Converting plastic into food: Tackle Starvation and Pollution simultaneously

The African continent is currently facing one of the worst food crisis in the past four decades, affecting countries like Kenya, Nigeria, Ethiopia and Somalia, according to the British Red Cross. This situation has arisen due to the limited feasibility of increasing meat production, which has been deemed as necessary to meet future demand. Meat production reached 331 megatons in 2015, and is projected to increase to 455 megatons by 2030, however, the current methods of livestock husbandry heavily rely on land use, making it difficult to increase production.

In addition to the food crisis, plastic pollution is also a major environmental concern. Over 70% of plastic waste ends up in landfills and oceans, as current recycling methods are either expensive, inefficient or impractical. This is a major cause for concern, as the amount of plastic being produced globally is expected to increase, and the situation is likely to get worse.

To address both of these crises, a new biological upcycling method for polyethylene terephthalate (PET) plastic waste is being proposed. This method involves using microorganisms to convert the plastic into edible microbial protein powder, also known as Single-cell protein (SCP). This innovative solution provides a potential solution for both the food crisis and plastic pollution.

Currently, there are four standard degradation methods for recycling plastics: mechanical recycling, energy recovery, chemical recycling, and biological recycling. The pros and cons of these methods have been summarized in the literature. The biodegradation process begins when a microorganism attaches itself to the plastic, and the extracellular enzymes expressed by the microorganisms depolymerize the plastic. The rate of degradation is influenced by various factors such as the physical and chemical composition of the material, environmental conditions, molecular weight and degree of crystallinity of the PET polymers, and ambient conditions in the environment.

Chemical & Biological Degradation

The biodegradation process initiates when a microorganism attaches itself to plastic. However, the process begins under the condition that extracellular enzymes expressed by the microorganisms depolymerize the plastics. As the enzymes' reactivity highly depends on the polymer crystallinity, microbes must overcome those obstacles to start the degradation process. Biodegradation rates are influenced by abiotic factors: physical and chemical composition of the material, environmental conditions [37], molecular weight and degree of crystallinity of the PET polymers, and ambient conditions in the environment [38]. Thus, the biodegradation rate must be increased to enhance the proposal's feasibility, which could be done by biotechnological improvements, coupling to chemical or mechanical deconstruction. PET-degrading microorganisms are found in different biomes, such as marine, soil and sewage environments. Although PET depolymerization is considered to be a rate-limiting step, many organisms can degrade the monomeric building blocks of PET. The pathway is shown below (Figure 2). PET degradation starts with PET deconstruction by PETases, while MHETases further convert the intermediate into terephthalic acid (TPA) and ethylene glycol. Microorganisms that can produce PETases are well discovered. Ideonella sakaiensis can produce PETases and encodes the complete pathway for PET assimilation, and Liu and his colleagues found a Delftia species that could completely degrade PET. The problem here is the low activity of natural PETases. A study found that wildtype PETase [leaf-branch compost cutinase (LCC)] vastly outperformed the PETase produced by Ideonella sakaiensis (Figure 3) but can only degrade 30% of the PET in 6 days under controlled conditions, showing the activity of the enzymes naturally found is low. To address the problem, engineered thermostable PETases are constructed, deconstructing 90% of the PET in 10 hours compared with other controlled groups under identical conditions (Figure 4).

Figure 2

Figure 3

Figure 4

To further boost the degradation rate, chemical techniques such as hydrolysis, alcoholysis, aminolysis and glycolysis can be used to depolymerize the plastic, recovering monomers. The chemical deconstruction of polymer aims to reduce cost and increase efficiency. Aminolysis is well-suited for hybrid thermochemical-biological processing. Microbes are then used to upcycle the deconstructed products (monomers or oligomers) into cell biomass/products. [] However, it is currently unknown whether microbes can biodegrade the terephthalamide produced during aminolysis.

Instead of using single-strains, microbial communities are another approach to improve PET biodegradation. The latest research shows that synergistic cooperation between microbes allows for complete or more efficient degradation of plastics and contaminants, due to division of labor or increased substrate accessibility.

Utilizing microbial cells as a source of protein

Producing enough protein-rich foods to meet demand is one issue facing the world's food supply. Now, meat and dairy products are the main sources of protein. However, the projection of increasing meat production to 455 megatons by 2030 has been deemed infeasible. Although some plants contain a high amount of protein, they must be grown more efficiently to meet dietary demands. Microorganisms, on the other hand, are rich in protein. Algae and bacteria contain more than 50% of dry weight protein, and fungi comprise 30-50% of dry weight protein.

Moreover, microbial cells consist of valuable nutrients. For example, algae contain lipids (omega-3 fatty acids, carotenoids, and triacylglycerides) and vitamins (A, B, C, and E) [78,81,86]. Single-cell protein (SCP) can be extracted from these microbes. Large-scale SCP production reduces CO2 emissions by 6-99% while using only 1% of the water that conventional food production needs. These statistics demonstrate the vast benefits on resource preservation of using SCP an alternative protein source.

Nonetheless, nucleic acids present in both bacteria and fungi posted potential threat to human health, symptoms such as gout and kidney could result from the consumption of high concentrations of nucleic acids. Therefore, an additional neutralization step is needed to ensure the SCP produced by the microbes is safe. Further biological testing of microbial toxins and animal/human testing is needed.

Future Projection

The degradation and recycling method proposed that if PET waste (32 million tons per year []) was converted with 100% efficiency into human food, we could supply ~4% of the global carbon consumption, which is a rough surrogate for energy needs. Although further consideration suggests the conversion of PET into food source would not be a major contributor to the global food supply. However, the extra food produced theoretically could still fulfill the urging need of food for “~36% of the 820million who are estimated to be hungry globally”, which is still a significant approach on global hunger problem.