Biodegradation of polylactic acid: Use of a thermophilic microbial consortium

Abstract

Materials made of polylactic acid (PLA) bioplastic, which includes the disposable foodware on the Sonoma State campus, are marketed as compostable. Currently, however, they cannot be commercially composted in a feasible way due their slow rate of degradation. We are researching methods to speed the rate of PLA breakdown through the optimization of physicochemical conditions that enhance the degradative activities of microbial consortia in liquid cultures. For our experiments, media for testing the degradation of shredded PLA consisted of a basal salts medium supplemented with gelatin as a source of nitrogen and as an inducer of PLA-degradative enzymes (proteases). The degradative microbial community was derived from a combined inoculum of PLA-associated composts and soils. Results were collected by filtering cultures through a 2-mm screen, gently rinsing with distilled water, and determining the dry weight of the retained fraction. In initial experiments, thermophilic cultures showed a faster rate of degradation at temperatures ≥63°C (i.e. the glass transition temperature for PLA, which makes the polymers more accessible to degradative enzymes) compared to 55°C. We found that cultures maintained at 63°C for three months showed degradation to the point where there was no harvestable PLA, whereas the uninoculated controls still had a substantial amount of PLA (43.3 ± 4.2% retainment; mean ± SD, n = 3). We are currently conducting metagenomic analysis to determine the dominant members of the thermophilic PLA-degrading microbial community. Furthermore, we are investigating whether bioenergy, as methane-rich biogas, can be extracted through anaerobic digestion of spent thermophilic cultures. Large-scale development of this technology could allow institutions, including Sonoma State, to treat and potentially obtain valuable byproducts from the PLA materials they consume.

Results

Key words:  zero-carbon plastics, sustainability, compost, water quality, microbes