After residing for a few seconds in the mouth, the emulsions are swallowed, which involves exposure to intense shear effects in pharynx, oesophagus and
finally stomach (Weisbrodt, 2001b), and the behaviour of emulsion droplets during this integrated movement is largely unknown. The emulsion would remain in the stomach for a short period (from a few minutes to a few hours) depending on the nature of the food (e.g. chemical composition, droplet size, microstructure, pH, ionic conditions and rheological properties) (Malagelada & Azpiroz, 1989). In the gastric tract, the emulsion is mixed with the digestive juices at a highly acidic pH containing various minerals, enzymes (both proteolytic and lipolytic) and also subjected to mechanical agitation due to peristalsis in the stomach
(Weisbrodt, 2001a; Ekmekcioglu, 2002; Kalantzi et al., 2006; Pal et al., 2007). Some of the important factors, possibly affecting emulsion stability during gastric processing, are discussed below:
2.7.2.1 Acidic pH and ionic strength
The pH of fasted human stomach ranges from 13 (Ekmekcioglu, 2002; Kalantzi et al., 2006). However, the pH varies significantly depending on the composition and quantity of ingested food and also differs among individuals (McClements et al., 2009; Singh et al., 2009). Generally, the pH of the stomach first increases from ~ 1.7 to ~ 6.0 immediately after consuming a neutral liquid meal, before returning back to the highly acidic fasting value of the stomach (Kalantzi et al., 2006), owing to the mealstimulated secretion of gastric acid (Gardner et al., 2002). During this gradual decrease in pH from neutral to acidic, most of the milk proteinstabilized emulsions are expected to undergo substantial changes in droplet characteristics in terms of incomplete to full charge reversal as well as some possible aggregation effects around the pI, as pI ranges from ~ 4.55.2 for most of the milk proteins (Singh et al., 2009).
Generally, the osmolality and ionic strength of fasted state (empty stomach condition) are ~ 190 mOsm/kg and ~ 100 mM respectively with concentration of important ions being: Na+: 70 ± 30 mM, K+: 13 ± 3 mM, Ca2+: 0.6 ± 0.2 mM, and Cl: 100 ± 30 mM. (Lindahl et al., 1997). When the meal enters into the stomach, there is a significant change in the osmolality and ionic strength of the stomach contents due to addition of more ions and solutes from the meal as compared to that of the fasted state. Authors have reported that the osmolality of stomach
increases from ~ 140 mOsm/kg in an empty stomach to ~ 560 mOsm/kg in fed state (after consuming a liquid meal). Osmolality gradually decreases as a function of gastric emptying time (Kalantzi et al., 2006). High ionic strengths might contribute to electrostatic changes in the proteinstabilized emulsions, which might result in saltinduced aggregation of emulsion droplets, especially with reference to proteinstabilized droplets.
Clearly, the drastic change in pH and ionic strength in the stomach could result in changes in emulsion droplets via flocculation or change in the interfacial layers.
2.7.2.2 Biochemical factors
Apart from the variable pH and ionic strengths, the proteinstabilized oil-in- water emulsions are also exposed to various biochemical agents, mainly enzymes (pepsin, and gastric lipases), gastric mucin, other partly digested food or residual food components of the previous meal (e.g. proteins, polysaccharides, phospholipids etc.), which might result in modification of the adsorbed protein layers and droplet characteristics, finally affecting the emulsion stability.
2.7.2.2.1 Pepsin
Pepsin is a protease, which preferentially cleaves the peptide bonds involving aromatic amino acids (phenylalanine, tyrosine and tryptophan), and other hydrophobic residues (such as leucine) and to a lesser extent bonds involving acidic amino acids (such as glutamic acid) (Tang, 1963; Weintraub et al., 1971; Lehninger et al., 1993; Sweeney & Walker, 1993; Blackman, 1994). However, pepsin does not cleave bonds containing valine, alanine or glycine.
Caseins, owing to their flexible random coil structure, are highly susceptible to hydrolysis by pepsin in their native state (Modler, 1985; Guo et al., 1995). However, whey proteins particularly β-lg, because of its highly folded globular conformation, is largely resistant to gastric digestion as well as in vitro peptic hydrolysis in its native state (Reddy et al., 1988; Schmidt & Poll, 1991; Schmidt & van Markwijk, 1993). Modifications of the native structure, using thermal and chemical treatments of β-lg solutions to render them accessible to physiological
enzymes, have been extensively studied (Reddy et al., 1988; Guo et al., 1995; Kananen et al., 2000; Chevalier et al., 2002). However, little information on the conformational changes of β-lg in the adsorbed state with respect to its proteolytic digestion, especially at pH 12 by pepsin, is available in the literatures. Milk proteinstabilized emulsions would supposedly undergo major physicochemical changes in the stomach in terms of droplet aggregation or coalescence due to the possible action of pepsin on the interfacial layer.
2.7.2.2.2 Gastric lipase
Limited degree of lipolysis (~ 1030%) of the ingested triacylglycerols occurs in the stomach, which can be mainly attributed to the gastric lipase activity
(Armand, 2007; Armand et al., 1994; Armand et al., 1999; Bauer et al., 2005; Carrière et al., 1993; Layer & Keller, 2005; Pafumi et al., 2002). Human gastric lipase, secreted by the chief cells located in the fundic mucosa of the stomach
(Moreau et al., 1988; Moreau et al., 1989) is a globular glycoprotein of molecular weight ~ 50 kDa and is generally stable over a broad range of pH values (pH 27) (Mukherjee, 2003; Bauer et al., 2005; Carrière et al., 2005; Armand, 2007).
Generally, gastric lipases are known to hydrolyse emulsified triglycerides containing medium chain fatty acids better than that of the ones with long chain fatty acids (Mu & Hoy, 2004). They preferentially cleave at sn3 ester bonds of triacylglycerols resulting in sn1, 2 diacylglycerols and free fatty acids (Moreau et al., 1988; Armand et al., 1996a; Carrière et al., 2005). Some of these lipid digestion products could competitively displace the initial emulsifier materials from the emulsion interface either fully or partially (Armand et al., 1994; Pafumi et al., 2002). Moreover, the initial extent of lipolysis by gastric lipases in the stomach has also been proposed to facilitate subsequent hydrolysis by pancreatic lipase in the intestines via promoting droplet breakup, stimulating hormonal release, increasing binding of colipase and solubilizing the lipid digestion products (Mu & Hoy, 2004; Bauer et al., 2005).
However, gastric lipases, normally present at 0.51 µM levels in human stomach
(Mcclements et al., 2009) are assumed to play an insignificant role quantitatively, where lipid digestion in healthy human adults is concerned (Fave et al., 2004; Bauer et al., 2005; Layer & Keller, 2005). They are reported to be substantially functional for individuals suffering from pancreatic lipase insufficiency. Moreover, these acid-stable lipases are positively charged in the harsh acidic pH of the gastric conditions, as their pI varies from 6.6 to 7.9 depending on their isoforms (Miled et al., 2005). The quantitative importance of gastric lipase in lipolysis of ingested milk proteinstabilized emulsions in healthy human adults remains to be elucidated.
2.7.2.2.3 Gastric mucin
The presence of highly glycosylated mucin (molecular weight > 106 kDa), which forms a selfassociated gellike structure at gastric conditions (pH 13 and at high mucin concentrations of ≥ 20 mg/ml) has an important physiological role of protecting the stomach from digesting itself (Nordman et al., 2002; Lee et al., 2005; Bansil & Turner, 2006). Importantly, mucin is the first barrier, with which any nutrient molecule must interact (Bansil & Turner, 2006). Mucins are largely charged species, due mainly to the glutamic acid and aspartic acid residues in the polypeptide backbone (pKa ≈ 4) (Cao et al., 1999) as well as the sialic acid
residues (pKa≈ 2.6) and sulphate groups (pKa≈ 1) present in the oligosaccharide
side chains (Waigh et al., 2002).
Recently, Lee et al. (2005) investigated the role of ionic strength and pH on the conformation of pig gastric mucin and its binding properties to polydimethylsiloxane (PDMS) surfaces. Authors predicted that the pI of mucin roughly lies between 2 and 3, indicating that the net charge on the mucin molecule significantly varies as the pH is altered. Lee and others (2005)
explained the adsorption mechanism of mucin to PDMS on the basis of hydrophobic binding sites of polypeptide backbone of mucin which becomes anchored to the PDMS surface. It was reported that at pH 2, the tertiary structure of protein part of the mucin was altered (as pH was around pI), which exposed the hydrophobic groups, thus allowing stronger binding of mucin to the PDMS
surfaces. Therefore, it might be expected that mucin can act as a potential surface active agent, competitively displacing or binding to the protein interfaces in emulsified lipid during gastric digestion, thus altering the droplet behaviour. Although a great deal of work on molecular approaches to enhance mucoadhesion, including, polyelectrolytic interactions (chitosans, polyacrylic acid, etc.) or hydrogen bonds (hydrogels) (Harding et al., 1999), and disulphide binding (thiolated polymers) (Bernkop-Schnürch et al., 1999; Leitner et al., 2003) has been carried out in pharmaceutical sciences; the interactions (if any) between gastric mucin and emulsified food lipids are still largely unknown. In general, several systematic studies have been carried out on the behaviour of drug based microemulsions in gastrointestinal tract (Calvo et al., 1997; MacGregor et al., 1997; Anal et al., 2003; AnnMarie et al., 2004). In contrast, the behaviour of food based emulsion droplets in gastric lumen remains largely unexplored. Although in vitro enzymatic digestion studies have been reported in food systems (Gauthier et al., 1986; Picot & Lacroix, 2004), but no systematic studies on the flocculation or disruption behaviour of milk proteinstabilized emulsion droplets in the complex gastric environments (effects of variable pH, ionic strengths, pepsin and mucin) together with the role of peristaltic shear effects have been reported to date. Intuitively, some of the simulation approaches used in pharmaceutical research might be intelligently applied in food emulsion science to gain insights about the behaviour of the food colloids, when interacting with the gastric secretions.