Starch
Plant cell amyloplasts (leucoplasts)
Amyloplast
Image courtesy of Beginner’s Guide to Molecular Biology (www.
res.bbsrc.ac.uk/molbio/guide/cell.html)
Plastids
• Chloroplast
– Responsible for synthesis of chlorophyll
• Chromoplast
– Responsible for synthesis of carotenoid
pigments (carotene, lycopene, etc.)
• Amyloplast (leucoplast)
– Responsible for synthesis of starch
Leucoplasts (amyloplasts)
Image courtesy of Plants and Society, Levitin and McMahon
(www.life.umd.edu/pbio100/contact1.html)
Starch granules
Synthesized in the amyloplasts
Potato starch, normal light
Potato starch, polarized light
Birefringence indicates that the granules are semicrystalline
Interior structure of the granules is still not well
understood
Potato starch granules under
polarized light
Hilum -where synthesis
of the granule
began
Image courtesy of Univ. of York, Inst. For Applied Biology
(www.york.ac.uk/org/macromol/)
Corn starch granules
Unmodified corn
starch
Corn
Potato
Tapioca
Oat
Rice
Wheat
Other
starches
(Magnification =
1000x)
Amylose
Alpha-1,4
A linear polymer of D-glucose (actually it is known
to contain a few very short and widely spaced
branches)
Much lower in molecular weight than amylopectin
Amylopectin
Alpha-1,6
Highly branched polymer of D-glucose
Much higher molecular weight than amylose
Amylopectin
Reducing end
Note highly branched
structure
Amylose/amylopectin ratio
• Generally, about one part amylose to
every three parts of amylopectin for
“normal” grain sources
• “Waxy” varieties contain 0% amylose
and 100% amylopectin
– Used in non-gelling starch applications,
starch-thickened frozen products, and many
modified starches
Amylose/amylopectin ratio
• High amylose varieties
– 60-85% amylose, the rest is amylopectin
– Useful as binders and film formers
Starch granule composition
Normal starch
AP:A ~ 3:1
Waxy starch
100% AP
High amylose starch ~ 60‐85% A
Starch degrading enzymes
Beta amylase--an exo enzyme
Maltose
Maltose
Beta-amylase
AP
Maltose
Limit dextrin
phosphorylase
glucoamylase
D-glucose
Starch degrading enzymes
• Endo enzymes
– Attack interior of the molecule
– May attack AP on either side of a branch
point
– Example: alpha-amylase
Debranching enzymes
Alpha 1,6 branch point
Amylopectin
R enzyme (malted barley)
Pullulanase
Isoamylase (yeast)
Gelatinization
• Swelling and disorganization of starch
granules heated in water
• Measures of gelatinization
–
–
–
–
–
Swelling of granules
Increased viscosity
Increased translucency
Increased solubility
Disappearance of birefringent granules on
heating
Gelatinization temperature
o
Starch
Range ( C)
Potato
56-66
Corn
62-72
Sorghum
68.5-75
Wheat
52-63
Gelatinization temperature range is characteristic for
each type of starch, that is for each botanical source.
Starch gelatinization
Raw
starch
Amylopectin
Heat and water
+
Swollen starch
Also see www2.hawaii.edu/lynn/main.html
Amylose
Gelatinization micrographs
Unmodified corn
starch
Corn starch, 65 C
Images courtesy of www2.unl.ac.uk/~hx14marshar/
cho.htm#Starch gelatinisation
Gelatinization micrographs
Corn starch, 70 C
Corn starch, 85 C
Images courtesy of www2.unl.ac.uk/~hx14marshar/
cho.htm#Starch gelatinisation
Starch gelation
swollen
amylose
collapsed
cool
Junction zone
water
water
water
Starch gel
Starch gelation
• Amylose is the “glue” that holds the gel
together
• Therefore, waxy starches do not gel
– They form thick, cooked pastes and are
frequently the starting material in the
production of modified food starches
Starch retrogradation
Junction zones;
not too large
Starch over-retrogradation
(uglification)
Large junction zones
Gelatinization, gelation and
uglification
Starch Gelatinization and
Pasting, Gelation, and
Uglification
Go to Slide Show mode and click to begin
RapidViscoAnalyzer
Ideal RVA graphs
Cooled paste
stability
Peak
viscosity
Breakdown
Setback
Viscosity
Time/temperature
RVA graphs of different starches
(idealized)
Corn
Cross-linked
Viscosity
Waxy corn
Time/temperature
RVA of grain starches
RVA of tuber starches
RVA of cross-linked starches
Factors affecting gelatinization,
retrogradation, and gel formation
• Sugar
– Competes for water and plasticizes junction
zones; decreases gelatinization and gel
strength
• Acid
– Hydrolysis of acid sensitive glycosidic
linkages produces smaller pieces of starch
molecules; decreases gel strength
Factors affecting gelatinization,
retrogradation, and gel formation
• Stirring/shearing/pumping
– Collapses swollen granules; this decreases
gel formation and gel strength
Modified starches
• Waxy
– All amylopectin, no amylose
• Pregelatinized
– Cold water dispersible (instant puddings)
Modified starches
• Acid modified (thin boiling)
– Limited hydrolysis of starch
– Decreases maximum viscosity
– Gives excellent cooled gel strength, e.g. gum
drop texture
Modified starches
• Cross-linked
– Restricts granule swelling
– Limits maximum viscosity
– Makes granules much less fragile
• Derivatized
– Ethers or esters
– Used to prevent or control syneresis. Good
for freeze-thaw stability