Food Chemistry
Chapter 17 in Green / Damjii
F.4: Colour
Homework
• Read F4 – Colour - pp. 481-486
• Do Qs 24-33
• on p 491-492
F.4.1: Distinguish between a dye and a
pigment
• DYE
– Colouring materials that are synthetic or from
other natural sources
– Food dye =
• food grade, water soluble colour
• Natural ex = saffron, paprika, caramel
• Artificial = tartrazine (see right)
aka Yellow 5
F.4.1: Distinguish between a dye and a
pigment
• PIGMENT
– Colouring materials naturally present in cells of
plants and animals (… in foods)
• Examples:
–
–
–
–
–
Anthocyanins
Carotenoids
Chlorophyll
Heme
melanin, hemoglobin, myoglobin
F.4.2: Explain the occurrence of colour in
naturally occurring pigments
COLOUR (aka COLOR) is due to …
• absorption of certain frequencies of visible light
– by the extensive delocalized pi bonds
• reflection of other frequencies of light that
stimulate the retina in the eye
EX – Spinach
• red and blue light are absorbed
• green light is reflected
F.4.3:
Describe the range of colours and sources of the
naturally occurring pigments anthocyanins, carotenoids,
chlorophyll, and heme.
Anthocyanins
[flavanones]
Carotenoids
[Astaxanthin]
Source
Color(s)
berries; beetroot ; red
cabbage; flowers
[red grapes; berries]
Red – pink – purple – blue
All living things; algae;
carrots, bananas ; tomatoes;
saffron
[lobsters; crabs; salmon]
Yellow – orange – red
[red]
[red]
Chlorophyll
green plants; green vegetables green
heme
red blood cells of higher
animals (meat)
Red
(red w/ oxygen; purple-red
w/o oxygen; brown-red
when oxidized)
anthocyanins
Anthocyanins
[flavanones]
Source
Color(s)
berries; beetroot ; red
cabbage; flowers
[red grapes; berries]
Red – pink – purple – blue
[red]
anthocyanins
carotenoids
Carotenoids
[Astaxanthin]
Source
Color(s)
All living things; algae;
carrots, bananas ; tomatoes;
saffron
[lobsters; crabs; salmon]
Yellow – orange – red
[red]
carotenoids
Carotenoids
[Astaxanthin]
Source
Color(s)
All living things; algae;
carrots, bananas ; tomatoes;
saffron
[lobsters; crabs; salmon]
Yellow – orange – red
[red]
carotenoids
chlorophyll
Source
Chlorophyll
Color(s)
green plants; green vegetables green
heme
heme
Source
Color(s)
red blood cells of higher
animals (meat)
Red
(red w/ oxygen; purple-red
w/o oxygen; brown-red
when oxidized)
F.10.1:
Compare the similarities and differences in
the structures of the natural pigments: anthocyanins,
carotenoids, chlorophyll and heme.
Similarities:
• all have extensive delocalized pi bonds
• most have ring systems – some fused
• many have –OH groups attached
F.10.1:
Compare the similarities and differences in
the structures of the natural pigments: anthocyanins,
carotenoids, chlorophyll and heme.
Differences:
• Overall shape
– Anthocyanins, Chlorophyll, heme – more compact
– Carotenoids – long and stringy
• some contain N and are capable of forming metal
complex ions
– Chlorophyll (Mg 2+)
– Heme (Fe 2+)
F.10.2:
Explain why anthocyanins, carotenoids, chlorophyll
and heme form colored compounds while many other organic
molecules are colorless.
COLOUR (aka COLOR) is due to …
• absorption of certain frequencies of visible light
– by the extensive delocalized pi bonds
(alternating single and double bonds)
• As delocalization increases, the energy split between the
bonding and anti-bonding pi orbitals becomes smaller,
shifting the absorbed light into the visible region.
• reflection of other frequencies of light that
stimulate the retina in the eye
F.10.3: Deduce whether anthocyanins and
carotenoids are water- or fat-soluble from their
structures.
• Anthocyanins – water soluble
– Multiple –OH groups
• can hydrogen bond with water
• Carotenoids – fat soluble
– Long hydrocarbon chains
• Insufficient –OH groups to overcome HC chain
F.4.4: Describe the factors that affect the color
stability of anthocyanins, carotenoids,
chlorophyll and heme.
• pH
– impacts anthocyanins & chlorophyll (H+ replaces
magnesium)
• Formation of complex ions
– impacts anthocyanins (cooking in metal pans),
F.4.4: Describe the factors that affect the color
stability of anthocyanins, carotenoids,
chlorophyll and heme.
• Temperature
– can impact all groups – particularly denaturing
proteins
• Oxidation
– Impact carotenoids (saturation of chain); heme
(binding to oxygen and oxidation of iron)
F.4.5: Discuss the safety issues associated
with the use of synthetic colorants in food.
Concerns:
• Synthetic dyes are biochemically active
– Can negatively impact health
• toxicity is easy to prove
• chronic health effects are difficult to determine
• Special concern about carcinogenic effects
– Most are NOT typically used in foods
– Standards vary from country to country
• Malachite green and sudan red are generally banned
F.4.6: Compare the two processes of nonenzymatic browning (Maillard reaction) and
caramelisation that cause the browning of food.
NOTE: Browning usually involves BOTH
processes… except for those foods that do
not have amino acids or proteins
sugar  toffee
sugar  crème brulee
Maillard Reaction
• Grilling meat, toasting bread, malting barley, making
fudge
– (also self-tanning treatments – imagine that !)
• Occur at temperatures > 140°C
Maillard Reaction
• aldehyde group (from sugar) reacts with
amino group (from AA, peptide, or protein)
– Rate depends on particular amino acids used
• Lysine is more reactive
– (found in milk – so it browns readily – fudge)
• Cysteine is less reactive
• MANY products
– smaller molecules = aromas & flavors
– initial products then polymerize to form brown
pigments
• melanoidins
Caramelization
• Occurs in foods with high carbohydrate
concentration
– Sugars
Caramelization
• When heated…
– Carbohydrate molecules dehydrate and form
polymers
• many products
• polymers have brownish color
– With continued heating…
• form carbon and water
• Cn(H2O)n  n C + n H2O
Caramelization
• Rate varies depending on sugar
–Fructose (in fruits) is easiest to caramelize
• Extreme pH (high and low) promotes
caramelization