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Chapter 6

Unit 4, Chapter 6 - Complete Notes.docx
Unit 4, Chapter 6 - Complete Notes.docx

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University of Guelph
Food Science
FOOD 2010
Massimo Marcone

UNIT 4 – CHAPTER 6: 6.1 – Food Colour Chemistry Color: describes a perception of a physical attribute of food arising from sensations • Sensations originating from rods + cones found in retina of eyes o Rods – sensitive to lightness + darkness o Cones – sensitive to red, green, or blue color • Light striking food gets: absorbed, reflected, or transmitted o Reflected light = color of food Surface • Colour o Hue – actual colour name (red, blue, green) o Chroma (saturation) – clarity + purity of colour o Intensity – range from lightness to darkness of colour • Appearance o Shiny + glossy = result of reflected light o Cloudy + translucent = related to transmission of light through food objects Electromagnetic spectrum of visible light • Wavelengths from 380-780nm (red, orange, yellow, green, blue, indigo, violet) o Object reflecting all colours = seen as white o Object absorbing all colours = seen as black • Ex: Broccoli = green o Absorbs white light (all wavelengths) except for light rays that produce green (reflects green wavelength ~500nm) PIGMENT MOLECULES Chromophoretic compounds: constitute a structurally diverse group and possess extremely complex chemical and physical properties • Colours classified based on chemical structures • 5 major groups of natural food pigment molecules o 4 plant kingdom  Lipid soluble 1. Chlorophylls 2. Carotenoids – bright orange colour  Water-soluble 3. Anthocyanins 4. Betalains o 1 animal kingdom 1. Heme protein myoglobin – meat colour Myoglobin: single peptide, globular protein • Contains: globin protein part + prosthetic group “heme” o Globin = polypeptide tertiary structure = amino acids  Attaches to heme in myoglobin and hemoglobin o Heme = iron porphyrin ring complex  Porphyrin ring complex – composed of 4 pyrrole units (connected by methane bridges) • Pyrrole units have N atoms in the corners • Fe present in centre of ring covalently bonded to the 4 N atoms of the 4 pyrrole units • 5 coordination site connects Fe to N atom of globin protein in myoglobin th • 6 binding site can bind any atom that is able to donate a pair of electrons (ex: oxygen)  Oxidation of porphyrin ring = yellow/green colour o Has the ability to bind oxygen because of Globin structure + Heme Meat + Colour changes 1. Upon slaughter o Oxygenated myoglobin is no longer formed  shift in equilibrium toward unoxygenated myoglobin Mb + O 2- -> O M2  Fe in unoxygenated myglobin exists in reduced state (ferrous iron Fe2+) • Oxidation + reduction of Fe in myoglobin is linked to colour changes in meat 2. After slaughter, meat is cut and exposed to air o Unoxygenated myoglobin’s iron (ferrous) loosely binds O 2  Converting myglobin  oxymyoglobin = bright red colour  Oxygenation: Ferrous myoglobin binds oxygen at 6 bbinding site + O 2 becomes part of pigment complex 3. Continuous exposure to air o Oxidation occurs + production of metmyglobin = grayish/brown colour  Metmyoglobin: aged meat exposed to air, Ferrous changes to Ferric (oxidized state) altering colour 4. Point of purchase o Choleglobin + sulfmyoglobin = bacterial action on contaminated meat = green colours  Packaging material, number of bacteria, species of animal, animal’s age, amount of myoglobin in animal tissue, particular muscle source Nitrate-cured meats • Myogloin reacts with nitric oxide (NO)  produces nitric oxide myoglobin = bright pink-red • When cured meats are cooked, retains pinkish/reddish colour o Cooking  heat  denature/coagulated nitrosymyoglobin = formation of nitrosohemochrome = pink/red colour • Storing cured meats = brown colour o Storing  Oxygen + light exposure = oxidation of ferrous  ferric iron = brown colour COLOUR CHEMISTRY OF FRUITS AND VEGETABLES Fruits and Vegetables • Naturally occurring plant pigments absorb + reflect light at certain wavelengths = colour o Contain conjugated double bonds: alternating single + double carbon-to- carbon bonds (C=C-C=C-C=C) • • Pigment molecules: o 3 groups based on structure: 1. The phenolic-based pigments (anthocyanins, anthoxanthins, betalains) • Anthocyanins: water-soluble flavonoid compounds that range in colour from deep purple  orange o Flavonoid: chemically related phytochemicals that contain 2 phenol rings + intermediate ring of variable structure (ex. Anthocyanin + anthoxanthin) o pH sensitive: red @ acidic  colourless @ pH4  blue @ 7 (neutral) o Structure: aglycone molecule + sugar molecule (attached by glycosidic bond) o Anthocyanidin: free aglycone molecule (without the sugar)  Examples: pelargonidin (red pigment), cyaniding (red-blue), delphinidin (blue) • Anthoxanthins: colourless/white pigments that can become yellow o Slight contribution to food cololur • Betalains: 2 types of water-soluble plant pigments o Betacyanins: covers 50 violet-red pigments  Examples: betanin (glycoside found in beets, R-sugar-group = lucose)  Glucose – O – R betanidin (R-sugar-group = hydrogen)  H – O – R o Betaxanthins: covers 20 yellow pigments  Examples: vulgaxanthin 2. Carotenoids • Fat soluble plant pigments • Contains o Carotenes: Hydrocarbons  Examples: beta-carotene (from carrots) + lycopene (from tomatoes) o Xanthophylls: oxygenated carotenoids containing alcohol, carbonyl, or other functional groups  Examples: lutein • All carotenoids except lycopene (from tomatoes) have beta-ionone rings: cyclic structural units, which the functional groups are attached • Red, orange, and yellow from resonance in isoprene units found on carotenoid structure 3. Chlorophylls • Green lipid-soluble plant pigments containing porphyrin rings bound to magnesium-Mg (similar to myoglobin) o Phytol alcohol attached via ester linkage to 1 (of the 4) pyrrole groups nd o Methyl alcohol to 2 pyrrole group • Metal-coordinated porphyrin rings • Resonance in porphyrin ring structures (chlorophyll, myoglobin, hemoglobin) + conjugated double carbon chains = colour o Resonance electrons move across atoms that have alternating single + double bones  movement across carbon-to-carbon  colour • Chlorophyll a + Chlorophyll b o Many chlorophyll a&b derivatives because alterations from thermal processing/exposure to extreme pH environments  Structural changes = alters colour o Degraded a&b to produce the substance pheophytin using chlorophyllase  To make pheophytin: requires acid + heat to remove Mg atom and replace with H atom in chlorophyll • Chlorophylls a & b + heat, acid  pheophytin a & b  Chlorophyllase: cleaves phytol group to produce chlorophyllide and loses magnesium atom • Chlorophyllide: tints cooking water green • Losing Magnesium atom creates chlorophyll derivative pheophorbide which changes colour from green  gray-brown o Examples: in canned green beans + brined cucumber pickles THE COLOUR CHEMISTRY OF FOOD COLORANTS Colorant: pigment used to pass on colour to food/beverage • 1900’s 1. Natural colorant • Naturally derived o Examples: annatto extract (containing 2 carotenoids: bixin and norbixin) turmeric (a dried herb) paprika oleoresins cochineal extract caramel color 2. Synthetic (artificial colorant) FD&C COLORANTS • FDA does not recognize any of today’s colorants as “natural” even if they are directly obtained from nature o All color ingredients = additives • Today’s society 1. Certified as FD&C colorants • Colorants certified safe by the FDA for use in foods, drugs and cosmetics • Structure: o Contain phenolic rings with double bonds & various functional groups  Example: Allura Red – 3 phenolic rings 1 ring contains OH group attached to it 2 ring contains SO Na3group 3 ring contains: one SO Na group, one OCH , and one 3 3 CH3group • SO 3a group makes colorant molecule more water soluble (easier to incorporate into food/drinks and excrete from body) • Food colour suppliers manufacture: a. Dyes: water soluble chemicals used to colour entire food products  Example: Lollipops b. Lake: insoluble powder formed by precipitation of water-soluble food colorant, derived from dyes. Used to color surface of foods/fat-based products  Example: Chocolates 2. Exempt from certification colorants • Annatto  caramel o Annatto: from the seeds of Bixa orellana, a tropical evergreen o Caramel color: carefully controlled heat treatment of sugars to create brown “burnt sugar color”  Not a flavor, but simple a coloring agent used at low concentrations in foods/beverages  Example: dextrose (glucose), sucrose, invert sugar, lactose, malt syrup, molasses (these all contain glucose) • Cochineal & grape skin extract  paprika o Cochineal: an anthraquinone (type of polyphenol) used in 5000BC by Egyptian women to color their lips treated dried bodies of cochineal insects to produce a red solution, alcohol evaporates  cochineal extract (high in carminic acid content) is used as colorant  Example of Cochineal pigment: carminic acid  Carminic structure: polyphenolic acid w/ 3 rings + COOH group 6.2 – Food Flavor Chemistry Flavor: property of food material and the receptor mechanisms of the human body • Taste + aroma • 4 basic/true tastes: sweet, salty, sour, bitter o Tastant molecules bind to the tongue receptor cells + interact with specific proteins in these sense cells  Interaction creates disturbance in molecular geography of surface (allows interchange of ions)  This reaction is followed by electrical depolarization in the sense cells, initiating a nerve impulse CHEMICAL STRUCTURE & TASTE • Substances need to be water-soluble in order to produce a taste o Sour = all food acids o Salty = sodium chloride  Salty + bitter = potassium bromide o Bitter taste = larger atoms + alkaloids (nitrogen-containing organic compounds from plants)  Example: quinine, picric acid, heavy metal salts o Sweet = sugars + lead acetate, saccharin, aspartame, sugar alcohols Pungency: sensation of “spicy heat” or “chemical heat” caused by specific chemicals (primarily cruciferous vegetables and chili peppers) on the mouth and lips • Caused by pungent molecules, varied by the intensity and duration according to the active chemical component present o When pungent foods are consumed, endorphins are also released = sensation of pleasure amidst the pain o Pungent substance example: Capsaicinoid family  Capsaicinoids: pungent alkaloid compounds found in chilies • Examples of Capsaicinoid: Capsaicin + dihydrocapsaicin Cooling Sensation • The sensation of coolness (for example, caused by chewing spearmint/peppermint gum) o Methol + isomers of menthol = cause of cooling sensation  Menthol = crystalline cyclic alcohol  “minty” flavor + aroma • Used in: baked goods, chewing cum, confectionery, frozen dairy products, jams, jellies, nonalcoholic beverages, soft candy  Polyols/Sugar alcohols = polyhydric alcohol counterparts of sugars • Sugar: maltose  Polyol: maltitol • Sugar: mannose  mannitol • Sugar: sucrose  sorbitol: most frequently used polyol because of humectant properties, in shredded coconu
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