How exactly is ammonium sulfate formed in a Kjeldahl analysis?
Ammonia is basic so:
$$\ce{NH3 +H+&lt;=&gt;NH4+}$$
Sulphuric acid is acidic(since it's name is itself so acidic!):
$$\ce{H2SO4&lt;=&gt;H+ +HSO4- &lt;=&gt;2H+ +SO4^2-}$$
Now the final reaction would be like:
$$\ce{2NH3 +H2SO4&lt;=&gt;NH3 +NH4+ +HSO4- &lt;=&gt;2NH4+ +SO4^2- &lt;=&gt;(NH4)2SO4}$$
And remeber since proteins are polymers of peptides which itself are molecules consisiting of both a carboxylic ($\ce{-COOH/-COO-}$) and amino ($\ce{-NH3/-NH4+}$) group, therefore with strong heating ammonium sulfate is expected.