Does Miso Go Bad

Few people know that miso is actually a fermented food product. This means that it is still living. That begs the question: does miso go bad? Although miso that is sold in a commercial context is vacuum packed, it becomes much less stable once it is opened. Let’s explore miso’s longevity and provide some storage tips to extend its lifespan.

Miso Background

Miso is created through a fermentation process that has the potential to function as a natural preservative if the proper conditions exist. Since miso is alive, it will continue to ferment, albeit very slowly, while it remains in its packaging. You can tell that miso is alive by the fact that it generates gasses and a general puffiness when stored inside of a bag. The quick cook version of miso is typically pasteurized ahead of packaging to limit the production of harmful carbon dioxide. All sorts of synthetics and chemicals like ethyl alcohol, bleaches and food coloring are commonly added to miso in an attempt to slow the aging process.

Miso soup

Image used under Creative Commons from Guilhem Vellut

Shelf Life of Miso

All miso packages have a “best by” date printed onto them. You will enjoy flavorful miso if you consume it before this date. If you scour the world wide web, you will find stories from those who have kept miso pastes in the refrigerator for years without any noticeable changes, molds or other signs of deficiencies. Yet miso and miso pastes do not last forever.

Miso is a fermented product with salt that allows it to last quite a while. Korean miso is typically sold in a plastic tub that allows it to stay softer than other types of miso when refrigerated. While its color will darken over time, it will still taste good and not cause sickness unless stored for an extraordinarily long period. Although older miso will not go bad for a significant period of time, it will slowly deteriorate. Some have reported that they have consumed miso up to a full year beyond its “best by” date. However, these individuals limited the miso’s exposure to fresh air. Its shelf life depends on the miso’s specific moisture and salt content. It is important to note that some versions of miso have various perishable food products added like seaweed stock, fish and salt. In general, the light colored versions of miso tend to last 9 months. Miso with darker hues tends to last about a year and a half.

How to Extend the Life of Miso

After you open your package of miso, transfer it to a airtight container. Close it tightly and store it in the refrigerator. An airtight seal combined with the cold air of a refrigerator will slow its aging process. Some store miso in the freezer to extend its lifespan. Others report that they cover their miso with a plastic wrap, similar to guacamole, and pop it right into the refrigerator as soon as possible. Dedicated foodies often choose to dehydrate or freeze dry miso for an even longer lifespan.

The Japanese have been dehydrating miso for about half a century. They use a process referred to as “spray drying”. This requires the mixing of miso with water. Once combined, the mixture is run through an atomizer. At this point, the miso solidifies as a powder. Since this is a fairly complicated and laborious process, most prefer to freeze dry miso to extend its longevity. You can quick-freeze your miso on trays that are vacuum-dried for a couple of hours. However, this requires significant effort and the process is fairly expensive. Yet it succeeds in preserving the wonderful miso flavor.

How to Tell if Your Miso has Gone Bad

You will be able to tell that your miso has gone bad if you find mold on it. Sometimes, a thin layer of mold will generate across the top or sides of miso, even if it is stored in the refrigerator. While it is possible to scrape this layer of mold off and consume the rest of the unaffected miso, doing so is not recommended.