# Probiotics in the Sourdough Bread Fermentation: Current Status
## 5 Enzymes in Sourdough
### Hydrolases
#### 5.4.2 Cellulase, Phytase, and Xylanase for Mineral Bio-Accessibility Improvement in Bread  
Cellulases are classified into endoglucanases (act in the middle of cellulose) and exoglucanases (cleave the extremities of the polymer). The endoglucanase is the endo-\(\beta\)-1,4-glucanase (EC 3.2.1.4), cleaving intramolecular bonds of \(\beta\)-1,4-glycosidic. The exoglucanases are the \(\beta\)-glucosidase (EC 3.2.1.21) and exo-\(\beta\)-1,4-glucan cellobiohydrolase (EC 3.2.1.91), which cleave ends of the cellulose and glycosidic terminals (liberating Cellobiose), respectively [216]. Endocellulase acts on cellulose and \(\beta\)-glucan substrates, removing insoluble arabinoxylans, contributing to gluten network formation [217]. Subsequently, there is a decrease in the hardness of the bread and, consequently, an improvement in the sensory evaluations. Cellulase action may also provide an anti-staling effect, which may be related to alterations in water distribution between the starch-protein matrix [182]. In the study carried out by [PERSON] et al. (2014) [218], the authors added \(\beta\)-glucanase to a mixture of barley flour (30%) and wheat flour (70%) with approximately 1.5% glucan as the starting material. As a result, they noticed an improvement in the maneuverability of the dough, an increase in softness and elasticity, an increase in the specific volume of the bread, and a reduction in hardness. The enzyme phytase or myoinositol-hexaphosphaphe phosphohydrolase is present in plants, bacteria, yeasts, and filamentous fungi. It hydrolyzes phytate molecules and releases inorganic phosphate, dephosphorylation-dependent inositol esters, and minerals linked in the phytate structure [219]. Two classes of phytase are categorized depending on where the hydrolysis of phytate begins, -3-phytage (EC 3.1.3.8) or 6-phytage (EC 3.1.3.26); the 3-phytage being the one that removes the orthophosphate group from the position C3 characteristic of microorganisms. Two classes of phytase are found depending on the position where the hydrolysis of phytase -3-phytage (EC 3.1.3.8) or -6-phytage (EC 3.1.3.26). The 3-phytase removes the orthophosphate group from the position C3 characteristic of microorganisms [220]. Xylan is the main hemicellulose component of hardwoods and accounts for approximately 30% of the woody cell wall. Xylanas (EC 3.2.1.8) are enzymes that hydrolyze this polysaccharide. They are of two types: \(\beta\)-1,4-endoxylanase cleaving internal glycosidic linkages, and \(\alpha\)-D-xylosidase which cleaves xylo-oligosaccharides, and xylobiose forming xylose, a disaccharide of two xylose molecules [211]. It is important to highlight that xylanase from various sources has different mechanisms. Two mechanisms can summarize the improvement in bread quality due to the addition of xylanase: (i) the removal of arabinoxylans from gluten alters the distribution of water between gluten proteins and arabinoxylans; (ii) pentosan destruction and viscosity reduction effect [182]. [PERSON] et al. [221] reviewed that wheat is a reservoir of minerals, mainly in the aleurone tissue between the endosperm and the seed coat. However, these minerals are complexed with phytate molecules that reduce their bio-accessibility. Xylanase and cellulase showed satisfactory results to facilitate access into the aleurone tissue for the action of phytase, improving iron bio-accessibility [222]. Although some aspects of mineral assimilation by the human body need to be considered, such as diet, blood flow, gut epithelium, and intestinal microbiota [223], some LAB and yeasts may have phytate activity that improves mineral bio-accessibility in sourdough bread [79; 224; 225; 226]. _Bifidobacterium_ strains have demonstrated the ability to increase the accessibility of iron through its phytate activity during bread fermentation [107]. Pentosanase is a hemicellulose that acts on pentosans, particularly hydrolyzing water-unextractable arabinoxylan (WUA) into water-extractable arabinoxylan (WEA).