Hybrid Carrageenans

Isolation, Chemical Structure, and Gel Properties

Loic Hilliou1    Institute for Polymers and Composites/I3N, University of Minho, Guimarães, Portugal
1 Corresponding author: email address: loic@dep.uminho.pt

1 Introduction

Carrageenans are natural polymers contained in specific species of red seaweeds belonging to the Gigartinales order. These are polysaccharides showing a variety of chemical structures, resulting from a complex interplay between the seaweeds species, the seaweed life stage, and the extraction process used to recover the polysaccharide. Among the various types of carrageenans showing different gelling or viscosity enhancement properties in aqueous solutions, hybrid carrageenans have recently received increased interest (van de Velde, 2008). The latter is motivated by the steadily increasing demand for gelling additives for food and nonfood application, which puts under pressure the farming of seaweeds producing kappa-carrageenan (K) and iota-carrageenan (I) (Bixler, 1996; Bixler & Porse, 2011). Thus, alternative algal resources for carrageenan production are highly demanded (Bixler & Porse, 2011; McHugh, 2003), and seaweeds producing hybrid carrageenans can be a solution to the issue triggered by the market. Recently, hybrid carrageenans were found to positively replace mixtures of K and I used in niche application in dairy food (Bixler, Johndro, & Falshaw, 2001; Villanueva, Mendoza, Rodrigueza, Romero, & Montaño, 2004). In spite of the industrial need and interest in using hybrid carrageenans, there is a lack in the literature for the structural and mechanical characterization of hybrid carrageenan gels (van de Velde, 2008), which explains why the relationships between the hybrid carrageenan chemical structure, the gel microstructure, and the gel mechanical properties are not yet understood.

This chapter focuses on the gel properties of hybrid carrageenan and addresses the relationships identified between the seaweeds biology, the extraction parameters, the chemical structure, and the gel properties. First, the biology and chemical composition of seaweeds producing hybrid carrageenan will be described. Then, the chemical structure of gelling hybrid carrageenan will be introduced together with the general proposed mechanisms for gel formation for K and I in the presence of salt. As this chapter is concerned with gelling hybrid carrageenan, two types of polysaccharides are solely discussed, namely, kappa/iota-hybrid carrageenan (KI) and their biological precursor kappa/iota/mu/nu-hybrid carrageenan (KIMN). Thus, the gel formation builds up on the mechanism devised for K and I. Then, the extraction process used to isolate the polysaccharide and the effect of extraction parameters on the macromolecular structure and chemistry of recovered polysaccharides will be discussed. With the characterized hybrid carrageenan in hand, gels will be formed and the rheological technique used to analyze their mechanical properties will be introduced before describing the effects of salt and polysaccharide concentrations on gel setting and elastic properties. Then, the interplay between hybrid carrageenan chemical structure and the gel properties will be discussed. Finally, the effect of steady flow on the gel setting and gel properties will be addressed as its relevance to the delivery of new food ingredients such as fluid gels or microgels (Garrec & Norton, 2012) and to the industrial processing of carrageenan is bright.

2 Chemical Structure and Gel Mechanism

2.1 Seaweeds chemistry

Before tackling the chemical structure of KI and KIMN, it is imperative to look at the chemical composition of seaweeds belonging to the Gigartinaceae, Petrocelidaceae, and Phylophoraceae families which are the major carrageenophytes used for the production of gelling hybrid carrageenan (see for instance the Stancioff's diagram in Bixler, 1996). Fourier transform infrared diffuse reflectance spectroscopy (DRIFT) is a versatile spectroscopic method which boosted the chemical analysis of seaweeds as no sample preparation but grinding dried seaweeds is required. DRIFT was applied to screen for the chemical composition of Gigartinales by Chopin, Kerin, and Mazerolle (1999). This extensive study performed on more than 50 species of the Gigartinales order confirmed that the chemical composition of seaweeds depends on its life stage, vegetative, or reproductive and in the latter case depends also on the gender of the gametophyte of specific seaweeds. Vegetative Gigartinales seaweeds are made of highly sulfated and nongelling carrageenan of the lambda-type, whereas the reproductive life stage produces K, I mu-carrageenan (M) and nu-carrageenan (N) in fronds and thalli of female and male gametophytes. The disaccharide units corresponding to these carrageenans are displayed in Fig. 2.1. This general picture was reached in earlier studies which relied on the isolation of polysaccharides with the inherent polymer chemical modification associated with the extraction process (see the tables in Chopin et al., 1999 where a direct comparison between DRIFT results and reports from the literature is offered). The impact of such modification on the qualitative chemical analysis of compounds contained in the seaweed was pointed recently in a study on Mastocarpus stellatus (Azevedo et al., 2013). DRIFT spectra of the native extracts obtained without alkali treatment showed all characteristic bands of K, I, M, and N disaccharide units, whereas both fronds and thalli did not show the specific band assigned to I, showing up at 805 cm− 1. The relative content in K, I, M, and N is however difficult to assess with this semiquantitative spectroscopic technique and thus hardly shows that the ratio between K and I is specific to each seaweed. This is illustrated in Fig. 2.2 where the DRIFT spectra of M. stellatus and Chondrus crispus hand collected on the Northern Portuguese coast are displayed, together with the spectra of commercial K and I (Sigma-Aldrich, Germany). Fronds and thalli of seaweeds were scratched on the DRIFT accessory pad of an FTIR spectrometer (Spectrum 100, PerkinElmer Ltd., UK), whereas powders were directly laid on the accessory. Both M. stellatus and C. crispus show the diagnose bands for I (805 cm− 1) and K (930 and 845 cm− 1). However, assessing whether C. crispus contains more K than M. stellatus is hard since ratios of K over I band intensities are 1.4 ± 0.2 for C. crispus against 1.1 ± 0.3 for M. stellatus, with errors computed from the averaging of five replicates from different fronds. Thus, one relies on extracting the polysaccharide from the seaweed and analyzing by 1H NMR the KI which are soluble in water to compute such ratios. A nice example of such exercise can be found in van de Velde et al. (2005) where three seaweed species harvested on the Portuguese coast showed K over I ratios ranging from 0.02 to 1.

2.2 Hybrid carrageenan macromolecular structure

The chemical structure of hybrid carrageenan has been vividly debated between two schools, and it is the author's opinion that the issue has been only recently solved by the publication of two critical papers (Guibet et al., 2008; van de Velde, Peppelman,...