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Sunday, December 23, 2007

The Plot Thickens

So, I've spent some time on the simple saccharides and now its time to move to the more complex carbohydrate structures. Although I've touched on these before, today we're going to dig a little deeper.

Polysaccharides are chains of saccharide molecules which form polymers. They can range from a few hundred to many thousand molecules long and can be straight or branched chains. The majority of the polysaccharides that we are familiar with are plant based. Plants, via photosynthesis, take carbon dioxide (CO2) from the air, water (H2O) from the ground and energy from the sunlight (through cholorphyll) and convert it to oxygen (O2) and carbohydrates to store energy (starches) and provide structure (cellulose).

Starch is comprised of glucose chains with alpha linkages (molecules are attached at the first, aka alpha, carbon) and comes in two types of structures: straight chains (amylose) and branched chains (amylopectin). Plants store these molecules in granules in the cells. Starches are not sweet, aren't solubile in cold water, but do swell (gelatinize) when the solution is heated. We start to break down starches through our saliva which contains amylase (the suffix -ase denotes the enzyme). Amylase breaks the starch into smaller units but this process stops once the stomach acids become involved; they inactivate the enzyme. The final breakdown of starch occurs in the small intestine where pancreatic enzymes finish the job and the glucose molecules are absorbed into the bloodstream.

Now, fiber is also a polysaccharide, and it is differenciated into soluble and insoluble forms. Insoluble fiber is characterized by chains with beta linkages (molecules attached at the second, aka beta, carbon). This is a small but significant difference because we can't break these linkages and insoluble fibers reach our large intestine intact. They include cellulose, hemicellulose and ligin and are found in whole grains, bran, and vegetables. Insoluble fibers leave our body in essentially the same form they entered, that's why they are important - they add bulk to our diet and help carry waste out.

Soluble fibers, as you've probably guessed, are soluble in water and include pectin and gums. Soluble fibers are also chains, but instead of sugar molecules, they are sugar acids. You find them in items like citrus fruits, apples, strawberries, oats, legumes, guar and barley. Although we can't break soluble fiber down, the bacteria in our gut can and do; this fermentation provides us with beneficial short-chain fatty acids.

So, now you know the basics of the polysaccharides and how they are processed in the body; next time I'll discuss how and why these are used by food technologists in your favorite foods. Until then - stay saucy!

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