Food Chemistry
Chapter 17 in Green / Damjii
Chapter X in Nuess
Chapter X in Oxford Study
(no resource in Chang)
F.3: Shelf Life
Homework
• Read F3 – Shelf Life - pp. 477-481
• Do Qs 12-23
• on p 491-492
F.3.1:
Explain the meaning of the
term shelf life.
Length of time a product can be stored without
degradation* of the food item to the point that
it is undesirable or unfit for consumption.
* change in flavor, smell, texture, and appearance or growth of
organisms
F.3.2: Discuss the factors that affect
the shelf life and quality of food.
1. CHANGE in WATER CONTENT
•
Moist foods lose water
•
•
•
Food becomes dry
Change in texture
Inc oxidation due to inc exposure to air – this …
–
–
–
•
dec nutrient value
Discoloration of surface
Rancidity
Dry foods gain water
•
•
•
Food becomes moist
Change in texture
More vulnerable to microbial degradation
F.3.2: Discuss the factors that affect
the shelf life and quality of food.
2. Chemical Reaction
• Chemical changes within food may produce
•
•
•
•
pH changes (lead to sour taste)
formation of other chemicals with undesirable flavors
color changes
decrease in nutritional value
3. Light
• Provides energy for photochemical reactions
•
•
•
Rancidity
Color fading
Oxidation of nutrients – esp vitamins
F.3.2: Discuss the factors that affect
the shelf life and quality of food.
4. Temperature
• Inc in temp = inc in reaction rate
5. Combination of factors
• Inc in temp and/or change in pH
tends to disrupt forces (bonds & IMFs)
holding protein and carbohydrate groups
together… this tends to reduce the amount of
water that type of food can hold… which then
impacts texture, softness, etc.
F.3.3: Describe the rancidity of fats.
Rancidity: development of unpleasant
smells in fats and oils… often accompanied
by changes to texture and appearance.
Can be…
1) Hydrolytic
2) Oxidative
F.3.4: Compare the processes of
hydrolytic and oxidative rancidity in lipids.
1) Hydrolytic
• Break ester bond - requires
− Lipase (an enzyme) [often produced by microorganisms]
− Heat [i.e. deep frying]
− Moisture (water)
• Forms fatty acids with unpleasant odors / taste
–
–
Butanoic acid ; hexanoic acid; octanoic acid
In milk / butter
• Forms fatty acids that change texture
–
–
palmitic acid ; stearic acid; oleic acid
in chocolate / cocoa butter
F.3.4: Compare the processes of hydrolytic
and oxidative rancidity in lipids.
2) Oxidative (auto-oxidation)
• Reaction of carbon-carbon double bond with
oxygen from air… using a free radical mechanism.
• This requires…
− Unsaturated lipid (i.e fish oils)
− Oxygen
• Is enhanced by…
–
–
Light (helps produce the free radicals)
Enzymes (usu produced by micro-organisms)
• Forms a wide variety of products – many with
unpleasant odors / taste
F.3.4: Compare the processes of hydrolytic
and oxidative rancidity in lipids.
F7.1:
Describe the steps in the free-radical chain
mechanism occurring during oxidative rancidity.
Mechanism for Oxidative Rancidity: (p 479)
• Initiation (w/ light; homolytic fission)
•
Propagation
•
Termination
F.3.5: Describe ways to minimize the rate of
rancidity and prolong the shelf life of food.
1) PACKAGING
• Opaque, colored glass bottles
–
reduce exposure to light
• Gas-impermeable wrapping film
–
reduce exposure to oxygen and water
• Vacuum packaging
–
reduce exposure to oxygen and water
• fill package with inert gas
– i.e. nitrogen
F.3.5: Describe ways to minimize the rate of
rancidity and prolong the shelf life of food.
2) STORAGE
•
Refrigeration
–
•
In the Dark – limit access to light
–
•
reduce rate of reactions
reduce rate of photo-oxidation
Reduce Water Level by Drying or Smoking
–
–
reduce rate of hydrolytic rancidity
discourage growth of micro-organisms
F.3.5: Describe ways to minimize the rate of
rancidity and prolong the shelf life of food.
3) ADDITIVES
•
Salting reduces water content (osmosis)
–
•
High Sugar content reduces water content (osmosis)
–
•
(preserves)
Sulfur dioxide & Sodium Sulfites are reducing
agents that prevent oxidation reactions that cause
browning
–
•
(bacon)
(fruit products)
Sodium and Potassium Nitrites and Nitrates are
reducing agents that prevent oxidation reactions
–
(curing meats)
F.3.5: Describe ways to minimize the rate of
rancidity and prolong the shelf life of food.
4) ANTI-MICROBIAL AGENTS
•
Discourage growth of micro-organisms in food
•
•
Pickling – vinegar makes environment too acidic for
micro-organisms
Organic acids and their salts
–
–
•
Benzoic acid / sodium benzoate added to fruit juices
Propanoic acid / sodium & calcium salts added to bread and
cheese
Fermentation produces ethanol which limits growth of
bacteria
– Wine keeps better than juice; distilled spirits keep
better than wine (higher % alcohol)
F.3.6: Describe the traditional
methods used by different cultures to
extend the shelf life of foods.
•
•
•
•
•
•
Fermentation
Pickling
Salting
Preserves
Drying
Smoking
F.3.6: Describe the traditional methods used by
different cultures to extend the shelf life of foods.
• Fermentation
– Beer
 “beer is stabilized…beer is one of the first beverages identified
that does not readily spoil…beer is stabilized by the
fermentation process…beer becomes resistant to action by
many other microorganisms…particularly true when air is
excluded from the beer by storage in air-tight containers.”
– Cheese
 Fermentation can lower pH to a level that inhibits bacterial and
enzyme activity… for instance, in the production of cheese,
lactobacilli are added as a starter to change the lactose (milk
sugar) into lactic acid… the resulting lower pH inhibits further
deterioration
F.3.6: Describe the traditional methods used by
different cultures to extend the shelf life of foods.
• Pickling
– Gherkins, Onions
 Addition of vinegar retards growth of bacteria and fungi
 Vinegar = %5 ethanoic acid… undissociated acid is responsible for
inhibition of microbial growth
– Yogurt
 Contains lactic acid
• Salting
– Salt is added to food (one of the oldest methods of preserving)
 Salt dissolves… makes solution… bacteria in contact with
solution lose water to the solution via the cell membrane
through osmosis… cells dehydrate… incapable of cell division
F.3.6: Describe the traditional methods used by
different cultures to extend the shelf life of foods.
• Preserves
– Jams, Condensed milk
 high concentration of sugar to a food substance
 Sometimes the surface of these foods collects additional
water, reducing the concentration of the sugar, allowing
the growth of molds
• Drying
– Herbs, fruits, meats, and vegetables
 Removes water… one material that tends to reduce shelf
life
F.3.6: Describe the traditional methods used by
different cultures to extend the shelf life of foods.
• Smoking
– Meats, Fish
 Warm, smoke-containing air is passed over food and food gets
covered with condensed tar (mostly phenol and aldehydes)
 Wood smoke contains well over 200 compounds – including ethanol,
ethanal, methanol, methanal
 methanal is the major microbial
• Irradiation
– Crops, processed foods
 Beta and gamma radiation are used to irradiate food
 Inhibit deterioration processes, destroy bacteria and insects
 May also destroy vitamins and denature proteins
 May produced undesirable changes and impact eating quality
F3.7: Define the term antioxidant.
Antidoxidant – substance that can be added
to food to increase its shelf life by delaying the
onset of oxidative degradation.
React with oxygen-containing free radicals…
preventing degradation
F.3.8: List examples of common naturally
occurring antioxidants and their sources.
Antioxidant
Source
Vitamin C (ascorbic acid)
Citrus fruits, green leafy vegetables, green
peppers, broccoli, strawberries, raw
cabbage, potatos
Vitamin E (α-tocopherol)
[main lipid soluble antioxidant]
Carrots, squash, broccoli, sweat potatoes,
tomatoes, kale, cantaloupe, peaches,
apricots, nuts, seeds, soy beans, whole
grains, some vegetable oils (canola oil)
Β-carotene
Vegetables – carrots & broccoli
Fruits – tomatoes & peaches
Selenium
Trace element found in fish, shellfish,
meat, eggs, chicken, garlic and grains
F.3.11:
List some antioxidants found in the
traditional foods of different cultures that may have
health benefits.
Other foods rich in anti-oxidants include:
• green tea
• Turmeric
• oregano
• Blueberries
• Cranberries
• dark chocolate
F.3.11:
List some antioxidants found in the
traditional foods of different cultures that may have
health benefits.
Other foods rich in anti-oxidants include:
• green tea
• Turmeric
• oregano
• Blueberries
• Cranberries
• dark chocolate
F.3.9: Compare the structural features of
the major synthetic antioxidants in food.
Common SYNTHETIC antioxidants include:
• BHA: 2- and 3-tert-butyl-4-hydroxyanisole
• BHT: 3,5-di-tert-butyl-4-hydroxytoluene
• PG: propyl gallate
• THBP: tertbutylhydroquinone
Almost all of these have
• Phenolic type structure – alcohol group (hydroxyl)
attached to benzene
• This structure is NOT found in natural anti-oxidants
F.3.10: Discuss the advantages and
disadvantages associated with natural
and synthetic antioxidants.
Natural Antioxidants
Advantages
Synthetic Antioxidants
May reduce risk of cancer and More effective than natural antiheart disease
oxidants
Some are precursors to other
important molecules… Vitamin
C for hormones and collagen ;
β-carotene for Vitamin A
May boost immune system
and combat aging and agerelated diseases
Diet high in fruit and
vegetables also means diet
high in fiber and lower in fat -
F.3.10:
Discuss the advantages and disadvantages
associated with natural and synthetic antioxidants.
Natural Antioxidants
Disadvantages Less effective than synthetics
Can be expensive; may add
color and aftertaste to foods;
too much may actually be
harmful
Synthetic Antioxidants
Safety not yet ‘proven’
Are food additives… and therefore
require strictly enforced codes to
ensure safety … difficult to police in
developing countries
High levels of antioxidants
may lead to a bitter taste (i.e.
some green leafy vegetables)
Excessive suppression of oxidation
may not be healthy as some toxic
free radical mechanisms are used by
some cells in the immune system
F.8.1:
Explain the differences between the three
main types of antioxidants.
1) Free Radical Quenchers
 Species that react with free radicals to produce
less reactive free radicals
•
Natural free radical quenchers
–
•
Vitamin E (tocopherol)
Synthetic free radical quenchers
–
–
–
BHA
TBHQ
BHT
F.8.1:
Explain the differences between the three
main types of antioxidants.
2) Chelating Agents
 Form stable complex ions with transition metal
ions… reducing the opportunity for the transition
metals to react with hydroperoxides to form
more radicals
•
Many plants contain natural chelating agents
–
•
Rosemary, tea, mustard
Synthetic chelating agent
–
Salts of EDTA (ethylenediaminetetraacetic acid)
F.8.1:
Explain the differences between the three
main types of antioxidants.
3) Reducing Agents (electron donors)
 react with oxygen in food
–
decreasing oxidation of food
 react with hydroperoxides initially produced by
auto-oxidation – thereby decreasing free radicals
•
natural reducing agents
–
•
ascorbic acid & carotenoids
synthetic reducing agents
–
sulfur dioxide, sulfites, nitrites