Leftover food can be tasty and healthy but at best leftover food is as healthy as tinned food because many vitamins are lost if food isn't eaten promptly. Fruit and vegetables expecially are best eaten soon after they are cooked. Salads also are best eaten soon after everything has been cut up. Cooking enough for two days at a time is a bad idea except rarely when you are pressed for time. 
Here is what a website that understands food hygiene recommends. "Store leftovers in the fridge within two hours of cooking and eat them within three days." Also foods like milk and yoghurt and tinned food should be eaten within three days of opening the container.
Plan how to use a large carton of yoghurt or a large tin of food within three days before you open it. If you end up with yoghurt or other food that's been open too long put it into a dish early during cooking and boil it thoroughly. Some foods like cheese don't taste quite so good after thorough boiling but food safety matters. A good idea can be to cook whatever needs thorough cooking with something like potato or pasta that also needs fairly long cooking. That way you don't use more fuel than you would need anyway to cook the potatoes or pasta.
If you eat leftovers that were open too long uncooked:
- Most of the time nothing goes wrong you think what you did was OK.
- Just occasionally you may get a bout of digestive upset. It all settles quickly or slowly after drinking stuff like Tummy Tea.
- Even more rarely you might get a serious case of food poisoning needing serious attention from your doctor and possibly worse.
Those most at risk are young children, old people and people of any age with compromised immune systems for example because they are recovering from an infection. Is it worth the risk?
- ↑ Observant Jews eat their sabbath meal roughly a day after it was prepared, some use slow cookers. Others prepare their meal and cut up their vegetables a day in advance, put it into a cold oven with a timers set before the start of the sabbath. The oven heats up automatically at the time preset. The Jewish community are in general at least as healthy as their gentile neighbours. Cooking food slowly over a day won't be worse than heating something up a day after it's cooked. The Jewish community has about 5,000 years experience with they way of eating and observant Jews typically eat a great deal of fresh fruit and vegetables apart from the sabbath. Up to once a week you can probably eat food heated up from the previous day provided what you eat in between is generally nourishing and freshly cooked.
- ↑ SafeFood