Do tomatoes lose their nutritional value when they are made into tomato sauce? What about canned tomato sauce?
Nutritional changes in cooking foods (including tomatoes) are not simple to answer: cooking can reduce some nutrients, and it can make other ingredients more available.
Tomatoes are a case in point, as cooking has both effects. According to as study in the Journal of Food and Agriculture, as reported in Cornell Chronical, both effects happen. Vitamin C is reduced as it breaks down under heat, but certain lycoenes and anti-oxidants increased.
Tomato samples were heated to 88 degrees Celsius (190.4 degrees Fahrenheit) for two minutes, a quarter-hour and a half-hour. Consistent with previous studies, vitamin C content decreased by 10, 15 and 29 percent, respectively, when compared to raw, uncooked tomatoes. However, the research revealed that the beneficial trans-lycopene content of the cooked tomatoes increased by 54, 171 and 164 percent, respectively. Levels of cis -lycopene (which the body easily absorbs) rose by 6, 17 and 35 percent, respectively; and antioxidant levels in the heated tomatoes increased by 28, 34 and 62 percent, respectively.
See related: Does cooking in a pressure cooker destroy nutrients?