Cystine is a nonessential amino acid that acts as a nutrient and dietary supplement. It is very slightly soluble in water an din alcohol. it improves the biological quality of the total protein in foods containing naturally occurring intact protein. DL-Cystine is a racemic mixture of the proteinogenic amino acids L-cystine and the non-proteinogenic D-cystine. DL-cystine is used in the preparation of sulfur-containing dimeric and monomeric surfactants.