2009_Book_FoodChemistry

3 Lipids
3.1 Foreword
Lipids are formed from structural units with
a pronounced hydrophobicity. This solubility
characteristic, rather than a common structural
feature, is unique for this class of compounds.
Lipids are soluble in organic solvents but not
in water. Water insolubility is the analytical
property used as the basis for their facile
separation from proteins and carbohydrates.
Some lipids are surface-active since they are
amphiphilic molecules (contain both hydrophilic
and hydrophobic moieties). Hence, they are polar
and thus distinctly different from neutral lipids.
The two approaches generally accepted for lipid
classification are presented in Table 3.1.
The majority of lipids are derivatives of fatty
acids. In these so-called acyl lipids the fatty
acids are present as esters and in some minor
lipid groups in amide form (Table 3.1). The acyl
residue influences strongly the hydrophobicity
and the reactivity of the acyl lipids.
Some lipids act as building blocks in the formation
of biological membranes which surround
cells and subcellular particles. Such lipids occur
in all foods, but their content is often less than 2%
(cf. 3.4.1). Nevertheless, even as minor food constituents
they deserve particular attention, since
their high reactivity may strongly influence the
organoleptic quality of the food.
Primarily triacylglycerols (also called triglycerides)
are deposited in some animal tissues
and organs of some plants. Lipid content in such
storage tissues can rise to 15–20% or higher and
so serve as a commercial source for isolation of
triacylglycerols. When this lipid is refined, it is
available to the consumer as an edible oil or fat.
The nutritive/physiological importance of lipids
is based on their role as fuel molecules (37 kJ/g
or 9 kcal/g triacylglycerols) and as a source of
essential fatty acids and vitamins. Apart from
these roles, some other lipid properties are
indispensable in food handling or processing.
Table 3.1. Lipid classification
A. Classification according to “acyl residue”
characteristics
I. Simple lipids (not saponifiable)
Free fatty acids, isoprenoid lipids (steroids, carotenoids,
monoterpenes), tocopherols
II. Acyl lipids
(saponifiable)
Mono-, di-, triacylglycerols
Phospholipids
(phosphatides)
Glycolipids
Diol lipids
Waxes
Sterol esters
Constituents
Fatty acid, glycerol
Fatty acid, glycerol or
sphingosine, phosphoric
acid, organic base
Fatty acid, glycerol or
sphingosine, mono-, di- or
oligosaccharide
Fatty acid, ethane, propane,
or butane diol
Fatty acid, fatty alcohol
Fatty acid, sterol
B. Classification according to the characteristics
“neutral–polar”
Neutral lipids
Polar (amphiphilic) lipids
Fatty acids (>C 12 ) Glycerophospholipid
Mono-, di-, triacylglycerols
Glyceroglycolipid
Sterols, sterol esters Sphingophospholipid
Carotenoids Sphingoglycolipid
Waxes
Tocopherols a
a Tocopherols and quinone lipids are often considered
as “redox lipids”.
These include their melting behavior and the
pleasant creamy or oily taste that is recognized
by a receptor, which has recently been identified.
Therefore, there are all together six taste qualities
(cf. 8.6.1). – Fats also serve as solvents for certain
H.-D. Belitz · W. Grosch · P. Schieberle, Food Chemistry 158
© Springer 2009