L-Cysteine

L-Cysteine is a naturally occurring sulfur-containing amino acid that is essential to human protein metabolism. As a flour treatment agent (E920), it acts as a reducing agent in dough production, cleaving disulfide bonds in gluten proteins to relax and extensify the dough. This reduces mixing time, improves machinability at high speeds in industrial baking, and allows dough sheeting without tearing. It is used in crackers, biscuits, pizza bases, and certain commercial bread products.

JECFA evaluated L-cysteine in 1965 and did not establish a numerical ADI, as it is a normal dietary amino acid and an endogenous metabolite. The quantities used in flour treatment are small relative to natural cysteine intake from dietary protein.

L-cysteine is approved in the EU (E920), the United States, and most global markets. The primary consumer concern is sourcing: food-grade L-cysteine has historically been derived from human hair or duck and chicken feathers, in addition to synthetically produced or fermentation-derived versions. Labels typically do not specify the source, making it difficult for vegans, vegetarians, or those with religious dietary requirements to verify. Manufacturers supplying certified halal, kosher, or vegan products typically use fermentation-derived L-cysteine. Looksee assigns a limit rating reflecting this sourcing uncertainty.