Raka-Ray Agar Base is a selective medium for the isolation of lactic acid bacteria in beer and brewing processes. It yields very good results in the detection of lactobacilli in the fermentation processes of beer. These organisms can change the organoleptic characteristics of the beer through their metabolites. Detection is complicated because of the nutritional and environmental requirements of these organisms. For these reasons, several formulations have been described to optimize the medium and obtain good growth. Higher counts of lactobacilli in comparative tests have been obtained with this medium because it contains growth nutrients and stimulants such as liver extract, yeast extract, tryptone, N-acetylglucosamine and sorbitan monooleate. Maltose and fructose are added as sources of carbohydrates when certain lactobacilli canÂ´t use glucose. Selectivity is obtained by adding 3 g/l of phenylethanol, to inhibit Gram-negative bacteria, and cycloheximide to inhibit yeasts. Sulfate salts provide inorganic ions. Betaine hydrochloride is used as a growth stimulating agent. Diammonium hydrogen citrate and potassium phosphate are buffering agents. Potassium aspartate and potassium glutamate are additional sources of amino acids. Bacteriological agar is the solidifying agent. Lactobacillus fermentans grow as white-cream colonies. If the number of colonies on each plate exceeds 300, dilute sample 1:10 in sterile saline and re-test.
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