Nivaldo J. Tro

http://academic.cengage.com/chemistry/tro

Deborah Koeck • Texas State University, San Marcos

Chapter 18The Chemistry of Food

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

• 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

Simple Carbohydrates

Complex Carbohydrates• Most common complex carbohydrates (polysaccharides) are

starch and fiber. • Starch has alpha linkages.• Digestible

• 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.

Complex Carbohydrates

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 SummaryMolecular 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.