Than yam vendor knows the Inivisible Man is from the South, as the yam is a symbol of Southern soul food. The narrator is in New York (Harlem, cold and bitter. After he bites into the ham, he has an epiphany: he realizes he's been victimized by Dr. Bledsoe and he realizes he misses home (the South).
He embraces his cultural heritage as he bites into the good yam, "They're my birthmark...I yam what I am!" (266). A birthmark, here, is both a mark of distinctive identity and a blemish on one's body. In other words, a yam is a proud symbol and a negative stereotype of his Black Southern heritage. He, wrongfully following the lead of Dr. Bledsoe, had shame-facedly associated soul food with “field niggerism” (265), adopting instead to savor the more sophisticated delicacies of the house Negro (who caters for the white slave master) in order to escape the stereotype.
Since he has awakened, however, the narrator "no longer [feels] ashamed by things I had always loved" (266); he is no longer ashamed of his birthright, a poor black man from the Deep South. He continues, "But what of those things which you actually didn't like, not because to dislike them was considered a mark of refinement and education--but because you actually found them distasteful?...It involved a problem of choice" (266). It is a problem, according to Sartre, of authenticity. So, has he not liked yams because whites didn't think he should? Or because other blacks (namely house Negros) didn't think he should? Or because he (as a repressed field Negro) didn't think he should? To love yams to spite the whites would be inauthentic. To hate the yams to spite his blackness would be inauthentic. The narrator is caught in an existential dilemma.
After he takes a bite of the second yam, anticipating another sweet relic of home, it is frostbitten, leaving a bad taste in his mouth. Although he naively loved the yams of home, the yams handed to him in his newfound freedom of the North are at first sweet, and then foul. He is in the process of reinventing himself, casting off both the house and field Negro stereotypes. His authenticity then is not defined by being inauthentic to any ascribed stereotype, but by relishing what he wants to relish, regardless of stereotype. His existence as a yam lover or yam hater precedes his essence, which may hold that eating yams is shameful.