Nations hoped to build a body that would protect the vulnerable and end conflict.
But fighting off the attacks ahead of the first round of voting, Hosni insisted his comment was part of an angry exchange with supporters of the Muslim Brotherhood and was taken out of context.
He told France 24 television he had been referring only to “Israeli books that insult Islam,” which he had been accused of tolerating.
Hosni’s detractors include Auschwitz death camp survivor and Nobel laureate Elie Wiesel, who says his appointment would “shame” the global community.
But he is also behind efforts to rebuild a crumbling old synagogue in Cairo, Egypt’s capital.
At the Unesco headquarters in Paris, supporters hailed Hosni as “a man of peace” who would improve ties with Muslim countries.
Envoys started voting on Thursday for a successor to Japan’s Koichiro Matsuura as director-general.
Still in the race are Benita Ferrero-Waldner, the European Commissioner for External Relations, Irina Bokova, the Bulgarian ex-foreign minister, and Ecuador’s Ivonne Baki.
If voting goes to a fifth round there must only be two candidates.
The UN agency is mandated to promote global understanding through culture, education and science.