Investigating casein gel structure during gastric digestion using ultra-small and small-angle neutron scattering

Meltem Bayrak,Jitendra Mata,Jared K Raynes,Mark Greaves,Jacinta White,Charlotte E Conn,Juliane Floury,Amy Logan

doi:10.1016/j.jcis.2021.03.087

Abstract

This study aimed to understand the structural devolution of 10% w/w rennet-induced (RG) and transglutaminase-induced acid (TG) gels in H2O and D2O under in vitro gastric conditions with and without pepsin. The real-time devolution of structure at a nano- (e.g. colloidal calcium phosphate (CCP) and micelle) and micro- (gel network) level was determined using ultra-small (USANS) and small-angle neutron scattering (SANS) with electron microscopy. Results demonstrate that gel firmness or elasticity determines disintegration behaviour during simulated mastication and consequently the particle size entering the stomach. Shear of mixing in the stomach, pH, and enzyme activity will also affect the digestion process. Our results suggest that shear of mixing primarily results in erosion at the particle surface and governs gel disintegration behaviour during the early stages of digestion. Pepsin diffusivity, and hence action, occur more readily in the latter stages of gastric digestion via access to the particle interior. This occurs via the progressively larger pores of the looser gel network and channels created within the larger, less dense casein micelles of the RG gels. Gel firmness and brittleness were greater in the D2O samples compared to H2O, facilitating gel disintegration. Despite the higher strength and elasticity of RG compared to TG, the protein network strands of the RG gels become more compact when exposed to the acidic gastric environment with comparatively larger pores observed through SEM imaging. This led to a higher degree of digestibility in RG gels compared to TG gels. This is the first study to examine casein gel structure during simulated gastric digestion using scattering and highlights the benefits of neutron scattering to monitor structural changes during digestion at multiple length scales.

Highlights

The delivery of food nutrients and their benefits to the consumer are determined, to a large extent, by the microstructure of the food within which they are contained [75,80,82,95]
After 60 min gelation, the casein gels formed with Rennet-induced gel (RG) and prepared in both D2O and H2O were significantly firmer than transglutaminase-induced acid gel (TG), with G, around 6–8 times higher
Coupled with observations that show the formation of progressively smaller gel pieces in the stomach vessel over time, these results suggest the primary cause of TG degradation is due to surface erosion of the gel as opposed to protein hydrolysis and that the degree of digestion is proportional to the size and number of larger gel particles entering the stomach

Summary

Introduction

The delivery of food nutrients and their benefits to the consumer are determined, to a large extent, by the microstructure of the food within which they are contained [75,80,82,95]. The acid gel structure may be modified via the introduction of new covalent bonds [90], such as TG, created through the covalent crosslinking between glutamine and lysine residues of the aggregated casein proteins [30,84]. These different modes of casein gelation lead to the formation of unique nano- and microstructures which influence the functional properties of the gel, such as gel strength, elasticity, brittleness, resistance to shear and overall physical stability to phase separation [48,63,67,76,97,101]

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Discussion

Conclusion