Lipid Technologies and Applications

Frank D. Gunstone, Fred B. Padley, FredB. Padley

• Publisher: CRC Press
• Year: 2018
• eBook ISBN: 9781351435109
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

""Provides a comprehensive review of the major technologies and applications of lipids in food and nonfood uses, including current and future trends. Discusses the nature of lipids, their major sources, and role in nutrition.

1

Fatty Acids and Lipids Structure

Frank D. Gunstone

Mylnefield Research Services Ltd., Scottish Crop Research Institute, Invergowrie, Dundee, Scotland

I. INTRODUCTION

It is almost obligatory to start any book on lipids with an account of the chemical structure of the lipids and their component acids, and this book is not different in this respect. The following material is based on recently published fuller accounts [1,2], where further details may be sought if required.

There is no generally accepted definition of the class of natural products designated as lipids, but I adhere to the view that lipids are composed of fatty acids or closely related components such as the corresponding alcohols and the sphingosine bases. Expressed slightly differently, they represent those products of the acetate-malonate scheme of biosynthesis derived through the reductive pathway.

II. FATTY ACID NOMENCLATURE

Fatty acids are designated in several different ways. Despite the alternative descriptions set out here, many trivial names are still widely used. These names were often given before the chemical structure of the acid was elucidated and were frequently chosen to indicate the source of the acid. Examples include palmitic (from palm oil), oleic (from olive oil, Olea europea), linoleic and linolenic (from linseed oil), ricinoleic (from castor oil, Ricinus communis), and arachidic acid (from groundnut oil, Arachis hypogea). A trivial name may be assigned or may continue to be used because the systematic name is cumbersome, as for example with α-eleostearic acid, which is simpler than 9c11t13t-octadecatrienoic acid. Trivial names are easy to use, but they are not in themselves indicative of structure.

Systematic names are based on internationally accepted rules agreed to by organic chemists and biochemists. Those who know the rules can interconvert systematic names and structures. As a simple example, oleic acid is cis-9-octadecenoic or Z-9-octadecenoic acid. This is a carboxylic acid (oic) with 18 carbon atoms (octadec) and one olefinic center (en) that lies between carbons 9 and 10 (counting from the carboxyl end) and has the cis (Z) configuration, i.e.,

${\text{CH}}_3{({\text{CH}}_2)}_7\text{CH}=\text{CH}{({\text{CH}}_2)}_7\text{COOH}$

which may also be represented by the line drawing

Representations like this are increasingly popular. They are more useful when the number of double bonds and/or other functional groups is larger and the saturated sections of the molecule are short. The structure shown below is not easily recognizable as stearic acid (octa-decanoic) because the number of carbon atoms represented in this structure is not immediately apparent and has to be carefully counted. This difficulty is overcome in the shorter formulation shown to the right.

The line forms are useful because of their immediate visual impact; they are not convenient for tabulated data or for insertion into lines of text. Because the word form can be complex and clumsy, systematic or trivial names are sometimes abbreviated to two or three capital letters as in the following examples:

Gamma-linolenic acid	GLA
Arachidonic acid	AA
Eicosapentaenoic acid	EPA
Docosahexaenoic acid	DHA

Another way of designating fatty acids involves the use of numbers such as 18:2. This symbol describes an acid such as linoleic with 18 carbon atoms (assumed to be straight-chain) and two unsaturated centers (assumed to be cis-olefinic). Since there are many isomeric compounds that could be represented by this symbol, additional descriptors may be added thus:

$\begin{array}{cccc}18:2(9,12),& 18:2(9c,12c),& 18:2(9Z,12Z),& 18:2(n-6)\end{array}$

All these refer to the same acid. The first indicates the position of the two unsaturated centers in the C₁₈ chain with reference to COOH= 1. The second and third formulations confirm the cis or Z configuration of the double bonds. The symbol ∆ is sometimes added to show that numbering is with respect to the acid function. The fourth designation introduces a further concept in fatty acid nomenclature.

It is sometimes useful to designate double bond positions with respect to the CH₃ end group, and this is done with symbols such as ω6 or n-6, which indicate that the first double bond is on carbon 6, counting from the methyl group. In the absence of other information it is assumed that all the double bonds are methylene-interrupted and have the cis (Z) configuration. Symbols such as c, t, and e are used to show cis, trans, and ethylenic unsaturation (where configuration is not known or does not apply), and a (acetylenic) or y (ynoic) is used to show a triple bond.

III. FATTY ACIDS—MAIN STRUCTURAL FEATURES

The number of known natural fatty acids exceeds 1000, though only a relatively small number, perhaps 20–50, are of common concern. Based on a survey of these 1000 structures, and noting particularly the structures of those acids produced most commonly in nature, it is possible to make four general statements. Each of these is generally true, but there are exceptions to all four. The exceptions are frequently trivial, but sometimes they are significant. Though originally based on a survey of chemical, structures, it is now clear that these statements reflect the underlying biosynthetic pathways by which the acids are produced in nature.

1. Natural fatty acids, both saturated and unsaturated, are straight-chain compounds with an even number of carbon atoms in their molecules. This is true for the great majority of structures and for the more abundant acids. Chain lengths range from two to over 80 carbon atoms, though they are most commonly between C₁₂ and C₂₂. Despite the validity of this statement, acids with an odd number of carbon atoms (e.g., heptadecanoic, C₁₇) occur, as do those with branched chains (e.g., isopalmitic, anteisononadecanoic) or with carbocyclic units (e.g., sterculic, chaulmoogric).

2. Acids with one unsaturated center are usually olefinic compounds with cis (Z) configuration and with the double bond in one of a limited number of preferred positions. This is most commonly ∆9 (i.e., nine carbon atoms from the carboxyl group as in oleic) or n-9 (i.e., nine carbon atoms from the methyl group as in oleic or erucic acid). But double bonds occur in other positions (e.g., petroselinic, 6c-18:1) or have trans configuration (e.g., elaidic 9t- 18:1) or can be replaced by an acetylenic ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• Part I Introduction
• Part II Processing
• Part III Food Emulsions
• Part IV Nonaqueous Foods
• Part V Special Food Applications
• Part VI Nonfood Uses
• Index