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Cereals are a large source of biopolymers, where mainly the starch is used for food and feed. A rapidly growing cereal application is the production of biofuel, mainly produced from corn in the US. The starch is fermented to ethanol leaving spent grain rich in cereal proteins as a by‐product. The corn protein zein is currently extracted on a large scale and used in, for example, material applications. Similarly, pennisetin can be extracted from pearl millet, a crop critical for food security in sub‐Saharan Africa. The formation of viscoelastic melts is crucial for (bio)plastics production and the viscoelasticity, microstructure, and molecular properties of zein and pennisetin melts were determined here. The proteins were mixed with plasticizers (polyethyleneglycol or glycerol/citric acid) to form melts. The melts displayed a phase separated microstructure with protein‐rich and plasticizer‐rich regions with distinctly separate Tgs. The pennisetin melts formed cross‐links at temperatures above 60°C, which could be related to the high content of cysteine and methionine, as compared to zein. As a consequence, pennisetin melts showed a more thermocomplex behavior than zein melts. For zein melts, the mixture of glycerol and citric acid interacted with protein in addition to being a plasticizer causing a high‐molecular weight shoulder in the molecular weight distribution. The study showed that, although both zein and pennisetin form viscoelastic melts, the choice of plasticizer strongly affects both melt structure and physical properties. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012