Lactic Acid Bacteria Selective Agar Base is based on the formula of Saha, Sondag and Middlekauff for the detection of lactic acid bacteria in beer and brewing processes (1). It is recommended by European Brewing convention (EBC) and the American Society of Brewing Chemists for isolation of Lactobacilli (2, 3). The family Lactobacillaceae has members that are important spoilage organisms in the brewing process.
The original medium viz. Raka-Ray Medium (1) was developed to enable brewers to monitor in-process beer quickly and accurately for a wide range of organisms including pediococci. Further studies towards optimization of conditions of growth factors led to the modification of Raka Ray medium with the addition of sorbitan mono-oleate to stimulate growth of lactic acid bacteria and incorporation of sugars such as fructose as an essential carbohydrate source for Lactobacillus fructivorans and maltose for lactobacilli as it lacks the ability to metabolize glucose.
Casein enzymic hydrolysate provides the nitrogenous compounds, potassium aspartate and potassium glutamate are additional sources of the respective amino acids while diammonium hydrogen citrate buffers the medium.
The addition of phenylethanol and cycloheximide in the supplement (FD055) make the medium selective for the isolation of lactic acid bacteria in beer. Phenylethanol inhibits gram-negative organisms, while yeasts are inhibited by cycloheximide. Polysorbate 80 or sorbitan monooleate (in FD055), liver concentrate, yeast extract and N-acetyl glucosamine act as growth stimulating agents.
Fructose is the essential carbohydrate source for Lactobacillus fructivorans (4), maltose helps in detection of lactobacilli which cannot utilize glucose (5) whereas glucose is utilized by pediococci (6).
Storage and Shelf-life:
Store dehydrated medium and the prepared medium at 2 8°C.Use before expiry date on the label.
References:
1. Saha R.B., Sondag R.J. and Middlekauff J.E. (1974) Proceedings of the American Society of Brewing Chemists, 9th Congress, 1974.
2. Methods of Analysis of the ASBC (1976) 7th Edition. The Society, St. Paul Mn USA. 3. European Brewing Convention, EBC Analytical Microbiologica: Part II J. Inst. Brewing (1981) 87. 303-321.
4. Van Keer B., Van Melkebeke L., Vertriest W., Hoozee G. and Van Schoonenberghe E. (1983) J. Inst. Brewing 89. 361-363.
5. Lawrence D.R. and Leedham P.A. (1979) J. Inst. Brewing 85. 119.
6. Coster E. and White H.R. (1951) J. Gen. Microbiol. 37.15.