## 3 Protein Digestibility  
Protein digestion occurs in two phases, namely, the gastric and the intestinal phases, beginning in the stomach and ending in the small intestine. The stomach acts as a reservoir, regulating the release of nutrients into the small intestine, which serves as the primary site of hydrolysis and absorption [15]. In the human stomach, acids and pepsin facilitate the breakdown of proteins, while pancreatic and intestinal enzymes aid in protein digestion in the small intestine [14]. Understanding the sequential events and mechanisms involved in protein digestion is crucial for evaluating protein quality and optimizing nutrient uptake in the human body. The stomach environment undergoes temporal and spatial variations, which are critical in regulating digestion kinetics. Upon food entry, the stomach's pH becomes relatively high, and enzyme concentration remains low, with values depending on the buffering capacity [15]. Several complex processes occur in the gastric compartment during digestion, including enzymatic, physical, and chemical processes that are crucial in controlling digestion kinetics [16]. The gastric phase of digestion begins with the secretion of pepsinogen and hydrochloric acid (HCI) from the chief cells and parietal cells, respectively. HCl secretion causes a decrease in pH, leading to the secretion of pepsinogen, the zymogen of pepsin, and gastric lipase [15]. Pepsin, activated by gastric acid, hydrolys proteins, with a preference for cleaving large aliphatic or aromatic side groups, thus completing around 10-20% of protein digestion [16]. The stomach's acidic environment serves as a critical barrier against microbial pathogens as it inhibits or eliminates many microorganisms. However, aging is associated with a decline in gastric fluid secretion, leading to a reduction in stomach acidity and a condition known as achlorhydria, which can increase the risk of gastrointestinal infections [17]. The stomach connects the esophagus to the duodenum, the initial segment of the small intestine, and remains relatively stable at both its esophageal and duodenal junctions. Thus, after gastric digestion, the pylorus, which holds the chyme (semifluid mass of partly digested food), acts as a filter, permitting only liquids and small food particles to pass through the pylorus (2-3 mm) to the pyloric sphincter [18]. During gastric emptying, rhythmic mixing waves propel small portions of chyme through the pyloric sphincter into the duodenum. If too much chyme were released at once, it would exceed the small intestine's capacity to process it efficiently. The remaining chyme is pushed back into the stomach body, where it continues to mix. This cycle repeats as subsequent mixing waves move additional chyme into the duodenum [19]. The precise regulation of the rate of gastric emptying of chyme into the duodenum is critical for further digestion and absorption in the small intestines, which is regulated by feedback from the intestines via a variety of gastrointestinal hormones [18]. Upon entering the intestines, the chyme transitions to the intestinal phase, where pepsin is inactivated at a pH greater than 4.5 to prevent auto-digestion. As chyme empties into the small intestine, pancreatic proteases are secreted along with pancreatic bicarbonate. Key proteases of the small intestine have an optimal pH range of 7.0-9.0 [17]. The contents of the small intestine are thoroughly mixed, and the relatively high enzyme activity leads to the rapid production of hydrolysis products [15]. The resulting products of the gastric and intestinal phases are absorbed by enterocytes in the jejunum and, to a lesser extent, in the ileum, and then metabolized in various body organs [20; 21]. Bile, secreted from the gall bladder into the duodenum, is mostly reabsorbed in the distal ileum, with only approximately 5% passing into the colon [15]. The intestinal phase is also crucial in gastric emptying and protein digestion.