Elsevier

Carbohydrate Polymers

Volume 73, Issue 2, 19 July 2008, Pages 212-222
Carbohydrate Polymers

On the gelling behaviour of ‘nopal’ (Opuntia ficus indica) low methoxyl pectin

https://doi.org/10.1016/j.carbpol.2007.11.017Get rights and content

Abstract

Fully de-esterified pectin with excellent gelling properties was isolated in the sodium-salt form from fresh ‘nopal’ cactus (Opuntia ficus indica) pads (0.6% w/w yield of fresh weight) using an alkaline extraction medium in the presence of a sequestering agent. Sugars composition of the cactus pectin alkaline extract (CPAE) was: 85.4% uronic acids, 7.0% galactose, 6.0% arabinose and minor quantities of rhamnose and xylose. Features of the Fourier-transformed infrared spectrum were nearly identical to those of commercial citrus pectin as well as to homogalacturan-rich pectin isolated from prickly pear, lime peel, and sugar beet pulp. The gelling behavior of this material was studied as a function of amount of Ca2+ added and temperature by dynamic oscillatory rheology. Addition of Ca2+ at 85 °C was adjusted at varying stoichiometric ratios, R (=2 [Ca2+]/[COO]), namely 0.07, 0.21, 0.35 and 0.42 and fixed pectin concentration (16 g/L), and a temperature dependent behavior of the system on cooling was imposed. Evolution of the viscoelastic storage (G′) and loss (G′′ moduli on cooling revealed unequivocal set up of a gel network under a highly cooperative sol-gel transition at R  0.21. At greater R values, the Ca2+-mediated dimeric association of pectin chains led to formation of a gel network even at 85 °C. On heating, pectin gels melted partially at temperatures notably greater than those at which they were formed. Thermal hysteresis observed between the cooling and heating temperature traces is explained in terms of helix–helix aggregation. The gelling behavior of this system is interpreted in terms of the formation of two distinct types of junctions mediated by the stoichiometric amount of calcium (R). Namely, short ‘egg-box’ type junctions formed directly at high temperature on addition of calcium (limited zones are related to high mobility of the chains) and highly cooperative 21 helix junctions followed by aggregation formed at lower temperature under a thermal conformational transition driven by charge neutralization and lower chain mobility (related to stabilization by H-bonds).

Introduction

Pectin is widely used in the food industry as a hydrocolloid gelling gum. Commercially, it is commonly derived from fruit waste mainly apple and citrus peel. Generically, the term “pectin” represents a family of structural polysaccharides which occur as constituents of the primary cell wall of plant cells and intercellular regions of higher plants where they function as hydrating agent and cementing material of the cellulosic network (Lau, McNeil, Darvill, & Albersheim, 1985). The main pectin component is a central, linear backbone chain composed of α-d-galacturonic acid units linked by (1  4) glycosidic bonds. This linear (or ‘smooth’) structure is interrupted by highly branched regions (‘hairy’ zones) (Oosterveld et al., 2000, Thibault et al., 1993, Voragen et al., 1995).

Depending on the botanic origin and the extraction procedure used, the carboxylic groups are partially esterified with methanol, and in certain pectins the hydroxyl groups are partially acetylated. Neutral sugars, such as rhamnose, arabinose, galactose, xylose and glucose are also usually present in a 5–10% proportion of the galacturonic acid weight. These neutral residues comprise the highly branched side chains (arabinan and galactan), a small part of the central chain (rhamnose) or occur as contaminating polysaccharides (glucans and xyloglucans) (Rolin, 1993). Depending on the degree of esterification (DE, expressed conventionally as a percentage of the content of esterified units compared with the content of total uronic acids), pectins can form gel networks either in an acid medium and under high sugar concentrations (high DE pectins, more than 50%) or by interaction with divalent cations, particularly Ca2+ (low DE pectins, less than 50%) (Durand et al., 1990, Evageliou et al., 2000). In these gels, the macromolecules are cross-linked by divalent calcium ions (Thibault and Rinaudo, 1984a, Thibault and Rinaudo, 1984b, Thibault and Rinaudo, 1985, Thibault and Rinaudo, 1986, Turquois et al., 1999). In both cases, gelation and gel properties depend upon many factors, including pH, temperature, DE, sugar content, calcium content and pectin content.

The occurrence of polysaccharides that generically have been called “pectins” in various Opuntia species from Mexico has been documented for almost three decades (Villarreal, Rojas, Arellano, & Moreno, 1963), with yield of soluble pectin within a wide range of 0.13–2.64% in wet basis (1.00–23.87% in dry-weight basis). Two types of water-soluble materials can be extracted from Opuntia spp. pads and fruits, namely a mucilage material easily recognized as a slimy fluid that appears as soon as cacti is cut and a structural cell-wall component. The sugar residues composition and linkage geometry of the mucilage from the fruits and pads of Opuntia spp. cacti have long been studied using different chromatographic techniques (Amin et al., 1970, Majdoub et al., 2001, McGarvie and Parolis, 1979a, McGarvie and Parolis, 1979b, McGarvie and Parolis, 1981a, McGarvie and Parolis, 1981b, Medina-Torres et al., 2000, Trachtenberg and Mayer, 1981). However, up to now, the mucilage polysaccharides in Opuntia, do not seem to be chemically associated, either covalently or otherwise, to the structural cell-wall pectins (Goycoolea & Cárdenas, 2003). The physicochemical and rheological properties of mucilage extracted from Opuntia spp. have been studied by several Groups (Amin et al., 1970, Forni et al., 1994, Majdoub et al., 2001, McGarvie and Parolis, 1979a, McGarvie and Parolis, 1981a, Medina-Torres et al., 2000, Mindt et al., 1975, Trachtenberg and Mayer, 1981, Trachtenberg and Mayer, 1982). The evidence is consistent in that this material has no gelling capacity. By contrast, the polysaccharide material addressed in this study is the far less studied pectin extracted from the cell-wall with a very low DE and an unequivocal capacity to form a gel network in the presence of calcium ions. It is hypothesized that the underlying mechanism for this phenomenon is the consolidation of a gel network that involves the association of junctions mediated by Ca2+ crosslinking of two distinctive types, namely short dimeric “egg-box” type structures (Grant, Morris, Rees, Smith, & Thom, 1973) formed at high temperature followed by a highly cooperative mechanism comprising ordered perhaps 21 double helical structure with aggregation at low temperature. This study opens new horizons to the use of cactus pads as a potential source of pectin, with gelling properties of enormous technological importance to the food industry, particularly in arid and semi-arid zones.

Section snippets

Materials

A batch (∼10 kg) of fresh cactus (Opuntia ficus indica) pads or cladodes were obtained from a commercial plantation in San Pedro El Saucito, Hermosillo, Sonora. Commercial low-ester citrus pectin was a sample from DANISCO A/S (Copenhagen) (DE < 50%). Chemical reagents were of analytical grade supplied by Sigma Chemicals (Mexico, DF) except sodium hexametaphosphate that was from Monsanto Co. (St. Louis, MO, USA). Bi-distilled water was used throughout.

Pectin extraction and purification

Before pectin extraction, the fresh cactus pads

Chemical composition

The cactus pectin extract afforded by the alkaline process, referred here to as cactus pectin alkaline extract (CPAE), represents 0.6% of the fresh tissue weight. Even when some soluble pectin may have been lost during the first aqueous extraction treatment intended to remove mucilage, the yield obtained for the extraction process was considerably greater than that documented for pectin extracted from prickly pear peel, 0.12% (Forni et al., 1994). CPAE obtained consists of 85.4% uronic acids

Conclusions

Cactus pectin, extracted by means of an alkaline process assisted by a calcium sequestering agent (hexametaphosphate) previously used to extract pectin from other plant matrices, exhibits a low content of neutral sugars (and presumably low rhamnogalacturonan ramification) and inexistent esterification due to the alkaline pH. Addition of calcium at a high temperature to this material, at stoichiometric equivalency levels R (=2[Ca2+]/[COO−]), which varied between 0.07 and 0.42 and the subsequent

Acknowledgments

We are grateful to Consejo Nacional de Ciencia y Tecnologia (CONACyT, Mexico) for research Grant No. 29088 and to Centre Nacional de la Recherche Scientifique (CNRS, France) for an international co-operation grant.

References (51)

  • Y. Habibi et al.

    Structural features of pectic polysaccharides from the skin of Opuntia ficus-indica prickly pear fruits

    Carbohydrate Research

    (2004)
  • K. Haxaire et al.

    Predictive and experimental behaviour of hyaluronan in solution and solid state

  • A.A. Kamnev et al.

    Comparative spectroscopic characterization of different pectins and their sources

    Food Hydrocolloids

    (1998)
  • F. Kar et al.

    Effect of temperature and concentration on viscosity of orange peel pectin solutions and intrinsic viscosity-molecular weight relationship

    Carbohydrate Polymers

    (1999)
  • H. Majdoub et al.

    Prickly pear nopals pectin from Opuntia ficus-indica physico-chemical study in dilute and semi-dilute solutions

    Carbohydrate Polymers

    (2001)
  • D. McGarvie et al.

    analysis of the mucilage of Opuntia ficus-indica

    Carbohydrate Research

    (1979)
  • D. McGarvie et al.

    The acid-labile, peripheral chains of the mucilage of Opuntia ficus indica

    Carbohydrate Research

    (1981)
  • D. McGarvie et al.

    Methylation analysis of the mucilage of Opuntia ficus indica

    Carbohydrate Research

    (1981)
  • L. Medina-Torres et al.

    Rheological properties of the mucilage gum (Opuntia ficus indica)

    Food Hydrocolloids

    (2000)
  • Z.H. Mohammed et al.

    Kinetic and equilibrium processes in the formation and melting of agarose gels

    Carbohydrate Polymers

    (1998)
  • E.R. Morris et al.

    Chiroptical and stoichiometric evidence of a specific, primary dimerisation process in alginate gelation

    Carbohydrate Research

    (1978)
  • E.R. Morris et al.

    Conformations and interactions of pectins. I. Polymorphism between gel and solid states of calcium polygalacturonate

    Journal of Molecular Biology

    (1982)
  • A. Oosterveld et al.

    Effect of enzymatic deacetylation on gelation of sugar beet pectin in the presence of calcium

    Carbohydrate Polymers

    (2000)
  • L. Piculell et al.

    Organisation and association of k-carrageenan helices under different salt conditions

    International Journal of Biological Macromolecules

    (1997)
  • C. Rolin

    Pectin

  • Cited by (0)

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