Etiket Arşivleri: Caramelization
FOOD CHEMISTRY I
Assoc. Prof. Dr. Dilek Boyacıoğlu
MIDTERM EXAM I
1. What is the effect of aw on keeping quality of foods?
2. Why do sugars form a pyranose or a furanose ring? Justify your reasons.
3. Draw and give the general name of the structures related to D-glucose
(b) Gluconic acid
(c) Glucuronic acid
4. What is the Maillard reaction? Justify the implication of these reactions on food quality.
5. Give two chemical methods to control Maillard reactions in foods.
6. Give names of sugars and indicate the type of glycosidic linkage in the following
7. Explain the structure of starch considering fractions, type of linkages and basic units.
8. Outline the changes during gelatinization of starch.
9. Explain the implication of retrogradation in bakery goods.
10. Define the followings;
(e) Chiral center
(f) Anomeric carbon
(j) Optical activity
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