Why does yogurt smell like sulfur after 2 days?

Question

Noticed a slight sulfur or eggy smell from your yogurt? Learn what can cause this to happen and if it's a sign of spoilage.

Answer 1

Yogurt smells like sulfur after a couple of days due to the breakdown of proteins and the production of volatile sulfur compounds by certain bacteria present in the yogurt. This is more likely to occur if the yogurt is not stored properly or is contaminated.

Detailed Explanation:

The sour smell you notice in yogurt after a couple of days, often described as sulfurous or eggy, is primarily caused by microbial activity. Yogurt contains live and active cultures, which are beneficial bacteria. However, other microorganisms can also be present, either as contaminants or as part of the natural evolution of the yogurt's microbial ecosystem.

Here's a breakdown of the process:

1. Protein Breakdown: Bacteria break down proteins in the yogurt. This process, called proteolysis, releases amino acids.

2. Sulfur-Containing Amino Acids: Some amino acids, like cysteine and methionine, contain sulfur. When these amino acids are metabolized by certain bacteria, they produce volatile sulfur compounds (VSCs).

3. Volatile Sulfur Compounds (VSCs): These compounds, such as hydrogen sulfide (H₂S), dimethyl sulfide (DMS), and methanethiol, are responsible for the unpleasant sulfurous odor. Different bacteria produce different VSCs, contributing to the specific smell.

4. Storage Conditions: Improper storage, such as leaving yogurt at room temperature or not sealing it tightly, can accelerate bacterial growth and the production of VSCs.

5. Contamination: Contamination with undesirable bacteria or yeasts can also contribute to the sulfurous smell. These organisms may produce different metabolites that contribute to off-flavors and odors.

Essentially, the sulfur smell is a sign that the yogurt is undergoing further fermentation and spoilage. While some level of fermentation is normal, a strong sulfurous odor indicates that undesirable microbial activity is occurring.

Pro Tip:

To minimize the sulfurous smell, always store yogurt in the refrigerator at a consistent temperature (around 40°F or 4°C) and ensure the container is tightly sealed. This slows down bacterial growth and helps preserve the yogurt's quality for a longer period. Discard yogurt if the smell is very strong or if you notice any signs of mold or unusual discoloration.