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