Beware: Pumpkin Butter

You may have seen the recipes for apple butter posted in November, and perhaps you thought about also making pumpkin butter this season. If so, then please consider this: home canning is NOT recommended for pumpkin butter or any mashed or pureed pumpkin or winter squash.

If you are thinking, “but wait, I know I’ve seen recipes for pumpkin butter in the past”, then you are correct. However, as original editions of publications are revised to reflect current scientific discoveries, those older publications become outdated. In the 1994 revision of USDA’s Complete Guide to Home Canning, the only directions for canning pumpkin and winter squash are for cubed pulp. Recommendations for canning mashed winter squash were withdrawn and the statement “Caution: Do not mash or puree.” is even included in the directions.

Why the change? When evaluating recipes for pumpkin or mashed squash, research has found much variability in acidity (pH), viscosity (thickness), and water activity. These three factors are critical to the safety of canned products. If these factors are not known and predictable, then the product cannot be verified to be safe.

Still not convinced?  Here are a few more details:

  •  Pumpkin and winter squash are low-acid foods, meaning that they have a pH value higher than 4.6. Therefore, if Clostridium botulinum bacteria are present and survive processing, and the product has a high enough water activity, then the bacteria can produce toxin in the product. This particular toxin can cause botulism– a very serious illness which can result in death.
  • Research published in 1995 from the University of Missouri found batches made by the same formula to have extremely variable pH values and pumpkin butters produced by home canners were found to have pH values as high as 5.4 (low-acid).
  • Studies at the University of Minnesota in the 1970’s found so much variability in viscosity among different batches of pumpkin purees that a single processing recommendation to cover all the variability could not be calculated.

Basically, the concern is over variability. Even though a large quantity of sugar is often added to make pumpkin butter, it may not be enough to inhibit pathogens. Vinegar or lemon juice may be added to increase acidity (and thereby decrease pH), but with such variability in pH levels, a single recommendation cannot be made to ensure a safe product at this time.

For a response to the FAQ about canning mashed or pureed pumpkin or winter squash, go to the National Center for Home Food Preservation website.

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9 thoughts on “Beware: Pumpkin Butter

  1. Annabelle

    I’m wondering if it’s still safe in the freezer. because once you defrost it to eat it, and store it in the fridge, could the bacteria grow?

    what if I test the pH before freezing?

    1. nchfp Post author

      Yes, instead of canning, it is best to freeze pumpkin butter or mashed squash for long term storage. For short term storage of the prepared or opened product, refrigeration is recommended.
      In order to make the botulinum toxin, Clostridium botulinum bacteria require a moist, low-acid, anaerobic environment above recommended refrigeration temperatures. While in the frozen state, the bacteria are inactive. Toxin is not produced while food is in the freezer. Once opened and refrigerated when thawed, as long as the food is kept cold enough, then toxin formation is not a concern. The types of C. botulinum bacteria usually of concern in low-acid processed (canned) foods grow and produce toxin above the usually recommended refrigerated food storage temperature of 40 degrees F, so refrigeration and freezing are safeguards against growth and toxin formation. The other major types of C. botulinum grow between 38 and 113 degrees F; refrigeration above 38 degrees F (such as 40 degrees) may not be a complete safeguard against botulism in that case. These types that grow down to 38F, however, do have spores that are less heat resistant and tend to be a problem in either unheated or only pasteurized foods, not something like a pumpkin butter that would have received a long cooking period. But there is no guarantee your pumpkin butter preparation has only the very heat resistant type of C. botulinum. So, maintaining refrigeration temperatures of 38-40 F is best, and that might require using a thermometer in your refrigerator to make sure you are maintaining those low temperatures, checking in several locations or at least near the door opening or warmest part (and not keeping opened foods too long).
      Absence of oxygen, or an anaerobic environment, also is necessary for growth and toxin formation. All the spores need is a microenvironment, or immediate situation around them, that is anaerobic. So a container that might seem like it has plenty of oxygen could have anaerobic pockets around the bacteria. So the overall message is make sure your refrigerator is cold enough and do not use vacuum packaging as a means to store your opened pumpkin butter, so you avoid creating an obvious oxygen-free environment.
      There is no need to test the pH of your pumpkin to store it as a frozen or refrigerated product. Low-acid foods are frozen and refrigerated regularly; we count on sanitation and temperature for microbial control all these times. If you are asking to test the pH so you might indeed be able to can it, we still could not recommend a canning process. pH is not the only factor that determines what a process method and time would be; other ingredients, thickness, and preparation steps play roles in that determination also.

      1. Annabelle

        thanks for the info. I did a lot of reading this morning and am making it now. I used commercially prepared pumpkin puree, am cooking in the crockpot on low for 6-8 hours, added spices, sugar, brown sugar, lemon juice, apples, and apple cider vinegar. of course my enquiring mind is curious about the pH.too bad accurately testing it is so expensive! I still don’t do any canning, I stick to fridge and freezer but I take extra precautions as it seems many of the people I gift foods to are immunosuppressed.

        I will instruct my friends to check their fridge temp and ensure it stays cold enough. they’re already used to my weird freezer jam instructions anyway.

        today I did see Ball’s fruit fresh stuff, which is mainly ascorbic acid. dont think the pH is low enough but that got me wondering about other alternatives and if maybe someday there will be official guidelines for us pumpkin freaks.

      2. nchfp Post author

        Indeed, the product you are referring to, Ball® Fruit Fresh, is specifically designed to prevent browning of prepared fruits. Ascorbic acid helps to prevent oxidation (and therefore browning or other discoloration) from occurring when oxygen encounters exposed enzymes in cut fruits. At this time there is not a recommendation for using it to acidify any of our products for safety, as the amount that would be needed has not been determined. Different types of acids acidify fruit and vegetable tissues in different ways, and ascorbic acid is not as efficient as citric acid in dropping pH for most vegetables so it has not been included as an option in the research. We suggest that you follow the recommended uses that manufacturers provide when using products such as this; it is not an even exchange for citric acid for pH adjustment.

        We do not know of plans among our peers to test home-canned pumpkin products at this time, unfortunately. If appropriate research is conducted to yield a safe recipe and canning process for the home-canning of pumpkin puree or pumpkin butter, then hopefully the research will be open for discussion and review, and we will be able to accept or even participate in procedures being published and offered to the public.

    1. nchfp Post author

      Hi Daisy, good question for clarification.

      No, we don’t recommend that you can pumpkin butter in a pressure cooker, or a pressure canner. The reasoning is two-fold:

      1. USDA does not have recommended processes for canning in a small pressure cooker. Research with low-acid foods for USDA was conducted using large pressure canners, resulting in recommendations for use with canners that can hold at least four quart-size jars. Being smaller in diameter, having less metal and using less water, pressure cookers heat up and cool down much faster that their larger counterparts. The time it takes to heat up (come-up time) and cool down (cool-down time) is calculated into the total processing times established by USDA. So then, the reduced time in a pressure cooker as compared to a pressure canner might not provide enough heat for long enough to destroy targeted microorganisms. Such under-processing of a low-acid food like pumpkin butter could result in foodborne illness, including botulism.

      2. Even to can in a pressure canner, there has to be a specific tested recipe and process time available in order to control for botulism. With so much variability in the pH value and viscosity of pumpkin butters, research has not been able to calculate a processing recommendation for pumpkin butters. There is no tested recipe and process time for pumpkin butter, so there are no USDA recommendations for canning it.

      For more information, read ‘Burning Issue: Canning in Pressure Cookers’ at http://nchfp.uga.edu/publications/nchfp/factsheets/pressurecookers.html.

      1. Daisy Steele

        Thank you! That’s very informative. I guess freezing is the best preservation method for this particular item.

  2. Anne Zander

    This is a great article on Pumpkin Butter….lots of consumer phone calls and questions about this.

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