Zucchini can sometimes be bitter due to an overabundance of a toxin, curcurbitacin. Cucurbitacin is meant to steer mammals clear of zucchini. Usually the zucchini crop is properly developed and is not bitter. BUT, when weather is particularly dry and hot, or zucchini are oversized or large, the zucchini can taste bitter from its outer skin and the ends.