The Chemistry of Food Lecture 2 ( Dr AN Boa )

1º and 2º Structure The Chemistry of Food • Peptide bonds join amino acids →the primary structure is Lecture 2 the specific amino acid sequence of a protein • Ordered structures can be stabilized by H-bonds so forming secondary structures such as – Helices Chemistry in Context 5.4 Å O R = N N -C-C H 06525/06529/06509 H – β-sheets (antiparallel peptide strands) O O N = = C H C – disulfide bonds (R-S-S-R) N-C-C-N H H O = formed by oxidation of neighbouring O O C = = N Dr AN Boa C-C-N-C C H Cys residues (R=CH2SH) H 1 3 Proteins 3º Structure • Proteins are polymers of amino acids • Globular proteins Hydrophobic amino – Hydrophobic residues acids avoid contact peptide (amide) bond buried in interior Charged groups with water • Large fraction of avoid interior + hydrophobic units – Hydrophilic, charged + groups on outside + • Amino acids are chiral molecules (except glycine, where R = H) – Most compact structure Roughly spherical shape • Our DNA encodes for L amino acids only • Amino acids can be divided into groups depending on the side • Fibrous proteins chain functionality. – Broadly there are hydrophilic and hydrophobic side chains, – More hydrophilic, less – these can be subdivided into acidic, basic, polar, aromatic and aliphatic2 compact

Essential amino acids Muscle • ends blend in with tendon Bundle of muscle cells • Amino acids are needed as building blocks – for our body’s own proteins – for purines and pyrimidines (nucleic acids) Muscle cell (or fiber) – for porphyrins (e.g. haemoglobin) • surrounded by sarcolemma • surrounded by thin connective – for other important substances tissue layer • The liver can biosynthesise some amino acids but others must come from diet – the so-called essential amino Muscle fibril acids: Ile, Leu, Lys, Met, Phe, Thr, Trp, Val, His • Proteins are degraded by stomach and intestinal Sarcomere (myofilaments) enzymes (proteases) into individual components. 5 Actin and myosin 7 Myofibrillar Proteins Making Bread: forming dough • Actin • Doughs are viscoelastic (they flow and recoil) – small spheres arranged like string of beads • Structure largely from proteins called glutens – consist of gliadins and glutenins – two strings twisted together ⇒thin filaments – 30% of amino acid residues are hydrophobic • Myosin – shaped like golf clubs • Kneading bread subjects it to shear forces – cluster ⇒thick filaments – promote interactions between glutenin molecules – H- bonding, hydrophobic interactions, S-S bonds – creates elastic protein networks (films) which trap gas – viscosity enhanced by gliadins and starch molecules

Lipids Triglycerides • A structurally diverse range of compounds • Food fats/oils are primarily triacylglycerols (esters of fatty acids • Non-polar and hydrophobic with glycerol) • also commonly known as triglycerides – 3 fatty acid chains on a glycerol backbone – Fats (solid at room temperature) – Oils (liquid at room temperature) • Fatty Acids • Triglycerides • Steroids • Waxes • Phospholipids • Terpenes 9 11 Attributes of Food Lipids Fatty acids • Three major functions in foods • Many R groups are alkyl groups (no C=C) – Energy and health – these are called saturated fatty acids – Influence food flavours • free fatty acids contribute flavours • Other R groups are alkenyl groups (with C=C) – Texture – 1 to 6 double bonds • solid vs liquid – cis-double bonds, methylene interrupted • mixed with water (emulsions) • one double bond are called monounsaturates • Attributes determined by types and positions of fatty • two or more double bonds are called acids on glycerol backbone polyunsaturates

Naming fatty acids Lorenzo’s oil • Fatty acid chains are named using a specific numbering • True story and topic of 1992 film system. • Lorenzo’s oil is possible treatment for rare disease adrenoleukodystrophy (ALD) methylene interrupt – involves accumulated saturated, very long chain fatty acids (VLCFA) in blood stream • patient cannot break down these VLCFAs cis • especially C26:0 • synthesized from shorter dietary fatty acids – Lorenzo’s oil is 50:50 mixture of oleic acid (C18:1) and erucic acid (C22:1) • shifts metabolism to form long chain unsaturated fatty acids can be named as 18:2∆9,12 or as 18:2ω6,9 • Lorenzo’s oil may not affect later disease progression 13 15 Important Food Fatty Acids COLOURS Abbreviation Systematic Name Common Name Symbol The molecular basis of colour 4:0 Butanoic Butyric B 6:0 Hexanoic Caproic H Colour Wavelength (nm) Complementary Colour Light absorbed Colour observed 8:0 Octanoic Caprylic Oc 10:0 Decanoic Capric D Violet 400 12:0 Dodecanoic Lauric La Yellow 14:0 Tetradecanoic Myristic M Blue 450 16:0 Hexadecanoic Palmitic P Orange Green 500 18:0 Octadecanoic Stearic St 70 °C Red 18:1 9-Octadecenoic Oleic O 10.5 °C Yellow 550 18:2 9,12-Octadecadienoic Linoleic L -5 °C Violet 18:3 9,12,15-Octadecatrienoic Linolenic Ln Orange 600 20:0 Eicosanoic Arachadic A Blue Red 650 20:4 5,8,11,14-Eicosatetraenoic Arachadonic An Green 22:1 13-Docosenoic Erucic E 700

Colour: green Colour: reds, mauves, blues • Leafy green vegetables and • The colours in berries and apple skins contain similar fruits are due to the chlorophylls a and b anthocyanins • Used in photosynthesis • Contain anthocyanidin aglycone often based • Cooking (heating) results in: on the flavylium cation – loss of phytol side chain • Six natural variants (OH, H, OMe) of the aglycone forming chlorophyllide – Loss of magnesium to form – Can have various sugars attached via glycosidic link at position 3 brown pheophytins (over (sometimes at 5, rarely elsewhere) cooked cabbage) – Sugars often esterified at C6 with cinnamic acid derivatives (caffeic = p-OH + chlorophyll a, R = H m-OH; coumaric = p-OH; ferulic = p-OH + m-OMe) chlorophyll b, R = CHO 17 19 Colour: yellow and orange Colour: browns • Yellows and oranges are due to the carotenoid group of terpenes, • On cutting apples and potatoes brown rapidly due to the e.g. lycopene and β-carotene action of phenoloxidase on chlorogenic acid Lycopene caffeic acid CHLOROGENIC ACID • Oxidation of polyphenols form reactive quinones which β-carotene then polymerise forming brown pigments called melanins – Causes desirable browning in tea, cider, dried fruit etc. • Terpenes made from repeating isoprene units derived from – Tannins form as red wine matures – polymerisation of anthocyanins mevalonic acid. Having 8 isoprene units, the carotenoids are tetra- terpenoids (monoterpenes have 2 units).

Enzymatic browning PO = phenoloxidase PO PO polymerisation • Prevention of browning – Eliminating oxygen (storing in an inert atmosphere or vacuum) – Lowering enzyme enzyme activity • The optimum pH for phenoloxidase is 7, and so citric or malic acid can be added to retard browning • Briefly heat to deactivate enzyme (blanching) • Add reducing agents (ascorbic acid, sulfites) 21 The colour of your steak N N • Myoglobin – protein containing iron Fe2+ – purplish-red N – Fe2+ N Globin O2 N N • Oxymyoglobin Fe2+ – supermarket red – Fe2+ O 2 H2O N N N N Globin • Metmyoglobin – brownish-red Fe3+ – Fe3+ H O 2 N N Globin

Bir yanıt yazın

Başa dön tuşu