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| Figure 1. |
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Characteristics: A sliminess or stringy characteristic. The degree may vary from slightly increased viscosity to a stringy condition so pronounced that milk may be drawn out in long threads. It is not present at milking, but can become apparent after storage for several hours and is related to the growth of bacteria. The flavor of ropy milk may not differ from that of normal milk. It is not harmful, but its appearance is extremely unappetizing.
Causes: Ropiness is caused by gums or mucins formed by bacteria. These materials are closely related to the capsule or gelatinous membrane that surrounds the cells of some bacteria. Two common organisms that cause ropiness are
Alcaligenes viscolactis (viscous) and
Enterobacter aerogenes. Other known causes are strains of
Klebisella, Bacillus and
Pseudomonas. Some micrococci and strains of lactic bacteria (
Lactobacillus, Leuconostoc, Pediococcus and
Streptococcus) may also cause ropiness.
Sources of Contamination: Organisms responsible for ropy milk are usually picked up from improperly cleaned and sanitized udders and teats, contaminated water, feed or improperly cleaned and sanitized equipment including raw milk tanks, pumps, and tankers.
Testing for Ropy Milk: Unfortunately, there is not a modern-day scientific method for detecting the presence of “ropy” organisms in milk. Thus, industry must rely on a simple test that has been utilized for more than 50 years. This test involves incubating a small sample of milk at temperatures of 60 - 72°F for 24 to 36 hours and testing for ropiness by inserting a wooden pick into the milk and looking for the stringy condition of ropy or gently pouring from the vial looking for strands of rope or slime (Figure 1).
Control: Conditions that favor the growth and multiplication of bacteria (slow cooling and/or temperature abuse) magnify the problem. Most of the organisms that cause ropiness are Gram negative and destroyed by pasteurization. However, these organisms can circumvent the pasteurizer through splashes of raw milk and contribute to environmental contamination. Ropiness in pasteurized milk is sometimes not observed because the milk is consumed before sufficient bacterial growth has taken place to cause the defect. Many of the organisms that cause this condition are psychrotrophic and grow slowly at refrigerated temperatures. Some species of
Bacillus are psychrotrophic spore formers that survive pasteurization and cause ropiness, while other species cause spoilage during extended refrigerated storage. Once “seeded” in a plant, these organisms pose a serious threat to the quality of products produced. Rigorous cleaning/sanitizing procedures are required for their removal. The presence of ropiness in test products demands a thorough review of sanitation practices from the farm through the plant.
