BioFactors 26 (2006) 71–79 71 IOS Press Available lysine content in human milk: Stability during manipulation prior to ingestion D. Silvestre a , E. Ferrer a , J. Gay´ a a , E. Jare ˜ no b , M. Miranda c , M. Muriach c and F.J. Romero c,∗ a Nutrition Department, Cardenal Herrera-CEU University, Valencia, Spain b Centro de Salud Moncada, Sevei Valenci ´ a Salut, Valencia, Spain c Physiology, Pharmacology and Toxicology Department, Cardenal Herrera-CEU University, Valencia, Spain Received 8 November 2005 Accepted 23 November 2005 Abstract. Available lysine content is an indicator of protein quality and nutritional value of milk. Many studies have examined the effects of extraction, treatment and storage of human milk upon its components, though no references are found regarding the possible changes in milk quality as defined by its content in essential amino acids such as lysine. The present study investigates the available lysine content in human milk and the variations in lysine resulting from milk manipulation as follows: (a) Cold storage (refrigeration at 4 ◦ C for 48 hours, and frozen for 15 days at -20 ◦ C); (b) Thermal treatment under conditions of low (Holder)(63 ◦ C/30 minutes) and high pasteurization (75 ◦ C/15 seconds). The results obtained show a decrease in milk lysine concentration after storage in both refrigerated and frozen samples. Pasteurization causes a highly significant loss of available lysine. The lysine losses were greater on applying low pasteurization versus the more gentle conditions of high pasteurization. Conclusions: While manipulation through cold storage or thermal treatment does not affect the protein content of human milk, its protein quality is modified. When human milk must be subjected to hygienization, it is preferable to apply high temperature treatment (75 ◦ C, 15 seconds) than habitual pasteurization (63 ◦ C, 30 minutes). Keywords: Lysine, human milk, pasteurization, refrigeration, freezing 1. Introduction Lysine (Lys) is a limiting essential amino acid in many foods, and its abundance in milk determines its interest as a source of proteins of high biological quality. Lysine concentration in human milk (57–79 mg/100 ml) [1–5] is inferior to the concentration found in cow milk. The composition of mature human milk covers the nutritional needs of healthy infants up to 5–6 months from birth. European Directive EEC 91/321 [6] (Royal Decree 72/1998 according to Spanish legislation) establishes a lysine content of 6.7 g/100 g proteins in human milk as reference for defining the amino acid requirements of the nursing infant (RDA, 1991) [7] and as a basis for the elaboration of infant milk ∗ Address for correspondence: Dr. Francisco Javier Romero G´ omez, Universidad Cardenal Herrera-CEU, Avda. Seminario s/n, 46113 Moncada, Valencia, Spain. Tel.: +34 961369000; Fax: +34 961369013; E-mail: jromero@uch.ceu.es. 0951-6433/06/$17.00  2006 – IOS Press and the authors. All rights reserved