Texture changes and protein hydrolysis in different cheeses under simulated gastric environment

The impact of heat-set milk protein gel textures modified by pH on circulating amino acid appearance and gastric function in healthy female adults: a randomised controlled trial

Modification of dairy proteins during processing impacts structural assemblies, influencing textural and nutritional properties of dairy products, and release and availability of amino acids during digestion. By modifying only pH, acid heat-set bovine dairy gels with divergent textural properties were developed to alter protein digestion. In vitro assay confirmed faster digestion of protein from a firm gel (pH 5.65) versus a soft gel (pH 6.55). We hypothesised that firm gel (FIRM-G; pH 5.6) would result in greater indispensable amino acid (IAA) appearance in circulation over 5 h and corresponding differences in gastric myoelectrical activity relative to soft gel (SOFT-G; pH 6.2). In a randomised, single-blind cross-over trial, healthy females (n = 20) consumed 150 g of each gel; plasma amino acid appearance was assessed over 5 hours. Iso-nitrogenous, iso-caloric gels were prepared from identical mixtures of bovine milk and whey protein concentrates; providing 17.7 g (FIRM-G) and 18.9 g (SOFT-G) of protein per serving. Secondary outcomes included gastric myoelectrical activity measured by body surface gastric mapping, glycaemic, triglyceridaemic, and subjective appetite and digestive responses. Overall plasma IAA (area under the curve) did not differ between gels. However, plasma IAA concentrations were higher, and increased more rapidly over time after SOFT-G compared with FIRM-G (1455 ± 53 versus 1350 ± 62 μmol L-1 at 30 min, p = 0.024). Similarly, total, branched-chain and dispensable amino acids were higher at 30 min with SOFT-G than FIRM-G (total: 3939 ± 97 versus 3702 ± 127 μmol L-1, p = 0.014; branched-chain: 677 ± 30 versus 619 ± 34 μmol L-1, p = 0.047; dispensable: 2334 ± 53 versus 2210 ± 76 μmol L-1, p = 0.032). All other measured parameters were similar between gels. Peak postprandial aminoacidaemia was higher and faster following ingestion of SOFT-G. Customised plasma amino acid appearance from dairy is achievable by altering gel coagulum structure using pH during processing and may have minimal influence on related postprandial responses, with implications for targeting food design for optimal health. The Clinical Trial Registry number is ACTRN12622001418763 (https://www.anzctr.org.au) registered November 7, 2022.

Dynamic Stomach Model-Capillary Electrophoresis-ICPMS for Evaluation of Release and Transformation Behaviors of Arsenic Species from Microplastics during Digestion

Microplastics (MPs) can act as carriers of environmental arsenic species into the stomach with food and release arsenic species during digestion, which threatens human health. Herein, an integrated dynamic stomach model (DSM)-capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICPMS) is developed for online monitoring of the release and transformation behaviors of arsenic species loaded on MPs (As-MPs) in the simulated human stomach. The 3D-printed DSM with a soft stomach chamber enables the behaviors of gastric peristalsis, gastric and salivary fluid addition, pH adjustment, and gastric emptying (GE) to be controlled by a self-written program after oral ingestion of food with As-MPs. The gastric extract during digestion is introduced into the spiral channel to remove the large particulate impurity and online filtered to obtain the clarified arsenic-containing solution for subsequent speciation analysis of arsenic by CE-ICPMS. The digestion conditions and pretreatment processes of DSM are tracked and validated, and the release rates of As-MPs digested by DSM are compared with those digested by the static stomach model and DSM without GE. The release rate of inorganic arsenic on MPs is higher than that of organic arsenic throughout the gastric digestion process, and 8% of As(V) is reduced to As(III). The detection limits for As(III), DMA, MMA, and As(V) are 0.5-0.9 μg L-1 using DSM-CE-ICPMS, along with precisions of ≤8%. This present method provides an integrated and convenient tool for evaluating the release and transformation of As-MPs during human gastric digestion and provides a reference for exploring the interactions between MPs and metals/metalloids in the human body.

Interplay of egg white gel pH and intragastric pH: Impact on breakdown kinetics and mass transport processes

Egg white gels have been utilized as a model system to study protein breakdown kinetics based on physical and biochemical breakdown processes during in vitro gastric digestion. Additionally, the impact of regulating intragastric pH on the breakdown kinetic processes was investigated. The present study evaluated the impact of gel pH (based on the pH of protein dispersion prepared at pH 3, 5 and 7.5) and intragastric pH regulation (with or without adjustment to pH 2 during in vitro gastric digestion) on the effective diffusion of gastric juice components (water and HCl), gel softening kinetics during gastric digestion, microstructural analysis using micro- computed tomography and protein hydrolysis in the liquid and solid fraction of egg white gel digesta. Egg white gels were subjected to 30 s oral digestion and 15, 30, 60, 120, 180 or 240 min gastric digestion in a static in vitro gastric digestion model, with or without gastric pH adjustment to pH 2. The gel pH affected all the properties measured during gastric digestion and each gel pH represented a specific driving mechanism for protein breakdown. A lower gel pH (pH 3) demonstrated a higher diffusion of moisture and acid, resulting in faster softening (p < 0.05). An intermediate pH (pH 5) showed greater protein-protein interactions due to the proximity to the isoelectric point of egg white proteins, resulting in very slow softening during digestion (p < 0.05), and a higher pH (pH 7) resulted in higher acid diffusion, intermediate gel hardness and very slow softening kinetics (p < 0.05). The gastric pH adjustment during digestion of egg protein gels affected (p < 0.05) the equilibrium moisture and acid contents as well as protein hydrolysis. The study confirmed that there is an interplay between initial gel pH and the intragastric pH which affected the breakdown kinetics of egg white gels during the gastric digestion process.

Feasibility of using a realistic food bolus for semi-dynamic in vitro gastric digestion of hard cheese with pH-stat monitoring of protein hydrolysis

Oral processing of solid foods leads to boluses made of a human saliva and particles distributed in the size range ∼ 0 to 5 mm. However, studies on the release of nutrients from realistic solid food boluses during digestion are scarce because such mechanisms are difficult to investigate in vivo, and in vitro experiments generally recommend to extensively mince solid foods during the oral stage. Similarly, it has previously been shown that the peptic hydrolysis of protein solutions during in vitro gastric digestion can be monitored by acid titration in both static and dynamic pH conditions, but such approach has never been evaluated in the presence of particles of several millimetres in size. The first objective of the study was therefore to test the feasibility of using a realistic food bolus for gastric digestion studies with a pH-stat monitoring of proteolysis, using Emmental cheese as a solid food and with consideration of gastric acidifying kinetics. Degree of hydrolysis (DH) of proteins was monitored from two series of experiments performed in the presence and absence of pepsin. Other DH measurements, estimated from an independent approach based on the amount of free NH₂ groups (OPA method) contained by peptides released in the supernatant (UV absorbance) validated the pH-stat results. A second objective of this work was to test the possible influence of human saliva on gastric proteolysis (in comparison with a water-based bolus). Results showed that saliva slightly delayed initiation of proteolysis, which could be explained by the slightly higher initial pH of the saliva-based bolus, but had no statistical effects on pepsin activity. We conclude that acid titration with a pH-stat system can be a valuable approach to monitor the gastric in vitro proteolysis of realistic solid food boluses in dynamic pH conditions.

Effect of processing on in vitro digestibility (IVPD) of food proteins

Proteins are important macronutrients for the human body to grow and function throughout life. Although proteins are found in most foods, their very dissimilar digestibility must be taking into consideration when addressing the nutritional composition of a diet. This review presents a comprehensive summary of the in vitro digestibility of proteins from plants, milk, muscle, and egg. It is evident from this work that protein digestibility greatly varies among foods, this variability being dependent not only upon the protein source, but also the food matrix and the molecular interactions between proteins and other food components (food formulation), as well as the conditions during food processing and storage. Different approaches have been applied to assess in vitro protein digestibility (IVPD), varying in both the enzyme assay and quantification method used. In general, animal proteins tend to show higher IVPD. Harsh technological treatments tend to reduce IVPD, except for plant proteins, in which thermal degradation of anti-nutritional compounds results in improved IVPD. However, in order to improve the current knowledge about protein digestibility there is a vital need for understanding dependency on a protein source, molecular interaction, processing and formulation and relationships between. Such knowledge can be used to develop new food products with enhanced protein bioaccessibility.

Bioactive peptides in ripened cheeses: release during technological processes and resistance to the gastrointestinal tract

Milk proteins are recognized as the main source of biologically active peptides. Casein's primary structure contains several bioactive amino acid sequences on its latent inactive form. These potential active sequences can be released during cheese manufacture and ripening, giving rise to peptides with biological activity such as antihypertensive, antidiabetic, antioxidant, immunomodulatory, and mineral-binding properties. However, the presence of biopeptides in cheese does not imply actual biological activity in vivo because these peptides can be further hydrolyzed during gastrointestinal transit. This paper reviews the recent advances in biopeptide formation in ripened cheeses production, focusing on the influence of technological parameters affecting proteolysis and the consequent release of peptides. The main discoveries in the field of cheese peptide digestion through recent in vivo and in vitro model studies are also reviewed. © 2021 Society of Chemical Industry.

In vitro trypsin digestion and identification of possible cross-linking sites induced by transglutaminase (TGase) of silver carp (Hypophthalmichthys molitrix) surimi gels with different degrees of cross-linking

Surimi gels with different cross-linking degrees (18.52%, 34.67%, 62.87% and 79.11%) were prepared to identify the numbers and locations of lysine residues involved in TGase-induced cross-linking, and to reveal the quantity and location relationships among cross-linking degrees, cross-linking sites and digestion sites by using trypsin digestion, SDS-PAGE and LC-MS/MS methods. The results showed that with the increase in cross-linking degree from 18.52% to 79.11%, 1) the quantity of cross-linking sites gradually increased from 25 sites to 47 sites, 2) the main possible cross-linking domain moved from myosin head to rod, 3) the numbers of digestion sites first decreased from 1262 sites to 1194 sites, and then increased to 1302 sites, 4) the changes in the values of digestion sites were mainly concentrated in myosin rod and it was also the main region of digestion. This study can help exploring the relationship between enzymatic cross-linking and nutritional properties of food.

Achieving realistic gastric emptying curve in an advanced dynamic in vitro human digestion system: experiences with cheese-a difficult to empty material

Nowadays, in vitro digestion models have received growing interest in recent years to track the digestive fate of foods in the gastrointestinal tract. A major challenge in the development of more physiologically relevant in vitro gastric models is to simulate realistic gastric emptying. In this study, an advanced dynamic in vitro human gastric system was investigated for its potential in achieving the above. The mechanisms for controlling the gastric emptying rate by modulations of the peristaltic moving distance, the pylorus opening size/frequency, and the stomach tilting angle in relation to time are illustrated. With solid cheese, a difficult food material for emptying, different combinations of the operational parameters of the stomach system were evaluated. The system was steered to attain consistent gastric emptying curve with the theoretical data by optimizing operational parameters. By fitting the gastric retention data with a power-exponential model, which is a common approach for describing gastric emptying, the total meal achieved an average emptying half-time (t1/2) of 84.5 min and a curve shape coefficient (β) of 1.69, similar to the theoretical data reported in the literature, where the values of t1/2 and β were 85 min and 1.8, respectively (p > 0.05). Furthermore, the mean median particle size was significantly decreased from the initial 2.80 mm (cheese cubes) to the final 1.35 mm (p < 0.05). There are few particles greater than 2 mm observed in the emptied cheese digesta throughout the digestion process. These suggest the powerful gastric grinding and sieving capacity exhibited by the in vitro system. The current study demonstrates that a well-considered in vitro system can offer a reasonable approach for tracking the structural and physicochemical changes of foods during digestion in the stomach, which is practically meaningful.

Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions

Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type II collagen (IC II) and pepsin soluble undenatured type II collagen (SC II). Results showed that, after gastric digestion, especially under low pH conditions, some thick and short collagen fibers were dissolved from IC II, which was accompanied by the release of soluble protein with triple helix structure. As to SC II, when gastric digestion was performed under higher pH conditions, collagen fibers were mildly thinned and curved, and the triple helix structure was slightly destroyed. However, those changes hardly occurred during the intestinal digestion. Moreover, the undenatured type II collagen existing in SC II digestive supernatant was 1.2-12.4 times higher than that of IC II depending on the pH conditions. These results suggested that, it is more likely for SC II to exert its activity of relieving arthritis, and it should be taken when the pH of gastric environment is high.

Cheese as a functional food for older adults: comparing the bioactive properties of different cheese matrices following simulated gastrointestinal in vitro digestion

Age-related changes to the gastrointestinal tract (GIT) can impact how food is digested. Studying the effects of these changes can help identify functional foods for older adults. Cheese was digested using two simulated gastrointestinal in vitro digestion (SGID) models representing adult and elderly gastro-intestinal conditions. Antioxidant capacity was measured using DPPH, FRAP and TPC assays. The ability of cheese to inhibit digestive enzymes was determined by the α-glucosidase and lipase inhibition assays. Digestive aging influenced the bioactivity of cheese, as elderly digestates had significantly lower (p < 0.05) antioxidant, α-glucosidase and lipase inhibitory properties compared to adult digestates. However, soft cheese (feta, goats', brie) demonstrated greatest potential with comparable radical scavenging properties and lipase inhibition, greatest FRAP and α-glucosidase inhibitory potential. Despite age-related changes, the bioactive properties of cheese were evident following digestion with an older adult SGID model, suggesting cheese has potential as a functional food for older adults.

Destructuring and restructuring of foods during gastric digestion

All foods harbor unique length scale-dependent structural features that can influence the release, transport, and utilization of macro- or micronutrients in the human gastrointestinal tract. In this regard, food destructuring and restructuring processes during gastric passage significantly influence downstream nutrient assimilation and feelings of satiety. This review begins with a synopsis of the effects of oral processing on food structure. Then, stomach-centric factors that contribute to the efficacy of gastric digestion are discussed, and exemplified by comparing the intragastric de- and restructuring of a number of common foods. The mechanisms of how intragastric structuring influences gastric emptying and its relationship to human satiety are then discussed. Finally, recently developed, non-destructive instrumental approaches used to quantitively and qualitatively characterize food behavior during gastric destructuring and restructuring are described.

Dairy structures and physiological responses: a matter of gastric digestion

Digestion and health properties of food do not solely rely on the sum of nutrients but are also influenced by food structure. Dairy products present an array of structures due to differences in the origin of milk components and the changes induced by processing. Some dairy structures have been observed to induce specific effects on digestion rates and physiological responses. However, the underlying mechanisms are not fully understood. Gastric digestion plays a key role in controlling digestion kinetics. The main objective of this review is to expose the relevance of gastric phase as the link between dairy structures and physiological responses. The focus is on human and animal studies, and physiological relevant in vitro digestion models. Data collected showed that the structure of dairy products have a profound impact on rate of nutrient bioavailability, absorption and physiological responses, suggesting gastric digestion as the main driver. Control of gastric digestion can be a tool for delivering specific rates of nutrient digestion. Therefore, the design of food structure targeting specific gastric behavior could be of great interest for particular population needs e.g. rapid nutrient digestion will benefit elderly, and slow nutrient digestion could help to enhance satiety.

Advances and challenges in liposome digestion: Surface interaction, biological fate, and GIT modeling

During the past 50 years, there has been increased interest in liposomes as carriers of pharmaceutical, cosmetic, and agricultural products. More recently, much progress has been made in the use of surface-modified formulas in experimental food matrices. However, before the viability and the applications of nutrients in liposomal form in the edible field can be determined, the digestion behavior along the human gastrointestinal tract (GIT) must be clarified. In vitro digestion models, from static models to dynamic mono-/bi-/multi-compartmental models, are increasingly being developed and applied as alternatives to in vivo assays. This review describes the surface interactions of liposomes with their encapsulated ingredients and with external food components and updates the biological fate of liposomes after ingestion. It summarizes current models for the human stomach and intestine that are available and their relevance in nutritional studies. It highlights limitations and challenges in the use of these models for liposomal colloid system digestion and discusses crucial factors, such as enzymes and bile salts, that affect liposomal bilayer degradation.