Food microbiology — Dairy defect: In milk, what is the principal cause of ‘‘ropiness’’ (stringy, slimy texture) observed during bacterial spoilage under refrigeration or mild temperature abuse?

Introduction / Context:
Bacterial ‘‘ropiness’’ in milk is a classic quality defect of dairy microbiology. It presents as strings or long slimy threads when the milk is poured, and is often noticed by consumers as an abnormal viscosity or mouthfeel.

Given Data / Assumptions:

• The question asks for the principal cause of ropiness.
• Typical psychrotrophic or mesophilic bacteria (e.g., Alcaligenes, Enterobacter, Klebsiella, Lactococcus/Lactobacillus) can produce extracellular polymers.
• Milk composition remains otherwise normal at onset; the defect is microbially driven.

Concept / Approach:
Ropiness is primarily due to the secretion of extracellular polysaccharides (EPS), historically described as ‘‘gums’’ or ‘‘mucins.’’ These capsular or slime layers increase apparent viscosity and form threads. Proteinaceous or lipidic capsular materials are not the dominant chemical basis of ropy defects in milk.

Step-by-Step Solution:
Identify the defect: ropiness = slimy, stringy texture. Link to microbial physiology: many gram-negative coliforms and some lactic acid bacteria synthesize EPS. Major macromolecule responsible = polysaccharide (gums/mucins) from bacterial capsule or slime. Therefore, select the option citing extracellular polysaccharides.

Verification / Alternative check:
Standard dairy science texts attribute ropiness chiefly to EPS production; heat treatment and hygiene that reduce EPS-producing organisms mitigate the defect.

Why Other Options Are Wrong:

• Proteins: although proteins exist in capsules for some microbes, they are not the usual chemical basis of ropy milk.
• Lipids: bacterial lipid secretion does not explain the thread-forming viscosity.
• All of the above: over-broad and not reflective of the dominant mechanism.

Common Pitfalls:
Mistaking casein coagulation or fat separation for ropiness; confusing yogurt-like viscosity (intentional EPS) with spoilage EPS in fluid milk.

Final Answer:
Slimy capsular material from the cells, usually gums or mucins (extracellular polysaccharides)