These groups of compounds come from a variety of plants and are
often very complex in their composition. The only thing that they
have in common is that they are sticky and they are exuded or
extracted from plants.
Plants latexes and resins have been replaced by synthetic petroleum products to a large extent but the use of natural hydrogels has expanded.
Hydrogels are substances that modify the behavior of water. Gravy is an example of a hydrogel. Some common household hydrogels are starch, pectin and gelatin (animal-derived)
There are three classes of plant hydrogels: gums, pectins, and starches
1. Gums - polysaccharides of acid salts of sugars other than glucose, can be linear, branched or cross-linked. Metal ions of salts cause these compounds to associate with water. Gums are not digestible by humans (inert) and are used in foods, diet products, medicine, also used in paper, textile and petroleum industries.
Biological function of gums is to seal wounds and prevent infection of wounds by microrganisms, majority of gums are not harvested in this manner but are extracted from plant tissue or synthesized.
A. Foods - provide texture, body, thickening, retain moisture, used in sauces, ice cream, "crystals" of instant beverages, sandwich spreads, luncheon meats
B. Medicine - hold tablets together, disperse particles, laxatives, toothpaste, lotions.
C. Textiles - major use of gums, used as sizing agents which stiffen and strengthen threads during weaving, sizing is washed out into water and gums do not cause ecological problems as starch does since they are not as digestible as starch, also smaller quantities of gum are required.
D. Paper - only major use is to texturize paper for printing and water repelling (Butcher and freezer papers).
E. Petroleum industry - used to lubricate drill bits and secondary recovery, now use mostly synthetic gums
Sources of gums
Natural gum - natural gums can be obtained from wounded woody tissue or seeds
1. Gum arabic - Acacia senegal, native to W. Africa, 90%
comes from wild plants, used as foam stabilizer in beer, postage
stamp glue, candies, soaps, lotions, water colors, ink
2. Gum tragacanth - Astragalus, is resistant to acid but can be allergenic, used in mayonnaise, sandwich spreads, lotion, pills.
3. Gum kayara - Sterculia urens, occurs in India, used as dental adhesive, binder in luncheon meats, laxative, hair setting gels.
4. Gum ghatti - Anogeissus latifolia - from India, used mostly as emulsifier in waxes and fat-soluble vitamins.
1. Locust gum - Ceratonia silique (Fabaceae), also used as
source of carob, is used in ice creams, salad dressing and pie
2. Guar gum - Cyanopsis tetragonobolus (Fabaceae), from India, shrubby plant grown in Texas and Oklahoma, originally as cover crop for grazing and as a substitute for locust gum in WWII. Can be harvested mechanically, used mainly in production of paper.
Cellulose gum - semisynthetic, cellulose reacted with Sodium monochloroacetate, used widely in detergents, paper industry, latex paints, processed foods.
Pectins are used in jams and jellies, obtained from apple residue and citrus peels, function in plant cells as cementing substances.
Starch is extracted from seeds, tubers or roots, is a chain of glucose molecules. There are two forms, one in which the chains are linear (amyloses) and the other in which the chains are branched (amylopectins)
Starch is packed into insoluble granules, these granules have characteristic shapes and sizes depending on the source. High temperatures cause the granules to burst. If they are not dispersed before heating, they will clump----->lumpy gravy.
Sources of starch are corn, wheat, sorghum, arrowroot, cassava and sago.
More than half of the starch used annually is used in cardboard production, also used as a food thickener, paste, laundry.
Latex is a complex mixture of organic compounds which form an emulsion in water and are produced in cells called laticifers, found mostly in dicots. May be byproducts that plants secrete into laticifers as waste. Opium poppies produce latex but it is inelastic.
Natural rubber is a polymer of isoprene
Hevea brasilensis (Euphorbiaceae) is the source of most natural rubber, is harvested from wounded bark. The latex can be dissolved in hexane and applied to other substances such as fabrics------>raincoats.
Charles Goodyear discovered vulcanization in 1839, add sulfur which cross-links isoprene chain and improves elasticity.
Sources of natural rubber (Asia) were cut off in WWII and researchers found a synthetic rubber could be produced from petroleum. Natural rubber is still used to a large extent, even in tires, since it is more resilient than synthetic rubbers.
Another possible source of natural rubber is guayule - Parthenium argentatum, plant native to Chihuahuan Desert. Plants cannot be harvested until 7 years of age. There is some production in Mexico and it may eventually be produced in U.S. (Texas).
Latex can be obtained from other New World plants but primary source has always been Hevea brasilensis.
Resins are polymerized terpenes and volatile oils, are water insoluble, probably function in plant as herbivore deterrent, are secreted in canals or ducts. Used in incense (frankincense and myrrh are resins), embalming, lacquer, turpentine, rosin, amber.
Sources - major natural sources are pines, most resins are now synthetically produced.
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