Biodegradability and disintegration of multilayer starch films with electrospun PCL fibres encapsulating carvacrol

Highlights

• Disintegration patterns of multilayers with or without carvacrol (CA) were similar.

• Disintegration value for all the multilayers was 75–80% after 84 composting days.

• CA-free multilayers completely biodegraded after 25 days.

• CA incorporation limited the biodegradation of the multilayers to a maximum of 85%.

Abstract

The biodegradation and disintegration of thermoplastic starch multilayers containing carvacrol(CA)-loaded poly-(ε-caprolactone) electrospun mats were evaluated under thermophilic composting conditions for 45 and 84 days, respectively, and compared with non-loaded carvacrol films and pure starch films. Sample mass loss, thermogravimetric and visual analyses were performed throughout the disintegration test. The disintegration behaviour of all multilayers was similar, reaching values of 75–80% after 84 days. Biodegradation, assessed by carbon dioxide measurements, revealed that all the carvacrol-free films completely biodegraded after 25 composting days. However, the presence of CA notably affected the compost inoculum activity, thus limiting the biodegradability of the CA-loaded multilayers to a maximum value of around 85% after 45 days. Nevertheless, this value was close to that established by the standard ISO method to qualify as biodegradable material.

Introduction

The quantity of plastics produced in the first 10 years of the current century is likely to approach the quantity produced in the entire preceding century [1] and only a modest percentage of these packaging materials ends up being recycled (plastic recycling rates of 9.1% in the US in 2015 [2] and 40.9% in the EU in 2016 [3]). The usage and disposal of plastics is controversial and there are growing concerns about waste accumulation, problems for wildlife resulting from ingestion and the potential for plastics to transfer harmful chemicals to wildlife and humans. There are numerous studies alerting to the alarming levels of microplastics found in oysters [4], mussels [[5], [6], [7]] crabs [8] and fish [9,10] which move up through the food chain, ending up in the human body. Risk assessments developed by the European Food Safety Authority (EFSA) [11] have taken these considerations into account. However, perhaps the most important overriding concern is that our current usage is not sustainable [1].

Due to the environmental impact these plastics generate owing to their long degradation times, more effort is being made to develop packaging from biodegradable materials. This trend is aligned with consumer demand for more natural products for food contact materials. As a result, new materials have been developed, using biodegradable polymers from renewable sources, such as polysaccharides and proteins, and natural active agents of plant or marine origin.

Starch (S) is one of the most commonly studied, readily available carbohydrates, obtainable from renewable sources at relatively low cost. Starch films present very good oxygen barrier capacity, but due to their hydrophilic nature, films exhibit water sensitivity and poor water vapour barrier properties. Combining, in multilayer assemblies, starch layers with sheets of hydrophobic polymers - with high water vapour barrier capacity - would provide these materials, intended for food packaging, with more adequate barrier capacity for both water vapour and oxygen. In this sense, poly-(ε-caprolactone) (PCL) (a completely biodegradable aliphatic polyester) [12] has been combined with thermoplastic corn starch, forming bilayers with improved barrier properties when compared to neat starch films [13,14]. These multilayer films became active films with antimicrobial properties by incorporating carvacrol encapsulated into electrospun PCL layers [14]. The encapsulation of carvacrol in PCL mats by electrospinning showed better retention of the compound in this non-polar matrix, exhibiting higher encapsulation efficiency than starch [15]. Likewise, electrospinning is applied at room temperature which also contributes to preserve the compound against the potential deterioration and volatilization that can occur during the starch thermal processing. Carvacrol (CA) is a phenolic monoterpene, one of the major constituents of oregano and thyme essential oils [16]. It exhibits significant in vitro antimicrobial [[17], [18], [19]] and antioxidant activity [20,21], and has been approved as a food additive by Joint FAO/WHO [22] and as flavouring substance by EFSA [23]. It is currently being used as a bioactive in packaging materials [[24], [25], [26]]. Thus, the addition of active compounds to biopolymer layers confers antimicrobial and/or antioxidant properties, making these materials more attractive as food packaging candidates. Given the inhibiting effect of such antimicrobial compounds on microflora, it is likely that the biodegradation of these matrices could be altered by the presence of active compounds [27,28].

Composting is a form of organic recycling, based on the activity of the microbiota population, which breaks down the biodegradable parts of the waste, generating stabilized organic residue [29]. The resulting compost could be used as soil conditioner to increase soil productivity by replenishing some of its nutrients, and reduce the excessive use of synthetic fertilizers [27]. As pointed out by Balaguer et al. [27], not all biodegradable polymers are compostable, since compostability implies biodegradation by biological processes at a rate consistent with other known compostable materials, leaving non-visibly distinguishable or toxic residues. Therefore, evaluation of compostability includes three phases: disintegration, biodegradation and ecotoxicity. The biodegradability and compostability of developed active packaging materials made with biodegradable polymers can be affected by the presence of actives or blend interactions and cannot be assumed as such. It must be analysed to ensure that the newly created materials comply with the requirements specified by law [29].

The aim of this study is to assess the disintegration and biodegradation behaviour under laboratory composting conditions of starch-PCL multilayer films, incorporating or not carvacrol.