Polysaccharides (Fiber)

Polysaccharides (Fiber)

Fiber, also known as roughage or bulk, describes a group of indigestible polysaccharides. Unlike starch, the sugar units in fibers are held together by bonds the human digestive enzymes cannot break down. Most fibers, therefore, pass through the human body without providing energy. Fiber is found only in foods of plant origin, especially certain cereals, vegetables, and fruits. Plant cells rely on the fiber between their cell walls for structural strength.

A number of different laboratory methods are used to measure the amount of fiber in foods. The older technique consisted of treating a food with strong acid to simulate the environment of the stomach, and then treating it with a base to parallel the experience in the small intestine. The remaining weight of undigested fiber was measured as "crude fiber" and was listed in most food composition tables as "fiber". This rather imprecise method has been largely replaced by a process that measures dietary fiber. For every 1 gram of crude fiber, there are about 2 to 3 grams of dietary fiber.

Chemists classify fibers according to how readily they dissolve in water: soluble fibers dissolve in water, while insoluble fibers do not. The insoluble fibers of foods act as a sponge in the intestine by soaking up water. This increases the softness and bulk of the stool and may thereby decrease the risk of constipation, diverticulosis, and possibly colon cancer. Scientists have also suggested that soluble fibers may benefit health by lowering high blood cholesterol levels and reducing high blood glucose in certain kinds of diabetics. Foods containing fiber usually have a mixture of both soluble and insoluble fiber. Foods high in soluble fiber include dried beans, peas, lentils, oats, rice bran, barley, and oranges. Insoluble fibers are found predominantly in whole wheat (wheat bran) and rye products, along with bananas.

The most common fibers are cellulose, hemicellulose, and pectic substances. A few other types of fiber include vegetable gums, inulin, and lignin, the last of which is one of the few fibers that is not an indigestible carbohydrate.

Cellulose is one of the most abundant compounds on earth. Every plant cell wall is composed in part of cellulose, long chains of repeating glucose molecules similar to starch. Unlike starch, however, the chains do not branch, and the bonds holding the glucose molecules together cannot be digested by human enzymes. As a result, the cellulose fiber is not absorbed, provides no calories (kcal), and simply passes through the digestive tract. The digestive systems of herbivores such as cattle, horses, goats, and sheep have the proper enzymes to digest cellulose, allowing them to use the energy from glucose found in grass and other plants.

Hemicellulose is composed of a mixture of monosaccharides. The most common monosaccharides comprising the backbone of hemicelluloses are xylose, mannose, and galactose; the common side chains are arabinose, glucuronic acid, and galactose. Baking soda is sometimes added to the water in which green vegetables are boiled to maintain their color. Unfortunately, it breaks down the hemicellulose of the vegetables, causing them to become mushy.

Pectic substances found between and within the cell walls of fruit and vegetables include protopectin, pectin, and pectic acid. These substances act as natural cementing agents, and so are extracted from their source foods by the food industry for use in thickening jams, keeping salad dressing from separating, and controlling texture and consistency in a variety of other foods. Not all the pectic substances, however, can be used for gelling purposes, and the amounts that can be obtained vary depending on the ripeness of the fruit or vegetable. The pectin found in ripe, but not overripe, fruit is responsible for gel formation in jams. Protopectin and pectic acid are prevalent in unripe and overripe fruit respectively, and are insufficient themselves to cause gel formation.

Vegetable gums belong to a group of polysaccharides known as hydrocolloids. They are derived from three main sources: plant gums (gum arabic, gum karaya, gum tragacanth), seeds (locust bean gum, guar gum), and seaweeds (agar, alginates, and carrageenan). A bacterium (Xanthomonas campestris) serves as the source for xanthan gum. Gum fibers are composed of simple sugars used to thicken, provide viscosity, gel, stabilize, and/or emulsify certain processed foods. They impart body, texture, and mouthfeel to foods, while also making it less likely for dispersed ingredients to separate. The gums' "water-loving" nature combined with their ability to bind as much as 100 times their weight in water contribute a certain desirable appearance, texture, and stability to food products. Vegetable gums are normally sold as a dry powder and are used extensively as stabilizers in the production of low-calorie salad dressings, confections, ice cream, puddings, and whipped cream. Gums are also used in many frozen products because they control crystal growth, yield optimum texture, and make the food more stable in the freezing and thawing process. Typical applications in the food industry of a gum, specifically carrageenan. Agar gum can be used for quick-drying frostings and to reduce chipping or cracking in glazed doughnuts.

Inulin consists of repeating units of fructose with an end molecule of glucose. Although this fiber occurs naturally in over 30,000 plants, it is most commonly found in asparagus, Jerusalem artichoke, and garlic, but commercial processors extract it from the chicory root. lnulin is a soluble fiber that can be used by the food industry for giving a creamy texture to frozen dairy products such as no-fat or no-sugar ice cream, improving the textures of margarine spreads, and developing no-fat icings, fillings, and whipped toppings.

Lignin is the one fiber that is not a carbohydrate. Instead of saccharides, it consists of long chains of phenolic alcohols linked together into a large. complex molecule. As plants mature, their cell walls increase in lignin concentration, resulting in a tough, stringy texture. This partially explains why celery and carrots get tougher as they age. Boiling water does not dissolve or even soften the lignin.

KEY TERMS

Diverticulosis

An intestinal disorder characterized by pockets forming out from the digestive tract, especially the colon.

 

Phenolic

A chemical term to describe an aromatic (circular) ring attached to one or more hydroxyl (-OH) groups.

 

 

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