*2.1.2 Bioactive peptides*

*Prebiotics and Probiotics - Potential Benefits in Nutrition and Health*

activity were determined.

**2.1 Milk and its components**

**2. Background**

correlation between proteolytic activities, peptide concentrations, and antioxidant

Milk is composed of water, carbohydrates, lipids and proteins, as well as enzymes, vitamins, and mineral salts [6]. Fat is the component that varies the most in milk and is the main determinant of its physical and organoleptic properties [7]. Lactose is the major carbohydrate in milk; it is formed by glucose and galactose, two simple sugars that the human body uses directly as an energy source [8]. It participates in the synthesis of cerebrosides and glycoproteins; also it acts to facilitate the absorption of calcium. Lactose and other sugars in milk also favor the growth of probiotics in the intestine [9]. In milk, two main classes of proteins are identified: the caseins, which represent 80% of the total proteins, comprised of several types (αs1, αs2, β, K, and γ), and serum proteins (α-lactalbumin, β lactoglobulin, and small amounts of serum albumin, immunoglobulins, and protease-peptone) [10]. These proteins are separated by the acidification of milk at pH 4.6, the isoelectric point of caseins, which produces its precipitation [6]. The biological value of casein in the diet is due to its content of essential amino acids [7]. The protein concentration, as well as the concentration and amino acid sequence of each of the milk proteins, depends on where the species comes from. Casein micelles are smaller in goat's milk (50 nm) than that in cow's milk (75 nm); these caseins present in goat's milk contain more glycine, less arginine, as well as sulfur-containing amino acids. Another difference between these types of milk is that cow's milk is slightly acidic and goat's milk is almost neutral (pH 6.7) because it has higher protein content and different combinations of phosphates [5]; also in cow's milk the largest fraction of protein is comprised of αS1-casein; however, in the case of goat's milk, the larger fractions include β-casein and αS2-casein [11]. Of the mineral elements, the milk contains sodium, potassium, magnesium, calcium, manganese, iron, cobalt, copper, fluorides, iodides, and phosphorus. Of which calcium, copper, iron, magnesium, manganese, phosphorus, and zinc are in the highest concentration in the membrane of the fat globules. In addition, the milk contains vitamins such as A, D, E, K, B1,

B2, B6, B12, and C, carotenes, nicotinamide, biotin, and folic acid [7].

Proteolysis is the process of degradation of proteins by the breakdown of their peptide bonds. From the abasement of milk proteins, peptides and free amino acids are formed, which can later present diverse bioactivities in the organism [12]. The proteolytic system of lactic acid bacteria is basically composed of proteinases that initially cleave the milk protein to peptides, and then these peptides are divided into peptides and smaller amino acids by intracellular peptidases. Subsequently, the amino acids are catabolized, producing a variety of low molecular weight compounds responsible for the formation of odor and taste compounds in fermented

The initial step in the degradation process is carried out by the proteinase PrtP bound to the extracellular wall that decompose the proteins into peptides of 5–30 amino acids that are transported to the cells [2]. The action of the proteinases and peptidases provides the cells with peptides and free amino acids, which are

**18**

milks [13].

*2.1.1 Proteolytic activity*

The biologically active peptides derived from milk are initially in inactive form within the sequence of the precursor molecules but can be released in different ways: by hydrolysis with digestive enzymes such as pepsin, trypsin, chymotrypsin, etc., proteolysis by enzymes derived from proteolytic microorganisms, and by fermentation of milk with proteolytic starter cultures [14]. During the fermentation of milk, LAB are able to produce bioactive peptides by the fermentation process; this is because they contain an active proteolytic system that allows the degradation and release of amino acids from milk proteins [2]. This system consists of a series of different intracellular peptidases, including endopeptidases, aminopeptidases, dipeptidases, and tri-peptidases. The production of various bioactive peptides, including antimicrobial peptides, immunomodulators, and antioxidants, has been demonstrated through microbial proteolysis [14]. Bioactive peptides derived from milk are generally composed of 2–20 amino acids and become reactive after the release of the precursor protein. Several lactic acid bacteria such as *Lactococcus lactis* and *Lactobacillus helveticus* have been reported to release bioactive peptides through the fermentation process [15].
