CHEM215 Lecture Notes - Lecture 9: Staling, Monosaccharide, Amylopectin
What makes a good storage carbohydrate?
• must consist of a large number of monosaccharide units (polysaccharide) - to avoid
increases in osmotic pressure and cell damage
• Be formed quickly while the nourishment is available
• Be easy to breakdown when required
• Thus they are generally globular with many sites for enzyme access.
Starch:
• derived from plant sources such as corn, potato, rice and wheat - grain seeds,
tubers and unripe fruits.
• Starch is composed of long polymers of glucose units joined by å-1,4- glycosidic
linkages.
• Two main types of glucose polymers that make up starch:
-amylose: mainly straight chain; makes up 20-25% of starch; long chains of a-D-
glucopyranosyl resides linked as in maltose between their 1- and 4- positions.
-amylopectin: branched chain; much larger molecule than amylose; most glucose
units are joined by a-1—>4 glycosidic links, but 4-5% are joined by a-1—>6 links
which creates branches.
• The proportion of these chains in starch is important because the behaviour of
heated starch in water depends on whether the source is high in amylose or
amylopectin.
• Starch is stored as granules in nature, which come in many shapes and forms
depending on the biological origin.
• Amylose polymers tend to form a helical arrangement in the starch molecule with
the amylopectin polymers.
• Starch gelatinisation: starches do not form true solutions with water because the
molecules are too large. When starch is heated in water, the bonds joining
amylose and amylopectin are weakened which allows water molecules to move in
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and form H-bonds.
Starch and food: Bakery products:
• Amylases are able to break down the å(1—>4)-glycosidic linkages.
• Amylases are proteins, thus they will be deactivated at high temperatures.
• Factors influencing starch breakdown:
-availability of water
-presence of fat —> affects the ability of enzymes to access the substrates
• Shortbread—> this is made from flour, butter and sugar. Oil from the butter coats
the starch granules which limits the access of water - leads to a visible granule
structure.
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Document Summary
Amylose: mainly straight chain; makes up 20-25% of starch; long chains of a-d- glucopyranosyl resides linked as in maltose between their 1- and 4- positions. When starch is heated in water, the bonds joining amylose and amylopectin are weakened which allows water molecules to move in and form h-bonds. Starch and food: bakery products: amylases are able to break down the (1 >4)-glycosidic linkages, amylases are proteins, thus they will be deactivated at high temperatures, factors influencing starch breakdown: Presence of fat > affects the ability of enzymes to access the substrates: shortbread > this is made from flour, butter and sugar. Dextrins and maltodextrin: dextrins: low molecular weight carbohydrates produced by the hydrolysis of amylose. Inulin: storage fructan - (2 >1) linked polymers of -d-fructofuranose that contains approximately 20-30 fructose units. Found in higher plants and algae and in tubers. Sometimes used as a starch substitute for diabetics as the release of sugar is much slower.
