Saturday, November 15, 2014

Tara Gum: Better than Guar Gum?


Introduction

Tara gum is a natural hydrocolloid polysaccharide obtained from the endosperm of the seeds of Caesalpinia spinosa (Fam. Leguminosae), a tree native to Peru.  Similar to guar and locust bean gums, tara gum is a high molecular weight galactomannan with a mannose-to-galactose ratio of 3:1.  It is cold water swelling and pseudoplastic with viscosity similar to that of guar gum.  About 1000 to 1500 tons of tara gum is produced annually [10].  It is commonly used as a food additive in fruit products, baked goods, gluten-free bread, ice cream, frozen products, dairy products, gravy, pudding, and meat products.  It can be used with other hydrocolloids to improve gel properties, preserving favorable texture and functional properties in food products.



Caesalpinia spinosa (Fam. Leguminosae)

Caesalpinia spinosa is a thorny tree native to the Cordillera region of Peru and Bolivia in South America, where the fruit grows from April to December [1]. It can grow at an elevation of up to 3,000 meters and can tolerate dry climates and sandy soils.  It can grow to a height of several meters with spreading, grey-barked leafy branches of up to 35 cm.  The fruit is a flat oblong indehiscent reddish pod (Figure 1) containing 4 - 7 large round black seeds consisting of 22% endosperm by weight.  Pods are usually harvested by hand and typically sun dried prior to processing [1].

Medicinal uses of Tara include treatment for inflamed tonsils, wounds, fever, cold and stomach aches.  The fruit pods are being used to develop substitutes for phenol in phenol-formaldehyde adhesives [3]. Tannin concentration is highest in the pods, which are ground into a powder that can be used as an environmentally friendly tanning agent [3].  Tara seeds are a rich source of a high molecular weight polysaccharide gum that can be used as a functional ingredient in food products.  To produce commercial tara gum, the tara seeds are threshed, sieved, roasted, split, and sieved to produce a white to yellow powder that is at least 80% galactomannans [4] [5].



Structure

Tara gum is a galactomannan, which is a polysaccharide consisting of a mannose backbone with galactose side groups.  The backbone of tara gum is a linear chain of β(1,4)-linked D-mannopyranose units with α(1,6)-linked D-galactopyranose  units every third backbone unit (Figure 2), but this ratio can vary depending on various environmental factors [8] [12].  The backbone chain has been shown to be linked with occasional α(1,4)-galactopyranosidic side chains [14].  The molecular weight is on the same order of magnitude as other galactomannans such as guar gum and locust bean gum [7].  Literature data on 3:1 ratio galactomannans report experimental sample values with average molecular weights of 1 – 2 x 106, corresponding to 5000 – 10,000 backbone units [6].




The mannose-to-galactose ratio of tara gum galactomannans is around 3:1, with fairly regular blockwise substitution patterns.  However, they have been found to possess both random and blockwise substitution patterns [8].  The chains contain more enzyme-degradable sequences than randomly substituted chains from locust bean gum, suggesting they have a more regular substitution pattern [8].

Galactomannans in solution have an extended linear conformation and occupy large volumes of gyration, both of which depend on the degree of substitution [6] [10].  The galactose side groups tend to hydrogen bond with bare backbone regions, resulting in tightly coiled chains, especially in those with stretches without substitution (Figure 3).  Conversely, blockwise substitution creates a more elongated and stiffer chain (Figure 4) [6].  

Galactomannans show lyotropic and thermotropic liquid crystallinities at a certain concentrations in water [14].  The orthorhombic unit cell dimensions of tara gum galactomannans are a = 8.9, b = 24.17, and c (fiber repeat) = 10.46 Å [12].  This suggests that the galactomannan chains are organized into a lamellar structure held together by mannan-mannan interactions in the ac plane and largely identical with those in the native mannan.  The galactose side groups then protrude from either side of this sheet.  Calculation of the size of tara gum suggests a crystallite length of 10 nm [12].




Properties

Tara gum is a white to yellow odorless powder with composition that can vary significantly depending on the manufacturer.  FAO specifications of maximum levels for food-grade tara gum are as follows [7]:

Moisture 15%
Protein 3.5%
Fiber 2%
Ash 1.5%



Chemical

Tara gum is a non-ionic high molecular weight hydrocolloid polysaccharide composed primarily of a mannan backbone with galactose side groups linked every third mannose unit on average.  These chains are rich in exposed hydroxyl groups, which enable them to solubilize in water.  Specifically, the monosaccharide units contain cis-hydroxyls (2,3-positions on mannose and 3,4-positions on galactose), which along with a relatively linear configuration leads to strong chain interaction via hydrogen bonding [10].  Highly and regularly substituted chains inhibit this aggregation and promote solubility.  Heating gum in solution can disrupt these hydrogen bonds and further promote solubility.  Solubility is also promoted by a shorter chain length [9].

Tara gum is about 70% soluble in water at ambient temperature that produces 75% of its full viscosity potential [7].  Heating to 80 – 95 oC will fully solubilize the chains, and subsequent cooling produces opaque tan-colored solution with high viscosity at relatively low gum concentration without forming gels on their own, making tara gum an effective thickener and stabilizer in food systems [7] [12].  



Gel Formation

Tara gum is sufficiently substituted to inhibit gel formation by itself.  However, the addition of large amounts of compounds such as sucrose can reduce the available water to the point where chains can interact and form a gel.  Solutions of tara gum can create gels through crosslinking with certain metal cations as well as borax, which binds cis-hydroxyls at pH greater than 9.0 [7].  Weak gels can be created by freeze-thawing at 0.75% concentration, but these created a suspension of gel particles rather than a uniform gel. Tara gum can interact with other polysaccharides such as xanthan, agarose and kappa-carrageenan, forming gels with increased strength and elasticity with less syneresis [7].

Galactomannans interact with xanthan through destabilization of its helix, facilitating interaction (Figure 6) [31].  Galactomannans act to denature the helix-coil equilibrium of xanthan and displace ordered conformation [31].  The interaction of xanthan gum with galactomannans is dependent on several factors, including the degree of substitution, the ratio of the mixture, pH and ionic environment [31] [32].  Less substituted galactomannans react more strongly with xanthan [31].  Optimum gum ratios are approximately 80:20 guar gum:xanthan gum and 50:50 for locust bean gum:xanthan gum (Figure) [32].  Generally, the synergism with xanthan is at its maximum in deionised water at neutral pH and is reduced at high salt concentration and low pH [32].

Tara and xanthan gum mixture at 0.2% of total gum does not gel at room temperature, but can gel at 0 oC [33]. Stronger interaction occurred in mixtures containing deacetylated, deacylated, or native xanthan than with depyruvated xanthan.  The maximum dynamic modulus was obtained when the ratio of tara gum to xanthan gum was 1:2.  The dynamic properties using deacetylated and deacylated xanthan decreased rapidly above 25 and 20°C, respectively [33].




Rheology & Viscoelasticity

The viscosity of a fully dissolved 1% solution was measured at 5000 – 6000 cPs (Brookfield RVDVE, 20 RPM, Spindle 4, 25 oC) [30].  The viscosity of a cold water-dissolved 1% solution was measured at 3500 – 4000 cPs after 30 minutes and 4500 – 5500 cPs after 24 hours [30].

The intrinsic viscosity of tara gum has been measured at 14.96 dl/g at 25 oC for the crude powder and 16.46 for the purified gum, compared to 11.03 for locust bean gum [15].  The crude and purified gums had a viscosity average molecular mass of 2.31 and 2.53 x 106, respectively.  Another study found intrinsic viscosity and Mv to be 14.55 dl/g and 2.23 x 106, respectively [16].  Typical flow curves for tara gum solutions at different concentrations are shown in Figure 7 [15].  The behavior was shear-thinning with a Newtonian region at low shear rate.  At low shear rate, the disruption of chain interactions is balanced by the formation of new ones, resulting in no net change of entanglement, while at high shear rate, disruption predominates over formation and molecules align in the direction of flow, resulting in decreased apparent viscosity [15].



Mechanical spectra of tara gum solutions, along with those reference adjusted and superimposed in master curve format, are shown in Figure 8 and Figure 9 [15] [16].  At low frequency, the loss modulus (G”) is higher than the storage modulus (G’), while at higher frequency G’ predominates.  Both curves approach the characteristic slopes of 1 and 2 for G” and G’, respectively.  The G’-G” crossover frequency decreases as concentration increases as a consequence of increasing relaxation times [15].




Emulsion

The surface activity of tara gum solution is concentration dependent, similar to other galactomannans [16]. At higher concentration, the more activity was required to reduce surface tension, and at lower concentration, very weak surface activity was observed.  It is suggested that at lower concentrations (< 0.1% w/v), short chain saccharides could migrate preferentially to the surface, while at higher concentrations the activity might be a result of the behavior of macromolecules at the surface.  Higher surface active polysaccharides can reduce surface tension while simultaneously forming a steric layer around the droplet, forming a stable emulsion.  In an emulsion, polysaccharides assist with their water-holding and thickening properties. Tara gum had an emulsion capacity of 61% with a stability of 46%.  Tara gum formed a better emulsion than locust bean gum despite exhibiting lower surface activity, showing that surface activity is not the only factor determining emulsion properties [16].  




Phase Transition

The phase diagram of tara gum established using phase transition temperature is shown in Figure 10 [24].  Glass transition, cold crystallization, and melting temperature are observed in the system.  Tg and Tcc are found in MCdb ranging from 50 to 100%.  Tm is observed when MCdb exceeds 40%.  Tm 



Application

Tara gum is used in the food industry as a stabilizing, thickening, binding, and gelling agent [17].  It is growing in popularity as a food ingredient but is still much less prevalent than guar gum or locust bean gum.  Due to its roughly 3:1 mannose-to-galatose ratio, tara gum shows favorable rheological properties in a range of products [18].  It is highly viscous in solution with a short texture and is used in such foods as frozen desserts, baked goods, dairy products, fruit products, gluten-free bread, condiments, pudding, meat products, gravy and salad dressings [7] [18].  In frozen dessert it provides fat-like texture, improved stability and flow properties, and excellent heat shock protection.  It can be used synergistically with other hydrocolloids such as carageenans and xanthan gums to create strong and elastic gels with reduced syneresis as well as providing long term stable suspensions [18].

Tara gum has also been investigated for use in non-food application, including pharmaceuticals, cosmetics, marine coatings, and textiles [7] [19].  Tara gum carbamate has recently been studied for use in cotton printing, showing improved color strength compared to conventional thickeners without affecting the fastness properties [19].  It has also been studied as a sieving matrix in capillary electrophoresis [23].

The addition of tara gum affected the heat-set gelation behaviour of β-lactoglobulin solutions, the linear viscoelasticity, the large amplitude oscillatory shear behaviour and the microstructure of final gels, at pH 4.6 and 7.0 [22].  It’s possible to modify the structural and rheological properties of mixed β-lactoglobulin-galactomannan aqueous solutions by changing the ratio of the two biopolymers [22].  The addition of tara gum to hot pepper soybean paste increased the consistency index, dynamic moduli, and apparent viscosity, which obeyed the Arrhenius temperature relationship [25].  These studies emphasize the rheological effects that tara gum can have on a food system, and these can be manipulated during formulation depending on the desired effects [22].


Comparison to Other Galactomannans


Common galactomannans include fenugreek, guar, tara, and locust bean gums, which differ primarily according to their degree of galactose substitution (Figure 11) [18].  These molecular differences effect the properties of each gum, including solubility, gel formation, rheology, texture and synergism with other food components [12].  Guar and locust bean gums are the most popular gums currently in use, but tara gum is gaining popularity due to unique benefits that it possesses.

Similar to guar gum, tara provides cold-water solubility and attains maximum viscosity in aqueous food systems within several minutes, though some heating is required to fully hydrate tara gum [11] [18].  Upon heating, tara gum decreases in viscosity slightly more than guar gum, while locust bean increases in viscosity due to increased hydration.  Unlike locust bean gum, tara gum provides excellent heat shock protection with a fat-like texture, encouraging its use in frozen desserts.  Also, tara gum is required at levels 20-25% less than locust bean gum, potentially reducing overall costs of formulation [18] [21].  

Tara gum has similar rheological properties to guar gum but has some advantages.  Tara gum flows more smoothly, has a less slimy texture, has less detectable flavor, and has improved synergism with other hydrocolloids and freeze-thaw stability [20].  Tara gum shows intermediate acid stability between guar and locust bean gums, resisting depolymerisation at pH 3.5, compared to 3.0 and 4.0 for locust bean and guar gums, respectively.  Tara gum also shows high temperature stability, resisting up to 145 oC in a continuous process or 121 oC for 30 minutes in a batch process, showing superiority to guar gum [11].

Comparing the rheology of the four previously mentioned galactomannans, the surface activities followed the trend: fenugreek > guar > locust bean > tara [16].  Emulsion capacities and stabilities followed the trend: guar > fenugreek > tara > locust bean.  Viscosity and intrinsic viscosity followed the trend: guar > fenugreek > tara > locust bean.  Storage modulus (G’) followed the trend: fenugreek > guar > tara > locust bean, which is in agreement with the trend of molecular weight [16].  An economic analysis should be conducted to determine if any benefits outweigh the costs in the food product of interest.


Health Effects

Tara gum is a rich source of soluble fiber and is not digestible by humans, making it an effective no-calorie fat replacement [26].  Soluble dietary fiber very effectively absorbs water in the intestine, creating a thickened food consistency and slowing the rate of digestion and absorption, lowering postprandial glycemia and promoting satiety [27]. Galactomannas have been reported to reduce plasma cholesterol and promote fermentation in the large intestines, yielding short-chain fatty acids, which have shown to have beneficial effects on the colon through stimulation of blood flow and enhancement of electrolyte and fluid absorption.  Potential negative effects of tara gum include reduced mineral bioavailability and the distension and flatulence associated with fermentation of soluble fibers [27].

Tara gum showed no carcinogenic effect in rats and has even shown to have anti-cancer effects in humans [27] [29].  Galactomannans have been shown to have the ability to bind toxic substances and bring them out of the body.  Sulphated C-glycosylated galactomannan derivatives showed anti-cancer and anti-inflammatory properties [27]. Purified tara gum was shown to induce phenotypic maturation of monocyte-derived dendritic cells [28].   Functional experiments showed the loss of particulate antigen uptake in galactomannan-stimulated dendric cells and increased alloantigen presentation capacity.  Tara gum also increased protein and mRNA levels of pro-inflammatory cytokines.  Tara gum may have therapeutic application in clinical settings where a boosted immune response is desired [28].

The toxicity of tara gum was assessed by three 90-day animal feeding studies, two 2-year feeding studies, a 3-generation reproduction study with rats, and a reproductive toxicity and teratogenicity study [26].  Additionally an Ames reverse mutation assay with five Salmonella typhimurium strains was used to test for mutagenicity.  The feeding studies did not indicate any significant adverse effects at the maximum tested dietary level of 5 %, and the mutation assay results were negative.  It was concluded that tara gum meets the scientific standards required for classification as a GRAS food ingredient.

References
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