Acylation of agricultural protein biomass yields biodegradable superabsorbent plastics

The study addresses the need to replace petroleum-based superabsorbent polymers (SAPs) with sustainable, biodegradable alternatives derived from renewable feedstocks. Industrial side-stream proteins, particularly potato protein concentrate (PPC) produced after starch extraction, are abundant yet underutilized. Conventional PPC production involves harsh thermal and acidic treatments of potato fruit juice (PFJ) that cause extensive protein aggregation/cross-linking and co-extract toxic glycoalkaloids, limiting food use but potentially beneficial for materials. Prior efforts to create bio-based SAPs have used cellulose and polysaccharides grafted with acrylic acid, and several industrial protein streams (soy, wheat, potato) have been functionalized, typically via dianhydride acylation, to mimic synthetic SAP chemistry. This work investigates whether acylation of PPC with ethylenediaminetetraacetic dianhydride (EDTAD) can yield biodegradable SAPs with competitive swelling and retention, and whether shifting functionalization upstream to PFJ can reduce environmental impact while maintaining performance.

• Renewable biomass, particularly industrial protein side-streams, is a promising route to replace fossil-based materials due to sustainability advantages.
• PPC is produced at scale as a byproduct of potato starch extraction but is highly aggregated due to industrial coagulation, affecting solubility and structure; co-extraction of glycoalkaloids also occurs.
• Synthetic SAPs (polyacrylic acid networks) absorb 10–1000 g g−1 and dominate daily-care products; bio-based SAPs investigated include cellulose-derived and polysaccharide systems grafted with acrylic acid.
• Protein-based SAPs from soy, wheat, and potato protein demonstrate competitive performance; functionalization typically involves dianhydride acylation to introduce carboxyl groups and enhance swelling via electrostatic repulsion.
• Prior work showed PPC can be acylated to achieve superabsorbency; however, PPC’s harsh processing reduces functionalization efficiency while providing endogenous crosslinking beneficial for gel stability. This study builds on these findings by using EDTAD and exploring upstream PFJ functionalization to reduce greenhouse gas emissions.

Materials: Commercial PPC and PFJ from Lyckeby Starch AB (Sweden). EDTAD, NaOH, NaHSO3, (NH4)2SO4 from Sigma-Aldrich. Mildly extracted PPC (PPCm) prepared in-house via ammonium sulfate precipitation to compare with industrial PPC.

Protein content: Dumas method (Flash 2000 N/C), N×6.25 conversion.

Acylation of PPC: PPC dispersed to 2 wt% in water adjusted to pH 11 (1 M NaOH). Denaturation at 90 °C for 30 min, rapid cooling. Acylation at pH 12 with EDTAD added over 30 min at varying loadings (5, 10, 20, 25 wt% relative to PPC; and a 1:1 mass ratio), stirring 1.5 h while maintaining pH 12. Cleaning by centrifugation (3000 RCF, 10 min), supernatant replacement with pH 11 water, re-dispersion, repeat, then concentrate to 25% of initial volume, neutralize to pH 7–8 (1 M HCl). Dry at 50 °C overnight, grind. Reference PPC processed identically without EDTAD (PPC/Ref). Samples labeled PPC/XED.

Acylation of PFJ (in situ): Assume 2 wt% protein. Denature and acylate as above, then precipitate acylated protein at pH 3–3.5 (1 M HCl), centrifuge, wash at pH 3.5, concentrate to 25% volume, adjust to pH 7–7.5 (1 M NaOH). Autoclave the acylated protein suspensions at 120 °C for 5, 10, 15, or 25 min, dry at 50 °C, grind. Labeled PFJ/25ED/AX. Controls: PFJ/25ED (no autoclave), PFJ/Ref (dried PFJ), and sequence-variant where PFJ is autoclaved first then acylated (PFJ/A25/25ED).

Swelling (FSC) and retention (CRC): Tea-bag method (NWSP 240.0.R2). 100–200 mg powder placed in mesh bags, equilibrated, immersed in excess deionized water (MQw), saline (0.9 wt% NaCl), higher-conductivity saline (Saline A, 16 mS/cm), or defibrinated sheep blood for 1, 5, 10, 30 min and 24 h. Blanks used to correct bag uptake. CRC: centrifuge swollen bags at 250 RCF for 3 min. FSC and CRC calculated from mass balances.

SE-HPLC: Three-step extraction in SDS/phosphate buffer (pH 6.9) with sonication steps to evaluate extractability and polymeric (1–15 min) vs monomeric (15–26 min) fractions. Waters SEC-4000 column, 210 nm detection.

FT-IR: ATR-FTIR (PerkinElmer Spectrum 100), 4 cm−1 resolution, 32 scans. Amide I deconvolution (1700–1580 cm−1) with Gaussian fitting to quantify secondary structure fractions.

Morphology: FE-SEM of dry powders and lyophilized swollen gels; sputter coat Pt/Pd, particle size measured (ImageJ).

Particle charge density: Conductometric titration (SCAN-CM 65:02) after protonation and rinsing; titration with 0.1 M NaOH under N2.

Biodegradability and mould resistance: Soil degradation per ASTM D5988-03 in sealed containers, CO2 capture with KOH and titration over up to 97 days; positive control potato starch and blank soil. Mould/humidity uptake: powders stored at 100% RH; record visual mould and gravimetric moisture uptake over 1–2 weeks.

GHG assessment: Estimate energy/CO2 for PPC production steps avoided by PFJ in situ acylation: (i) PFJ heating (22→120 °C under pressure) and (ii) evaporation of water from 60% moisture PPC precipitate. Steam from natural gas (85% boiler efficiency), drying efficiency 60%, typical gas composition, heating value 49.4 MJ kg−1, carbon 74.2%. Annual PFJ volume ~10 million m3 from 3.7 Mt starch. Calculate CO2 savings per m3 PFJ and per kg bio-SAP produced.

• Acylated PPC (PPC/25ED) achieved high water free swelling capacity (FSC): ~15 g g−1 at 30 min and ~25 g g−1 at 24 h (≈2400% weight increase), an ~8–10× increase vs untreated PPC (~2 g g−1).
• Rapid kinetics: ~12 g g−1 (1200%) water uptake within 1 min.
• In saline/blood: PPC/25ED FSC ~9 g g−1 (saline), ~6 g g−1 (Saline A, 16 mS/cm), and ~4 g g−1 (defibrinated sheep blood). These are within ranges for protein-based SAPs and the highest reported for PPC-based absorbents.
• Retention (CRC after 30 min swell, 250 RCF, 3 min): PPC/25ED retained ~2.8 g g−1 in Saline A and ~3 g g−1 in blood (~47% and ~75% of their FSC, respectively). While PPC/25ED FSC in Saline A and blood was 28% and 20% of a commercial polyacrylic SAP, retention ratios and rate of uptake were of similar magnitude.
• EDTAD loading effect: Increasing EDTAD from 10 to 20 wt% raised 2400 s water swelling from ~8 to ~11 g g−1; 25 wt% yielded the highest swelling. A 1:1 EDTAD:PPC ratio did not further increase swelling vs 25 wt%, indicating reactive sites (e.g., lysines) were saturated.
• PFJ in situ acylation plus autoclaving: PFJ/25ED/A25 swelled to ~9 g g−1 at 30 min and ~10 g g−1 at 24 h, ≈3× higher than PPC/Ref and PFJ autoclaved without EDTAD (PFJ/A25). Autoclaving before acylation reduced swelling (PFJ/A25/25ED: ~6 g g−1 at 24 h), implying fewer available lysine/functional moieties after heat treatment.
• Material retention in tea-bag after 24 h water swell: For PFJ without EDTAD, autoclaving increased retained solids from 18% (no autoclave) to 40%, 61%, and 65% at 10, 20, 25 min respectively. For EDTAD-treated PFJ, >75% retained after 25 min autoclave.
• Protein extractability and structure: EDTAD acylation increased total extractable protein (e.g., PPC Ext.1 from ~25% to ~55%; PPCm from ~165% to ~210%). Proportion of monomeric fractions rose in PPC after acylation, indicating reduced cross-linking. FT-IR showed increased carboxylate/carboxylic signals and reduced strongly bonded β-sheet content in PPC/25ED (from 50% in PPC to 41%). Conductometric titration indicated reduced lysine availability post-acylation.
• Morphology: Dry PPC/25ED particles ~304±135 µm, smooth surfaces; swollen-then-lyophilized gels formed porous, foam-like fragments with interconnected pores (~20 µm walls).
• Mould resistance and humidity uptake: PPC and wheat gluten showed visible mould at 100% RH in 2 weeks, whereas PPC/25ED and commercial SAP showed no obvious mould. PPC/25ED absorbed 0.9 and 1.1 g g−1 at 1 and 2 weeks, respectively (commercial SAP: 2.6 and 3.7 g g−1).
• Biodegradability: PPC/25ED degraded rapidly in soil—~30% by day 10 and ~50% by day 97—comparable to potato starch control; commercial SAP showed no biodegradation after 97 days.
• GHG savings: Performing acylation directly on PFJ instead of PPC avoids energy-intensive coagulation/drying, saving an estimated ~325,000 tons CO2 per year globally (≈116,000 passenger vehicles), about 10% of upstream starch production life-cycle GHGs linked to the starch volume considered. Savings correspond to ~2.3 kg CO2 per kg of bio-SAP; potential additional 5–10% savings with heat recovery/CHP.

Acylation of industrial PPC with EDTAD effectively introduces carboxylate functionality, enhancing electrostatic repulsion and hydration, thereby markedly increasing water and physiological fluid absorption while leveraging endogenous PPC cross-links to maintain gel integrity without added toxic cross-linkers. Although absolute FSC in saline and blood is below that of commercial polyacrylates, the uptake rate and retention under centrifugal load are comparable, indicating practical applicability in absorbent products subject to pressure (e.g., diapers, sanitary pads). Order of processing is critical: acylating PFJ prior to autoclaving maximizes available reactive amino groups, producing hydrogels with substantially higher swelling and better material retention than autoclaving prior to acylation. Structural analyses (SE-HPLC, FT-IR) corroborate reduced cross-link density and increased solubility/monomer content post-acylation, while still forming cohesive networks. The acylated PPC’s resistance to mould at high RH and significant humidity uptake, together with rapid biodegradation in soil, address storage and end-of-life concerns that hinder synthetic SAPs. Environmentally, upstream implementation at the PFJ stage can avoid energy-intensive steps of PPC production, yielding considerable CO2 savings, aligning the technology with circular bioeconomy principles. Overall, the findings demonstrate a feasible pathway towards biodegradable, bio-based SAPs with tunable performance via acylation level and thermal aggregation control.

EDTAD acylation transforms potato protein side-streams into biodegradable superabsorbents. Industrial PPC acylated at pH 12, after denaturation, reached ~24–25 g g−1 water swelling (≈2400%) and showed meaningful saline and blood absorption with good retention, enabled by intrinsic PPC cross-links and without added cross-linkers. Upstream acylation of PFJ followed by autoclaving produced stable gels with up to ~10–11 g g−1 water swelling and minimal material loss, while potentially avoiding PPC production energy and delivering large GHG savings (~325,000 tons CO2 per year; ~2.3 kg CO2 per kg bio-SAP). The acylated PPC resisted mould and biodegraded to ~50% in 97 days, whereas commercial SAP did not degrade. These results support the viability of protein-based, biodegradable SAPs for daily-care and desiccant applications. Future work should target industrial scalability (e.g., recovery/reuse of unreacted EDTAD), long-term storage performance, optimization across relative humidities, and closing environmental loops via energy recovery and process integration.

• Performance gap vs commercial SAPs remains in absolute free swelling capacity in saline and blood, despite similar uptake kinetics and retention ratios.
• GHG assessment assumes equal energy footprints for acylation steps in PPC vs PFJ routes and neglects some process steps (e.g., pH adjustment, separations), introducing uncertainty; no heat recovery assumed for drying could overestimate savings if industry uses recovery.
• Autoclaving/acylation sequence critically affects functionalization efficiency; conditions may need optimization across industrial PFJ compositions.
• Potential presence and recovery of unreacted EDTAD require industrial-scale solutions; storage stability and performance over time under varying RH need further study.
• Some analytical indications (e.g., FT-IR of PFJ samples) showed subtle differences, suggesting complex interactions with residual starch that warrant deeper mechanistic study.