Acylation of agricultural protein biomass yields biodegradable superabsorbent plastics

The research aims to create sustainable and biodegradable alternatives to petroleum-based superabsorbent polymers (SAPs) using agricultural waste. SAPs are vital for hygiene products, but their petroleum origin poses environmental concerns. Potato protein concentrate (PPC), a byproduct of starch extraction, is identified as a promising renewable resource. The current industrial process for PPC production involves harsh chemical treatments, raising environmental and safety concerns. This study proposes a more sustainable approach by directly acylating potato fruit juice (PFJ) which is the precursor to PPC before the energy and chemical intensive industrial process which leads to PPC. This would leverage the protein content of PFJ to produce a biodegradable SAP, reducing reliance on fossil fuels and promoting a circular bioeconomy. The existing literature explores the use of proteins from various sources for creating bio-based SAPs; however, the focus on utilizing PPC obtained as a side-stream, while improving the sustainability of its production, is a unique contribution to the field.

Previous research has investigated using various renewable feedstocks for material production, focusing on proteins as side-streams from industrial processes. PPC, a significant underutilized agro-industrial side-stream, shows promise as a replacement for fossil-based materials. Past work has focused on PPC's foaming and emulsifying properties, its use in bio-based plastics (with glycerol as a plasticizer), and the synthesis of protein nanofibrils. The development of bio-based SAPs using proteins has also been explored, with studies using soy, wheat, and potato proteins showing competitive functional properties compared to synthetic SAPs. Typically, these proteins are functionalized by acylation using a dianhydride to mimic the chemical structure of synthetic SAPs. Previous work demonstrated PPC acylation achieving liquid super-absorbent properties, but this involved harsh conditions creating extensive crosslinking and potentially including toxic compounds.

The study involved two main approaches: acylation of commercially obtained PPC and in-situ acylation of PFJ. For PPC acylation, the powder was suspended in water, adjusted to pH 11, heated to 90°C for denaturation, then the pH adjusted to 12 before adding varying amounts of EDTAD (5, 10, 20, 25 wt%, and a 1:1 ratio). The mixture was stirred, centrifuged to remove unreacted EDTAD, neutralized, dried, and ground. A reference sample (PPC/Ref) was prepared without EDTAD. In-situ PFJ acylation followed a similar process, with the key difference that the acylation was performed on the PFJ before the industrial coagulation process to obtain the PPC. After the EDTAD addition and the reaction, the pH was adjusted to 3-3.5 using 1M HCL at room temperature to precipitate the proteins. The suspension was then centrifuged and washed. The pellet was then resuspended in MQw, the pH was adjusted to 7-7.5 using 1M NaOH and finally the proteins were separately autoclaved for 5, 10, 15, and 25 min at 120°C to precipitate the proteins. A non-autoclaved EDTAD-acylated PFJ sample (PFJ/25ED) and a dried fraction of the as-received PFJ (non-autoclaved and non-acylated, PFJ/Ref) were also prepared. The liquid free swelling capacity (FSC) was determined using the 'tea-bag test' with MQw, saline, and defibrinated sheep blood, measuring weight changes at different times. Centrifugal retention capacity (CRC) was also assessed. Protein extraction profiles were analyzed using size-exclusion high-performance liquid chromatography (SE-HPLC) to investigate protein aggregation. Fourier-transform infrared (FT-IR) spectroscopy characterized protein structure and functionalization. Conductometric titration determined the particle charge density. Scanning electron microscopy (SEM) examined particle morphology. Biodegradability was assessed using a soil degradation test measuring CO2 evolution. Mold resistance was evaluated by exposing samples to 100% relative humidity (RH). Finally, a greenhouse gas (GHG) assessment compared the emissions of the PPC acylation process with the in-situ PFJ acylation process. The amount of natural gas required to heat the PFJ and evaporate the water from the protein precipitate was calculated to estimate the additional energy needed if the acylation was performed on the PPC instead of the PFJ.

Acylation of PPC with EDTAD significantly increased water swelling capacity from 2 to 24 g/g (2400%), a tenfold improvement. The highest swelling was achieved with 25 wt% EDTAD. Acylation of PFJ, before industrial PPC production, also yielded increased swelling capacity (up to 9 g/g after 1 min). The acylated PPC exhibited rapid water absorption and maintained substantial liquid retention capacity under centrifugation, comparable to commercial SAPs in saline and blood. The in-situ acylation resulted in a protein network with a swelling capacity of up to 11 g/g in water, without significant material loss. SE-HPLC showed that the industrial PPC had a higher degree of protein aggregation than mildly extracted PPC. The acylation process increased the extractability of both monomeric and polymeric protein fractions in both PPC and mildly extracted PPC, suggesting a less cross-linked network. FT-IR spectroscopy confirmed EDTAD incorporation, and conductometric titration indicated reduced lysine content due to acylation. SEM revealed that the acylated PPC particles had a smooth surface initially, but after swelling and lyophilization, they formed a foam-like structure with interconnected pores. The acylated PPC showed significant resistance to mold growth compared to untreated PPC or wheat gluten. Biodegradation tests showed 50% degradation of acylated PPC within 97 days, while commercial SAP showed no degradation. The in-situ acylation of PFJ instead of PPC was estimated to reduce annual greenhouse gas emissions by approximately 325,000 tons of CO2, equivalent to the emissions from about 116,000 passenger vehicles.

The findings demonstrate the potential of using agricultural protein waste streams (PPC and PFJ) to create sustainable, biodegradable SAP alternatives. The substantial increase in swelling capacity achieved through EDTAD acylation addresses the key limitation of using protein-based materials for this application. The ability to achieve high swelling capacity without the use of toxic cross-linkers is a significant advantage. The in-situ acylation of PFJ offers significant environmental benefits by reducing energy consumption and GHG emissions associated with industrial PPC production. The mold resistance and rapid biodegradability of the acylated PPC further enhance its suitability for disposable hygiene products. The results suggest that the endogenous crosslinks in the industrial PPC contributes to a stable hydrogel without the addition of toxic crosslinkers. The improved swelling capacity is attributed to the increase in carboxylic acid/carboxylate groups resulting from the acylation. The study successfully addresses the research question of developing a sustainable, biodegradable SAP using agricultural waste by demonstrating both technical feasibility and environmental benefits. The superior performance in comparison to other biobased absorbents shows the potential for this material to compete with existing commercially available alternatives.

This study successfully produced biodegradable superabsorbent materials from potato protein side-streams, offering a sustainable alternative to petroleum-based SAPs. Acylation with EDTAD significantly improved swelling capacity, and in-situ PFJ acylation offered substantial GHG emission reductions. The material demonstrated biodegradability, mold resistance, and promising liquid retention properties, making it suitable for disposable hygiene applications. Future work should focus on optimizing the acylation process for industrial scalability, including EDTAD recovery and long-term material performance.

The study focused on laboratory-scale production. Further research is needed to evaluate the scalability and cost-effectiveness of the method for large-scale industrial production. Long-term stability and performance under varying environmental conditions require further investigation. The biodegradation testing was conducted under specific laboratory conditions which might not fully reflect the variability in natural environmental settings.