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Lecture 3

Week 3

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University of Guelph
Food Science
FOOD 2010
Massimo Marcone

Principles of Food Science FOOD*2010 Chapter 4 (p. 91-119) 4.1 is pretty much a general chemistry recap: if you are pretty fluent in chemistry, feel free to skim it or jump ahead to 4.2 . If you are really unfamiliar with chemistry, be sure to read the whole chapter  4.1 – The nature of matter • Food is made of matter (as is everything else) • Matter made of pure substances • Pure substances are found as either elements or compounds • Elements are the simplest pure substance and cannot be further broken down, they are the building blocks of all matter • Compounds are two or more elements chemically bonded together, but the properties of a compound are very different than the elements that it is composed of, compounds CAN be broken down into its elements under certain conditions • In food science we mostly deal with organic compounds which are mainly made up of the elements: carbon, oxygen, & hydrogen, but they can also contain sulphur, nitrogen, & phosphorus • A mixture is a combination of substances that are physically combined instead of chemically (for example salad dressings) Below is a diagram of a carbon atom to illustrate protons, neutrons and electrons. • Chemical bonds are the forces that hold atoms and molecules together. • Forming bonds requires energy • Electrons are the only part of the atoms involved in bonding. (Protons and neutrons are NOT involved in bonding) • Atoms want a stable outer (or ‘valence’) electron shell, and having a “full” shell is the most stable, this desire to be stable will drive atoms to take electrons, give away electrons or share electrons (which ever uses the least amount of energy. ie, an atom would not take 7 electrons when it could just give up one) • Covalent bonding: when atoms share electrons to fill their valence shell. There can be a single, double or triple covalent bond. This occurs in many food substances. • Ionic bonding: when atoms transfer electrons to fill their valence shell. The atom that donates the negative electron, becomes a positive ion (cation). The atom that accepts the negative electron becomes a negative ion (anion) • Hydrogen bonding: when atoms slightly share electrons in a very weak covalent bond, as one of the atoms ‘hogs’ the electron. Hydrogen bonds occur between molecules, not within molecules. 4.2: Chemical reactions in foods Composition reaction: A + B  AB - food molecules produced this way Decomposition reaction: AB  A + B - food molecules breaking down in processing, digestion, or by enzymatic reactions Enzymatic reactions: • Enzymes occur only in living tissues (plant, animal, microbial) • Can change colour, texture, flavour, odour of food (either desirable or undesirable) • Enzymes are specialized protein molecules and biological catalysts (speed up chemical reactions) • Each enzyme is specific to its own substrate • The active site is the area on the enzyme where catalytic activity occurs, where the reactivity is determined by specific functional groups • Enzyme + Substrate Enzyme-Substrate complex Enzyme + Products • The above equation shows that the reaction can go in any direction based on what is favourable • Activation energy is the amount of energy needed to convert the substrate into the enzyme- substrate complex • An enzyme lowers the amount of activation energy needed for the reaction to proceed • Temperature, pH, and amount of substrate present affect the if and how much the enzyme is used. • Zero-order reaction = high amount of substrate, and therefore the reaction proceeds as fast as the enzyme will allow. • Enzymatic Hydrolysis - breaking down large food molecules into smaller fragments (includes carbohydrases, lipases, proteases…. The –ase ending indicates an enzyme) • Enzymatic oxidation-reduction – when the enzyme causes chemical change in the food. Oxidation: oxygen added or hydrogen removed. Reduction: losing oxygen or gaining hydrogen. • Enzymatic polymerization – enzymes often combine long polymers from monomers (small subunits) in proteins and carbohydrates through condensation reactions Nonenzymatic Reactions: the following do not rely on enzymes • Addition reactions – carbon to carbon double and triple bonds are more reactive, the bond is broken and another molecule is added in (often hydrogen in food science) • Oxidation-reduction reactions – electron transfer. In general adding oxygen = oxidation, adding hydrogen = reduction • Condensation – joining two subunits and losing a water • Hydrolysis – breaking apart two subunits with the addition of water 4.3: Functional groups • Arrangements of a few atoms that have special properties 4.4: The Chemical and Functional Properties of Water • The oxygen has an affinity for electrons, and this creates a polar nature. This is the reason for its boiling point, freezing point and other critical characteristics. • Salvation and dispersing action: see picture to the right. This occurs with substances that are also polar water in foods: moisture content and water activity (not the same thing) • moisture – the absolute amount of water in food in relation to its other components • absorbed water – water associates itself around and in between hydrophilic substances • bound water – exists tightly chemically bound, and does not have the typical properties of water • water activity (a ) w o measured availability of water that can enter into microbial, enzymatic or chemical reactions o Measures relative humidity o Determines shelf life of food o Temperature dependant o As the percentage of bound water increases, water activity decreases o a w P/P 0 where P = vapour pressure of food P 0 vapour pressure of pure water o values from 0 to 1; 0 = no water activity, 1 being pure water • water in emulsions o emulsion is a system containing two liquids that do not mix, ie oil and water • water and heat transfer – water is able to act as a conductor of thermal energy to food molecules 4.5: The Chemical and Functional Properties of Food Acids Food acid structure: • often a carboxylic acid, differing in the location of the –COOH and the structure its attached to • functionally act as flavour enhancers or antimicrobial agents • acids can benefit some foods because they are hygroscopic (low attraction for moisture) Acid strength • acids donate/lose protons (H+) • food acids are weak acids and do not dissociate/separate very much (the H+ in strong acids separates completely) -- + • foods with –COOH will also contain a small amount of COO and H • dough softening: –S - S- bonds are very strong, without the H+ donation from fumaric acid these bonds would form in the dough and make it very tough - organic salts also dissociate/separate in solution - a buffer is a solution of a weak acid and its salt, together they form a buffer system and resist severe changes in pH Leavening • = the production of gas by: yeast fermentation, acid + baking soda, or by heating salts • The key is the production of gases that cause the food product to rise • Leavening acids help baking soda to release CO by neutralizing it 2 • Baking soda/sodium bicarbonate (NaHCO ) is a3kaline/basic and ionizes, producing HCO 3— • Leavening acids then convert the ion into H CO2whic3 then naturally produces  H O + CO 2 2 • Baking powder is used alone because it is made up of baking soda and a leavening acid (cream of tartar) already and only needs water in order to rehydrate Food Acidity • Acidity affects taste and sourness, and pH • pH tells us the hydrogen ion(H+) concentration; Recall: all acids lose a proton(H+) the more tendency they have to lose a proton/ dissociate/ionize the stronger the acid and the lower the pH • pH = -log[H+] • pH scale: values from 0 to 14, with 7 being neutral • 0-----------------7---------------------14 acidic neutral basic • Acid foods = pH below 4.6 • Acidified foods = low-acid foods that have acids added to them, with a final pH below 4.6, water activity above 0.85 • Low-acid foods = pH above 4.6 and water activity above 0.85 • Fermented foods= low acid foods subjected to certain microorganisms which produce acid during their growth. Final pH below 4.6 Chapter 5: Food Chemistry II: Carbohydrates, Lipids, Proteins 5.1 Food Carbohydrates The Structures of Sugars - All carbohydrates contain carbon, hydrogen and oxygen, which form the simple sugar building block - Simple sugars include both monosaccharides (ex. Fructose) and disaccharides (ex. Sucrose). A simple sugar is also called an organic alcohol, since it contains carbon atoms attached to an alcohol (- OH) group Monosaccharides - Triose = a monosaccharide containing 3 C atoms - Pentose = a monosaccharide containing 5 C atoms - Hexose = a monosaccharide containing 6 C atoms - Glucose, an aldose, is the most common monosaccharide in foods. Fructose, is contrast, is a ketose sugar. Disaccharides - The bond connecting two monosaccharides to form a disaccharide is a glycosidic bond - Sucrose, lactose and maltose are all disaccharides The Functional Properties of Sugars - The two functional groups of sugars are the carbonyl (-C=O) and the alcohol (-OH) groups. The carbonyl reduces activity and the Maillard browning reaction, which can cause colour and flavour changes. The alcohol is important for solubility and sweetness. Reducing Sugars - Sugars that contain the aldehyde or ketone carbonyl group are called reducing sugars – they react with other substances through redox reactions to yield a reduced substances plus the oxidized sugar molecule. All monosaccharides and certain disaccharides are reducing sugars. - Dextrose Equivalent (DE) is a measure of the percentage of glycosidic bonds hydrolyzed in disaccharides and polysaccharides, which indicates the level of reducing sugar present. The higher the DE, the more soluble and the greater the reducing ability of a sugar. Browning - Reducing sugars react with AAs through the Maillard reaction to produce brown colour pigments in foods. The brown pigments that form as called melanoidins. This is also known as non-enzymatic browning because enzymes are not part of the reaction. - Step 1: Condensaton. Step 2: Rearrangement. Colour and flavour development begins at this stage. Step 3: Polymerization. Large molecular weight melanoidins are produced. (See Fig. 5.3 on page 125) - Caramelization is the formation of brown caramel pigments as a result of applying heat energy (generally 200 degrees C) to sugars. It is also a non-enzymatic process. Crystallization - Crystallization implies org
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