As fall approaches, those end of season unripe tomatoes are still usable. Be sure to pick them before a frost or freeze if canning them.
Unripe, or green, tomatoes can be preserved just like ripe tomatoes. So when canning them they still require acidification. Here are some ideas to can green tomatoes.
Don’t want to can them, or it is after a frost or freeze? Then freeze them for later use.
In short, it is not a suggestion, but a requirement to add acid to home canned tomatoes. This is for water bath AND pressure canned tomatoes. Why you ask?
Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat. The prevention control in vegetables, meat and other naturally low-acid foods is by heat alone.
Tomatoes can have a natural pH above 4.6 (at least up to 4.8). But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend adding a small amount of acid so they can be treated as a food with a pH less than 4.6 for home canning. Therefore, they are suitable for boiling water canning when the acid is added. (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)
Sources: https://nchfp.uga.edu/publications/nchfp/factsheets/acidifying.html and www.bookstore.ksre.ksu.edu/pubs/MF1185.PDF
Tomatoes may have that tasty zing that makes them tart and tasty. But in reality, they are not as acidic as they seem, especially when canning tomatoes.
Tomatoes have a pH value around 4.6 which makes them unsafe to can by themselves, with many varieties above 4.6. All tomatoes must be acidified with either citric acid, bottled lemon juice, or vinegar with 5% acidity in both water bath and pressure canning processing.
Without this added acid, tomatoes will likely ferment and spoil. Learn more in Preserve it Fresh, Preserve it Safe: Tomatoes.
A recent foodborne illness outbreak occurred in northeast Kansas due to contaminated fresh tomatoes served at a church supper. Preliminary investigation results report 69 illnesses and 14 of those tested positive for Salmonella Newport. The tomatoes came from multiple sources so an environmental assessment is underway to determine the contamination source.
Salmonella Newport is the third most implicated microorganism in U.S. foodborne illness outbreaks. It tends to survive in extreme conditions such as low relative humidity, high temperature, and UV exposure. It also tends to survive on the skin surface of tomatoes. It can also become internal during plant growth as it could come from contaminated water and soil.
Therefore, care in handling fresh tomatoes from farm to fork is important. In the kitchen, take care in preventing cross contamination during tomato preparation by keeping surfaces and utensils clean and sanitized. All cut tomatoes should be stored in the refrigerator within two hours. Always wash your hands before and after handling any food as dirty hands are a significant source of foodborne illness.
Source: J. of Food Protection, Vol. 81, No. 7, 2018, pp. 1193-1213
Choose fresh, vine-ripened tomatoes that are at their peak ripeness. Over-ripe tomatoes are less acidic. The acidity level in tomatoes varies throughout the growing season. Tomatoes reach their highest acidity when they are still green and decrease in acidity until they reach their lowest acidity as they mature.
Canning is NOT a way to use damaged tomatoes or those from dead or frost-killed plant vines. These tomatoes may have a pH level greater than 4.6 and may have extra pathogens. The canning process time may not be enough to kill extra organisms. This could lead to a product that spoils and is unsafe to eat.
So you saved your tomato crop in the freezer. Can those frozen tomatoes be canned?
It is not recommended to can tomatoes that froze on the vine. This is because the acid content changes too much making them unsafe for canning. But tomatoes harvested prior to a fall freeze, then frozen, do not change in acidity. What does change is their texture and how they measure.
The best choice for canning previously frozen tomatoes is to make a well cooked product such as a stewed or crushed tomato product, or made into tomato juice or sauce.
It is not recommended to can them whole or quartered. They will pack into the jars differently, absorb moisture differently, and the heat transfers through the jars differently. This could lead to underprocessing and spoilage. Tomato canning recipes are based on fresh tomatoes.
Source: University of Georgia