The USDA's safety recommendations against canning pumpkin butter and other pumpkin purees at home.
Can I can pumpkin butter or pumpkin puree at home? I get asked this question a lot! You may can pumpkin in a cooked, cubed form; just not as a puree or ready-pumpkin pie filling. (see this page for instructions about how to can it in cubed form). Of course, you can freeze it, too, in cubed or puree or ready filling form (see this page for those instructions). And while I do have a recipe for canning pumpkin butter, I still keep the jars in the back of the fridge until I use them. Here's the reason!
According to The National Center for Home Food Preservation (a land-grant university consortium sponsored by the USDA, and considered to be the leading authorities on food safety science and food preservation research) . In 1989, the USDA's Extension Service revised recommendations and published the Complete Guide to Home Canning" (and I am quoting them here):
"Home canning is not recommended for pumpkin butter or any mashed or pureed pumpkin or winter squash, but we do have directions for canning cubed pumpkin. Pumpkin puree can be frozen or made into a spicy pumpkin leather.
There are not sufficient data available to allow establishing safe processing times for any of these types of products. It is true that previous USDA recommendations had directions for canning mashed winter squash, but USDA withdrew those recommendations.
Some of the factors that are critical to the safety of canned pumpkin products are the viscosity (thickness), the acidity and the water activity. Studies conducted at the University of Minnesota in the 1970's indicated that there was too much variation in viscosity among different batches of prepared pumpkin purees to permit calculation of a single processing recommendation that would cover the potential variation among products (Zottola et. al, 1978). Pumpkin and winter squash are also low-acid foods (pH > 4.6) capable of supporting the growth of Clostridium botulinum bacteria which can cause the very serious illness, botulism, under the right storage conditions. If the bacteria are present and survive processing, and the product has a high enough water activity, they can thrive and produce toxin in the product.
More recent research with pumpkin butter has been done at the University of Missouri. Pumpkin butter is mashed or pureed pumpkin that has had large quantities of sugar added to it, but not always enough to inhibit pathogens. Sometimes an ingredient such as vinegar or lemon juice is added to the formulation to increase the acidity (decrease the pH). However, pumpkin butters produced by home canners and small commercial processors in Missouri have had pH values as high as 5.4. In fact, the pH values seemed to be extremely variable between batches made by the same formulation (Holt, 1995).
It is not possible at this point to evaluate a recipe for pumpkin or mashed squash for canning potential by looking at it. At this point, research seems to indicate variability of the products is great, and in several ways that raise safety concerns. It is best to freeze pumpkin butters or mashed squash."
Obviously, pumpkin pie filling is essentially "pureed pumpkin" and similar to pumpkin butter. This means that neither the cooked pumpkin puree not the pumpkin pie filling (puree plus sugar and spices) would be candidates for safe home canning.
The University of Illinois Extension also says: "Canning pumpkin butter not a good idea, but try pieces or freezing. "
The University of Minnesota's extension service in October 2009, says: "Canning is not recommended for pumpkin butter or mashed/pureed pumpkin or winter squash. The density of the product prevents adequate heat transfer to the center of the jar and might allow harmful bacteria to survive."
The next questions asked are:
The reaspons are simple: commercial canning equipment is 10 ft tall and reaches pressures of 2 atmospheres or greater, which kills botulism spores. Home equipment can only kill the active botulism, but does nto destroy all (or enough) of the dormant spores - which are present everywhere!)
Again, without scientific lab data, you would not know how much would be the right amount to add; how long to process it, or even whether to use a water bath or pressure canner. Or in a pressure canner, what would be a safe pressure. You would need a lab with an autoclave, incubator and a high quality optical or scanning electron microscope so you can culture it and test for pathogens, and repeat the test a statically relevant number of times to ensure safe reproducibility.
Simply adding lemon juice (or vinegar) still will not overcome the density/heat penetration issue.
So if you were planning to just give it to your friends and family, remember botulism is odorless, tasteless and leaves no visible signs of contamination, and can be fatal.
The USDA and major universities say they have not yet find a set of home canning conditions that produce a reliably safe pumpkin puree or pumpkin butter. There may be a way to safely do it, but in their tests, they still find active botulism spores.
We hear about cases of fatal food poisoning every year: Taco Bell (E. Coli in undercooked hamburger); Tainted peanut butter, and spinach in 2009, 4 million eggs recalled for Salmonella in 2010 and so on. Cases of fatal or serious food poisoning from home cooking or canning rarely makes the news because it only affects one family and often the cause remains undiagnosed until much later.
Home food canning is perfectly safe, as long as you follow the lab-tested, approved recipes and directions, which presently do not include pumpkin butter. I'm going to wait until the food scientists tell me they have a recipe that can be shown to be safe. My family means more to me than a jar of pumpkin butter.