Transcript FOOD-CHEMISTRY-Lecture-3-Unit-2

FOOD CHEMISTRY-
Polysaccharides
BY
DR BOOMINATHAN Ph.D.
M.Sc.,(Med. Bio, JIPMER), M.Sc.,(FGSWI, Israel), Ph.D (NUS, SINGAPORE)
PONDICHERRY UNIVERSITY
III lecture
8/August/2012
Source: Collected from different sources on the internet and presented b y Dr Boominathan Ph.D.
Polysaccharides
2
Goals: Cellulose & Starch
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•
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Cellulose structure
Cellulose ingredients
Starch structure
Starch gelatinization
Modified starches
CH2OH
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
b
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
CH2OH
H
OH
cellulose
Cellulose, a major constituent of plant cell walls, consists
of long linear chains of glucose with b(14) linkages.
Every other glucose is flipped over, due to b linkages.
This promotes intra-chain and inter-chain H-bonds and
van der Waals interactions,
that cause cellulose chains to
be straight & rigid, and pack
with a crystalline
arrangement in thick bundles
- microfibrils.
Schematic of arrangement of
cellulose chains in a microfibril.
CH2OH
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
H
OH
cellulose
The role of cellulose is to impart strength and
rigidity to plant cell walls, which can
withstand high hydrostatic pressure
gradients. Osmotic swelling is prevented.
Cotton
Cotton fibres represent the purest natural form of cellulose, containing
more than 90% of this carbohydrate.
Cellulose
• Most abundant organic compound on the
planet
• Plant cell wall component
– Gives tensile strength to cell wall
CH 2 OH
O
O
O
OH
OH
OH
O
O
O
• Very high molecular weight insoluble
polymer of glucose
– b-1-4 glycosidic bonds
– These bonds give cellulose a very rigid straight
parallel chain that has extensive H-bonds
– It is mainly used to produce paperboard and
paper; to a smaller extent it is converted into a
wide variety of derivative products such as
cellophane and rayon.
– Converting cellulose from energy crops into
biofuels such as cellulosic ethanol is under
investigation as an alternative fuel source.
CH 2 OH
CH 2 OH
OH
OH
OH
AMYLOSE
v.s.
CH 2 OH
CH 2 OH
CH 2 OH
O
O
O
O
OH
O
OH
OH
O
OH
OH
OH
CELLULOSE
AMORPHOUS
REGIONS
CRYSTALLINE REGIONS
7
Cellulose
Hydrogen
Bond
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Cellulose
Properties
• Crystalline regions have very tight H-bonding
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–
–
–
Insoluble in water
No effect on viscosity (why?)
Little access to hydrolytic reagents and enzymes
Very tough texture
• Not digestible by humans
b-1-4 glycosidic bonds
Pass through digestive system
Contributes no calories
Some ruminants like cows and sheep contain certain
symbiotic anaerobic bacteria (like Cellulomonas) in the
flora of the rumen, and these bacteria produce
enzymes called cellulases that help the microorganism
to break down cellulose
– Dietary fiber
–
–
–
–
• Improve bowel movements
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Cellulose
• Uses in foods
– Unmodified cellulose is made from wood pulp or cotton (dry
powder)  very cheap
– Minimal effect on viscosity
– Added as "fiber" (breads and cereals)
• Non-caloric bulk (no flavor, color etc)
– Very little effect in foods
• Can improve function slightly by heating
–
–
–
–
Small number of H-bonds break
Slight swelling, softening
Only slightly soluble in water
No change in digestibility
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Cellulose
Cellulose can be modified to dramatically improve
its function and use:
A) Microcrystalline cellulose (MCC)
◦
Prepared by partial acid hydrolysis
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◦
◦

Non-crystalline regions are penetrated by acid and
cleaved to release the crystalline regions
Crystalline regions combine to form microcrystals
Still insoluble (all crystalline)
Limited food uses:
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

Stabilizes emulsions
Absorbs oils & syrups
Dry mixes - keeping them free-flowing
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Cellulose: Microcrystalline cellulose
(MCC)
•
Two main products of MCC
1. Powdered MCC
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•
•
Spray dried MCC
Forms aggregated
porous/sponge-like
microcrystals
Uses:
–
–
Flavor carrier
Anticaking agent in powders and
cheese
Cellulose
2. Colloidal MCC
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•
•
Mechanical energy applied after hydrolysis to
rip microcrystals apart to form small microaggregates
Water dispersible – similar function as food
gums
Food uses:
– Foam and emulsion stabilizer
– Pectin and starch stabilizer
– Fat and oil replacement
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Cellulose
CH
2
OH
CH
2
O
B) Methyl cellulose
• Cellulose treated with
NAOH to swell fibers and
then methyl chloride is
introduced:
CH
OH
2
OH
O
O
OH
O
O
OH
OH
O
OH
OH
OH
1] NaOH
2] CH
3
Cl
– Get methyl ether group
CH
2
OCH
CH
3
O
2
OCH
CH
3
O
3
O
O
OH
OCH
2
O
OH
OH
O
OH
OH
OH
Cellulose: Methyl cellulose
• Unique results:
– “Soluble” in cold water
• Methyl ether group breaks H-bonding
– Solubility  as temperature 
• Heating dehydrates the cellulose and
hydrophobic methyl ether groups start to
interact
• Viscosity increases and methyl cellulose forms a
gel
• Becomes soluble again on cooling
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Cellulose: Methyl cellulose
• Food uses:
– Thermogelation properties
• Fat/oil barrier in batters for deep fried food
applications
– The cellulose gels on heating and prevents fat uptake
• Holds moisture in food during thermal processing
• Acts as binder during thermal processing
– Fat replacer
• “”Methyl ether group””gives it fat-like properties
– Emulsion and foam stabilizer
• Due to increased viscosity (thickening effect)
– Film forming ability (e.g. water soluble bags)
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Cellulose
CH
OH
CH
2
C) Carboxymethyl
cellulose (CMC)
2
CH
OH
OH
2
O
O
O
O
OH
O
OH
OH
O
• Cellulose treated with
NaOH
to swell fibers and then
chloroacetic acid is
introduced:
OH
OH
OH
1] NaOH
2] ClCH
pH DEPENDENT
-
– Get carboxymethyl ether
group
CH
2
O CH
2
CO
2
O
COOH
-
-
CH
2
2
O CH
2
CO
CH
2
2
O
2
CO
2
O
O
OH
O CH
O
OH
OH
O
OH
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OH
OH
Cellulose: Carboxymethyl cellulose
• Food use:
– Major use: non-digestible fiber in dietetic foods
– Hot and cold water soluble
– Weak acid  properties affected by pH due to
carboxyl group
• COOH  COO• Negative charge leads to repulsion between CMC
making it a good thickening and stabilizing agent
• repulsion = viscosity
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Cellulose: Carboxymethyl cellulose
• Food uses (cont.)
– Common stabilizer in ice cream
• Retards ice crystal formation
– Foam stabilizer
– Tends to interact with proteins due to charge,
increasing their viscosity & solubility
• Used to stabilize milk proteins in milk
– Can form gels and films
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Hemicellulose
• Hemicellulose is a polysaccharide related to
cellulose that comprises nearly 20% of the
biomass of most plants.
• In contrast to cellulose, hemicellulose is derived
from several sugars in addition to glucose,
especially xylose but also including mannose,
galactose, rhamnose, and arabinose.
• Hemicellulose consists of shorter chains – around
200 sugar units. Furthermore, hemicellulose is
branched, whereas cellulose is unbranched.
Bio-fuel: Ethanol Production Today
BRAZIL
sugarcane
(sucrose)
Cosgrove, 2006
extract
Sugars
ferment
ethanol
USA
(starch)
Sugars
Hydrolyze
(enzymes)
ethanol
ferment
* Brazil and the US are the leaders in ethanol fuel production
They use the “easy way” to make ethanol.
* Today’s ethanol production is from sugar (mostly cane and beets)
and starch (corn in the US, wheat and barley in Europe). Yeast will
ferment that sugar into ethanol.
Chemical structure of starch
STARCH
Starch is the storage molecule plants use to provide energy for
seedling germination and growth. It is a simple repeating chain
of glucose molecules, a six carbon sugar that can be easily fermented.
Starch is easy to break down into glucose, using heat and the readily
manufactured enzyme amylase.
http://www.ucmp.berkeley.edu/monocots/corngrainls.jpg
http://www.scientificpsychic.com/fitness/carbohydrates1.html
The rest of the plant is mostly sugar too!
3 nm
Most people aren’t aware that woody biomass,
leaves and stalks area about 70% sugar.
That sugar is locked up in cellulose and hemicellulose –
the polymers that compose the
plant cell wall..
Polymerized glucose
The rest of the plant is mostly sugar too!
* Cellulose is also a chain of glucose, a 6 carbon sugar,
while hemicellulose contains both 5 and 6 carbon sugars.
* Cellulose contains more complex chemical
bonds of alternating glucose units, requiring three
different enzymes to break apart.
* And the 5 carbon sugars are a bit tricky to convert to
ethanol – a topic that we will discuss in a moment
Plant cell wall
Cell walls
fuel
Cellulose,
Hemicellulose,
+ lignin
Slow & expensive step
“recalcitrance”
enzyme
sugars
digestion
Cellulose microfibril
chemical
pretreatments
Fermentation
ethanol
Parallel strands of glucose polymers
Cosgrove, 2006
Not cost effective: Extensive research is ongoing to reduce the costs
Cell walls
fuel
• The difficulty in breaking down plant cell walls
into sugars is called “recalcitrance”. Current
methods to address recalcitrance depend on
relatively expensive enzyme cocktails after a high
temperature pretreatment that often includes
acid or alkaline chemicals to break down lignin.
• Pretreatment costs account for about 1/3 of the
conversion process, while enzymes might add
another 20% to the ethanol cost. Extensive
research is ongoing to reduce both of these costs.
Components of plant cell walls
This is the breakdown of typical plant cell
walls.
Biochemical strategies can convert the
cellulose and hemicellulose to ethanol, while
the residual lignin would most likely be
burned to provide heat and power for the
biorefinery, and export electricity to the grid.
Cellulose (6 carbon sugars)
Lignin (phenols)
Extractives
Ash
Hemicellulose
(both 5 and 6 carbon sugars)
(need modified microbe to
convert to ethanol) Chapple, 2006; Ladisch, 1979, 2006
Ethanol from glucose or xylose
Jeffries & Shi Adv. Bioch Eng 65,118
To convert the 5 carbon sugars from hemicellulose into ethanol requires a number
of complex steps. A few microorganisms have been modified so that that can ferment
both the 5 and 6 carbon sugars into fuels – this slide illustrates one of several strategies
that have been tried. Licensing an effective co-fermenting organism
(or organisms) is likely to be a critical factor in and business becoming a cellulosic
ethanol success.
Bio-fuel: Ligno-Cellulosic Ethanol Fact Sheet
Cellulosic Ethanol Production
Cellulosic Materials
Most plant matter is not sugar or starch, but cellulose,
hemicellulose, and lignin. The green part of a plant is
composed nearly entirely of these three components. To
convert cellulose to ethanol, two key steps must occur:
Agricultural Waste, Forest Waste, Municipal Solid Waste &
Dedicated Energy Crops
1) Saccharification: A variety of thermal, chemical, and
biological processes are used to break cellulose down
into sugars. This step is a major challenge.
2) Fermentation: The sugars must be fermented to make
ethanol, similar to the grain-to-ethanol process.
http://www.neeic.org
Benefits of Cellulosic Ethanol
 Access to wider array of potential feedstock, including
waste cellulosic materials and dedicated cellulosic crops.
 Greater avoidance of conflicts with land use for food
production
 Greater displacement of fossil energy per litre of fuel,
due to nearly completely biomass-powered systems.
 Much lower net greenhouse gas emissions than with
grain-to-ethanol production powered primarily by fossil
energy.
Greenhouse Gas Reduction Impacts
Producing ethanol from cellulosic feedstock has the potential to achieve greater
greenhouse gas (GHG) reductions than grain-based ethanol. The use of cellulosic feedstock
in producing ethanol has a “double value” in that the left over (mainly lignin) parts of the
plant can be used as process fuel
to fire boiler fermentation systems. This makes for both a relatively more energy-efficient
production process and a
more renewable approach since fossil energy use for feedstock conversion can be kept to a
minimum.
Production Costs
Typical estimates for net GHG emissions reduction from production and use of cellulosic
ethanol are in the range of 70% to 90% compared to conventional gasoline. Net GHG
reductions can be boosted even further if the electricity produced by cogeneration facilities
is used to displace coal-fired power on the grid.
Cellulosic ethanol requires much greater processing than grain or sugar-based ethanol, but feedstock costs for grasses and trees are generally lower. If targeted reductions in conversion costs
can be achieved, the total costs of producing cellulosic ethanol could fall below that of grain ethanol.
There are no large-scale commercial cellulosic ethanol plants currently in operation, by the National Renewable Energy Laboratory estimates that in the near-term, it would cost a large-scale
facility about $1.36 per gallon to produce cellulosic ethanol. The Department of Energy (DOE) has set a goal of bringing down the overall production costs to $1.07 per gallon by 2012.
Gasification
Gasification is an alternative production technology that does not rely on chemical
decomposition of the cellulose.
Instead of breaking the cellulose into sugar molecules, the carbon in the raw material
is converted into synthetic gas (syngas), a mixture of carbon monoxide and hydrogen.
The syngas can then be converted to diesel (via Fischer-Tropsch (FT) synthesis),
methanol, or dimethyl ether- a gaseous fuel similar to propane. Alternatively, the
hydrogen can be separated and used as fuel.
Currently, most interest exists in the production of diesel via FT synthesis- the same
technology used in gas-to-liquids and coal-to-liquids plants.
Research & Initiatives
 Millions of research dollars are focused on developing more efficient separation, extraction,
and conversion techniques.
 A key research area is enzymatic hydrolysis processes, which is believed to have the potential
to improve the efficiency and lower the cost of cellulosic ethanol production.
 The DOE recently awarded grants totaling $385 million over 4 years in 6 companies working
on cellulosic ethanol plants.
 The Department of Agriculture is seeking to increase its bioenergy financing to $161 million
from $122 million, including $21 million in loan guarantees for cellulosic ethanol plants.
 In the early part of 2007, venture capital firms, Wall Street, and even oil companies have
invested approximately $200 million in cellulosic ethanol development.