Y ELLOW M USTARD G UM

4.4.3 Y ELLOW M USTARD G UM

4.4.3.1 Source

Yellow mustard gum (YMG) is a mucilage deposited in the epidermal layer of yellow mustard seed (Sinapis alba). It is soluble in water and can be extracted from whole seed or seed coat (bran).

4.4.3.2 Method of Production

Extraction from Seed Yellow mustard gum is relatively easy to extract with water because it is in the epidermal layer of the testa and it is highly soluble in water. The whole seed can

be extracted with water with a seed-to-water ratio of 1:10. The viscous extract is filtered or centrifuged to remove any solid particles. The supernatant can be recov-

ered by precipitation in alcohol. The fibrous gelatinous material from alcohol pre- cipitation is redissolved and precipitated again in alcohol, to give a snow-white

fibrous product with a yield of ~5%. Extraction from Bran

Using whole seed as a source of YMG is not economically feasible since there is no practical use for the seeds after the water extraction. In North America, substantial amounts of mustard meats are used as food ingredients, while the bran is a by- product. Since mucilage is deposited in the epidermal layer of the seed coat, extrac- tion of mucilage from the bran could be commercially viable (1) to reduce the processing cost and (2) to allow utilization of the by-product. The yield of YMG from bran varied from 15 to 25%, depending on the extraction conditions. 128,129 The extraction process generally includes (1) defatting of the bran with a mixture of hexane, ethanol, and water; (2) water extraction (1:20 ratio) and separation by centrifugation; and (3) precipitation in ethanol and drying.

4.4.3.3 Chemistry and Structural Features

YMG contains 80.4% carbohydrates, 4.4% protein, and 15% ash. Dialysis against distilled water could reduce the ash content to 4.8% with a corresponding increase in carbohydrates from 80.4 to 91.1%. YMG is a mixture of complex polysaccharides containing six neutral sugars and two uronic acids. Of the monosaccharides, glucose (23.5%) is the predominant neutral sugar, followed by galactose (13.8%), mannose (6.1%), rhamnose (3.2%), arabinose (3.0%), and xylose (1.8%); the uronic acids are about 14.7%, including galacturonic acid and glucuronic acid. 129

Seed Polysaccharide Gums

4-–O-Me-β-D-GlcA

β-D-Gal p B β-D-Gal p

2)-α-L-Rha p-(1 4)-α-D-GalA-(1 2)-α-L-Rha p-(1 2)-α-L-Rha p-(1 4)-α-D-GalA-(1

C D C'

FIGURE 4.5 Structural features of yellow mustard rhamnogalacturonan. (Reproduced from Cui, W. et al., Carbohydr. Res., 292, 173, 1996. With permission.)

The crude YMG can be separated into two fractions according to solubility in water: a water-soluble (WS) and a water-insoluble (WI) fraction. The WS is the major fraction and exhibits shear-thinning flow behaviors observed for YMG. 129 Subsequent fractionation and structural analysis revealed that WS is composed of a neutral polysaccharide and an acidic polysaccharide. The neutral polysaccharide is essentially a cellulose-like polymer that contains 1,4-linked β-D-glucose as the backbone chain with ether groups (i.e., ethyl and propyl) randomly distributed at the C2-, 3-, and 6-positions. Ether groups present along the cellulosyl backbone chain behave as “kinks” that prevent the formation of crystalline structures due to interchain interactions, therefore rendering the polysaccharide water soluble. 52

The structure of the acidic polysaccharide is a rhamnogalacturonan, which has

a rhamnogalacturonan backbone with branches consisting of galactose and glucu- ronic acid. An average repeating structure unit was elucidated from methylation analysis, one- and two-dimensional nuclear magnetic resonance (NMR) spectros- copy, and characterization of oligosaccharides released from partial hydrolysis. The rhamnogalacturonan consists of →2)-α-L-Rhap-(1→4)-α-D-GalA-(1→ backbone chain, with side chains composed of glucuronic acid and galactosyl residues at the 4-position of 50% of the 2-linked α-L-rhamnosyl residues, as shown in Figure 4.5. 130

4.4.3.4 Functional Properties and Applications

YMG is the only naturally produced gum that resembles xanthan gum by (1) exhibiting shear-thinning flow behavior at low concentrations (0.3% and above), (2)

forming a weak gel structure, and (3) interacting synergistically with galactoman- nans. The shear-thinning flow behavior and weak gel structure of YMG are shown

in Figure 4.6. When yellow mustard gum is mixed with galactomannans, the viscosity of the mixed systems increases significantly, accompanied by the changes of viscoelastic characteristics. 4 YMG and locust bean gum (LBG) blends are prepared by mixing the two gums in the same concentration according to designed proportions and heating at 80°C for at least 30 min before rheological measurements. The 1:9 (LBG:YMG) blend exhibited gel-like properties where G' is much greater than G'' and the two moduli are less independent of frequency (Figure 4.7). The synergism

Functional Food Carbohydrates

100 .s)

G′ G′′ duli (Pa) ∗ , Pa η 10 η∗

′′) mo sity ( is ss (G co

1 xv

′), lo ple om .1

orage (G St

Frequency (Hz)

FIGURE 4.6 Mechanical spectrum of yellow mustard gum at 1.0% (w/w) and 23 o C.

1000 duli (Pa) ʹʹ) mo 100

ss (G ʹ) and lo 10

G ʹ G ʹʹ

orage (G St

Frequency (Hz)

FIGURE 4.7 Mechanical spectra of yellow mustard gum and locust bean gum at the ratio of 1:9 at 2% total polymer concentration, 23 o C.

of yellow mustard gum with galactomammans follows the order of LBG > guar gum > fenugreek gum (Figure 4.8), which is consistent with the order of manose:galactose ratios (4:1, 2:1, and 1:1, respectively). YMG–LBG mixtures can form a gel even at 0.1% (w/w) total polymer concentration. Both polymers behave as viscoelastic fluids at 0.1% polymer concentration. 4

The observed improvement in gelling behavior of YMG by addition of small amounts of LBG at low polymer concentrations (e.g., 0.1% w/w) suggests that the synergistic interactions that occurred between LBG and YMG are through a coop- erative association of long stretches of the two polymers into mixed junction zones. 131 That heating is necessary for the interaction suggests that the associative synergistic interactions between galactomannans and YMG require melting the ordered struc- tures in YMG and galactomannans. The proposed mixed-junction model for YMG and LBG is analogous to that of mixtures of xanthan and LBG. 131 Evidence has also

Seed Polysaccharide Gums

YMG-LBG at 0.1%

YMG -Guar at 0.1%

1.0 .8 dulus G' (Pa)

orage mo .2 St

YMG and Fenugreek Gum at 0.1% .2

0 20 40 60 80 100 120 Percentage of YMG in galactomannans

FIGURE 4.8 Storage modulus G' (Pa) of yellow mustard gum and three galactomannans at different ratios.

shown that the water-soluble (1 →4)-linked β-D-glucan is the active component that can synergistically interact with galactomannans.

Because of charged groups on the polysaccharide chains, the effects of pH and salt on viscosity are significant. Viscosity at a fixed shear rate (93.32 sec –1 ) increases

in both the lower and higher pH regions. An increase in temperature results in a continuous reduction in viscosity. 129

The surface active property of YMG makes it one of the best stabilizers for oil/water emulsion-based products. 129 Increasing YMG concentration up to 0.05% can substan- tially reduce the surface tension. The surface tension and interfacial activities of some plant hydrocolloids (guar and locust bean gums, etc.) are ascribed to the presence of proteins. However, the surface activity of YMG and its fractions appears independent

Functional Food Carbohydrates

of the level of protein contaminant. Emulsifying capacity and emulsion stability of YMG and its fractions are much higher than those of most of the commercial gums. 129

The applications of gums/hydrocolloids in food systems are based on their functional properties. Since YMG has demonstrated rheological properties similar to those of xanthan gum, similar applications can be expected for YMG, including synergistic interactions with galactomannans. For example, a salad cream product using a YMG:LBG (9:1) blend (at 0.3% w/w gum concentration) exhibited favorable stability and rheological properties comparable to those of commercial products containing xanthan gum with or without alginates. 132

A commercial salad dressing produced in New Zealand was stabilized with yellow mustard mucilage. There is a substantial amount of commercial ground yellow mustard products that contain ~5% YMG and are used in processed meats as condiments or bulking agents. The addition of a small amount of LBG to commercial yellow mustard flour will significantly increase the gelling strength and improve the rheological behavior of the products. For example, LBG was added to yellow mustard flour before being blended with meat systems, enhancing the rheological characteristics of the meat products.

Yellow mustard gum was also found to improve the texture and stability of a number of different plant starches. A strong synergy was observed between mixtures of yellow mustard gum and gelatinized wheat, rice, and pea starches. 133,134

A marked increase in viscosity was observed when yellow mustard gum was added to wheat and rice starches, which was attributed to entanglement with amylose released from the starch granule as well as with swollen amylopectin molecules. 134 In addition, syneresis of both of these starches was substantially decreased in the presence of the gum. Similar results were also observed for buckwheat, pea, and corn starches. 135 The ability of yellow mustard gum to modify the properties of plant starches should find applications in food and nonfood products.

Yellow mustard gum has recently been commercially produced in Canada for food and cosmetic applications. A skin care moisturizing lotion was prepared using YMG according to a formula including 15% YMG preparation. 136 The prepared moisturizing lotion has a yield stress and a shear-thinning flow behavior with a favorable hand feeling compared with lotions prepared with other commercial gums. 136 The solution of yellow mustard gum has a high viscosity in a wide range of pHs. It is an ideal stabilizer for salad dressings and fruit juice concentrates.

4.4.3.5 Physiological Properties and Health Benefits

Yellow mustard gum is a dietary fiber that is expected to exert some physiological effects typical for dietary fibers, including regularizing colonic function, normal- izing serum lipid levels, and attenuating the postprandial glucose response, and perhaps suppressing appetite. A study by Begin and coworkers 137 examined the effect of YMG and other soluble fibers on glycemia, insulinemia, and gastrointes- tinal function in the rat. They found that YMG, guar gum, oat β-glucan, and carboxymethylcellulose all significantly decreased postprandial insulin levels at

45 min, indicating a slowdown in glucose absorption. YMG decreased insulinemia primarily by delaying gastric emptying, while the other fibers increased intestinal

Seed Polysaccharide Gums

contents and consequently decreased absorption. 137 Viscosity was considered the major contributing factor to the improved insulin status at peak time. Since vis- cosity of YMG increases at acid pH, it exerts a stronger gastric effect than the other fibers examined. Jenkins and coworkers demonstrated that incorporating mustard fiber into white bread at levels not affecting palatability had a modest but significant effect of reducing the glycemic index of the bread in both normal and diabetic human volunteers. 138

A significant reduction in percent peak rise in post- prandial blood glucose was also observed.

The anticancer potential of yellow mustard gum was recently observed by Eskin et al. using male Sprague Dawley and female Zucker obese rats as models of sporadic and obesity-associated colon cancer, respectively. 139 At 8 weeks of age, rats were injected with azoxymethane, a specific colon carcinogen, and maintained on a basal diet with or without 5% yellow mustard gum for 6 weeks. A significant (p < 0.05) decrease in the number of advanced aberrant crypt foci (ACF > 7 crypts) was found in the case of Sprague Dawley rats, which accounted for a 50% reduction compared to the control diet. In the case of the Zucker obese rats, a 60% inhibition in the development of large colonic ACF was observed. These studies pointed to the potential of yellow mustard mucilage as a nutraceutical in the treatment of colon cancer.