Page 22 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

226 Biopolymers

thickener or gelling agent in soups and desserts. Starch is by molecular association, of a loose polymer network,
broken down, mostly in the small intestine, to oligosaccha- although in some cases aggregation of helices may be in-
rides and ultimately glucose. The most important enzymes volved. Some barley β-glucans can thus cause filtration
involved in this hydrolysis are α-amylases (also found problems in the brewing industry. There is evidence that
in plants and microorganisms) that bring about mainly increased cereal β-glucan in the human diet can play a
random scission of α-(1 → 4)-glucosidic bonds to yield role in lowering blood cholesterol levels that are consid-
oligosaccharides. Other enzymes then catalyze further hy- ered undesirably high.
drolysis to glucose. Some starch, particularly after heating Many structural tissues in living organisms appear to be
and cooling of foodstuffs, is more resistant to attack and constructed from fibers embedded in an amorphous ma-
reaches the large intestine where it is fermented by the res- trix. In plant cell walls, the fibers are cellulose while the
ident bacteria. This “resistant starch” is now believed to matrix consists of a complex arrangement of proteins and
play a part in prevention of some bowel diseases and may many polysaccharides. Historically, the matrix polysac-
contribute to lowering of blood cholesterol levels. Starch charides have been considered either as pectic substances,
and starch derivatives are used in the textile and laun- soluble in hot water or dilute acid solution, or hemicellu-
dry industries—in the first as a warp size to strengthen loses, soluble in alkali. The nature of the polysaccharides
the yarn or as a finish to give a polish to thread, in the in each group varies with the type of plant under study,
second to stiffen clothing and some household linens. and also with the age and kind of tissue and even with the
Starch products are important in the paper industry for part of the cell wall being investigated.
strengthening the paper and improving the surface quality. Pectins are found in the primary cell walls and intercel-
Starch dextrins, prepared by heating dry starch, are widely lular spaces in land plants. These polymers confer elastic-
used as adhesives on labels, envelopes, postage stamps, ity on, and hold water in, the cell walls and may be co-
etc., while minor outlets for starch are the oil industry valently bonded in the plants to arabinogalactans, which
(in drilling muds), ore refining (as a flocculating agent), have galactose backbones and branches containing arabi-
smelting (as a binder in refineries), and biodegradable nose or arabinose and galactose. Pectins occur abundantly
plastics. in fruits and vegetables, and are extracted commercially
The production of alcohol for human consumption is from citrus fruit peel, apples, sugar beet, and sunflower
based on the enzymic degradation of starch to glucose, heads. They are polysaccharides where the main chains
followed by conversion of the glucose into ethyl alcohol. contain galacturonic acid and rhamnose, and side chains
Alcohol for use as a fuel can also be made in this way. contain fucose, xylose, arabinose, and galactose. The
Glucose and partially hydrolyzed starches (glucose structure is very complex. There may be stretches of unin-
syrups) are made by enzymic and/or acid hydrolysis and terrupted (1 → 4)-linked α-D-galacturonic acid residues
find wide use in the food and confectionery industries. and so-called “hairy” regions containing a high propor-
Glucose is also used in pharmacy and medicine as an in- tion of rhamnose in the main chain. The neutral sugars fu-
stant energy source. High fructose syrups, on a weight cose, galactose, arabinose, and xylose are attached as side-
basis sweeter than glucose syrups, are produced com- chains, mostly on the rhamnose, and may occur as single
mercially from glucose solutions by passage over an im- monosaccharides or be part of long chains, often formed
mobilized enzyme, glucose isomerase. The possibility mainly of arabinose. The galacturonic acid units may be
of using glucose, from starch hydrolysis, as a feedstock acetylated, but the most important feature of pectins is the
for biodegradable polyester production is currently un- degree to which these uronic acid residues are present as
der investigation (Section D). Further, the incorporation methyl esters. High-methoxyl pectins, with a high degree
of starch polysaccharides into nonbiodegradable plastics, of esterification, and low-methoxyl pectins, prepared by
to improve their rate of break-down in the environment, is longer extraction with acid, have different properties and
also being studied. are both commercially useful. The two types of pectin
In addition to cellulose, other polymers of β-linked are capable of forming gels; the high-methoxyl pectin re-
glucose units exist; these are the β-glucans. Some, from quires a large quantity of added sucrose for gel forma-
microorganisms and seaweeds, contain mostly (1 → 3) tion, and so fruit pectins are important in the jam and
interunit links and seem to be food-reserve polysaccha- jelly and confectionery industries. Low-methoxyl pectin,
rides. Others are linear molecules containing (1 → 3) and on the other hand, carries a large number of acid groups
(1 → 4) bonds in the ratio 1 : 3 and have a structural role and the polysaccharide chains cannot associate well un-
in, for example, endosperm cell walls of barley kernels. less the negative charges associated with these acid groups
Soluble β-glucans of this type can give solutions of high are neutralized. These pectins form gels in the presence
viscosity in water, presumably because of the formation, of calcium ions and are believed to give an “egg box”