Fat & Lipids

With the analysis of lipids we attempt to get an answer to the following questions:

1. what fatty acids (or alcohols or amines) are present in the sample?
2. what different lipid classes are present?
3. what is the fatty acid composition of each separate lipid class?
4. how are the acyl chains associated in individual lipid molecules?

This section contains first some general information on lipids that is essential for a better understanding of the extraction and analytic methods described in the further sections.

• This section: Generalities about fats and lipids
  • Terminology
  • Physiological function
  • Classification
• Next sections:
  • Major lipid groups
    • Glycerides
    • Phospholipids
  • Sample preparation for lipid extraction
  • Extraction and separationof fats and lipids
  • Gas Chromatography (GC)

Terminology

In feedstuff chemistry the words fat, lipid and oil are sometimes used synonymously. Tables of feed composition often refer to the crude fat level, by which is meant the material which can be removed from the-feed by ether extraction. The term crude lipid content can also be used. The word lipid is a general term which covers sterols, waxes, fats, phospholipids and sphingomyelins. The terms oil, fat, and wax, reflect the increasing melting points of these lipid components.
Many of the vitamins are fat soluble and will be extracted by ether - thus the term crude lipid content.

Physiological functions

• Fatty acid esters of glycerol are the primary means by which animals store energy. Fish are able to metabolize lipids readily, particularly when deprived of food. Dietary lipid has two main functions - as a source of energy and as a source of its component fatty acids, some of which are essential (i.e. cannot be synthesised by the animal itself) dietary components for the growth and survival of the recipient animal. Lipids are also important factors in the palatability of feeds. The triglycerides are one form of fatty acid esters with glycerol and make up the majority of dietary and body fat. As already mentioned they have several roles in the body: energy storage, energy production, satiety value, insulation, vitamins, essential fatty acids, flavor and texture.
• Phospholipids are components of cellular membranes.
• Sphingomyelins are found in brain and nerve tissue compounds.
• Sterols are important components of, or precursors of, sex and other hormones in fish and shrimp.
• Waxes form important energy storage compounds in plants and in some animal components.

Classification

I. "Simple" Carboxylic esters

1. Fats or glycerides (esters of fatty acids with glycerol e.g. acylglycerols)
   • Monoglycerides
   • Diglycerides
   • Triglycerides
2. Waxes (including sterol esters : esters of fatty acids with alcohols other than glycerol)

II. Complex carboxylic esters

1. Glycerophospholipids
2. Glycoglycerolipids
3. Glycoglycerolipid sulfates

III. Complex lipids (containing amides)

1. Sphingolipids
2. Glycosphingolipids

IV. Precursor and derived lipids

1. Acids (including phosphatidic acid and bile acids)
2. Alcohols (including sterols)
3. Bases (Sphinganines, etc.)

V. Hydrocarbons

1. Straight-chain
2. Simple branched
3. Polyisoprenoid

VI. Lipid vitamins and hormones with multiple functional groups not clearly falling into any of the above classifications