Cysteine, L-Cysteine, Cysteine Hydrochloride, Cysteine HCl, E920, E921, 52-89-1, 7048-04-6, 52-89-1

Cysteine, L-Cysteine, Cysteine Hydrochloride, Cysteine HCl, E920, E921, 52-89-1, 7048-04-6, 52-89-1

Comprehensive Technical Data Sheet: L-Cysteine Hydrochloride

1. Product Description and Identification

L-Cysteine Hydrochloride is the hydrochloride salt of the semi-essential, sulfur-containing amino acid, L-Cysteine. It is a versatile biochemical compound widely utilized across the food, pharmaceutical, and cosmetic industries due to its unique reducing properties and role in protein structure.

• Common Names: L-Cysteine Hydrochloride, L-Cysteine HCl, L-Cysteine Hydrochloride Monohydrate, Cysteine Hydrochloride, Cysteine Monohydrochloride.

• INCI Name: Cysteine HCl

• E-Number:

  • E 920: Generally refers to L-Cysteine and its hydrochloride (anhydrous form).

  • E 921: Specifically refers to L-Cysteine Hydrochloride Monohydrate.

• Chemical Name: (2R)-2-amino-3-sulfanylpropanoic acid hydrochloride

• CAS Number: 52-89-1 (This is the primary CAS for the anhydrous hydrochloride form. The monohydrate form has a separate CAS: 7048-04-6).

• Molecular Formula: C₃H₈ClNO₂S

• Molecular Weight: 157.62 g/mol (for anhydrous); 175.63 g/mol (for monohydrate).

2. Physical and Chemical Properties

Understanding these properties is crucial for proper handling, formulation, and storage.

• Appearance: White or colorless crystalline powder.

• Odor: Characteristic, but very faint. It is often described as odorless in its pure, dry state, though some may detect a slight sulfurous note.

• Optical Rotation: [α]²⁰/D = +5.6° to +8.0° (in 5M HCl). This confirms its L-enantiomer purity.

• Solubility (Crucial Property):

  • Water: Highly soluble. Solubility is approximately 280 g/L at 20°C. This excellent aqueous solubility is a key reason the hydrochloride salt is preferred over the free base form in many industrial applications.

  • Ethanol: Slightly soluble.

  • Ether and Acetone: Practically insoluble.

• pH: A 1% aqueous solution typically has a pH between 1.5 and 2.0, indicating its acidic nature due to the hydrochloride content.

• Thermal Stability: It does not have a distinct melting point. It undergoes decomposition upon heating, typically above 175°C (for monohydrate, losing water first) and completely decomposes above 260°C.

• Stability & Reactivity:

  • pH Sensitivity: It is most stable under acidic conditions.

  • Oxidation: This is the most critical chemical property. In neutral or alkaline (basic) conditions and in the presence of atmospheric oxygen, L-Cysteine is readily oxidized. Two L-Cysteine molecules form a disulfide bond (S-S), creating L-Cystine. This reaction is reversible.

    • 2 L-Cysteine ⇌ L-Cystine

  • Hygroscopicity: The anhydrous form is slightly hygroscopic. The monohydrate form is stable in ambient humidity. It should be stored in a cool, dry place in tightly sealed containers to prevent oxidation and moisture absorption.

3. Production Methods (Origin)

The source and method of production are of paramount importance for regulatory compliance (Halal, Kosher, Vegetarian/Vegan) and consumer acceptance.

1. Historical Method (Protein Hydrolysis - Now Largely Obsolete):

   • L-Cysteine was traditionally extracted by hydrolyzing keratin-rich proteins. The most common source was human hair, but animal hair, feathers, and hog bristles were also used.

   • This process involved treating these materials with concentrated hydrochloric acid, followed by multiple purification steps.

   • This method has been largely abandoned by major manufacturers due to ethical concerns, potential impurities, high costs, and religious restrictions (non-Halal/non-Kosher).

2. Modern Method (Microbial Fermentation - Current Industry Standard):

   • Today, the vast majority of the world's L-Cysteine is produced via microbial fermentation, a process similar to how beer, wine, or antibiotics are made.

   • Specially engineered, non-pathogenic strains of bacteria or yeast (such as E. coli or Candida species) are cultivated in large fermentation tanks.

   • These microorganisms are fed a nutrient-rich broth made from renewable, plant-based sources like corn syrup (glucose), beet molasses, or starch.

   • Through their metabolic processes, the microbes produce and excrete L-Cysteine.

   • The L-Cysteine is then harvested, purified through crystallization and chromatography, and finally reacted with hydrochloric acid to form the highly soluble L-Cysteine Hydrochloride salt.

   • Advantages: This method yields a pure, consistent product that is free from animal-derived components, making it suitable for Halal, Kosher, and vegetarian/vegan markets.

3. Enzymatic Method:

   • A less common method involves using enzymes to convert a precursor molecule (e.g., 2-amino-Δ²-thiazoline-4-carboxylic acid) into L-Cysteine.

4. Applications and Functions

A. Food Industry (as Additive E920 / E921)

• Flour Treatment Agent / Dough Conditioner (Primary Use): This is its most significant application in food.

  • Mechanism: Gluten, the protein network in wheat flour, gives dough its elasticity. This network is held together by disulfide bonds (S-S bonds) between cysteine residues in gluten proteins.

  • Action: L-Cysteine Hydrochloride acts as a reducing agent. It breaks these disulfide bonds, weakening the gluten structure.

  • Result: This reduces the dough's elasticity and viscosity (makes it more extensible and less "rubbery"). The dough becomes softer, relaxes faster, and is easier to machine and shape. This is particularly useful for:

    • Crackers and Biscuits: Prevents shrinkage during baking and ensures a uniform shape.

    • Pizza Dough: Makes it easy to stretch into thin bases without tearing.

    • Puff Pastry: Improves extensibility.

    • Noodles: Improves texture.

• Flavor Enhancer and Processing Aid:

  • In savory flavors, it can act as a precursor for meaty and roasted flavors, especially when heated with reducing sugars (Maillard reaction).

  • It is used in the production of bouillon and meat extracts. By scavenging oxygen, it creates an anaerobic environment that can be beneficial for certain fermentation processes or help prevent the oxidation of fats.

• Antioxidant:

  • It can prevent the enzymatic browning (oxidation) of fresh-cut fruits and vegetables like apples, potatoes, and avocados.

B. Pharmaceutical and Healthcare Industry

• Precursor to Glutathione: L-Cysteine is the rate-limiting substrate in the biosynthesis of Glutathione, one of the body's most powerful intracellular antioxidants. Glutathione is crucial for protecting cells from oxidative damage, supporting the immune system, and detoxifying harmful substances.

• Mucolytic Agent: L-Cysteine and its more stable derivative, N-Acetylcysteine (NAC), are used to break down disulfide bonds in the glycoproteins of mucus. This reduces the viscosity of thick, sticky phlegm, making it easier to cough up. This is used in treating respiratory illnesses like bronchitis and cystic fibrosis.

• Detoxification: It aids in phase II liver detoxification, helping to conjugate and eliminate toxins, including heavy metals like lead and mercury, and harmful metabolites like acetaldehyde (from alcohol metabolism).

• Nutritional Supplement: As a semi-essential amino acid, it is included in parenteral nutrition (IV feeding) and protein supplements for wound healing, muscle recovery, and overall health.

C. Cosmetics and Personal Care Industry

• Hair Waving/Straightening (Cold Wave): The mechanism is identical to its use in dough. It penetrates the hair shaft and reduces the disulfide bonds in the keratin protein. This softens the hair, allowing it to be reshaped around rollers (for curls) or stretched flat (for straightening). After shaping, an oxidizing neutralizer is applied to reform the disulfide bonds, locking the hair into its new shape.

• Skin Care:

  • Collagen Support: L-Cysteine is a building block for collagen, the main structural protein in skin that provides firmness and elasticity.

  • Antioxidant Protection: In topical formulations, it can help protect the skin from free radical damage caused by UV exposure and pollution.

5. Safety and Regulatory Status

L-Cysteine Hydrochloride is generally recognized as safe (GRAS) by the U.S. Food and Drug Administration (FDA) when used as a nutrient and/or as a dough conditioner in accordance with good manufacturing practices. It is also an approved food additive in the European Union (E920/E921) and by the Joint FAO/WHO Expert Committee on Food Additives (JECFA).

Other Names / Synonyms for L-Cysteine Hydrochloride

L-Cysteine Hydrochloride is known by various names depending on the context (chemistry, food industry, trade, or daily use). Below is a comprehensive list of its synonyms and identifiers.

1. Common & Short Names

• L-Cysteine (The most common short name, though technically the base form)

• Cysteine

• L-Cysteine HCl

2. Names Indicating the Salt Form (Hydrochloride)

These names specify that the compound is the hydrochloride salt of L-Cysteine.

• L-Cysteine Hydrochloride (IUPAC / Scientific name)

• Cysteine Hydrochloride

• Cysteine HCl

• L-Cysteine Hydrochloride Monohydrate (The form containing one water molecule)

• Cysteine Monohydrochloride

3. Systematic Chemical Names (IUPAC)

These are formal names used in chemical nomenclature.

• (R)-2-Amino-3-mercaptopropionic acid hydrochloride

• (2R)-2-amino-3-sulfanylpropanoic acid hydrochloride

• α-Amino-β-thiopropionic acid hydrochloride

4. Abbreviations & Symbols

• Cys (Standard 3-letter abbreviation for Cysteine)

• C (Standard 1-letter abbreviation for Cysteine)

• L-Cys-OH

5. Regulatory & Identification Codes

• E Number: E 920 (Generally refers to L-Cysteine and its Hydrochloride, anhydrous form)

• E Number: E 921 (Specifically refers to L-Cysteine Hydrochloride Monohydrate)

• CAS Number: 52-89-1 (The unique Chemical Abstracts Service number for the anhydrous form)

• CAS Number: 7048-04-6 (The unique CAS number for the Monohydrate form)

• EC Number: 200-157-7 (European Community number for 52-89-1)

• FEMA Number: 3263 (Flavor and Extract Manufacturers Association number, used in flavorings)

6. Other Language Equivalents

• French: Chlorhydrate de L-cystéine

• German: L-Cystein hydrochlorid

• Spanish: Clorhidrato de L-cisteína

• Turkish: L-Sistein Hidroklorür, Sistein HCl, Sistein Klorhidrat

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