Lipids

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Principles

Introduction

Lipids along with proteins and carbohydrates are one of the major structural components of living cells. It plays an important nutritional role in human diet.  It is a concentrated source of energy, providing 9 kcal per gram of fat.  In contrast, each gram carbohydrates and proteins provides only  ~ 4 kcal.  In addition, fat is the carrier of fat soluble vitamins, A, D, E, and K.  Lipids are also important building blocks of cell membranes, hormones, retinol, bile salts, etc.  Last but not least, ingestion of fat signals satiety and provides us with the sense of fullness.

Lipids, as fats and oils, demonstrate unique properties in foods.  Sensory attributes such as flavour, texture, mouthfeel, and appearances of foods are affected by their melting points and crystalline structures.  In addition, fat also interacts with other food constituents to create desirable properties.

Now, just close your eyes and think about the following:

aroma of a bacon strip in your morning breakfast
smoothness of a chocolate snack
richness of a cheese cake slice
flaky crust of an apple pie
crispiness of potato chips
creaminess of your bowl of cream of mushroom soup
All of the above are results of fats in foods.  Fat is an important part of a nutritious and delicious meal!
 



Physical Properties

Lipids are a chemically diverse group of molecules. Some lipids are long chain hydrocarbons with polar ends (such as fatty acids) and others are combinations of porphoryin rings (such as cholesterol).  Nevertheless, almost all lipids are insoluble in water and are soluble only in organic solvents such as chloroform, benzene, and ether.

Fats Vs Oils

Food Scientists often differentiate fats and oils by their physical state at room temperature. Fats, which consist mainly of saturated fatty acids, are solid at room temperature. These include beef tallow and lard. Oils are liquid at room temperature. This is because they are made up of mainly long chain unsaturated fatty acids. This unsaturation inhibits close packing and keeps the lipids form crystallizing at room temperature.
 
 
Fats - Solid

Saturated fatty acids have crystallization stabilized by hydrophobic interactions.
Oils - Liquid

Mixtures of saturated and unsaturated fatty acids. The cis-double bonds of the unsaturated fatty acids introduce bends in the hydrocarbon tail and inhibit close packing. 

Triacylglyceride


Fatty acids are the building blocks of triacylglyceride which is the most common molecule found in fats and oils. In each triacylglyceride, there is a glycerol molecule with three fatty acids attached by ester bonds.  Fatty acids are long chain hydrocarbons with polar ends.  When the carbon chain is completely hydrogenated, the fatty acid is referred to be a Saturated Fatty Acid.  An Unsaturated Fatty Acid contains double bonds along the carbon chain.
 
 

The compositions of fatty acids in different fat vary and can affect the physical properties of the fat.  In general, fats containing more saturated fatty acids have higher melting points; fats containing a higher percentage of unsaturated fatty acids have lower melting points and are liquid at room temperatures.
 
 

Common fatty acids in foods and source classification:
Fatty Acid oo oo Melting Point (oC) Common Source
Common Name Systematic Name Chemical Abbreviation oo oo
Lauric Dodecanoic C12:0 43.6 Coconut
Myristic Tetradecanoic C14:0 53.8 Coconut and Milk Fat
Palmitic Hexadecanoic  C16:0 62.9 Animal and Vegetable 
Stearic Octadecanoic  C18:0 69.9 Animal and Vegetable
Arachidic Eicosanoic C20:0 75.2 Peanut Oil
Palmitoleic 9-Hexadecenoic C16:19 -1.5 Milk Fat
Oleic 9-Octadecenoic  C18:1w9 14.0 Animal and Vegetable
Linoleic 9,12-Octadecenoic C18:2w6 -11.0 Animal and Vegetable
Linolenic 9,12,15-Octadecatrienoic C18:3w3 -30.0 Linseed and Rubberseed
Arachidonic 5,8,11,14-Eicosatetraenoic C20:4w6 n/a Lecithin and Lard

 

As used in the above table, three nomenclature systems are commonly used to describe fatty acids.  They are common name, systematic names, and chemical abbreviation.

Chemical Abbreviation:

  • The first letter C represents Carbon
  • The number after C and before the colon indicates the Number of Carbon
  • The letter after the colon shows the Number of Double Bond
  • The letter n (or w) and the last number indicate the Position of the Double Bonds


    • Positional Distribution of Fatty Acids in Fats

    Distribution patterns of fattys acids in triacylglycerol vary among animals and plants.  In general, long polyunsaturated fatty acids are preferentially located at the center position, while the short chain acids in milk fat are located at the sn-3 position.
     


    Essential Fatty Acids

    Although our bodies are capable to synthesize a number of fat compounds from sugars and proteins, linoleic (C18:2n6) and linolenic (C18:3n3) acids must be ingested from diets; n-6 and n-3 fatty acids are essential.  Subsequent elongation and desaturation of these fatty acids will lead to the production of a number polyunsaturated fatty acids (PUFAs).


     

    Metabolic Pathways for Polyunsaturated Fatty Acids
     

    Through the conversion of desaturase and elongase enzymes, linoleic (C18:2n6) and linolenic (C18:3n3) acids are converted to arachidonic acid (AA; C20: 4n6) and decosahexaenoic acid (DHA; C22:6n3), respectively.  These PUFAs are thought to play important roles in supporting normal growth and in developing cell structures in the brain, peripheral nerve growth, and are precursors of bioactive metabolites such as prostaglandins, thromboxanes, and leukotrienes.
     
     

    Since the desaturation and elongation of both n-6 and n-3 as well as the n-9 series utilize the same enzymes, the proportion of dietary n-6 and n-3 fatty acids will affect the amount and the type of metabolites produced.

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