I take it you don't eat vegetables, then, Bathsheba, which is where nitrates come from.
Vegetables are the main source of nitrate in the diet and contribute to 70 to 90 per cent of the total nitrate intake. Green leafy vegetables, such as lettuce and spinach, naturally contain higher levels of nitrate than other foods.
Cooking food is processing it. Look at the label and see what is added. You can buy cooked meats with very little additives.
As you eat a lot, consider buying a turkey or turkey crown, cooking it and freezing slices in suitable portions. It would probably be cheaper. :)