What you see: Black discoloration on your shrimp. The tails are turning black and amorphous black blobs have formed where the shell segments connect.
What it is: Melanosis! Also known as “blackspot.”
Eat or toss: Eat! You’re seeing a natural, biochemical reaction between the shrimp and air. It’s harmless and is similar to browning on apples, avocados and other types of produce. But, it could (or could not) be a sign the shrimp is older, so give it a sniff while you read on.
Why do some shrimp have dark spots on their shells
When shrimp are removed from the water, a series of chemical reactions takes place. Much like the way an apple slice or guacamole eventually turns brown after contact with oxygen, certain areas of a shrimp will blacken after exposure to air. Known as melanosis (after a family of pigments that also color human hair and skin), this phenomenon tends to be most apparent on the head, tail, the spots where the shrimp’s shell segments connect, and the swimmerets (yep, those are little shrimp legs!).
Just as an apple slice with some browning is still fine, this color change doesn’t impact the safety, flavor or texture of the shrimp.
It is, however, possibly a sign that shrimp is older, as melanosis can take time—between minutes and days—to appear. But, the speed with which melanosis appears can be impacted by innocuous factors like the type of shrimp, the season, and where the shrimp is in its molting cycle, or more concerning issues like injuries, rough handling, or improper storage temperature.
Here are some shrimp harvested about 24 hours earlier. They’re still fresh and will taste great, but you can already see some black melanosis on them:
And here are some shrimp that are 12 days old. They smell bad and won’t taste good. They also have more obvious melanosis:
The bottom line: it’s tough to reach any conclusions about the eating quality of shrimp simply because it’s sporting melanosis.
Evaluate shrimp freshness with a simple sniff test
“If there’s a problem or there’s an age issue, the smell is going to tell you before the melanosis does,” said Julie Anderson Lively, executive director of the Louisiana Sea Grant College Program. “One of the advantages and disadvantages of seafood is seafood will smell very quickly when it starts to spoil.“
Chemical treatments prevent melanosis
Consumers don’t see melanosis terribly often because shrimp producers typically dip shrimp in a chilled bath of a sulfite solution. The chemical basically blocks the chain of reactions that cause melanosis and thus keeps the shrimp looking pristine. Without the treatment virtually all shrimp would develop the inky blobs of melanosis.
But sulfites, which are used as preservatives in products ranging from raisins to canned vegetables, aren’t ideal. Some people are allergic to them and they can cause dangerous sulfur dioxide build up on shrimping vessels. Seafood Source reported that between 1970 and 1993, the U.S. Coast Guard attributed 11 deaths and 32 injuries to sulfur dioxide in unventilated holds on shrimp boats.
Some alternatives are available. A chemical, 4-hexylresorcinal, which can be derived from kiwis, is effective, but also more expensive. In a possible win-win, some researchers are studying how chemicals extracted from by-products of certain agricultural commodities could prevent melanosis. For example, the peel and seed from avocados or the pulp and pits left after extracting oil from olives could provide low-cost options.
Is melanosis a problem that needs to be solved?
Melanosis, with its creeping and expanding rounds of black, is certainly unattractive and it’s hard to blame consumers for avoiding it. Still, a seafood manager argued in the New York Times that so-called blackspot is actually a good thing. If you see melanosis, he said, you can be certain the the shrimp wasn’t treated with chemicals.
But, given the many variables that can affect melanosis development, Lively said it’s best to evaluate the shrimp on a case by case basis.
“If I’m buying shrimp from a supplier I know, maybe someone who comes to our farmers market, I know he’s doing the best in cold chain management. So if he has some blackspot I’m not going to be concerned at all. If I’m at a really inexpensive grocery store where I may not know where the shrimp came from I’m more likely to go buy elsewhere because I don’t know if they’re old shrimp or they’re just untreated.”
- Julie Anderson Lively. Executive Director. Louisiana Sea Grant College Program. Associate Professor. Louisiana State University. School of Renewable Resources.
- Preventing Melanosis in Shrimp. Hervé Lucien-Brun. October 2005. Responsible Seafood Alliance. Global Seafood Advocate.
- Shrimp dealers weigh benefits of new solution for melanosis. Chris Loew. Seafood Source. May 25, 2017.
- Preventing Black Spots From Appearing on Healthy Shrimp. Julie Anderson Lively and Evelyn Watts. Louisiana State University Ag Center. LSU College of Agriculture. December 2018.
- Phan, D., Bui, T. H., Doan, T., Nguyen, N. V., & Ly, T. H. (2021). Inhibition of Melanosis in Whiteleg Shrimp (Litopenaeus vannamei) during Refrigerated Storage Using Extracts of Different Avocado (Persea americana Mill.) By-Products. Preventive nutrition and food science, 26(2), 209–218. https://doi.org/10.3746/pnf.2021.26.2.209
- Shrimp and Chemicals: What you need to know. Melissa Clark. New York Times. Oct. 15, 2019.
- Melanosis in Shrimp/Prawn. Technical Bulletin. Bureau of Agriculture and Fisheries Standards. Philippines.
- Studies on some enzymes and the effect of modifield atmosphere packaging and some chemicals of quality changes of black tiger and white shrimps during refrigerated storage. Chapter 1. Ruangnalin Thepnuan. Prince of Songkla University Faculty of Agro-Industry (Food Technology). Chapter 1 available here: https://kb.psu.ac.th/psukb/bitstream/2553/1571/7/292762_ch1.pdf
- Melanosis in crustaceans: A review. Alex Augusto Gonçalves, Adriene Rosceli Menezes de Oliveira. LWT – Food Science and Technology. September 2016