Sesame chutney thickens upon cooling due to the natural oils and solids in sesame seeds solidifying and binding together as the temperature decreases. This process creates a denser consistency.
Sesame seeds are rich in oils and proteins. When you grind sesame seeds to make chutney, these oils are released and emulsified with the other ingredients, such as water, spices, and sometimes yogurt or tamarind. The heat generated during the grinding process and any subsequent cooking helps to keep these components in a relatively fluid state.
However, as the chutney cools, the sesame oil begins to solidify. The proteins and other solids present in the sesame seeds also tend to bind together. This combination of oil solidification and solid binding increases the viscosity of the chutney, causing it to thicken. The extent of thickening depends on the proportion of sesame seeds used, the amount of liquid added, and the temperature to which it cools. The higher the sesame seed content, the more pronounced the thickening effect will be.
If you prefer a thinner consistency even after cooling, add a little warm water or lemon juice to the chutney and mix well before serving. This will help to loosen the solidified oils and solids, restoring a smoother texture.