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Worksheet 17.2
Chapter 17: Food chemistry – fast facts
F.1 Food groups
 A food is any substance, whether processed, semi-processed or raw that is intended for human
consumption, and includes beverages, chewing gum and any substance that has been used in the
manufacture, preparation or treatment of ‘food’. It does not include cosmetics, tobacco or
substances used only as drugs (Codex definition 2005).

Nutrients are any substance obtained from food and used by the body to provide energy and to
regulate growth, maintenance and repair of the body’s tissues. Proteins, lipids, carbohydrates,
vitamins, minerals and water are considered nutrients.

Fats and oils are triesters (triglycerides) formed from three long-chain fatty acid (carboxylic
acid) molecules and one glycerol molecule.

The nature of the R group determines the physical and chemical properties of the lipid. Fats are
solid at room temperature; oils are liquids.

The simplest carbohydrates are monosaccharides. They contain one carbonyl group (C = O) and
at least two hydroxyl (–OH) groups, and have the empirical formula CH2O. Monosaccharides are
the building blocks of disaccharides and polysaccharides.

Disaccharides are formed from the condensation reaction of two monosaccharides.

Polysaccharides are condensation polymers formed from monosaccharides with the elimination
of water molecules. Glucose is the most important monomer of the naturally occurring
polysaccharides. Starch and cellulose are examples.

Proteins are condensation polymers of 20 2-amino acids. All proteins contain C, H, O and N and
some also have P and S in their chemical composition.
F.2 Fats and oils
 Most naturally occurring fats contain a mixture of saturated, mono-unsaturated and polyunsaturated fatty acids and are classified according to the predominant type of unsaturation
present.

Saturated fatty acids, which are often animal in origin, are carboxylic acids with the general
formula CnH2n + 1COOH.

The carbon chain is made from only single carbon–carbon bonds with carbon atoms bonded in a
tetrahedral arrangement which allows the chains to pack closely together.

The van der Waals’ forces are sufficiently strong between the chains to make the compounds solid
at room temperature.
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
Unsaturated fats contain the carbon–carbon double bond. This produces a ‘kink’ in the chain,
which prevents the molecules from packing closely together and reduces the intermolecular forces.

Unsaturated oils, which are generally vegetable in origin, are liquids.

The greater the number of C = C double bonds, the greater the separation between the chains and
the lower the melting point.

Solid fats are more likely to be crystalline, more saturated and/or have longer fatty acid
hydrocarbon chains. Mono-unsaturated (olive, canola and peanut) and poly-unsaturated fats
(safflower, sunflower, corn, fish, linoleic, linolenic) are liquids, and saturated fats (palm, coconut,
lard, butter, shortening) are solids at room temperature.

The melting point of fatty acids increases with increasing relative molecular mass and increasing
degree of saturation. Fats and oils are chosen for cooking on the basis of their melting temperature.
For example, cocoa butter melts at close to body temperature, and fats chosen for cake-making
melt over a wide range of temperatures.

Cis fatty acids have hydrogen atoms on the same side of the carbon–carbon double bond.

The cis isomer is the most common form of unsaturated fat; the trans form only occurs in animal
fats and in processed unsaturated fats, such as margarine.

The molecules of the cis isomer cannot easily arrange themselves side by side to solidify, so they
tend to have lower melting points than the corresponding trans isomer.

Trans fatty acids have hydrogen atoms on opposite sides of the carbon–carbon double bond.

The cis and trans isomers are examples of geometric isomers.

Saturated fats are more stable than unsaturated fats. The carbon–carbon double bonds in
unsaturated fats react with oxygen (auto-oxidation), hydrogen (hydrogenation), light (photooxidation) and enzymes/heat/water (hydrolysis).

The addition of hydrogen to the carbon–carbon double bond of a fatty acid in the presence of heat
(140–225ºC), pressure and a finely divided metal catalyst (Zn, Cu, Ni) increases the degree of
saturation, which can be partial or full.
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The advantages of hydrogenating fats and oils:
The disadvantages of hydrogenating fats and oils:


Mono- and poly-unsaturated fats are healthier
for the heart than saturated fats.

In partial hydrogenation, trans fatty acids can
form which are hard to metabolize,
accumulate in fatty tissue, and are difficult to
excrete from the body. They increase levels
of LDL (bad) cholesterol and are a lowquality energy source.
Changes a liquid oil to a semi-solid or solid,
which makes the melting point of an
unsaturated fat more like that of a saturated
fat.

Decreases the rate of oxidation (stability
increases with increasing saturation).

Increases hardness.

Controls the feel and plasticity (stiffness).
F.7 Oxidative rancidity (auto-oxidation)
 Oxidative rancidity occurs by a free radical mechanism. The key intermediates are
hydroperoxides (ROOH), which degrade to volatile aldehydes and ketones with strong off
flavours. They undergo further oxidation and decomposition to produce even more free radicals.



Initiation
The strong carbon to hydrogen bond of an
unsaturated fatty acid R – H is first broken
homolytically.
R — H  R● + H●
Propagation
Peroxide radicals are formed which react
with more unsaturated fatty acid molecules
to form hydroperoxides and other alkyl free
radicals.
R● + O2  ROO●
ROO● + RH  R● + ROOH
Termination
The chain reaction is terminated when two
free radicals combine to form non-radical
products.
R● + R●  R — R
R● + ROO●  ROOR
ROO● + ROO●  ROOR + O2
sunlight
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
The hydroperoxides react to form aldehydes
and ketones responsible for the unpleasant
smells and taste of the rancid food.
F.3 Shelf life
 A food reaches its shelf life when it no longer maintains the expected quality desired by the
consumer because of changes in flavour, smell, texture and appearance (colour, mass) or because
of microbial spoilage.

Chemical factors that cause a decrease in the shelf life include:

Water content: exposure to air can make a food dry and change its texture. This increases the
rate of oxidation and leads to rancidity, discolouration of the surface and a general decrease in
nutrient value. Alternatively if dry foods become moist they are susceptible to microbial
spoilage.

Chemical changes occurring within the food can result in a pH change (e.g. becoming sour),
produce undesirable flavours, change colours and lead to a loss of nutrients.

Light provides energy for the photochemical reactions which lead to rancidity, the fading of
the colour and the oxidation of nutrients, particularly vitamins.

An increase in temperature increases the rate of reactions which degrades the food. A
decrease of temperature slows the rate of these reactions which lead to spoilage.

Rancidity is the production of flavours
in lipids that our senses perceive as off,
because they have a disagreeable smell,
taste, texture or appearance.

Hydrolytic rancidity is due to the
breaking down of a lipid into its
component fatty acids and glycerol.

It takes place more rapidly in the presence of enzymes (lipase), heat and moisture.

In deep frying, for example, the water present in food and high temperatures increase the rate of
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hydrolysis.

Examples of off-flavoured fatty acids include:

butanoic, hexanoic and octanoic acid in rancid milk.

palmitic, stearic and oleic acids give chocolate an oily or fatty flavour.

lauric acid gives palm and coconut oil, in cocoa butter substitutes, a soapy flavour.

butanoic acid in butter.

Oxidative rancidity is due to the oxidation of fatty acid chains, typically by the addition of
oxygen across the carbon–carbon double bond of the unsaturated fatty acid. Volatile, unpleasanttasting aldehydes and carboxylic acids are formed in these free-radical reactions.

The oxidation of unsaturated fats by molecular oxygen, which occurs in air in the absence of
enzymes, is called auto-oxidation.

Oily fish, such as mackerel and herring, contain a high proportion of unsaturated fatty acids and
are prone to oxidative rancidity. Extensive oxidation can lead to some polymerization with
consequent increases in viscosity and browning.

The rate of rancidity can be reduced and the shelf life extended by:
Processing
Packaging reduces contact with
oxygen in the air by:
•
refrigeration of dairy products
•
using an inert gas
•
reducing light levels during
storage
•
hermetic sealing
•
removing air from packaging
•

reducing moisture levels by
adding salt or sugar, or smoking
Adding additives:
•
Na2SO3, NaHSO3 and citric acid
reduce non-enzymic browning
•
curing meat with NaNO2 and
NaNO3 fixes colour and inhibits
micro-organisms
•
C6H5COOH and C6H5COONa
are antimicrobial agents in fruit
juices, carbonated beverages,
pickles and sauerkraut
•
sorbic acid, propanoic acid,
calcium propanoate and sodium
propanoate delay mould and
bacterial growth in breads and
cheeses.
•
CH3COOH and C6H5COOH add
flavour and delay mould and
bacterial growth in pickled
meats and fish.
Traditional methods of extending the shelf life include fermentation, preserving, pickling, salting,
drying and smoking.
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
An antioxidant is a substance that delays the onset or slows the rate of oxidation. It is used to
extend the shelf life of food.

Naturally occurring antioxidants include:

Vitamin E: a fat-soluble vitamin found in foods such as wheat germ, nuts, seeds, whole
grains, green leafy vegetables and vegetable oils like canola and soya bean.

Vitamin C (ascorbic acid): a water-soluble vitamin found in citrus fruits, green peppers,
broccoli, green leafy vegetables, strawberries, red currants and potatoes.

β-carotene is found in carrots, squash, broccoli, sweet potatoes, tomatoes, kale, cantaloupe,
melon, peaches and apricots.

Selenium is found in fish, shellfish, red meat, eggs, grains, chicken and garlic.

Synthetic antioxidants include BHA, BHT, PG, THBP and TBHQ.

Almost all have phenolic-type structures: that is a hydroxyl group attached to a benzene ring.

The tertiary butyl group, which has three methyl groups bonded to one carbon atom, is found in
BHA, BHT and TBHQ.
The advantages of antioxidants in food:
The disadvantages of antioxidants in food:

Some consumers perceive natural
antioxidants to be safer because they occur
naturally in food.

Some consumers perceive synthetic
antioxidants to be less safe because they are
not naturally occurring in food.

Naturally occurring vitamins C, E and
carotenoids reduce the risk of cancer and
heart disease by inhibiting the formation of
free radicals.

Natural antioxidants are more expensive than
synthetic antioxidants, can add colour and an
aftertaste to food, and can be less effective at
slowing down the rate of rancidity.

Vitamin C is vital for the production of
hormones and collagen.


β-carotene can be used as an additive in
margarine to give colour (yellow) and act as a 
precursor for vitamin A.
Synthetic antioxidants need to be regulated
by policies and legislation to ensure their safe
use in food.
Policies regarding the labelling and safe use
of food additives can be difficult to
implement and monitor internationally.

They enhance the health benefits of existing
foods and boost overall health and resilience.

Many substances traditionally used in different cultures to promote good health are rich in natural
antioxidants.

Examples include: green tea, turmeric, oregano, blueberries, cranberries and dark chocolate, which some
claim reduce levels of LDL cholesterol, lower blood sugar levels and blood pressure, and prevent cancer.
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F.8 Antioxidants
 Antioxidants (AH) inhibit the formation of free radicals in the initiation step of auto-oxidation or
interrupt the propagation of the free-radical chain.

Free-radical quenchers form stable and less reactive free radicals:
ROO + AH  ROOH + A
Examples include BHA, BHT, TBHQ and tocopherols.

Chelating agents reduce the concentration of free metal ions in solution.
 Examples include salts of EDTA and plant extracts (rosemary, tea, ground mustard).

Reducing agents (electron donors) decrease the concentrations of oxygen.
 Examples include ascorbic acid (vitamin C) and carotenoids.
F.4 Colour
 A dye is a food-grade, synthetic, water-soluble colorant.

A pigment is a naturally occurring colorant found in the cells of plants and animals.

Foods have colour because of their ability to absorb light in the visible region of the
electromagnetic spectrum. Other colours are reflected or emitted and we see the complementary
colour.

Naturally occurring pigments include anthocyanins, carotenoids, chlorophyll and heme.
Anthocyanins

The anthocyanins all have very similar three-ring C6C3C6 structures, with conjugated double
bonds, but vary in the number and position of the hydroxyl groups and alkoxy side chains.

They are a sub-class of flavonoids responsible for a range of colours in fruits and vegetables
including yellow, red and blue.

They are the most widely distributed pigment in plants and are present, for example, in
strawberries, plums, cranberries, blueberries and raspberries.

Many anthocyanins are red in acidic conditions and turn blue at higher pH.
Carotenoids

The carotenoids are derived from a 40-carbon polyene chain, with, in some cases, ring structures at
the ends of the chain.

The colours range from yellow to orange to red. They are present in bananas, carrots, tomatoes,
watermelon, red/yellow peppers and saffron.

They are the most widespread pigments in nature with the large majority produced by algae.

They act as a precursor for vitamin A.
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
Those found in fruit and vegetables contribute 30–100% of the vitamin A requirement in humans.

Red astaxanthin when complexed with protein gives the blue or green hue found in live lobsters
and crabs and the pink colour of salmon.
Heme and chlorophyll

Heme and chlorophyll both contain a planar heterocyclic porphyrin
ring which consists of four pyrrole rings linked by a single bridging
carbon atom.

The porphyrin ring acts as a polydentate ligand and forms a dative
covalent bond with a central metal ion.

The central ion is Mg2 + in chlorophyll and Fe2 + in heme.

The compounds absorb light in the visible region of the spectrum, as
they have alternate single and double (conjugated) bonds.

Chlorophyll is the green pigment responsible for the colour of
vegetables.

There are two closely related forms of chlorophyll that have different R groups. Chlorophyll a has
a methyl group (CH3) and chlorophyll b has an aldehyde CHO group.

Chlorophyll absorbs the red and blue light needed for photosynthesis.

Heme is the red pigment found in the red blood cells of higher animals.

Myoglobin, an extremely compact heme protein found in muscle tissue, is responsible for the
purplish-red colour of meat.

The colour stability of naturally occurring pigments is affected by degree of oxidation,
temperature, pH and the presence of metal ions.
Stability of anthocyanins
 In aqueous solution anthocyanins exist in four possible structural forms, depending on the pH and
temperature.
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They are most stable and highly coloured at low pH and temperature.

They form deeply coloured coordination complexes with Fe3 + and Al3 + ions, when the fruit is
exposed to metal cans.

They become less stable when exposed to heat, causing a loss of colour and browning.
Stability of carotenoids

The multiple conjugated carbon–carbon double bonds are susceptible to oxidation catalyzed by
light, metals and hydroperoxides, allowing them to behave as antioxidants.

The bread-making properties of flour improve with prolonged storage as carotenoids are bleached
to give the bread a more ‘attractive’ whiter crumb.

Oxidation leads to a loss of vitamin A activity and produces ‘off’ odours.

The carotenoids are stable up to 50°C and at pHs in the range 2–7. They are not generally
degraded by processing.

The naturally occurring trans isomer rearranges to the cis isomer when heated.
Stability of chlorophyll
 The green colour of vegetables can fade to yellow and brown as they are cooked, owing to the
thermal instability of chlorophyll, which depends on pH.

When heated in acidic solution, the cell membrane of the plant deteriorates, and Mg2 + ions are
replaced by two H + ions. The COOC20H39 ester group is hydrolyzed to leave a brown colour.

Chlorophyll is more stable in alkaline solution.

This cell degradation caused by heat also makes chlorophyll more susceptible to photodegradation.
Stability of heme

In muscles heme is associated with the purple–red protein myoglobin molecule, which binds to
oxygen molecules to form the red oxymyoglobin molecule:
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
Fe2 + is more stable than Fe3 + ion in the non-polar environment provided by the side chains in the
complex. The red oxymyoglobin does, however, undergo a slow auto-oxidation reaction to form a
Fe3 + complex with an undesirable brown colour known as metmyglobin:

The stability of colour and the rate of brown metmyglobin formation is decreased if the meat is
stored in oxygen-free conditions.

Air is removed and meat is stored in CO2.

There are safety concerns over the use of synthetic colorants. Different colorants are permitted in
different countries which makes international legislation difficult.

Cooking often causes food to turn brown. The two processes involved are the Maillard reactions
and caramelization.

Caramelization occurs with foods of high carbohydrate content and low nitrogen content.

The process of caramelization starts with the melting of the sugar at temperatures above 120°C.

The compounds are dehydrated and double bonds are introduced. The small sugar molecules react
together by condensation reactions to produce polymers with conjugated double bonds, which
absorb visible light to give brown colours.

Smaller volatile molecules are also formed by fragmentation reactions, which give the food unique
flavours and fragrances.

Caramelization produces desirable colours and flavours in bakery goods, coffee, soft drinks, beer
and peanuts.

Undesirable effects occur if all the water is removed and carbon is produced: CnH2mOm  nC +
mH2O.

The highest rate of browning occurs in fructose, as caramelization starts at a lower temperature.

The rate and products of caramelization can be controlled by pH.

Examples include the browning on the top of baked egg dishes.

Maillard browning involves a complex series of reactions between amino acids and reducing
sugars, usually at increased temperatures.

The first step is the condensation reaction of a reducing sugar, such as glucose, with an amino
acid, which leads to the replacement of a C = O in the aldehyde group of the sugar by a C = N—R
bond.

The larger the sugar, the slower the reaction with amino acids.
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
Lysine is the amino acid which reacts the fastest and causes darkest colour as it has two amino
groups.

Cysteine, with only one amine group and a sulfur group, produces the least colour of the amino
acids.

A series of dehydration, fragmentation and condensation reactions result in a complex mixture of
products.

The reaction is affected by pH, type of amino acid and sugar, temperature, time, presence of
oxygen, water activity and other food components.

Examples include heating sugar and cream to make toffees and fudges.
F.10 Chemical structure and colour
Anthocyanins

They contain the characteristic C6C3C6 flavonoid skeleton with conjugated double bonds. They
differ in the number of hydroxyl and/or methoxy groups present, the types, numbers and sites of
attachments of sugars to the molecule, and the types and numbers of aliphatic or aromatic acids
that are attached to the sugars in the molecule.

Examples include quercetin.
Carotenoids

The majority are derived from a 40-carbon polyene chain, which may be terminated by cyclic endgroups and may be complemented with oxygen-containing functional groups.

The hydrocarbon carotenoids are known as carotenes, while the oxygenated derivatives are known
as xanthophylls.

Examples include - and -carotene, vitamin A.
Heme and chlorophyll

They contain a planar heterocylic unit called a porphin, whose structure contains a cyclic system
of conjugated double bonds.

Porphins with substituents in positions 1 to 8 are called porphyrins.
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Heme
Chlorophyll
•
•
This is a magnesium porphyrin complex with the
original double bond between positions 7 and 8 now
saturated and an R group on C3.
It is found in two forms, chlorophyll a and b, which
differ in the substituent R group. In chlorophyll a, R is
a CH3 group; and in chlorophyll b, R is a CHO group.
•
•
Myoglobin is the primary pigment in muscle tissue
and hemoglobin is the pigment in blood.
Myoglobin is a complex of globin (a protein) and
heme (a porphryin ring containing a central iron
atom).

The part of an organic molecule responsible for absorbing UV and visible radiation is called a
chromophore, and generally includes unsaturated groups, such as C = C, C = O, N = N, NO2 and
the benzene ring.

A compound is more likely to absorb visible light and so appear coloured when it contains a
conjugated system of alternate C = C and C–C bonds with the π electrons delocalized over a
larger area.

The colour of the anthocyanins is due the conjugated π system of electrons in the flavonoid
C6C3C6 rings.

The colour of the carotenoids is due to conjugation in the long hydrocarbon chain.

The porphin ring is an extended π system and so absorbs visible light.

Anthocyanins are water-soluble and carotenoids are fat-soluble.
F.5 Genetically modified foods
 A genetically modified food is one derived or produced from a genetically modified organism.

The food is either substantially different or essentially the same in composition, nutrition, taste,
smell, texture and functional characteristics, to the conventional food.
Benefits of GM foods
•
•
•
•
•
•
•
•
•
•
•
Improved flavour, texture and nutritional value.
Longer shelf life.
GM organisms can be more resistant to disease and pests.
Increased crop yields and feed efficiency in animals.
Increased resistance to herbicides and fungicides.
Environmentally ‘friendly’ bio-herbicides and bio-insecticides can be
produced.
Conservation of soil, water and energy.
Improved waste management.
Increased production of substances which can improve human
health. such as vitamins A and C; anti-cancer substances and
vaccines.
Decreases amounts of unhealthy fats.
GM plants grow in a wider range of climatic conditions.
Concerns about GM foods
•
•
•
•
Uncertainties about the outcomes
of genetic modifications given its
relatively recent development.
Increased allergies (for people
involved in the processing of GM
foods).
Risks of changing the composition
of a balanced diet by altering the
natural nutritional quality of foods.
Contamination of ‘normal’ crops or
the wild population by pollen from
GM crops.
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F.6 Texture
A dispersed system is a kinetically stable mixture of one phase in another largely immiscible
continuous phase.

Emulsifiers help with the formation of emulsions and foams.

An emulsifier generally has a polar head which is
hydrophilic and is attracted to the water, and a non-polar
tail which is hydrophobic.

They are soluble in fat and water and act as the interface (surface) between the liquid, solid and
gas phases in the dispersed system.

Mechanical energy (beating,
mixing) is needed to make an
emulsion of oil and water.

Lecithin present in egg yolk is widely used as an emulsifier. It is added to oil and water mixtures
to make mayonnaise and other salad dressings.

Stabilizers such as trisodium phosphate, Na3PO4, are added to prevent the emulsions from
separating out into the separate phases.
F.9 Stereochemistry in food
 There are three different conventions used for naming the different enantiomers.
+ (d) and – (l) (based on direction of rotation of polarized light)

An enantiomer that rotates the plane of polarized light clockwise is dextrorotatory,
labeled + or (d).

An enantiomer that rotates the plane of polarized light anticlockwise is levorotatory,
labeled – or (l).
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D
and L (Based on configuration around chiral carbon)

A compound made from D-glyceraldehyde without changing the configuration of the chiral carbon
is labeled the D-enantiomer.

A compound made from L-glyceraldehyde is labeled as the L-enantiomer.

What you would see if you were looking at
the carbon atom with the H atom pointing
away from you in D-glyceraldehyde:

The naming of the amino acids called the CORN rule. The system depends on the arrangement in
space of the COOH, R, NH2 groups and H atom around the asymmetric carbon, with the hydrogen
atom again pointing away from the viewer. If the CORN groups are arranged clockwise, then it is
the D-enantiomer; if they are arranged anti-clockwise, it is the L-enantiomer.
R and S
 This system is used mainly when dealing with substances other than carbohydrates and amino
acids. Each chiral, or asymmetric carbon centre, is labeled R or S, according to the Cahn–Ingold–
Prelog (CIP) priority rules.

The atoms bonded to the chiral carbon are ranked in order of increasing atomic number. If two or
more atoms have the same atomic number, the next atoms are used to rank the substituents.
Double bonds count as double.

The molecule is viewed with the lowest ranking substituent pointing away from the observer.

If the priority of the remaining three substituents decreases in a clockwise direction, it is assigned
the R-form; if priority decreases in an anticlockwise direction, it is the S-form.

Different enantiomeric forms found in food often have different smells, tastes and toxicity.

+ (d)-carvone tastes of caraway seeds and dill and – (l)-carvone tastes of spearmint.

Most naturally occurring amino acids are in the D-form. The D amino acids taste sweet; the
L amino acids are tasteless.

Most naturally occurring sugars exist in the D form and are sweet. + (d)-limonene smells of
oranges and – (l)-limonene smells of lemons.

Synthetically made foods often contain a racemic mixture of each enantiomer.

Natural raspberry flavour is due to R-alpha-ionone; synthetic raspberry flavourings contain both
the R- and S-isomers.
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