Name: Hai Truong
Course: Chem 367
Final paper:

Applications of Guar galactomannan in food and oil field industries


Cluster_bean-guar-Cyamopsis_psoralioides-Cyamopsis_tetragonolobus-TAMIL_NADU73.jpg
Image 01: Guar bean cluster (21)

I. Introduction:

Guar galactomannan is a kind of plant polysaccharides that has applications in many fields, but this research paper just focuses only on food and oil field (12). Guar polysaccharide, also called guar gum, is extracted from seed of guar bean. Guar is grown in many parts of the world. It is mainly cropped annually in India, and Pakistan, with fewer amount produced in United States, Australia, and Africa (4, 7). Every year, Unites States imports over 91 million lbs of guars from India and Pakistan, countries that produce 60% amount of guar around the world in 2003 (4). Guar gum (Cyamopsis tetragonoloba) is contained in endosperm of guar bean (4).
The structure of guar gum polysaccharide comprises of a linear backbone of β(1,4)-linked D-mannose units with various amounts of α (1,6)-linked D-galactose side chains (12). And the ratio of mannose to galactose is 2:1 (12). Because guar gum is generally odorless, tasteless, and colorless, have low energy value and digestibility, it is used as key additive in food product to control quality of food and to increase value of nutrition (8). Because guar is high molecular weight hydrophilic polymer, it is also used as functional ingredient in food industry. It can modify and control the rheological properties of food product. A small addition of guar in food formulation can enhance viscosity and stability of dispersions (6). Due to low cost and characteristics of hydrating fast in cold water to form highly viscous solution at low concentration, it was used in oil field as thickener, stabilizer, and emulsifier (12). The molecular weight is around 220,000 to 300,000 (12). Industrial-grade guar gum is mainly used in oil field as drilling fluids, while the purified grade is mainly used in food and pharmaceutical industry (12).

Screen_shot_2010-12-03_at_12.42.25_PM.png
Figure 01: Basic structure of Guar gum (2)

II. Viscosity of guar polysaccharides:

One of the most significant behaviors of guar gum is that it can hydrate and build up viscosity at low concentration. At the Oxford College of Engineering, Bangalore, INDIA, several guar solutions were prepared in the concentration ranging from (0.05-0.3% w/v) of guar for intrinsic viscosity measurement (12). Intrinsic viscosity values of those solutions were measured by using Ostwald viscometer at various pH from 2-13.4. From intrinsic viscosity values, the of isolated molecules was determined by using equation:
Screen_shot_2010-11-30_at_10.50.05_PM.png(12)
The intrinsic viscosity was determined by taking specific viscosity at various concentration and extrapolating the concentration to c=0 (12).
The concentration dependence is also expressed by Huggins equation as:
Screen_shot_2010-11-30_at_10.52.21_PM.png (12)
Where k =Huggins constant
From the intrinsic viscosity, viscosity average molecular weight Mv was estimated using Mark-Houwink’s equation
Screen_shot_2010-11-30_at_11.11.49_PM.png(12)
Where
η= Intrinsic viscosity obtained from the graph.
K = Proportionality constant
M = Molecular weight of the polymer
a = Shape factor

K and a are also called as Mark-Houwink constants and their values are obtained from the literature as K = 3.8*10-4 and a = 0.732 (12). The molecular weight at different pH of guar gum were tabulated in table below:

Screen_shot_2010-11-30_at_11.27.09_PM.png
Table 01: The molecular weight at different pH of guar gum (12)
As seen above, the intrinsic viscosity value was maximum at pH 6.4, and it tends to decrease when pH is above or below 6.4. Hence, the acidic or basic condition may attributed to partial hydrolysis of polysaccharides and also cause the variation in the dielectric constant of solvent resulting in a decrease in the molecular weight (12). In addition, molecular weight is directly proportional to intrinsic viscosity, so decreasing in molecular weight leads to reducing in viscosity. Hence, viscosity of guar depends on its existing molecular weight and solution pH. In the next step, a profile of viscosity changes of solutions at different guar concentration over 5 hours hydration was plotted (12).

Screen_shot_2010-12-03_at_8.14.43_PM.png
Figure 02: Viscosity developments at shear rate 0.5s^-1 (12)
Apparently, higher guar concentration will result in higher optimum viscosity after 5 hours. Even though the measurements was taken over five hours, all solution nearly reach their maximum viscosity within only one hour. These four solutions was prepared in increment of 0.2% guar difference, but the differences in viscosity were not even. At around 1% concentration, an increment of 0.2% can cause double viscosity (between red and blue line).

III. Applications in food industry

Guar bean is used as food in daily diet. There are many researches around the world on the combination of wheat flour and guar for its advantageous behavior hypocholesterolemic and hypoglycemic agent in reducing diseases (4). Guar is a nutritious agent due to high soluble fiber value that can reduce and prevent high glucose concentration. Soluble fiber is especially a need for daily diet to lower cholesterol and glucose level (4). Dietary fiber is the potential material for fighting disease in human health. There are two kinds of fiber: soluble and insoluble. Insoluble fiber helps increase feeling of fullness after the meal and prevent over intake of nutrition per serving. Soluble fiber, like guar, can hydrate in intestine to increase amount of water in the stool, which can help decrease amount of cholesterol and glucose after meal (4). The appearance of fiber results in increasing secretion of bile acid into faces that lower absorption of dietary lipid and fatty acid synthesis in liver. It also suggests that an addition of 10g fiber intake daily can help reduce risk of disease by 17% (4). Hence, guar gum, as dietary fiber, can help people to manage weight loss, especially in obesity or diabetics. The fiber helps people have fullness feeling that decreases food intake per serving and increase the release of fat and nitrogen. Gel forming fibers are more effective than non-gel forming fiber in weight control because gel-forming fiber hydrates immediately in intestine, more water was obtained in the diet (4). Volume of food serving is bigger than is actual value, which contributes to fullness feeling and reduce amount of calorie intake as well as glucose absorbed. In a study, guar was proved more effective than glucose and bran groups in managing weight (4). Finally, viscous gel form of guar help to slow down the breakdown of food in intestine and cause delay in absorption of nutrients. (4)

In the portable water treatment, guar gum can be effectively used as flocculant aid (9). Commonly, polyacrylamide serves as popular flocculant aid. But there is potential danger that polyacrylamide can release toxic arylamide monomer in drinking water raising health problem. Hence, the health risk of population should be concerned. Due to the lack of reliable analytical technique to detect potential toxic substance, guar gum can be used as a sufficient alternate for suspicious polyacrylamide. Furthermore, it is not only safe to use, but also improve quality of treatment system by reduce raw water turbidity from 26.5 to 1.0 (9). Guar is considered as "green" alternate.

There are more and more people pay careful attention on nutrition facts printed on food product's cans. Nutrition becomes a main theme against high fat disease. Meatball in daily meal is a high fat product. Typically, meatball contains approximately 20% to 30% fat, so it is essential to reduce this amount to 10% (20). However, qualities of meatball in the market are not about fat level, but also flavor, texture and mouth feel. Reducing fat will also result in increasing the toughness of meatball and decreasing acceptability of the product (20). To balance between healthiness and deliciousness, food researcher discovered that guar bean is one of the solutions. By adding guar bean to recipe, toughness would be decreased in cooked meatball. Also, guar can improve a quality of adhesiveness in raw meatball. In a study, it is observed that fat reduction, from 30% to 12% or 5%, caused a decrease in adhesiveness of meatballs (20). Fat and heat when cooking can decrease the adhesiveness of meatball (20). Guar gum can hydrate well and form viscous gel that can improve adhesiveness of raw and cooked meatball as well. Lastly, guar gum also has a significant effect on springiness of meatball with fat level at 15% and 10%. When fat is reduced from 30% to 10% or 15%, the springiness increase because of protein component. However, the springiness of cooked meatballs will decrease with addition of 1% guar gum (20).

Polysaccharides, included guar gum, are often used to stabilize oil-in-water emulsion and control food rheological properties of food dispersion (8). Rheology is defined as study of fluid flow (12), so guar, a thickener, can control the dense texture of liquid based food. Guar is generally odorless, tasteless, colorless, so it is easily used in egg yolk product to obtain the desire texture. Egg yolk product includes mayonnaise and salad dressings, in general, bakery products and cakes, custard, etc (8). In these products, emulsion stability is important because it maintains the homogeneity of the product. For example, it would be bad if mayonnaise bottle has two layer, thick-creaming layer on top and liquid like on the bottom. In a study, two products were prepared with and without guar gum. It was showed that in the presence of guar gum, the extent of creaming decreased significantly with increasing guar gum concentration (8). Creaming is a movement of substances in a solution to the top or bottom depending on substances' density relative to solvent, so creaming is emulsion- unstable. Hence, decreasing creaming means improving emulsion stability. This can be explained that high concentration of guar gum attributed to the increase in continuous phase viscosity, and higher viscosity solution can hold its particle suspended evenly throughout the solution. Guar gum is sometime used in soups, desserts, and pie-fillings (12). An example is guar addition in hot pepper soybean paste (HPSP). HPSP is comprised of solid particles (pepper or soybean powder) dispersed in a continuous liquid phase (6). Guar, with its ability to form gel phase, can increase gel quality of HPSP and disperse pepper and soybean particles thoroughly in paste.

An important application of guar gum is an alternate for gluten in rice cake (19). Due to the fact that Celiac disease patients are allergy to gluten, which is related to inflammation of the small intestine leading to malabsorption of several important nutrients and damage of intestinal mucosa (19). The only treatment for this disease is to avoid food that contains gluten. Hence, gluten-free product is at high demand because celiac disease is increasing: “As much as one in 200 to one in 350 of the population of Europe, one in 250 to one in 500 of the population of the USA, and one in 300 to one in 500 of the population of Turkey suffer from celiac disease (Tandoruk, 2005)” (19). To deal with this situation, polymeric substance, such as hydrocolloids, is used instead to mimic the viscolastic properties of gluten (19). Furthermore, a research found that addition of guar gum or other gum in formulation of cake can improve quality of gluten-free cake. Firstly, guar gum can reduce weight loss of cake baked in the oven (19). Higher concentration of guar gum, the more it can decrease weight loss of baked cake in oven (19). Hydrocolloid guar also increases hardness of baked cake during storage after baking (19). However compared to other gum, such as xanthan, guar is most effective in increasing cake hardness. Two graph below show effect of different gum at different concentration on hardness of baked cake during storage in microwave–infrared oven (19)

Screen_shot_2010-12-01_at_4.06.17_PM.png
Figure 03: Effects of gums and storage times on hardness of the cakes baked in Microwave-Infrared oven (19)
As seen on graph, cake contains guar gum (0.1%) show greater hardness compared to xanthan gum or gum blend.

In public health issue, egg-associated salmonellosis is considered as a significant problem in the United States and European countries. A normal-look egg can contain a bacterium Salmonella enteritidis that causes egg-consumers illness if raw or undercooked egg is used. This disease may come from infection of Salmonella when laying hens are stressed under bad condition that impair the immunological function and increase the susceptibility of layers (10). One of the reasons that cause laying hen stress is using long-term feed withdrawal to induce molting of late-phase laying hens. Many commercial egg producers in the United States used this method to stimulate multiple laying cycles and restore egg quality (10). One way to prevent this disease and improve the quality of products in egg production is to inducing molt in laying hen involves full-feeding grain and legume by products rich in indigestible plant fibers, such as guar meal. Mannose backbone unit of guar gum is effective in prevent Salmonella cells to bind and colonize on intestinal epithelial cells of laying hen and cause diseases in eggs (10). A research has shown that when hens is molted by 20% guar meal diets, the number of Salmonella Enteritidis colonization on layers’ organs is significantly reduced (10).

IV. Application in oil field

a. Fracturing process in oil field

One of the most important applications of guar glactomannan in oil field is fracturing fluid. Once the oil field is detected and drilled into rock formation deep down underground or ocean bottom, oil and natural gas will be released from that drill. Then the goal is to open the crack wider to get maximum flow of oil fluid. To maximize fracture, a process of fracturing will be performed. The best way to fracture is hydraulic fracturing using fracturing fluid. Fracturing fluid is in form of high viscous gel that will be pumped into pre-existing fractures or flow pathway. This fracturing fluid contains guar, cross-linking agent, proppant and water (15). Proppants are sands or coarse particles. Then, an extremely high pressure will be applied in a short amount of time (in hours). The pressure must be high enough to fracture rock and lead to the penetration of proppant and gel into fracture (15). There are two roles for fracturing fluid containing proppant. First one is to break apart rock formation, and second role is to carry those proppants (sands or coarse particles) into rock formation. When the rock is cracked and filled with this fluid, pressure is released. The release of pressure will allow the closure of fracture onto proppants. When gel part is removed by clean-up process, proppants remain between fractures will play a role as a permeable pack that keep fracture open for oil production. In clean-up process, guar is removed from proppant-packed area by thermal degradation or chemical breakers (15). High temperature of oil formation will degrade gel to low viscous fluid that can be pumped back out of oil field. Hence, the role of guar gel is very important in successfulness of the hydraulic fracturing process (15) . It must satisfy several requirements as fracturing fluid. Firstly, it must have sufficient viscosity under high pressure to break, propagate, suspend and penetrate the proppant into fractured rock formation (15). Secondly, because fracturing process lasts for several hours at extremely high temperature, gel must resist to thermal degradation for at least few hours to maintain its high viscous behavior. Thirdly, it has to be easy to remove from the formation. Guar is the most popular polymer to use as main component of fracturing fluid. Guar itself cannot form highly viscous gel. Even those guars can hydrate fast and from viscous solution at low concentration, it also can cross-link with a cross-linking agent and other additives to yield a good gel. Common cross-linker used is boron, borax or zirconium. Additives are used to maintain optimum viscosity at high temperature during fracturing process.

b. Guar gum derivatives

Because the structure of guar gum have three hydroxyl groups on D-mannose or D-galactose sugar units, it is possible these polysaccharides can be modified to yield different behaviors such as solution clarity, alcohol solubility, or thermal resistances (2). The maximum degree of substitution (DS) in guar gum is three. The most popular guar derivatives include Carboxymethyl guar (CMG), Hydroxypropyl guar (HPG), or Carboxymethylhydroxypropyl guar (CMHPG).


Screen_shot_2010-12-02_at_10.49.11_AM.png
Table 02: Guar gum derivatives and their structures (7)
Carboxymethyl guar is modified guar consists of a 1,4- b-D-mannose backbone with 1,6-a-D-galactose side groups (1). It is modified by adding a carboxymethyl group on galactose side chain of each guar unit. Its chemical structure is shown below:

Screen_shot_2010-12-03_at_8.27.13_PM.png
Figure 04: Carboxymethyl Guar (CMG) structure (1)
Hydroxypropyl guar (HPG) is the most commonly used derivative of guar gum. HPG is formed by reaction of native guar gum with propylene oxide in the presence of an alkaline catalyst (5). As a result, it is composed of a linear backbone of (1-4)-beta linked D-mannose units with (1-5)-alpha-linked D-galactose units randomly attached as side chains (14).

Screen_shot_2010-12-03_at_8.23.42_PM.png
Figure 05: Hydroxylpropyl Guar (HPG) (16)
Carboxymethyl hydroxypropyl guar (CMHPG) is also a derivative of guar gum. It is synthesized by reacting HPG with sodium monochloroacetate. As a result, CMHPG can hydrate and resist to thermal degradation better than HPG and guar (16).

Screen_shot_2010-12-03_at_8.25.53_PM.png
Figure 06: CMHPG molecular structure (16)

c. Crosslinking system

Even though guar can hydrate well at small concentration, it is more effective to use cross-linking agent to build up. With a small amount of cross-linkers such as borate ion, viscosity can be increase up to many times (11). In other word, with cross-linker, smallest concentration of guar can form a very stable cross-linked gel (13). Hence, the first advantage is cost-effective. In addition, build up viscosity by increasing concentration can cause damaged oil formation. The most widely known crosslinking agents are borate ion and zirconium ion (2). Borate ion is usually used to cross-link with native guar or HPG while zirconium is used to cross-link with ionic guar, such as CMG or CMHPG (13). Borate ion is used at low temperature (about 25C) while zirconium can be used at extremely high temperature (about 205C) (2). Borate cross-linker is only activated in solution at pH 8-11. Because boric acid is weak acid, and pH has to be high enough that borate ion can exist and involve in gelling process (18). However, if pH above 11, different forms of borate complex is occurred and also do not take part in cross-linking process (2). At pH 8-11, borate ion is mostly appeared and takes place in the process. Borate is distinguished from other cross-linkers because its cross-linking with guar is a reversible complexation reaction (11). This reaction can be reserved by mechanical shear rate or lower pH(11). At lower pH, below 8, borate ion is trapped by hydrogen ion and is not available for cross-linking (18). Cross-linking is understood by the interaction of borate ion to two guar units at adjacent cis-hydroxy groups on the mannose backbone(11):

Screen_shot_2010-12-02_at_9.21.22_AM.png
Figure 07: The equilibrium of borate ion complexation with cis-hydroxy pairs on a Guar Gum (2)
Zirconium ion cross-linker is widely used since 1970s because it is flexible in wider range of pH from 1 to 12 and temperature range from 37C to 205C (2). However, when temperature reach above 205C, cross-linking occurs so fast and is followed by thermal degradation. Usually, zirconium cross-linker containing triethanolamine can maintain viscosity up to 121C, while cross-linker containing hydroxylalkylated ethylenediamine can maintain viscosity above 121C to 135C (17).
This cross-linker is introduced to solution in form of chelating complex, such as lactate or citrate (3). Unlike borate, there are three possible position that zirconium ion can cross-link those polysaccharides together. The first ways is by hydrogen bond, hydrogen bonding occurs normally at pH 4-10 between non-ionic guar derivatives such as pure guar gum or HPG and zirconium chelate. However, hydrogen bond is easily broken by high shear rate and reformed over long period of time (2).

Screen_shot_2010-12-02_at_9.24.13_AM.png
Figure 08: Hydrogen-bonding mechanism for zirconium-guar (2)
The second way to cross-link is by covalent bonding. Covalent bond is formed between ionic guar such as CMG or CMHPG with cross-linker by carboxylate group on polysaccharides. This covalent cross-linking is a lot stronger than hydrogen linking, and not easily broken by high shear rate (2).

Screen_shot_2010-12-02_at_9.24.23_AM.png
Figure 09: Covalent bonding mechanism for zirconium-guar (2)
The third ways to cross-linking is linking between colloidal particles formed by partial hydrolysis of metal chelate at high pH above 11 with guar molecules.
In summary, guar and its derivatives with high potential for different method of forming strong gel solution are perfect to sever as fracturing fluid in oil industry (2).

V. Conclusion

Guar gum comprises of many qualities: low cost, well hydration, high stability, high nutrition, and modifiable ability that makes it high demand in many applications, especially in food industry and oil field.

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