P SYLLIUM M UCILAGE

4.4.1 P SYLLIUM M UCILAGE

4.4.1.1 Source

Psyllium is the common name of the plant genus Plantago. The seed coat (husk or hull) of the plant contains high levels of mucilage (dietary fiber) and has a

long history for medicinal use. Psyllium husks from Plantago ovato and Plantago psyllium are commercially produced in several European countries, Pakistan, and India, while the U.S. is the world’s largest importer of psyllium husk, mainly for pharmaceutical firms to make fiber-based laxative products. Recently, there has been a significantly increased interest in psyllium due to the FDA’s approval that producers of certain foods containing soluble fiber from psyllium seed husk (PSH), such as some breakfast cereals, may claim that these foods, as part of a diet low in saturated fat and cholesterol, may reduce the risk of coronary heart disease (CHD). 89

Functional Food Carbohydrates

4.4.1.2 Method of Production

Psyllium mucilage is produced by mechanical milling/grinding of the seed coat of psyllium seed, and the yield is about 25% of the seed weight. The milled seed coat is a white fibrous material that absorbs water quickly to give clear, colorless mucilaginous gel.

4.4.1.3 Chemistry and Structural Features

Psyllium mucilage is a heteroxylan consisting of (1 →3)- and (1→4)-mixed-linked β-D-xylopyranosyl backbone chains, with side chains attached at the 2- and 3- positions of the 1,4-linked β-D-xylopyranosyl residues. The side chains are com- posed of β-D-xylopyranosyl and α-L-arabinofuranosyl residues. In addition, a pectic fraction may also exist containing 1 →4-linked α-D-GalpA residues and small amounts of 1,2,4-linked Rhap and 1,3- 1,6-, and 1,3,6-linked Galp. 90

4.4.1.4 Functional Properties and Applications

Psyllium gum does not dissolve completely in water but swells to a mucilagenous dispersion with the general appearance of wallpaper paste. A 2.0% psyllium gum dispersion exhibits gel-like structure, similar to that of xanthan gum, which generates

a weak gel network by entanglement or association of rigid and ordered molecular structures. Increasing the concentration of psyllium gum from 1 to 2% gives a significant increase in gel strength. However, freshly prepared solutions/dispersions of psyllium gum (1%) show flow properties similar to those of disordered coils, with

a Newtonian plateau at low shear rate. Upon aging, psyllium gum solutions form cohesive gels and show obvious syneresis. The gels continue to contract on storage over long periods (to about 30% of their original volume after 3 months). This contraction process can be accelerated by freezing and thawing cycles. However, psyllium gum is stable in high-salt solutions (e.g., 2.5 M NaCl) that are formed by neutralization of the alkaline extract over a prolonged storage, with no evidence of gelation or precipitation.

Psyllium mucilage is used in ice creams and frozen deserts as a thickener and stabilizer. It is also used with other gums in bakeries to replace wheat gluten. For example, in preparation of gluten-free bread, psyllium gum and hydroxypropylme- thylcellulose (HPMC) at 2 and 1%, respectively, are added to rice flour to give a

loaf volume close to that of hard wheat control. 91 The effectiveness of the psyl- lium–HPMC in bread making is attributed to the network stabilizing properties of psyllium that stabilize the gas cells formed during proving, and preventing them from collapsing during the initial stages of heating in the baking oven. Further stabilization of the gas cells is due to the gelation of HPMC. Psyllium mucilage is also used in preparing highly soluble fiber foods, such as ready-to-eat breakfast

cereals and nutritional bars for their cholesterol-reducing effect. 92 In the U.S., the challenge is to incorporate the required amount of psyllium into one serving of a food product in order for the cholesterol-lowering claim to be allowed on the label. 92

Seed Polysaccharide Gums

Reducing gelling properties or water-holding capacity will help to include more psyllium in a food product. To achieve this, several techniques have been used to modify psyllium mucilage, such as physical, mechanical, and enzymatic means. 93 Lower-grade psyllium husk is used in landscaping as a water-binding agent for improving water retention of newly seeded grass areas or improving the transplanting success of woody plants. Psyllium husk is also used as an environmental friendly binder for fixing joints of interlocks in the landscape industry.

4.4.1.5 Physiological Effects and Health Benefits

Psyllium mucilage exhibited significant bioactivities in both animal models and human subjects. For example, psyllium has been shown to reduce total and LDL

cholesterol in animals and humans. A meta-analysis of 12 studies on 404 adults with mild to moderate hypercholesterolemia concluded that psyllium-enhanced cereal product reduced total and LDL cholesterols in 5 and 9%, respectively. 94 Other studies demonstrated that psyllium mucilage lowers plasma LDL cholesterol levels by 6 to 20% in mildly hypercholesterolemic individuals. Animal studies have been carried out to elucidate the mechanisms by which psyllium mucilage

brings about a reduction of cholesterol. 95 Psyllium mucilage increased the activity of cholesterol 7 alpha-hydrolylase, which is the rate-limiting enzyme in bile acid synthesis in guinea pigs. 96 Diet intake containing 10 to 20% psyllium mucilage significantly increased the gastrointestinal mucin level in rats, which correlated with the reduction of cholesterol. The increase of mucin level caused by psyllium

mucilage may protect these organs as well as alter nutrient absorption. 97 The mechanism of the LDL cholesterol-lowering action of psyllium was examined in the hamster, and the major effect is exerted at the level of LDL cholesterol

production. 95 In a human study, 125 patients with type 2 diabetes were treated with 5 g of psyllium mucilage for 6 weeks. It was found that the levels of fasting plasma glucose, total plasma cholesterol, LDL cholesterol, and triglyceride were significantly reduced (p < 0.05), while the HDL cholesterol level increased sig-

nificantly (p < 0.01). 98 It was also demonstrated that intake of psyllium reversed the hypercholesterolemic effect of trans fatty acids in rats. 99

Many epidemiological and experimental studies suggest that a low-fat, high- fiber, high-calcium diet has a strong protective effect against colon cancer. The

combination of wheat bran and psyllium mucilage exhibited a synergistic effect in inhibiting earlier phases of carcinogenesis. 100

Psyllium gum has been used as a demulcent in dysentery, erosion of intestines, dry coughs, burns, excoriations, and inflammations of the eyes. 101,102 It is extensively

used as a bulk laxative. The water-holding capacity and gelling property of psyllium gum can be used to delay and reduce allergic reactions by holding toxins and allergens in the gel structure. Although there were no adverse effects for consumption of psyllium seed or husk, the recommended daily intake is between 10 and 30 g/day in divided doses. It is also recommended that psyllium mucilage be kept well hydrated before taking. However, some individuals may be allergic to psyllium mucilage; thus, caution must be taken in this regard. 103,104

Functional Food Carbohydrates