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Structural properties of diluted alkali-soluble pectin from Pyrus communis L. in water and salt solutions

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The self-assembly and gelation of low-methoxyl diluted alkali-soluble pectin (LM DASP) from pear fruit (Pyrus communis L. cv. Conference) was studied in water and salt solutions (NaCl and CaCl2, constant ionic strength) without pH adjustment at 20 ◦C.

Carbohydrate Polymers 273 (2021) 118598 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Structural properties of diluted alkali-soluble pectin from Pyrus communis L in water and salt solutions ´ ska , Piotr Pieczywek , Monika Szyman ´ska-Chargot , Jolanta Cie´sla *, Magdalena Koczan Justyna Cybulska , Artur Zdunek Institute of Agrophysics, Polish Academy of Sciences, Do´swiadczalna 4, 20-290 Lublin, Poland A R T I C L E I N F O A B S T R A C T Keywords: Cation effect Diluted alkali-soluble pectin Dynamic light scattering Gel point Pear fruit pectin Self-assembly The self-assembly and gelation of low-methoxyl diluted alkali-soluble pectin (LM DASP) from pear fruit (Pyrus communis L cv Conference) was studied in water and salt solutions (NaCl and CaCl2, constant ionic strength) without pH adjustment at 20 ◦ C The samples at different LM DASP concentrations were characterized using rheological tests, Fourier-transform infrared spectroscopy, dual-angle dynamic light scattering and atomic force microscopy LM DASP from pear fruit (Pyrus communis L.) showed gelling ability The indices (aggregation index and shape factor) based on light scattering may be useful for the characterization of structural changes in polysaccharide suspension, particularly for the determination of a gel point The results obtained may be important for the food, cosmetic and pharmaceutical industries where pectin is used as a texturizer, an encap­ sulating agent, a carrier of bioactive substances or a gelling agent Introduction Pectin is an important component of the plant cell wall which affects the texture of fruits (Paniagua et al., 2014) Three associated poly­ saccharides are the main components of this biopolymer: the linear homogalacturonan (HG) which consists of (1–4)-α-D-galacturonosyl units with different degrees of methyl-esterification; type I rhamnoga­ lacturonan where the (1–2)-α-L-rhamnopyranosyl units in the backbone of the molecule have side branches which contain the (1–5)-α-L-arabi­ nofuranosyl or/and (1–4)-β-D-galactopyranosyl residues; and type II rhamnogalacturonan, the backbone of which is composed of at least eight (1–4)-α-D-galacturonosyl units and twelve different types of sugars with dozens of different linkages being present in branched chains (Schols & Voragen, 1994; Schols, Voragen, & Colquhoun, 1994) Pectin is a safe, readily-available and relatively inexpensive biopolymer It is a very functional material that is used as a prebiotic carrier, an encapsulating agent, a texturizer and a component of com­ posites for 3D printed food in the food industry, a carrier of active substances in the drug delivery systems, a binder of radioactive com­ pounds, a sorbent of metals (medicine and water purification) as well as a tissue scaffold for tissue engineering (Moslemi, 2021) The main source of commercial pectin are citrus fruits and apples, and which is wellknown to form a gel (Moslemi, 2021) However, the pear (Pyrus communis L.) fruit which contains over 14% wt of carbohydrates (Itai, 2007) may be an alternative to those which are commonly used Pyrus communis L is one of the main commercial species in Europe, North and South America, Africa and Australia (Food and Agriculture Organization of the United Nations, 2019) Pectin extracted from the Pyrus communis L fruit has not been thoroughly investigated till date, and this includes its behaviour in aqueous dispersion in the presence of various cations The sequential extraction of polysaccharides from the plant cell wall provides an opportunity to obtain the pectin fractions soluble in various liquids, e.g water-soluble, chelator (cyclohexanetrans-1,2-diamine-N,N, N′ ,N′ -tetraaceate; CDTA)-soluble, or sodium carbonate-soluble pectin (also known as diluted alkali-soluble pectin, i.e DASP) (Gawkowska, Cybulska, & Zdunek, 2018) These fractions differ in the chemical structure of their constituent macromolecules (Pos´e, Kirby, Mercado, ´ ska-Chargot & Zdunek, 2013; Zdunek, Morris, & Quesada, 2012; Szyman Kozioł, Pieczywek, & Cybulska, 2014) and also in their physicochemical properties (Zhu et al., 2018, 2017) Moreover, the composition of the cell wall depends on the stage of the growth and development of the plant and the plant organs The amount of water- and chelator-soluble pectin is reported to increase and that of DASP to decrease during ripening of Pyrus communis L., cv Blanquille This is accompanied by simultaneous decrease in the degree of methyl esterification (MartínCabrejas, Waldron, & Selvendran, 1994) DASP refers to the pectin * Corresponding author E-mail address: j.ciesla@ipan.lublin.pl (J Cie´sla) https://doi.org/10.1016/j.carbpol.2021.118598 Received March 2021; Received in revised form 18 August 2021; Accepted 19 August 2021 Available online 24 August 2021 0144-8617/© 2021 The Author(s) Published by Elsevier Ltd This is an (http://creativecommons.org/licenses/by-nc-nd/4.0/) open access article under the CC BY-NC-ND license J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 which is rich in rhamnogalacturonan I and is bound to the plant cell wall by ester linkages (Brummell, 2006; Pos´e, Kirby, Mercado, Morris, & Quesada, 2012) The macromolecules of DASP revealed the ability to form a network on mica It was shown previously that this fraction is important for maintaining the mechanical properties of the plant cell wall during the postharvest storage of pear fruit (Zdunek, Kozioł, Cybulska, Lekka, & Pieczywek, 2016; Zdunek, Kozioł, Pieczywek, & Cybulska, 2014) The DASP fraction from pear fruit is rich in arabinose For Pyrus communis, cv Barlett, the content of this sugar ranges from 59 to 84 mol% of neutral sugars, depending on the sunlight conditions during ripening as well as the conditions for postharvest storage (Raffo, Ponce, Sozzi., Vincente, & Stortz, 2011), whereas for cv S Bartolomeu it is about 50 mol% of cell wall sugars (Ferreira, Barros, Coimbra, & Delgadillo, 2001) The changes in the dimensions of the DASP macro­ molecules are dependent on the ripening stage, postharvest storage duration and condition of the fruit (Paniagua et al., 2014) The length of the molecules and their ability to form a network are reduced during postharvest storage due to the enzymatic degradation of polysaccharides (Cybulska, Zdunek, & Kozioł, 2015; Paniagua et al., 2014; Pieczywek, Cybulska, & Zdunek, 2020; Zdunek, Kozioł, Pieczywek, & Cybulska, 2014) The gelation of pectin in liquid media is one of the most important utilitarian attributes of this polysaccharide, which affects the use of pectin in the food, cosmetic and pharmaceutical industries According to IUPAC (International Union of Pure and Applied Chemistry) terminol­ ogy, gelation is a process of passing through the initial network forma­ tion (gel point) to form a chemical or physical polymer network (gel) which expands throughout the whole volume of the liquid Usually, the viscosity tending to infinity is an indicator that the gel point has been reached A network can be obtained due to crosslinking as well as through the physical aggregation of polymer chains Crosslinking is the chemical interaction of active sites or functional groups of macromole­ cules that leads to the formation of branching point for at least four chains in a macromolecule The aggregation of chains occurs due to formation of hydrogen bonds, ionic interactions, and hydrophobic in­ teractions (McNaught and Wilkinson, 1997) The spontaneous rear­ rangement of macromolecules into ordered superstructures dispersing in liquid, which occurs due to physical interactions, is defined as selfassembly In nature this process results in the development of biopolymer fibres and cell membranes (Dahman, Caruso, Eleosida, & Hasnain, 2017) The process of pectin gelation is affected by many factors such as the chemical structure of the biopolymer (the molecular size, number and arrangement of the side chains and the degree of methyl-esterification), density of electrical charge on the macromolecule, pH and composition of the dispersing medium, temperature, etc (Moslemi, 2021) In the case of low-methoxyl (LM) pectin, to which DASP belongs, this process may occur: et al., 2015; Fang et al., 2008; Fraeye et al., 2010; Ventura, Jammal, & Bianco-Peled, 2013) The structural reorganization of macromolecules in a liquid is usuư ăm, Schuster, & Goh, ally characterized using rheological tests (Stro 2014), scanning electron microscopy (Basak & Bandyopadhyay, 2014) or atomic force microscopy (Pos´ e, Kirby, Mercado, Morris, & Quesada, 2012; Zdunek, Kozioł, Pieczywek, & Cybulska, 2014) The aggregation index (AI), which is based on the dynamic light scattering data, can be useful for description of the self-assembly of DASP in the water (Gaw­ kowska, Cie´sla, Zdunek, & Cybulska, 2019b) The AI is calculated by the subtraction of the mean hydrodynamic diameter which is determined using the back scattering (Zave,backward) from the mean hydrodynamic diameter which is determined using the forward scattering data (Zave, forward), and next, dividing the obtained difference by the mean hydro­ dynamic diameter determined from the back scattering (Zave,backward): ( )/ AI = Zave,forward − Zave,backward Zave,backward (1) AI was initially proposed for the determination of protein aggrega­ tion (Zetasizer Nano Application Note, 2010) In the case of the systems which are transparent to light and contain the self-assemblies, the value of AI is higher than zero For the ideal homogeneous systems, the light scattering is the same in all directions, and AI is Considering the polysaccharides dispersed in the liquid, AI equal to may reflect the regular three-dimensional distribution of macromolecules in the bulk of the liquid An increase in the concentration of the apple DASP in water makes AI values negative A value of − reflects the lack of light transmittance through the sample which corresponds a well networked structure (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b) Assuming that for the spherical particles or the regular three-dimensional distri­ bution of particles in the bulk of the liquid, the ratio of diameters ob­ tained on the basis of both the back and forward light scattering is equal to 1, a shape factor (SF) is proposed in the present work to determine the deviation from this ideal state SF is the ratio of a shorter mean hydro­ dynamic diameter to a longer one among those determined using the back and forward light scattering The closer the SF value is to 1, the rounder is the shape of the dispersed particles, or more homogeneous is three-dimensional structure formed by DASP In the case of diluted systems, low values of SF indicate the presence of elongated particles For the concentrated suspensions, the SF value tending to (the ‘loss’ of one dimension) corresponds to a decline in the sample transparency Therefore, AI = and SF = corresponds to the homogeneous threedimensional distribution of macromolecules/particles in the liquid, and the DASP concentration at AI = − and SF ~ reflects the gel point It was hypothesized that DASP originating from pear fruit is able to form a network in the liquid (spontaneously in water and with the participation of cations in salt solution) and the gel point can be deter­ mined using the indices based on the light scattering (i.e AI and SF) This work could establish DASP from Pyrus communis L fruit as a functional material for use in food, pharmaceuticals, and in environ­ mental engineering The investigations were performed over a wide range of DASP con­ centrations to observe and identify the changes in samples properties Rheological tests, Fourier transform infrared (FT-IR) spectroscopy, dy­ namic light scattering (DLS), and the analysis of images obtained from atomic force microscopy (AFM) were carried out A scheme of the in­ vestigations is shown in Fig S1 a) at a low pH when the acidic functional groups of the macromolecules are un-dissociated, the electrostatic repulsion between them is reduced, and the macromolecules can self-organize due to the for­ mation of hydrogen bonds (Capel, Nicolai, Duranda, Boulenguer, & Langendorff, 2006; Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019a; Yuliarti & Mardyiah Binte Othman, 2018), b) in the presence of monovalent cations when the neutralization of negative electrical charge of macromolecules reduces the intermo­ lecular repulsion (Fishman, Chau, Kolpak, & Brady, 2001; Wang et al., 2019; Wehr, Menzies, & Blamey, 2004; Yoo, Fishman, Savary, & Hotchkiss Jr., 2003), c) in the solution of divalent cations when crosslinking occurs due to the formation of both rod-like junction zones and point-like links between the pectin chains and the monocomplexes; the mechanism of gelation covers the monocomplexation of divalent cations by macromolecules and the ‘egg-box’ dimers formation by mono­ complexes without a clearly visible lateral association (Assifaoui Materials and methods 2.1 The DASP dispersions Lyophilized DASP was obtained (sequential extraction; fraction sol­ uble in 50 mM Na2CO3 and 20 mM NaBH4) from pear fruit (Pyrus ´jec, Poland) and characterized communis L., cultivar ‘Conference’, Gro J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 ´ ska, Pieczywek, Cybulska, & Zdunek, 2021) previously (Cie´sla, Koczan The water content in the lyophilized samples was ~8 wt.% The mo­ lecular weight of the DASP, which was determined using the static light scattering method, was 532 ± 11 kDa The degree of methylesterification (DM) was ~3% The lyophilized DASP contained about 70.77 ± 0.09 mg of Na and 3.62 ± 0.07 mg of Ca per g of dry sample ´ ska, Pieczywek, Cybulska, & Zdunek, 2021) The (Cie´sla, Koczan monosaccharide (mannose: 2.0 ± 0.5 mol%; rhamnose: 5.3 ± 0.0 mol%; glucose: 0.7 ± 0.2 mol%; galactose: 17.6 ± 0.1 mol%; xylose: 4.4 ± 0.1 mol%; arabinose: 34.1 ± 1.4 mol%; fucose: 0.5 ± 0.0 mol%) and uronic acid (galacturonic acid (GalA): 33.3 ± 1.6 mol%; glucuronic acid: 2.3 ± 0.3 mol%) content was determined using high-performance liquid chromatography (HPLC) The GalA (mol%) to rhamnose (mol%) ratio, pointing out to the contribution of homogalacturonans versus rhamno­ galacturonans, was In the case of DASP from Pyrus communis L cv De Cloche, this value was ~40 (Brahem, Renard, Gouble, Bureau, & Le Bourvellec, 2017) For the studied DASP, the ratio of the sum of arabi­ nose (mol%) and galactose (mol%) to the rhamnose content (mol%), corresponding to the hairy regions (degree of rhamnogalacturonan branching), was 10 The literature data show values ranging from (cv S Bartolomeu; Ferreira, Barros, Coimbra, & Delgadillo, 2001) to 8–32 (De Cloche and Barlett cultivars; Brahem, Renard, Gouble, Bureau, & Le Bourvellec, 2017; Raffo, Ponce, Sozzi., Vincente, & Stortz, 2011) A detailed description of the isolation and characterization of DASP from Pyrus communis L., cv Conference is placed in the Supplementary material The DASP dispersions (1.8 ⋅ 10− 4–1.8 ⋅ 100% w/v) in the ultrapure (MilliQ) water and the NaCl and CaCl2 solutions (ionic strength of 30 ´­ mM, corresponding to the previously studied systems; Cie´sla, Koczan ska, Pieczywek, Cybulska, & Zdunek, 2021) were prepared, mixed for 24 h (20 ◦ C) and then analysed concerning which FT-IR wavenumbers contribute the most to the sep­ aration of samples/observations 2.4 Determination of the particle size in dispersions Zetasizer Nano ZS (633 nm He–Ne laser light; Malvern Ltd., Mal­ vern, UK) was used to characterize the particle size of DASP in the liquid media at 20 ◦ C The results of dynamic back light scattering (173◦ ) were analysed by the apparatus software to determine the relaxation time (τ) (International Standard ISO 22412, 2017) The hydrodynamic diameter of particles was measured in six repli­ cations at the detection angle of 173◦ (back scattering) and 13.7◦ (for­ ward scattering) to calculate AI (Eq 1, Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b) and SF, i.e indices based on the results of dynamic light scattering A non-linear estimation with the least squares method was applied to describe the dependencies of AI and SF on the DASP concentration (Statistica 12, StatSoft, Cracow, Poland) The models which were best fitted to the experimental data were chosen 2.5 AFM analysis of the DASP samples The DASP dispersions (60 μl) were drop-deposited onto a freshly cleaved mica base of 10 × 10 mm (EMS, Hatfield, PA, USA) and distributed using a spin coater (SPS-Europe B.V., Midden Engweg 41, NL-3882 TS PUTTEN, The Netherlands) The air-dried samples were analysed at ambient temperature (20–22 ◦ C) and at the relative humidity of 26–30% A Multimode with a Nanoscope V controller (Bruker, Billerica, MA, USA) and automatic PeakForce Tapping mode (ScanAsyst) was applied A silicon pyramidal tip on a nitride cantilever (nominal radius: nm; nominal spring con­ stant: 0.4 N/m; Bruker) was used The scanning parameters were: the area of μm2 (aspect ratio 1:1, μm × μm), the resolution of 512 × 512 points and the linear velocity of 0.9 Hz For each sample, images were obtained and the heights of the AFM topographic images were analysed The AFM image processing steps are precisely described in the Supplementary material section and shown graphically in Fig S2 The height of the molecules was defined as the maximum value within a by pixel window around each sampling point The individual line sections were also characterized by their lengths The images of individual nonbranched and not intersecting objects were applied to calculate the persistence length (P) of the molecules using a measurement of the mean square of the end-to-end distance (R) as a function of distance along the chain contour (l) (Rivetti, Guthold, & Bustamante, 1996): ⎛ ⎞⎞ ⎛ 〈 2〉 2P l − ⎝1 − e 2P ⎠ ⎠ R = 4Pl⎝1 − (2) l 2.2 Rheological measurements The viscous behaviour of the DASP dispersions was investigated with a Discovery Hybrid Rheometer (TA Instruments, New Castle, DE, USA) using a plate-and-plate geometry (20 mm diameter, 0.8 mm gap) at 20 o C The measurements were carried out in triplicate at a shear rate ranging from 10− to 102 s− The Ostwald-de Waele (σ = b˙γ n ) and the Herschel-Bulkley (σ = b˙γ n + C) models (where b is the consistency index, n – is the flow index and C – is the yield stress) were used to describe the dependence of shear stress (σ) on shear rate (˙γ ) (Bourne, 2002) 2.3 Determination of the FT-IR spectra The FT-IR spectra of DASP dispersions (the same amount of each sample) were recorded by Nicolet 6700 FT-IR spectrometer with Smart Multi-Bounce HATR with a 10 reflection ZnSe crystal (Thermo Scienti­ fic, Waltham, MA, USA) The ultrapure (MilliQ) water spectrum served as a background The spectral range was 4000–650 cm− (resolution of cm− 1) and 200 scans were accumulated twice (20 ± ◦ C) Due to the high level of noise, which was visible on the spectra of lowconcentration dispersions, the next calculations and analyses were performed for selected samples (1.8 ⋅ 10− 3, 1.8 ⋅ 10− 2, 4.6 ⋅ 10− 2, 1.8 ⋅ 10− 1, 4.6 ⋅ 10− 1, 9.2 ⋅ 10− and 1.8 ⋅ 100% w/v) at a spectral range of 1800–900 cm− This wavenumber range containing the most valuable ´ ska, Szyman ´ ska-Chargot, & Zdunek, 2016; spectral information (Chylin ´ ska-Chargot, Chylin ´ ska, Kruk, & Zdunek, 2015) was used for Szyman PCA analysis Multivariate statistical analyses of the spectra data were performed using Unscrambler 10.1 (Camo Software AS., Norway) The NIPALS algorithm was used and the maximum number of components which explained the spectral variability of the samples was The result of the PCA analysis is the score plot which is a summary of the rela­ tionship between the observations (samples) and each data point rep­ resents a single spectrum and the loadings plot, which gives information For the DASP concentrations at which the surface of the mica was no longer visible nor the individual molecules, samples were described by their surface properties, namely surface roughness Ra (the arithmetical mean deviation of the assessed profile, Eq 3) and Rq (the root mean squared deviations of the profile, Eq 4), defined as (International Standard BS EN ISO 4287, 2000): Ra = ∑1 ∑ y=N− x=M− |z(x, y) − z | MN x=0 y=0 √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ √ ∑1 ∑ y=N− √ x=M− ̿ Rq = √ [z(x, y) − z ]2 MN x=0 y=0 (3) ̿ (4) where: z(x,y) – the height of the image at the x, y point; z – the average height of the image, and M, N – the total number of sampling points in the x and y directions The P, Ra, Rq and lengths of the objects were not applicable for the J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 characterization of the visual appearance of DASP with CaCl2 In these cases only the heights of the visible objects were calculated Above 1.8 ⋅ 10− 1% of DASP in CaCl2 solution the samples were too stiff to be dropdeposited on mica consistency index and to a simultaneous decrease in the flow index value For DASP in the CaCl2 solution an increase in the DASP content over 4.6 ⋅ 10− 2% (i.e a decrease in the Ca2+/COO− mole ratio below 8.2) resulted in a huge increase in the consistency index value with the decreasing value of flow index This was probably connected with a reduction in the distance between nanoparticles, changes in the nano­ particle structure, the possible formation of hydrogen bonds and Cabridges between macromolecules of adjacent nanoparticles and finally, the network formation At the highest concentration of DASP (1.8 ⋅ 100%) in the water, NaCl solution and CaCl2 solution there were ob­ tained the following values of consistency index: 21.28 ± 0.01, 54.33 ± 0.03 and 103,490 ± 10,620 mPa⋅sn, respectively Simultaneously, the flow index values were: 0.95 ± 0.01, 0.88 ± 0.00 and 0.19 ± 0.02 The increase in the consistency index and decrease in the flow index with the DASP concentration corresponded to the process of gelation Similar relationships were found for the aqueous dispersion of DASP from onion at a concentration which increased from 0.5 to 2.0% (Zhu et al., 2017) Other authors (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b; Karaki, Aljawish, Muniglia, Humeau, & Jasniewski, 2016; Stră om, Schuster, & Goh, 2014) pointed out that an increase in the content of LM pectin in the aqueous dispersion can lead to a significant increase in viscosity connected with weak gel formation The process started at a pectin concentration above 0.1% (Gawkowska, Ciesla, Zdunek, & Cybulska, ăm, Schuster, & Goh, 2014) or even 1.0% (Karaki, 2019b), 0.5% (Stro Aljawish, Muniglia, Humeau, & Jasniewski, 2016), depending on the pectin source and the pH conditions A high content of GalA in pectin resulted in the process of gel formation occurring in the acidic envi­ ronment without the presence of divalent cations (Gilsenan, Richardson, & Morris, 2000) 2.6 Statistical analyses The influence of DASP concentration and the dispersing medium composition on the selected properties of the studied samples was analysed using the two-factors ANOVA and post-hoc Tukey HSD test at the 0.05 significance level (Statistica 13.1, StatSoft, Cracow, Poland) Results and discussion 3.1 Rheological properties of DASP dispersions The relationship between the shear stress and the shear rate (10− 1–102 s− 1) obtained for the DASP dispersions in different media is shown in Fig S3 The Ostwald-de Waele and the Herschel-Bulkley models (Bourne, 2002) were fitted to the measurement data The re­ sults are summarized in Tables S1 and S2 All of the dispersions tested revealed the character of non-Newtonian liquids For most of the DASP dispersions at a low flow rate, the samples containing colloidal particles of polysaccharide behave as sticky liquids An increase in the flow rate reduces the interactions between macromolecules leading to thinning In the case of DASP concentrations ranging from 9.2 ⋅ 10− to 1.8 ⋅ 100% in the CaCl2 solution, shear thinning was observed over the full range of the applied shear rate, thereby revealing pseudo-plastic behaviour Probably with the increasing shear rate the network structure was rearranged into the DASP nanoparticles from which it was formed This reduced the frictional resistance that was noticed as the shear thinning The influence of DASP concentration on the values of both the con­ sistency and flow indices is graphically shown in Fig The effect of both the DASP content and the dispersing medium composition was visible Generally, for the low-concentration systems (≤1.8 ⋅ 10− 2%) the values of consistency index increased in the following order of dispersing media: CaCl2 < H2O < NaCl, whereas for the flow index it was: H2O = NaCl < CaCl2 This revealed that in the CaCl2 solution the salting-out process took place and the nanoparticles of DASP were pre­ sent in the liquid (Li, Liao, Thakur, Zhang, & Wei, 2018) Up to the DASP concentration of 1.8 ⋅ 10− 1%, the consistency and flow indices were constant for the dispersions in water and in NaCl solution Next, an in­ crease in the amount of DASP led to a significant increase in the 3.2 FT-IR spectra of the DASP dispersions FT-IR spectra were collected for DASP dispersed in water, NaCl so­ lution and CaCl2 solution (Fig S4) in order to obtain information about the functional groups of macromolecules and thence – about the inter­ molecular interactions A detailed description of the spectra is included in the Supplementary material In brief, in the case of pectin, the most striking feature is the region between 1800 and 1500 cm− which pro­ vides insight into the esterified (–COOCH3) and undissociated carboxyl groups (–COOH) (band at 1760–1730 cm− 1), non-esterified carboxyl groups and stretching vibration of the carboxylate ion (–COO− ) (1650–1550 cm− 1) (Filippov, 1972; Zhao et al., 2018) However, in the case of DASP, all these groups undergo de-esterification during the Fig The relationship between a) the consistency index, b) the flow index and concentration of diluted alkali-soluble pectin (DASP) of pear fruit (Pyrus communis L., cv Conference) which was obtained by fitting the Herschel-Bulkley model to the measurement data; different letters indicate significantly different results (twoway ANOVA and post-hoc HSD Tukey test, p < 0.05) J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 fraction separation in alkaline solution leading to a diminished band with the maximum at around 1740 cm− (Paniagua et al., 2017) The principal component analysis (PCA) is one of the most commonly used chemometric methods for data reduction and the exploratory analysis of high-dimensional data sets (Fig 2.) The score and loading plots are obtained as a result of the analysis The score plot presents sample grouping due to their spectral similarity, while the loading plot provides information concerning which wavenumbers have the greatest influence on samples scattering along the principal component (PC) axes The PCA analysis of the measured spectra was performed in the range of 900–1800 cm− since this region had the greatest influence on variability between the samples The PCA score plots obtained for dispersions in water and also the NaCl and CaCl2 solutions are presented in Fig 2a–c In the case of DASP in water, PC1 explained nearly 100% of the variability encountered (Fig 2a) Loadings related to the PC1 scores as a function of wave­ number presents the variance of FT-IR spectra (Fig 2e) Therefore, the scatter of points related to the samples alongside the PC1 axis reflects the changes in the DASP concentration In the case of DASP dispersions in the salt solutions, PC1 explained the majority of the variability observed (above 90%) The PC2 component explained only 1% (Fig 2b) and 8% (Fig 2c) of the variability for the NaCl and CaCl2 solutions, respectively As is the case with aqueous dispersion, the PC1 loadings reflected the spectral variance of the sample and the scatter of points alongside the PC1 axis reflected the changing content of DASP For DASP in the NaCl solution the samples were divided into two groups as regards the PC2 scores The positive influence had the PC2 loadings with the maximum at 1633 cm− which was probably connected to the solvated Na+ ions The wavenumbers at 1727, 1266, 1120 and 1077 cm− exhibited the negative influence Previously, the band at 1727 cm− was related to the acetylated carbonyl groups vibration and that at 1266 cm− was assigned to the stretching of (C–O–C) in acetyl ester (Fig 2f) (Syn­ ˇ kova ´, Matˇ ytsya, Copı́ ejka, & Machoviˇc, 2003) However, in this case, it is more likely to be due to the vibration of carbonyl groups with the attached metal ions The bands at 1120 and 1077 cm− can be assigned to the ring and side group vibrations (C–C), (C–OH), (C–H) (Synytsya, ˇ kova ´, Matˇ Copı́ ejka, & Machoviˇc, 2003) In the case of the PC2 scores obtained for DASP in the CaCl2 solution, the scatter is more visible – the points denoting samples with the DASP concentration ranging from 1.8 ⋅ 10− to 9.2 ⋅ 10− 1% formed a large group while those denoting the samples with 1.8 ⋅ 10− 3% and 1.8 ⋅ 100% of DASP were placed sepa­ rately The bands at 1102 and 1008 cm− were probably connected with the solvated Ca2+ ions, the uronic acid content and the backbone stretching (Fig 2g) Positive influence on the PC2 scores was exhibited by the wave­ numbers around 1734, 1465 and 1274 cm− which can be assigned to the esters of carboxyl groups While the esterification of carboxyl groups was impossible in this case, the most probable was that these wave­ numbers denoted the vibration of Ca2+ ions attached to them The PC1 (which explained 98% of variability) vs PC2 (1% of vari­ ability) of the PCA score plot of DASP dispersions in different media are shown in Fig 2d Three clusters can be separated The first one contains the highly-concentrated DASP samples (9.2 ⋅ 10− and 1.8 ⋅ 100% in H2O, 9.2 ⋅ 10− and in the NaCl solution, and 1.8 ⋅ 100% in the CaCl2 solution) The next two areas are placed close to each other but the samples can be divided into the separate groups containing the samples with a tendency towards intermolecular interactions (4.6 ⋅ 10− 1% in H2O, 1.8 ⋅ 10− and 4.6 ⋅ 10− 1% in NaCl, 4.6 ⋅ 10− 2, 4.6 ⋅ 10− and 9.2 ⋅ 10− 1% in CaCl2) and those in the form of a low-concentration dispersion (1.8 ⋅ 10− 3–1.8 ⋅ 10− 1% in H2O, 1.8 ⋅ 10− 3–4.6 ⋅ 10− 2% in NaCl, 1.8 ⋅ 10− and 1.8 ⋅ 10− 1% in CaCl2) The sample of 1.8 ⋅ 10− 3% DASP in the CaCl2 solution was an outlier with the most negative values of PC1 and PC2 The loadings related to both PC types are presented in Fig 2h Once again the PC1 loadings had only positive values and reflected the vari­ ance of spectra, which in turn is related to the concentration of DASP The PC2 loadings had both positive and negative values The greatest negative influence on the scattering of sample points along the PC2 had the wavenumbers: 1724–1726, 1465, 1379 cm− 1, which probably denoted the vibration of the carboxyl bands modified by ions (Ca2+ and Na+ (Schiewer & Balaria, 2009), and 1274 cm− related to the CH2 groups vibrations While the negative influence on the scores had the wavenumbers at 1623 cm− (very broad band, probably resulting from Ca2+ and Na+ ions binding to the carboxylic groups (Schiewer & Balaria, 2009), 1102 and 1017 cm− (vibration of (C–O), (C2–C3), (C2–O2), (C1–O1) in uronic acids in the galacturonic acid backbone) What is interesting, the points denoting DASP in water alone were scattered along the PC1 axis, but very close to of PC2 which means that this component did not have any influence over the variability of those samples The analysis of the FT-IR spectra showed that the DASP con­ centration modified the sample properties and that both the Na+ and Ca2+ interacted with the carboxylic groups of the GalA units The FTIR spectra for the three DASP concentrations (4.6 ⋅ 10− 2%, 4.6 ⋅ 10− 1% and 1.8 ⋅ 100%) representing the low-concentration dispersions and the systems where self-assembly and gelation occurred, respectively (Fig S5a–c), were analysed in order to highlight the differences between the samples At a given concentration the most striking differences were visible for the 1800–1500 cm− region In the case of low-concentration samples, the bands: 1729, 1625 and 1465 cm− were probably the result of Ca2+ and Na+ ions binding to the carbomethoxy groups (Fig S4a) (Guo, Duan, Wang, & Huang, 2014) While at 4.6 ⋅ 10− 1% (Fig S4b), the spectra of DASP in water and in the NaCl solution revealed a lower in­ tensity band at 1741 cm− than that at 1593 cm− in contrast to the spectrum of DASP in the CaCl2 solution where the band at 1593 cm− was barely visible In the case of 1.84 ⋅ 100% DASP (Fig S4c), the most noticeable detail was the lack of a 1741 cm− band for the dispersion in the CaCl2 solution (the weak band was present for the dispersions in H2O and the NaCl solution) It can be generalized that the carboxyl groups are strongly involved in the interactions between the macromolecules at the increasing DASP concentration In water, this is connected with their dissociation degree, whereas in the salt solutions — it is influenced by the interactions with the cations However, while Na+ can bind with one carboxylate group, Ca2+ can form intermolecular or intramolecular bonds This corresponds ´ ska, Pieczywek, well to the previously published data (Cie´sla, Koczan Cybulska, & Zdunek, 2021) 3.3 Structural properties of DASP dispersed in liquids 3.3.1 Characterization of the DASP suspension using indices based on light scattering The mean relaxation time obtained from the DLS method was affected (p < 0.05) by the DASP content in all of the systems studied (Fig 3a) Across the full range of DASP concentrations examined, its values were the highest in the CaCl2 solution, pointing out to the pres­ ence of particles larger than those in the water and Na+ solution For DASP in the salt solutions, the particle size did not change as the con­ centration ranged from 1.8 ⋅ 10− to 4.6 ⋅ 10− 2% A further increase in the concentration resulted in a significant extension in the relaxation time For the DASP in water, the most significant increase in the relax­ ation time started from the concentration of 1.8 ⋅ 10− 2% At the DASP concentration range of 4.6 ⋅ 10− 2–4.6 ⋅ 10− 1%, the relaxation time in the NaCl solution was shorter than that in water but this difference dis­ appeared with the further increase in concentration The smaller value of the hydrodynamic diameter (i.e a shorter relaxation time) in the NaCl solution as compared to water was also determined for the citrus pectin by Lima, Soldi, and Borsali (2009) (100 mM NaCl) and Schmidt, Schütz, and Schuchmann (2017) (85 mM NaCl), suggesting the dense packing of macromolecules in the presence of this salt An extension of the relax­ ation time accompanies gelation because the particle motion is hindered by intermolecular interactions and the network formation (Horne, Hemar, & Davidson, 2003) The aggregation index (AI) (Gawkowska, Cie´sla, Zdunek, & J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 Fig Results of the PCA analysis of the FT-IR spectra obtained for dispersions of diluted alkali-soluble pectin (DASP) of pear fruit (Pyrus communis L., cv Con­ ference) Score plots of dispersions in a) H2O, b) NaCl solution, c) CaCl2 solution, and d) all samples, presented together with the loadings plots (e–h), respectively J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 content ranging from 1.8 ⋅ 10− to 4.6 ⋅ 10− 2% the AI value was constant A further increase in the DASP concentration decreased the AI until a value close to − was obtained at the highest concentrations In the case of DASP in water, at the concentration ≤ 4.6 ⋅ 10− 2% the mean values of AI were in the range of 0.3–0.7 (excluding the value of 1.3 at 1.8 ⋅ 10− 2%) These values were lower than those previously determined for the apple pectin (AI of 1.9–3.3, concentration ≤ 1.0 ⋅ 10− 2% (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b) This indicates that the structure formed in the solution by the pear DASP was looser than that of the apple DASP AI equal to 0, indicating the existence of the homogeneous three-dimensional system, was obtained at the concen­ tration of 3.0 ⋅ 10− 1%, which was slightly lower than the value of 3.3 ⋅ 10− 1% as determined by Gawkowska, Cie´sla, Zdunek, and Cybulska (2019b) for the apple For the pear DASP dispersed in the salts solutions (the ionic strength of 30 mM), the ranges of AI values in the presence of Ca2+ and Na+ were different In the case of the NaCl solution, at the DASP concentration ≤ 4.6 ⋅ 10− 2% the mean values of AI ranged from 1.3 to 1.8, suggesting that the particles were more compact and more distant from each other than those dispersed in the water AI equal to was obtained at the con­ centration of 6.6 ⋅ 10− 1% In contrast to the DASP dispersions in water and the NaCl solution, the values of AI in the CaCl2 solution were negative over the full range of the concentrations used This was prob­ ably the result of the presence of nanoparticles or flocks (Basak & Bandyopadhyay, 2014) causing samples turbidity even at the lowest DASP content It was shown by Jonassen, Treves, Kjøniksen, Smistad, and Hiorth (2013) that the addition of NaCl (50 mM) to the pure aqueous pectin solution did not modify the transmittance whereas the divalent cation caused its reduction At the DASP concentration ≤ 4.6 ⋅ 10− 2% the AI values oscillated around − 0.5, next they decreased and a value of close to − was obtained at about 2.5 ⋅ 10− 1%, corresponding to 1.8 Ca2+/COO− mole ratio (Fig 3b) The total lack of the sample transparency to light (i.e AI = − 1) corresponds to the presence of the pectin network (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b) The calculated DASP concentration referring to such a structure in water and in NaCl solution was about 5.0 ⋅ 100% and 3.5 ⋅ 100%, respectively In the case of the apple pectin dispersed in water, it was approximately 3.3 ⋅ 100% but the other equation was used to perform the calculation (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b) The shape factor (SF) was also applied to evaluate the effect of DASP concentration on the structure formed in the dispersions (Fig 3c) The parameters of the equations describing the dependence of SF on the DASP content are summarized in Table S4 For the diluted systems (DASP concentration ≤ 4.6 ⋅ 10− 2% for H2O and the NaCl solution and the concentration ≤ 1.8 ⋅ 10− 2% for the CaCl2 solution) no significant effect of the DASP concentration on SF was observed The particles present in the NaCl solution (SF ~ 0.40) were slightly more elongated compared to those in the H2O and CaCl2 solution (SF ~ 0.55) A further increase in the DASP concentration led to an increase in the SF value The value of reflecting a regular shape/structure was determined at the DASP concentration of 2.0 ⋅ 10− 1, 6.0 ⋅ 10− and 4.9 ⋅ 10− 2% in H2O, NaCl and CaCl2 solutions, respectively An increase in the DASP content resulted in a decrease in the SF value which tended to with decreasing both the distance between the particles and the transparency of the samples The dispersed particles become indistinguishable in a network (SF ~ 0) It was assumed that the values of SF < 0.01 pointed out to the network structure obtaining (i.e the gel point) They were determined for the DASP concentration ≥ 5.0 ⋅ 100%, 4.0 ⋅ 100% and 5.5 ⋅ 10− 1% (Ca2+/COO− mole ratio of 0.98) in H2O, NaCl and CaCl2 solutions, respectively The application of both AI and SF indices allowed for the determi­ nation of the ranges of concentration of DASP from the pear fruit cor­ responding to negligible interactions between the dispersed particles (≤4.6 ⋅ 10− 2%), obtaining the homogeneous a three-dimensional structure in the liquid (2.0 ⋅ 10− 1–3.0 ⋅ 10− 1%, 6.0 ⋅ 10− 1–6.6 ⋅ 10− 1% and 4.9 ⋅ 10− 1% for dispersion in water, NaCl and CaCl2 solutions, Fig Relationship between a) the relaxation time (expressed as log(τr)), b) the aggregation index (AI), c) the shape factor (SF) and the concentration of diluted alkali-soluble pectin (DASP) from pear fruit (Pyrus communis L., cv Conference) in different media; bars indicate the standard deviation; different letters mean significantly different results (two-way ANOVA and post-hoc HSD Tukey test, p < 0.05) Cybulska, 2019b) was applied to monitor the structural changes in the DASP dispersions The relationship between AI and the DASP concen­ tration had a similar shape for all of the systems studied (Fig 3b) and was described using non-linear regression (Table S3) At the DASP J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 respectively) and a gel point (5.0 ⋅ 100%, 3.5 ⋅ 100–4.0 ⋅ 100% and 2.5 ⋅ 10− 1–5.5 ⋅ 10− 1% for dispersion in water, NaCl and CaCl2 solutions, respectively) (Fig S7) This was possible due to a wide range of the DASP concentrations studied The use of these indices in combination ´­ with the previously determined physicochemical ones (Cie´sla, Koczan ska, Pieczywek, Cybulska, & Zdunek, 2021) gives a possibility of multidirectional characterization of the behaviour of polysaccharides dispersed in the liquids and the optimization of the gelation conditions persistence length of DASP in water (129 ± 92 nm) was over twice as low as in the NaCl solution (363 ± 185 nm) Starting from the DASP concentration of 1.8 ⋅ 10− 2%, branched structures were formed in the NaCl solution and the network was visible in the water In the NaCl solution, a regular network was observed at the concentration of 1.8 ⋅ 10− 1% In general, the DASP macromolecules in the water were slightly lower and shorter than those in the NaCl solution (Fig 4b and c) Up to the DASP concentration of 4.6 ⋅ 10− 2% in water the height of the molecules on mica (0.63 ± 0.32 nm–1.14 ± 0.30 nm) was not significantly affected by the DASP concentration The values obtained corresponded to the results (0.3–1.0 nm) previously reported by Zdunek, Kozioł, Pieczywek, and Cybulska (2014) A further increase in the con­ centration caused an increase in the height value (4.63 ± 1.63 nm at 4.6 ⋅ 10− 1%) Considering that the distance between the O1 and O4 oxygen atoms in GalA is 0.500–0.597 nm (Cybulska, Brzyska, Zdunek, & ´ ski, 2014), the single molecules and bimolecular forms of DASP Wolin were present in the diluted aqueous dispersion For the DASP concen­ tration of 4.6 ⋅ 10− 1%, the height of DASP on mica corresponded to the 3.3.2 The AFM images of DASP Analyses of the AFM images of air-dried DASP samples were per­ formed (Figs 4, and S6, Table S5) to verify the results of the light scattering measurements The range of DASP content presented in Fig was limited to 1.8 ⋅ 10− 1% due to non-feasibility for the more concen­ trated dispersions in the CaCl2 solution (a gel) The images of 4.6 ⋅ 10− 1% DASP in the water and NaCl solution are shown in Fig S6 For the DASP concentrations of up to 4.6 ⋅ 10− 3% in H2O and NaCl solution the unbranched separated chains were visible The mean Fig a) The AFM images of diluted alkali-soluble pectin (DASP) from pear fruit (Pyrus communis L., cv Conference), b) height and c) length of macromolecules on mica; bars indicate the standard deviation; different letters indicate significantly different results (two-way ANOVA and post-hoc HSD Tukey test, p < 0.05) J Cie´sla et al Carbohydrate Polymers 273 (2021) 118598 separation between the molecules, when compared to the aqueous dispersion In the case of the increasing content of DASP in water, a regular network was formed on mica due to the decreasing distance between single molecules and bimolecular forms as well as the overlapping of chains A similar effect of concentration was observed for the DASP from apple (Gawkowska, Cie´sla, Zdunek, & Cybulska, 2019b), LM pectin (Zareie, Gokmen, & Javadipour, 2003) and alginate (Wang, Wan, Wang, Li, & Zhu, 2018) In the case of DASP in the NaCl solution, the presence of electrolyte facilitated the interactions between the macromolecules leading to the formation of elongated tri- or tetra-molecular structures even in the low-concentration systems An increase in the DASP content resulted in the formation of branched structures, the further overlapping of which led to a network formation The greater persistence length and larger dimensions of molecules in the NaCl solution than in water corresponded well to the AI values (Fig 3b) which were also higher in the presence of this salt Up to the DASP concentration of 4.6 ⋅ 10− 2%, the SF value (Fig 3c) in the NaCl solution was lower than in the water, thereby indicating that the par­ ticles in the salt solution were elongated Only the values of the relax­ ation time, indicating that at the concentration ≥ 4.6 ⋅ 10− 3% the particles in the water were larger than those in the NaCl solution, may be inconsistent However, the slightly longer relaxation time was probably due to less freedom of movement for the high number of di-molecular forms in the water as compared to the lower number of tri- or tetramolecular structures located far from each other in the NaCl solution (Horne, Hemar, & Davidson, 2003; Lima, Soldi, & Borsali, 2009; Schmidt, Schütz, & Schuchmann, 2017) The AFM images of DASP in the CaCl2 solution revealed the presence of small particles, the height of which on mica increased from 1.18 ± 0.29 nm (i.e representing the diameter of macromolecules) to 4.91 ± 3.49 nm (i.e 9–14 macromolecules) at the DASP concentration which ranged from 1.8 ⋅ 10− 4% to 1.8 ⋅ 10− 3% At the concentration of 4.6 ⋅ 10− 3% the value reached 0.95 ± 0.26 nm (i.e macromolecules) and further to 2.00 nm (i.e about macromolecules) at still higher con­ centrations (1.8 ⋅ 10− 2%–1.8 ⋅ 10− 1%) In the presence of Ca2+ both intramolecular and intermolecular bridges formation led to the attain­ ment of different-sized flocks (visible as particles after drying) The in­ teractions between flocks at the DASP concentration higher than 1.8 ⋅ 10− 1% resulted in gel formation (Basak & Bandyopadhyay, 2014) While, at a low content of pectin and a high concentration of divalent cations, precipitation (Han et al., 2017) and the nanoparticles formation due to the cation chelation inside the coils of macromolecules (Jonassen, Treves, Kjøniksen, Smistad, & Hiorth, 2013; Wei et al., 2009) may occur Shrinking of the coiled macromolecules of polysaccharide after the addition of Ca2+ was determined by Sagou, Rotureau, Thomas, and Duval (2013) Moreover, the stirring applied during the samples prep­ aration could result in the formation of soft gel particles dispersed in the liquid phase (Einhorn-Stoll, 2018) For the DASP concentrations ≥9.2 ⋅ 10− 1% the networks formed in the NaCl solution and water were indistinguishable (Fig 5a) Therefore, the roughness of the samples surface was analysed (Fig 5b; Table S5) At the DASP concentration of 9.2 ⋅ 10− 1% the roughness of DASP dispersed in the water and NaCl solution was similar but for the higher content of DASP the values obtained in the water were lower than those obtained in the salt solution Moreover, in the presence of Na+ an in­ crease in the DASP concentration led to the roughness increase A sig­ nificant increase in roughness was observed by Gawkowska, Cie´sla, Zdunek, and Cybulska (2019b), Zareie, Gokmen, and Javadipour (2003) and Wang, Wan, Wang, Li, and Zhu (2018) when a gel structure was formed The obtained results show the potential of DASP extracted from the fruit of Pyrus communis L cv Conference to gelation in the water and salt solutions This provides the opportunity for further studies concerning the possible application of DASP in food, cosmetics or in the pharma­ ceutical branches of industry Fig a) The AFM images of diluted alkali-soluble pectin (DASP) from pear fruit (Pyrus communis L., cv Conference) at the concentration range of 9.2 ⋅ 10− 1%–1.8 ⋅ 10− 1%; b) The surface roughness (Ra) defined as the arithmetical mean deviation of the assessed profile of DASP films deposited on mica; bars indicate the standard deviation; different letters indicate significantly different results (two-way ANOVA and post-hoc HSD Tukey test, p < 0.05) diameter of 8–10 macromolecules In the case of DASP in the NaCl so­ lution, the mean height of the molecules increased slightly from 1.69 ± 0.75 nm (i.e 3–4 macromolecules) to 4.02 ± 1.94 nm (i.e 7–10 mac­ romolecules) with the DASP concentration increasing from 1.8 ⋅ 10− 4% to 1.8 ⋅ 10− 1%, with the next increase in the concentration to 4.6 ⋅ 10− 1% resulting in an increase in the height to 11.32 ± 4.14 nm (i.e 20–26 macromolecules) The molecule length in water (~20 nm) was not significantly affected by the DASP concentration For dispersions in the NaCl solution an increase in the mean value from 21 to 63 nm with the DASP concentration increasing from 1.8 ⋅ 10− 4% to 4.6 ⋅ 10− 3% was observed but a further increase in the DASP content to 4.6 ⋅ 10− 1% led to a decrease in the length to 19 nm (Fig 4c) When comparing the structural changes of DASP dispersed in the water and the NaCl solution, it can be generalized that at the concen­ trations lower than 1.8 ⋅ 10− 2% the presence of NaCl led to an increase in the length and height (thickness) of the DASP molecules A further in­ crease in the DASP concentration formed a lower density network in NaCl solution as indicated by the greater lengths of the molecules measured between the branching points and the higher spatial Carbohydrate Polymers 273 (2021) 118598 J Cie´sla et al Conclusions Cybulska, J., Zdunek, A., & Kozioł, A (2015) The self-assembled 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(1997) IUPAC Compendium of Chemical Terminology (The "Gold Book".) (2nd ed.) Oxford: Blackwell Scientific Publications, ISBN 0-9678550-9-8 https://doi.org/10.1351/goldbook Online version (2019-) created by S J Chalk Moslemi, M (2021) Reviewing the recent advances in application of pectin for technical and health promotion purposes: From laboratory to market Carbohydrate Polymers, 254, Article 117324 Paniagua, C., Pos´e, S., Morris, V J M., Kirby, A R., Quesada, M A., & Mercado, J A (2014) Fruit softening and pectin disassembly: An overview of nanostructural pectin modifications assessed by atomic force microscopy Annals of Botany, 114, 1375–1383 Paniagua, C., Santiago-Dom´enech, N., Kirby, A R., Gunning, A P., Morris, V J., Quesada, M A., & Mercado, J A (2017) Structural changes in cell wall pectins during strawberry fruit development Plant Physiology and Biochemistry, 118, 55–63 Pieczywek, P M., Cybulska, J., & Zdunek, A (2020) An atomic force microscopy study on the effect of β-galactosidase, α-L-rhamnosidase and α-L-arabinofuranosidase on the structure of pectin extracted from apple fruit using sodium carbonate International Journal of Molecular Sciences, 21, 4064 Pos´ e, S., Kirby, A R., Mercado, J A., Morris, V J., & Quesada, M A (2012) Structural characterization of cell wall pectin fractions in ripe strawberry fruits using AFM Carbohydrate Polymers, 88, 882–890 LM DASP from the pear fruit (Pyrus communis L., cv Conference) showed gelling ability in aqueous medium and in solutions of mono- and divalent cations without pH adjustment at room temperature indicating its utility in the food, cosmetic or pharmaceutical industries Both AI and SF indices based on the light scattering can be useful for characterizing the structural changes of the DASP dispersions This was confirmed by the results of the relaxation time and rheological tests as well as by the analyses of the FT-IR spectra and AFM images The indices can be applied for the determination of the gel point This may be useful for the optimization of gelation conditions CRediT authorship contribution statement Jolanta Cie´sla: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing – original draft, ´ ska: Investigation, Writing – review & editing Magdalena Koczan Validation, Writing – original draft, Writing – review & editing Piotr Pieczywek: Investigation, Validation, Data curation, Visualization, Writing – original draft, Writing – review & editing Monika Szy­ ´ ska-Chargot: Investigation, Validation, Data curation, Visualiza­ man tion, Writing – original draft, Writing – review & editing Justyna Cybulska: Investigation, Resources, Writing – review & editing Artur Zdunek: Supervision, Funding acquisition, Writing – review & editing Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements This study was supported by the National Science Centre, Poland (Project No DEC-2015/17/B/NZ9/03589) Appendix A Supplementary data Supplementary data to this article can be found online at https://doi org/10.1016/j.carbpol.2021.118598 References Assifaoui, A., Lerbret, A., Uyen, H T D., Neiers, F., Chambin, O., Loupiac, C., & Cousin, F (2015) Structural behaviour differences in low methoxy pectin solutions in the presence of divalent cations (Ca2+ and Zn2+): A process driven by the binding mechanism of the cation with the galacturonate unit Soft Matter, 11, 551–560 Basak, R., & Bandyopadhyay, R (2014) Formation and rupture of Ca2+ induced pectin biopolymer gels Soft Matter, 10, 7225–7233 Bourne, M C (2002) Food texture and viscosity: Concept and measurement (2nd ed.) 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