Evaluation of an in vitro system to simulate equine foregut digestion and the influence of acidity on protein and fructan degradation in the horse′s stomach
When developing in vitro digestion, that simulates the natural digestion processes of horses, it is essential to know about equine physiological processes. As equine saliva has no digestive enzyme activity (Al Jassim and Andrews, 2009), enzymatic degradation of the food starts in the stomach. The secretion of hydrochloric acid into the stomach induces an acidic gastric environment (pH 2–5) (Coenen, 1992; Husted et al., 2008; Glatter et al., 2016c). The low pH causes both denaturation of proteins and presumably acidic hydrolysis of polysaccharides (Ince et al., 2013). Pepsinogen is the most important enzyme in the stomach. After secretion into the acidic environment of the lumen, pepsinogen is activated to pepsin. Pepsin splits denatured proteins hydrolytically into polypeptides. The highest metabolic activity of pepsin was measured in a potent acidic environment (pH 1–4) between at least 37 and 42 °C (Scharrer and Wolffram, 2004; Worthington and Worthington, 2011). The pancreatic enzymes (peptidase, nuclease, amylase, lipase) are secreted into the lumen of the small intestine, where the pH is increased up to pH 7–8 (Mackie and Wilkins, 1988; Glatter et al., 2016c) by pancreatic secretions. Amylase is particularly relevant for the hydrolytic degradation of starch as it splits the unbranched 1.4‐α‐ glycosidic linkages (Kienzle et al., 1994; Santos et al., 2010).
The apparent digestibility of forages in the foregut of horses was investigated in vivo earlier, using the mobile bag technique. Thus, an apparent crude protein digestibility of 52% was detected for haycubes in the horse′s foregut (Moore‐Colyer et al., 2002; Brøkner et al., 2012). Monosaccharides and disaccharides are almost completely absorbed pre‐caecally. In addition, a small fraction of cell wall carbohydrates, like hemicellulose and cellulose as well as fructans, is degraded in the stomach and small intestine by microbial fermentation (Al Jassim et al., 2005; Coenen et al., 2006; Perkins et al., 2012). Only 5–11% of the non‐starch polysaccharides (NSP), and 8–22% of the NDF, was lost pre‐caecally when using the mobile bag technic for analyses by Moore‐Colyer et al. (2002) and Brøkner et al. (2012) respectively. Fructans are oligomeric or polymeric polysaccharides and consist of β‐linked D‐fructofuranosyl units. Inulins are (2‐1) linked fructans, levans are (2‐6) linked with partial (2‐1) linked branches, and graminans comprise both types of linkages (Vijn and Smeekens, 1999). Depending on the degree of polymerization (DP), inulin‐type fructans were classified as short chain fructo‐oligosaccharides, fructo‐oligosaccharides and inulin with mean DP around 3.6, 4 and 12 respectively (Glatter et al., 2016a). This classification is common for inulin‐type fructans only. Therefore, a distribution of the chain length is also necessary to classify the other fructans which are of such great importance for digestive processes. Levans and graminans, specific grass fructans, are suspected of causing digestive disorders if they reach the horse′s hindgut in excessive quantities (Longland and Byrd, 2006). The question of how grass fructans are digested pre‐caecally is still being discussed. It is assumed that fructans are not degraded by endogenous enzymes (Nilsson et al., 1988). However, Ince et al.