What you see: Your RAW potatoes are foaming. And/or, when you squeeze one, it’s as soft as mashed potatoes. Other potatoes may be sitting in pools of liquid.
What it is: The work of soft rot bacteria and/or the consequences of being frozen.
Eat or toss: Discard (compost, ideally) potatoes that are squishy or oozing anything. Inspect the rest and if they’re still firm and foam-free, they’re probably fine. Just give them a good scrub with a brush under running water.
Why did these potatoes go squishy and start to foam?
The weekend before Thanksgiving, Stephanie Miller purchased some Yukon gold potatoes at her local farmers market in Washington, DC. She, of course, had plans for mashed potatoes.
But something else was cooking in these spuds.
As she got ready for Thanksgiving, Stephanie noticed that some of the potatoes were, uh, foaming. Others rested in pools of a mysterious liquid. She squeezed another and it was like, somehow, it had already been mashed, but in the worst possible way. Check it out:
Stephanie hurried out to get some new potatoes, thus saving Thanksgiving at the Miller household. But, luckily for us, she sent images and video of the ordeal over to EatOrToss. We consulted with University of Idaho potato specialist Nora Olsen, who identified two possible explanations for the squishy, sputtering spuds: either the potatoes were damaged by frost or a too-cold refrigeration misfire; and/or microbes known as “soft rot bacteria” settled in. Soft, indeed.
In either case though, Olsen said that any unaffected potatoes would still be OK to eat. You’ll know they’re still OK if they’re firm, not oozing anything and don’t have any obvious injuries on the surface. Scrub them clean and use them quickly.
Freezing injury damages potato cells
Many fruits and vegetables, potatoes included, do strange things at cold temperatures (examples: eggplants, plums, avocados). Freezing temperatures are even worse, causing potato cells to break down. This means that materials inside the potatoes’ cells leak, changing the potatoes texture from firm to, it would appear, paste.
Sometimes, when fruits and vegetables are hurt by cold temperatures, the damage doesn’t show up until the produce starts to warm. That warming could have begun the moment that Stephanie stashed the potatoes on her counter. Before then, maybe there was a frost. Maybe the farmer’s cooler malfunctioned and got too cold. The potatoes would have looked fine until they arrived in Stephanie’s comparatively balmy kitchen.
Soft rot bacteria can turn potatoes to mush
Another explanation for these foaming potatoes is destruction wrought by a group of microbes dubbed “soft rot bacteria.” A number of species fall under this umbrella, though Olsen identified pectobacteria as the most likely culprit—in potatoes they tend to induce softened, tan or cream-colored flesh and and cause slimy or watery substances to ooze out of the tuber.
When a potato is damaged by cold or freezing temperatures, soft rot can more easily invade, but the bacteria can also settle in all on its own, slipping in through bacteria-friendly doors like wounds (wounds commonly occur during harvest) or lenticels (breathing pores).
The bacteria find their way to potatoes via soil, water and weeds, and can even hitch rides on insects and equipment. And, although we just talked about the dangers of cold temperatures, if a farmer stores potatoes somewhere too warm and just a little wet, soft rot bacteria might have felt especially welcome to a Thanksgiving feast of their own.
Soft rot bacteria can make a potato’s flesh “macerated”–sounds a lot like what Stephanie encountered when she squeezed a potato in the video above. The bacteria degrade molecules that bind plant cells together, which causes the tissue to fall apart, according to the University of Wisconsin.
Stephanie also reported that the potatoes didn’t smell bad – that also tracks with soft rot bacteria. In isolation, it’s odorless. However, other microbes, which do generate odors, can settle in after soft rot and then create some stinky smells.
What’s leaking out of these potatoes?
So why all the liquid? While potatoes seem drier than, say, tomatoes, they’re actually filled with water that’s tightly packed within their cells (that’s why raw potatoes are so hard!). When those cells break down, whether by microbes or freezing temperatures, the liquid escapes.
“It’s so much water,” Olsen said. She said the pools that formed below Stephanie’s potatoes were probably liquid from the potatoes mixed with any liquid that had been expelled by bacteria as they stuffed their one-celled faces with potato. Eww.
It’s OK to compost rotten potatoes
Stephanie, author of Zero Waste Living: The 80/20 Way, isn’t one to waste food and was particularly distraught about having to throw away her Yukon Golds. But that led to another tricky question. Could composting diseased potatoes spread disease in her backyard vegetable garden?
In this case, Olsen said not to worry. Soft rot bacteria, is “in your soil is already,” she said. “We have it everywhere. I would definitely put those in my compost,” she said.
Soft rot is common and devastating
Stephanie isn’t alone in her foaming potato struggles. Soft rot is very common among a wide range of produce. According to the University of Wisconsin, “Bacterial soft rots are a group of diseases that cause more crop loss worldwide than any other bacterial disease.”
They come by this designation in part by being able to thrive in various conditions. Writing for Growing Produce, Phil Nolte notes that, as “facultative anaerobes,” soft rot bacteria can operate with or without oxygen. When a waterlogged field or even a film of moisture cuts potatoes off from oxygen, the potatoes become especially vulnerable and soft rot bacteria can really thrive.
As Nolte wrote, “The ability to exist between worlds, with or without oxygen, affords a unique niche for this group of extremely important pathogens.”
- Nora Olsen. Professor and potato specialist. University of Idaho.
- Soft rot diseases of potatoes. Andrew Taylor. Department of Primary Industries and Regional Development. Government of Western Australia. Last updated August 2017.
- Bacterial Soft Rot and Blackleg. University of California, Agriculture and Natural Resources. Integrated Pest Management Program. Last updated March 2019.
- Bulletin #2493, Potato Facts: Blackleg and Bacterial Soft Rot. By Steven B. Johnson, Ph.D., Extension Crops Specialist. Cooperative Extension Publications. University of Maine. 2015.
- Potato (Solanum tuberosum)-Bacterial Soft Rot, Blackleg, and Lenticel Rot. By K. Frost and C. M. Ocamb. Pacific Northwest Pest Management Handbooks. Last updated March 2023.
- Bacterial Soft Rot. Institute of Agriculture and Natural Resources. University of Nebraska – Lincoln.
- Tuber Soft Rot, Blackleg and Aerial Stem Rot. Noah Rosenzweig. Michigan State University Extension. April 15, 2016.
- Storing potatoes for quality and food safety. Michelle Jarvie, Michigan State University Extension – November 2016
- A Few Potatoes in My Bag are Leaking Liquid And Smell Bad. Do I Have to Throw Them All Away? Ask Dr. Potato. The Idaho Potato Commission.
- Potato soft rot (296). Pacific Pests, Pathogens & Weeds. Australian Centre for International Agriculture Research.
- Bacterial Soft Rot. Science Direct.
- Why Some of the Most Dangerous Potato Diseases are Successful. Phil Nolte. Growing Produce. August 9, 2017
- Abu-Obeid, I. , Khlaif, H. and Salem, N. (2018) Detection and Identification of Potato Soft Rot Pectobacterium carotovorum Subspecies carotovorum by PCR Analysis of 16S rDNA in Jordan. Agricultural Sciences, 9, 546-556. doi: 10.4236/as.2018.95037.