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Numerous studies have reported a 20-50% reduction of the glycaemic response to starch-rich meals by low-pH drinks or foods. Although different possible explanations have been put forward, our work using bread and lemon juice (pH≈2.3) as a case study has pointed to the acid-induced inhibition of salivary α-amylase as the most likely underlying mechanism. Therefore, our aim was to study the digestion of a different subset of such meals to further investigate this hypothesis. Four meals have been replicated in the lab both with and without their pH-lowering ingredient. Their initial pHs were measured and they were then digested in vitro using the semi-dynamic INFOGEST protocol. Each digestion included oral, gastric and intestinal phases and experiments were conducted in triplicate. Samples were collected at different time-points to monitor the extent of starch hydrolysis and the MW weight distributions of the polysaccharide hydrolysates throughout the experiments. Low pH meals exhibited a marked reduction of the proportion of starch digested compared to their neutral-pH counterparts. The agreement between these results with those of previous human studies confirms our earlier observations and offers new perspectives for the development of strategies to improve the glycaemic responses to starch-rich foods.

Background Saliva may play a role in digestion through its enzymatic content or physico-chemical impact on the food bolus or chyme. Salivary proteins also interact with tannins, which could counteract their effect on digestive enzymes. We here evaluated the impact of saliva on gastric proteolysis of solid or liquid dairy products, including one contaning tannins.MethodSwiss-type cheese (C), milk in water (M) or milk in black tea (T), all in presence of human saliva vs water, were digested using semi-dynamic (C) and static conditions (M,T), respectively. Proteolysis was monitored throughout gastric digestion by OPA assay in all samples. The degree of hydrolysis was also calculated for cheese using acid titration data.ResultsFor cheese, the presence of saliva delayed slightly the initiation of proteolysis. OPA measurements suggested a small proteolytic inhibition by saliva. For milk, the presence of tannins increased gastric proteolysis (possibly by denaturing the pepsin substrates) but saliva had no substantial impact on OPA measurements, whether the matrix contained tannins or not.Conclusion In the conditions used, saliva had a moderate, if any, impact on gastric digestion of dairy products. Due to the documented effect of saliva on emulsions stability, the study could be extended to lipolysis.

Objectives : Buriti (Mauritia flexuosa) is an under-utilized Amazonian fruit with a high content in phenolic compounds. Although polyphenols have health benefits, they may also hinder protein digestibility but salivary proteins might counteract this negative effect through binding to polyphenols. The objective of this work was to describe the impact of saliva on protein digestibility of yoghurt containing buriti.Methods : A water extract of buriti flour was mixed with human saliva (LeeBiosolutions) at estimated concentrations varying from 0.001 to 0.05 g polyphenols (GAE)/ml. The particle size distribution in the mixtures was measured by laser diffraction. Pepsin and trypsin activity were measured in presence or not of the buriti extract. Furthermore, an in vitro model of digestion was applied to yoghurt or to yoghurt with buriti flour (1:4 w/w). In the oral phase of digestion, the two food products was mixed with human saliva or simulated salivary fluid SSF (ionic solution) at a ratio of 1:1. The standardized INFOGEST static model was then performed. The gastric and intestinal phases of digestion were conducted using pepsin and pancreatin, respectively. Digestive proteolysis was assessed by measuring accessible NH2 groups in samples over time by the OPA method.Results : Buriti extract also had no significant effect on pepsin or trypsin activity. However, when added to yoghurt, buriti flour had a large inhibiting impact on digestive intestinal proteolysis. Interestingly, the presence of saliva instead of SSF during digestion of buriti-containing yoghurt increased slightly the digestive proteolysis, both in the gastric and intestinal phase. Finally, buriti extract had no detectable impact on particle size in saliva (D50 ~ 45µm), suggesting no major aggregation of salivary proteins.Conclusions : Compared to SSF, saliva improved protein digestibility of yoghurt containing buriti flour, but is unclear whether this occurred through binding of salivary proteins to polyphenols. Further investigations are required.

Gastrointestinal digestion is a complex dynamic process consisting of mechanically and enzymatically breaking down the food structures into nutrients which are usable by the organism. Monitoring food degradation during the different digestion phases is essential for understanding the mechanisms regulating the digestion of specific nutrients and the impact of food structure on nutrient absorption. In last years, in vitro systems simulating digestion processes [1] have been developped, making it possible to resolve several difficulties associated with in vivo experiments, espetially biological complexity and ethical concerns. MRI is a highly promising non-invasive approach for both in vivo and in vitro digestion research [2] as it can be used to obtain information on the status and amount of water and lipid protons in foods. These information can be used for spatially resolved measurements of multi-scale structural features and composition of food, as demonstrated in recent studies on simplified food such as gelsThe present study aimed to evaluate MRI for monitoring digestion of a complex meal composed of bread, cheese and water. All macronutrients (saccharide, protein, lipid) were gathered in such a meal, with particularities: bread samples containing mainly starch and to a lesser extent (10%) proteins while cheese mainly contained proteins and lipids. Food destructuration associated to erosion and hydrolysis were studied using a semi-dynamic protocol simulating a gastrointestinal human digestive cycle. The study was performed on a 1.5 T MRI scanner (Avanto, Siemens). Evolution of morphological and physico-chemical properties of the particules and the liquid were investigated using: 1) 2D T2 maps 2) 3D fat fraction maps obtained by water-fat separation approach and 3) 3D morphological images obtained by ultra short echo time sequence. Combining different MRI image modalities (Fig. 1), it was possible to investigate separately several phases of the digesta, i. e. supernatant, large food pieces (cheese and bread crust) and molecules or small fragments in suspension which laid deposited at the bottom of the vessel during the MRI measurement.

International audience; Pepsin, secreted by the stomach’s parietal cells, is the first protease encountered during digestion. The stomach slowly acidifies thefood intake and it is generally said that pepsin is active at acidic pH only (pH ≤ 4), with an optimum around pH 21. However, the impact of pH on gastric proteolysis has not been studied on complex mixes of alimentary proteins, only on few neat globular proteins. The goal of this study was to compare the kinetics of peptic hydrolysis of dairy caseins and egg white proteins as a function of both pH and in-vitro gastric digestion time.

International audience

Introduction et but de l’étude: La pepsine, sécrétée dans l’estomac, est la première protéase rencontrée par les protéines alimentaires au cours de la digestion. L’estomac acidifie lentement les bols alimentaires et il a été montré que le pH gastrique a un impact majeur sur les cinétiques de protéolyse 1.En effet, il est généralement admis que la pepsine n’est active qu’à pH acide, pour pH ≤ 5, avec un maximum d’activité autour de pH 2 pour beaucoup de substrats 2.Elle est également représentée comme un diacide 3, ce qui semble vrai lorsqu’elle agit sur l’hémoglobine, substrat de référence pour mesurer son activité. Ce comportement en fonction du pH n’a pas été validé sur des mélanges de protéines alimentaires telles que les caséines, principales protéines du lait, ou les protéines du blanc d’œuf. L’objectif principal de cette étude est de comparer les profils d’activité de la pepsine en fonction du pH sur les caséines du lait de vache (CA) et les protéines du blanc d’œuf (BO).Matériel et méthodes: Les cinétiques d’hydrolyse de protéines de BO entier et de micelles de CA par la pepsine, à différents pH compris entre 1 et 8, ont été suivies au pH-stat pendant 2h, avec quantification du degré d’hydrolyse final (DH) par la méthode OPA. Les vitesses initiales ont été calculées à partir de ces cinétiques entre 0 et 5 minutes. Les concentrations en substrat et en pepsine étaient les mêmes pour chaque expérience, et l’environnement salin était celui du protocole INFOGEST 4. Les CA se présentent en structures supramoléculaires, les micelles, qui précipitent à pH acide. Une étape d’acidification à pH 4 des CA a permis d’étudier des suspensions d’agrégats tous formés dans les mêmes conditions. L’aspect et la taille de ces agrégats ont été observés par microscopie confocale et par morpho-granulométrie.Résultats et Analyse statistique: Les cinétiques d’hydrolyse obtenues suivent, dans leur majorité, des profils de type loi de puissance en fonction du temps. Pour les protéines de BO, le DH final est maximal à pH1 (14,4%) et diminue de manière linéaire en fonction du pH,jusqu’à atteindre des valeurs négligeables pour les pH ≥6. A l’inverse, les valeurs de DH finaux sur les CA ne sont pas significativement différentes sur la gamme de pH 1 à 5 (7,5 ± 1,4 %) puis diminuent à partir de pH 6 et atteignent des valeurs négligeables à pH 8.Le rapport entre la vitesse d’hydrolyse initiale et le DH final des CA est supérieur à celui des protéines du BO quel que soit le pH, suggérant une cinétique d’hydrolyse plus rapide pour les CA.Ces résultats montrent que les caséines du lait,dont le pH se situe un peu au-dessus de 6, n’ont pas besoin d’être longuement acidifiées pour que l’hydrolyse par la pepsine puisse commencer dans l’estomac, atteignant une activité quasi optimale de la pepsine dès pH 5.Conclusion: Les cinétiques d’hydrolyse par la pepsine en fonction du pH diffèrent donc entre un substrat de type protéines globulaires comme l’œuf, et les caséines du lait. Le profil d’activité de la pepsine en fonction du pH est également différent de celui du standard hémoglobine. En outre, contrairement à ce qui est généralement admis, la pepsine peut hydrolyser certains substrats à des pH plus hauts que pH 5.

Mathematical modelling of food digestion. STLOpendays

Les dernières décennies ont vu une augmentation spectaculaire, à l’échelle internationale,des pathologies liées à l’aliment et à l’alimentation. Le surpoids et ses conséquences tellesque les maladies cardiovasculaires ou le diabète de type 2 touchent environ 1,5 milliard d’adultes dans le monde et la plupart des prévisions prévoient que ce chiffre atteindra 3 milliards d’individus à l’horizon 2030. On estime à près de 3 millions le nombre de personnes décédant chaque année du fait de leur surpoids ou de leur obésité. En France, 6,5 millions de personnes sont considérées comme obèses. La proportion des personnes obèses est ainsi passée de 8,5 % à 14,5 % entre 1997 et 2009. L’augmentation de la prévalence est observée dans toutes les tranches d’âge de la population, y compris chez les séniors, avec une exacerbation chez les femmes (15,1 %) par rapport aux hommes (13,9 %). L’augmentation de l’obésité est majoritairement liée à (1) une plus grande consommation d’aliments très caloriques riches en graisses et (2) un manque croissant d’activité physique en raison de la nature de plus en plus sédentaire de nombreuses formes de travail, de l’évolution des modes de transport et de l’urbanisation. Malgré de multiples campagnes de sensibilisation, force est de constater qu’il est difficile de contraindre les personnes en surpoids à accroître leur activité physique. Une des solutions possibles serait donc de proposer au consommateur une offre alimentaire moins calorique, tout en présentant les mêmes propriétés de rassasiement et de satiété.

The most common hypothesis to explain the influence of food structure on digestion rates is that the proteolysis may be first limited in the stomach by the pepsin activity within the protein structures, as formed during food processing and possibly modified during gastric digestion. It is known that the gastric juice penetrates the food matrix and assists in digestion, but how the acidity and enzyme of the gastric juice affect disintegration of food is still far from being fully understood. There is, indeed, a real lack of methods that allow monitoring the evolution of food micro and nanostructure in the course of digestion. A novel time-lapse synchrotron deep-UV microscopy methodology was developed that made use of the natural tryptophan fluorescence of proteins. It enabled the monitoring in situ of the microstructural changes of protein small gel particles ( 1mm) during simulated gastric digestion. Two dairy gels with an identical composition, but differing by the coagulation mode, were submitted to static in vitro gastric digestion. The kinetics of gel particle breakdown were quantified by image analysis and physico-chemical analyses of digesta. In parallel, the fluorescence recovery after photobleaching (FRAP) technique in confocal microscopy was adapted to investigate the diffusion behaviour of fluorescently labelled pepsin in casein gels of increasing protein concentrations, allowing to form protein networks with different microstructures. The results confirm the tendency of rennet gels, but not acid gels, to form compact protein aggregates under acidic conditions of the stomach. Consequently, the kinetics of proteolysis were much slower for the rennet gel, confirming the hypothesis of a reduced pepsin accessibility to its substrate. Indeed, pepsin diffusivity was quantified (by FRAP) as significantly lower in stronger gels, because of the increasing steric hindrance in the gel microstructure as the protein concentration increased. Moreover, the proteolysis kinetics of the gel particles followed an exponential trend, and while the particle size decreased progressively, their morphology remained unchanged, suggesting that external erosion was the predominant mechanism of the enzymatic breakdown of dairy gels in these experimental conditions.

The tools developed in soft matter physics are of particular interest to characterize food micro- and nanostructures and monitor how they evolve under various conditions. For instance, X-ray scattering from dairy products has provided decisive knowledge on the internal structure of casein micelles, the main dairy protein, and on their response to industrial processing (de Kruif ACIS 2012).Food structure can modify the release of nutrients during digestion, and should therefore be considered when evaluating nutritional properties or related health effects. In vivo experiments on mini-pigs showed that enzymatic dairy gels (curd-type, made with rennet) led to a remarkable slowdown of the kinetics of amino-acid absorption, compared to milks and acid dairy gels (yogurt type) of identical composition (Barbe Food Res. Int. 2014). This is attributed to delayed gastric retention arising from acid-induced rearrangements of the protein network of enzymatic gels. Results obtained by SAXS at synchrotron SOLEIL on SWING beamline show, at a nanometer scale, the structural evolution of the gels under in vitro gastric digestion. The evolution of the SAXS spectra during digestion is very informative. -calcium-phosphate nanoclusters, which provide a well-defined signature in the 0.03-0.15 Å-1 range (size ~ 2.5 nm), are immediately dissolved upon HCl addition. This is the main reason why enzymatic gels, but not acid gels (for which nanoclusters are already dissolved), undergo important microstructural rearrangements upon acidification.- one can follow quantitatively how the large signal from casein particles at low q progressively disappears during protein hydrolysis. - a signature appears suggesting that molecular edifices of few nanometers, which could result from peptides self-assemblies, are created during digestion.Finally, the evolution at large distances (scattering at low q), is slower for rennet gels than for acid gels, showing a structure effect on digestion kinetics.

Gastric digestion of food is the result of physical disintegration and enzymatic reactions leading to the release of nutrients. Depending on the gelation conditions, food protein networks with specific structural and rheological features can be formed. Several in vivo and in vitro studies have shown an influence of the macro- and microstructure on the kinetics of milk protein hydrolysis. Despite growing interest to modulate food structure to potentially control digestion rates, little is known concerning the fundamental mechanisms by which the structure of dairy gels affects the digestion kinetics.An original time-lapse synchrotron deep-UV microscopy method was developed using the natural tryptophan fluorescence of proteins. It allowed monitoring in situ the microstructural evolution of protein gels during simulated gastric digestion. Two dairy gels of identical composition but different coagulation modes (acid and rennet) were submitted to static in vitro digestion with the aim of decoupling the acidification process from the enzymatic action of pepsin. Automated analysis procedures of the collected images allowed to model the kinetics of disintegration of sub-millimetric gel pieces. Results confirmed the propensity of rennet gels, but not acid gels, to undergo large microstructural modifications and form compact protein aggregates (syneresis) under acidic conditions. Consequently, the kinetics of proteolysis was much slower for the rennet gel, reiterating the hypothesis of a pepsin accessibility reduced to the surface of solid casein substrates (Fig 1). Finally, the morphological shapes of particles were almost preserved during the full digestion process and the kinetics of disintegration could be fitted by a simple exponential model. Therefore, we concluded that the erosion mode is probably the predominant mechanism during the gastric digestion of dairy gels.