to fermentation by osmophilic yeasts. Yeast fermentation is negligible when the water contents is less than 17.1%, while between 17.1 and 20% fermentation depends on the count of osmophilic yeast buds. 

Extract of malt with or without cod-liver oil is liable occasionally to ferment owing to the presence of osmophilic yeasts. The yeasts are naturally present in the original raw materials and some growth can take place during... 

Weigh about 1 g of the extract of malt and disperse into 9 ml of sterile saline. From this first dilution prepare further ten-fold serial dilutions in saline and inoculate 1 ml of each (representing 0 1 g, 0 01 g and so on of the original sample) into a bottle of the sterilised medium. Incubate at about 25° for seven days. Each bottle of inoculated medium showing gas production (easily observed on swirling gently) indicates growth of an osmophilic yeast. 

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