5.1: The Chemist's View of Fatty Acids and Triglycerides

Lipids (3):

1. triglycerides

2. phospholipids

3. sterols

• characterized by: insolubility in water

• fat soluble vitamins

• composed of CHO (carbon, hydrogen, oxygen)

  • more carbons & hydrogens than oxygens

  • more energy per gram

Chemical Structure of Triglycerides: E

• 1 glycerol

• 3 fatty acids (chains of carbon atoms) attached

  • 4-24 (even numbers) carbons long

  • 18 carbon long ones are most common in foods and abundant in food supply

    • stearic acid: simplest of the 18 carbon fatty acids, bonds between its carbons are all alike, saturated fatty acid

  • saturated or unsaturated

    • 1 or more points of unsaturation: monounsaturated or polyunsaturated

  • 18 carbon polyunsaturated fatty acids:

    1. omega-3 fatty acids, linolenic acid

    2. omega-6 fatty acids, linoleic acid

       • essential fatty acids that the body cannot make

       • primary member of a family of longer chain fatty acids

         • regulate blood pressure, blood clotting, and other body health functions

Fatty Acids

• all have the same basic structure

  • one end: chain of carbon & hydrogen atoms with an acid group (COOH)

  • other end: methyl group (CH3)

• differ from one another

  • length of their carbon chains

  • number & location of their double bonds

Length of the Carbon Chain

• naturally occurring fatty acids contain even numbers of carbons in their chains

  • up to 24 carbons in length

Long chain (more than 12 carbons) fatty acids:

• meats, seafood, and vegetable oils most common in diet

Smaller amounts of medium chain (8-12 carbons) & short chain (up to 6 carbons) fatty acids:

• dairy products

The Number of Double Bonds

• saturated fatty acid:

  • fully loaded with hydrogen atoms

  • contains only single bonds between its carbon atoms

• every carbon must have 4 bonds

  • 2 carbons form 1 double bond, yielding oleic acid

• point of unsaturation:

  • the double bond of a fatty acid

  • hydrogens are missing

  • at least 1 double bond is an unsaturated fatty acid

  • there is an oxygen attached to a carbon instead of a hydrogen

• oleic acid:

  • 18 carbon monounsaturated fatty acid

  • abundant in olive and canola oil

• polyunsaturated fatty acid

  • 2 or more carbon-to-carbon double bonds

    • linoleic acid

    • linolenic acid

The Locations of Double Bonds

• fatty acids differ in:

  • length of their chains

  • degrees of saturation

  • locations of their double bonds

• chemists identify polyunsaturated fatty acids by:

  • position of the double bond closest to the methyl (CH3) end of the carbon chain (omega number)

  • omega-3 fatty acid: a polyunsaturated fatty acid in which the closest double bond to the methyl (CH3) end of the carbon chains is 3 carbons away

  • omega-6 fatty acid: a polyunsaturated fatty acid in which the closest double bond to the methyl (CH3) end of the carbon chain is 6 carbons away

Omega-3 and Omega-6 Fatty Acids Similarities and Differences

• similarities

  • location of the double bond closest to the methyl end

• differences

  • lengths and numbers of double bonds

• monounsaturated fatty acids

  • belong to omega-9 group

  • closest and only double bond 9 carbons away from the methyl end

  • oleic acid

    • most predominant monounsaturated fatty acid in the diet

Triglycerides

• triglycerides

  • lipids composed of 3 fatty acids attached to a glycerol molecule

  • made in a series of condensation reactions

    • combine hydrogen (H) from glycerol & hydroxyl group (OH) from a fatty acid

  • contain a mixture of more than one type of fatty acid

Characteristics of Solid Fats and Oils

• 3 chemical features of a fatty acid affect the characteristics of foods and the health of the body-

  1. short or long

  2. saturated or unsaturated

  3. closet double bond is at carbon 3 or carbon 6

Firmness

• degree of unsaturation influences the firmness of fats at room temperature

• vegetable fats

  • polyunsaturated liquids (oils)

  • cocoa butter, palm oil, palm kernel oil, and coconut oil are saturated

    • firmer than most vegetable oils because of their saturation

    • softer than animal fats because of their shorter carbon chains (8-14 carbons long)

• saturated animal fats

  • solid

• the shorter the carbon chain → the softer a fat is at room temperature

• at room temperature-

  • saturated fats → solid

    • fats found in butter and other animal fats

  • unsaturated fats → liquid

    • fats found in vegetable oils

Stability

• the degree of unsaturation influences stability

• all fats become spoiled when exposed to oxygen

• bad smell and taste of fat-

  • oxidation

  • exposure to heat and light

  • microbial growth

• polyunsaturated fats-

  • spoil easily because their double bonds are unstable

• monounsaturated fats-

  • less susceptible to spoil

• saturated fats-

  • most resistant to oxidation and least likely to spoil

• manufacturers can protect fat-containing products against rancidity-

  1. products are sealed in airtight, nonmetallic containers, protected from light, and refrigerated

  2. add antioxidants (additives BHA, BHT, vitamin E) to compete for the oxygen and protect the oil

  3. products undergo hydrogenation

Hydrogenation

• hydrogenation: some or all points of unsaturation and saturated by adding hydrogen molecules and creates trans-fatty acids

• 2 food-processing advantages-

  1. protects against oxidation, prolonging shelf-life, by making polyunsaturated fats more saturated

  2. alters the texture of foods by making vegetable oils more solid (margarine and shortening)

• makes margarine spreadable, pie crusts flaky, and puddings creamy

• most often, fat is partially hydrogenated (creating trans-fatty acid)

  • remaining double bonds after processing change their configuration from cis to trans

Trans-Fatty Acids

• naturally, most double bonds are cis (hydrogens next to the double bonds are on the same side of the carbon chain)

• trans-fatty acids-

  • only a few fatty acids (milk and meat products) naturally occur as this

  • hydrogens next to the double bonds are on opposite sides of the carbon chain

  • behave like saturated fats

  • raises blood cholesterol and the risk of heart disease

• a relatively small amount of trans fat in the diet comes from natural sources

• natural trans fat exert little effect on blood lipids

• conjugated linoleic acids-

  • naturally occurring trans-fatty acids

  • offer health benefits

  • not counted as trans fat on food labels