Hydrogels, Gums, and Pectins

David S. Seigler Department of Plant Biology University of Illinois Urbana, Illinois 61801 USA seigler@life.illinois.edu http://www.life.illinois.edu/seigler

OUTLINE: HYDROGELS, GUMS AND PECTINS Importance o Widely used o Non-nutritive Botanical o Algae o Legumes o Other families

Properties o Gels o Thickeners o Absorbents Economics o Starch utilization Isolation

Reading CHAPTER 10 IN THE TEXT, p. 243 Although the materials in this chapter may not be overly familiar, if everyone tries reading labels on a number of common food products, cosmetics, and beverages, many of the names will appear.

Hydrogels Hydrogels or water modifying substances are widely used to "thicken" various products. Both gelatin and starch have these properties. The three main types from plants are gums, pectins, and starches. In the plant these serve different functions.

Agar Agar comes from a red alga. Several species are used. In Japan, the agar is isolated by freezing and thawing naturally, but this substance is isolated by other methods in various other countries. Used in culture media, laxatives, pharmaceuticals, and foods as thickener and stabilizer.

Drying of agar, a red alga Courtesy Dr. Larry Hoffman

Algin Algin comes from the brown algae Laminaria and Macrocystis. Algin is in dentrifices (tooth paste), shaving creams, and hand lotions. The viscosity changes with calcium ion concentration.

Microcystis, a brown alga, kelp Courtesy Dr. Larry Hoffman

Irish moss, Chondrus crispus, a red alga Carrageenin comes from Irish moss, a red alga, Chondrus crispus. This polysaccharide is used mostly as an ice cream stabilizer, but also for a thickening and suspending agent in other food products. R. Bentley and H. Trimen, Medicinal Plants, London, Churchill, 1880.

Gums Gums are wound or injury products of woody plants. In nature, they probably seal wounds. Chemically, gums are polysaccharides containing acidic groups. Pg. 245 for structures. The acidic groups interact with small amounts of (especially) calcium, magnesium and potassium. Gums make viscous solutions or gels in water.

Many important gums are listed on pg. 244. In general, humans do not have the ability to digest gums. The gums pass through humans more or less unaltered. Gums are widely used in the paper, textile, and petroleum industries. Gums, such as gum arabic, are used for microencapsulation of medicinal and food products.

In foods, gums are used to give the products the proper texture or body. These polysaccharides stabilize emulsions, retain moisture, thicken liquids and suspend particles.

In frozen products, gums help prevent the formation of ice crystals. Gums are used to coat many "instant mixes" so that the mixes don't react with water from the air and become all globbed together. In medicine, gums are used to hold some tablets together (some grades of starch are also used for this purpose) and are also used to coat the little beads of medicine in "time release formulations" (they're not the only things used there, however).

Gums also are used to make things like toothpaste "gel". In the paper and textile industries, they are used to fill irregularities and pores in the surface of products and give the product a nice smooth feel. Gums also help to make a nice smooth image in printing. These are called "sizing agents" or "sizes".

Some types of paper use gums to make the paper more water resistant, but this has largely been replaced by plastics today. In textiles, gums also make the threads somewhat stiff during the weaving process (as sizes). When you wash the clothes, the gums all wash out. Starch is more often used as a sizing agent because it is cheap compared to gums.

Gums have also been used in oil well drilling operations Gums have also been used in oil well drilling operations. They serve as lubricants (tannins which we will discuss later are also used for making "oil well drilling mud"). The gums keep the particles that are removed by grinding suspended and the particles are pumped out of the hole. Many of the gums used for these purposes are synthetically derived.

The most important (and most expensive) plant gum is gum arabic or gum acacia that comes from Acacia senegal (and other species). Pg. 247 and 248.

Acacia senegal, Fabaceae R. Bentley and H. Trimen, Medicinal Plants, London, Churchill, 1880.

Orchard of Acacia senegal trees in the Sudan Courtesy Dr. Abraham Krikorian

Wounding of branches and gum exudate Courtesy Dr. Abraham Krikorian

Courtesy Dr. Abraham Krikorian Grading gum in market Courtesy Dr. Abraham Krikorian

Most of the gum is from the Sudan. Ninety percent of the gum arabic today still comes from wild trees. These trees are wounded, the bark peeled back and blobs of gum allowed to form; the balls of gum are then collected.

We use gum arabic daily. One of the common uses is in soft drinks. As mentioned in the text, gum arabic is used for beer making to stabilize the foam. Although used as an adhesive although the main adhesive on stamps was derived from starch until recently. This has changed as new issues of stamps have other types of adhesives.

Gum drops and similar candies of good quality almost always include gum arabic. This material is used in many cosmetics, and hand lotions, as well as in some paints and in inks. Also as a size, but too expensive for most uses of this type. Often used in pharmaceuticals. For preparation of various instant mixes.

Currently, industrial users are trying to locate better gums from more regular and controllable sources.

Gum tragacanth, Astragalus gummifer, Fabaceae Some other plant gums, such as gum tragacanth from Astragalus gummifer, Fabaceae or Leguminosae, are fairly good. That gum is mostly produced in Iran and Iraq. R. Bentley and H. Trimen, Medicinal Plants, London, Churchill, 1880.

All these gums are hand harvested from trees All these gums are hand harvested from trees. Cut into the center of the trees near the base, and ribbons of gum are exuded. The trees or shrubs that produce these gums can't be grown easily. Gum tragacanth is mostly used in the food industry; there are no really satisfactory synthetic substitutes.

Carob or locust gum, Ceratonia siliqua (Fabaceae) Although true gums come from woody tissue, two gums found in seeds of legumes are of considerable importance. Pg. 249. Carob has been grown for thousands of years in the Near East. The food that the prodigal son fed to the swine in the Bible is almost certainly the fruits of this plant.

Ceratonia siliqua, carob, Fabaceae Courtesy Dr. B. E. van Wyk

Ceratonia siliqua, carob

Carob is best known to us as a substitute for chocolate. The Egyptians used it for an adhesive for mummy bindings. The endosperm from the seeds is isolated and ground into a powder.

The seed is about 35% gum. Locust gum is widely used in the food industry in ice creams, salad dressings, and pie fillings. Apparently still eaten directly by poor in the Near East. This is the kind of locusts that John the Baptist ate in the wilderness.

Guar gum (Cyamopsis tetragonolobus) (Fabaceae) Guar has also been cultivated for thousands of years in India. Interestingly, all the wild relatives of guar are found in Africa. Guar can be raised as an annual crop and is cultivated in North Texas and southern Oklahoma.

Courtesy Dr. Ted Hymowitz Guar field in Texas Courtesy Dr. Ted Hymowitz

Guar (Cyamopsis tetragonolobus) (Fabaceae)

Can be handled with equipment used for other crops and grown cheaply. Guar gum is widely used. It is not as good for many purposes as gum arabic (or other gums), but is often substituted because of cost and availability. Guar gum is widely used in the paper industry. Also used in fire hoses because it makes the water flow faster.

Modified starches and cellulose Carboxymethylcellulose is synthetically derived from cellulose. This gum is widely used, as it is inexpensive. Used in laundry detergents because it keeps the dirt from settling back on the clothes. Also used in the paper industry. Used in latex paints to make them go on well with rollers.

Pectins Pectins are water soluble and make viscous solutions or gels, similar in composition to the gums mentioned above. Pectins also form gels under certain conditions.

In plants, pectins are the substances that stick the plant cell walls together. For this purpose, pectin is often associated with calcium ions. There are also enzymes "pectinases" that degrade pectins. The number of methoxy groups present is important in determination of the properties of any particular pectin.

As fruits mature, pectinases usually break down the pectins between cells and make the fruit soft and edible. The peels of many fruits serve as convenient sources of pectins. Most commercial pectins come from "apple pomace" or waste from manufacture of apple products (10-15% pectins), citrus wastes (20-30% pectins) and from sugar beet processing.

Pectins are extracted by heating the plant materials in water (60-95 C) and at an acidic pH (2.5). The pectins are precipitated with ethanol and removed by centrifugation. As is true for gums, humans are incapable of digesting pectins. However, bacteria in our gut can. We still lose out, because we don't get any particular benefit from it.

At the grocery store, "Sure Gel" and "Certa" are pectin products that are widely used to make jellies, preserves, jams etc. Over 75% of the pectins used are used for this purpose. Many candies (especially gum drops) involve pectins as a gelling agent. Also used in ice cream, cosmetics, adhesives, and in hardening steel. Used in many foods as a thickener and stabilizer.