Download Chemistry in Focus 3rd edition Tro

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.