Transcript Resource, Materials and Environment

Resource, Materials and
Environment
Lesson 2: Food Chemistry
Food Components
Protein: Amino acids, Peptide and
Polypetide
Carbohydrates
Fats and oils
Food Additives
Food Production
Proteins
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proteins are
essential parts of all
living organisms
Amino acids
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Building blocks for protein
Properties of amino acids
Differentiation of amino acids,
techniques
Chromatography and electrophoresis
Peptide bond
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Peptide group
Dehydration synthesis (condensation)
reaction forming a peptide group
Peptides are synthesized by coupling the
carboxyl group or C-terminus of one amino
acid to the amino group or N-terminus of
another.
Properties of amino acids
Due to the co-existence of the NH3+and the COO-,
amino acids exhibit amphoteric behaviour, for
instance alanine reacts both with acids and bases;
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There are
twenty
standard amino
acids proteins
by living cells.
Different
sequencing of
amino acids
results in
different type
of proteins.
Properties of amino acids
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Amino Acid is slightly acidic. For
alanine, the following reaction
occurs in pure water:
Properties of amino acids
Isoelectric point
The pH at which an amino acid carries no
net electric charge, this quantity can be
determined by an analytical technique
called electrophoresis
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Isoelectric Point
Separation of amino acids
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Electrophoresis
Based on the mobility of ions in an electric field.
Separation of amino acids
Thin Layer Chromatography (TLC)
Useful for separating organic compounds.
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Stationary Phase
A static phase, usually a solid or a liquid
adsorbed on a solid supported
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Separation of amino acids
Mobile phase
A dynamic phase, the components to be separated
is carried through the stationary phase by the
mobile phase
Mobile
phase
Gas
Stationary
phase
Liquid
name
abbreviation
Gas-liquid
GLC
Liquid
Solid
Liquid-solid LSC
Gas
Solid
Gas-solid
GSC
Joining amino acids: Formation of
Peptide
Condensation reaction through
elimination of water molecules::
CH3CH(NH2)COOH
alanine +
H2NCH2COOH
glycine →
CH3CH(NH2)=OC-NHCH2COOH+H2O or
H2NCH2=OC-NHCH(COOH)CH3+H2O
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Which one yields more?
Peptide bond
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Partial double bond character
C-N bond length(1.32 A。 ) is shorter
R groups are arranged trans to each
other to prevent repulsion
Rotation is restricted about the C-N
bond
Peptide bond
Peptide bond
Peptide bond
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Unit, Residue: Each amino acid in the
peptide
Dipetide: form from two units
Tripeptide: formed from three units
Polypeptide: any difference from a
protein?
Structure of protein
Primary structure
 Secondary structure:
Long chains of amino acids will commonly
fold or curl into a regular repeating
structure.
Structure is a result of hydrogen bonding
between amino acids within the protein.
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Protein as a polymer
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Resonance
structures of the
peptide bond that
links individual
amino acids to form
a protein polymer.
A protein is
polypeptides more
than about 50 amino
acids long.
Structure of protein molecule
Different reconstructed illustrations of protein molecule
Primary structure of Protein
Here is an example sequence of amino acids in a
protein.
It also shows the abbreviations commonly used.
Secondary Structure
Common secondary structures are:
a-helix
β-pleated sheet
 Secondary structure adds new
properties to a protein like strength,
flexibility…
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a-helix
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High strength
Low solubility in
water
Proteins (a-helix example)
β-pleated sheet
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Silk fibroin
Stack like
corrugated
cardboard for extra
strength
β-pleated sheet (Secondary
structure)
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Held together by hydrogen bonding between adjacent
sheets of protein
Effect of temperature and pH on
proteins
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Both will alter the 3-D shape of a
protein if you go beyond a ‘normal’
range.
Disorganized protein will no longer act
as intended – denatured.
They will clump together – coagulate.
Example: frying in an egg, reason for
HCl in stomach
An example of Tertiary Structure:
Collagen
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About one third of all protein in humans
Provide strength to bones, tendon, skin, blood
vessels.
Form triple helix-tropocollagen
Collagen and Vitamin C
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Major use of Vitamin C is for making collagen
Scurvy – disease from lack of Vitamin C results
in skin lesions, bleeding gums and fragile blood
vessels.
Protein hormone examples
Carbohydrates
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Carbo (carbon)+ hydrate (water)
Made up of C,H,O moleucles
Can be classified into monosaccharides,
disaccharides and polysaccharides
Carbohydrates are the most abundant
biological molecules, and fill numerous roles
in living things, such as
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storage and transport of energy (starch, glycogen)
structural components (cellulose in plants, chitin in
animals).
Carbohydratesmonosaccharides (I)
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Better known as “simple sugar”
Has reducing power
Common examples are glucose,
galactose and fructose
Source of energy in our body
Carbohydratesmonosaccharides (II)
Glucose molecule
Disaccharides (I)
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Form by dehydrating reaction between
two molecules of monosaccharides
Has the general formula of C12H22O11
Disaccharides (II)
Sucrose (table sugar)
(glucose+fructose)
Lactose
(sugar in milk)
(glucose+galactose)
Maltose
(glucose+glucose)
Polysaccharides
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Polysaccharides have a general formula
of Cn(H2O)n-1 where n is usually a large
number between 200 and 2500. The
general formula can also be represented
as (C6H10O5)n where n=100-3000.
Examples include starch, glycogen,
cellulose, chitin.
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Starch in flour and
bread is an example
of polysaccharides
Fats and fatty acids
Oil
Butter
Fats and fatty acids
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Fats and oils provide our body with energy
and essential fatty acids (carboxylic acids)
R,R’,R’’ are hydrocarbon chains
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Esters of propane-1,2,3-triol and fatty acids
Hydrolysis of fats
Reaction with sodium
hydroxide
Fatty Acid Structure
Long chain monocarboxylic acids
CH3(CH2)nCOOH
Size range:C12C24
Always an even number of carbon
Saturated: no double bonds
Unsaturated: one or more double bonds
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Hydrogenation of unsaturated
fatty acids
Hydrogenation
RCH=CHCH2CH2COOH—(H2,Ni)→
RCH2CH2CH2CH2COOH
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Used to convert unsaturated vegetable oils to
margarine
Unsaturated fatty acids: an
example
Elcosanoids
 All are unsaturated
 All have twenty carbons
 Some are Essential Fatty Acids
 Can’t be produced by the body
 E.g. linolenic acid and linoleic acids
Iodine value
• Unsaturated fat is considered desirable in the
diet
• A quantitative measure of the degree of
unsaturation in fats and oils
• Based on the reaction:
-CH=CH- + I-I →-CHI-CHI-
Defined as the number of grams of iodine that
reacts with 100 grams of fats/oils
The higher the value is, the greater the degree
Of unsaturation in the fat or oil.
Fats / Oils
Iodine Values
Butter 牛油
25-30
Dripping 肉油
35-65
動物脂肪
Lard 豬油
45-65
Vegetable oils
Coconut oil 椰子油
8-10
棉子油及豆油
Cotton seed oil and soya oil
80-140
Ground-nut oil 花生油
85-105
Olive oil 橄欖油
80-90
Almond oil 杏仁油
90-110
Corn oil 粟米油
115-130
Animal fats
植物油
Some common fatty acids
Common
IUPAC Name
MP
Formula
Lauric月桂酸
n-dodecanoic
44
C11H23COOH
Palmitic棕櫚酸
n-hexadecanoic
63
C15H31COOH
Stearic硬脂酸
n-octadecanoic
70
C17H35COOH
Palmitoleic棕櫚油酸
Cis-9-hexadexenoic
0
C15H29COOH
Oleic油酸
Cis-9-octadecenoic
16
C17H33COOH
Linoleic亞油酸
Cis,cis,9,12octadecadienoic
5
C17H31COOH
Presence of double bonds reduces melting point.
Food Additives
Chemical added to preserve and/or
improve the appearance of food
 Several Classes:
Colourings, Antioxidants, Flavour
Enhancers, Preservatives, Sweeteners,
Emulsifiers, stabilizers, and thickeners,
and etc.
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Principles of food preservation
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Removal of moisture
Altering temperature
Changing pH value
Use of osmotic process
Use of chemical additives
Some Food Preservation
Techniques
Irradiation
Expose food to ionizing radiation
Drying and
Dried in the sun or in special ovens or
Dehydration freeze driers
Refrigeration In chilling, kept at 0-4oC
Canning
Salting
Pickling in
vinegar
Use of food
additives
Cooked under pressure in hermetically
sealed container
Treated with salt or strong salt solution
Kept in vinegar
Substances which control microorganisms and chemical spoilage
Some classes of food additives
Preservatives Prevent microbial growth and spoilage
Antioxidants
Prevent rancidity of fats and oils
Emulsifier,
stabilisers,
and thickener
Anticaking
agents
Humectants
Give texture, blend, smoothness and other
consistencies; stabilize oil-water mixtures.
Leavening
agents
Bleaches
Make food light in the texture
Keep food fast flowing, prevent caking in
humid weather
Retain moistures
Confer white colour to food
Some common food additives
Some common flavourings
Flavour
Ginger
Grape
Lemon
Orange
Pear
Spearmint
Vanilla
Spicy
Food Additive
Ginger oil
Methyl anthranilate
Citral
Orange oil
Amyl butyrate
Carvone
Ethyl vanillin
Ethyl cinnamate
E numbers of food additives
Type of additive
E number
Colourings
Most begin with 1
Preservatives
Most begin with 2
Flavourings
Not numbered
Antioxidants
300-321
Emulsifiers and stabilisers
Acids, bases and buffers
E322 and some numbers
between E400 and E495
Most begin with 5
Sweeteners
Most begin with 4 or 6
Flavour enhancer
• MSG (Monosodium-L-glutamate monohydrate)
• Chemically isolated by a Japanese in 1908
• Excess intake linked to with the symptoms known as
“Chinese Restaurant Syndrome”.
The need to
preserve food
Food
additives
Principles of food
preservation
Killing of microorganisms
Inhibition of
microbial
growth
Different
functions
Ratardation of
chemical
changes
Possible
menace
Monitoring and
legislation
Common food
preservation
techniques
Principle of
BHA/BHT as
antioxidant
Regulations on Color Additives in
US
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Food and Drug Administration (FDA)
responsible for regulation all man-made
color additives
Color additive certification as approval
process: assures the safety, quality,
consistency and strength.
Example: Caramel used in sauces, soft
drinks, and baked foods.
Regulations on Food Additives
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All foods have to be labeled with
ingredients.
In some countries, such as US, all foods
should be labeled with specific food
additives.
MSG has been given green light as a
food additive.
Wikipedia links
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Food chemistry
Carbohydrates
Protein
Lipid
Enzymne
Vitamins
Dietary minerals