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Food Chemistry . Chapter 17 in Green / Damjii. Homework. F.4: Colour. 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 =
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Food Chemistry Chapter 17 in Green / Damjii
Homework F.4: Colour • 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.
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 non-enzymatic browning (Maillard reaction) and caramelisationthat 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 • aldehydegroup (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