Sun-cured tobacco, Bastam, Iran Curing and subsequent ageing allow for to the gradual oxidation and degradation of carotenoids in tobacco leaf. This creates certain compounds from the tobacco leaves and gives a sweet hay, tea, rose oil, or fruity aromatic flavor that contributes to your "smoothness" of the smoke. https://broadfar.com/oraco-global-trends-in-heat-not-burn-products/