Plastics are mostly high molecular weight synthetic polymers. Their production started in around 1950, and since then it has increased from 1.5 million tons to almost 370 million tons in 2019 [1]. This increase is mainly due to the manufacturing of single-use disposable plastic materials, which represent about 50 % of all plastic products. The massive increase and accumulation of these plastics, which exhibit extremely low to no biodegradability, have led to the pollution of various parts of the planet, including soil, oceans, and air [2].

More sustainable alternatives to conventional plastics are bio-based plastics made from renewable sources, such as plants, microorganisms, or other biological materials [3]. Currently, bio-based plastics account for only about 1 % of the total amount of plastics produced annually; however, this production is expected to gradually increase from 2.11 million tons in 2019 to 2.43 million tons in 2024 [4]. The most common bio-based plastics are produced from polylactic acid (PLA), polyhydroxyalkanoates (PHA), starch, and cellulose. These plastics are characterized by their biodegradability and have less of an impact on the environment. In these plastics, the corresponding glycosidic and ester bonds are readily catalyzed by microorganisms, which makes polymer fragmentation faster [5].

PLA is one of the most widely used bio-based biodegradable polymers for applications in agriculture, biomedicine, and as a packaging material. This is due to its wide availability, good biodegradability, and mechanical properties [6,7]. However, the applicability of PLA has been relatively limited because its thermal distortion temperature, hardness, and degradation rate are unsatisfactory [8]. In addition, its degradability is null or very low in natural environments at an ambient temperature [9].

PLA degradation requires suitable temperature conditions such as those achieved in the composting process [10,11]. Composting is currently the most used technique to valorize organic wastes due to its low operational cost and the additional production of a stabilized substrate called compost [12].

Biodegradation of PLA in the composting process occurs in two stages. Initially, heat, moisture, and microorganisms from composting attack the PLA chains and break them apart, producing low molecular weight polymers and, eventually, lactic acid. Finally, the microorganisms present in the composting process mineralize the oligomer fragments and lactic acid to generate methane and carbon dioxide under anaerobic and aerobic conditions, respectively [13,14]. Furthermore, the rate of PLA biodegradation depends on several influencing factors, such as temperature, moisture content (MC), aeration, carbon/nitrogen (C/N) ratio, and the pH of the composting environment [15,16].

Although multiple investigations have explored the effects of turning aeration and the initial C/N ratio of the composting process [[17], [18], [19]], to our knowledge, no specific studies have been carried out to investigate the influence of these parameters on the biodegradation of commercial polymers such as PLA.

The aim of this study was to analyze the effects of turning aeration and the initial C/N ratio on PLA biodegradation, to decrease the biodegradation time of the PLA. Three types of aeration by turning were used to generate aerobic conditions and thus to achieve a relevant effect on the growth of microorganisms during the composting process. The types included: (i) without turning (A0); (ii) one turn per week (A1); (iii) two turns per week (A2). Moreover, three C/N ratios (20, 30, and 40) were used as the ratio influences the availability of nutrients and the rate of biodegradation of the materials.

Furthermore, this work explores not only the quantitative degradation of the PLA but also any changes at the surface of the PLA, thus providing a comprehensive analysis of its behavior under different composting conditions.