Pinching dynamics, extensional rheology, and stringiness of saliva substitutes.

Saliva substitutes are human-made formulations extensively used in medicine, food, and pharmaceutical research to emulate human saliva's biochemical, tribological, and rheological properties. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, gleeking, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on measured values of shear viscosity. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response, which cannot be evaluated or anticipated from the knowledge of shear rheology response. In this contribution, we comprehensively examine the rheology of twelve commercially available saliva substitutes using torsional rheometry for rate-dependent shear viscosity and dripping-onto-substrate (DoS) protocols for extensional rheology characterization. Even though most formulations are marketed as having suitable rheology, only three displayed measurable viscoelasticity and strain-hardening. Still, these too, failed to emulate the viscosity reduction with the shear rate observed for saliva or match perceived stringiness. Finally, we explore the challenges in creating saliva-like formulations for dysphagia patients and opportunities for using DoS rheometry for diagnostics and designing biomimetic fluids.

Stabilization of myoglobin from different species (produced by cellular agriculture) using food-grade natural and synthetic antioxidants.

Cellular agriculture products, like myoglobin, are increasingly used by the food industry to provide desirable sensory properties to plant-based meat substitutes. This study elucidated the physicochemical properties and redox stability of myoglobin from both natural (equine) and cellular agriculture (bovine, sperm whale, and leopard) sources. The electrical characteristics and water-solubility of the different myoglobin samples were measured from pH 2.5 to 8.5. The isoelectric point of the myoglobin samples depended on the species, being pH 5.5 for equine, pH 4.5 for leopard and bovine, and pH 6.5 for sperm whale. All myoglobin samples had a solubility greater than 80% across the entire pH range studied. All myoglobin solutions appeared red and had two peaks in their UV-visible absorbance spectra after one day, which is consistent with oxymyoglobin formation. Equine myoglobin at pH 8 was selected to study its redox and color stability over time, where the oxymyoglobin oxidative status closely paralleled with the redness of the solutions. The effects of antioxidants (ascorbic acid, caffeic acid, catechin, gallic acid, quercetin, taxifolin, Trolox, and 4-methylcatechol) on the redox and color stability (redness) of the equine myoglobin (pH 8.0) was also studied. Antioxidants with low reduction potential values (ascorbic acid and quercetin) were particularly effective at enhancing the color stability of oxymyoglobin. The computational modeling study showed that amino acids on the myoglobin interacted with antioxidants through hydrogen bonds. The insights obtained may have important implications for the use of cellular agriculture to produce myoglobin for food applications.

Rheology and dispensing of real and vegan mayo: the chickpea or egg problem.

The rheology, stability, texture, and taste of mayonnaise, a dense oil-in-water (O/W) emulsion, are determined by interfacially active egg lipids and proteins. Often mayonnaise is presented as a challenging example of an egg-based food material that is hard to emulate using plant-based or vegan ingredients. In this contribution, we characterize the flow behavior of animal-based and plant-based mayo emulsions, seeking to decipher the signatures that make the real mayonnaise into such an appetizing complex fluid. We find that commercially available vegan mayos can emulate the apparent yield stress and shear thinning of yolk-based mayonnaise by the combined influence of plant-based proteins (like those extracted from chickpeas) and polysaccharide thickeners. However, we show that the dispensing and dipping behavior of egg-based and vegan mayos display striking differences in neck shape, sharpness, and length. The ratio of apparent extensional to shear yield stress value is found to be larger than the theoretically predicted square root of three for all mayo emulsions. The analysis of neck radius evolution of these extension thinning yield stress fluids reveals that even when the power law exponent governing the intermediate pinching dynamics is similar to the exponent obtained from the shear flow curve, the terminal pinching dynamics show strong local effects, possibly influenced by interstitial fluid properties, finite drop size and deformations, and capillarity.

Ultrathin Micellar Foam Films of Sodium Caseinate Protein Solutions.

Sodium caseinates (NaCas), derived from milk proteins called caseins, are often added to food formulations as emulsifiers, foaming agents, and ingredients for producing dairy products. In this contribution, we contrast the drainage behavior of single foam films made with micellar NaCas solutions with well-established features of stratification observed for the micellar sodium dodecyl sulfate (SDS) foam films. In reflected light microscopy, the stratified SDS foam films display regions with distinct gray colors due to differences in interference intensity from coexisting thick-thin regions. Using IDIOM (interferometry digital imaging optical microscopy) protocols we pioneered for mapping nanotopography of foam films, we showed that drainage via stratification in SDS films proceeds by the expansion of flat domains that are thinner than surrounding by a concentration-dependent step-size, and nonflat features (nanoridges and mesas) form at the moving front. Furthermore, stratifying SDS foam films show stepwise thinning, such that the step-size and terminal film thickness decrease with concentration. Here we visualize the nanotopography in protein films with high spatiotemporal resolution using IDIOM protocols to address two long-standing questions. Do protein foam films formulated with NaCas undergo drainage via stratification? Are thickness transitions and variations in protein foam films determined by intermicellar interactions and supramolecular oscillatory disjoining pressure? In contrast with foam films containing micellar SDS, we find that micellar NaCas foam films display just one step, nonflat and noncircular domains that expand without forming nanoridges and a terminal thickness that increases with NaCas concentration. We infer that the differences in adsorbing and self-assembling unimers triumph over any similarities in the structure and interactions of their micelles.

Flow-induced alignment of protein nanofibril dispersions.

HYPOTHESIS: Protein nanofibrils (PNF) resulting from the self-assembly of proteins or peptides can present structural ordering triggered by numerous factors, including the shear flow. We hypothesize that i) depending on the contour length of the PNF and the magnitude of the shear rate applied to the PNF dispersion, they exhibit specific orientation, and ii) it is possible to predict the alignment of PNF by establishing a flow-alignment relationship. Understanding such a relationship is pivotal to improving the fundamental knowledge and application of fibril systems. EXPERIMENTS: We use ß-lactoglobulin PNF aqueous dispersions with different average contour lengths but equal persistence lengths. We employ simple shear-dominated microfluidic devices with state-of-the-art imaging techniques: flow-induced birefringence (FIB) and micro-particle image velocimetry (µ-PIV), to probe the effect of shear flow on PNF alignment. FINDINGS: We provide an empirical relationship connecting the birefringence Δn (quantifying the extent of PNF alignment), and the Péclet number Pe (correlating the shear rate of the flow relative to the rotational diffusion of PNF) to understand the flow-alignment behavior of PNF under shear-dominated flows. Furthermore, we assess the alignment and flow profile of PNF at both high and low flow rates. The length of PNF emerges as a controlling parameter capable of modulating PNF alignment at specific shear rates. Our results shed new insights into the hydrodynamic behavior of PNF, which is highly relevant to various industrial processes involving the fibril systems.

Effects of beverage carbonation on lubrication mechanisms and mouthfeel.

The perception of carbonation is an important factor in beverage consumption which must be understood in order to develop healthier products. Herein, we study the effects of carbonated water on oral lubrication mechanisms involved in beverage mouthfeel and hence taste perception. Friction was measured in a compliant PDMS-glass contact simulating the tongue-palate interface (under representative speeds and loads), while fluorescence microscopy was used to visualise both the flow of liquid and oral mucosal pellicle coverage. When carbonated water is entrained into the contact, CO2 cavities form at the inlet, which limit flow and thus reduce the hydrodynamic pressure. Under mixed lubrication conditions, when the fluid film thickness is comparable to the surface roughness, this pressure reduction results in significant increases in friction (>300% greater than under non-carbonated water conditions). Carbonated water is also shown to be more effective than non-carbonated water at debonding the highly lubricious, oral mucosal pellicle, which again results in a significant increase in friction. Both these transient mechanisms of starvation and salivary pellicle removal will modulate the flow of tastants to taste buds and are suggested to be important in the experience of taste and refreshment. For example this may be one reason why flat colas taste sweeter.

Improvement of emulsifying behavior of pea proteins as plant-based emulsifiers via Maillard-induced glycation in electrospun pea protein-maltodextrin fibers.

Heat-treated electrospun pea protein isolate (PPI)-maltodextrin fibers containing glycated PPI were analyzed for their interfacial tension and emulsifying properties compared to unheated electrospun PPI-maltodextrin fibers. Interfacial tension at the oil-water-interface of the heated fibers was higher (19.2 ± 0.1 mN m-1) compared to the unheated fibers (16.3 ± 1.4 mN m-1) due to the covalently bound hydrophilic maltodextrin in the glycoconjugates. Applied in oil-in-water emulsions (10% w/w oil, 0.7% protein, 103.4 MPa, 3 passes), unheated PPI-maltodextrin fibers produced large droplets (72-259 µm) with multimodal distributions in the pH range of 2-7. The largest droplet size was at pH 4, which was around the pI of PPI. Emulsions were also prone to flocculation, which was most probably caused by a depletion flocculation mechanism due to an excess of maltodextrin in the aqueous phase. In contrast, emulsions made with heated PPI-maltodextrin fibers were monomodal (36-55 µm) at pH 2-7 and only showed a minor increase in droplet size close to the pI of PPI. The improved properties of heated PPI-maltodextrin fibers were ascribed to the enhanced steric repulsion caused by the covalently bound maltodextrin. The results indicate that Maillard-induced glycation of PPI with maltodextrin in electrospun fibers can be used as a novel method to improve the properties of PPI as a plant-based emulsifier.

The role of saliva in oral processing: Reconsidering the breakdown path paradigm.

We discuss food oral processing research over the last two decades and consider strategies for quantifying the food breakdown model, originally conceptualized by Hutchings and Lillford. The key innovation in their seminal 1988 paper was shifting the focus from intact food properties, measured in the lab, toward strategies to capture the dynamic nature of eating. This has stimulated great progress in the field, but a key aspect missing in oral processing research is the conversion of the Hutchings and Lillford breakdown path conceptual model into quantifiable parameters considered in the context of physiological factors such as saliva and oral movements. To address these shortcomings, we propose the following analysis: Hutchings's and Lillford's definitions of "Structure" and "Lubrication" are incomplete and they comprise many and varied physicochemical properties. We offer, here, a deeper analysis of each parameter, and propose strategies for researchers to consider in their quantification as an update of the Hutchings and Lillford Breakdown path.

Ring Shear Tester as an in-vitro testing tool to study oral processing of comminuted potato chips.

Oral processing of solid foods is an extremely dynamic and complicated activity that involves multiple processes in tandem such as comminution, mixing, dilution, hydration and enzymatic breakdown that gradually transform the food from a morsel or a bite to a bolus that is ready for swallowing. It is hypothesised that just after "first bite" and initial particle reduction and hydration of solid brittle foods, the response to deformation of food particles is analogous to studies on the flowability and cohesion of wetted powders, which are effectively characterised using a Ring Shear Tester (RST). We examine this hypothesis and determine whether the RST measures properties of solid snack foods (potato chips or crisps, PCs) that are relevant to their dynamic sensory response, which includes capturing the effect of hydration on comminuted PCs. The RST is found to differentiate PCs obtained from different manufacturing sources (e.g. baked versus fried), and its measurements of cohesion and friction can be considered in context of the structure and composition of the PCs as well as oral processing. Remarkably, RST measurements for this small set of PC samples correlate with several sensory attributes that arise during mastication, which includes Sharpness and Ease of Clearance. This study highlights the potential of the RST as a new tool for oral processing research.

Enabling the Rational Design of Low-Fat Snack Foods: Insights from In Vitro Oral Processing.

Texture perception is conceptualized as an emergent cognitive response to food characteristics that comprise several physical and chemical properties. Contemporary oral processing research focuses on revealing the relationship between the sensory perceptions and food properties, with the goal of enabling rational product design. One major challenge is associated with revealing the complex molecular and biocolloidal interactions underpinning even simple texture percepts. Here, we introduce in vitro oral processing, which considers oral processing in terms of discrete units of operation (first bite, comminution, granulation, bolus formation, and tribology). Within this framework, we systematically investigate the material properties that govern each specific oral processing unit operation without being impacted by the biological complexity of the oral environment. We describe how this framework was used to rationally design a low-fat potato chip with improved sensory properties by investigating the impact from adding back, to a low-fat potato chip, a small amount of oil mixed with the surface-active agent polyglycerol polyricinoleate (PGPR). The relevance of instrumental measures is validated by sensory assessment, whereby panelists ranked the perceived oiliness of three different types of potato chips. The sensory results indicate that perceived oiliness was higher when a low-fat potato chip was supplemented with an additional 0.5% (w/w) topical coating (the coating comprised 15%, w/w, PGPR in oil) compared to the unaltered low-fat potato chip. The perceived difference in oiliness is hypothesized to correspond to the dynamic friction measured in vitro with a saliva-coated substrate in the presence and absence of PGPR. The study illustrates how dividing oral processing into distinct units provides a rational approach to food product design focused on controlling key sensory attributes.

Formation of Whey Protein Isolate (WPI)-Maltodextrin Conjugates in Fibers Produced by Needleless Electrospinning.

Glycation of proteins via the first stage of the Maillard reaction is capable of improving their stability but not economically feasible yet. This work reports the glycation of whey protein isolate (WPI) with maltodextrin at a high yield after heating electrospun fibers made from the reactants. Glycoconjugates were characterized by Fourier transform infrared spectroscopy (FTIR) and SDS-PAGE. The binding ratio between WPI and maltodextrin was assessed via the free amino groups. The molecular weight of the conjugates and the reaction yield were studied by size exclusion chromatography. The impact of different mass ratios between WPI and maltodextrin in the fibers (5:95, 10:90, 20:80, and 25:75 w/w) was investigated. With increasing WPI content, the binding ratio of maltodextrin decreased from ∼2.1 to ∼1.2. Preferably small polysaccharides (2-13 kDa) from the maltodextrin reacted. Protein specific reaction yields of up to 44.52 ± 7.46% w/w were demonstrated in all WPI-maltodextrin fibers after heating.

The impact of the molecular weight of dextran on formation of whey protein isolate (WPI)-dextran conjugates in fibers produced by needleless electrospinning after annealing.

The conjugation reaction of electrospun fibers of a mixture of whey protein isolate (WPI) and dextran using different molecular weights (40, 70, and 100 kDa) and mixing ratios was studied. This study includes the electrospinnability of a mixture of WPI and dextran, and the conjugation reaction between them via the initial stage of the Maillard reaction. The WPI-dextran fibers were characterized using optical and transmission electron microscopy. The covalent attachment of dextran to WPI was confirmed using sodium-dodecyl-sulfate-polyacrylamide gel-electrophoresis with protein and glycoprotein staining. Both 70 and 100 kDa of dextran and WPI at mixing ratios of 2 : 1 and 3 : 1 in phosphate buffer (30 mM, pH 6.5) were electrospun using needleless electrospinning. The solution concentration of the mixture was 50 wt% (33.3/37.5 wt% for dextran/16.5/12.5 wt% for WPI). The optimal conjugation conditions chosen from the experiments were a mixture of dextran (70 kDa)-WPI at 3 : 1 (75% relative humidity, 60 °C, 48 h).

Influence of hydration and starch digestion on the transient rheology of an aqueous suspension of comminuted potato snack food.

Oral processing of most foods is inherently destructive: solids are broken into particles before reassembly into a hydrated bolus while salivary enzymes degrade food components. In order to investigate the underlying physics driving changes during oral processing, we capture the transient rheological behaviour of a simulated potato chip bolus during hydration by a buffer with or without α-amylase. In the absence of amylase and for all oil contents and solids weight fractions tested, we find a collapse of the transient data when graphed according to simple Fickian diffusion. In the presence of amylase, we find effects on the transient and pseudo steady state bolus rheology. Within the first minute of mixing, the amylase degrades only ≈6% of the starch but that leads to an order of magnitude reduction in the bolus elasticity, as compared to the case without amylase. Thus, for an in vitro bolus, only a small amount of starch needs to be digested to have a large impact on the bolus rheology very soon after mixing.

Influence of Cosolvent Systems on the Gelation Mechanism of Globular Protein: Thermodynamic, Kinetic, and Structural Aspects of Globular Protein Gelation.

How thermostability and gelation of globular protein are affected by cosolvent systems present in food systems is critical to understanding their functionality. The expression of these functional attributes depends on the molecular structure and thermal-mechanical history of the protein, as well as its chemical environment. To improve the design of processing protein-containing food systems, one must fully understand the thermodynamic, kinetic, and structural impact of cosolvent on globular protein gelation. This review focuses on the impact of weakly interacting neutral cosolvent systems (for example, sugars and polyols) on the gelation of globular proteins. The physicochemical mechanisms by which these cosolvent systems can modulate protein gelation are highlighted from a thermodynamic, kinetic, and structural point of view.

Combined influence of NaCl and sucrose on heat-induced gelation of bovine serum albumin.

The combined influence of a strongly interacting cosolvent (NaCl) and a weakly interacting cosolvent (sucrose) on the heat-induced gelation of bovine serum albumin (BSA) was studied. The dynamic shear rheology of 4 wt % BSA solutions containing 0 or 20 wt % sucrose and 0-200 mM NaCl was monitored as they were heated from 30 to 90 degrees C at 1.5 degrees C min(-)(1), held at 90 degrees C for 120 min, and then cooled back to 30 degrees C at -1.5 degrees C min(-)(1). The turbidity of the same solutions was monitored as they were heated from 30 to 95 degrees C at 1.5 degrees C min(-)(1) or held isothermally at 90 degrees C for 10 min. NaCl had a similar effect on BSA solutions that contained 0 or 20 wt % sucrose, with the gelation temperature decreasing and the final gel strength increasing with increasing salt concentration and the greatest changes occurring between 25 and 100 mM NaCl. Nevertheless, the presence of sucrose did lead to an increase in the gelation temperature and final gel strength and a decrease in the final gel turbidity. The impact of NaCl on gel characteristics was attributed primarily to its ability to screen electrostatic interactions between charged protein surfaces, whereas the impact of sucrose was attributed mainly to its ability to increase protein thermal stability and strengthen the attractive forces between proteins through a preferential interaction mechanism.