Food safety and the neglect of the unpeeled potato

Judging from the shortage of canning jars and lids in grocery stores recently, the COVID-19 pandemic has spurred fresh interest in home canning (a potentially dangerous practice that has declined since its peak in 1943); and because the public expects doctors to know everything about health, it may be educational for physicians to consider the tragic case of the unpeeled potato.

I recently read a Facebook post in a food preservation group. A woman had just preserved 17 pints of venison stew, carefully pressure treating it at ten PSI for a full 75 minutes; however, the next morning, she realized that she had made a tragic mistake which instantly transformed her delicious venison stew into trash: She had not peeled the thin-skinned Yukon Gold potatoes! Appending her post, numerous comments confirmed: Never preserve unpeeled potatoes! With accompanying condolences, she was told to take this difficult lesson to heart and throw it all away. With timid hope, the woman proposed a possible solution; that is, removing the potatoes and reprocessing the stew with peeled potatoes. Again, the response from the canning choir came in perfect harmony: You cannot retreat any preserved food unless you do it within two hours of the initial treatment, and since this period of time had already expired, she had to throw it all away.

As someone with a research background in the biological sciences, my skeptical mind began to spin out of control. Peeled potatoes, when alone, are processed for 35 minutes at ten PSI. Unpeeled potatoes (thin-skinned Yukon Gold, no less), however, were apparently still dangerous after 75 minutes at 10 PSI. How could this be? So, I dangerously dipped my big toe into the boiling water of the home canning group. Accustomed to relying on randomized controlled trials and experienced in leading discussions of study validity in journal club conferences with my medical residents, I had to ask: Where did this recommendation come from? After all, the potato peel contains important nutrients. Do we really have to throw it out?

Although I was quickly offered links to some random online articles that clearly stated: “You must peel potatoes before canning,” which I dismissed as “not science,” I was eventually referred to the National Center for Home Food Preservation at the University of Georgia, and I breathed a sigh of relief. Alas, now I would certainly find the answer.

Unfortunately, no. Apparently, the canning processes for potatoes that have been rigorously tested have all been performed on peeled potatoes, purportedly because that is how most people preferred to eat them at the time of the original testing. Thus, the unpeeled potato has sadly been neglected. Notably, some sources suggest a theoretical risk of potato peels; that is, their irregular, dimpled surfaces harbor the bulk of potato bacteria (including Clostridium botulinum spores), allegedly making unpeeled potatoes more difficult to fully sterilize; an untested theory.

According to NCHFP sources, however, the main factors in safe, low-acid, food preservation are heat penetration and duration, and the main foods in which heat penetration is known to be poor are pureed or mashed foods (which don’t have reliable heating dynamics), so these foods are simply not recommended items for home canning. In the case at hand, however, it seems likely that a chopped, unpeeled potato floating in soup broth is much less likely to evade the sterilizing heat. However, safety first; and safety in the canning world means the following:

  1. Only use an approved canning recipe.
  2. Do not make any modifications to the recipe. Refer to #1.
  3. Do not make up your own canning recipes. Refer to #1.

The NCHFP apologizes to us gently: “Experimentally determining safe processing times for home-canned foods is a lengthy, expensive and time-consuming process, which explains why there are fewer home-canned processes available than many people would like. In short, there is no easy formula to work out processing times without experimentation and analysis that take into account how each food product heats in a particular canning situation.”

Importantly, they note, “there have not been the resources to do the amount of experimentation needed.” In fact, I contacted the NCHFP, and they replied, stating that they do not even have an active laboratory today to test new canning recipes due to a lack of funding.

To follow up on the second part of the case at hand, we should address the rule that preserved foods cannot be retreated if 2 hours have elapsed since the initial treatment. I found no particular reference to this being researched, but I assume it has to do with the risk of botulinum spores activating and producing toxins within that 2-hour window of time. That would be a pretty bold spore in my view, and I would also note that the botulinum toxin (a protein) is actually destroyed by boiling for 5 minutes, according to the World Health Organization, which would be properly destroyed during reprocessing, it seems. Once again, it just hasn’t been studied.

Food safety is serious, but statistically, we have a much greater risk of food poisoning from other sources (norovirus > 5 million and salmonella > 1 million annual cases of illness, each causing > 10,000 deaths annually). In comparison, home-canned food causes less than 100 cases of botulism per year. In a review of botulism cases from 1975 to 2009 (34 years!), a total of 61 deaths were attributed to food-borne botulism (< 2 per year), despite the likelihood that many home canners do not “follow the rules.” Of course, any preventable death is a tragedy.

But it begs the question: When it comes to something so basic as food and self-sufficiency, why wouldn’t there be federal funding to settle these issues once and for all, instead of leaving a bunch of terrified home canners throwing out 17 pints of delicious venison stew? We’ve sent spaceships to Mars. We’ve developed designer drugs for diseases. We’ve split the atom. Yet, alas, we still don’t know if it is safe to preserve the unpeeled potato.

David M. Mitchell is a hospitalist.

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