Why does yogurt develop a slimy top layer?

Question

Noticed a slimy layer on top of your yogurt? Learn what can cause this to happen and if it's a sign that your yogurt has gone bad.

Answer 1

A slimy top layer on yogurt is usually caused by the growth of harmless surface yeasts or bacteria that thrive in slightly acidic environments and consume lactose, producing exopolysaccharides (EPS) which create the slimy texture. Proper storage and handling can minimize this.

Detailed Explanation:

The development of a slimy top layer on yogurt is a common phenomenon, and it's generally not harmful, although it can be unappetizing. Here's a breakdown of the reasons why it occurs:

1. Microbial Activity: Yogurt contains live and active cultures, primarily bacteria like Streptococcus thermophilus and Lactobacillus bulgaricus. Sometimes, other microorganisms, such as certain yeasts or bacteria, can also be present. These organisms can grow on the surface of the yogurt, especially if there's a small amount of air exposure.

2. Exopolysaccharide (EPS) Production: Some of these surface-growing microorganisms produce exopolysaccharides (EPS). EPS are essentially long chains of sugar molecules that create a viscous, slimy texture. These are produced as a byproduct of their metabolism, often when they're consuming lactose (milk sugar) at the surface of the yogurt.

3. Surface Conditions: The surface of the yogurt can have slightly different conditions than the rest of the yogurt. It's more exposed to air, which can affect the pH and moisture levels. This can create a favorable environment for the growth of these EPS-producing microorganisms.

4. Storage Conditions: Improper storage, such as fluctuating temperatures or leaving the yogurt container open for extended periods, can encourage the growth of these surface organisms.

While the slimy layer is usually harmless, it's always a good idea to inspect the yogurt for other signs of spoilage, such as mold growth or an off odor. If you notice anything unusual, it's best to discard the yogurt.

Pro Tip:

To minimize the formation of a slimy top layer, store your yogurt in the refrigerator at a consistent temperature, and ensure the container is tightly sealed after each use. Avoid leaving the yogurt out at room temperature for extended periods.