F LAXSEED G UM

4.4.2 F LAXSEED G UM

4.4.2.1 Source

Flaxseed is the seed of flax plant (Linum usitatissimum), a member of the Linaceae family. Flaxseed has a flat and oval shape with average dimensions of 5 mm in length, 2.5 mm in width, and 1.5 mm in thickness. 105 The seed coat (testa) contains

a thick mucilage (epidermis) layer that can be easily identified by microscopy. This mucilaginous material is a secondary cell wall polysaccharide that is soluble in cold

water. The high content and easy extraction of mucilage make flaxseed gum a potential commercially viable gum.

4.4.2.2 Method of Production

4.4.2.2.1 Extraction of Gum from Flaxseed Flaxseed gum’s characteristic of easy dissolution in cold water allows the use of mild conditions to extract the gum from raw materials. A typical extraction procedure is to soak the seeds in water at various temperatures with stirring for 3 to 16 h. 52 Temperature and time have a significant effect on the yield and composition of flaxseed gum. 106 For example, higher temperature could increase the gum yield from

5 to 9%, but it also increases the content of proteins in the gum. 106 The protein contaminants can be partially removed by treatment with Vega clay. 106,107 There is an optimum extraction condition that gives a relatively high yield of gum (ca. 8%) with low levels of protein contaminants (<8%). 108 The optimum condition was iden-

tified as a temperature of 85 to 90°C, a pH between 6.5 and 7.0, a water:seed ratio of 13:1, and an extraction time of 2.5 to 3 h. 108

The aqueous extract is then filtered or centrifuged to remove solid particles, and the supernatant is precipitated in organic solvent, followed by drying and

grinding to obtain powder products. Spray drying is an alternate way of drying flaxseed gum. However, the final products often have lower viscosity due to the

high outlet temperature. 109

4.4.2.2.2 Extraction of Flaxseed Gum from Flaxseed Meal Since linseed oil is the primary product of flaxseed, a large amount of flaxseed meal

is produced as a by-product from the oil crushing industry. Solvent-extracted meal could be separated by air or screened to obtain kernel and hull fractions. 110 The hull fraction is then extracted with water (water:solid ratio, 30:1; pH 4.5; temperature,

60 to 80 o C). The extracted liquid is centrifuged or filtered, adjusted to pH 7, con- centrated by rotary evaporation, and spray dried. The meal cake can also be extracted with 5% sodium chloride, followed by centrifugation with activated carbon, and alcohol precipitation. The flaxseed gum produced from commercial meals can be useful as an emulsifying agent for chocolate milk and other food products because it frequently contains high levels of proteins. Iron salt can be added to the extraction solvent to prevent extraction of tannin pigment. 111

Seed Polysaccharide Gums

4.4.2.2.3 Extraction from Hull Because the mucilage is deposited in the outer layer of the seed coat, it will be more efficient to extract the gum from the hull if it can be separated from the kernel. A recently patented technology enables the separation of flaxseed into a hull fraction and a kernel fraction on an industrial scale. 112,113 Flaxseed hull produced by this technology is rich in fiber, lignans, and other nutrients. The hull is first extracted with organic solvents to give a high-lignan product, and the residue is further extracted with water to produce flaxseed gum. The extracted gum can be recovered by spray drying or precipitation in organic solvents.

4.4.2.3 Chemistry and Structural Features

Flaxseed gum contains 50 to 80% carbohydrates, 4 to 20% proteins, and 3 to 9% ash. The large variation in chemical composition is mostly due to the raw materials, such as varieties and growing conditions, and, more importantly, to the form or part of the material used for extraction (e.g., whole seed, hulls or meal, etc.). The extraction solvent, pH, temperature, and other processing conditions also have a significant influence on the chemical composition of flaxseed gum. 106,107,114–119 Table

4.2 displays the variations of chemical and monosaccharide composition of flaxseed gums extracted from different flaxseed cultivars and breeding lines. 116 Flaxseed gum has two major fractions: a neutral polysaccharide mainly com- posed of xylose, arabinose, and galactose, and an acidic polysaccharide consisting of D-galactose, L-rhamnose, and D-galacturonic acid. The neutral arabinoxylan has

a (1 →4)-β-D-xylosyl backbone to which arabinose and galactose side chains are attached at positions 2 and 3. 115 The acidic polysaccharide has a backbone consisting of (1 →2)-linked α-L-rhamnopyranosyl and (1→4)-linked D-galactopyranosyluronic

TABLE 4.2 Variation of Monosaccharide Compositions of

Flaxseed Gum from Different Cultivars 116

Flaxseed Gums a

Uronic acid (%) b 21.0 25.1 23.9 15.7

Sugar composition (%) c

Rhamnose 21.2 27.2 25.6 12.8 Fucose

2.1 3.6 8.2 3.7 a Flaxseed gums were extracted from four cultivars/breeding lines.

b On dry base. c Relative composition.

Functional Food Carbohydrates

TABLE 4.3 Intrinsic Viscosity of Flaxseed Gum and Its Fractions 114

Flaxseed Gum a Intrinsic Viscosity (ml g –1 , in 1 M NaCl)

Norman

CFG b 483.0

Note : CFG = crude flaxseed gum; NFG = neutral flaxseed gum; AFG = acidic flaxseed gum.

acid residues with side chains of fucose and galactose. The ratio of L-rhamnose, L- fucose, L-galactose, and D-galacturonic acid is about 2.6:1:1.4:1.7. 119,120 The molec- ular weight of the neutral polysaccharide fraction is much higher than the acidic fraction, as revealed by size-exclusion chromatography, which is in agreement with their hydro- dynamic volumes (as indicated by intrinsic viscosity), as shown in Table 4.3. 114

4.4.2.4 Functional Properties and Applications

Flaxseed gum exhibits Newtonian flow behavior at low concentrations and shear- thinning flow behavior at high concentrations. However, the broad variation in chemical composition allows some flaxseed gum to exhibit stronger rheological properties, such as formation of gel, while others may behave like a viscoelastic fluid. 113 There is an apparent correlation between the structural features of flaxseed gum and its rheological properties: gums that contain high levels of neutral polysaccharides (arabinoxylan) are more viscous and exhibit shear-thinning flow behavior; in contrast, gums that contain higher levels of acidic polysaccharide have a much lower viscosity and exhibit Newtonian flow behavior. 116 Evidence has shown that the neutral arabinoxylan is responsible for the high viscosity and shear-thinning flow behavior due to its higher molecular weight. In contrast, the acidic polysaccharide fraction has a much smaller molecular weight, and therefore exhibits lower viscosity and Newtonian flow behavior. The roles of neutral and acidic polysaccharides were verified by examining their molecular weight and rheological properties separately. 115,118

Dynamic oscillatory rheological tests showed that flaxseed gums extracted from different varieties exhibited wide variations in their viscoelastic properties. It can

be a typical viscoelastic fluid or a real gel when examined at the same concentration (1 to 3%). 118 The viscoelastic properties of flaxseed gum are also correlated to the

amount and molecular weight of the neutral fraction (arabinoxylan). 114,118

Seed Polysaccharide Gums

The pH of the gum solution has a significant effect on the flow behavior and viscosity of flaxseed gum. The lowest viscosity for flaxseed gum is observed at pH

2. As the pH increases, the viscosity increases steadily until pH 8, at which the viscosity is three times its value at pH 2. Further increase of pH results in decrease of viscosity. 106 Depending on the source and chemical composition of the gum, the effect of pH on solution viscosity varies slightly, but generally follows the trends of high viscosity at neutral pHs and lower viscosities in both high and low pH regions.

Similar to other gums, flaxseed gum can be used as a thickener and stabilizer in food products. Flaxseed gum affects bread-making properties including pasting,

dough rheology, and baking. Adding flaxseed gum to bread formulations improved the grain texture of the bread loaves. 121 The cells were more elongated, and the

texture was silky and softer after storage. The arabinoxylan component may have played a role in its ability to delay the firming. The gum may have a mellowing

effect on the gluten, which allows for greater expansion during the fermentation and baking stages. 121

The addition of flaxseed gum could improve muffin height and volume, without changing the texture significantly; 0.5% flaxseed gum (flour basis) could replace 0.1% xanthan gum or guar gum to produce muffins with good volume and texture. 122 In the evaluation of the effect of flaxseed gum on the stability of a model salad dressing, pH affected the stability of the emulsion the most. The most stable emulsion occurred at pH 6, while the least stable occurred at pH 2; this observation correlates with the viscosity dependence of pH: flaxseed gum exhibits the highest viscosity at pH 6 and the lowest viscosity at pH 2, as described previously. Flaxseed gum appears to act as a steric stabilizer, which means sufficient adsorption of flaxseed gum is required to cover the particle surface completely to prevent two particles from approaching one another. 122

Flaxseed gum is also considered to be a better water-in-oil emulsifier than Tween

80, gum arabic, or gum tragacanth. Concentrations of 0.5 to 1.5% flaxseed gum are suitable to stabilize oil/water emulsions. In food applications, flaxseed gum has been used as egg white substitutes in bakery products and ice creams. The strong buffering action of flaxseed gum also makes it useful in the manufacture of fruit drinks. Other applications of flaxseed gum include use in the printing, textile, and cigar industries.

A paper product using flaxseed gum as a deflocculant has good tensile and flexural (tear) strength.

4.4.2.5 Physiological Properties and Health Benefits

A diet supplement with partially defatted flaxseed reduced the level of LDL choles- terol in serum, and flaxseed gum was identified as the likely responsible active ingredient. 123 It has been suggested that flaxseed gum could also be used in medicinal preparations — ointments. Pastes containing flaxseed gum are effective in the treat- ment of furunculosis, carbunculosis, impetigo, and ecthyma. 124 Flaxseed gum has been used as a bulk laxative, a cough emollient agent, and a stabilizer in barium sulfate suspensions for x-ray diagnostic preparations. 125,126 Tablets prepared with flaxseed gum have improved disintegration and a slower rate of drug release. The stringy and fast drying properties of flaxseed gum make it suitable in hairdressing

Functional Food Carbohydrates

preparations, hand cream formulations, and denture adhesives. At 2.5% concentra- tion, flaxseed gum is a good base for an eye ointment.

Flaxseed gum solution has been used as a saliva substitute due to its lubricating and moisture-retaining characteristics, resembling those of natural saliva. 127 It is effective for patients suffering from dryness of the mouth (xerestomia), and partic- ularly suitable for reducing the dryness of the mouth at night. 127