Rasa is the fourth tanmatra, produced with shabda, sparsha, rupa and gandha from the tamasic aspect of ahamkara, and it is the newest quality apas, water, contributes once the elemental sequence reaches its fourth stage. Water, in this account, already carries sound, touch and form by inheritance from the elements before it, and rasa is specifically water's own addition, the reason later Indian medical and culinary writing treats liquid, and the water content of food generally, as the primary carrier of flavour.
Rasana, the organ correlated with rasa, is built to register exactly this one tanmatra, no more and no less, following the strict one-organ-one-tanmatra discipline that runs through all five Jnanendriyas. Nothing about rasana's design in the Samkhya scheme allows it to double as a general chemoreceptor picking up smell as well as taste, even though ordinary experience often blends the two; the classification insists on keeping rasa and gandha, taste and smell, as separately fundamental tanmatras, correlated with separately fundamental organs, rasana and ghrana.
The word rasa carries a long afterlife well beyond Samkhya's technical use, becoming central to later aesthetic theory, where it names the emotional flavour or essence a work of art evokes in an audience, and to Ayurvedic medicine, where it names both taste itself and the essential nutritive fluid extracted from digested food. Samkhya's narrower, tanmatra-specific sense of rasa is one root among several the word's much wider later career grew from.