# The Effect of High-Pressure Treatment and Skimming on Caprine Milk Proteins
## 1 Introduction  
Proteins are susceptible to the influence of high pressure and this determines the applicability of high-pressure treatment in dairy production [1; 2; 3; 4; 5]. High-pressure treatment weakens hydrophobic and electrostatic interactions, and it strengthens or weakens hydrogen bonds, depending on the applied pressure. Hydrogen bonds are damaged under exposure to \(\geq\)700 MPa. Chemical bonds can be arranged in the following descending order based on their sensitivity to high pressure: hydrophobic interactions > electrostatic interactions > hydrogen bonds > covalent bonds. High-pressure treatment affects mainly quaternary (hydrophobic and non-covalent interactions), tertiary (ionic and hydrophobic interactions) and secondary protein structures (intra- and intermolecular hydrogen bonds). Exposure to pressure higher than 200 MPa leads to the unfolding and reassociation of polypeptide chains. The pressure-resistant properties of proteins with a secondary structure (\(\alpha\)-helices or \(\beta\)-sheets) are determined mainly by the presence of hydrogen bonds in molecular conformations [2; 3; 4; 6; 7; 8].  
High-pressure treatment induces the following changes in milk proteins: (1) the size of casein micelles changes [1; 6; 9; 10; 11; 12]; (2) the proportions of soluble casein fractionsincrease in the following order: \(\kappa\)\(\sim\)\(\beta\)\(>\)\(\alpha_{\rm s1}\)\(>\)\(\alpha_{\rm s2}\)[3; 13; 14]; (3) the soluble form of calcium and phosphorus content of milk increase [3; 11; 13; 14]; (4) whey proteins are denatured in the following order: lactoferrin \(>\)\(\beta\)-lactoglobulin \(>\) immunoglobulin \(>\) serum albumin \(>\)\(\alpha\)-lactalbumin [1; 15]; and (5) milk proteins interact with milk fat globule membrane components [16].  
Pressure-induced changes in protein structure influence the functional attributes of proteins, such as the ability to stabilize emulsions, bind water, and form gels. These changes are particularly important in the production of emulsions, ripened cheese, cottage cheese, and fermented milk products (such as yogurt), where the chemical, functional, and nutritional properties of proteins play a pivotal role [2; 3; 4].  
Caprine milk casein is characterized by a higher content of \(\beta\)-casein and \(\kappa\)-casein, a lower content of \(\alpha\)-casein, in particular \(\alpha_{\rm s1}\)-casein, in comparison with cow's milk [17; 18; 19]. Pressure-induced changes in milk proteins appear to be a crucial process in caprine milk. Caprine milk is more abundant in soluble casein than cow's milk [18], and \(\beta\)-casein in caprine milk is also dissociated under exposure to pressure or low temperature. The low hydration of casein micelles combined with the high content of ionized calcium and low citrate content contribute to the low colloidal stability of caprine milk [17; 18]. High-pressure treatment induces the denaturation of caprine \(\beta\)-Lg, enhances protein retention in the curd [20; 21], and increases the content of non-soluble minerals and proteins in caprine milk [13].  
High-pressure treatment is applied in the food processing industry to prolong the shelf life of raw materials and food products. High-pressure technology offers an alternative to thermal preservation, pasteurization, and sterilization in dairy production, and it enables the development of novel products with desirable textures, flavors, and functional properties [2; 3; 4; 5]. Most research studies investigating the effects of high-pressure treatment on proteins were conducted on cow's milk, whereas caprine milk proteins attracted less interest [3; 11; 13; 20].  
The aim of this study was to determine the effect of high-pressure treatment and skimming of HP-treated milk on the content of nitrogen compounds, protein composition, and color of caprine milk.