Where is starch present in grains?
- cereal grains
- pulses/ legume chains
What surrounds starch
Starch in grains is embedded in a complex matrix (enzyme access limited)
* Protein matrix surrounds starch
* Cell wall surrounds starch and protein
Types of polysaccharides
homo - polymerized with same monomer
hetero - polymerized with different monomers
Composition of starch molecules
- major: starch
- minor: protein, lipid, minerals (found in granules)
Classification of starch based on the amylose content
- Waxy <10%, w/w - swelling power is higher so more visciostiy
- Regular ~18-25%, w/w
- High Amylose >40%,w/w - can retrograde quickly to form gel
Two types of starch molecules
- amylose: 1-4 alpha glycosidic link; linear
- amylopectin: 1-4, 1-6 alpha glycosidic links; branch points are nearly 5% of total bond structures
gel stability of amylose and amylopectin
- amylose is unstable: Linear so retrograde fast so gel more fast changing in texture since retrogradation is more efficient and linear molecules can pack more tightly and associate.
- amylopectin is stable: Branched nature so does not retrograde as well so gel is more stable for longer.
HiMaize
High amylose corn flour
* Variety of corn grounded into powder 50-70% amylose; usually sold in mass production of bread
making. High in resistance starch so hard to release and used in low glycemic food application
General amylose content of grains
- Waxy, regular and high amylose corn and barley starches available
- Wrinkle pea (a legume) starch has >70% amylose
- Pulse/legume starches usually show higher amylose content, >35% up to 42%
How do rice grains vary in amylose content
- Short grain/Sticky rice Japonica Rice (High amylopectin) - more sticky: Upon cooling after cooking in excess water, slower retrogradation of
amylopectin /starch leads to lower amounts of resistant starch formation in short grain rice that leads to faster digestion and higher GI. - Long grain/Fluffy rice Indica Rice (High amylose) -High degree of packing and difficult for enzymes to act on it: Upon cooling after cooking in excess water, retrogradation of starch leads to
higher amounts of resistant starch formation in long grain rice that leads to slower digestion and lower GI.
Fine Structure of Amylopectin
Cluster Model
C chain is centre most chain and then branches into the B chains which extends from one to the other which then go into the smaller A chains (outermost chain). Potential to form double helical structure through H bonding. So can pack themselves where A chain is because of the linear formations through double helix formation very tightly so potential to form a highly ordered region. Branching is not linear so considered an unordered or amorphous region
* C-chain carries the ONLY reducing end
* All other tips are non-reducing
* Highly packed and ordered = crystalline
* Less associated = amorphous
backbone of starch granules
amylopectin
* Mainly amylopectin structure and blocklet is the base unit to form base layers of growth rings of granule. Blocklets pack themselves and then amylose runs between and through the blocks to bind them together
Starch Granule Architecture
Intermolecular hydrogen bonds stabilize the ultra-structure
How does amylase break down starch?
Highly packed crystalline lamellae/regions of starch known as resistant starch type 2 (RS2) Resist enzyme hydrolysis - amylase enzyme enter starch granule interior through amorphous lamellae
* Enzymes put holes on top and then penetrate in and hydrolyzed them. Enzymes mainly search for less associated regions such as the amorphous region. Then radiate up and down to hydrolyze crystalline regions. Crystalline resist hydrolysis but eventually get in. They are type II resistant starch. Enzyme selectively penetrate the blockers through amorphous part
Where do amylose molecules exist within the starch ultra-structure?
- Free amylose in the amorphous region - random coil
- Amylose-lipid complex in the amorphous region
- Co-crystalized with the amylopectin crystals
What is used to determine amylose content of starches?
Iodine blue technique
How to determine amylose bound to lipids
How are amylose co-crystallized with amylopectin?
Run through the blocklets themselves and into the helix structure
* dominant in pulse starch
* High amylose content foods
* Resistant to amylase hydrolysis and cooking
What grains commonly have co-crystallization?
highly present in higher amylose containing pulse, corn and barley starches. Hence, high amylose starches are digested slowly by amylase enzymes (also called as slowly digestible starches – good for low glycemic food formulation)
V-Amylose Crystals
Clusters of Amylose–Lipid Complex can associated with each other through H bonding since the outside is hydrophilic between the amylose which get packed and so enzyme cannot hydrolyze the crystals very well.
* RS5 type of resistant starch.
* Melting temperature = 90-110o C
How does Amylose/lipid structure influence structure
- Makes bread softer
During brad making take the wheat flour and mix and knead the dough and then allow it to proof and then cut and bake. The starch will geletinze during baking anf amylose are released and can pack tightly on their own when cooled and for stronger textured bread. If fat added though the amylose forms a complex with the fat instead so changing conformation from linear to helix conformation and cannot pack as strongly so bread becomes softer.
Is Native Starch Soluble in Water?
- YES - Individual amylose and amylopectin molecules are soluble in water
- NO - In native starch granules, amylose and amylopectin molecules are tightly associated to each other – hence not soluble in cold water
What does starch gelation include?
Includes both
* gelatinization (heating)
* retrogradation (cooling)
What occurs in gelatinization?
Heat starch in excess water
* granule swelling due to hydration
* Crystal/double helical melting of amylopectin
* Amylose leaching
