Food Chem (HL)

Option F: FOOD CHEMISTRY

F1 Food Groups Food – Any substance, processes, partially processed, or raw that is intended for human consumption. Includes drink, gum, and any substance which

has been used in the manufacture, preparation or treatment of ‘food’ but does not include cosmetics, tobacco, or substances used only as drugs

Nutrients – Any substance obtained from food and used by the body to

provide energy, regulate growth, maintenance and repair of the body’s tissues. Include: Carbohydrates, Proteins, Lipids, Vitamins, Minerals, and Water

1. Carbohydrates

Have empirical formula: CH2O Simplest carbohydrates are monosaccharides Monosaccharides contain: 1 carbonyl (C=O), at least 2 –OH groups

(for polymerization), and 3-6 carbons (to make a ring) Monosaccharides w/ formula C5H10O5 are known as pentoses (ribose) Monosaccharides w/ formula C6H12O6 are known as hexoses (glucose)

Ex. Straight Chain Glucose Ring Form Glucose CHO

Monosaccharides can undergo condensation rxns to make dissacharides and polysaccharides

Disaccharide examples include lactose and maltose Polysaccharide examples include starch and cellulose

Ex. Glucose + Fructose yields Sucrose (monosacc) (monosacc) (dissacharide) 2. Proteins

are polymers made up of chains of 2-amino acids amino acids have general formula H2NCHRCOOH where R is the side

chain which may be the same or different from other R groups

Ex. H2N-CH-C-OH H-N-CH-C-OH H-N-CH-C-OH H2N-CH-C--N-CH-C--N-CH-C-OH + 2H2O Amide Link (Peptide Bond)

3. Lipids (Fats & Oils)

Are triesters/triglycerides formed from condensation rxns btw propane-1,2,3-triol (glycerol) with 3 chains of carboxylic acids (fatty acids)

General formula of a fat or oil:

H2C-O-C-R1 R1, R2, and R3 are long chain H2C-O-C-R2 chain hydrocarbons which may be the same or different H2C-O-C-R3

The R Group

R group determines physical & chemical properties of the lipid Typically 15-25 carbon atoms long

A) Fats

Fats are solid at RT (butter, lard, shortening) R groups contain only saturated carbons (do not have C=C double bonds)

B) Oils

Oils are liquid at RT R Groups contain at least 1 C=C double bond Mono-unsaturated lipids have 1 C=C double bond

(olive oil, canola oil, peanut oil) Polyunsaturated lipids have more than 1 C=C

(sunflower oil, fish oil)

Balanced Diet: 60% carbohydrate 20-30% protein 10-20% lipid Vitamins & Minerals included in 3 above 2 dm3 water

too much or little results in malnutrition F2 Fats & Oils Structures of Saturated & Unsaturated Fatty Acids Most naturally occurring fats contain a mix of saturated, mono-

unsaturated, & polyunsaturated fatty acids Common fatty acids (page 146 in SG)

Name #of C atoms/ molec

# of C=C

bonds

M.P. (°C)

Saturated Fatty Acids Lauric Acid CH3(CH2)10COOH 12 0 44.2

Myristic Acid CH3(CH2)12COOH 14 0 54.1 Palmitidc Acid CH3(CH2)14COOH 16 0 62.7 Stearic Acid CH3(CH2)16COOH 18 0 69.6

Unsaturated Fatty Acids Oleic Acid CH3(CH2)7CH=CH(CH2)7COOH 18 1 10.5

Linoleic Acid CH3(CH2)4CH=CHCHCH2=CH(CH2)7COOH 18 2 -5.0

Unsaturated fatty acids my be in cis-form or trans-form

cis,cis-linoleic acid trans,trans-linoleic acid

Physical properties of Fats & Oils The melting point and the degree of crystallization (solidification) depends

on: 1) The length of the hydrocarbon chains

Mp of fatty acids increases w/ increasing molar mass Ex. Stearic (saturated) acid w/ 18 C: mp = 69.6°C Lauric (saturated) acid w/ 12 C: mp = 44.2°C

2) The degree of unsaturation

Saturated fatty acids have higher mps than unsaturated fatty acids

The more unsaturated, the lower the mp and less crystalline it would be

Ex. Stearic Acid Linoleic Acid 18 carbons 18 carbons no C=C bonds 2 C=C bonds mp = 69.6°C mp = -5.0°C

3) Geometric Isomerism around the double bonds

Saturated fatty acids have tetrahedral sp3 shape that forms a rigid backbone (109.5° apart)

This leads to a crystalline structure packed closely together by strong vdW’s forces

If C=C double bonds are present, there will now be a 120° bond angle that interferes w/ the tetrahedral straight rigid chain producing a “kink” in the chain

This leads to a less rigid structure and lowering the mp Trans isomers can pack more closely together resulting in higher mp

than cis isomers Chemical Properties of Fats & Oils

Unsaturated fats are less stable than saturated fats C=C double bonds can react w/ water (hydrolysis) in presence of heat or

enzymes. C=C can also react w/ oxygen (auto-oxidation), light (photo-oxidation), and hydrogen (hydrogenation)

Hydrogenation of Unsaturated Fats

C=C + H2 C-C

unsaturated saturated

Advantages of saturation: Increases mp, hardness, and chemical stability by making them less

susceptible to oxidation Margarine is manufactured this way

Disadvantages of saturation: Oils that contain only cis-fatty acids may undergo partial hydrogenation

to produce trans-fatty acids Trans-fatty acids behave more like saturated fats which are high in

cholesterol, harder to metabolize and excrete, therefore accumulate as fatty tissue. They are also a lower quality energy source

F3 Shelf Life a food reaches its shelf life when it no longer maintains the expected

quality due to changes in flavour, smell, texture, and appearance, or because of microbial spoilage

Chemical Factors that Affect Shelf Life 1) Water Content

change in water content causes loss of nutrients, browning, and rancidity

loss of water dries food and changes texture dry foods become vulnerable to microbes if they absorb water

2) Change in pH

causes changes in flavour (sour w/ low pH), colour, browning, and loss of nutrients

3) Light Exposure

causes rancidity, vitamin loss, and fading of colour 4) Temperature

higher temperature increases rate of other forms of spoilage

5) Exposure to Air increases the rate of oxidation

Rancidity – is the perception consumers have of lipids, those that our senses perceives as off because of a disagreeable smell, taste, texture or appearance A) Hydrolytic Rancidity

hydrolysis of the triester (w/ water) by breaking down a lipid into its component propane triol and its fatty acids

reverse of esterification where water is added takes place more rapidly in the presence of enzymes (lipase), heat and

moisture deep frying (high temp) increases rate of hydrolysis

H2C-O-C-R1 H2C-OH HO-C-R1 H2C-O-C-R2 + 3H2O H2C-OH + HO-C-R2 H2C-O-C-R3 H2C-OH HO-C-R3 Lipid propane-1,2,3-triol + fatty acids (polyester) (alcohol) (carbox acids)

Examples of off-flavoured fatty acids: butanoic, hexanoic, octanoic acid in rancid milk palmitic, stearic,and oleic acid give chocolate oily flavour lauric acid gives palm/coconut oil a soapy flavour butanoic acid (smelly feet) in butter

B) Oxidative Rancidity

due to oxidation of fatty acid chains addition of O2 across the C=C double bond of unsaturated fatty acids oily fishes have unsaturation and are prone to oxidation process catalysed by light and enzymes process proceeds by free radical mechanism

Free-Radical Chain Mechanism

Initiation – unsaturated lipid exposed to light (photo-oxidation) - initial Ea very high - hemolytic fission btw C and H bond forming free radicals R-H R• + •H (RH = unsaturated fatty acid) Propagation – free radicals continue to form chain rxns - propagate in presence of O2 to form peroxide radicals (R-O-O•) - ROO• may react to form hydroperoxides (R-O-O-H) R• + O2 R-O-O• peroxide radical R-O-O• + H-R R-O-O-H + R• hydroperoxide

- O2 may initially react w/ alkene in R-H to produce a hydroperoxide directly

R-H + O2 ROOH

- the weak O-O bond in the hydroperoxide then breaks either photochemically or by catalysis w/ TM ions to form/degrade to volatile aldehydes and ketones w/ strong off flavours (spoiling of food)

Termination - occurs when 2 free radicals combine to form non- radical products

R• + R• R-R R• + ROO• ROOR ROO• + ROO• ROOR + O2

Methods to minimize rate of Rancidity and prolong shelf life A) Packaging

using inert gas to minimize contact w/ O2, covering food using low gas permeable packaging or hermetic sealing minimize amount of air in headspace by keeping jars full

B) Processing

limit lipase hydrolysis by storing dairy products at low temperatures (refrigeration)

reduce light exposure by storing in a dark place keeping moisture levels low during processing by adding salt, sugar, or

smoking radiation (gamma or X rays) to destroy microorganisms

C) Additives

sodium sulphite, sodium hydrogensulphite, citric acid to delay the onset of non-enzymic browning

sodium and potassium nitrite and nitrate for curing meats, fixing colour, and inhibiting microorganisms

sodium benzoate and benzoic acid as antimicrobial agents in fruit juices, carbonated beverages, pickles

sorbic acid, propanoic acid, calcium propanoate and sodium propanate for delaying mould and bacterial growth in breads and cheeses

ethanoic acid and benzoic acid for delaying mould and bacterial growth in pickled meats and fish products and to add flavour

F4 Colour The Colour Wheel

An object’s colour is seen as it absorbs visible light You don’t see the absorbed portions (wavelengths) of light but will see

the transmitted portions The observed transmitted light is known as the complimentary colour Ex. Pumpkins are orange. They absorb blue light.

Dye - A food-grade synthetic water-soluble colourant. Pigment - A naturally occurring colourant found in the cells of plants and

animals.

Synthetic Colourants, Dyes

Many foods contain dyes for colour, and flavour Identified by numbers Some have proven to be carcinogenic Diff countries have diff regulations on acceptable dyes. International

legislation needed for food trade.

Naturally Occurring Pigments

1) Anthocyanins The most widely occurring pigments in plants Responsible for the pink, purple and blue colours in fruits and

vegetables, including cranberries, blueberries, strawberries and raspberries

2) Carotenoids The most widespread pigment in nature Large majority produced by algae. Act as a precursor for vitamin A. Colours range from yellow to orange to red, including bananas, carrots,

tomatoes, watermelon, red/yellow peppers and saffron Red astaxanthin, when present as a complex with protein, gives the blue

or green hue found in live lobsters and crabs and pink colours of salmon. 3) Chlorophyll The major light-harvesting pigments for photosynthesis found in green

plants. 4) Haem The red pigment found in red blood cells and muscle tissue.

* Myoglobin is responsible for the purplish-red colour of meat.

Factors that Affect The Colour Stability Factors should include the effects of oxidation, temperature change, pH

change and the presence of metal ions. Students should analyse absorbance spectra that demonstrate these effects.

1) Anthocyanins In aqueous solution, equilibrium reaction exists between the four

different structural forms depending on the pH and temperature. They are most stable and most highly coloured at low pH and temperature.

(A) (AH+) (B) (C)

quinonoid flavylium carbinol base chalcone (blue) (red) (colourless) (colourless)

They form deeply coloured coordination complexes with Fe3+ and Al3+

ions, a source of which can be the metal cans to which the fruit is exposed; this causes a discolorations in canned fruit. They become less stable when exposed to heat, causing a loss of colour and browning.

2) Carotenoids

The presence of multiple unsaturated carbon-carbon double bonds makes carotenoids susceptible to oxidation catalysed by lights, metals and hydroperoxides. Oxidation results in the bleaching of colour, loss of vitamin A activity and off odours.

They are stable up to 50C and in the pH range of 2-7, and, therefore, are not degraded by most forms of processing. With heating, the naturally occurring trans isomer rearranges to the cis isomer.

3) Chlorophyll

Reaction with heat depends on pH. In a basic solution (pH 9), chlorophyll is stable, and in an acidic solution (pH3) it is unstable. When heated, the cell membrane of the plant deteriorates, releasing acids, which decrease the pH. This results in the magnesium atom being displaced by two hydrogen ions, resulting in the formation of olive-brown pheophytin complex. This cell degradation during heating also makes the chlorophyll more susceptible to photo-degradation.

4) Haem

During oxidation, oxygen binds to purple-red myoglobin (Mb), and red oxymoglobin (MbO2) forms. In Mb and MbO2) the heme iron is in the Fe2+ state. Mb and MbO2 can be oxidized, through auto-oxidation, changing the heme iron from Fe2+ to Fe3+. In the Fe3+ state, it is called metmyoglobin (MMb) and has an undesirable brown-red colour. Interconversion between the three forms occurs readily.

(MbO2) (Mb) (MMb)

oxymyoglobin myoglobin metmyoglobin (red, Fe2+) (purple-red, Fe2+) (brown, Fe3+)

The stability of colour and the rate of brown MMb formation from

auto-oxidation can be minimized if the meat is stored in conditions free of oxygen by using packaging films with low gas permeabilities. Air is removed from the package and a storage gas (100% CO2) is injected.

Non-enzymatic Browning (Maillard Reaction) and Caramelization That Cause the Browning of Food

Comparisons should include the chemical composition of the foods affected, factors that increase the rate of the browning, products and examples.

1) Maillard reaction

Chemical composition of the foods affected condensation reaction between an amino group on an amino acid or protein and a reducing sugar (glucose or lactose). The presence of the amino acid lysine results in the most browning colour and cysteine the least colour. Foods containing lysine, for example, milk, brown readily.

Example include:

- Heating sugar and cream to make toffees, caramels and fudges - milk chocolate

Products include: - desirable and undesirable colours (characteristic golden-brown

colour is desirable) - change in smell and flavour (caramel aroma)

2) Caramelization

Chemical composition of the food affected foods with a high carbohydrate content, especially sucrose and reducing sugars, without nitrogen-containing compounds. Factors that increase the rate of the reaction are acid- or base- catalysed at pH above 9 or below 3; a temperature above 120C that occurs during the baking and roasting of foods with a high sugar content

Examples include the browning on the top of baked egg dishes

Products include:

- volatile caramel aromas - brown caramel-coloured products

F5 Genetically Modified Foods Genetically modified (GM) food

derived of produced from a genetically modified organism. The food can be substantially different from or essentially the same as the conventional food, in terms of composition, nutrition, taste, smell, texture and functional characteristics.

Benefits and Concerns of using GM foods A) Potential Benefits

1) Crops

Enhanced taste and quality, reduced maturation time, increase in nutrients and yields, improved resistance to disease, pest and herbicides, enrichments of rice with vitamin A.

2) Animals

Increases resistance, productivity and feed efficiency, better

yields of milk and eggs, improved animal health. 3) Environment

“Friendly” bio-herbicides and bio-insecticides, conservation of soil, water and energy, improved natural waste management.

B) Potential Concerns

Links to increased allergies (for people involved in their processing) The risk of changing the composition of a balances diet by altering the

natural nutritional quality of foods.

B) F6 Texture Dispersed System – a kinetically stable mixture of 1 phase in Food in another largely immiscible phase Dispersed Systems: 1) Solid-Liquid Dispersions

Suspensions solid particles dispersed in a liquid blood (solid red & white cells remain suspended in plasma), molten

chocolate

Gels liquid particles dispersed in a solid fruit jelly where the water is trapped in protein mix

2) Liquids Dispersed in Liquids

Emulsions stable blend of 2 immiscible liquids mayonnaise (oil droplets in an aqueous system), cream

3) Gas Dispersed in Liquids Foams gas bubbles trapped in liquid medium whipped cream or eggs, beer

Aerosols liquid droplets suspended in a gas (smell in food)

Emulsifiers

substances which aid the dispersion of immiscible droplets and stabilize them to prevent them from separating or forming large chunks

to make an emulsion, oil, water, an emulsifier and mechanical energy (beating or mixing) are needed

Two Types of Food Emulsions:

1) Water in Oil Emulsions (butter)

Dispersion of water droplets in an oil phase

2) Oil in Water Emulsions (milk & salad dressing)

Dispersion of oil droplets in a continuous water phase C) Action/Function of Emulsifiers

1) Help with the formation of emulsions and foams 2) Act as the interface (surface) between the liquid, solid, gas phases in

the disperses system 3) To be soluble in fats (oils) and water

Common Emulsifiers

Lecithin (egg yolk), milk protein, salts of fatty acids

F8 Antioxidants Antioxidant – A substance that delays the onset or slows the rate of oxidation. Used to extend shelf life of food

Naturally Occurring Antioxidants

1) Vitamin C (absorbic acid)

Citrus fruits, green peppers, broccoli, green leafy vegetables, strawberries, red currants, potatoes

2) Vitamin E (tocopherols)

Wheat germ, nuts, seeds, whole grains, green leafy vegetables,

vegetable (canola) oil, soya beans

3) B-carotene

Carrots, squash, broccoli, sweet potatoes, tomatoes, kale, cantaloupe, melon, peaches, apricots

4) Selenium

Fish, shellfish, red meat, eggs, grains, chicken, garlic

Synthetic Antioxidants

1) butylated hydroxyanisole, BHA

2) butylated hydroxytoluene, BHT

3) propyl gallate, PG

4) trihydroxybutyrophenone, THBP

5) tert-butylhydroquinone, TBHQ All examples of synthetic antioxidants given have:

phenolic group (-OH joined directly to a benzene ring) tertiary butyl group, -C(CH3)3 (a 3° C) both above are good free radical scavengers (good at stopping radical

propagation) they react and remove free radicals during oxidation thus prolonging

shelf life

Advantages of Antioxidants in Food

naturally occurring vitamins C, E and carotenoids reduce risk of cancer & heart disease by inhibiting formation of free radicals

vitamin C is vital for production of hormones & collagen B-carotene can be added to margarine to provide yellow colour and act

as sign for vitamin A synthesis Believed to enhance the health effects of other foods and boost overall

health and resilience Disadvantages of Antioxidants in Food

consumers perceive synthetic antioxidants to be less safe because they do not occur naturally

natural antioxidants more expensive/less effective than synthetic and can also add unwanted colour and leave aftertaste to food

synthetic antioxidants classified as food additives and need to be regulated to ensure their safe use in food, may be difficult to implement in developing countries

Antioxidants in Traditional Food

many traditional food found in diff cultures contain natural antioxidants vit C & carotenoids found in many types of fruit & veges flavonoids (citrus, green tea, red wine, oregano, dark chocolate) linked

to lowering LDL (bad) cholesterol and blood sugar levels which reduce high blood pressure and prevent the development of cancerous cells

Three Main Types of Antioxidants (HL)

1. Free Radical Inhibitors

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 or non radical products

AH include BHA, BHT, TBHQ and tocopherols (vit E)

Examples: R + AH R-H + A RO + AH R-O-H + A

ROO + AH R-O-O-H + A R + A R-A RO + A R-O-A

2. Complexing / Chelating Agents

form irreversible complex w/ metal ions (Fe2+, Al3+) to reduce metal ion concentrations so that the metal ions in solution are unable to catalyze the oxidation rxns (catalysts for making hydroperoxides in oxidative ranc.)

this complexing is called chelating chelating agents include ethylenediaminetetracetic acid (EDTA), plant

extracts (rosemary, tea, ground mustard) Examples include salts of EDTA and plants extract (rosemary, tea, ground mustard)

EDTA may inhibit oxidation of Fe2+ Fe3+ in raw beef

EDTA

3. Reducing Agents (oxidizes and loses e-s itself but causes others to reduce and gain e-s)

Electron donors and remove or reduce concentrations of oxygen Examples include ascorbic acid (vitamin C) and carotenoids

Ex. Oxidation of vitamin C

F9 Stereochemistry in food Enantiomers - 2 distinct spatial arrangements of a molecule - each containing a chiral carbon - are non-superimposable mirror images - have identical physical & chemical properties except for rotation of polarized light C C

There are three different conventions used for naming the different Enantiomeric forms. 1. (+) or d and (-) l notation

+ or d (dextrorotatory) - or l (laevorotatory) Rotatates the plane of polarized light clockwise or to the right

Rotates the plane of polarized light counter clockwise or to the left

Has a positive rotation value Has a negative rotation value

labels stereoisomers according to the direction they rotate the plane of polarized light

however, this convention provides no indication of the spatial arrangement of molecule

2. D,L Notation (unrelated to d and l )

based on the absolute configuration (spatial arrangement) of the 2 enantiomers

D and L system is commonly used for carbohydrates (sugars) and amino acids

notation referenced to 2,3-dihydroxypropanal (glyceraldehyde)

C – C – C – H

Ex. D,L notation of Glyceraldehyde C C

D-glyceraldehyde L-glyceraldehyde

Ex. “CORN” Rule when naming amino acid stereoisomers

arrange substituents COOH, R, NH2 w/ H in back if CORN arranged clockwise D-enantiomer if CORN counterclockwise L-enantiomer

D-alanine L-alanine

3. R,S Notation

used by chemists when dealing w/ stereoisomers other than carbohydrates or amino acids

substituents are labeled around the chiral carbon atom according to CIP rules (Cahn-Ingold-Prelog)

CIP priority rules: 1) substituent w/ highest atomic # = 1

lowest atomic # = 4 2) if atoms are same, look at the 2nd, 3rd, etc. 4) view molecule w/ lowest ranking substituent behind or pointing away from you 5) if substituents clockwise around C R-enantiomer counterclockwise S-enantiomer Ex. 2-hydroxy propanoic acid C – C – C – OH

C C

R-lactic acid S-lactic acid

Ex. 2-bromobutane

R-2-bromobutane S-2-bromobutane Note: Knowing the direction of rotation of plane polarized light alone does

not identify whether a stereoisomer is D,L or R,S. It is also impossible to predict the direction of rotation for a given absolute configuration

Properties Enantiomers in Food the different enantiomeric forms vary in their tastes, odours and toxicity mst naturally occurring sugars exist in the D form and are sweet most naturally occurring amino acids are in the L form (D- amino acids

taste sweet, L-amino acids are tasteless) Ex. R-(-)-carvone S-(+) carvone D-(-)-carvone L-(+) carvone

Spearmint Taste Caraway / Dill Taste

Ex. +(d)-limonene smells like oranges -(l)-limonene smells like lemons

natural raspberry flavour is due to R-alpha-ionone and synthetic raspberry

flavourings contain both the R and S isomers other synthetically made foods often contain a racemic mixture of each

enantiomer (50:50) one tragic example was the drug thalidomide where one enantiomer caused

severe defects in unborn children

F10 Chemical Structure and Colour Observing Colour in Organic Molecules

When electrons are excited, they must fluoresce back down to a lower energy level. When they do this, they will emit energy.

The amount of energy involved w/ organics is usually quite high, therefore would only emit in the UV region, thus being colourless

Some organics, especially ones that have extensive conjugation (many alternating C=C double bonds in benzene rings that have delocalized pi electrons) will have their electrons absorb then emit energy in the visible region

Good examples of natural pigments that have delocalization of electrons (have benzene rings) include: anthocyanins, carotenoids, chlorophyll, and heme

Anthocyanins

Contain the characteristic C6C3C6 flavonoid skeleton with conjugated double bonds (pi electrons). The more extensive the conjugation, the lower the energy (longer the wavelength) of the light absorbed. This can be exemplified using cyaniding. In acidic solution it forms a positive ion and there is less conjugation than in alkaline solution where the pi electrons in the extra double bond between the carbon and oxygen atom are also delocalized. The difference in colour depending on pH explains why poppies,

which have acidic sap, are red whereas cornflowers, which also contain cyaniding but have alkaline sap, are blue. 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 number of aliphatic or aromatic acids that are attached to the sugars in the molecule. Examples include quercetin.The basic flavonoid C6C3C6 backbone is essentially non-polar. As more polar hydroxyl groups are added the potential for them to form hydrogen bonds with water molecules increases and many anthocyanins, such as cyaniding with several –OH groups, are appreciably soluble in water for this reason.

Carotenoids

In carotenoids the conjugation is mainly due to a long hydrocarbon chain (as opposed to the ring system in anthocyanins) consisting of alternate single and double carbon to carbon bonds. The majority are derived form a 40-carbon polyene chain, which may be terminated by cyclic end-groups and may be complemented with oxygen-containing functional groups. The hydrocarbon carotenoids are known as carotenes, while the oxygenated derivatives are known xanthophylls. Examples include - and -carotene, vitamin A. - and -carotene and vitamin A are all fat soluble and not water soluble. Although vitamin A does contain one polar hydroxyl group the rest of the molecule is a large non-polar hydrocarbon.

Heme and Chlorophyll

Contain a planar heterocyclic unit called a porphin whose structure contains a cyclic system of conjugated double bonds. The cyclic system’s carbon atoms are sp2 hybridized. This results in a planar structure with extensive pi conjugation. Porphins with substituents in positions 1 to 8 are called porphyrins.Porphins contain four nitrogen atoms. The non-bonding pairs of electrons on the nitrogen atoms enable the porphin to form coordinate bonds with metal ions.

Chlorophyll: This is a magnesium porphyrin complex with the original double bond between positions 7 to 8 now saturated and an R group on C3. It is found in two forms: chlorophyll a and b, which differ in substituent R group. In chlorophyll a, R is a CH3 group and in chlorophyll b, R is a CHO group.

Heme: Hemoglobin is the oxygen carrier found in mammalian blood.

Myoglobin is the primary pigment in the muscle tissue and hemoglobin is the pigment in blood. Myoglobin is a complex of globin (a protein) and heme (porphyrin ring containing a central iron atom).

Explain why anthocyanins, carotenoids, chlorophyll and heme form coloured compounds while many other organic molecules are colourless.

The nature of chromophores, conjugation effects and characteristic absorptions are required. Students should understand how the wavelength of energy absorbed relates to the colour the food appears.

Solubility

Anthocyanins have many –OH groups attached to their ring structure. They can therefore form hydrogen bonds w/ water thus tend to be water soluble

Carotenoids have long hydrophobic hydrocarbon chains that will prove to be more nonpolar than polar. They will tend to be insoluble in water, but soluble in oils and fats