Which form of "kein" you have to use depends on the noun after it (here: Zucker):
1) Is the noun masculine, feminine or neuter singular or is it plural? (here: "Zucker" is masculine singular)
2) Which case is used for the noun? (here: accusative)
See also this chart of the different forms of "kein": http://de.wiktionary.org/wiki/kein
You aren´t negating sugar. "Kein" is used to negated noun without articles.
May it come in handy