Protein composition and micelle size of donkey milk with different protein patterns: Effects on digestibility

Qualitative and quantitative analysis of the protein profile of milk samples from 14 Ragusano donkeys was carried out. Isoelectric focusing (IEF) analysis revealed three IEF protein patterns: individual variability was related both to casein (one deviating sample, without αS1-casein) and whey proteins (two deviating samples, without β-lactoglobulin II). The observed polymorphism reflected the different distribution of the protein fractions (casein or whey protein content) but did not affect the size and zeta-potential of casein micelles. A simulated in vitro digestion, carried out in a two-step digestion assay with human gastric juice and duodenal juice, was also performed to provide a direct estimation of donkey milk protein digestibility. Individual IEF variability had an effect on digestibility only in the two samples lacking the β-lactoglobulin II, revealing a more rapid degradation of the β-lactoglobulin fraction when it consists of the single β-lactoglobulin I, particularly in the gastric digestion.