Surface Roughness and Hydrodynamic Boundary Conditions

We report results of investigations of a high-speed drainage of thin aqueous films squeezed between randomly nanorough surfaces. A significant decrease in hydrodynamic resistance force as compared with predicted by Taylor's equation is observed. However, this reduction in force does not represents the slippage. The measured force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at the intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to be independent on the separation and/or shear rate. Our results disagree with previous literature data reporting very large and shear-dependent boundary slip for similar systems.Comment: Revised versio

Lubrication by biomacromolecules: mechanisms and biomimetic strategies

Biomacromolecules play a key role in protecting human biointerfaces from friction and wear, and thus enable painless motion. Biomacromolecules give rise to remarkable tribological properties that researchers have been eager to emulate. In this review, we examine how molecules such as mucins, lubricin, hyaluronic acid and other components of biotribological interfaces provide a unique set of rheological and surface properties that leads to low friction and wear. We then highlight how researchers have used some of the features of biotribological contacts to create biomimetic systems. While the brush architecture of the glycosylated molecules present at biotribological interfaces has inspired some promising polymer brush systems, it is the recent advance in the understanding of synergistic interaction between biomacromolecules that is showing the most potential in producing surfaces with a high lubricating ability. Research currently suggests that no single biomacromolecule or artificial polymer successfully reproduces the tribological properties of biological contacts. However, by combining molecules, one can enhance their anchoring and lubricating capacity, thus enabling the design of surfaces for use in biomedical applications requiring low friction and wear

Functional categorisation of dietary fibre in foods: Beyond 'soluble’ vs ‘insoluble’

© 2018 Elsevier Ltd Background: Diets rich in dietary fibre are associated with multiple health benefits, but there is often only a restricted understanding of the mechanisms underlying these associations. This limits the ability to select or design foods for specific nutritional purposes. Traditionally, the diverse physical and chemical forms of dietary fibre have only been categorised as either soluble or insoluble. Scope and approach: In this review, the physicochemical properties that have been proposed to be responsible for the biological functionality of dietary fibres in the digestive tract are summarised and classified. The extent to which these properties follow naturally from categorisation into soluble vs insoluble forms are then assessed. Based on this analysis, a new approach to functional categorisation of dietary fibres is proposed. Key findings and conclusions: The physicochemical properties of dietary fibre components that are relevant to digestive tract functionality can be classified under the headings of binding, structuring, and transport barriers. Major nutritional outcomes such as control of macronutrient digestion or the nature of residual digesta that are available for fermentation by the large intestinal microbiota depend on combinations of these physicochemical properties in ways which are not readily reflected by a soluble vs insoluble fibre definition. An alternative approach is proposed based on 2D mapping of dietary fibre materials as a function of molecule/particle size and local density. This effectively separates diverse fibre materials and can be linked semi-quantitatively with functionally-important properties

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 conceptualised by Hutchings and Lillford . The key innovation in their seminal 1988 paper was shifting the focus from intact food properties, measured in the lab, towards 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 short comings, 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

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

Texture perception can be conceptualized as an emergent cognitive response to several of the physical and chemical properties of a food. Contemporary oral processing research is focused on revealing the relationships between the sensory perceptions and the food properties, with the goal of enabling rational product design. One major challenge is the complexity of molecular and biocolloid interactions, underpinning even simple texture properties. Here, we will introduce the in vitro oral processing approach, which divides oral processing into discrete units of operation (first bite, comminution, granulation, bolus formation, and tribology) and then systematically investigates the material properties that govern each specific oral processing unit operation without the added complexity inherent to biological systems. We will describe how we used the approach to rationally design a low-fat potato chip 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 was validated by sensory assessment wherein panelists ranked the perceived oiliness of three different types of potato chips. The sensory results indicated that perceived oiliness was higher when the low- fat potato chip was supplemented with a 0.5% by weight topical coating (0.5% by weight 15% by weight PGPR in oil mixture) compared to the unaltered low-fat potato chip. The perceived difference in oiliness was found to correspond to in vitro transient friction of saliva in the presence and absence of PGPR. These results illustrate how dividing oral processing into distinct phases allows one to more readily align sensory and in vitro measures, allowing for integration of the two disciplines and more rational design when modifying macronutrients

Responsive polysaccharide-grafted surfaces for biotribological applications

The elucidation of biolubrication mechanisms and the design of artificial biotribological contacts requires the development of model surfaces that can help to tease out the cues that govern friction in biological systems. Polysaccharides provide an interesting option as a biotribological mimic due to their similarity with the glycosylated molecules present at biointerfaces. Here, pectin was successfully covalently grafted at its reducing end to a polydimethylsiloxane (PDMS) surface via a reductive amination reaction. This method enabled the formation of a wear resistant pectin layer that provided enhanced boundary lubrication compared to adsorbed pectin. Pectins with different degrees of methylesterification and blockiness were exposed to salt solutions of varying ionic strength and displayed responsiveness to solvent conditions. Exposure of the grafted pectin layers to solutions of between 1 and 200 mM NaCl resulted in a decrease in boundary friction and an increase in the hydration and swelling of the pectin layer to varying degrees depending on the charge density of the pectin, showing the potential to tune the conformation and friction of the layer using the pectin architecture and environmental cues. The robust and responsive nature of these new pectin grafted surfaces makes them an effective mimic of biotribological interfaces and provides a powerful tool to study the intricate mechanisms involved in the biolubrication phenomenon

How hydrocolloids can control the viscoelastic properties of acid-swollen collagen pastes

The interactions between proteins and polysaccharides are of considerable importance in the food industry. In this study, the effect of adding non-charged methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), medium (GM) and high (GH) molecular weight guar gum and negatively charged sodium carboxymethylcellulose (CMC) was investigated on the rheological and thermal properties of acid-swollen collagen pastes, as a function of collagen concentration. Dynamic frequency sweeps showed that the addition of the hydrocolloids, except for CMC, increased the storage modulus (G′) and loss modulus (G″) of collagen pastes at all collagen levels investigated. MC, HPMC, GH, and GM increased the collagen pastes' storage modulus to a similar extent. In contrast, the values of the loss moduli were found to be markedly different for each hydrocolloid. The loss factor (tan δ) showed that incorporating the hydrocolloids made the pastes less elastic than pure collagen pastes. The phase transition temperature of collagen measured during the temperature sweep tests was not affected by MC, HPMC, GH, and GM. However, CMC shifted the transition temperature of collagen to higher temperatures. Micro Differential scanning calorimeter (microDSC) results showed that the presence of MC, HPMC, and guar gums did not affect the denaturation temperature (Td) and enthalpy (ΔH). In contrast, the addition of CMC increased the denaturation temperature and denaturation enthalpy of collagen pastes. By fixing the hydrocolloids' viscosifying power and changing the collagen concentration, collagen pastes with different hydrocolloids show different viscoelastic properties

Instrumental characterization of xanthan gum and scleroglucan solutions: Comparison of rotational rheometry, capillary breakup extensional rheometry and soft-contact tribology

Xanthan gum and scleroglucan, two rod-like polysaccharide hydrocolloids, are compared using a wide range of instrumental techniques and methods: steady shear flow, small amplitude oscillatory shear, first normal stress difference, capillary break-up and soft-contact tribology. The aqueous solutions of these two hydrocolloids with similar flow and viscoelastic profile show marked differences in capillary break-up time and apparent extensional viscosity. This result correlates with differences in first normal stress difference and, to a lesser extent, Stribeck curve behaviour. Formulating the hydrocolloids in concentrated sucrose solution (40 wt%) shifts relaxation profiles to longer times which greatly diminishes differences in rheological and lubrication behaviour. With exception of capillary break-up tests, no other methods showed statistically significant differences between the polysaccharides dissolved in the viscosified matrix. The toolbox of techniques is also applied to probe interactions of xanthan gum and scleroglucan with human whole saliva and bovine submaxillary mucin. We report no specific interactions between either hydrocolloid and salivary proteins and suggest that any cumulative effects must stem from specific sets of linear and non-linear rheological properties of saliva/hydrocolloid mixtures

Investigating the influence of pectin content and structure on its functionality in bio-flocculant extracted from okra

© 2020 The Author(s) Okra extract is known to have potential application as a bio-flocculant for wastewater treatment. However, no research to date has given insight into the components responsible for the flocculating ability of okra extract or its flocculating mechanism. The work presented here addresses this knowledge gap showing that pectin, especially pectin homogalacturonan (HGA) regions, appear to be the polysaccharides responsible for the flocculating ability of okra extract. The way pectin works in flocculation may be best explained by a polymer bridging mechanism. Specifically, a linear relationship between okra bio-flocculating ability and pectin homogalacturonan region to rhamnogalacturonan-I region weight ratio (HGA/RG-I) was found (y = 2.0x+47.6, R2 = 0.93, when GalA content > 300 mg/g extract), which was also validated using commercial citrus peel pectin

Development of a separated-dough method and flour/starch replacement in gluten free crackers by cellulose and fibrillated cellulose

Two strategies were combined and applied in this study to achieve a desired structure and texture of gluten free crackers and to reduce the calorie content. The first strategy is increasing structural heterogeneity of crackers and doughs and a separated-dough method was developed. A butter dough and a water dough were prepared separately and mixed together and the influence of mixing time was investigated. In the second strategy, which is the incorporation of a structuring material, powdered cellulose and fibrillated cellulose were incorporated in formulation to replace flour and pregelatinised starch with enhanced health benefits of low calorie and high fibre. Powdered cellulose played the role of the skeleton of the gluten free crackers. A laminar structure was observed in crackers when powdered cellulose was initially added to the butter dough. The crackers exhibit high thickness, hardness and fracturability and sharp sound emission which are typically observed in wheat crackers. Pregelatinised starch can be replaced by fibrillated cellulose at a lower addition level