In vivo digestion of bovine milk fat globules: Effect of processing and interfacial structural changes. II. Upper digestive tract digestion Sophie Gallier ⇑ , Xiang Q. Zhu, Shane M. Rutherfurd, Aiqian Ye, Paul J. Moughan, Harjinder Singh Riddet Institute, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand article info Article history: Available online 14 June 2013 Keywords: Bovine cream Milk fat globule membrane In vivo intestinal digestion Confocal microscopy Transmission electron microscopy abstract The aim of this research was to study the effect of milk processing on the in vivo upper digestive tract digestion of milk fat globules. Fasted rats were serially gavaged over a 5 h period with cream from raw, pasteurised, or pasteurised and homogenised milk. Only a few intact dietary proteins and peptides were present in the small intestinal digesta. Significantly (P < 0.05) more longer chain (C P 10) fatty acids were present in the digesta of rats gavaged with raw (448 mg g À1 digesta dry matter (DDM)) and homog- enised creams (528 mg g À1 DDM), as compared to pasteurised and homogenised cream (249 mg g À1 DDM). Microscopy techniques were used to investigate the structural changes during digestion. Liquid–crystalline lamellar phases surrounding the fat globules, fatty acid soap crystals and lipid–mucin interactions were evident in all small intestinal digesta. Overall, the pasteurised and homogenised cream appeared to be digested to a greater extent. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Milk is an important food in the human diet, not only as the sole source of nutrition for infants but also as a source of energy in the form of fat, protein and carbohydrate for children and adults. Milk fat has a highly diverse fatty acid profile containing more than 400 different fatty acids (Michalski, 2009). Milk contains 98% of its tri- glycerides in the form of globules surrounded by a complex inter- face, the milk fat globule membrane (MFGM), a trilayer of phospholipids with proteins, glycoproteins, cholesterol, glycolipids and vitamins. The diameter of the milk fat globules ranges between 0.2 and 15 lm, with an average diameter of 4 lm(Michalski, Mi- chel, Sainmont, & Briard, 2002). Recent studies have focused on the structure and composition of the MFGM (Dewettinck et al., 2008; Gallier, Gragson, Jimenez-Flores, & Everett, 2010; Gallier, Gragson, Jiménez-Flores, & Everett, 2012; Vanderghem et al., 2011). The main MFGM proteins are mucin 1 (MUC 1; 200 kDa), xanthine oxidase (XO; 150 kDa), cluster of differentiation (CD) 36 (76–78 kDa), butyrophilin (BTN; 67 kDa), adipophilin (52 kDa) co-migrating with periodic acid Schiff (PAS) 6/7 (48–54 kDa) and fatty acid binding protein (13 kDa). MUC1 plays an important role in the intestinal immune system. It is able to inhibit infection by rotavirus and common enteropathogenic bacteria and prevent mother–child transmission of HIV (Le et al., 2012). PAS 6/7 and CD36 also improve the intestinal immune system and bind to pathogens. Therefore, the resistance of MFGM glycoproteins to digestion or their partial degradation into bioactive peptides dur- ing digestion is critical for their action in the gastrointestinal tract. Raw milk is usually processed for safe human consumption and extended shelf-life. Commercially available milk is commonly pas- teurised and homogenised. Thermal treatments such as pasteurisa- tion and ultra-high temperature processing lead to denaturation of some MFGM proteins (Ye, Singh, Taylor, & Anema, 2004) and inter- actions between the MFGM and whey proteins (Michalski, 2009). homogenisation involves the rupture of the MFGM and a concom- itant reduction in the size of the globules to less than 1 lm, thus creating a larger oil–water interface. As the MFGM is not present in sufficient quantity to cover this new interface, caseins are able to adsorb onto the interface (Michalski, 2009). The changes in com- position and structure of the milk fat globule surface that occur during heat treatment or homogenisation will also have an impact on interfacial processes such as interactions with the food matrix and the process of lipolysis. The process of triglyceride digestion in the gastrointestinal tract begins with lingual or gastric lipase (depending on the species) which hydrolyses 5–40% of the dietary lipids in the stomach and 7.5% in the duodenum. Pancreatic lipase then hydrolyses much of 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.06.019 Abbreviations: DDM, digesta dry matter; RC, raw cream; PC, pasteurised cream; PHC, pasteurised and homogenised cream; MFGM, milk fat globule membrane; MUC1, mucin1; XO, xanthine oxidase; PAS, periodic acid Schiff; CD, cluster of differentiation; BTN, butyrophilin; FABP, fatty acid binding protein; LCFA, long chain fatty acid; SDS–PAGE, sodium dodecyl sulphate polyacrylamide gel electro- phoresis; DIC, differential interference contrast; CLSM, confocal laser scanning microscopy; TEM, transmission electron microscopy. ⇑ Corresponding author. Tel.: +64 (0)6 356 9099x81612; fax: +64 (0)6 350 5655. E-mail address: S.Gallier@massey.ac.nz (S. Gallier). Food Chemistry 141 (2013) 3215–3223 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem