'Zein' protein is related to which crop?

Cereal and oilseed storage proteins are classified by solubility under the Osborne scheme: albumins (water-soluble), globulins (salt-soluble), prolamins (alcohol-soluble), and glutelins (dilute acid/alkali-soluble). Each major crop's seed is dominated by one specific storage protein from this scheme, and that protein acts as a species marker for the crop.

Zein is the prolamin-class storage protein that makes up roughly half of the total protein in maize (corn) kernel endosperm. It is notably deficient in the essential amino acids lysine and tryptophan, a well-documented nutritional limitation of maize-based diets. Since zein is the name specifically given to the maize prolamin, it maps only to Maize among the crops offered.

Cross-checking each other crop's own storage protein confirms this by elimination:

• Soybean's dominant storage proteins are the globulins glycinin and beta-conglycinin, not a prolamin.

• Oat's prolamin is called avenin, a separate protein from zein even though both are prolamins.

• Paddy (rice) grain protein is dominated by oryzenin, a glutelin, not a prolamin at all.

So the crop is Maize.