Author
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Lawton Jr, John |
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Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/10/2003 Publication Date: 1/12/2004 Citation: Lawton Jr, J.W. Plasticizers for zein:their effect on tensile properties and water absorption of zein films. Cereal Chemistry. 2004. v.81. n.1. p. 1-5. Interpretive Summary: Zein, a protein from corn, can be cast into films. However, films made of 100% zein are brittle under normal conditions. To make zein films more flexible, small molecules called plasticizers are added to zein prior to casting. Zein is also hygroscopic, meaning it can gain or lose water to reach equilibrium with ambient air. Water can also be a plasticizer for zein. Zein needs some plasticizers to make it flexible at room conditions, but too much plasticizers will make zein films too weak to be usable. This work was done to test different plasticizers and understand how the plasticizers affect the amount of water absorbed by the zein. The water absorption proficiency of these plasticizers have ambiguous consequences. A hydrophilic plasticizer that readily absorbs water will be helpful at low humidities where the added water will make the zein more flexibile, but may be detrimental at high humidities where it absorbs water, rendering the zein film too weak to be useful. This basic work will allow us to tailor zein films to specific applications and relative humidities. Technical Abstract: Zein will gain or lose moisture to reach equilibrium with ambient air. Cast zein films are brittle at room conditions so plasticizers are added to make them more flexible. The amount of water absorbed or lost in these films is known to be affected by relative humidity of the ambient air. However, little is known about the affect plasticizers have on water absorption of these films. Cast zein films were plasticized with either glycerol (GLY), triethylene glycol (TEG), dibutyl tartrate (DBT), levulinic acid (LA), polyethylene glycol 300 (PEG), and oleic acid (OA). Mechanical properties and moisture content (MC) of the films were measured after one week of storage at 3, 20, 50, 70, 81, and 93% relative humdity (RH). There were large differences in the amount of water absorbed by the films which, depended on the plasticizers contained in the film. At 3% RH, all films contained low amounts of water. Films with no plasticizers contained 2.4% water while films with DBT and TEG contained 3.1 and 4.8% water, respectively. When the films were stored at 50% RH, the MC of the films increased to 7.1%, 4.6% and 8.1% for films with no plasticizers, DBT and TEG, respectively. At high RH the differences in absorbed water were even greater among zein films containing the various plasticizers. At 93% RH, the absorbed water in the film was 20.7% for films containing no plasticizers, 16% for DB, and 40% for TEG. Mechanical properties of the films were affected by the absorbed water. Films containing hygroscopic plasticizers like TEG absorbed too much water at high RH and became weak, but they absorbed enough water at lower RH to not be brittle. While films containing the hydrophobic plasticizer DBT were brittle at intermediate RHs, they had good mechanical properties at high RHs. All films were brittle at the low RH of 3% regardless of the plasticizer used. OA and GLY were poor plasticizers for cast zein films. |
