Review:: Indicators of damage of protein quality and nutritional value of milk

The nutritional value of milk can be negatively affected by thermal treatments. The quality of the proteins can be modified as a consequence of a set of reactions, the most important of which are the interactions between protein and carbohydrates, also known as Maillard reactions. These reactions can also occur when the storage conditions of moisture and temperature are unfavorable. The different thermal treatments involved in processing milk (pasteurization, sterilization, drying, etc.) lead to different steps of the Maillard reaction and therefore to the formation of different compounds that can be useful as chemical markers of milk alteration, or more exactly of protein alteration. Estimating these markers is useful in monitoring the changes, especially those related to proteins, that happen during processing and storage. A review of the literature on the markers used to evaluate the modifications that milk undergoes during thermal treatment and subsequent storage was carried out, focusing mainly on proteins. Special attention was paid to the analytical aspects. The chemical markers taken into account can be classified into two groups according to their relation with the Maillard reaction: 1. Specific markers of Maillard reaction: undesirable compounds (furosine, lisinoalanine, histidinalanine, furfurals, melanoidins); loss of nutrients (available lysine). 2. Non-markers of Maillard reaction: galactose; lactulose; protein-reducing substances; denaturation of protein; in vitro digestibility of proteins; others-pH, viscosity free fatty acids.