Corn fiber gum: A potential gum arabic replacer for beverage flavor emulsification☆
Introduction
Corn fiber gum (CFG) is an alkaline extract of corn “fiber”, which is the main low value by-product of the corn wet and/or dry milling process. Corn fiber from these sources is primarily composed of cell-wall non-starch polysaccharides derived from corn kernel pericarp and/or endosperm tissues. CFG is an arabinoxylan (hemicellulose B) with unique high solubility and low viscosity. Corn fiber obtained from the wet milling industry, commonly known as “white fiber” is a mixture of coarse and fine fibers. The coarse fiber originates from the kernel pericarp or hull and the fine fiber is the inner cellular fiber from seed endosperm (Singh, Doner, Johnston, Hicks, & Eckhoff, 2000). Corn bran is a by-product from the commercial corn dry milling process and can also be called corn pericarp fiber as it originates only from this portion of kernel. The hemicelluloses are poorly defined and have long been neglected from an applications point of view, perhaps due to being low-viscosity gums, a property not seen as desirable by many food industries. However, it is now recognized that there is a need for gums with improved secondary characteristics that allow them to be used as bulking, bodying, emulsion stabilizing, and protective colloid agents, i.e., in applications in which low-viscosity, high-solid gum solutions are required. It has been suggested that if CFG could be produced commercially, it could be used as an adhesive, thickener and stabilizer (Wolf, MacMasters, Cannon, Rosewell, & Rist, 1953) and as a film former and emulsifier (Whistler, 1993; Woo, 2001). However, despite repeated attempts by numerous researchers and corn millers to develop a commercial product, none of the past attempts led to a suitable quality product at an affordable price. Further, the detailed evaluation of the viscosity and emulsification properties of pure CFG has not been accomplished. Part of the purposes of this report is to characterize these properties. Gum arabic, a natural exudate of acacia trees, is widely used in the food industry. It has a unique combination of excellent emulsifying properties and low solution viscosity despite its high molecular weight. So far this gum is considered to be the best gum in a dilute oil-in-water (O-in-W) emulsion system (Garti, 1999). However, almost all the world's gum arabic supply comes from three production areas in the Sahelian region of Africa. About 75% is produced by the Republic of the Sudan; most of the remainder comes from Senegal, Mauritania, and Nigeria. It is produced by trees which take at least 6 years to establish and which grow in a belt just below the Sahara desert (Glicksman, 1983). The exudate masses are picked by hand. Its supply is variable, uncertain and subject to climatic, economic and political conditions in the region. World demand for gum arabic is increasing and import into the US alone has increased from 6250 to 7500 metric tons from 1989 to 1994 (National Trade Data Bank, Department of Commerce). Thus the availability of an effective domestic substitute for gum arabic would be desirable for US producers of citrus O-in-W beverage emulsions and stable flavor powders. In addition, creation of a high-valued gum from a low-valued corn milling by-product could benefit US corn growers and processors.
CFG has been reported to be a homogeneous, predominantly carbohydrate molecule (Whistler & BeMiller, 1956; Montgomery & Smith, 1957), with the following sugar composition reported by various groups: d-xylose (48–54%), l-arabinose (33–35%), galactose (7–11%), and glucuronic acid (3–6%), (Doner, Chau, Fishman, & Hicks, 1998; Hespell, 1998; Suguwara, Suzuki, Totsuka, Takeuchi, & Ueki, 1994; Saulnier, Marot, Chanliaud, & Thibault, 1995a; Whistler & BeMiller, 1956). The gum structure is highly branched with a β-(1–4)-xylopyranose backbone and α-l-arabinofuranose residues as side chains on both primary and secondary hydroxyl groups (Saulnier et al., 1995a). Most of the d-glucuronic acid residues are linked to the O-2 position of xylose residues of the main xylan backbone (Montgomery & Smith, 1957). Galactose and some xylose residues are attached to the arabinofuranosyl branches (Whistler & Corbett, 1955).
CFG, like many hemicelluloses, is not extractable with water but can be extracted with alkali and alkaline H2O2 from plant cell walls and so it is believed that it is cross linked with other cell wall components. Covalent linkages, ionic and hydrogen bonding between the cell wall components have been claimed by Carpita and Gibeaut (1993). The possible linkages between hemicellulose and the cell wall components are mentioned in detail by Doner and Hicks (1997) explaining that ferulic, diferulic and p-coumaric acids play a very important role in cross linking them together (Saulnier, Vigouroux, & Thibault, 1995b). It is also clear that polyphenolics such as lignin can form alkali resistant linkages (ether linkages) with hemicelluloses, which can be cleaved by alkaline hydrogen peroxide treatment. In the cell wall, arabinoxylan and protein are also closely associated as supported by the finding of a stable linkage between hemicelluloses and protein in corn bran (Saulnier et al., 1995a) and rye bran (Ebringerova, Hromadkova, & Berth, 1994). The present investigation is carried out to study the emulsification properties of CFG from different corn milling fractions and to relate their structural characteristics with their ability to stabilize emulsions. In addition, the effect of extraction conditions on the emulsion properties of CFG was also studied.
Section snippets
Materials
Corn fiber samples were kindly provided by ADM Research, Bunge (North America, Bunge Milling, Inc., St. Louis, MO) and Cargill Central Research (Minneapolis, MN). They were oven dried by the suppliers before shipping. Fiber samples were ground to a 20-mesh particle size using a Wiley mill and extracted with hexane to remove oil (Moreau, Powell, & Hicks, 1996). Starch was removed from the 20-mesh de-oiled fiber by treating with Termamyl α-amylase (a gift from Novo Nordisk Bioindustrials, Inc.,
Isolation of CFG
CFG-1 and 2 were extracted from wet milled pericarp fiber (WPF), dry milled pericarp fiber (DPF); and wet milled pericarp and endosperm fiber (WPEF) according to the scheme shown in Fig. 1. This extraction procedure is a slight modification of the published isolation method for CFG (Doner et al., 1998; Doner & Johnston, 2001). Corn fiber produced with any milling procedure contains some percentage of oil which was removed by hexane extraction (Moreau et al., 1996). A considerable amount of
Conclusions
CFGs extracted from WPF, DPF and WPEF of corn are similar in sugar composition but differ in protein content and sugar branching. CFGs extracted from WPEF and WPF have higher protein contents than those extracted from DPF. CFG-2 from each source has a higher protein content than the corresponding CFG-1. Both CFG-1 and 2 from WPEF are more branched (Ara/Xyl ratio in Table 3) than the other CFG samples. Emulsions prepared with CFGs from WPEF and WPF have higher stability than CFGs from DPF at
Acknowledgement
The authors are pleased to acknowledge Michael Kurantz for the protein, moisture and ash determination and Kyle Beery from ADM Research, Ting Carlson from Cargill Central Research, and Wil Duensing from Bunge Milling, Inc. for providing corn fiber samples.
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