BACTERIA IN MILK

Waikato Independent, Volume XLV, Issue 6234, 4 May 1949, Page 7

BACTERIA IN MILK

COMMON CAUSES OF SOURING % Milk drawn from normal healthy cows should be free from bacteria as it leaves the, udder. But since there are many opportunities for contamination with bacteria from the milkingshed surroundings and from the milking machine itself, it is not surprising to find that all milk and cream, on arrival at a dairy factory or milk plant, contains a very mixed bacterial population. Bacteria make their presence known by causing the. milk to go sour, or by altering its qualitv in some other way (states G.J.H. in the latest News Circular of the N.ZDairy Research Institute). The most important group of organisms are the lactic streptococci which ferment the milk sugar and produce lactic acid—they cause the milk to turn sour. These bacteria usually predominate over all others in average milk after it has been kept for more than twelve hours, and especially if it has not been well cooled, because they are fast growers, and the conditions in milk just suit them. Speciallyselected strains of lactic streptococci are used as starters to produce, the controlled souring action required in cheese manufacture. Another group of bacteria —the coliform organisms—run the lactic streptococci a close second. They come from animal dung and from plants, and they also tend to sour milk. They form not only lactic acid, but some gas and a variety of objectionable flavours—effects which are seen (or .smelled) in a bad cheese-curd test sample. Whenever milk is carelessly produced, or a milking machine is allowed to become dirty, large numbers of coliform organisms gain access to the milk, and thus these bacteria are universally regarded as “indicator organisms”; i.e„, their presence indicates a lack of hygiene on a farm (or in a milk plant), .with a consequent possibility that many other objectionable or disease-producing bacteria may also have entered the milk. The lactic acid bacteria and coliform types together usually prevent the extensive growth of other types of bacteria which do not like a high acidity. 'Occasionally, however, in a milk kept for household consumption the “putrefactive” bacteria (of which there are many types) get the upper hand and the milk “goes bad” instead of souring. This usually occurs in a milk which has been pasteurised, or in a raw milk which has been very carefully produced. It is due to the destruction of the lactic acid types, and the survival of a small number of “putrefactive” types which—usually after a period of keeping longer than usual-—decompose some of the casein to give objectionable, flavours.