At the beginning of this scene, the stage directions say that John comes in, tastes the stew, then puts more salt in it. I think that we see the importance of this a bit later in the scene. We see this when John compliments Elizabeth on how well the stew is seasoned. She blushes with pleasure.
To me, what is going on here is that John is making a real effort to try to patch up their marriage. He feels guilty about his affair and he wants to make Elizabeth happy whenever he can. So, here, he makes sure that the stew will taste the way he likes it so he can compliment her.
I agree with pohnpei.
Proctor wanted more spice...literally for his food and figuratively in his love life. Perhaps Elizabeth's bland soup matches her bland devotion that caused him to stray. He secretly spices things up for himself in both instances. I am being slightly facetious, of course.