The Collaborative International Dictionary of English v.0.48:

Humin \Hu"min\, n. [L. humus the earth, ground.] (Chem.)
   A bitter, brownish yellow, amorphous substance, extracted
   from vegetable mold, and also produced by the action of acids
   on certain sugars and carbohydrates; -- called also humic
   acid, ulmin, gein, ulmic or geic acid, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

humin
    n 1: a black humic substance that is not soluble in water