1.Celery
Celery with its high nitrate content can become toxic if you heat it the second time and eat it. People usually use celery to prepare soups. Just like spinach, celery also contains nitrates that transform into nitrites after being reheated. If you have made a soup with some celery in it, you should either eat the whole soup immediately, or remove the celery before you warm the soup. The same thing goes for carrots.