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Nivaldo J. Tro http://academic.cengage.com/chemistry/tro Chapter 18 The Chemistry of Food Deborah Koeck • Texas State University, San Marcos You Are What You Eat • Our body parts are ultimately composed of atoms borrowed from food. – We use these atoms for about 75 years and then return them to the environment. You Are What You Eat • Between the mouth and the rectum, food is plundered of its nutritionally valuable molecules. • Some molecules are oxidized to meet the body’s energy needs, while others are rearranged to make the necessary structures and components of the body. Carbohydrates • Sugars, starches, and fibers in food • Simple carbohydrates are easily and efficiently transported in the bloodstream. – Remember glucose, fructose, and sucrose from Chapter 16 – Lactose: Milk sugar Simple Carbohydrates • Glucose – Exclusive fuel of the brain – 4 Calories of energy per gram • Fructose – Sweetest of all sugars Simple Carbohydrates • Sucrose – Digested to glucose and fructose, then into bloodstream • Lactose – Digested to glucose and galactose – Lactose intolerance • Lack of lactase, the enzyme that breaks down lactose • Causes discomfort, flatulence, diarrhea Complex Carbohydrates • Most common complex carbohydrates (polysaccharides) are starch and fiber. • Starch has alpha linkages. • Digestible Complex Carbohydrates • Fiber has beta linkages – Indigestable • Fiber increases food bulk without adding significantly to caloric content. • Good for the digestive tract, but can be overdone • Carbohydrates should compose 45–64% of total caloric intake. Proteins • The body produces only half of the amino acids it needs. • The remaining come from food. – Meat, cheese, eggs, milk, grains, legumes, and nuts – During digestion, proteins are cut into amino acid components, which then enter the bloodstream. • Cells use amino acids to construct their own proteins. • Proteins can be metabolized at 4 Calories per gram, but the body does this only as a last resort. Workhorse Molecules • Physical structure, enzymes, hormones, oxygen transport, and antibodies • Proteins need ALL amino acids for proper functioning. – Must be obtained in the right proportions in the diet – Should compose 30% of total caloric intake • Complete proteins – Meat – Proper combinations of legumes and grains or nuts Fats and Oils • Dietary sources are primarily triglycerides. • From Chapter 16 – Saturated fatty acid side chains • Solids • From meats – Unsaturated side chains • Liquids • From plants • Sources: Meat, vegetable oils, butter, cheese, cream, egg yolks, ice cream Fat Metabolism • Slower for fats than for carbohydrates • Lingering sense of fullness after ingestion • Reassembled to triglycerides before entering bloodstream – Nonpolar – Need lipoproteins to be carried in bloodstream • Transported for dismantling and reassembly into other fats – Used for energy directly (9 Calories per gram) – Stored in fat cells Cholesterol • Nonpolar compound found in animal foods • Integral part of hormone synthesis – Liver makes it • Excessive amounts lead to deposition on arterial walls. – Arteriosclerosis • Blockage leads to heart attack and/or stroke Lipoproteins • Carry cholesterol in the bloodstream • Classified by their density • Low-density lipoproteins (LDL) – Transport cholesterol from the liver – Tend to deposit cholesterol on arterial walls • High-density lipoproteins (HDL) – Transport cholesterol to the liver – Tendency to reduce cholesterol deposition • Fats should compose less than 20% of total caloric intake. Caloric Intake and the First Law • Food provides energy for our bodies. • It must either be used or stored. • According to the first law of thermodynamics: Energy intake (food) = energy expended (used) + energy stored (fat) Energy Intake Energy Expended • Energy required to stay alive – Basal metabolism – Heart pumping, body temperature maintenance, and breathing – 0.5 Cal/hour per pound of body weight • Energy associated with exercise – Varies according to activity level Calories Expended Energy Stored • If energy intake exceeds energy used, the remainder is stored as fat. • Every 3500 Cal of excess intake means storage of 1 lb of fat. – Fat is energetically dense. • For every 3500 Cal energy output, the body burns 1 lb of fat (as long as fat is available). Vitamins • Essential in the diet, but have little to no caloric value • Helpers in cell functions • Nonpolar or fat soluble – Vitamin A, vitamin D, vitamin E, vitamin K • Polar or water soluble – Vitamin B, vitamin C Fat-Soluble Vitamins: Vitamin A • Vision, immune defense, and maintenance of body lining and skin • Part of retinal pigments that absorb visible light • Liver, fish liver oils, milk, butter, eggs • Beta carotene in plant pigments – Carrots, squash, tomatoes, cantaloupe • Too much can have detrimental effects. Fat-Soluble Vitamins: Vitamin D • Promotes absorption of calcium into the blood to be used for bone structure • Body can synthesize vitamin D with sun exposure. • Milk is vitamin D-fortified for children. • Too much vitamin D can cause deposition of Ca in soft body tissue. • Vitamin D deficiency causes rickets, which results in bone deformities. Fat-Soluble Vitamins: Vitamin E • Antioxidant; prevents oxidative damage, especially to cell membranes • Widespread presence in food – Deficiencies are rare • Low toxicity Fat-Soluble Vitamins: Vitamin K • Necessary for the synthesis of four proteins involved in blood clotting. • Present in leafy green vegetables and milk; synthesized by intestinal bacteria – Infants lack this bacteria, so they are given vitamin K at birth. • Overuse can result in blood clotting and subsequent brain damage. – Available only by prescription Water-Soluble Vitamins: Vitamin C • • • • • Synthesis of connective tissue (collagen) Protection from infection Absorption of Fe Antioxidant Deficiency is called scurvy – Common among early sailors – “Limeys” Water-Soluble Vitamins: Vitamin C • Citrus fruits, leafy green vegetables, cantaloupe, peppers, tomatoes, mangoes • Water soluble – Excess is easily excreted. • Overuse can lead to nausea and cramping B Complex Vitamins • Thiamine, riboflavin, niacin, vitamin B6, folate, and B12 • Central role in metabolism, protein synthesis, and cell multiplication • Spread through all food groups – B complex-containing grain hulls are removed during processing. – Milled grain frequently enriched • Deficiency results in irritability, depression, abnormal heart action, and skin problems Minerals • Elements other than carbon, hydrogen, nitrogen, and oxygen that are needed for good health • Many are present in the body as ions rather than neutral atoms. The Major Minerals • Compose about 4% of the body’s weight • Calcium – Structural material for bones and teeth, nerve signal transmission, and blood clotting – Persistent deficiency leads to osteoporosis • Phosphorus – Bound with Ca in bones and teeth, energy metabolism, and DNA • Sodium – Body fluid level regulation – Persistent excess leads to hypertension • Potassium and magnesium – Electrolyte balance in and around cells The Minor Minerals • Present in trace amounts • Iodine – Involved in regulation of basal metabolic rate – Deficiency results in goiter • Iron – Composes a critical part of hemoglobin – Deficiency leads to anemia • Zinc – Enzyme functioning • Others – Selenium, chromium, and copper Food Additives • Long history (salt, sugar, vinegar) • Shift from rural to urban lifestyles requires preservation, packaging, and shipping • All additives regulated by FDA (25¢ of every consumer dollar spent) • GRAS: Generally Recognized as Safe – Salt, sugar, spices, sodium benzoate, and EDTA – New additions: Simplesse and Olestra • Five categories of additives exist. Antimicrobial Agents • Added to prevent the growth of bacteria, yeasts, and molds – Botulism: A few nanograms will kill an adult • Salt (meat and fish) and sugar (jams and jellies) – Dehydrating conditions • Sodium benzoate, potassium sorbate – Canned, bottled, and packaged foods • Sodium nitrite – Makes meat pink – Inhibits growth of bacteria (including the one responsible for botulism) – Recent scrutiny Antioxidants • Added to prevent oxidation • Unsaturated oils will oxidize in air – Products are volatile aldehydes, ketones, and acids – Foul odors and flavors—rancid • Common antioxidants are vitamin C, BHA, and BHT – Laboratory animals had lower cancer rates and longer lifespans than those without antioxidants in their diet. • BHA and BHT can be toxic. • Sulfites – Wine and fruits • EDTA – Immobilizes metal ions that often catalyze oxidation Fruit and Fruit Juice Oxidation • Fruits and fruit juice oxidize • Foul smells can develop • Brown color develops Artificial Colors • Added to improve appearance • Natural plant pigments • Synthetic dyes – Use is purely aesthetic – Red dye no. 3 – Delaney Clause • Forbids the addition of any substance shown to cause cancer in animals, regardless of dose. Artificial Flavors and Flavor Enhancers • Artificial flavors – Can be natural or synthetic versions of the same molecule – Sweeteners • Sugar, corn syrup, aspartame – Plant flavors • Wintergreen, peppermint, ginger, vanilla, almond extract • Flavor enhancers – No flavor of their own, enhance flavors in general – MSG (monosodium glutamate) Stabilizers • Added to improve and preserve the physical characteristics of food • Humectants – Added to keep food moist • Anti-caking agents – Added to powders to keep them dry and granulized • Emulsifiers – Added to keep polar and nonpolar ingredients mixed Molecules Used to Grow Crops • • • • Sunlight Water Carbon dioxide PHOTOSYNTHESIS: – Light + 6CO2 + 6H2O C6H12O6 + 6O2 • Plants need other elements from the soil to synthesize lipids, nucleic acids, and proteins. Primary Nutrients • Fields that get repeated use to grow crops become depleted of some of the necessary elements. • They must be replaced by fertilizers. Potassium • Used by plants in its ionic form, K+ • Chemical potassium fertilizers – Potassium chloride: KCl – Potassium sulfate: K2SO4 • Mineral deposits of potassium are located in Germany, Russia, and Canada. Nitrogen • Used by plants for the synthesis of amino acids • Normally absorbed as ammonium ions (NH4+) or nitrate ions (NO3-) – Can’t be used in its elemental form • Fixed nitrogen – Crop rotation • Soybeans and peas grow nitrogen-fixing bacteria in their roots. – Nitrogen fertilizers • Haber process (3H2 + N2 2NH3) • Urea, ammonium nitrate, ammonium sulfate Phosphorus • Absorbed in ionic forms, as phosphates – H2PO4– HPO4– Made by treating phosphate rock (calcium phosphate with sulfuric acid) to form superphosphate Fertilizer • Most fertilizers are numbered to indicate the percentage of nitrogen, phosphorus (as P2O5), and potassium (as K2O). – For example: 5, 15, 5 Secondary Nutrients • Secondary nutrients – Calcium, magnesium, and sulfur • Micronutrients – Boron, chlorine, copper, iron, manganese, molybdenum, sodium, vanadium, and zinc Molecules Used to Protect Crops • Used to control pests and weeds that carry disease, damage crops, or provide a nuisance • One third of the world’s total crops are destroyed by pests or weeds. • Insecticides • Early ones were highly toxic (Hydrogen cyanide) • DDT and HCB came next – Stunning successes – Relatively nontoxic to humans Molecules Used to Protect Crops • Bioamplification – Levels of DDT contamination concentrated as DDT moved up the food chain – American bald eagle almost driven to extinction – The chemical stability of chlorinated hydrocarbons allowed them to accumulate in soil and water supplies. Modern Insecticides • Organophosphates – Malathion and parathion – Broad-spectrum – More toxic to humans and pests than chlorinated hydrocarbons – BUT they degrade quickly in the environment • Carbamates – Carbaryl and aldicarb – Narrow-spectrum – Toxic but degradable Herbicides • Used to kill weeds and unwanted plants that compete with and dominate crop species • 2,4,5-T and 2,4-D are defoliants Agent Orange • Agent Orange is a 1:1 mixture of 2,4-D and 2,4,5-T • Defoliants are banned due to the unavoidable presence of dioxin as a contaminate. – Dioxins are both toxic and carcinogenic. Triazine Herbicides • Atrazine – Destroys weed in cornfields – Low toxicity to mammals • Metolachlor – Soybean and corn crops – No accumulation/bioamplification • Paraquat – Marijuana – No accumulation/bioamplification Chapter Summary Molecular Concept • Carbohydrates • Proteins • Fats/oils • Fat-soluble vitamins • Water-soluble vitamins • Antioxidants Societal Impact • The average North American diet is higher in protein and fat than the ideal diet. • The North American diet is often high in caloric content, so many North Americans have a tendency to be overweight.