Conventional sericin extraction processes are primarily based on hydrolytic methods that use high-temperature water (95-120°C) and pressure, often in combination with alkaline substances such as sodium carbonate or soaps. These methods present several environmental issues.
First, traditional alkaline degumming generates wastewater with a high pollution load, characterized by high BOD (biochemical oxygen demand) and COD (chemical oxygen demand) values. It is estimated that for every kilogram of raw silk processed, up to fifty liters of highly contaminated wastewater are produced.
The use of aggressive chemicals not only partially degrades the protein, reducing its commercial value, but also contributes to water pollution. The treatment of this wastewater requires additional resources and energy processes, increasing the overall ecological footprint.
Methods employing high temperatures involve significant energy consumption, contributing to greenhouse gas emissions and the depletion of non-renewable resources. It is calculated that the traditional degumming process requires approximately 3-4 kWh of energy per kilogram of treated silk.
The thermal and chemical degradation of sericin during these processes alters its molecular structure and reduces its bioactive properties, limiting its potential value-added applications. This represents not only a waste of resources but also a missed opportunity for economic valorization. In this context, technological innovation has initiated a paradigm shift towards more sustainable approaches, which aim to preserve the integrity of the protein while simultaneously reducing the environmental impact of the extraction process.
Eco-sustainable extraction methods
Ultrasound-assisted extraction (UAE) is currently one of the most promising technologies for the eco-sustainable recovery of sericin. This method exploits the phenomenon of acoustic cavitation to facilitate the separation of sericin from fibroin fibers without resorting to excessive temperatures or aggressive chemical reagents.
The process is based on the application of ultrasonic waves (frequencies typically between 20 and 100 kHz) in an aqueous medium containing cocoons or silk fibers. The ultrasound generates microbubbles in the liquid which, upon imploding, create microscopic shock waves and high-velocity jets. This phenomenon produces localized mechanical effects that promote the solubilization of sericin.
Recent studies have shown that UAE can reduce extraction times from hours to minutes, with a decrease in energy consumption of up to 70% compared to conventional methods. Research conducted at the University of Shanghai has demonstrated that ultrasonic extraction at 40°C allows for sericin yields comparable to those of high-temperature degumming (90-100°C), but with a better-preserved amino acid profile and superior functional properties.
A further advantage of UAE is the significant reduction in the volume of water required, with consequent minimization of generated wastewater. The process can be optimized by modulating parameters such as ultrasonic power, temperature, pH, and treatment time, allowing the method to be adapted to different qualities of raw material and final product requirements.
Life cycle analyses (LCA) have shown that the implementation of UAE on an industrial scale could reduce the carbon footprint of the extraction process by up to 45%, representing a significant step forward towards sustainability in the valorization of sericulture by-products.
Microwave effect
Microwave-assisted extraction (MAE) utilizes electromagnetic energy to generate heat through the rotational movement of polar molecules, mainly water. This rapid and uniform heating of the system allows for efficient solubilization of sericin with significantly reduced processing times.
Unlike conventional heating that transfers heat from the outside to the inside of the material, microwaves generate heat directly within the matrix, creating an inverted thermal gradient that favors the migration of sericin into the solution. This phenomenon, known as the non-thermal microwave effect, contributes to improving extraction efficiency even at moderate temperatures.
Recent research conducted at the Textile Technology Institute of Prato has shown that microwave extraction at 60°C for just 10 minutes can achieve sericin yields comparable to traditional alkaline degumming (90-95°C for 30-60 minutes). Moreover, the sericin extracted with MAE has shown a higher average molecular weight and greater antioxidant activity, advantageous characteristics for cosmetic and biomedical applications.
From an environmental perspective, MAE allows for a 40-60% reduction in energy consumption and an 80-90% decrease in processing time compared to conventional methods. The ability to operate at neutral or slightly alkaline pH (7-8) significantly reduces the pollutant load of wastewater, facilitating its treatment or reuse in closed-cycle processes.
Recent technological developments have allowed for the design of continuous flow MAE systems, which facilitate industrial implementation and precise control of process parameters, representing a significant evolution towards large-scale production of sericin with sustainability criteria.
Evident environmental benefits
The use of supercritical fluids, particularly supercritical carbon dioxide (SC-CO?), represents a cutting-edge technology for the eco-sustainable extraction of sericin. A supercritical fluid is a substance that, when brought beyond its thermodynamic critical point, exhibits properties intermediate between those of a liquid and a gas, combining the high solvent power of the former with the low viscosity and high diffusivity of the latter.
Supercritical CO? (critical temperature 31.1°C, critical pressure 73.8 bar) offers numerous environmental advantages: it is non-toxic, non-flammable, readily available, economical, and completely recoverable at the end of the process. Moreover, its low critical temperature makes it ideal for the extraction of thermosensitive biomolecules such as sericin.
The extraction process requires the use of a polar co-solvent, typically water or ethanol in small percentages (5-15%), to improve the solubility of sericin in supercritical CO?. Studies conducted at the University of Bologna have demonstrated that this approach allows for obtaining sericin fractions with high purity and intact biological properties, particularly suitable for high-value pharmaceutical and biomedical applications.
The environmental advantages of SC-CO? extraction are significant because they lead to the elimination of toxic organic solvents, the total absence of contaminated wastewater, naturally reduced water consumption, and above all, the obtaining of a product free from chemical residues.
Technical analyses show that, despite the higher initial costs for high-pressure equipment, SC-CO? extraction can be economically advantageous in the medium to long term, especially considering the savings in wastewater treatment costs and the greater valorization of the final product. Recent advances in process engineering have also allowed for reducing operating costs through energy recovery systems and optimization of pressurization cycles.
Enzymatic extraction
The enzymatic approach to sericin extraction is another promising path in the perspective of the circular bio-economy. This method exploits the selective action of proteolytic enzymes to hydrolyze the bonds connecting sericin to fibroin fibers, allowing for a highly specific recovery of the protein under mild conditions.
The process mainly uses proteases (such as papain, bromelain, trypsin, or specific serine proteases) in aqueous solutions at controlled pH (6.5-8.5) and moderate temperatures (30-60°C). This enzymatic selectivity allows for preserving both the integrity of fibroin in the main fiber and the macromolecular structure of the extracted sericin.
Research conducted at the Silk Biotechnology Center in Sozhou, China, has demonstrated that the enzymatic approach can enable the extraction of sericin fractions with specific molecular weights and targeted functionalities, paving the way for customized productions for various industrial applications.
The most recent innovations in the field concern the development of optimized enzymatic cocktails and the use of enzyme immobilization techniques on reusable supports, which further reduce the costs of the process and increase its sustainability.
From an economic perspective, although the cost of enzymes still represents a limitation, the growing availability of biocatalysts produced through recombinant technologies is progressively reducing this barrier, making the approach increasingly competitive compared to conventional methods.
Hybrid technologies and integrated approaches
The most recent evolution in the field of eco-sustainable sericin extraction is represented by the development of hybrid technologies that combine two or more of the previously described methods, exploiting the synergies between different approaches to simultaneously optimize extraction efficiency, product quality, and environmental sustainability.
Particularly promising is the integration between ultrasound and enzymes, where acoustic cavitation improves the penetration and effectiveness of enzymatic action, reducing processing times and the amount of enzymes needed. This combination can increase the extraction yield by up to 15-20% compared to using individual methods, while maintaining operating temperatures below 50°C.
Similarly, the coupling between microwaves and enzymatic extraction allows for exploiting the non-thermal effects of microwaves to increase the catalytic activity of enzymes, enabling operation at lower enzymatic concentrations and lower temperatures.
An innovative methodology is also represented by multi-stage sequential extraction systems, which allow for recovering sericin fractions with different molecular and functional characteristics. For example, a first stage of moderate hydrothermal extraction (60-70°C) can recover the lower molecular weight sericin fractions, followed by an enzymatic treatment for the medium molecular weight fractions, and finally by ultrasound-assisted extraction for the fractions more tenaciously bound to the fiber.
This fractionated approach not only optimizes the overall recovery of the protein but also allows for obtaining differentiated products for specific industrial applications, maximizing the added value of the entire process. The higher molecular weight fractions can be destined for high-value biomedical applications, while those with lower molecular weight can find use in cosmetics or the food industry.
The integration of extraction processes within biorefinery systems represents the most advanced horizon for the complete valorization of sericulture by-products. In these systems, in addition to sericin, other valuable components such as lipids, pigments, and bioactive compounds present in cocoons are recovered, in a circular economy perspective with zero waste.
Eco-compatible recovery and purification systems
Sustainable sericin extraction is not limited to the separation phase from fibroin but also includes downstream processes of recovery, concentration, and purification of the protein. Even in this area, technological innovation has developed low environmental impact solutions that preserve the quality of the final product.
Membrane technologies represent the most promising approach, replacing traditional methods of chemical precipitation or lyophilization, which are energy-intensive. Tangential ultrafiltration systems allow for selective concentration of sericin based on molecular weight, removing impurities and salts, with an energy consumption reduced by 60-70% compared to thermal evaporation.
Ceramic or eco-compatible polymer membranes offer high durability and regeneration possibilities, reducing operating costs and environmental impact. The most advanced systems integrate diafiltration processes that allow for modifying the salt composition of the protein solution without resorting to extensive dialysis, significantly reducing the volumes of water required.
For the drying phase, low-temperature spray drying technologies or supercritical drying are replacing conventional lyophilization, with energy savings of up to 80% and reduced processing times. These methods better preserve the functional properties of sericin, avoiding thermal denaturation and protein aggregation.
Particularly interesting is the application of eco-compatible non-solvent precipitation technologies, which use naturally derived alcohols such as ethanol and isopropanol, recoverable and reusable through low-energy distillation. This approach eliminates the use of toxic organic solvents traditionally employed in protein precipitation.
Waste valorization and process circularity
Another fundamental aspect of eco-sustainable sericin extraction is the management of process waste in a circular economy perspective. The most recent innovations have transformed what was traditionally considered waste to be disposed of into a valuable resource.
The wastewater from the extraction process, rich in low molecular weight protein fractions and oligopeptides, can be further processed using nanofiltration and reverse osmosis technologies to recover bioactive components usable as functional ingredients in cosmetic formulations or food supplements.
The solid residues of the process, mainly consisting of degummed fibroin fibers, represent a valuable raw material for technical textile applications or for the production of innovative biomaterials. The fibers partially hydrolyzed during enzymatic extraction show greater surface reactivity, ideal for advanced functionalization treatments.
In modern integrated plants, non-valorizable organic waste is conveyed to anaerobic digestion systems for biogas production, used as a renewable energy source to partially power the extraction process itself, in a virtuous cycle of energy self-sufficiency.
This integrated approach to waste valorization has drastically reduced the overall environmental footprint of the process, transforming a disposal problem into an opportunity for creating added value. Eco-efficiency analyses conducted at the Biomaterials Research Center in Lyon have demonstrated that the implementation of these circular systems can reduce disposal costs by up to 85% and generate a 25-30% increase in the overall economic value derived from the initial raw material.
Innovative applications of eco-extracted sericin
Sericin obtained through eco-sustainable extraction processes presents superior structural and functional characteristics compared to that recovered with traditional methods, opening new application perspectives in high value-added sectors.
In the biomedical field, eco-extracted sericin has demonstrated excellent properties as a biomaterial for tissue engineering. Its characteristics of biocompatibility, ability to support cell adhesion, and anti-inflammatory properties make it ideal for the creation of scaffolds for skin and cartilage regeneration. Preliminary clinical studies conducted at the University Hospital of Osaka have shown promising results in the use of sericin hydrogels for the treatment of chronic wounds and burns.
In the cosmetic sector, sericin extracted with enzymatic methods or with supercritical fluids has shown superior antioxidant and moisturizing activity compared to commercial counterparts, thanks to better preservation of active functional groups. Formulations containing this high-quality sericin have shown significantly greater efficacy in clinical tests of skin hydration and protection from environmental damage.
The food and nutraceutical industry is exploring the use of sericin as a functional ingredient with emulsifying, stabilizing, and antioxidant properties. Sericin extracted using eco-sustainable technologies maintains its bioactive properties intact, proving effective as an ingredient for functional foods aimed at intestinal health and microbiota modulation.
In the field of advanced materials, films and coatings based on eco-extracted sericin have demonstrated excellent oxygen and UV barrier properties, opening perspectives for bio-based and biodegradable food packaging. The combination with naturally derived nanoparticles has allowed for the development of composite materials with intrinsic antimicrobial properties, useful for applications in active packaging and medical devices.
