Dynamic in vitro gastric digestion behavior of goat milk: Effects of homogenization and heat treatments

The gastric digestion behavior of differently processed goat milks was investigated using a dynamic in vitro gastric digestion model, the human gastric simulator. Homogenization and heat treatment of goat milk resulted in gastric clots with highly fragmented structures. They also delayed the pH reduction during digestion, altered the chemical composition of the clots and the emptied digesta, promoted the release of calcium from the clots, and accelerated the hydrolysis and the emptying of milk proteins. The apparent density of the protein particles and the location of the homogenized fat globules changed during the digestion process, as shown in the emptied digesta of the homogenized goat milks. The effects of processing on the digestion behavior of goat milk were broadly similar to those previously reported for cow milk. However, the overall gastric digestion process of goat milk was more affected by homogenization than by heat treatments.     

Intraluminal immunoreactive caseinomacropeptide after milk protein ingestion in humans

Caseinomacropeptide (CMP) is a 64 amino acids peptide which is the first product released after κ-casein hydrolysis. The present work investigates the kinetics delivery of CMP in human jejunal lumen during the digestion of intrinsically [¹⁵N]-labelled casein, whey protein, yoghurt and pea flour meal. Effluents were collected through a naso-intestinal tube and analysed for the enrichment in [¹⁵N] to evaluate the dietary nitrogen fraction. Detection and quantification of CMP was performed by an inhibition Elisa procedure. No trace of CMP was detected in the ileum of volunteers after the ingestion of the casein meal. The results showed that CMP appears in the jejunal effluents within the first 20 min after meal ingestion at a level varying from meal to meal. During digestion of whey protein, CMP appeared rapidly as a single peak and in high amounts, whereas it is discharged slowly in moderate proportions with the casein meal. These results demonstrate that CMP is emptied from the stomach in significant amounts during milk products digestion and support the hypothesis that food-born peptides could exert a physiological function. Moreover, in the present study a relation could be assumed between the amount of CMP in the meal and the stimulation of luminal endogenous nitrogen secretion. However, the specific physiological activity of CMP in humans, particularly on the digestion process, requires further studies.     

Effect of homogenization and heat treatment on the behavior of protein and fat globules during gastric digestion of milk

The effects of homogenization and heat treatment on the formation and the breakdown of clots during gastric digestion of whole milk were investigated using a human gastric simulator. Homogenization and heat treatment led to formation of coagula with fragmented and crumbled structures compared with the coagulum formed from raw whole milk, but a larger fraction of the protein and more fat globules were incorporated into the coagula induced by action of the milk-clotting enzyme pepsin. The fat globules in the whole milk appeared to be embedded in the clots as they formed. After formation of the clot, the greater numbers of pores in the structures of the clots formed with homogenized milk and heated whole milk led to greater rates of protein hydrolysis by pepsin, which resulted in faster release of fat globules from the clots into the digesta. Coalescence of fat globules occurred both in the digesta and within the protein clots no matter whether they were in homogenized or heated milk samples. The formation of clots with different structures and hence the changes in the rates of protein hydrolysis and the release of milk fat into the digesta in the stomach provide important information for understanding the gastric emptying of milk and the potential to use this knowledge to manipulate the bioavailability of fat and other fat-soluble nutrients in dairy products.     

Gastric digestion of milk protein ingredients: Study using an in vitro dynamic model

The coagulation behavior and the kinetics of protein hydrolysis of skim milk powder, milk protein concentrate (MPC), calcium-depleted MPC, sodium caseinate, whey protein isolate (WPI), and heated (90°C, 20 min) WPI under gastric conditions were examined using an advanced dynamic digestion model (i.e., a human gastric simulator). During gastric digestion, these protein ingredients exhibited various pH profiles as a function of the digestion time. Skim milk powder and MPC, which contained casein micelles, formed cohesive, ball-like curds with a dense structure after 10 min of digestion; these curds did not disintegrate over 220 min of digestion. Partly calcium-depleted MPC and sodium caseinate, which lacked an intact casein micellar structure, formed curds at approximately 40 min, and a loose, fragmented curd structure was observed after 220 min of digestion. In contrast, no curds were formed in either WPI or heated WPI after 220 min of digestion. In addition, the hydrolysis rates and the compositions of the digesta released from the human gastric simulator were different for the various protein ingredients, as detected by sodium dodecyl sulfate-PAGE. Skim milk powder and MPC exhibited slower hydrolysis rates than calcium-depleted MPC and sodium caseinate. The most rapid hydrolysis occurred in the WPI (with and without heating). This was attributed to the formation of different structured curds under gastric conditions. The results offer novel insights about the coagulation kinetics of proteins from different milk protein ingredients, highlighting the critical role of the food matrix in affecting the course of protein digestion.     

Functional Significance of Bioactive Peptides Derived from Milk Proteins

Bioactive peptides can be defined as protein fragments with potential biological activities. Milk proteins are precursors of many different biologically active peptides. Bioactive peptides from milk proteins are considered potential modulators of various regulatory processes in the body. They mediate physiological functions in cardiovascular, nervous, gastro intestinal and immune systems. The functional significance of bioactivities depends on peptide fragment. Bioactive peptides encrypted in major milk proteins are latent within the sequence of the parent protein molecule. They can be liberated by (i) gastro intestinal digestion of milk, (ii) fermentation of milk with proteolytic starter cultures, and (iii) hydrolysis by proteolytic enzymes. In relation to their mode of action, bioactive peptides may reach target sites at the luminal side of intestinal tract, or after absorption, in peripheral organs. The production, functionalities, and mode of action of bioactive milk peptides as well as latest peptide products and ingredients are reviewed.     

Structural changes of bovine milk fat globules during in vitro digestion

An in vitro digestion model that simulated gastric and intestinal fasting conditions was used to monitor the physical, chemical, and structural changes of fat globules from raw bovine milk. During in vitro gastric digestion, the fat globules were stable under low-acidic conditions. Some peptides and β-lactoglobulin were resistant to proteolysis by pepsin. Phospholipids, proteins, and peptides stabilized the globules in the stomach model. During in vitro intestinal digestion, most of the β-lactoglobulin and residual peptides were hydrolyzed by trypsin and chymotrypsin, and the lipolytic products, released from the hydrolysis of the triglyceride core of the globules, led to destabilization and coalescence of the globules. By accumulating at the surface of the fat globules, the lipolytic products formed a lamellar phase and their solubilization by bile salts resulted in the formation of disk-shaped micelles. This study brings new interesting insights on the digestion of bovine milk.     

The heat treatment and the gelation are strong determinants of the kinetics of milk proteins digestion and of the peripheral availability of amino acids

This study aimed to determine the kinetics of milk protein digestion and amino acid absorption after ingestion of four dairy matrices by six minipigs: unheated or heated skim milk and corresponding rennet gels. Digestive contents and plasma samples were collected over a 7 h-period after meal ingestion. Gelation of milk slowed down the outflow of the meal from the stomach and the subsequent absorption of amino acids, and decreased their bioavailability in peripheral blood. The gelled rennet matrices also led to low levels of milk proteins at the duodenum. Caseins and β-lactoglobulin, respectively, were sensitive and resistant to hydrolysis in the stomach with the unheated matrices, but showed similar digestion with the heated matrices, with a heat-induced susceptibility to hydrolysis for β-lactoglobulin. These results suggest a significant influence of the meal microstructure (resulting from heat treatment) and macrostructure (resulting from gelation process) on the different steps of milk proteins digestion.     

Structural changes in milk from different species during gastric digestion in piglets

This study investigated the structural and physicochemical changes that occur in milk, a naturally designed complex structured emulsion, during gastric digestion using the bottle-fed piglet as an animal model. The gastric digestions of cow, goat, and sheep milk were compared in male piglets euthanized at different postfeeding times to collect the stomach chyme. The cow and noncow milks separated into curd (aggregated caseins) and liquid (mostly soluble whey) phases in the piglet's stomach. For milk from all the species, the curd remained longer in the stomach because of its slow disintegration, whereas the liquid phase emptied readily. The majority of the fat globules were found to be entrapped within the protein network of the curd. The rate of release of fat globules was strongly dependent on the breakdown of the surrounding protein network of the curd. The consistency of the gastric curds changed as digestion progressed, with goat and sheep milk curds having relatively softer curd consistency and less fused protein networks, especially toward the end of digestion. This might have led to the lower protein and fat retention in the goat and sheep milk curds and relatively faster gastric emptying of these nutrients from goat and sheep milk in comparison to cow milk. This in vivo study provided new and enhanced understanding of the mechanisms of the gastric digestion of milk from different species. It may have implications for developing bioinspired structures for the controlled digestion and delivery of nutrients.     

Dietary sodium butyrate supplementation increases digestibility and pancreatic secretion in young milk-fed calves

The aim of this study was to test, in 8 calves fed milk formula based on soybean protein, the ability of sodium butyrate (SB) supplementation to improve nutrient digestibility and daily pancreatic secretions and to modify the kinetics of these secretions. Additionally, effects of duodenal SB infusion were evaluated. Plasma levels of gastrin, secretin, and cholecystokinin were measured. Butyrate supplementation in milk formula increased nutrient digestibility and total daily pancreatic secretions. For juice volume, this increase was most important from 12 to 17 h after the morning meal. During the 3-h postprandial period, oral SB supplementation reduced the physiological decrease of postprandial pancreatic secretion (while duodenal digesta flow rate was maximal) and had a minor effect on plasma gut regulatory peptide concentrations. Compared with the diet without SB, ingestion of SB stimulated pancreatic secretion. Taken together, these results could explain the measured increase in nutrient digestibility. The data obtained after duodenal SB infusion did not indicate an effect on pancreatic secretion, apart from elevated lipase output compared with control. The mechanisms responsible for these events are not known and circulating gut regulatory peptides do not seem to be implicated. Our work brings new results regarding SB as a feed additive in young calf nutrition.     

Invitro digestion and absorption efficiency of homogenised milk lipids

The effects of homogenisation, heat treatment and their order on in vitro gastric/pancreatic digestions, and in vitro absorption of milk lipids, expressed as free fatty acids, were investigated. Milk, cream or mixture of homogenised/heat‐treated cream and skim milk were subjected to gastric digestion at pH 2 or 6, followed by pancreatic digestion and cell absorption by CaCo‐2 cells. Heat treatment after or prior to homogenisation affected in vitro digestion efficiencies of milk lipids. The presence of milk proteins led to reductions in the levels of gastric/pancreatic digestion of homogenised milk lipids. Absorption of FFA was not affected by gastric digestion pH.     

Gastric proteinase digestion of caseins in newborn pups of the mouse

Casein micelles of mouse milk consist of alpha-, beta-, gamma-, and kappa-caseins. By digestion with alkaline phosphatase, they were separated as an independent band by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). The compositions of alpha-, beta-, gamma-, and kappa-caseins were 24.3, 25.1, 9.4, and 41.2% in colostrum, and 36.8, 15.6, 11.9, and 35.7% in mature milk, respectively. Zero-day-old pups were allowed to access either colostrum or mature milk, and the aggregated milk in the stomach was analyzed by SDS-PAGE. Caseins in colostrum were digested more rapidly and efficiently than those in mature milk. Among the seven peptides present in the aggregated caseins, four peptides were colostrum-specific and derived from alpha- and gamma-caseins. It was expected that colostrum-specific and soluble peptides were generated from alpha- and gamma-caseins through gastric proteinase digestion. Amino acid sequence analysis and the pH of the aggregated milk suggested that caseins in the stomach were digested by a chymotrypsin-like proteinase. Caseins in colostrum were different from those in mature milk, with respects to the casein composition as well as the gastric proteinase sensitivity. It is concluded that the lactating mice on the day of parturition supply particular caseins to their young.     

Structural changes in cow,goat, and sheep skim milk during dynamic in vitro gastric digestion

Coagulation of milk in the stomach is the first crucial step in its digestion. Using a human gastric simulator, the dynamic gastric digestion of goat and sheep skim milk were compared with that of cow skim milk, focusing particularly on their physical characteristics. The gastric contents were analyzed for changes in dry matter and microstructure, and the extent of protein digestion. The study revealed that the skim milk from all species formed a curd within the first 15 min of gastric digestion, at which time the pH was ~6.1 to 6.3. Compared with cow skim milk, the dry matter contents of the clots formed from goat and sheep skim milk were lower and higher, respectively, which was due to the differences in their total solids and protein contents. Microstructural analysis showed that, as digestion progressed, the clot structure became more cohesive, along with a decrease in moisture content, which in turn affected the breakdown and hydrolysis of caseins by pepsin; this phenomenon was similar for milk from all species. However, the extent of moisture retained in the sheep skim milk clot appeared to be lower than those of the cow and goat skim milk clots. In addition, the relative firmness of the sheep milk clot was higher than those of the cow and goat milk clots at the end of gastric digestion. The pattern of protein hydrolysis by pepsin was similar for the milk of all species, despite the differences in the proportions of different proteins. The study provided insight into the coagulation kinetics of goat and sheep skim milk under in vitro gastric digestion conditions.     

Angiotensin converting enzyme-inhibitory activity of peptides isolated from Manchego cheese. Stability under simulated gastrointestinal digestion

In this study, several peptides, which had previously been identified in active HPLC fractions from Manchego cheese, were synthesised and their angiotensin converting enzyme (ACE)-inhibitory activities were measured. From 11 peptides, which were selected based on their structures, only two, VRYL and KKYNVPQL, showed considerable ACE-inhibitory activity with IC₅₀ values of 24.1 and 77.1 μ , respectively. Subsequently, the impact of the gastrointestinal digestion on ACE-inhibitory activity was evaluated. Some of the peptides selected were resistant to the incubation with pepsin followed by hydrolysis with a pancreatic extract. The ACE-inhibitory activity after simulated digestion did not change drastically except for peptide _s2-CN f(195-204) (TQPKTNAIPY) that exhibited an activity 6 times greater after simulated digestion. In contrast, after simulated digestion, the activities of peptides VRYL and KKYNVPQL decreased. The peptides not hydrolysed by gastrointestinal enzymes and peptide VRYL, which was only partly hydrolysed, were incubated with ACE and were found to be true inhibitors of the enzyme and to have a competitive inhibition pattern.     

Digestion of meat proteins in a human-stomach: A CFD simulation study

Understanding gastric digestion mechanisms is important for the design of functional foods. In this study, we have investigated the meat-protein digestion in human-stomach by using a CFD method. The gastric motility is modeled with a dynamic mesh. The disintegration of large food particles in an acidic environment is simulated using a reaction-diffusion-convection model. A food matrix is used to model the large food-particles. The numerical results show that the digestion and emptying become faster when the meat is treated at a higher temperature. The digestion rate is reduced considerably when the gastric motility or the H⁺ secretion is weakened due to a stomach disorder. TACs stimulate backflows which enhance the transport of enzymes and H⁺, thereby accelerating the digestion process. Due to the flow resistance by the food matrix made of large food particles, liquid gastric contents are emptied in a pathway close to the stomach inner-surface. Large food-particles are mainly disintegrated in the region next to the stomach inner-surface. Therefore, the characteristic length scale of species transport (for enzymes or H⁺) should be the size of food matrix, instead of the size of large food-particles.     

大豆粕蛋白酶酶解条件和产物分析研究

以水解度为衡量指标,通过正交试验设计,对木瓜蛋白酶、胰蛋白酶和As1.398蛋白酶酶解大豆粕的最佳水解条件进行了筛选.试验结果表明:木瓜蛋白酶水解大豆粕的最佳条件为pH7.0、温度60℃、时间5 h、酶浓度5%.胰蛋白酶水解大豆粕的最佳条件为pH7.0、温度50℃、时间7 h、酶浓度5%;As1.398蛋白酶水解大豆粕的最佳条件为pH6.5、温度50℃、时间7 h、酶浓度5%.对酶解产物进行超滤和SDS-PAGE电泳分析表明,因酶解条件不同,大豆粕酶解产物中蛋白质和肽的组成及其数量也不同;酶的种类不同,大豆粕酶解产物组成也不同;大豆粕水解度越高,其酶解产物中小分子肽数量越多.     

Activity against Listeria monocytogenes of human milk during lactation. A preliminary study

Human milk samples from three healthy donors were investigated in order to evaluate the antibacterial activity during lactation against Escherichia coli ATCC 25922 and Listeria monocytogenes. The concentration of the main human-milk antimicrobial proteins (lactoferrin (LF), lysozyme (LZ) and secretory immunoglobulin A (sIgA)) was determined by ELISA. Results showed that human milk exhibited antibacterial activity against List. monocytogenes, although it was weakly active against Esch. coli ATCC 25922. The observed antilisterial activity was positively correlated with LZ concentration. In addition, the effect of gastrointestinal proteases, at different pH conditions, that prevail in the stomach of infants (pH 2.0-6.5), on antilisterial activity and protein degradation was evaluated. Hydrolysis with pepsin at pH 4.0-6.5, followed by treatment with pancreatic enzymes, resulted in a decreased hydrolysis of LZ, LF and sIgA and an enhanced antibacterial activity against List. monocytogenes. It is suggested that partial degradation of certain milk proteins at the gastrointestinal level may produce peptides that could act synergistically with the remnant intact proteins.     

Peptidome comparison following gastrointestinal digesta of bovine versus caprine milk serum

Infant formula is used as a supplement for newborns. Although bovine milk-based infant formulas dominate the market, caprine milk-based infant formula has attracted increasing attention because of its lower allergenicity. This study compared the digestive peptidome of bovine and caprine milk serum proteins by using in vitro infant simulating conditions. The result showed that the degradation pattern of milk proteins was similar, whereas the digestive rates of milk proteins differed between bovine and caprine milks. Several proteins, such as α-lactalbumin (LALBA), β-lactoglobulin (LGB), serum amyloid A protein (SAA1), glycosylation-dependent cell adhesion molecule 1 (GLYCAM1), and lactotransferrin (LTF), released more peptides during digestion of caprine milk serum than during digestion of bovine milk serum; however, more peptides derived from α_S1-casein (CSN1S1) were found in bovine digesta. In addition, antimicrobial-related peptides were mostly only found in caprine intestinal digesta. The results of this study may be useful in understanding the digestion characteristics of milk serum proteins and providing guidance on the improvement of infant formula.     

Functional role of bioactive peptides with special reference to cheeses

Growing perception about diet in relation to health has extended the necessity to explore the biologically active components present in native foods. In this review, bioactive peptides released from cheeses that may have important physiological functions are discussed. Bioactive encrypted peptides can be generated from precursor milk proteins during food processing via enzymatic hydrolysis and fermentation. Generally, biofunctionalities of peptides are latent within precursor proteins. Bioactive peptides liberated from cheeses exhibit numerous potential therapeutic roles: for example, angiotensinogen‐converting enzyme (ACE) inhibition, antioxidant, anti‐thrombotic, anti‐microbial, anti‐cancer and anti‐inflammatory. This article critically focuses on the functional roles of bioactive peptides derived from different cheeses.     

Mild thermal treatment and in-vitro digestion of three forms of bovine lactoferrin: Effects on functional properties

In-vitro digestion behaviour of iron saturated (holo)-, native- and iron depleted (apo)- bovine lactoferrin (LF) was studied in simulated oral, gastric and intestinal digestion conditions. All LFs were hydrolysed to <10 kDa peptides; 93% of iron was released from holo-LF within 2 h of gastric digestion. Iron binding ability of all the three LFs was decreased by 80–90% during gastric digestion, while antioxidant activities at the end of gastric digestion were 3–8 times higher than those at the end of oral and intestinal digestion. Antioxidant activity of apo-LF was higher than that of holo-LF after gastric and intestinal digestion. Gastric and intestinal digestion of all the three forms of LF produced negatively charged peptides or amino acids. There were only minor differences among the three forms of LF in terms surface charge, hydrolysis patterns and iron binding/release characteristics between heated and unheated LF samples after gastric digestion.     

以不同体外模拟系统评价大豆蛋白的胃消化特性

采用静态消化模型和半动态消化模型,评价经透析处理和未经透析处理的大豆蛋白的胃消化特性。结果表明：在静态消化模型中,透析对大豆蛋白的胃消化过程没有影响;但在半动态消化模型中,未经透析的大豆蛋白pH值、排空内容物的干物质质量和游离氨基的含量与经透析处理的大豆蛋白存在一定差异,表明未经透析处理的大豆蛋白保留较多大豆中的成分,使其缓冲能力更强,胃排空滞后,蛋白质消化速率减慢。此外,静态消化模型和半动态消化模型有不同的优势,所以可以根据不同的研究目的采用不同的消化模型。这项工作将为大豆蛋白产品的开发和健康效应提供新的见解。