The darkening of Jowar roti (bread) is due to over baking of roti on the pan. This dark matter is equivalent to biochar / charcoal formed from sorghum. This roti also turned crispy, when consumed it is good for health.
In our daily food, there is a possibility of some charcoal appearing, either intentionally or by chance charcoal is produced. This charcoal consumed is of great value to us. The Indian chapattis baked on pans, most often produces bits of charcoal as patches. Indian tandoor roti also has bits of charcoal in it. Some times rice over cooked will have charcoal formed at the base in contact with bottom of the utensil. Chicken tikka has chicken charcoal. Peanuts over roasted will have peanuts charcoal. Bread twice toasted would have lots of charcoal formed, good for curing upset constitution. This charcoal produced in the food has a typical bitter taste and burnt smell. If children reject the food with bits of this charcoal, mothers encourage them to consume it, saying they will have black hair. In case of dysentery people are given such over cooked rotis or bread with charcoal formed.
This site is dedicated to discover the use and values of biochar / charcoal for health.