Food Chemistry

1. Food Chemistry IB Chemistry Option F

2. Nutrients • A food is any substance we deliberately ingest for nourishment, ideally containing one or more nutrients • A nutrient is a component of food used by the body to provide energy or to grow and repair tissue • Carbohydrates (sugars) • Lipids (fats/oils) • Proteins • Water • Vitamins and minerals • Lack of nutrients leads to malnourishment

3. Food Groups • 5 Main food groups: • Grains– complex carbohydrates (polysaccharides), vitamins and minerals • Fruit – simple carbohydrates (mono and disaccharides), vitamins and minerals • Vegetables – simple and complex carbohydrates, vitamins and minerals • Meat/Beans/Fish/Poultry – proteins, vitamins and minerals, fats and oils • Dairy – simple carbohydrates, proteins, fats and oils

4. Nutrients (carbohydrates) • Monosaccharides (ONE sugar): • One carbonyl group (C=O) • At least two hydroxyl groups (-OH) • Empirical formula of CH2O (carbo: C and hydrate: H2O) • Ex. Glucose: C6H12O6 • Ex. Fructose: C6H12O6 glucose fructose

5. Nutrients (carbohydrates) • Disaccharides (TWO sugars) form when two monosaccharides come together, releasing a water molecule (condensation reaction) • “New bond” is called a glycosidic linkage between the two original monosaccharides • Polysaccharides (MANY sugars, a.k.a. complex carbohydrates) are the combination of many simple sugars through glycosidic linkages + H2O

6. Nutrients (fats and oils) • Fats and oils are both classified as triglycerides: • Glycerol (CH2OHCHOHCH2OH) combines with three fatty acid molecules (fatty = long, nonpolar hydrocarbon chain and acid = carboxylic acid) • Also called triesters because of the formation of an ester group when an alcohol group (from glycerol) combines with a carboxylic acid group (from fatty acid)

7. Nutrients (fats and oils) Formation of a triglyceride:

8. Nutrients (proteins) • Proteins are polymers of amino acids • 20 different amino acid “monomers” form long chains of near infinite combinations • Typical protein is approximately 300 amino acids, but can be made from less or more • Peptide bond (a.k.a. amide linkage) bonds amino acids together + H2O

9. Nutrients (water and vitamins/minerals) • Do not provide energy • Water is used for transport and various metabolic processes • Vitamins and minerals are also important in metabolic processes and as enzyme cofactors

10. Fats and Oils • Properties of a fat (chiefly melting point and chemical stability) are related to: • The degree of unsaturation (# of double bonds in the hydrocarbon chains) • More double bonds = lower melting point and more reactive • Length of the hydrocarbon chains • Shorter chains = lower melting point • Whether the chain is cis or trans isomerized around double bonds • cis = lower melting point

11. Fats and Oils • Saturated – all C-C single bonds • Fatty acid has a regular, zig-zag shape due to geometry around sp3 hybridized carbon atoms • Monounsaturated – one C=C double bond • Can be cis or trans isomerized • cis means hydrogen atoms are on same side of double bond • trans means hydogen atoms are on opposite sides of the double bond • Polyunsaturated – multiple C=C double bonds • Can be either cis or trans isomerized • A “fat” is a tryglyceride that is solid at room temperature • Usually saturated, monounsaturated, or trans unsaturated • An oil is a triglyceride that is liquid at room temperature • Usually polyunsaturated

12. Fats and Oils • Fats with longer hydrocarbon chains are able to make more van der Waals forces between each other, making molecules harder to separate • Higher melting point • Saturated fats have “straight” chains that pack closely and allow for lots of contact between molecules, making molecules harder to separate • Higher melting point • Trans-unsaturated fats have straight chains just as saturated fats do • Higher melting point • Cis-unsaturated fats have irregularly-shaped, less “straight” chains. Do not pack as closely, molecules are easier to separate • Lower melting point

13. Fats and Oils

14. Fats and Oils • “Degree of Crystallization” is related to the melting point of a fat • Higher melting point means a higher degree of crystallization (fat is more likely to be solid at room temperature) • Increases with: • Increasing degree of saturation • Increasing amount of trans-unsaturation • Higher molecular mass • Naturally-occuring triglycerides most commonly tend to be cis-unsaturated

15. Fats and Oils • cis-unsaturated fats are typically the healthiest • Lower melting point allows for less buildup of plaque in the arteries that can cause stroke or heart attack • trans-unsaturated fats are less healthy; do not commonly occur naturally, are harder to metabolize and buildup in fatty tissue • Cause an increase in LDL cholesterol (Low Density Lipoprotein or “bad” cholesterol

16. Fats and Oils • Unsaturated oils are more reactive due to the possibility of addition reactions across the double bonds • Are thus less stable and keep less well than saturated fats • Prone to auto-oxidation in the presence of light (photo-oxidation), which is a reaction of the fats with atmospheric oxygen • Also more prone to hydrogenation (addition of hydrogen), hydrolysis (breaking of the fat back into glycerol and fatty acids) and microbial degradation

17. Fats and Oils • Unsaturated fats are often artificially hydrogenated/partially hydrogenated: • Increases degree of saturation: • Allows for control of texture because melting point is affected • Improves stability and shelf life as reactivity is decreased • Improves cooking technique where more solid fats are needed • Hydrogen gas is added over a solid nickel catalyst • Double bonds converted to single bonds and degree of saturation increases • Drawback - decreases the health value, as fats are healthier when mono- or poly-unsaturated Ni(s)

18. Shelf Life • Foods gradually become unfit for consumption due to: • Spoilage (growth of organisms) • Changes in texture, smell, flavor or appearance • Undesirable processes caused by: • Change in water content • Chemical reactions • Exposure to light • Changes in temperature • Shelf life is the length of time a product can be stored without these undesirable changes occurring

19. Shelf Life • Change in water content: • Causes texture change • Loss of water increases exposure to air and thus oxidation • Causes rancidity and discoloration • Increase in water encourages microbial growth and spoilage • Chemical reactions/temperature change: • Increased temperature increases rates of “harmful reactions” • Changes in pH or temperature affect the amount of water in a food • Souring with decrease in pH • Changes color • Can decrease nutritional value • Light: provides energy for “harmful” chemical reactions

20. Shelf Life • Rancidity is a common type of food degradation • Unpleasant textures, smells, and flavors of fats and oils • Two types of rancidity: • Hydrolytic rancidity – ester bond in fat is broken yielding free fatty acids (reverse of formation of a fat) • Oxidative rancidity – oxygen reacts near the C=C double bonds in unsaturated fats

21. Shelf Life • Hydrolytic rancidity of fats and oils • reverse of fat formation, uses water (hydrolysis = “water” “splitting”) to split triglycerides back into glycerol and fatty acids • Encouraged by: • Lipase – an enzyme produced by microorganisms • Deep frying – encourages reaction of fats with moisture in food • Releases free fatty acids • 4 – 8 carbon fatty acids have a powerfully pungent aroma/flavor • palmitic, stearic, oleic, lauric acids give a soapy, fatty feel to foods

22. Shelf Life • Oxidative rancidity of fats and oils • Reaction of atmospheric oxygen with fats and oils initiates complex process producing highly reactive free radicals • Products include unpleasant smelling/tasting byproducts • More of an issue with increasing degree of unsaturation (more C=C double bonds to react with) • Encouraged by • the presence of light (photo-oxidation) • Enzymes produced by microorganisms

23. Shelf Life • Oxidative rancidity of fats and oils occurs in 3 steps: • Initiation by exposure to light – produces highly reactive hydrocarbon radicals (species with an unpaired electron) • R-H  R· + H· • Propagation – radicals produce other radicals • R· + O2 R-O-O· • ROO· + R-H  R-O-O-H + R· • Termination – radicals encounter one another and end the reaction • R· + R·  R-R • R-O-O· + R-O-O·  R-O-O-O-O-R • R· + R-O-O·  R-O-O-R (R-H = unsaturated fat or oil R· = hydrocarbon radical images on next slide)

24. Shelf Life • Initiation

25. Shelf Life • Propagation

26. Shelf Life • Termination

27. Shelf Life • Shelf life is prolonged by hindering spoilage processes • Packaging • Opaque or darkened packages block light • Can be gas impermeable to limit exposure to oxygen and water • Can be filled with inert gases or vacuum packed (no gases) • Storage • Low temperatures slow harmful reactions • Smoking or drying foods removes water and hinder microbial growth • Additives • Salt or sugar added to remove water and hinder microbial growth • KNO3 or NaNO3 salts are reducing agents and can prevent harmful oxidation reactions • Anti-microbial agents • Pickling - Organic acids and their salts (ex. benzoic acid and benzoate salts) make pH unfavorable for microbial growth • Fermentation – production of alcohol; hinders microbial growth

28. Shelf Life • Antioxidants delay oxidative degradation processes by reacting with oxygen to contain free radical formation • Can occur naturally: • Vitamin C (ascorbic acid) – citrus and green vegetables • Vitamin E (tocopherol) – nuts, seeds, grains, canola oil • Beta-carotene – carrots, broccoli, tomatoes, peaches • Selenium – shellfish, meat, eggs, grains • Foods high in natural antioxidants: green tea, blueberries, cranberries, dark chocolate, turmeric, oregano • Can be synthetic additives, but may have harmful side effects • BHA • BHT • PG • THBP • TBHQ these are all based arounda phenol group, which is not necessarily foundin natural antioxidants: (Full structures in data booklet)

29. Shelf Life • Types of antioxidants: • Free radical quenchers: • React with radicals to produce less reactive radicals (HA = quencher) R-O-O· + HA  R-O-O-H + A· • Chelating agents • Form very stable complex ions with transition metals (which can produce radicals) • Found naturally in rosemary, tea, and mustard • Salts of the organic acid EDTA are added as artificial chelating agents • Reducing agents • React with oxygen or hydroperoxides • Vitamin C or carotenoids are natural reducing agents

30. Color • Foods are colored by either pigments or dyes: • Pigments occur naturally • Dyes are added artificially and must be tested for safety • Dyes or pigments will absorb a range of light frequencies and reflect others • The color we see in a dye or pigment is the result of the colors of light reflected, not absorbed • Ex. chlorophyll in green leaf vegetables absorbs red and blue light, reflecting green • Molecular structures all involve extensive delocalized pi bonding (shown on next slide) • This system of delocalized pi bonding (conjugated system) is responsible for the color we see

31. Color • Some of the most common natural pigment groups are: • Anthocyanins • Ex. Cyanidin • Carotenoids • Ex. Beta-carotene • Chlorophyll • Ex. Chlorophyll-A • Heme • Ex. Heme B group β-carotene cyanidin chlorophyll-A heme B group

32. Color • Anthocyanins • Responsible for reds, pinks, blues in berries, beets, flowers (Flavonones, which give color to red grapes and berries, are closely related to anthocyanins) • 3-ring structures with varying numbers of OH groups in varying positions • Structure and therefore color is related to: • pH • predominantly red at low pH (acidic), blue at high pH (basic), and colorless/pale yellow at neutral pH • Temperature • May break down at high temperatures and cause browning • Exposure to metal • Form complexes with iron and aluminum and can change color • Color also affected when anthocyanins bond to sugars

33. Color • Color of anthocyanins is pH dependent (they are acid/base indicators):

34. Color • Carotenes • Responsible for orange, yellow, and red colors in foods like carrots, bananas, tomatoes, and saffron • Characterized by long hydrocarbon chains that often have carbon rings on the ends • Have nutritional value: • precursors to vitamin A (important in vision) • act as antioxidants • Relatively stable during food processing, but presence of C=C double bonds in hydrocarbon chain opens them up to oxidative degradation as seen in fats • Causes discoloration • Prevents carotenoids from being able to be converted to vitamin A

35. Color β-carotene

36. Color • Astaxanthin is a red pigment similar to carotenoids found in lobster, crab, and salmon • Bonds to proteins in the live animal and gives a blue/green color • When heated, bond to protein is broken which modifies the structure and frequency of light absorbed, appears bright red • Also an antioxidant

37. Color • Chlorophyll • Green pigment responsible for plant photosynthesis • Found in green vegetables • Structure is centered around a porphyrin ring – a planar ring system with 4 nitrogen atoms surrounding a central metal atom (magnesium in the case of chlorophyll) • Two forms: chlorophyll A and chlorophyll B • In chlorophyll B, an aldehyde side chain replaces a methyl side chain • When cooked, plant cells release acids • Causes an H+ ion to replace magnesium atom in center of structure, causing a color change to an olive/brown • Opens the possibility for photodegradation (breakdown by light)

38. Color (replaced with an aldehyde in chlorophyll B) chlorophyll A

39. Color • Heme • Pigment found in red blood cells • Structure is similar to chlorophyll, but there is an iron atom in the center of the porphyrin ring • Found in the protein myoglobin, responsible for oxygen transport • Bright red when bound to oxygen, but slow process of autoxidation converts iron from +2 to +3, changing color to brown (less desirable) (myoglobin metmyoglobin) • Color change can be prevented by vacuum packing or packing with an inert gas like CO2

40. Color heme group in myoglobin

41. Color • Color in foods is a result of the structures seen in anthocyanins, carotenoids, chlorophylls, and hemes • All of these structures include an extensive network of delocalized pi-bonds • Alternating single and double bonds is called a conjugated system • The greater the extent of pi bond delocalization, the closer together in energy bonding and antibondingorbitals become • All molecules absorb light energy as it promotes e-s between molecular orbitals (bonding and antibondingorbitals) – energy required is proportional to distance between orbitals (less distance = less energy) • As orbitals are close together in a conjugated system, low energy light (in the visible region) is absorbed for e- promotion • Color in these groups is the product of visible light absorption - color we see is complementary to the color(s) absorbed • Molecules without an extensive delocalized pi bond network (conjugated system) lack color, as orbitals are further apart and only higher energy light promotes e-s (ex. the anthocyanincarbinol)

42. Color • Some pigments are water soluble because the structures contain a large amount of -OH groups (H-bonding) • Anthocyanins • Some pigments are fat soluble (i.e. not water soluble) because their structures contain little or no -OH groups (no H-bonding) • carotenoids

43. Color • Many synthetic dyes are biochemically active and could be potentially harmful • Short-term toxicity easy to test and categorize, but long-term effects are more difficult to study • Use of dyes in foods must be regulated but regulations are not standardized internationally, posing issues in trade

44. Color • Cooking foods often leads to browning • Two processes responsible: • Maillard reactions – combination of sugars and proteins • Caramelization – dehydration of sugar • Both involve removal of water molecule(s)

45. Color • Maillard reactions: • Sugars and proteins within the food combine in a condensation reaction: • Aldehyde group in sugar (O atom) combines with amino group in protein (2 H atoms) • Initial condensation products polymerize to form brown-colored melanoidins • Maillard reactions only happen > 140° C • Rate depends on amino acid present (ex. lysine reacts faster than cysteine) initial condensation product

46. Color • Caramelization: • Happens in foods high in carbohydrates • Sugars dehydrate (lose H2O) at high temperatures, leaving behind C (dark brown color) • Browning intensifies the longer food is cooked, eventually burning it (pure carbon is black) • Rate depends on: • Sugar type (fructose in fruits caramelizes very quickly) • pH – extremes (high and low) promote caramelization

47. Genetically Modified Foods • Produced when organisms with modified DNA are used in food production • Used to: • Provide pest or disease resistance: • Bt corn: contains toxin from bacillisthuringiensisthat kills insect pests • Fungal-resistant potatoes • Nematode-resistant bananas (nematode = worm) • Improve quality and range of crop • Development of higher-yielding rice varieties • Development of corn that can grow in dryer environments • Produce medicines or other products in large quantities • Use of chickens that have been modified to lay eggs containing human interferon (combats tumors and viruses) • Use of cows to produce milk rich in omega-3 fatty acids (polyunsaturated fats essential to growth, development, brain function) Moth larva 

48. Genetically Modified Foods • Drawbacks: • Are GM foods safe? • Will GM food production alter the natural ecosystem? • Do we understand enough about genetic modification? • GM foods: • Can cause allergic reactions in some people • Have a slightly different composition from natural foods – alters diet • Produce altered pollen – might escape and cross with natural species • Long-term effects might be catastrophic, unknown so far

49. Texture • Food texture is related to physical properties: • Hardness • Elasticity • Viscosity • These properties can be altered by: • Cooking • Use of dispersed systems • Dispersed system = a stabilized, macroscopically homogeneous mixture of two immiscible phases • (this means two substances that would not ordinarily mix “appear to” on a macroscopic level (although at the molecular level they are still separate)

50. Texture • Many types of dispersed systems, name depends on the physical states of the substances mixed • Liquid/Solid or Solid/Liquid: • Solid particles suspended in a liquid is called a suspension • Ex. Blood (blood cells suspended in plasma) • Liquid dispersed throughout a solid medium is called a gel • Ex. Fruit Jelly (water trapped in a solid protein matrix) • Liquid/Liquid: • Stable blend of two liquids that don’t mix is called an emulsion • Ex. Mayonaisse (oil droplets suspended in aqueous system) • Liquid/Gas or Gas/Liquid: • Gas bubbles trapped in liquid medium is called a foam • Ex. Whipped cream or egg whites • Liquid droplets suspended in gas are called aerosols • Ex. Aromas from food carried through the air