THE CELLULOSE DERIVATIVES

Transcription

1 THE CELLULOSE DERIVATIVES Cellulose Gum, Methylcellulose, Methylhydroxypropylcellulose, and Hydroxypropylcellulose Chemistry, Functionality, and Applications Andrew C. Hoefler Brookside Consulting Ltd 321 Mill Pond Lane Oxford, PA

2 This presentation will cover the important food grade cellulose derivatives, starting with cellulose gum or "CMC". Figure 1 is a diagrammatic representation of a cellulose molecule. Note that each glucose unit in the cellulose chain has three hydroxyl groups, each of which is capable of hydrogen bonding to an adjacent molecule. In the bottom of Figure 1, we indicate cellulose more pictorially as a series of circles connected together in a long, linear chain. Figure 2 shows a group of cellulose molecules in water. Because of the abundance of hydroxyl groups, and their ability to hydrogen bond to a neighboring molecule, the chains are bound tightly together. Water molecules, at any temperature, cannot force their way in between the chains to hydrate them, thus cellulose is water insoluble ( which is just as well, since most of our houses are made of wood. I certainly would not want my house to dissolve the next time it rains! ) 2

3 Figure 3 (previous page) illustrates the reaction for the manufacture of CMC. It is essentially a two step process. In the first step, cellulose is suspended in alkali to open the bound cellulose chains, allowing water to enter. Once this happens, the cellulose is then reacted with sodium monochloroacetate to yield sodium carboxymethyl cellulose. An idealized unit structure of CMC is depicted in figure 4. The CMC shown here has a D.S. (Degree of Substitution) of 1.0. If the remaining two hydroxyl groups on this unit became substituted, the D.S. would be 3.0. A D.S. of 3.0 is the theoretical maximum one could attain. Figure 5 is a pictorial representation of CMC molecules. Note that the carboxymethyl groups protrude from the cellulose backbone, such that the hydroxyl groups of the backbone cannot get close enough to hydrogen bond to each other. The result is that even in the dried state, water can slip in between the CMC molecules and hydrate them, causing them to "peel apart" from each other and go into solution. As a result, cellulose gum is water soluble while cellulose is not. 3

4 Figure 6 depicts the nomenclature for Hercules cellulose gum. The specific product described is cellulose gum type 7H3SXF. The "7" stands for the degree of substitution. In the food industry, there are "7" and "9" types of substitution. The pharmaceutical industry also has a "1.2" type to work with. The "H" signifies a high viscosity grade, and the "3" is a reference point which defines the maximum viscosity of the gum in a 1% solution at 25C (in this case, 3000 centipoise). There are "L", "M", and "H" types, representing low, medium, and high viscosity respectively. The "S" stands for special rheological properties (smooth flow). There are "S" types for smooth flow, and "O" types for tolerance in acidic systems. Both of these types show considerably less thixotropy than the randomly substituted regular types of cellulose gum (more will be said about this later). The "X" stands for fine grind material, while a "C" would indicate a coarse particle size, and no letter would indicate a "regular" particle size. The "F" represents food grade (FCC), while a "P" would be pharmaceutical grade (USP). Some typical viscosity values are shown in Figure 7. Please note that "L" and "M" types are measured at a 2% concentration, while "H" types are measured at 1%. Figure 8 shows the concentration versus viscosity relationship in a more visual fashion. 4

5 The effect of the Degree of Substitution on the properties of CMC is shown in figure 9. Tolerance to salt increases and tendency towards thixotropic behavior decreases as the degree of substitution increases. The slight change in density and hydroscopicity are of no importance to a food technologist. There is another factor which is as important as the Degree of Substitution, and that is the "Uniformity of Substitution", which is shown visually in Figure 10. The "smooth" or non-substituted regions of a non-uniformly substituted molecule behave just like cellulose because they are still cellulose! These regions can hydrogen bond to a similar region on an adjacent molecule, leading to the buildup of a loose gel network (Figure 11). 5

6 This buildup is time dependent, and is called "thixotropy". The loose gel network can be disrupted by shearing the CMC solution, but upon standing under no shear conditions the network will reform over time. Visually, the difference between uniformly and non-uniformly substituted CMC solutions can be seen in Figure 12. Smooth flowing CMC types are desirable for food systems such as syrups or frostings where smooth consistency is a must. Thixotropic CMC would find use in structured, grainy foods such as sauces or purees. 6

7 Cellulose gum is probably the fastest gum to hydrate in cold water. Consequently, it is the gum most like to form lumps when dispersed into water, due to it's rapid swelling in water. To overcome the problem described above, four procedures are recommended in Figure 13: Method 1: direct addition: Here the gum is added directly to the vortex of a vigorously agitated body of water. The rate of addition should be slow enough to keep the particles separated, but fast enough so that all of the gum is added before the vortex disappears. The reason for this is that it is extremely difficult to thicken an already viscous solution of cellulose gum by adding more dry powder. The direct addition method is usually encountered in highly controlled processing situations. Method 2: dry blending: In this method, the CMC is dispersed with other dry ingredients, such as sugar, prior to their addition to aqueous systems. The other particles serve to keep the CMC particles away from each other. Commonly, one part of CMC is mixed with five to ten parts sugar to effectively prevent lumping. The dry mix beverage is a classic example of this dispersion technique. Method 3: dispersion in a water miscible non-solvent: Cellulose gum may be dispersed in glycerine, ethanol, or propylene glycol and the slurry is then added to water. An off-shoot of this method is to disperse the gum in corn syrup, and then add the mixture to water with the aid of agitation. 7

8 Method 4: mixing device: Another method for the addition of cellulose gum to food systems in plant operations is the use of a stainless steel mixing device (figure 14). The gum is fed through a smooth wall funnel into a water jet eductor, where it is dispersed by the turbulence of water flowing at high velocity. Each particle is individually wetted out to give a uniform solution. Under optimum conditions, cellulose gum leaving the eductor is about 80-90% hydrated. ADD THE GUM FIRST! (Figure 15) This is a general rule to follow when adding cellulose gum to water in all food formulations. Any electrolytes that are dissolved in the water first can greatly inhibit the hydration of cellulose gum and lower its efficiency. 8

9 As an example of the importance of order of addition, Figure 16 is a graph of CMC viscosity versus salt concentration. In one case, the CMC was dissolved in the water before the salt, and the salt had a minimal effect on the viscosity of the solution. In the other case, the CMC was dissolved AFTER the salt, and the resulting final viscosity was much lower, especially as the salt concentration increases. Figure 17 gives an idea of how cellulose gum is effected by increasingly stronger salt solutions, and by the uniformity of substitution. Going from distilled water to 4% sodium chloride drops the viscosity by a factor of about 12 for 7HF, and by about 3 for the more evenly substituted 7H3SF. The proportions are similar when going to a saturated salt solution (last column). 9

10 Figure 18 shows the effect of some other ions on the viscosity of a CMC solution. The Aluminum salt actually increases the viscosity of CMC because it has the steric capability of gelling CMC. Unfortunately, for taste reasons, this has little application in the food industry. Figure 20 shows that the viscosity of CMC, like most other water soluble polymers, decreases with increasing temperature. Under normal conditions, this effect is reversible (ie: raising or lowering the solution temperature has no permanent effect on the viscosity characteristics of the solution). However prolonged heating at extremely high temperatures will permanently degrade the cellulose gum (depolymerization) which results in a viscosity decrease. What this means to the food technologist is that CMC is not particularly retort stable, even at neutral ph values. 10

11 Figure 21 indicates that CMC, like most food gums, is pseudoplastic. This means that the apparent viscosity will decrease at increasing shear rates, because the long molecules will line up in the direction of the shear, giving less resistance to flow. The effect is totally reversible. As soon as the shear is stopped, the viscosity returns to its original value because the molecules assume random positions relative to each other. CMC is more tolerant to the presence of ethanol than most other food gums (Figure 22). This makes cellulose gum useful for cordials and other low alcohol content beverages which require optical transparency. 11

12 CMC will give a synergistic viscosity increase with other hydrocolloids such as guar or locust bean gum (Figure 23). If one were to mix a 1% guar solution of 3800 centipoise with a 1% CMC solution of 4000 centipoise, the net result is not the 3900 centipoise average of the two; it will be closer to 6500 centipoise. There are more average "collisions per second" between unlike molecules, which results in this synergistic viscosity increase. The key functions of cellulose gum in food systems are to provide viscosity / thickening, to "organize" the water, to form flexible films, and to control the rate and size of crystal growth. 12

13 The availability of cellulose gum in different viscosity grades, particle sizes, special rheological grades, and combinations thereof permits tailor-made application of CMC to many different food systems. The following is a brief discussion of some of these applications: Cake mixes CMC is used to improve the moisture retention in cake mixes, as a dried out cake is quite objectionable organoleptically. High D.S. types are preferred in cake mixes for maximum moisture binding. CMC also controls batter viscosity, imparts tolerance during mixing, protects against leavening loss, improves cake volume, and controls the uniformity of the cross sectional grain of the cake. For ease of mixing, fine grind types of CMC are preferred in cake mixes for rapid entry into solution. The other formula ingredients provide a dispersion method for the CMC. Frostings and Icings CMC may be used in frostings and icings to toughen the film, prevent sticking to the package, and reduce sugar crystal growth (graininess). In ready-to-spread frostings CMC helps stabilize the emulsion and adds creaminess. Most important, CMC prevents the icing or frosting from drying out. Uniformity substituted CMC (S types) are recommended to give a smooth icing or frosting. Pie fillings In starch based pie fillings, the addition of a small amount of CMC will prevent cracking control syneresis and firm the texture. The use of uniformity substituted O types of CMC are preferred for stability in acidic fillings such as in a lemon pie filling. Dairy products CMC was originally pioneered in ice cream and today this application still remains as one of the largest single uses for the gum. In ice cream CMC prevents ice crystal growth (sandiness), inhibits lactose crystal growth, imparts mix viscosity and body to the finished product, gives correct meltdown, and provides freeze/thaw stability (heat shock control). The use of coarse particle size types of CMC are preferred for ice cream applications (dispersion) because of poor mixing 13

14 conditions commonly encountered in dairies. CMC is utilized as a stabilizer in many other dairy products such as egg nog, soft serve ice cream, milk shakes, and ice cream ripples. Pancake syrup CMC enjoys widespread use in regular, reduced calorie and dietetic pancake syrups. Here the excellent clarity, viscosity ability compatibility with sugar and non-caloric characteristics of the gum are put to good use. Dry mix beverages The ability of CMC to hydrate rapidly and viscosity in aqueous systems for body and mouthfeel is used in instant breakfast drinks instant fruit drinks hot cocoa mixes and low calorie dry mix beverages. Uniformity substituted low or medium viscosity fine grind types of CMC are most frequently used in these products in order to minimize "fish eye" formation. High viscosity types of CMC are not recommended in these products regardless of particle size since higher molecular weight types take longer to dissolve and are more prone to form fisheyes if dispersion and energy input (stirring) are not optimum. Pet foods and animal feed In semi-moist pet foods, CMC facilitates extrusion, binds moisture, and improves the cosmetic appearance of the product. In dry gravy-forming pet foods CMC is "dusted" onto tallow coated "kibble" with other ingredients, so that upon reconstitution a rich viscous shiny gravy evolves. Another animal food application for CMC is its use as a physical binder in pelleted animal feeds. A small amount of low viscosity CMC in the product holds the pellet together and prevents accumulation of fines in the product package during shipment. Additionally, the gum assists the extrusion process during manufacture of the pellets and helps reduce energy consumption by the pellet mill. CMC greatly modifies the behavior of water in sugar solutions (figure 26). Combinations of sugar and CMC display a significant "boost" in viscosity that is believed to be the consequence of a crowding mechanism. Cellulose gum decreases the tendency towards syneresis in high sugar food systems by serving as a water binder. Most importantly, CMC also reduces the rate of sugar 14

15 crystal growth and crystal size in concentrated sugar systems. This functionality becomes important in confectionery applications such as fondants (Figure 27 below). Just as CMC controls sugar crystal growth in confectionery applications, it controls ice crystal growth in ice cream the same way. Texturally, it is desirable to have a large number of small ice crystals (smooth) rather than a small number of large ones (sandy). A few cautions about using cellulose gum in food products: Exposure to UV light and entrained air in a food system should be minimized to prevent degradation of the gum. Molecular oxygen will cause the gum to breakdown by a free radical mechanism similar to that, which occurs during the autoxidation of lipids. The presence of cations (calcium, iron, and aluminum) will accelerate the process. Therefore it is recommended that a sequestrant such as sodium hexametaphosphate be used in systems where CMC is exposed to air and cations. Our next topic is methylcellulose and methylhydroxypropylcellulose. 15

16 For methylcellulose, some of the hydroxyl groups on the glucose ring have been converted to methyl ether groups. Note that this makes methylcellulose non-ionic, where CMC was anionic. Also, the methyl ether groups are far more hydrophobic than the carboxymethyl groups on CMC. Methylhydroxypropylcellulose has both methyl ether groups and hydroxypropyl groups where hydroxyl groups used to be on the cellulose backbone. MHPC is also non-ionic like MC. Hydroxypropyl groups also can be attached to other hydroxypropyl groups ("stacked"), besides attaching to the glucose ring hydroxyl sites. This can lead to other changes in solution behavior that we will discuss later on. 16

17 The Aqualon division of Hercules refers to their MC and MHPC products by the trade name "Benecel", and it is their nomenclature that will be explained. The letter "M" indicates methylcellulose, while the letters "MP" indicate methylhydroxypropylcellulose. The first digit inidcates the average percent by weight of hydroxypropyl groups in the product. The second digit is the viscosity, and the third digit is the power of ten to multiply the second digit by. For example, "M043" would be methylcellulose with a viscosity of 4000 centipoise as a 2% solution. "MP823" would be methylhydroxypropylcellulose containing 8% (w/w) hydroxypropyl groups, and would have a 2% viscosity of 2000 centipoise. The unique property of MC and MHPC is thermal gelation. Most gum solutions become thinner when heated. MC and MHPC solutions do become thinner when heated, but only to a point. Above that critical temperature, they will gel and become a solid mass. Upon cooling, they will revert to transparent but viscous solutions again. If heated too far above their gelation point, they will flocculate (precipitate) and settle out. Upon cooling, the flocculent material will redissolve in the water. 17

18 At room temperature, there is a layer of "organized" water molecules surrounding the MC molecule. This "water of hydration" is associated with the hydroxyls on the cellulose backbone, and extends out the thickness of several water molecules. This organized water layer is large enough to prevent the hydrophobic ("water avoiding" or "oil soluble") methyl ether groups from approaching each other close enough to start clinging together. As the temperature of the solution is raised, the "organized" water layer around the MC molecules becomes thinner. This is why the solution initially becomes thinner as it is heated. When the water layer reaches a certain critical "thinness", the methyl ether groups can now get close enough to each other to try and cling together to avoid the remaining water (or to minimize entropy regarding the organized water). These methyl group "clumps" form junction zones, which results in the buildup of a gel structure, in a manner analogous to the hydrogen bonding together of unsubstituted regions of CMC molecules to form a thixotropic gel structure. 18

19 The amount of hydroxypropyl groups will influence the gel point (gelling temperature) of the methylcellulose. As a general rule, the higher the percentage of HP groups, the higher the gel point (temperature) and the softer the gel structure. It is assumed that the size of the HP groups will affect how thin the water layer must be before the methyl groups can cling together. The concentration of MC or MHPC in water affects the temperature at which the gum precipitates, or its "flocculation point". It is thought that the increased crowding that accompanies increased concentration leads to increased statistical odds of inter-chain hydrogen bonding, and thus precipitation. Most food applications do not call for more than one half percent MC or MHPC, so this is less of a concern for the food technologist. 19

20 The key functions of MC and MHPC in food systems are that the gums provide thickening / viscosity when cold (below body temperature) and can form rigid gels when hot (60 degrees C and higher). They are good film formers, as most hydrocolloids are, and they create optically transparent water solutions. Because they are water soluble, the MC films make excellent oil barriers in deep fat fried foods. Most interesting is the surface activity of MC and MHPC. Usually we associate whipping agents as being water soluble proteins such as gelatin and egg whites, for these molecules posses both hydrophilic and hydrophobic areas along their chains. Since MC and MHPC also posses both hydrophilic and hydrophobic areas along their chains, they make excellent whipping agents when compared to most other hydrocolloids. The most well known food applications of MC and MHPC involve deep-frying, where the thermal gelation of MC and MHPC can lead to greater mechanical strength while hot. Also, there is less oil pickup in fried foods due to the oil barrier properties of MC and MHPC. This would include all forms of batter and breaded meat items, onion rings, chicken nuggets, etc. Extruded food also may benefit from the thermal gelation properties of these hydrocolloids. Whipped / aerated products can make use of the whipping and self stabilizing properties of the methylcelluloses, while soups and sauces benefit from the thickening properties. The best quality "veggie burgers" are made from soy proteins plus MC and carrageenan. The MC provides hot strength to the burger, while the carrageenan provides gel strength at the lower temperature ranges. 20

21 Hydroxypropylcellulose has hydroxypropyl groups where hydroxyl groups used to be on the cellulose backbone. HPC is also non-ionic like MC. Hydroxypropyl groups also can be attached to other hydroxypropyl groups besides attaching to the glucose ring hydroxyl sites. The Aqualon division of Hercules refers to their HPC products by the trade name "Klucel", and it is their nomenclature that will be explained. One gets increasing viscosity going down the list of designations. Please note that they do not follow alphabetical order. 21

22 The key functions of HPC in food systems are that it can provide thickening / viscosity when cold. It is a good film former, as most hydrocolloids are, and can create optically transparent water solutions. Most interesting is the surface activity of HPC. Usually we associate whipping agents as being water soluble proteins such as gelatin and egg whites, for these molecules posses both hydrophilic and hydrophobic areas along their chains. Since HPC also possesses both hydrophilic and hydrophobic areas along its chains, it makes an excellent whipping agent. The major current food applications of HPC are as a whipping / stabilizing agent for whipped nondairy toppings, and as a surface film agent on confectionery items. Please note that HPC does not have a gelled state when hot. When an HPC solution is heated to the "gel point", it flocculates (Precipitates) out of solution. It does redissolve upon cooling of the water. 22

23 Here is an example of how the type of side chain can affect hydrocolloid properties. In the above figure there are listed three different cellulose based gums. All three have the same average backbone length, but all three have different side units. At 25 degrees Celsius, all three of these gums have the same 1% (w/w) water viscosity in distilled water, as measured with a Brookfield viscometer. Now, let us heat all three of the 1% water solutions to a temperature of about 70 degrees Celsius. As the 1% CMC solution is heated to 70 degrees Celsius, it becomes thinner, and the Brookfield viscosity drops to about 1200 centipoise. This is a reversible process. If the solution is cooled back down to 25C, it will immediately go back to a viscosity of 3000 centipoise again. The reason for the viscosity drop with increasing temperature is that the layer of "organized" water molecules around the CMC molecule becomes smaller as the temperature rises, making collisions with a neighbor somewhat less likely. When the methyl cellulose solution approaches 70 degrees, it actually 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 3000 centipoise at 25C. As the hydroxypropylcellulose solution approaches 70 degrees, it precipitates out of solution and makes a white layer on the bottom of the container. Upon cooling back down towards 25C., the hydroxypropyl cellulose will begin to redissolve in the water. So we have three gums with identical backbone structures of cellulose, but they each have different side chains, which leads to three very different behaviors when the solutions are heated to 70C 23

24 In summary, the cellulose derivatives can offer the food technologist a wide range of behaviors and textures under varied conditions of temperature., and can provide stability under a wide range of conditions as well. Andrew C. Hoefler Brookside Consulting 321 Mill Pond Lane Oxford, PA ahoefler@aol.com 24