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FOOD CHEMISTRY: COLOUR

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Adi I

on 19 March 2014

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Transcript of FOOD CHEMISTRY: COLOUR

FOOD CHEMISTRY: COLOUR
Colour
The Stability of Natural Pigments
ANTHOCYANINS
Safety Issues with Synthetic Colouring Agents
Common Features of Molecules
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Non enzymatic browning; Carmelization versus Maillard Reaction
Advantages and Disadvantages
Caramelization
The Maillard Reaction
Azo linkage - acts as a chromophore
Sulfonic acid groups
- promotes solubility
Conjugated Aromatic Rings
Example; Tartrazine in fruit based products
Different mixtures of anthocyanins are responsible for the range of colours exhibited by flower petals and fruit
Changes in the pH level can cause transitions in the anthocyanin structures
Increasing the pH
A proton is lost
A water molecule is accepted to form the carbinol pseudo-base
Decrease in colour intensity
Forms the quinoidal base (weak purple)
Ionized quinodial form (deep blue)
When pH levels are high, the carbinol-pseudo base gives rise to a small proportion of the colourless chalcone form
When pH values are greater than 3:
React with colourless flavanoids present in plant tissues
Flavanoids enhance the intensity of the colour and stabilize the blue quinodal form
An addition of a hydroxide ion to any cationic heterocycle produces pseudo-bases
Can form complexes with metal ions
(Ex. Fe3+(aq) and Al3+(aq))
Temperature
Increasing = the structure is less stable and destroyed
Causes the loss of initial colour and a brown colour develops
CAROTENOIDS
Are highly unsaturated molecules
Examples of degradation pathways:
isomerisation
oxidation
decomposition
Isomerisation:
molecules are promoted from the trans from to the cis form through heat, light and acids
Oxidation:
is catalyzed by light, metals, enzymes, and hydroperoxides
This results in the bleaching of colour, the production of unpleasant odours, and loss of vitamin A
HEME
Fresh red meat darkens when water is lost from the cut surface concentrating the pigments and making them look darker
Myglobin:
a heme-containing protein that acts as a store of oxygen for the muscle
Becomes
purple
because of the presence of a heme group containing an iron (III) ion
Advantages
To prevent the conversion of metmyoglobin : a supply of oxygen is needed
Disadvantages
The oxidization to brown metmyoglobin is caused by the surface of oxymyoglobin becoming deoxygenated
Solution: using 'breathing film'
When meat is cooked: the myoglobin turns brown because it oxidizes the iron (III) ion at the centre of the molecule within the heme group
brighter
greater chemical stability
cost
a wider range of
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Regulations are not internationally set.
Testing doesn't cover allergies and intolerance.
Most stable and most highly coloured at low pH levels and temperatures
Two types of "colour" in food:
Dye
: a food-grade synthetic water-soluble colour, commonly used in food additives
Natural
eg. caramel, chlorella, saffron, paprika
Artificial
eg. tartrazine, sunset yellow FCF, fast green FCF
Pigment
: a naturally occurring colour found in the cells of plants & animals
Eg. anthocyanins, carotenoids,chlorophyll, melanin, hemoglobin, myoglobin
CHLOROPHYLL
Is stable in basic solutions, and unstable in acidic solutions
The cell membrane can deteriorate when heated, decreasing the pH
Results in the formation of olive-brown pheophytin complexes
Makes it more suspectible to photo-degradation
A colour wheel can be used to determine the colour of a pigment, and the colour absorbed by a pigment
The colour absorbed by a pigment is complementary to the colour of the pigment itself, A.K.A the colour reflected by the pigment
Complementary = complementary colours, the colours opposite to each other on the colour wheel
eg. if violet, red, and orange light are absorbed, then blue, green, and yellow (the respective complementary colours) are reflected. The middle colour is perceived, hence the pigment appears green.
Therefore, chlorophyll should absorb red light the most, because we perceive it to be green, which is its complementary colour
Occurs during dry heating and roasting of foods where..
The concentration of carbohydrates are high
Amines and other nitrogen compounds are absent
The appearance of colour in naturally occurring pigments is possible because of their ability to reflect and absorb different wavelengths of visible light
Light is measured in wavelengths
For the visible light spectrum, different wavelengths = different colours
When white light shines on a pigment, some of its wavelengths can be absorbed or reflected, depending on the pigment’s preference
Examples include; baked egg dishes, caramelized onions, roasted peanuts
Carmelization - carbohydrates are heated, usually producing a desirable odour, flavour and colour
Pale Amber
Golden Brown
Chestnut Brown
First, the sugar will melt (usually at high temperatures, of about 160°C)
Temperatures
At extremely high temperatures, the sucrose is completely dehydrated and carbon is formed
Carotenoids are the most widespread pigments, found in many areas:
They’re responsible for red and yellow pigments in food
Eg. paprika, yellow pepper, bananas, tomatoes, saffron, watermelon, butter, egg yolks & margarine
Beta-carotene is responsible for the orange in carrots
Increasing the rate
A temperature above 120°C, for baking or roasting foods with a high sugar content
acid- or base- catalyzed with a pH above 9 or below 3
Steps of the reaction
Boiling/Foaming
Carotenoids are found in algae, which provide protection against sunlight damage
They help absorb blue light, aiding in photosynthesis
Carotenoids can act as a precursor for vitamin A synthesis in humans
Red astaxanthin, a carotenoid, when present as a complex with protein, gives the purple-green found in raw lobsters and crabs and the pink colour of salmon
Chlorophyll is the major pigment in green plants, existing as two forms: chlorophyll-a and chlorophyll-b
It absorbs sunlight for photosynthesis and is present in the chloroplasts of the plant leaves’ cells
Anthocyanins are responsible for the pigmentation in flowers, vegetables, and fruits
Eg. blue cornflowers, red roses, strawberries, blueberries, red apples, etc.
Heme in the hemoglobin of blood and myoglobin is responsible for the purple-red in raw meat
A muscle more used is a darker colour, as it requires more myoglobin
Melting
Decomposition
Condensation
Isomerization and Dehydration
Fragmentation and Polymerization
The Maillard Reaction - different from Caramelization
Condensation Reaction; an amino groups acts as a neucleophile
begins at a lower temperature, with lower concentration, and is accelerated by alkaline conditions
Reactivity Rules
Monosacccharides > disaccharides
Amino acids also differ in reactivity...
Lysine produces the most colour, and cysteine has the least.
Foods that contain lysine-rich protein like Milk proteins are likely to brown very readily.
Cysteine also breaks down during the cooking to release the gases: hydrogen sulfide and ammonia
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