F4 Colour

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F4 Colour

F.4.1 Distinguish between adye and a pigment.

F.4.2 Explain the occurrenceof colour in naturally occurring pigments.

F.4.3 Describe the range of colors and sources of naturally occurringpigments.

F.4.5 Describe the factorsthat affect the color stability of anthocyanins, heme,carotenoids & chlorophyll

F.4.5 Discuss the safety issues associated with theuse of colorants in food.

F.4.6 Compare theprocesses of non enzymatic browning andcaramelisation.

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Dye Pigment

A food gradesynthetic

water solublecolorant that

can be addedto food to

enhance orchange itsnatural color .

A naturally occurringcolorant found in the

cells of plants andanimals.

Chemical families:porphyrins,

carotenoids, flavonoids,chlorophyll, heme andmyoglobin.

Dye versus Pigment

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What causes Colour?

Dyes and Pigment molecules withconjugated (alternating single and

double) carbon-carbon bonds.

The π electrons of the double bond aredelocalized and spread out along the

adjacent molecules.

Colorless organic molecules do not have

conjugated carbon-carbon bonds.

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The delocalized electrons absorb visible light when the energy of theincoming light photon matches the energy difference between theelectrons ground state and excited state.

The electron becomes excited and transition from a lower-energy orground electron configuration to a higher-energy electronconfiguration.

When the electrons drops back down to its ground state it releases/reflects/transmits visible light with a complementary wavelength.

The color we see has a complementary relationship with thecolor of the visible light absorbed.

Molecules that do not absorb light in the400-700nm visible light range are colorless.

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Pigment molecules differ in color dueto differences in the amount of conjugation.

There is a positive relationship betweenthe amount of conjugation and the

wavelength of visible light absorbed.

As the amount of conjugationincreases, the wavelength of light

absorbed increases changing thecomplementary color reflected.

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Anthocyanins

Sub-class of flavonoids Orange-red, red-blue colors Foods: strawberries, berries

(blueberries, cherries, blackberries), grapes and some vegetables, such as egg-plant andavocado.

Water soluble – form hydrogen bonds with water due to thepresence of multiple hydroxylgroups.

Three phenol groups (benzenering) with conjugated carbon-carbon bonds and varyingnumbers of hydroxyl groups.

Glucose molecules bond to theoxygen atom on the second phenolgroup. Anthocyanins are found infoods high in sugar content.

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Anthocyanin Stability

Unstable with pH, temperature, in the presence of UV light and Fe3+

& Al3+ ions. Most stable at low pH (acid) and low temperature. Equilibrium exists between four different forms. Position of

equilibrium affected by adding acid or alkali or by changing thetemperature causing different colored forms to predominate.

Page 35 of the Data Booklet shows the equilibrium relationship

between the four different colored structures at 25°C and pH. At high temperature during cooking anthocyanins decompose. The

position of equilibrium moves to the right, resulting in a loss of colordue to the predominance of the colorless structure.- Blue quinonoidal base (A), stable at pH 6-7- Red flavylium cation (AH+), stable at pH 1-2

- Colorless carbinol psuedobase (B), stable at pH 4-5- Colorless chalcone (C), stable at pH > 7

Make good natural acid-base indicators. E.g red cabbage is red in acidand blue and purple in base.

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Carotenoids

Color: Yellow to orange-redpigments Food sources: carrots, oranges,

salmon, tomatoes, lobster, yellow peppers

Solubility: Fat soluble andinsoluble in water. Long conjugated

hydrocarbon chain which outweighsthe small number of polar hydroxylgroups.

Functions: Acts as antioxidants.Can be converted/precursor to

Vitamin A (retinol) in the body andso is considered a nutrient. Vitamin

A needed for good vision (productionof light sensitive cells in the retina)and to hydrate skin cells. Carotenescontribute 30-100% of our Vitamin A requirement.

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Color Stability in Carotenoids

Stable in pH 2-7. At low pH color will change from red-orange to orange-yellow due to the formation of alternatestructure.

Stable up to 50°C and therefore color is not degraded by most forms of food processing.

C=C double bonds mean that they are easily oxidized by oxygen in the air causing discoloration, loss of Vitamin A and off odors.

Metals from knives and cutting surfaces and UV light

also catalyze the oxidation. Food need to be kept wrapped, kept away from light and

metal and kept in the refrigerator.

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Chlorophyll

Color: Green to olive green Sources: green leafy plants

(spinach), algae Function: Photosynthesis (food

production) in Plants. Solubility: Fat soluble in pure

form due to lack of hydroxyl groupto hydrogen bond with water.

Structure: A mixture of molecules each with a different R group. Ring structure called aporphyrin coordinately bonds 4

nitrogen atoms to a central Mg2+ion. Square planar shape.

Attached to ring IV is a phytolgroup (C20H39 carbon chain) Structure page 36 of Data Booklet

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Colour Stability inChlorophyll

Unstable - affected by heat,oxygen gas (in air), UV lightand pH.

Fresh foods containingchlorophyll should be stored

away from heat, light and airso color changes do notoccur.

Canning and preserving in jars will change color of

chlorophyll becauseprocessing requires heatingto boiling to kill micro-organisms.

Olive-brown

Pheophytin-a

Bright green chlorophyll-a

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When green leafy plants are exposed to heat, the cell walldeteriorates releasing H+ ions, lowering the pH. H+ ions displace

the Mg2+ ion replacing it with two H atoms forming the olive- brown pheophytin-a pigment. This coloration often occurs in

canned and preserved vegetables like peas and beans.

Chlorophyll also reacts with OH- ions (source NaOH) to give the bright green chlorophyllin pigment.

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Heme

Function: transport of oxygen forcellular respiration.

Structure:

Central Fe (II) ion which bonds to 6other groups (called coordination

sites) 4 of the 6 coordination sites

occupied by nitrogen atoms from aporphyrin ring.

5th coordination site occupied by thenitrogen atom from a protein

molecule. 6th site is available to bind an O2

molecule.Heme molecule

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Hemoglobin Myoglobin

Structure: Four proteinchains and four heme

groups that carry oxygen from the lungsto the tissues.

Source: red blood cells

Function: Carriesoxygen in the blood totissue

Structure: Single proteinchain and one heme

group that transportsoxygen in tissue - musclecells (page 35 of DataBooklet)

Source: Muscle tissue

Function: Carries oxygenin the tissues

Hemoglobin vs. Myoglobin

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Hemoglobin

Four protein chains and fourheme groups

MyoglobinOne protein chain and oneheme group.

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Colour Stability in Myoglobin

When animal tissue comes in contact with oxygen in the airthe myoglobin exits in three different structures.

Myoglobin (Mb) is purple-red with Fe(II) oxidation state. Onexposure to oxygen in the air it forms bright red oxymyoglobin(MbO2), also Fe(II). This is the color of freshly cut meat.

Mb-Fe2+ + O2

Mb-Fe2+-O2

With extended exposure to oxygen the Fe2+ ion is auto-oxidized (loses an electron) to form Fe3+ - the metamyoglobin(MMb) structure. MMb has an undesirable brown in color.

Mb-Fe2+ Mb-Fe3+

Choose bright red colored meat – it is the freshest. To keep

meat from forming metamyoglobin store in an oxygenreduced atmosphere – low gas permeability film, vacuumpacked or replace oxygen with an inert gas.

Fe2+ more stable than Fe3+.

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Synthetic Colorants in Food

Use of synthetic food colorings/dyes in food is notregulated by international standards, so countries havetheir own regulations concerning their use.

In the US FD&C numbers are given to approved syntheticfood dyes that do not exist in nature. In the European

Union, E numbers are used for all additives, both syntheticand natural. Other countries have their own numberingsystem.

The Codex Alimentarius Commission (FAO and WHO)sets food standards, and codes of practice that can be used by international governmental and non-governmental

organizations (NGO). Some synthetic food dyes have been linked to health

problems – hyperactivity in children. Research into thehealth effects is still in its infancy.

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Natural food dyes, commercially produced and allowed in food in the

US include:

Caramel coloring (E150),used in cola products. Green dye made from chlorellaalgae (chlorophyll, E140). Cochineal (E120), a red dye derived from the cochineal

insect. Betanin extracted from beets. Turmeric (curcuminoids, E100). Saffron(carotenoids, E160a). Paprika (E160c)

Often provided in highly purified form, for increasedstability.

Artificial coloring approved in the US:

FD&C Blue No. 1 – Brilliant Blue, E133 (Blue) FD&C Blue No. 2 – Indigotine, E132 (Dark Blue)

FD&C Green No. 3 – Fast Green, E143 (Blue-green)

FD&C Red No. 40 – Allura Red AC, E129 (Red)

FD&C Red No. 3 – Erythrosine, E127 (Pink)

FD&C Yellow No. 5 – Tartrazine, E102 (Yellow)

FD&C Yellow No. 6 – Sunset Yellow, E110 (Orange)

Also a list of delisted artificial coloring, no longer permitted.

(FD&C Blue No. 1, Red No. 40, Yellow No. 5 and Yellow No. 6 usedto color Starbursts)

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Enzymatic Browning Non-Enymatic Browning

Chemical reaction which occurs in foods

containing the enzymepolyphenoloxidase,

Results in theproduction of brown

colored pigments.

Browning of food without enzymes

via the Maillardreaction andcaramelization.

Browning of Food

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Maillard Reaction

Food Composition: Food containing protein and carbohydrates (specifically areducing sugar).

Complex reaction mechanism. The first step involves a condensation reaction between an amino acid and a reducing sugar (glucose, fructose, sucrose, lactose)followed by a series of dehydration, fragmentation and condensation reactions toform brown pigmented products called melanoidins. Also causes flavor and smellchanges.

Factors effecting rate of browning: pH (4-7 optimum), type of amino acid(lysine most & cysteine least brown), type of reducing sugar, temperature (widerange), time, presence of oxygen and water.

E.g. boiling milk, frying meat and foods that involve cooking sugar and protein(often from milk) like fudge, caramel toffee, milk chocolate, baking bread.

Features of the Product: Desirable brown color, flavor and smell. Reducesnutritional value of food as amino acids and sugar is lost during reaction.

Food affected: Frying meat. Heating sugar and milk to make fudge, caramel

toffee, and milk chocolate. Brown crust on baked bread, roasted coffee beans. Browning meat

http://www.bbc.co.uk/food/get_cooking/techniques/003001.shtml

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What is a Reducing Sugar?

Sugars exist in solution as an equilibriummixture of open/straight-chain and closed-ring (cyclic) structures.

In the straight chain form, the C1 atomcontains the C=O bond (carbonyl group)either as a ketone or aldehyde. In the ring(cyclic) structure, the carbonyl carbon, C1is the one which is attached to the O of thering and an OH group.

Straight chained sugars that have analdehyde or ketone group can act as areducing agent in a chemical reaction.

Reducing agent – means the sugar isoxidized – gains oxygen atoms & loseselectrons in a chemical reaction.

Reducing sugars are: glucose, fructose,lactose, maltose (not sucrose)

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Caramelization

Food Composition: During dry heating and roasting of foodhigh in sugar and without proteins/amino acids.

Complex reaction mechanism. At its most simple the processinvolves melting sugar at a temperature > 120°C in order todehydrate (remove water)

Factors the increase rate of browning: pH <3 & >9,temperature >120°C, type of sugar (fructose best).

Feature of product: desirable brown color, caramel/ butterscotch smell and flavor. Undesirable effects – burnedsugar smell and taste, and black color. Occurs when heated fortoo long at high temperatures causing all the water is removedfrom the sugar producing carbon.

CnH2nOm nC + mH2O Foods affected: roasted vegetables, color and flavor of cola

beverages, topping on baked egg dishes.

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Activity 1

When hydrogen is added across the double bond of lycopene (the yellow/orange) pigment found in tomatoes,the number of C=C double bonds decreases. Predict andexplain the color change that would be observed in thetomato.

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Activity 2

1. The color of Cherry Kool-Aid is the result of added an artificial dye.Explain why makes cherry

Kool-Aid, appear red to your eye?

2. Using structures on Pages37-38 of the Data Bookletpredict whether β-caroteneand lutein would appearorange or red.

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Activity 3

1. What color areanthocyanins at pH 0-6and pH 7-14.

2. Using Bronsted-Lowry theory and the equationon page 35 of the DataBooklet explain how thataddition of ethanoic acidand sodium bicarbonate

changes the color of redcabbage. Account for thepurple color.

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Activity 4

α and β-carotene absorb visible light in the430-490nm range andreflect/transmit yellow-orange light.

The carotenoid found inlobster shell is calledastaxanthin (page 38 of Data Booklet). Suggesta reason why lobstersare blue-green whenlive and orange-red when cooked.

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Activity 5

Astaxanthin is responsible forthe natural pink color of shrimp. It contains C: 80.54%,H: 8.72%, and O: 10.74% by mass.

1. Distinguish between apigment and a dye

2. Calculate the empiricalformula for Astaxanthin

3. Given that the Molar Mass of Astaxanthin was found to be620.00 gmol-1 determine the

molecular formula.4. How are the empirical andmolecular formula related?

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Activity 6

A UV-Visible spectrometer/spectrophotometer measuresthe amount of visible lightabsorbed by a coloredpigment. This can be read as Absorbance versus wavelength.

Maximum absorbancecorresponds to the wavelength of light absorbed.

The color seen / reflected isits complementary color.

Beer’s Law can be used todetermine the concentration.

The absorbance spectrum of chlorophyll-ataken from a spectrophotometer is shownalongside.

1. Determine λ max .What does itrepresent?

2. Explain why chlorophyll appears green.

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Activity 7

Normally metal ions form ionic compounds withnon-metal ions. Complex ions are an exception to

this rule. Chlorophyll is a complex ion because eachof the N atoms on the four rings of the porphyrin

donates its lone pair of electrons to the central Mg2+

ion forming a coordinate covalent bond. Theporphyrin rings are called ligands.

1. Define a coordinate bond and ligand.

2. Name another molecule with this structure.

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Activity 8

1. Compare the structural features of chlorophyll andheme.

2. Deduce and explain their solubility in water.

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Erythrosine is a cherry-pink/red synthetic dye used tocolor foods like cherries,candy bars, and baked goods.It has been linked tohyperactive behavioral

disorders in children, thyroidcancer in rats. Banned in inNorway. Identified using thefollowing coding.

* FD&C Red No. 3

* E127 (Food Red 14)* Indian Standards No. 1697

Outline some of the issuesrelated to the use of this dye.

Activity 9

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Activity 10

1.

Write an equation to show thecondensation reaction thatoccurs between glucose andcysteine. Give the structure of the three possible amadoricompounds formed.

2. Distinguish between the type

of browning that occurs in bread and bananas. Explaineach process.

3. Compare the Maillard browning and caramelizationin terms of the chemicalcomposition of the food and

the products formed.4. Write an equation for the

burning of sucrose duringcaramelization.

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Activity 11

Deduce with a reason the type of browning:

1. Caramel candy or milk candy is a made by boiling milk, sugar, butter, vanilla essence, water, and glucose or corn syrup. It isheated in a pot up to 120 °C.

2. Many cultures have a baked egg desertmade with cream, eggs and sugar (crème

brulee, crème caramel, flan, egg tarts).Typically sugar is sprinkled on the top &heated to a hightemperatureuntil it browns.

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F4 Colour