## 4 Protein Quality Evaluation Methods
### Ex Vivo Models  
In vitro absorption models range from basic membrane-based systems to more advanced cell-based models, while ex vivo models utilize intestinal tissues. Still, their viability time is limited to the typical intestinal transit time [15]. To overcome this limitation, ex vivo models employ gastrointestinal fluids from humans as digestive agents rather than using commercially available enzymes derived from animals. The enzymes in juice can differ in activity or specificity from commercial enzymes as they are composed of different isoforms. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Food** & **Process** & **Total Protein (\%)** & **Protein Digestibility (\%)** & **References** \\ \hline Barley & Raw & 10–12 & 80 & [48; 49] \\ Barley & Milled & 10–12 & 65 & [48; 49] \\ Bean & Heated & 23–25 & 53 & [48; 50] \\ Coconut & Cake & 20–24 & 57 & [48; 51] \\ Cowpea & Raw & 23–32 & 86.6 & [52; 53] \\ Horsebeans & Raw & 25–28 & 86 & [48; 54] \\ Lentils & Raw & 23–5 & 77 & [10; 48] \\ Linseed & Raw & 17–22 & 77 & [48; 55] \\ Linseed & Expeller & 34–35 & 75 & [48; 56] \\ Lupins & Raw & 32–44 & 87 & [48; 57] \\ Maize & Raw & 8–11 & 82 & [48; 58] \\ Maize & Chemical/Heated & 8–11 & 82 & [48; 58] \\ Maize & Brand & 9–11 & 74 & [58; 59] \\ Maize & Milled & 8–11 & 83 & [58; 59] \\ Maize gluten & Meal & 60–70 & 90 & [59; 60] \\ Millet & Raw & 11–19 & 85 & [59; 61] \\ Millet & Pearl milled & 11–19 & 85 & [59; 61] \\ Niger seeds & Raw & 20–24 & 63 & [59; 62] \\ Oats & Raw & 10–20 & 76 & [59; 63] \\ Oats & Milled & 9–20 & 60 & [59; 63] \\ Peas & Raw & 19–20 & 79 & [50; 59] \\ Potato protein & Concentrated & 79–80 & 90 & [59; 64] \\ Rape seed & Raw & 19–26 & 72 & [59; 65] \\ Rape seed & Expeller & 32–37 & 72 & [59; 66] \\ Rice & Dehulled & 4–18 & 95 & [59; 67] \\ Rice & With hulls & 4–18 & 76 & [59; 67] \\ Rice & Husk & 5–9 & 39 & [59; 68] \\ Rye & Raw & 5–13 & 77 & [59; 69] \\ Sesame seed & Expeller & 41–45 & 84 & [59; 70] \\ Sorghum & Raw & 6–9 & 84 & [59; 71] \\ Sorghum gluten & Meal & 43–58 & 86 & [59; 72] \\ Soybean & Expeller & 44–47 & 87 & [59; 73] \\ Soybean & Hulls & 13–22 & 54 & [59; 74] \\ Soybean & Heated & 36–40 & 82 & [59; 75] \\ Sunflower seed & Expeller & 33–37 & 80 & [59; 76] \\ Trticale & Raw & 9–16 & 85 & [59; 77] \\ Wheat & Raw & 11–14 & 89 & [59; 77] \\ Wheat & flour & 11–14 & 91 & [59; 77] \\ Wheat gluten & Meal & 78–82 & 100 & [59; 78] \\ Wheat & Bran & 12–19 & 68 & [59; 79] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Protein digestibility of different plant protein sources measured using in vivo models. Human gastric juice (HGJ) contains pepsin and gastric lipase, while human duodenal juice (HDJ) consists of proteolytic and lipolytic enzymes, different inhibitors, and bile salts. To reduce inter-individual variability related to enzymes and other juice components, the use of gastrointestinal juice pools is suggested. Therefore, ex vivo models have the potential to accurately replicate human digestion by incorporating juices that contain various enzymes, inhibitors, and salts. Ex vivo absorption methods involve using animal or human gut segments, which provide a more accurate representation of the intestinal wall's morphological and physiological features. This includes the presence of all relevant cell types and architecture and a mucus layer that allows for the simulation of possible further hydrolysis by brush border membrane enzymes. As a result, ex vivo absorption models are a more realistic representation of in vivo absorption compared to in vitro models [80,81]. Ex vivo models of intestinal absorption offer a comprehensive assessment of the morphological and physiological characteristics of the intestinal wall, including the presence of all relevant cell types, the architecture, and the mucus layer, enabling the investigation of nutrient or bioactive compound transport, permeability, absorption, and mucus layer interactions [80]. The approach's limitation is the tissue's limited viability, usually between 1 and 3 h, depending on the presence or absence of muscle layers.