Summer squash including zucchini (aka courgettes) is a low-acid food. To can summer squash or zucchini, you need to do it as an acidified condiment such as pickles, relish, or crudités, and water bath or steam can the jars.
Otherwise, because summer squashes including zucchini are low-acid vegetables, they would have to be pressure-canned, and the catch is, you can’t currently do that.
The problem occurs because of how mooshed up and squished together summer squash and zucchini get: the extreme high temperatures in pressure canning cause it to disintegrate and compact together, which creates density issues in the jar, which makes heat penetration unpredictable. This means that the heat required to sterilize the food might not reach everywhere adequately inside the jar.
The Clemson Cooperative Extension explains,
Slices or cubes of cooked summer squash will get quite soft and pack tightly into the jars. The amount of squash filled into a jar will affect the heating pattern in that jar and may result in inadequate processing and an unsafe product.” 1
It’s true that there used to be USDA-provided instructions for pressure-canning summer squash, but they were withdrawn in 1988 2. It’s not that it was proven unsafe: it’s that when they retested the old directions in 1988 during a complete revision of the USDA guides, the results were not coming up consistently safe. And, when they attempted to locate the paper records holding the data behind the previously recommended processing times, the documentation could not be found:
Documentation for the previous processing times cannot be found, and available reports do not support the old process. Attempts to reproduce the old process did not result in adequate heating to ensure safety.”3.
And, owing to a lack of funding, there have not been the resources since then to re-tackle the topic, and consequently, there are currently no tested and recommended as safe procedures.
Due to the lack of tested recipes, the canning of summer squash without the addition of vinegar (for pickling) is no longer recommended.”4
So, since they can’t guarantee at what processing point it will become safe for everyone under all possible circumstances, they have withdrawn their instructions. The USDA’s modern home canning division just does not guess and say, “Give it a whirl; you might get lucky — or not.”
So to be clear, unless you pickle it in some way, there is currently no research-based proven safe way to feed home canned summer squash or zucchini to yourself, your family or your friends.
Exceptions to the rule
That being said, summer squash and zucchini are safely incorporated as an ingredient into some pressure canning recipes.
For instance, the USDA 2015 Guide has a recipe for pressure canned Tomato and Zucchini. 5
Ball has a recipe for pressure canned mixed vegetables which calls for zucchini. 6
This is fine; the issue is not that summer squash and zucchini are inherently evil; it’s the unpredictable density when you process it all together by itself in a single jar, for which there are currently no recommendations that will cover all variables (and mistakes) to keep everyone safe.
Zucchini. Penn State Extension. 14 July 2012.
Why is canning summer squash or zucchini not recommended? In: Frequently Asked Questions. National Center for Home Food Preservation.
Preserving Summer Squash. Clemson Cooperative Extension. Accessed March 2015 at http://www.clemson.edu/extension/food_nutrition/canning/tips/26preserving_summer_squash.html ↩
“Summer squash canning was removed from USDA complete guide [ in 1988 ]” — Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015. ↩
Tomatoes with Okra or Zucchini. United States Department of Agriculture (USDA). 2015. Complete guide to home canning. Agriculture information bulletin No. 539. Available at: http://nchfp.uga.edu/publications/publications_usda.html (Accessed Feb 2016). Page 3-12 ↩
Ball Blue Book. Muncie, Indiana: Healthmark LLC / Jarden Home Brands. Edition 37. 2014. Page 113. ↩