Some Gum Fun
(from Osmosis 21, Summer 2001)

What possible connection could exist between Egyptian mummies, drilling for oil, paper manufacture, ice cream and shampoo? One answer lies in plant gums.

Gums are watersoluble polysaccharides. You already know about at least two polysaccharides - starch (a food store) and cellulose (a structural component of cell walls). Both these are made up of many units of the monosaccharide glucose, whereas guar bean gum and locust bean gum are synthesised from two other monosaccharides, galactose

and mannose. Xanthan gum is made up of mannose and glucuronic acid units. Look at some food wrappers - of yoghurts, mayonnaise or cakes - and you will see that these gums are often exploited in the food industry, where they are used as thickening and stabilising agents.

When solutions of some polysaccharides are mixed, they interact with each other. This can result in an increase in viscosity (‘gooeyness’), which becomes greater than each solution individually. Under certain conditions, they may even form a gel. This means they do not flow like a liquid and do not join up once broken.

In this experiment you investigate what happens to the texture when different polysaccharides are mixed. You do this by comparing viscosity before and after mixing together, followed by heating. You will need a syringe barrel with the nozzle cut off, a stop clock, boiling tubes and beakers. Follow the instructions in diagrams 1 to 6. The flow rate from the syringe gives you a measure of the viscosity of the solution - the slower the flow rate the higher the viscosity.

1.Preparing the gum solutions
• GG = guar gum (0.25%)
• LBG = locust bean gum (0.25%)
• XG = xanthan gum (0.25%)
• Record the ‘gelling’*of each gum

2. Filling the syringe barrel
• Put finger over the hole of the syringe
• Fill syringe to 20 cm³ mark with one gum solution

3. Measuring viscosity
• Remove finger and start timer when level reaches the 15 cm³ mark
• Stop timer when the level reaches the 5 cm³ mark and note the time
• Pour solution back into original beaker. Rinse beaker and syringe
• Repeat steps 2 and 3 for other gum solutions

4. Mixing some gums
• Label the boiling tubes 1, 2 and 3 and add the following solutions
      Tube 1 = 10 cm³ XG + 10 cm³ LBG 
     Tube 2 = 10 cm³ XG + 10 cm³ GG
      Tube 3 = 10 cm³ GG + 10 cm³ LBG
• Mix contents and record 'gelling’
• Repeat steps 2 and 3 and record viscosity for each mixture

5. Heating the mixed gums

• Place the boiling tubes in waterbath at 90 °C for 5 minutes
• Record the ‘gelling’ in each tube as you remove it from the waterbath
• Cool the tubes

6. Describing viscosity

• After cooling, record the ‘gelling’ of each mixture
• Repeat steps 2 and 3 and record viscosity for each mixture
(If too viscous, only record ‘gelling’)

* Gelling describes viscosity - flowing (yes/no/fast/slow), or if the gel breaks, is it permanent or does it re-join?

Gum Fun — a closer look at some industrial applications of polysaccharides

Many gums are extracted from plants and we know that ancient Egyptians used locust bean gum in the binding of mummies. The human race has a long history of exploiting plant polysaccharides - such as starch in foodstuffs and cellulose for clothing materials.

In more recent years, gums have been employed in quite a range of activities. These include oil well fracturing and drilling, in paper manufacture as strengthening agents, as thickeners and gels in blasting agents, in rheology(flow) control in latex paints and as thickeners in various cosmetics. The food industry makes use of gums most often as additives to control properties of a food product. As primary ice cream stabilisers, for example, gums confer heat-shock resistance, smooth meltdown, desirable texture, chewiness and inhibit formation of ice crystals. Dextran is another polysaccharide that has important medical applications.

Guar gum (GG) and locust bean gum (LBG) are extracted by grinding the endosperm portions of the seeds of the legume plants Cyamopsis tetragonolobus and Ceretonia siliqua respectively. Cyamopsis tetragonolobus is an annual plant that grows mainly in the arid and semi-arid regions of the Indian sub-continent and is also grown as a food crop for animals. Ceretonia siliqua (the carob tree) grows extensively in Spain and in other Mediterranean countries. Seeds from both species of plant contain polymers of galactomannan as the energy store in their endosperms. Galactomannans consist of a mannan backbone randomly substituted with galactose. In LBG the ratio of mannose:galactose is greater than in GG. Both LBG and GG consist of very long chains of repeating subunits.

Xanthan is a polysaccharide which can be harvested from an aerobic fermentation of the bacterium Xanthomonas campestris. Varieties of this bacterium parasitise a range of plants. Xanthan gum (XG) has a backbone of glucose units (as in the structure of cellulose), but with trisaccharide side-chains consisting of mannose and glucuronic acid. This gives a fairly rigid molecular structure which may be a single or multi-stranded right-handed helix. Such ‘exopolysaccharides’, produced by bacteria as slimes, may be associated with virulence, plant-microbe interactions, protection from desiccation or protection from attack by phage (bacterial viruses) or protozoa.

Aqueous solutions of all of these polysaccharides demonstrate high viscosity at low gum concentrations. Indeed, thickening of aqueous solutions is the most valued property of gums such as guar, locust bean and xanthan. Some gums demonstrate ‘viscosity synergism’ when combined with anionic surfactants and anionic polymers. This is a useful property for producing appropriate texture with minimum concentration of gum. Synergistic interactions also take place between different galactomannans and between galactomannans and certain linear polysaccharides such as xanthan.

If we focus again on the experiment described in this worksheet, when xanthan is mixed with guar gum or locust bean gum, the resulting solution demonstrates increased viscosity in comparison to the same concentration of any one of the gums. It is thought that the galactomannans of guar and locust bean link with helices of xanthan to form a more rigid molecular structure of a gel. If heated to 90 °C for a short time then cooled, each mixture forms a gel. These are true gels which do not flow and do not recover from mechanical damage. Synergistic interaction between locust bean gum and xanthan is more pronounced than with guar gum.

For post-16 students, in their current specifications, storage and structural functions of polysaccharides are usually exemplified by starch, glycogen and glucose. The background to this practical helps to broaden their perspective because it introduces unfamiliar storage polysaccharides in plants and structural ones in bacteria. It also captures their interest as it describes applications relevant to every-day life. All three of these gums are common food additives, used to provide good flavour release and enhance texture (leading to good ‘mouthfeel’). By looking at labels on food products, students will see that these gums are often used in sauces, in products such as prawn cocktails and also in salad dressings where they help to stabilise oil and water emulsions. Gels are found particularly in canned pet foods to provide the ‘cling’ factor.

References

Industrial Gums, third edition: edited by Whistler R.L. and BeMiller J.N.; Academic Press 1993. ISBN 0-12-746253-8

Biotechnology of Microbial Exopolysaccharides: Sutherland I.W.: Cambridge University Press 1990. ISBN 0-521-36350-0

Practical Fermentation: a Guide for Schools and Colleges: John Schollar and Benedikte Watmore. Society for General

Microbiology. ISBN 0 9536838 0 X

SAFETY: Do not inhale powder when making up gums solutions and take care with 90 ºC water bath and with tubes and their contents when they are removed from bath.

Kath Crawford, SAPS
ICMB,University of Edinburgh

This investigation is based on an experiment used by Professor I Sutherland of Edinburgh University.

© SAPS This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose.

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