Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt Rheological surface properties of commercial citrus pectins at different pH and concentration Noemi Baldino, Olga Mileti, Francesca R. Lupi, Domenico Gabriele * Department of Information, Modeling, Electronics and System Engineering, (D.I.M.E.S.) University of Calabria, Via P. Bucci, Cubo 39C, I-87036, Rende, CS, Italy ARTICLE INFO Keywords: Interfacial properties Dilatational rheology Pectin Molecular weight Methoxylation degree ABSTRACT The interfacial activity of commercial low-methoxy (LM) and high-methoxy (HM) pectins from citrus peel was investigated at air-water interfaces by focusing on the role of their molecular weight (MW) and degree of methoxylation (DM). A pendant drop tensiometer was used to carry out transient interfacial tension measure- ments and small amplitude oscillations. Different pectin concentrations (ranging between 0.00001 g/100 g and 5 g/100 g) and pH conditions (4 and 6) were used during the tests. It was observed that citrus pectins are characterised by interesting surface properties that could allow potential practical uses. Experimental results evidenced that MW affects the diffusion of molecules towards the interface, whereas other investigated para- meters (i.e. surface tension, adsorption rate, dynamic moduli) seem more dependent on DM and a clear de- pendence on MW was not observed. pH conditions modify intermolecular interactions in the bulk and surface layer, even if their effects are related to the fraction and distribution of carboxylic groups along the chain. As a consequence, a complex dependence on investigated parameters was observed and no clear relationship was obtained. Nevertheless, among tested commercial samples LM pectins exhibited the most interesting properties and this behaviour seems related to the intermolecular interactions occurring among them. 1. Introduction Pectin is a complex mixture of polysaccharides that consist mainly of galacturonic acid (GalA) (at least 65% according to the definition of “pectin” given by the Joint FAO/WHO Expert Committee on Food Additives (JECFA, 2009)) even if the fine structure of the single com- ponents can be significantly different (Willats, Knox, & Mikkelsen, 2006). Three main pectic polysaccharide domains are identified (Willats et al., 2006): Homogalacturonan (HG), where GalA units are covalently α (1 → 4) linked to form a linear backbone; Rhamnoga- lacturonan I (RGI) consisting of the repeating disaccharide [→4)-α-D- GalA-(1 → 2)-α-L-Rha-(1→] to which glycan units are connected to rhamnose component; Rhamnogalacturonan II (RGII) having a HG backbone where complex side chains are connected to GalA units. The GalA units of the pectin chain are variously methylesterified and this significantly affects the structural properties of this biopo- lymer. As a consequence, the degree of methoxylation (or methyles- terification, DM) is used to classify pectin into high-methoxyl (HM) pectins, containing more than 50% esterified carboxyl groups (DM > 50), and low-methoxyl (LM) pectins, with < 50% esterified carboxyl groups (DM < 50) (May, 1990). Pectin is produced, at industrial level, by extraction from apple pomace and citrus peels, even though other sources are available such as sugar beet and sunflowers head residues, cocoa husks, potato pulp, soy hull, etc. (Thakur, Singh, & Handa, 1997; Yancheva, Markova, Murdzheva, Vasileva, & Slavov, 2016). It is mainly used for its gelling ability in the food industry in jams, fruit preparations, desserts and confectionery, and as a viscosity en- hancer and stabilizer in beverages and, moreover, as a stabilizer of acid milk drinks and in dairy products (Dickinson, Semenova, Antipova, & Pelan, 1998; May, 1990; Schmidt, Schütz, & Schuchmann, 2017; Yancheva et al., 2016). In fact, HM pectin can form a three-dimensional network through hydrogen bonds and hydrophobic interactions between the methyl- ester groups whereas LM pectin gelation occurs mainly thanks to ionic linkages with divalent ions (typically calcium ions) (de Cindio, Gabriele, & Lupi, 2016). It is considered as a “safe” food ingredient, therefore there is no maximum acceptable daily intake and it is only recommended to use it at a level not higher than the necessary amount, according to good manufacturing practice (de Cindio et al., 2016). Moreover, pectin is a source of dietary fibre and therefore can be considered a healthy in- gredient due to the ability to lower cholesterol as well as having several beneficial effects on the colon (Willats et al., 2006). https://doi.org/10.1016/j.lwt.2018.03.037 Received 4 January 2018; Received in revised form 13 March 2018; Accepted 14 March 2018 * Corresponding author. E-mail address: domenico.gabriele@unical.it (D. Gabriele). LWT - Food Science and Technology 93 (2018) 124–130 Available online 15 March 2018 0023-6438/ © 2018 Elsevier Ltd. All rights reserved. T