In: Biology
Milk is a complex nutritional source that contains proteins (mainly casein) in an aqueous solution of lactose and minerals. Bacterial enzymes alter the media and may bring about various changes. Litmus is added to the medium to detect pH changes that may occur as a result of these enzymatic reactions. Above a pH of 8.3 litmus is blue, while below a pH of 4.5 litmus is red. The major milk substrates capable of transformation of the milk sugar- lactose and 3 milk proteins: casein, lactalbumin & lactoglobulin. Litmus milk now forms an excellent differential medium in which microorganisms can metabolize milk substrates depending on their enzymatic complement. Litmus is incorporated into the medium to distinguish among the metabolic changes produced in milk.
A number of results may be obtained in the Litmus Milk test, making it a rather complex test. There are four main reactions: lactose fermentation, litmus reduction, casein coagulation, and protein hydrolysis. Some or all of these reactions may occur at the same time, and many of them can have more than one reaction (for example, casein coagulation can be from a solid clot, as in an acid clot, or from the casein being converted to a solid curd in the bottom below a liquid whey (as in curd formation).
Litmus was not included in manuals because Litmus milk is a complex medium that can produce a diversity of results. Because of this, litmus milk can give quite unreliable results or not trust worthy in all the cases i.e., the results often give negative results. Therefore, the results of this test should not be used to solely identify a species, other tests must be used along with a close fitting but not necessarily an exact match to the expected Litmus results to confirm your identification. Thus, it would be advised to use litmus milk as a confirmatory test but not a definitive test (except as a last resort).
Rennet curd is known to be soft & semisolid and will flow slowly if tube is tilted. Some organisms produce rennin, an enzyme that acts on casein to form paracasein, which in the presence of calcium ions is converted to calcium paracaseinate & forms an insoluble clot. This is a soft and semisolid clot, that flow slowly and are known as Rennet curd. A clot formation is simply recorded as “clot” and cannot clearly differentiate between a clot and curd formation in this medium.