Milk-Powder Agar for the Determination of Bacteria in Milk

Abstract

Before presenting descriptions of new media for the determination of bacteria in milk, we wish to call attention to the fact that the media described give not only quantitative but, to a certain extent, qualitative results. If the new media achieved merely the same results as those now well known, we should not feel warranted in taking up the reader's time. It is believed that the time has passed when one can be satisfied with total counts only. Mere quantitative results in bacteriological milk analysis can not be interpreted satisfactorily unless something of the history of the milk is known. Often this "something" is missing; then one must interpret the count as due to this, that, or the other condition or a combination of them. This is not an attempt to discredit the total bacterial count, for it is very valuable as it stands, in milk-control work, but it is hoped that some of the media described in this paper will increase its value. The media extensively used in milk-control work have been (1) infusion agar, described in the standard methods of the American Public Health Association in 1910, and (2) extract agar, later adopted as a standard medium, its formula having been published by the association in 1916. Many who use the extract agar realize that it does not give the high counts which are obtained on the infusion agar, but use it because it is the standard medium for milk analysis. Even the members of the Committee on Standard Methods recognized the fact that extract agar is defective, as is shown by the state-565

Do different standard plate counting (IDF/ISSO or AOAC) methods interfere in the conversion of individual bacteria counts to colony forming units in raw milk?

“…According to Ayers and Mudge (), the methods that use conventional agar to determine bacteria in milk may contain lower bacterial count, mainly of lactic acid bacteria, when compared with powdered milk agar. The authors made the comparison of determining bacteria count in milk samples plated in conventional agar and with the addition of dried milk powder and found that of the 36 samples of milk plated in 22 cases, the counts were higher in powdered milk agar.…”

Detection and Enumeration of Spore-Forming Bacteria in Powdered Dairy Products

“…Total bacterial counts and spore counts, although informative, are not without their limitations. Almost a century ago it was highlighted that different media will result in different bacterial counts ( Ayers and Mudge, 1920 ) and that more than just quantitative data is needed with respect to contamination of dairy products, in order to determine the significance of the contamination ( Ayers and Mudge, 1920 ). The use of various heating methods is somewhat redundant in terms of identification of different species ( Miller et al, 2015 ).…”

Factors which Influence the Growth of Heat-treated Bacteria

“…The literature contains numerous references to the appreciably higher plate counts on pasteurized dairy products obtained by the use of improved media (Ayers and Mudge, 1920;Bowers and Hucker, 1935;Bradfield, 1937;Nelson, 1940;Phelan, 1936;Sherman, 1916;Yale, 1938). In milk, the organisms in question usually are present as mixed cultures, and the increases in count usually have been attributed to the ability of an increased number of organism types to develop on the improved media (Bowers and Hucker, 1935;Bradfield, 1937;Foltz and Bushnell, 1940;Yale, 1938).…”