Milk protein polymorphism in Amiata donkey

•Milk protein polymorphisms in Amiata donkey were highlighted.•The IEF analysis showed the presence of null alleles for β and αS1-casein (CN).•Genetic variability affected β, αS1-CN, lysozyme and β-lactoglobulin content in milk.•In addition, for the first time we quantified the κ-casein in donkey milk.•Our results give a contribution to the knowledge of nutritional quality of donkey milk. Interest in donkey milk for human nutrition has been growing steadily, especially in the therapeutic-diet field, due to its similarity to human milk. Caseins are one of the main allergenic components of milk. Whereas cow milk has a high casein content, in donkey milk the casein concentration is lower and is characterized by a greater digestibility. In fact, recent studies have shown that donkey milk can be consumed by children with cow milk protein allergy. The aim of this study was to evaluate the protein profile of individual samples of Amiata donkey milk and to quantify the different protein fractions in order to better understand the characteristics of donkey milk. Protein fractions of 57 individual whole milk samples were analysed by isoelectric focusing (IEF). The main milk protein fractions were quantified both by IEF gel image analysis and directly on milk using the Agilent 2100 Bioanalyzer. The IEF analysis identified 10 different electrophoretic patterns, which differ from each other in terms of the presence of bands that correspond to homozygous or heterozygous genotypes, as well as the presence/absence of specific protein bands. In addition the quantification of different caseins (αS1, β and κ-CN) and whey proteins (β-LG, α-LA and LZ), highlighted a significant phenotypic variability among individuals which seems to be related to the patterns identified. In particular, different casein:whey protein ratios were identified among jennies and this is known to affect the nutritional, allergenic and technological properties of milk. These findings could be useful in the selection of donkeys with specific milk protein polymorphisms.