Effect of milk heat treatment on molecular interactions during the process of Qishta, a Lebanese dairy product

Protein–protein cross-linking and protein-fat interactions in the traditional Lebanese dairy product, Qishta, were investigated. Liquid chromatography coupled to mass spectrometry was used for the detection of lysinoalanine (LAL) and lanthionine (LAN) during the production of Qishta, which is made by heating whole milk in an open shallow vessel for more than 2 h and harvesting the surface aggregates formed. LAL and LAN cross-links were seen in Qishta at concentrations higher than those in milk, indicating their importance in the gel formation and therefore their impact on Qishta texture. Disulphide bridges were also involved in Qishta formation. The amino acid residues (cysteine, serine or threonine) involved in the β-elimination and in dehydroalanine formation were identified. Confocal scanning laser microscopy showed the role of fat globules in the gel structure.