Post Fermentation Infection – What It Is and How to Deal With It

If you've ever opened a fermenting jar and smelled something sour, funky, or downright rotten, you’ve probably faced a post fermentation infection. It happens when unwanted microbes slip in after the main fermentation is done. The result? Off‑flavors, cloudiness, and sometimes a batch that’s totally undrinkable. The good news is you can catch it early and often still salvage the batch.

Common Signs of an Infection

First, trust your senses. A sharp, vinegar‑like aroma or a sudden fizz that wasn’t there before usually means bacteria or wild yeast are at work. Look for a thin film on the surface – a pellicle – that can be white, brown, or even greenish. If the liquid turns cloudy or develops strange bubbles, that’s another red flag. Taste a tiny sip; if it’s overly sour, metallic, or has a ‘wet dog’ taste, you’re dealing with an infection.

Don’t forget visual clues on the container. A cracked seal, loose lid, or a gasket that has dried out can let contaminants in. Even a small dust particle can be a gateway for wild microbes, especially in warm rooms where they thrive.

Steps to Save Your Batch

1. **Isolate the problem** – Move the suspect batch away from healthy ones. This prevents cross‑contamination.

2. **Adjust acidity** – Raising the acidity can slow down unwanted microbes. Add a measured amount of food‑grade acid (like citric acid) and re‑taste.

3. **Cold‑crash** – Drop the temperature to near freezing for 24‑48 hours. Cold temperatures cause many bacteria to become inactive and settle to the bottom, making it easier to rack the clear liquid off.

4. **Racking** – Carefully siphon the clear liquid into a clean, sanitized container, leaving sediment and any film behind. Use a fine‑mesh filter if you have one.

5. **Sanitize again** – If you plan to continue fermenting, give the new container a good sanitizing rinse. This reduces the chance of another infection.

6. **Monitor** – Over the next few days, keep checking for any new off‑smells or clouds. If everything stays clean, you can finish the batch as planned.

When working with coffee fermentation, the same rules apply. Coffee beans can be fermented to develop flavor, and an infection can ruin subtle notes. Keep the fermentation vessel sealed, use clean water, and work in a cool, dark spot. If you notice a strange odor during the coffee wash, follow the steps above before moving the beans to drying.

Prevention is always cheaper than cure. Start each batch with fresh, high‑quality starter cultures, and always sterilize your tools. Store lids and gaskets in a dry place, and check seals before you begin.

In the end, a post fermentation infection is just another hiccup in the brewing journey. With a nose for the odd smell and a few simple steps, you can turn a potential loss into a learning experience – and maybe even a new flavor twist if you decide to keep the funky notes on purpose!