Hi Richard,

It is very difficult to track down the information that led to the skinning tomato recommendation. I have pulled and read a lot of the old research articles and here is what I am comfortable concluding:

Commercial canning of tomatoes was a large and growing industry in the US at the turn of the 20^th century (early 1900s). Home canning procedures for tomatoes began being published in the early 1910s. Scalding and peeling the tomatoes has been a consistent recommendation since this time. The primary reason seems to be that the skins will turn tough and increase the bitterness of the canned product; removing the skin would also reduce the level of pesticides in the canned product. However, there are other references that say that peeling the tomatoes will reduce the amount of bacteria and spores that end up in the jar and will reduce the likelihood of spoilage and increase the safety of the product. Due to the consistent history of using peeled tomatoes in canning recipes, processing conditions using skin-on tomatoes have never been evaluated. It is possible that the skins have minimal to no impact on the thermal transfer, but this has not been verified.

In the 1950s, research demonstrated that the pH of some tomatoes was in the danger zone (pH > 4.6). At the same time, the commercial tomato industry was seeing an increase in bacterial spoilage of canned tomato products. A combination of changes in tomato varieties (with potentially lower acid) and harvesting practices (hand harvesting à machine harvesting) were considered to be likely contributors to spoilage. One theory was that mechanical harvesting increased the amount of soil (and therefore bacteria) on the surface of the tomatoes before they went into the processing facility. The thorough washing procedures used by the tomato processors could not effectively reduce the higher concentration of bacteria spores from the surface of the tomato. Some of these spoilage organisms raised the pH of the tomato product to a high enough level that supported the germination of C. botulinum spores, resulting in botulism toxin formation. This “situation” led to the industry petitioning FDA to change the legal definition (standard of identity) of canned tomatoes to allow for acidification (initially denied in 1959, accepted in 1966).

During this time, there were additional studies to help to characterize and understand the microbiological problems associated with canned tomato products. One such study (Samish et al 1962) evaluated the relative bacterial concentration on the surface and flesh of various fruits and vegetables, including tomatoes. Tomatoes were a particularly interesting case compared to the other produce tested (cucumber, broad beans, garden peas). The stem scar and the underlying central core high in bacterial counts, including sporeformers. The remainder of the flesh of the tomato were low in bacteria.

In the 1970s, there were several outbreaks of botulism associated with home canned tomato products (tomatoes and tomato juice). Representatives from the CDC, National Canners Association, FDA, and USDA met to discuss the outbreaks and provided updated recommendations for acidification of home-canned tomato products. At the time, 12 outbreaks of botulism had been linked to home-canned tomatoes and recommendations on acidification of home canned tomatoes were provided. This seems to be the last major revision to the recommendations for canning tomatoes.

Based on all of this, we encourage people to follow the recommendations from USDA and NCHFP:

“Peeling tomatoes may seem like an unimportant extra step, but the texture of the skin was determined to be undesirable and product testing did not include considerations of how the skin would alter the final product safety.

So, scald, peel, and chop tomatoes as described in the procedure. Our canning recommendations are meant to be followed as written, since that is how they were developed and changing ingredients or steps may influence not only the quality but also the safety of the final product.”

I wish the data was better documented, but this is what I could find.

Please let me know if you have additional questions. Thanks for using Ask Extension.

Joy