Biodegradability Study of Native Corn Thermoplastic Starch In Laboratory Scale: Influence of Plasticizer Concentration, Ageing, Sample Superficial Area and Soil Source

Aerobic biodegradation of different samples of native corn thermoplastic starch (TPS) were accordingly evaluated with standard test method ASTM D 5988 – 03 in order to establish the effect of plasticizer concentration, ageing, sample superficial area and soil source in TPS biodegradation.

Samples were prepared using glycerin as plasticizer, varying its concentration at 30%, 35% and 40%. Dry native corn starch and glycerin were premixed in a blending machine and then extruded in a single screw extruder; pellets and TPS film were obtained after double-step extrusion. Pellet samples were aged during 70 and 90 days in polyethylene bags at room temperature.

Two different soils from a local landfill (Doña Juana, Bogotá/Colombia) and germination peat were used as degradation matrix. Soils were characterized for amylolytic microorganisms, mesophilic aerobic bacteria and fungi, as well as for physical-chemical properties as pH, carbon and moisture content.

Biodegradation rate was assessed by exposing the TPS samples to the soil and measuring the carbon dioxide evolved by microorganisms as a function of time. Carbon dioxide production was measured for TPS samples and was expressed as a percentage of the calculated initial carbon content of samples.

We found that for non-aged samples the increasing plasticizer concentration lead to a higher biodegradation rate. On the other hand, for aged samples, plasticizer concentration negatively affects degradation rate. It was also seen that samples superficial plays a big role in biodegradation rate while no significant difference was observed for different soils.