LMDA – Lee’s Multi-Differential Agar
By Carli Severson
Microtechnologist
The Brewing Science Institute
In a perfect world, a brewer would only have one organism, the culture yeast, present during fermentation. Even though the brewing environment is home to many bacterial species and wild yeasts, this is a realistic goal, under strict sanitization, aseptic practices, and use of modern day resources. The growth of bacteria and wild yeast can contribute unwanted flavors to wort and beer. The degree to which it can affect flavor depends on the stage of brewing at which it occurs.
Main Types of Wort & Beer Spoilage Bacteria
The main types of wort and beer spoilage bacteria include: Flavobacterium, Enterobacter, Escherichia, Lactobacillus, Pediococcus, Acetobacter, Acetomonas, and Zymomonas (3). Wild yeast strains come in two groups: Saccharomyces and Non-Saccharomyces (4). Both bacteria and wild yeast can cause turbidity, haze, slime formation, over-attenuation, souring, and off flavors in the product. There are many kinds of media available to determine if there is contamination by wild yeast or spoiling bacteria.
Lee’s Multi-Differential Agar (LMDA) is a medium for both the identification and enumeration of microorganisms that are commonly found in the brewing industry. LMDA allows for visual interpretation of bacteria and wild yeast; each type has distinct traits. Differentiating characteristics include color, surface appearance, shape, and possible formation of a halo zone around bacterial colonies (3).
LMDA includes an extensive list of ingredients in minuscule amounts: tomato juice broth, peptonized milk, dextrose, calcium pantothenate, a pH lowering agent, calcium carbonate, a wetting agent, bromocresol green, a yeast suppressing agent, and agar (3). Bromocresol green turns yellow to indicate acid production (4). The acid dissolves the calcium carbonate creating a halo around the colony, indicating the degree to which the acid has been produced (4). Both ingredients work together to differentiate between true acid producers and slight acid producers. LMDA plates can be used aerobically or anaerobically depending on which bacteria the brewer wants to detect. Incubation is done under anaerobic conditions, except when acetic acid bacteria are of major interest, then aerobic incubation is used (1).
Cycloheximide, an antibiotic, can be added to LMDA to make the media selective to bacteria and wild yeast. With cycloheximide, one can suppress the growth of most 
brewing yeasts without interfering with bacteria and wild yeast growth. Cycloheximide is shown to be a teratogen; a drug or other substance capable of interfering with the development of a fetus, causing birth defects (1). With increased concerns as a health hazard, nystatin is a suitable alternative sought to replace cycloheximide. Research is being done to question its effectiveness compared to the traditional additive.
Previous differential media allowed one to differentiate grossly different microbial types, such as yeast and bacteria, or to differentiate between limited types of bacteria (2). LMDA allows professionals to identify microorganisms and in turn, assists them in gauging the seriousness of the problem and locating the source of the contamination to eliminate it.
To order LMDA plates, use the link: https://brewingscience.com/product/lmda-brewing-bacteria-40-shipping-30-sterile-pre-poured-plates/
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References
1. American Society of Brewing Chemists. Report of Subcommittee on Microbiological Controls. Proc. 1975.
2. Lee, S.Y., inventor; 1974 Apr. 1. Multi-Differential Agar Culture Medium. United States patent US 3,878,050
3. Lee, S. Y., Jangaard, N. O., Coors, J. H., Hsu, W. P., Fuchs, C. M., & Brenner, M. W. (2018). Lee’s Multi-Differential Agar (LMDA); A Culture Medium for Enumeration and Identification of Brewery Bacteria, Proceedings. Annual meeting – American Society of Brewing Chemists, 33:1, 18-25, doi: 10.1080/00960845.1975.12007044
4. Spedding, G. (2000 Seminar). Microbiological Media for Bacteria and Wild Yeast Detection in the Brewery.