INTRODUCTION TOFOOD GUMS
CHEMISTRY, FUNCTIONALITY, AND APPLICATIONS
HERCULES INCORPORATEDFood Gums Group500 Hercules Road
Wilmington, DE 19808-1599
Andrew C. HoeflerManager, Food Gums Technical Service
(302)[email protected]
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Food hydrocolloids ( or "gums" for short ) are carbohydrates of relatively high molecular weight,when compared to ingredients such as sucrose or the various forms of corn syrup. They are usuallypolysaccharides, although there are one or two exceptions.
Figure 1 is a representation of a typical food hydrocolloid. It is a long chain (sometimes branchedbut usually linear) of sugars, with what we call "substituents" protruding from the main chain. Thechain can be anywhere from several hundred to several thousands of sugar units long. The typeof monosaccharide will help determine certain properties, such as whether or not the gum is acidstable, while the type, number, and distribution of substituents will help determine whether the gumis a thickening agent versus a gelling agent, as well as other properties which will be discussed aswe go through the rest of this lecture.
The top portion of Figure 2 is a pictorial representation of the chain of glucose units which constitutecellulose. Please note that there are a significant number of hydroxyl groups protruding from eachsugar unit in the chain, even though they are not shown here. These hydroxyl groups will hydrogenbond with adjacent cellulose chains, and will do this so well that water of hydration cannot separatethe chains from each other. As a result, cellulose is water insoluble.
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If we derivatize cellulose into cellulose gum, what we do is replace some of the hydroxyl groups withcarboxymethyl groups, which are attached to the glucose rings via an ether linkage. The bottom ofFigure 2 shows a pictorial version of this carboxymethyl cellulose chain. The short pieces stickingout from the backbone represent the carboxymethyl units, which take the place of some of thehydroxyl groups along the chain. Again, although the remaining hydroxyl groups aren't displayedin the pictorial diagram, remember that there are many of them present along the chain. Theseprotruding carboxymethyl groups prevent the cellulose backbones from getting close enough toeach other to hydrogen bond, even when in the dry (not hydrated) state. This leaves enough spacebetween the backbones for water to slip in and hydrate (dissolve) these molecules. As a result,cellulose gum IS water soluble.
Figure 3 shows some of the typical substituents or "side units" present on food gums, all of whichserve a similar purpose to the carboxymethyl groups on cellulose gum. Most food gums conformto the basic structure of a long, linear chain with substituents protruding from the main chain. Again,this prevents excessive hydrogen bonding of the parallel "backbone" chains when they are in thedry state so that water of hydration can "slip in" and "peel" these molecules apart (hydrate them). Please note that a substituent can be as small as a carboxyl or sulfate group (pectin andcarrageenan respectively), or it can be as large as an additional sugar (guar and locust bean gum)or group of sugars (xanthan). Note that food gums are all either non-ionic or anionic; there are nocationic food gums at present.
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The factors which affect gum properties are listed in Figure 4 (previous page). The effect ofmolecular wight is obvious. A roughly linear gum molecule randomly tumbles through space whileit is solution, sweeping out a certain volume in its path. If one doubles the length of the molecule(ie, doubles the molecular weight), then it sweeps out eight times the volume it did before. Thismeans that it is eight times as likely to collide with a neighbor than before, and restrict the neighbor's"sweeping" volume. These collisions and restrictions are what we experience as resistance to flowor viscosity. The monosaccharide composition determines such factors as acid stability over time, whether a gum is a gelling agent or a viscosity agent (which is also true of the type and number ofside chains), and the distribution of the side chains usually affects cold water solubility and certainsynergies with other gums (more about that later on).
Here is an example of how the substituent or side chain can affect a hydrocolloid's properties. InFigure 5 there are listed three different cellulose based gums. All three have the same averagebackbone length, but all three have different side units. At 25 degrees Celsius, all three of thesegums have the same 1% (w/w) water viscosity in distilled water, as measured with a Brookfieldviscometer. Now, let us heat all three of the 1% water solutions to a temperature of about 70degrees Celsius (Figure 6, below).
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As the 1% CMC solution is heated to 70 degrees Celsius, it becomes thinner, and the Brookfieldviscosity drops to about 1200 centipoise. This is a reversible process. If the solution is cooled backdown to 25C, it will immediately go back to a viscosity of 3000 centipoise again. The reason for theviscosity drop with increasing temperature is that the layer of "organized" water molecules aroundthe CMC molecule becomes smaller as the temperature rises, making collisions with a neighborsomewhat less likely. When the methyl cellulose (Benecel ) solution approaches 70 degrees, itactually changes into a white, opaque, rigid gel that can be cut with a knife and will retain its shape. As soon as this gel begins to cool off, it will again revert to a transparent liquid of about 3000centipoise at 25C. As the hydroxypropyl cellulose (Klucel ) solution approaches 70 degrees, itprecipitates out of solution and makes a white layer on the bottom of the container. Upon coolingback down towards 25C., the hydroxypropyl cellulose will begin to redissolve in the water. So wehave three gums with identical backbones structures of cellulose, but they each have different sidechains, which leads to three very different behaviors when the solutions are heated to 70C.
There are three basic types of carrageenan: Kappa, Iota, and Lambda. All three types are extractedfrom various species of red seaweed, and they all have in common a backbone of galactose sugarunits. They differ in the number and location of sulfate units and anhydro bridges. Kappacarrageenan (Figure 7 above) has one sulfate for every two galactose units on the chain, and oneanhydro bridge. The sulfate groups increase carrageenan's water solubility, while the anhydro
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bridge tends to decrease its water solubility.
Next, is Iota carrageenan (Figure 8, previous page), which has two sulfates and one anhydro bridgeper every two galactose units. Thus, it could be said that Iota carrageenan is more water solublethan Kappa carrageenan.
Third, is Lambda carrageenan (Figure 9), which has three sulfates and no anhydro bridge per twogalactose units, and it is the most water soluble form of the three.
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Figure 10 is a summary of these structural differences, and their effect on the cold water solubilityof the different carrageenan types. Also, Kappa is the strongest gelling agent of the three, preciselybecause it is the least water soluble of the three (because it has the least water solubility, it is theone most like to drop halfway back out of solution into the gelled state).
In figure 11, we have a description of the textures of the three carrageenan types, which are quitedifferent from each other, even though the molecules are only different by their sulfate groups andanhydro bridges. Kappa is a rigid, brittle gel, while Iota is a shear reversible, elastic, cohesive gel,and Lambda wouldn't appear gelled to the average consumer, but merely thickened.
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Carrageenan is about five times more effective in milk than it is in water (Figure 12). This isbecause carrageenan can incorporate the casein micelles in milk right into its gel structure. Giventhe high concentration of casein micelles in milk, this allows a gel network to be built with far fewercarrageenan molecules than could be done with most other hydrocolloids. This tends to makecarrageenan very cost effective for the dairy industry.
Figure 13 is a diagram of a portion of a pectin molecule. Pectin is the methylated ester ofpolygalacturonic acid. It is commercially extracted from citrus peels and apple pomace under mildlyacidic conditions. Each ring is a molecule of galacturonic acid, and there are 300 to 500 such ringsin a typical pectin molecule, connected in a linear chain. You can see five such galacturonic acid
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units in Figure 13. Please note that three of the five are in the methyl ester form, while the other twoare in the acid form. This represents a degree of methoxylation of 3 out of 5, or 60 percent. You willsee the term abbreviated as "DM" or "DE", which is short for degree of esterification. Both terms areinterchangeable, and they refer to the percentage of acid groups which are present in the pectinmolecule as the methyl ester.
By FCC definition, any pectin of 50% DE or greater is a High Methoxyl pectin, while anything undera DE of 50% is low methoxyl pectin. The two types of pectin will gel for completely different reasons,as indicated in Figure 14. HM pectin gels due to high soluble solids and low pH conditions, asindicated on the graph as a light grey line. As the DE of a pectin is lowered, it begins to lose it'sability to gel under these conditions. The black line is for the ability to gel with divalent ions (usuallycalcium ions in food systems). This is the hallmark of LM pectin. Please note that as the DE israised, the pectin will eventually lose it's ability to gel with calcium. Also note that a pectin with a DEaround 50% will posses characteristics of both types. Figure 15 (top of next page) summarizes theeffect that the degree of esterification has on the gelling conditions required for high ester and lowester pectin.
A few more terms need to be defined at this point (Figure 16). DP is short for the degree ofpolymerization, which is a fancy way of saying molecular weight or chain length. As a hydrocolloid
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molecule becomes longer, it sweeps out a much greater volume as it randomly tumbles in solution,leading to increased collisions with its neighbors, which results in an increase in viscosity. Also, thelonger the molecule, the slower it is to hydrate initially, because it must disentangle itself from itsneighbors.
DS is degree of substitution, or number of side units per unit length of the main chain. As the DSincreases, more and more of the backbone structures are held apart from each other and cannothydrogen bond to each other, which tends to speed up hydration. Also, the higher the DS, the morelikely it is that the side units are evenly distributed along the chain, rather than occurring in clumps.
This brings up another factor in hydrocolloid chemistry, one which is not usually easy to measure,yet it can have a profound effect on the behavior of gums in certain applications, and that is the
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uniformity of substitution (Figure 17). The top pictorial diagram shows an evenly substitutedmolecule, while the bottom shows an unevenly substituted molecule. We refer to the areas of thebackbone without side units as "smooth regions", while the areas where all the side units arecrammed together are known as the "hairy regions".
A classic example of this "even" versus "uneven" substitution is the comparison of guar gum versuslocust bean gum (Figure 18). Both gums are galactomannans, but locust bean gum is highlyunevenly substituted, while guar gum is evenly substituted. As a result, locust bean gum is not coldwater soluble (it will swell somewhat in cold water) while guar gum is cold water soluble. Locustbean gum will form a synergistic gel with xanthan gum, while guar gum does not. Kappacarrageenan gels are normally very brittle and non-elastic. When locust bean gum is added tokappa carrageenan, the resulting gel is far less brittle and far more elastic than kappa alone, whileguar gum will not produce this change. The mechanism for the interaction of locust bean gum withxanthan or kappa carrageenan is the hydrogen bonding of the smooth regions of the locust beanmolecule with the helixes in solution of either xanthan or kappa carrageenan - in the case ofxanthan, this results in a gel structure, and in the case of kappa carrageenan, this results in a gelstructure that is far more elastic and flexible.
This is a review of the proper means of making gums solutions (Figure 19). If you try to stir a
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teaspoon of almost any type of hydrocolloid into a beaker of water, you will get one large, stickylump floating around in your beaker. If you are patient, and are willing to stir for several days, thelump will eventually dissolve and go completely into solution. Most of us don't have the luxury of thatmuch time to dissolve our gums. This difficulty in dispersion holds true for all hydrocolloids.
The key to lump-free gum hydration is to remember the following: Separate the gum particles fromeach other JUST BEFORE they hit the surface of the water. Figure 20 shows a comparison of thehydration of the sugar in your morning coffee versus pectin. A sugar particle enters the water, andbegins to dissolve from the outside in. The sugar particle becomes smaller with time as themolecules hydrate and float away, and within minutes all the sugar is dissolved. Pectin and othergums DO NOT WORK THIS WAY!!! When a pectin particle hits the water, it rapidly absorbs waterlike a sponge and the particle swells to many times it's original size. I think of it as going "SPROINK"as it hits the water, becoming hundreds of times larger. When it has swelled to a certain size, thenthe pectin molecules begin to unravel themselves from the outside surface, and float away from theparticle, being now completely hydrated.
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If the pectin particles are right next to each other when they contact the water (Figure 21), then theyall try to "SPROINK" at the same time, and weld themselves together into one large, slow to hydratelump that has very little surface area relative to its mass.
If the pectin particles are all slightly separated from each other when they contact the water (Figure22), then they all have enough room to go through their initial expansion without getting stuck to aneighbor, and will then generally completely hydrate within five to fifteen minutes.
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There are several ways to achieve this slight separation (Figure 23). The first is the use of a polymerdisperser, such as the Hercules Eductor Funnel. Here, the pectin particles are separated by astream of air just before they contact the water. Second is the dry blending of 5 parts sugar to 1 partpectin. When this is dispersed into water, the sugar particles (which don't go "SPROINK") separatethe pectin particles, allowing the pectin to expand without contacting a pectin neighbor. Third is theuse of non-solvents, such as vegetable oil, glycerine, or 80% solids 42DE corn syrup. Withnon-solvents, the pectin particles are wetted and separated from each other but cannot swell. Fourthis the use of high shear, where the rapidly moving water separates the gum particles. Also, if lumpswere to form, the high level of mechanical work being done will break up lumps and ensure quickhydration. This method is typified by devices such as the Warring Blender , the Cuisinart foodprocessor, the Breddo Likwifier , and the Clover Triblender .
Figures 24 and 25 are a list of some of the functions of hydrocolloids in food systems. These rangefrom thickening to providing suspension of particulates to crystallization stabilization to waterbinding. If one looks at Figures 24 and 25, it becomes apparent that all of these functions "boildown" to one or more of three things:
1. Doing something to the texture of the product.2. Doing something to increase the stability of the product, either over time or under special conditions such as high temperatures.3. Fat or calorie reduction, either by fat replacement or fat holdout.
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Next, we will rank the various food gums by the properties listed above in Figure 27:
In terms of solution clarity (Figure 28), nothing can beat cellulose derivatives. They are, as a rule,optically transparent in solution. Also, carrageenan, gelatin, gellan gum, and agar are nearlyoptically transparent. Pectin and refined locust bean gum are also very nearly optically transparent,while the remaining food hydrocolloids range from slightly hazy to completely opaque(microcrystalline cellulose).
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The next three slides (Figures 29, 30, and 31) are tables showing the solubility of the foodhydrocolloids at various temperatures in distilled water. This data helps to make clear which gumsare cold soluble and which are not, and also indicates the approximate gelling temperature of methylcellulose.
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These tables also indicate that the cations associated with carrageenan can control whether or notit is cold water soluble as well.
Figure 32 lists all the food gums divided broadly by whether they are gelling agents possessingsome three dimensional network, or viscosity agents which rely on random collisions of moleculesin solution.
Remember that the suspension of particulates (Figure 33), either solid particles like cocoa inchocolate milk, or liquid oil droplets in salad dressing, requires a gelling agent or a hydrocolloidhaving a yield point in solution.
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Figure 34 groups the gums by whether or not they can be construed as "Natural". Most foodhydrocolloids are naturally occurring plant substances, and the chemically modified gums aremodifications of natural substances, rather than completely synthetic polymers.
Carrageenan and pectin (Figure 35) are the two gums capable of stabilizing protein particles in afood system, such as the casein in milk. If the pH of the system is above the isoelectric pH of theprotein (around 4.6 for casein), then carrageenan should be used. If the pH is below the isoelectricpH, then pectin should be used.
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Figure 36 lists the relative acid stability of the food gums, with pectin, gellan, and xanthan leadingthe pack. Gum arabic, gum tragacanth are also very acid stable, and suprizingly, so is locust beangum, perhaps due to its uneven substitution. Guar, propylene glycol alginate, et al are in the faircategory, while cellulose derivatives (particularly cellulose gum) and carrageenan are at the bottomof the list. Carrageenan is not recommended for use below a pH of around 4.0, particularly if theproduct is pasteurized. Cellulose gum will hold up at a pH of 3.8 or higher at room temperature fora period of about two years, only losing about half its viscosity during that time.
Figure 37 lists some ballpark cost ranges for the various food gums, which were current at the timeof this printing. Remember that the real cost in a food formula is "cost-in-use", which is cost perpound times use level.
I would like to thank you for your attention over the last 45 minutes, and if you have any questionsabout food gums, feel free to contact me at "[email protected]" or visit our website at:
http://www.herc.com/foodgums/index.htm
