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editions of publications are revised to reflect current scientific discoveries, those older publications become outdated.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 Recommendations for canning mashed winter squash were withdrawn and the statement “Caution: Do not mash or puree.”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.