Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres Characterisation of rheology and microstructures of κ-carrageenan in ethanol-water mixtures Zhi Yang a,b , Huijuan Yang a,c , Hongshun Yang a,b, ⁎ a Food Science and Technology Programme, c/o Department of Chemistry, National University of Singapore, Singapore 117543, Republic of Singapore b National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China c Synergetic Innovative Centre of Food Safety and Nutrition, Key Laboratory of Meat Processing and Quality Control, Ministry of Education, Key Laboratory of Animal Products Processing, Ministry of Agriculture, Jiangsu Innovative Centre of Meat Production and Processing, College of Food Science and Technology, Nanjing Agricultural University, Nanjing 210095, PR China ARTICLE INFO Keywords: Polysaccharide Sol-gel transition Rheology Solvent quality Network strength Small angle X-ray scattering ABSTRACT The effects of ethanol (up to 20 wt%) on the rheological properties and structural characteristics of κ-carra- geenan gel were investigated by Field Emission Scanning Electron Microscopy (FESEM) and Small Angle X-ray Scattering (SAXS). Both the sol-gel and gel-sol transition temperatures shifted to higher degree (from 36.8 ± 0.5 to 52.5 ± 1.4 °C and from 51.2 ± 0.6 to 67.0 ± 0.5 °C, respectively) upon 20 wt% ethanol addition (P < 0.05). The critical relaxation exponent n and the critical gel strength S g obtained from Winter-Chambon criterion decreased and increased, respectively as the ethanol concentration increased. The κ-carrageenan gel was formed due to the formation of fibrillar networks, and the fibrillar density increased upon ethanol addition via FESEM. Moreover, upon 20 wt% ethanol addition, the average radius of gyration of κ-carrageenan strand increased from 1.18 ± 0.03 of control to 1.55 ± 0.02 nm by SAXS. A mechanism underlying the effect of ethanol on the κ-carrageenan gelation was proposed based on coil to double helix transition followed by the helix aggregation. 1. Introduction Kappa-carrageenan is a natural sulphated polysaccharide obtained from red seaweed predominately including Chondrus, Gigartina, and various Eucheuma species (Necas & Bartosikova, 2013). It has been traditionally used as thickener, stabilizer, and texturing agent in food, cosmetic, and pharmaceutical industries (Zhang et al., 2017). Recently, κ-carrageenan has been applied in bioactive encapsulation (Xu et al., 2014), wound healing (Aramwit, 2016), and multifunctional packaging (Rhim, 2013) due to its distinguished biocompatibility and biodegrad- ability. Kappa-carrageenan forms a thermoreversible gel in aqueous solu- tion. It is generally considered that the gelation involves two steps: the initial random coil to helix transition and the aggregation of double helices upon introduction of mono or double positive ions such as K + , Na + , and Ca 2+ (Şen & Erboz, 2010). Many factors affect the sol-gel transition and the rheological and microstructural properties of the final gel. These factors include raw source from which κ-carrageenan was extracted (Yang et al., 2011), molecular weight (Leiter, Mailänder, Wefers, Bunzel, & Gaukel, 2017), introduction of ions (Robal et al., 2017), and presence of sugar and polyols (Gekko, Mugishima, & Koga, 1987). To date, most studies on κ-carrageenan gelation are in aqueous systems (Sow, Kong, & Yang, 2018). There are quite few investigations on the influence of mixed water and non-aqueous solvents (e.g. ethanol and methanol) on the rheological and microstructural properties of polysaccharide hydrogel, although this kind of study is pretty mean- ingful and important from both application and theoretical perspec- tives. For example, ethanol is usually employed in hydrogel based wound healing material primarily due to its antiseptic effect. The co- presence of κ-carrageenan and ethanol is not rare in food and beverage industries. Besides that, high concentrations of ethanol or methanol (usually higher than 50 wt%) are usually employed to decrease the solubility and precipitate κ-carrageenan from aqueous extracts. How- ever, the effect of low concentration of ethanol (~20 wt%) on the sol- gel transition of κ-carrageenan remains unknown. Introducing ethanol or methanol is an effective way to tune solvent quality of biopolymer aqueous system (Wang & Padua, 2010). The addition of ethanol up to 15% increased the mechanical strength of calcium alginate gels (Hermansson, Schuster, Lindgren, Altskär, & https://doi.org/10.1016/j.foodres.2018.03.016 Received 7 January 2018; Received in revised form 2 March 2018; Accepted 4 March 2018 ⁎ Corresponding authors at: Food Science and Technology Programme, c/o Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore. E-mail address: chmynghs@nus.edu.sg (H. Yang). Food Research International 107 (2018) 738–746 Available online 07 March 2018 0963-9969/ © 2018 Elsevier Ltd. All rights reserved. T