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Home / News / Industry News / What Is Silk Made Of? Fibroin, Sericin, and the Fiber Processing Explained
Roughly 90 percent of the world's commercial silk comes from the cocoon of one insect: Bombyx mori, the domesticated mulberry silkworm. Silk is not a cellulose fiber like cotton, and it is not a synthetic polymer. What silk is made of is biochemical: a continuous protein filament built from two proteins, fibroin and sericin, which the silkworm secretes as a liquid and pulls into a thread as it hardens in air. A single cocoon yields a filament up to 900 meters long, and that filament becomes the raw material for woven fabrics, sewing threads, and decorative yarns.
90%of commercial silk worldwide is mulberry silk from Bombyx mori
300 to 900 mof continuous filament can be reeled from one undamaged cocoon
70 to 80%of the finished fiber is the structural protein fibroin
This composition is not trivia. Every quality a buyer evaluates, from luster and drape to dye uptake and wash durability, can be traced back to silk's protein chemistry. When a supplier describes a material as silk-like, what that really means is that another fiber has been engineered to imitate part of that protein-driven behavior.
What Is Silk Made Of at the Molecular Level
At the molecular level, silk is made of a fibrous protein core called fibroin, wrapped in a gummy protective protein called sericin.
Fibroin accounts for 70 to 80 percent of the fiber's weight and forms the structural core. Its amino acid sequence is simple and repetitive: about 43 percent glycine, 30 percent alanine, and 12 percent serine. This allows the chains to fold into tightly packed beta-sheet crystals, which give silk its strength and its characteristically smooth surface.
Sericin makes up most of the remaining 20 to 30 percent and acts as an adhesive gum that bonds the fibroin strands and cements the cocoon wall. Raw silk straight from a cocoon feels stiff and dull because of sericin. Once the gum is dissolved away during degumming, the fiber turns soft, lustrous, and ready to take dye.
Core definitionSilk is a natural protein fiber. Its core strand, fibroin, carries the fiber's mechanical strength, while the surrounding sericin layer holds the cocoon together and is removed during textile processing.
| Component | Share of fiber | Structural role | Processing fate |
| Fibroin | 70 to 80% | Crystalline filament core | Retained; forms the degummed silk fiber |
| Sericin | 20 to 30% | Gum-like coating and binder | Dissolved in hot soap or water baths |
| Waxes, fats, minerals | 1 to 3% | Natural surface impurities | Removed during reeling and scouring |
Fibroin: the load bearerArranged in near-perfect beta-sheet crystals, it gives silk its tensile strength and handles strain without permanent deformation.
Sericin: the sacrificial glueAmorphous and water-soluble, it protects the filament inside the cocoon but is stripped away in degumming so the fiber feels soft.
How a Silkworm Spins Liquid Protein Into Fiber
A silkworm produces liquid silk proteins in two modified salivary glands and forces them through a spinneret in its head, where they harden into a double filament.
The posterior part of the silk gland secretes fibroin; the middle part adds sericin around it. When the larva is ready to pupate, it presses the spinneret against the cocoon surface and draws its head in a figure-eight pattern for about three days, repeating the motion 250,000 to 300,000 times to build the cocoon.
Each cocoon is one uninterrupted filament, typically 300 to 900 meters long. Because the filament is continuous rather than spun from short staples, silk yarn keeps an unusually smooth surface, which is the physical basis of its sheen.
One kilogram of raw silk requires roughly 5,500 silkworm cocoons. A single undamaged cocoon is a seamless fiber up to 900 meters long.
- Fibroin is secreted as a viscous liquid and aligns into crystals as it is drawn, similar to a stretched polymer chain locking into place.
- Sericin is wrapped around the fibroin core before the filament leaves the spinneret, which is why raw silk is a composite rather than a single protein.
- Diet and rearing conditions slightly alter the protein balance, which is why silk from different harvests can differ in luster and strength.
From Cocoon to Reeled Yarn: The Three Processing Stages
Raw silk reaches textile mills as a continuous reeled thread, and the three critical stages are stifling, reeling, and throwing.
Each stage preserves or removes part of the fiber's natural structure. Overheating the cocoons, reeling too fast, or leaving sericin residues will damage evenness, luster, and dyeing behavior.
Stifling the cocoon
Cocoons are heated or steamed to kill the pupa before it emerges and cuts through the shell, preserving the filament as one continuous length.
Reeling the baves
Workers brush the cocoon to find the filament end, then combine filaments from three to eight cocoons onto a reel. The combined strand becomes one raw silk thread with a controlled diameter.
Throwing and degumming
The reeled thread is twisted into yarn to control hand-feel and strength, then boiled in soap or enzyme baths to strip the sericin. Degummed silk loses about 20 to 25 percent of its weight and takes on its familiar soft, brilliant character.
Before degummingStiff handle, dull surface, yellowish tint; sericin still shields the filament from abrasion and makes even dyeing difficult.
After degummingSoft and drapable, bright luster, white to cream color; dye uptake is consistent and the fiber develops its trademark whisper.
What Is Silk Made Of Compared With Silk-Like Fibers
Most silk substitutes imitate silk's luster or drape with cellulose or synthetic polymers, but none replicate the fibroin protein structure that governs silk's strength and moisture behavior.
The market is full of materials that look like silk; the difference is composition. Rayon is regenerated cellulose, polyester is a petroleum polymer, and metallic yarns are metal-coated polymer films. Buyers who know what silk is made of can judge these alternatives against the product's real needs.
| Fiber | Raw material | Polymer type | Dry tenacity (g/den) | Moisture regain |
| Mulberry silk | Silkworm cocoon | Protein (fibroin plus sericin) | About 4.8 | About 11% |
| Rayon (viscose) | Wood pulp | Regenerated cellulose | About 2.7 | 11 to 13% |
| Polyester | Petroleum | Polyethylene terephthalate | About 5.2 | About 0.4% |
| M-type metallic yarn | PET film plus aluminum coating | Metal-coated composite | 2.5 to 4.0 | Under 1% |
Rayon comes closest to silk's moisture feel and drape but loses 40 to 55 percent of its strength when wet. Polyester is stronger and dries quickly, yet its low moisture regain builds static and feels less breathable. Metallic yarns such as our M-type series are not silk replacements; they add bright highlights and surface structure that no plain silk can match.
At JinDun Textile Co., Ltd., which has been in textile manufacturing since 2006, these differences drive daily sourcing decisions. Buyers who want a silk hand and drape at a controlled cost choose rayon printed woven fabric for blouses. Knitwear programs that need a silky plush surface use imitation mink yarn instead of silk boucle. Decorative lines pair metallic yarn with silk grounds for extra body and luster.
Rayon Printed Woven Fabric for Versatile Garment UseThis woven rayon offers a flat, stable surface with customizable prints, ideal for blouses and other apparel where silk-like drape at controlled cost is needed.View Product → 
Imitation Mink Yarn with Durable Non-Shedding PileThis feather yarn provides a thick, plush surface with customizable fiber composition and counts, suited for knitwear, scarves, and accessories requiring silky loft.View Product → 
M Type Metallic Yarn for Decorative TextilesMade from slit metalized polyester film, this superfine yarn adds luster and body to high-grade fabrics, knitwear, and crafts, with multiple widths and stock colors.View Product →Why Silk's Composition Drives Its Performance
Nearly every measurable silk property, such as tenacity, elasticity, and moisture behavior, comes from the arrangement of fibroin protein chains.
Alternating crystalline and amorphous regions within fibroin carry load and absorb energy, producing a fiber with tensile strength comparable to high-grade steel at equal weight and an elongation at break of about 10 to 20 percent.
- Tenacity: dry value near 4.8 g/den, with the beta-sheet structure giving exceptional strength per unit of weight.
- Moisture regain: about 11 percent, which makes silk breathable, comfortable in warm weather, and low in static charge.
- Dyeing: the reactive amino groups in fibroin accept acid dyes with excellent depth and fastness.
- Care limits: prolonged UV exposure, perspiration, and strong alkalis degrade the protein, so silk needs gentler washing than polyester.
These numbers also explain why manufacturers blend silk with other yarns rather than pretend another fiber is identical. For decorative trims, the comparison runs in the other direction: how metallic yarn compares with other decorative threads when a design calls for light and movement that silk alone cannot deliver.
Frequently Asked Questions About Silk Composition
Is silk an animal fiber or a plant fiber?
Silk is an animal fiber. It is produced by silkworm larvae as a protein secretion and contains no cellulose, so it behaves differently from cotton or linen in strength, moisture, and dyeing.
What protein is silk made of?
The filament consists of fibroin, which forms the strong crystalline core, and sericin, a soluble gum that coats and binds the fiber in the cocoon. After degumming, the remaining fiber is almost entirely fibroin.
Why does real silk feel different from rayon or polyester?
Silk's protein chains absorb about 11 percent moisture, move with the wearer, and produce low static. Rayon shares silk's moisture feel but loses strength when wet; polyester is stronger but hydrophobic and less breathable.
Can silk be produced without silkworms?
Laboratory fermentation can create silk proteins from engineered microbes, but commercial apparel grades still come almost exclusively from reeling Bombyx mori cocoons. Spun silk and waste silk are made from broken filaments that cannot be reeled.
