**2.2 Lactic acid bacteria**

Lactic acid bacteria are microorganisms that have various applications, including the fermentation of foods such as milk, meat, and vegetables. These bacteria, in addition to contributing to the biopreservation of foods, help to improve taste, smell, texture, and nutritional quality [18]. In addition, beneficial effects on health are attributed to them through the direct effects of live microorganisms known as probiotics as well as indirect effects during fermentation, in which these microorganisms participate in the generation of secondary metabolites such as peptides with biological activities [19].

There are several genera of LAB, which are classified as homofermentative and heterofermentative based on the final product of their fermentation. The homofermentative produce lactic acid as a product of the fermentation of glucose. On the other hand, the heterofermentative produce lactic acid in addition to other products such as acetates, ethanol, and carbon dioxide as a product of its fermentation [18]. The LAB are characterized by Gram-positive cocci or bacilli, catalase and oxidase negative, facultative anaerobic, non-sporulated, and non-motile. In addition, they are tolerant acid, being able to grow some at pH values as low as 3.2 and others at values up to 9.6; however, most grow between pH of 4 and 4.5 [19].

#### *2.2.1 Lactobacillus acidophilus*

There are many probiotic bacteria that are used for human consumption, although the most used are *Lactobacillus* spp. These lactic acid bacteria have been used for food preservation through fermentation for hundreds of years, in addition to providing flavor and texture, and they increase the nutritional value and are also found in the gastro-intestinal tract of humans [20]. In addition, they are a key factor in the processes of competitive exclusion and immunomodulation carried out by commensal organisms. *Lactobacillus acidophilus* is a Gram-positive, non-sporeforming bacterium, homofermentative anaerobic, and catalase negative, 2–10 μm in diameter, which has an optimal growth temperature of 37°C and is a typical intestinal bacterium in humans [21]. This microorganism is not part of the natural flora of milk and acts on it very slowly, which is why it is essential to avoid contamination during the manufacture of a product [8].

*Lactobacillus acidophilus* uses the glycolysis or Embden-Meyerhof-Parnas route (EMP) to ferment hexoses and produce lactic acid. Lactic acid does not contribute to the aroma because it is odorless, but it helps the sour taste of dairy products [13]. *Lactobacillus acidophilus* LA-1/LA-5 is one of the main species of microorganisms that can potentially be used as probiotic cultures in dairy products. Some examples in the market of fermented milk products that include *Lactobacillus acidophilus* are Bioghurt, Aktifit, Actimel, Bifilac, Kaiku, and Kefir [22].

#### **2.3 Fermented beverages**

Today, the main function of fermented milk is to prolong shelf life, improve flavor and digestibility, and manufacture a wide range of dairy products. In fermented milk products, probiotic bacteria can act in the treatment of some infectious, atopic, and tumoral diseases, among others [23]. Fermented milks can be classified based on different criteria, among them are its fat content, the concentration of milk, separation of the whey, the use of milk from different species, and the type of fermentation process. Based on the type of fermentation, there are the products with a lactic fermentation such as ymer, langfil, viili, yogurt, and acidified milk. Among the products in which lactic fermentation is combined with the production of alcohol are kefir and koumiss [8]. Today, the main function of fermented milk is to prolong shelf life, improve flavor and digestibility, and manufacture a wide range of dairy products. In fermented milk products, probiotic bacteria can act in the treatment of some infectious, atopic, and tumoral diseases, among others [23]. Fermented milks can be classified based on different criteria, among them are its fat content, the concentration of milk, separation of the whey, the use of milk from different species, and the type of fermentation process. Based on the type of fermentation, there are products with a lactic fermentation such as ymer, langfil, viili, yogurt, and acidified milk. Among the products in which lactic fermentation is combined with the production of alcohol are kefir and koumiss [8].

**21**

*Comparision of Antioxidant Activity of Cow and Goat Milk During Fermentation…*

Cow's milk (10 L) and goat's milk (10 L) were separately subjected to a heat treatment at 95°C for 20 minutes and then cooled down to 37°C. Four treatments were prepared by making three batches of each of them, all incubated at 37°C until a pH of 4.5. Two of the treatments were fermented with *Lactobacillus acidophilus* LA-5 (Chr. Hansen) at 1%, only one of them with goat's milk and the other with cow's milk; the other two treatments that were the controls were not added with probiotics, and their fermentation occurred due to thermodynamic microorganisms (persistent after pasteurization). The beverages were kept refrigerated at 8°C

The physicochemical analysis was formed by fat (%), nonfat solids (%), density

point (°C), and electrical conductivity (Ms/cm) was performed in triplicate of the raw material (cow's and goat's milk) in the Lactoscan Milk Analyzer (Lactoscan SA,

It was determined based on the norm NOM-155-SCFI-2012 [24], taking 10 mL of sample, 20 mL of distilled water, and two drops of phenolphthalein, carrying out the titration with 0.1 N NaOH. The calculation of titratable acidity was carried out

where V = milliliters of 0.1 N NaOH solution, spent in the titration; N = normality of the NaOH solution; M = volume of the sample in mL; 90 = lactic acid

The samples were treated as described by Donkor [25], in which 5 mL of each was taken and mixed with 10 mL of 0.75% trichloroacetic acid (TCA), passing the mixture through filter paper (Whatman No. 1 of 150 mm), obtaining the filtered beverages (FB), which were frozen (−20°C) until analysis. The filtrates were carried out at 0, 7, 14, 21, and 28 days to determine the proteolytic activity, the total peptide

The proteolysis of each of the FB was determined in triplicate based on the reaction of the free primary amines (NH3) with O-phthaldialdehyde (OPA) and b-mercaptoethanol, according to the Church method [26]. The OPA reagent was prepared as follows: 25 mL of 100 mM sodium tetraborate, 2.5 mL of 20% sodium dodecyl sulfate (SDS), 40 mg of OPA in 1 mL of methanol, 100 mL of b-mercaptoethanol, and capacity to 50 mL with tridestilated water. For the readings, 100 mL of each sample was taken and mixed with 2 mL of the OPA reagent by inversion of the

⁄L) = \_

(V)(N)(90)

<sup>M</sup> (1)

Acidity(g

), lactose (%), protein (%), total solids (%), added water (%), and freezing

*DOI: http://dx.doi.org/10.5772/intechopen.88212*

**3.1 Treatment of fermented beverages**

**3. Materials and methods**

during their shelf life.

(kg/m3

equivalent.

**3.2 Physicochemical analysis**

Milkotronic Ltd., Bulgaria).

using the following equation:

**3.4 Preparation of the filtrates**

**3.5 Proteolytic activity**

concentration, and the antioxidant activity.

**3.3 Determination of titratable acidity**
