Browning Reactions The Chemistry of Food Lecture 3 • Caramelization high temps sugar brown pigments + flavours • The Maillard Reaction Chemistry in Context reducing sugar + amino acid brown pigments + flavours 06525/06529/06509 • Other enzyme-mediated browning reactions can take Dr AN Boa place, which do not involve sugars – see lecture 2 1 3 FLAVOURS Caramelization • acid / base catalysed (organic acids / water) • We normally associate taste as the property of liquids, solids and gases in solution detected by the sensory • any sugar – here glucose cells on the tongue and oral cavity …. • … and associate aroma with the volatiles substances detected by the olfactory system of the nose. • Very few flavours allow a clear distinction between these two. Sweetness Astringency Saltiness Pungency Sourness Meatiness Bitterness Fruitiness (synthetic flavours) (off-flavours / taints) 2 CARAMEL FLAVOURS 4

The Maillard Reaction Meatiness AMADORI REARRANGEMENT • Water soluble, non-volatile taste components – Known as ‘umami’ by the Japanese • Monosodium glutamate, MSG, umami1 – Levels of 10-35 mg/100 g meat • Inosine monophosphate, IMP, umami2 – Levels of 1-200 mg/100 g • deamination and further isomerization leads to several – Also, but less important, reactive dicarbonyl compounds (HMF and others) GMP (0-10 mg/100 g) • cyclic products polymerize to form brown, insoluble poorly defined high MW materials. 5 7 The Maillard Reaction (continued) Meatiness • Acrylamide, a probable • Many volatile aromas present in meats carcinogen and agent known to cause neurological – Over 650 identified in beef damage, has been detected in – Most common are hydrocarbons (18%) many roast and fried foods. and non cyclic sulfur compounds (11%) • Believed to arise from the Maillard reaction involving asparagine. • Levels detected very variable, but best technique for extraction and analysis still a matter of debate. breakdown of S-containing amino acids ROASTED MEAT FLAVOURS 6 8

Sweetness Extended theory • Sweetness is found in many types of molecules (not just • An extended theory of sugars), and relative sweetness is normally compared to sweetness includes a sucrose hydrophobic γ-site • Natural sugars – sucrose (1.0); glucose (0.76); fructose (1.52) • Also artificial sweeteners – sodium cyclamate (30); acesulpham K (140); aspartame (200); saccharin (350); 1-n-propoxy-2-amino-4-nitrobenzene (4000) 9 11 Schallenberg’s Sourness “saporous unit” theory • Sweet molecules contain H-bonding groups such as hydroxyls, • Sourness assumed to be linked with acidic solutions amines etc. • However the presence of unionized organic acids (i.e R- CO2 H) is more important for the taste of sourness – citric, malic, tartaric (grape), isocitric, oxalic, acetic, lactic 0.3 nm • In foods: – Sourness of vinegar due to acetic acid, but also adds importantly to aroma, such as with fish and chips – Lactic acid in pickled foods such as sauerkraut comes from bacterial fermentation of the sugars in the vegetables – Sodium lactate is used in salt and vinegar flavoured crisps • Geometry of so-called “saporous” units crucial for interaction with a “sweetness” receptor 10 12

Bitterness Phenolics: Seville oranges • Several classes of compounds exhibit bitterness • Phenolics in the form of flavanoids are source of • Taste buds at back of tongue responsive to bitterness in citrus fruits. – group 1 and 2 halide salts – Naringin is a bitter sugar-flavanone conjugate found in Seville oranges. Its bitterness is detected at 1:50,000 dilution. – certain phenolics • KBr is both salty and bitter – Halide salts with the sum of their ionic diameters greater than KBr are bitter, if the sum is less then they are salty NaCl (0.556) < KBr (0.658 nm) < KI (0.706) < MgCl2 (0.850) – The non-sugar unit attached to sugar known generally as the aglycone 13 – Here the sugar is neohesperidose, and the aglycone is naringenin 15 Bitterness Phenolics: beer • Many plants contain molecules which we perceive as • Before the fermentation stage of the brewing process, the flowers of very bitter the hop plant, Humulus lupulus, are added to the wort – Nicotine, atropine, emetine, quinine • Hops are added to add both flavour and bitterness • Role in plants unknown….. – but many have undesired pharmacological properties – physostygmine (eserine) the boil • Animals’ ability to perceive these have undoubtedly evolved to avoid eating these plants – Quinine antiplasmodial agent used to prevent and cure malaria by consumption of tonic waters • The humulones are converted to the more soluble and more 14 bitter iso α-acids in the boil 16

Pungency: Pungency: onion and garlic chillies, peppers, ginger • Chillies • Garlic and onions – belong to Allium species (Capsicum frutescens) • Early (19th century) studies using steam distillation – Capsaicin and isolated non-odorous species dihydrocapsaicin – Gave rise to trivial name of ‘allyl’ for the prop-1-enyl unit • Black pepper (Piper nigrum) – Piperine • Gentler extraction processes (lower temp, ethanol as • Ginger solvent) isolated more interesting molecules! (Zingiber officinale) – Gingerols and shogaols – ketones similar to capsaicin with hydroxy or alkene groups in a variable length aliphatic side chain 17 19 Pungency: mustards and brassica Pungency: onion and garlic • Pungency in certain raw vegetables [mustards, (horse)radish, • Garlic: on cell rupture allinase reacts with alliin, a derivative of the cabbages etc.] are due to thiosugar derivatives – glucosinolates amino acid cysteine dimerise myrosinase glucosinolates distinctive garlic smell • Onion: allinase reacts with an isomer of alliin pungency molecules isomerise • When cells are damaged, such as in cutting or chewing, the action of onion lachrymatory factor myrosinase triggers the breakdown of glucosinolates 18 20

Conclusions • Carbohydrates: saccharides • Proteins: amino acids and peptides • Lipids: oils and fats • Colourings • Flavour chemicals …..affect the physical and chemical properties, and of course our enjoyment, of foodstuffs

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