# Probiotics in the Sourdough Bread Fermentation: Current Status
## 5 Enzymes in Sourdough
### Hydrolases
#### 5.4.4 Peptidase and Implications for the Gluten Network  
Peptides hydrolyze peptide bonds of proteins and peptides. They are still known as proteases, proteinases, and proteolytic enzymes [234]. The International Union of Biochemistry and Molecular Biology (IUBMB) classifies peptidases with criteria based on reaction and catalytic type. Endopeptidases (EC 3.4.21-99) hydrolyze internal peptide bonds, while exopeptidases (EC 3.4.11-19) remove amino acids from the ends of peptide chains. These enzymes have a complex classification involving the reaction and catalytic type and the evolutive relations. The MEROPS platform is a complete site about peptidases, including their complete classification [234]. Gluten is a complex protein responsible for wheat bread's most peculiar characteristics, such as expandability and retention of gas produced in fermentation. Its quality is more important than quantity [235]. Gluten structure varies with the cultivar and cultivation conditions [236]. It is composed of gliadin (\(\alpha\), \(\beta\), \(\gamma\), \(\omega\)), glutenin, and polypeptide fractions that combine with gliadin or glutenin [237]. Figure 4 seeks to demonstrate the complexity of gluten formation and the water effect. Gluten forms a network retaining carbon dioxide during baking, making dough expand. It is responsible for dough's rheological characteristics due to gliadin, which ensures the dough's viscosity, and glutenin, with the property of elasticity. During sourdough fermentation, the partial hydrolysis of gliadin and glutenin proteins occurs because of the acidification and activation of cereal peptidases. In addition, endogenous flour peptidases became activated at a low pH reducing the gluten disulfide bonds [38]. Cysteine proteinases in wheat grain are capable of hydrolyzing both gliadins and glutenin. The hydrolysis of glutenins during sourdough fermentation results in depolymerization and subsequent solubilization [238]. Endopeptidases may be applied to improve the rheological properties, and one example is the endo- and exopeptidases from _A. oryzae_. The fungi peptidases are employed to modify wheat gluten by limited proteolysis resulting in better rheology and shorter dough mixing for a higher loaf [239]. Lactic acid bacteria also exhibit proteolytic activity, decreasing the content of native gluten [240; 231; 241]. Several studies have focused on LAB peptidases and demonstrated that in different ways, that LAB in sourdough fermentation has a proteolytic activity for gluten hydrolysis [60]. Celiac disease or gluten enteropathy is an autoimmune disease caused by gluten-containing food and results in those with it to adhere to a strict gluten-free diet. One characteristic of gluten is that it is resistant to proteolysis by human digestive peptidases [242]. There is considerable interest in producing gluten-free bread that is specific for celiac consumers. The gliadin fraction, especially \(\alpha\)2-gliadin, has been considered the most allergenic part of gluten [243]. Another possible treatment is enzyme therapy, in which peptidases are ingested with food to increase the hydrolysis of resistant immunogenic peptides containing proline and glutamine amino acids [244]. The hydrolysis of some parts of gluten could favor the appearance of allergen terminals and cause more harm than benefit to a celiac person [243]. On the other hand, hydrolyzing gluten to reduce the allergenic components potentially benefits non-celiac people [69]. A group of LAB hydrolyzed gluten, but not the most allergenic part in the central region of alpha-gliadin. However, other allergenic proteins were degraded, improving the wheat digestibility [245]. A bioprospection analysis of gliadin-cleaving proteolytic activity among 20 _Lactobacillus_ strains showed that the most active strain, _L. casei_, was able to hydrolyze the 33-mer immunogenic peptide of \(\alpha\)-gliadin by 82% in 8 h and completely in 12 h [246]. Bread digestibility might be positively affected by long sourdough fermentation [148]. _L.