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.