Ropiness throughout the milk may be caused by any of the following kinds of baterias:
- Enterobacter aerogenes, E cloacae, Klebsiella and rarely Escherichia coli. Ropiness caused by Enterobacter usually is worse near the top of the milk.
- Certain strains of lactic acid bacteria. Lactobacillus casei, L. bulgaricus , and l. plantarum may also produce ropiness. Most of these lactic bacteria can grow in long chains, characteristic that supposedly contributes to the stringy condition of the milk.
- Miscellaneous other bacteria among the alkali formers, micrococci, streptococci and bacilli. Ordinarily these bacteria would be suppressed by acid formers.
Since the sources of the bacteria causing ropiness are water, manure, utensils and feed, the reduction or elimination of contamination from these sources helps prevent ropiness. Adequate pasteurizationof milk readily destroys most of these kinds of bacteria.
Spoilage of Milk Products
Frozen desserts include ice cream, ice milk, frozen custards, sherbets etc. The ingredients may be various combinations of milk, cream, evaporated milk, condensed milk, dried milk, flavors, sweetening agents. Any of these may contribute microorganisms to the product and affect the quality of the dessert as judged by its bacterial content or its content of specific kinds of bacteria such as the coliforms. The desserts are not ordinarily subject to spoilage however as long as they are kept frozen. The only important types of spoilage take place in the ingredients before it is frozen no spoilage problems should result unless it is held at temperature above freezing for a considerable time, when souring by acid forming bacteria can take place.
Many of the defects of butter originate in the cream from which it is made especially when the cream has been held for several days on the farm before collection by the creamery. During this time lactic acid bacteria, gas formers and other spoilage organisms may grow and be followed by molds e.g Geotrichum candidum. Lactose fermenting occasionally, yeasts, which are present only occasionally may develop high gas pressures in the can of cream.