Why the scent of mouth gets dirty after not eating for a long time?
This is caused by a build up of ketone bodies in your blood which lowers the pH of your body and can cause bad breath. Ketone bodies are a response of your body breaking down fatty acids for energy instead of glucose when in a low food intake (fasting) state. This is also true for morning breath since your body has been in a fasting state.
The way to prevent this is to eat the proper amount of calories needed for your body to function normally.