CONICET | Buscador de Institutos y Recursos Humanos

The aim of this work was to perform a complete thermal characterization of biodegradable films based on yeast biomass. Films from commercial baker?s yeast biomass were prepared from a dispersion at 10% wt/vol (dry basis), treated with a combination of high pressure homogenization and thermal treatment. Glycerol was added as plasticizer (10, 20 and 30% wt/wt dry basis). Dispersions were poured onto Petri dishes and casted at 50 ºC (18 hours). Thermogravimetric analyses were carried out at 20 ºC min-1 from 50 to 600 ºC (N2). Maximum degradation rate temperature (Tmax) and initial degradation temperature (T90) were calculated. In order to study the thermal degradation of yeast films, sample residues were obtained at different temperatures from 120 to 275 ºC and further analysed by infrared spectroscopy (FTIR) in the range of 4000 to 400 cm-1. Glass transition temperature and other thermal events were determined by Modulated-temperature differential scanning calorimetry (MDSC), heating samples in hermetically sealed aluminium pans, from -80 to 200 ºC at 10 ºC min-1, 40 sec and ±1°C. Plasticized samples were conditioned at 21 ºC and 0% relative humidity (RH), while samples with no glycerol were also conditioned at RHs: 43, 75 and 90%.T90 was affected by the presence of glycerol and drop until 150 ºC at 20% glycerol. Maximum decomposition rate temperature was encountered nearly 309 °C and attributed to β-glucan pyrolysis and massive decomposition of proteins and other organic compounds, since FTIR results showed the disappearance of the amide II band at 1543 cm-1 and a clear separation of 1031 and 1076 cm-1 bands was evidenced, reflecting the sample decomposition, at temperatures higher than 225 °C. No appreciable changes were observed in residues obtained after heating samples from 120 up to 200 ºC. Determination of thermal events in MDSC thermograms from microorganisms is a complex task due to the variety of molecules present and the complexity of their interactions. However, it was possible to determine glass transitions in the reversible heat flow, while two endotherms were observed in the non-reversible heat flow maybe due to transitions of glucans in those fully dried samples. When samples were exposed to higher RHs, the energy related to the second endotherm decreased until completely disappearance at 90% RH. The amount of glycerol added (10 and 20%) did not have influence on endotherms, but 30% of glycerol a unique endothermic event was described. In conclusion, appreciable changes in thermal transitions were observed at different RH corresponding to the polymer state under certain conditions of humidity and considerable thermal degradation was detected at temperatures higher that 200 °C for non-plasticized sample.

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