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Food Explainer: Why Does Fish Sauce Have an Expiration Date?

A fish sauce factory in Vietnam.

Photograph by Stefan via Wikipedia

The second entry in a new occasional series in which we demystify all manner of gustatory conundrums and culinary puzzles. If you have a food-related question you’d like answered, see the email address provided below.

Fish sauce, that pungent, savory Southeast Asian condiment, has been selling more slowly than usual in Vietnam lately. Home cooks, it seems, are making the staple last longer, possibly longer than the bottle advises. But why does fish sauce even have a use-by date? Hasn’t the sauce, by its very nature, already gone bad?

Yes, but the environment in which the fish protein degrades makes fish sauce—liquified fish that’s been fermented with salt—very different from, say, fresh fish that’s been left rotting on the kitchen counter for two days. The latter breaks down haphazardly, facilitated by a host of unruly microbes, and results in rank smells, fetid flavors, a slimy texture, and even potentially harmful toxins and contamination by disease-causing pathogens like salmonella and E. coli.

The fish-sauce-making process, on the other hand, is carefully controlled to prevent the growth of dangerous bacteria. First, whole, tiny fish (predominantly anchovies) are combined with salt in roughly a three-to-one ratio. The mixture is then tossed into either concrete or wooden tanks. Enzymes contained within the cells of the fish flesh degrade the proteins, fats, and other molecules into amino acids (compounds responsible for the deeply savory flavor in fish sauce), eventually liquifying the solid tissue. The salt accelerates this process, known as protein hydrolysis, by causing the fish cells to open up, which releases the enzymes.

The briny conditions in the tank keep most microbes and all pathogens at bay, but some harmless, extremely salt-tolerant bacteria, known as halophilic bacteria, survive. These occur naturally in the guts, mouth, and skin of the decomposing anchovies. They too break down the fish proteins, yielding various acids, alcohols, and nitrogen compounds that give fish sauce its characteristic cheesy, meaty flavors. But by the end of fish sauce’s six- to twelve-month fermentation period, the salt has managed to kill even the hardiest halophilic bacteria. At this point, the clear, amber liquid in the tanks is filtered, and any sediment is discarded. The fermentation process does such a good job of killing off bacteria that no pasteurization is required.

There is no legal requirement in the U.S. for fish sauce to bear an expiration date, but most manufacturers put dates on the label anyway, since people are more inclined to trust foods that tell us when they should be discarded. These dates tend to be three to four years from the date of manufacture, which is actually on the conservative side in light of industry guidelines. The Handbook of Indigenous Fermented Foods in the ASCA Countries (published by the Association for Science Cooperation in Asia, a science-policy organization) ascribes to the condiment a shelf life of five years.

This is isn’t to say fish sauce will always be good, either before or after the use-by date (though you should always refrigerate it after opening). It can deteriorate in quality over an extended period of time due to chemical reactions, resulting in color changes or the development of “off” flavors. On rare occasions mold or yeast might develop on the inside surface or lip of the bottle where there’s excess moisture and less salt. These growths are usually innocuous, but as with any food, if it looks strange, smells strange, or tastes strange, you should throw it out.

Food Explainer thanks Bob Hutkins, Professor of Food Microbiology at the University of Nebraska.

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Previously from the Food Explainer:
Why Is Cheese Yellow When Milk Is White?