The word "sweetens up" generally refers to the act of making something more pleasant, enjoyable or appealing. However, there are several antonyms that can be used to convey the opposite meaning of "sweetens up". Some of these antonyms include bitter, sour, astringent, tart, acrid, pungent, sharp, acrimonious, caustic, harsh, and unpleasant. These words can be used to describe situations, events or activities that are not meant to be enjoyed or liked, or to convey dissatisfaction or disappointment with something that was initially perceived as positive or appealing. Using antonyms can help to add depth and complexity to language use, and to better convey different shades of meaning in everyday conversation.