Science
in Archaeology.
Lipids and diets
by Andrew Hayes
Animal lipids
Land animal fats
contain mostly
saturated fatty acids.
Eg; Palmitic acid
(C16) and Stearic
acid (C18).
Fatty acids vary in animals due to species, age, sex,
diet and body part.
Saturated fatty acid side chains fit together much
better, so have higher melting points.
Consequently animal fats
are solids.
Plant lipids
Plant oils are rich in
unsaturated fatty
acids,
Eg; 1) Oleic acid, C16:1.
whose double bond is
between C9/10 and is
cis.
2) Linoleic acid, C18:2,
cis9/10 cis 12/13.
3) Linolenic acid, C18:3,
cis 9/10 cis 12/13 cis
15/16.
They also rich in
saturated palmitic acid
(C16:0).
The exact mix varies
with soil type and
climate.
Double bonds mean that
unsaturated fatty acid
side chains do not fit
together well, so have
lower MPs.
Plant and fish
lipids are
therefore liquid.
Food residues
After cooking food is often
left on the sides of the pot.
Residues easily removed from metal saucepans on
washing up.
But people once cooked with unglazed
pottery, eg Black Burnished pot from Bath.
Porous pots absorb
residues from food
cooked, prepared or
stored in them.
Lipids are insoluble in water, and therefore
once absorbed stay put.
• Samples are cleaned,
crushed and extracted
with solvents.
• What kind of solvents
should be used?
• Non-polar.
Or a 2:1
mixture
by
volume
of;
Dichloromethane
Eg; Chloroform
: Methanol
How could lipids be separated and
identified?
• Separation is achieved on the basis of
the different boiling points of fatty
acids using gas chromatography.
• Fats are often treated to form methyl or
trimethyl derivatives, which are easier
to separate.
• This is combined with mass
spectroscopy to identify specific fatty
acids.
How to identify what was on the menu.
Beef
or Lamb?
Start with determining the composition of fats and oils
from modern animals.
% Distribution of fatty acids
C;
C=C
12:0 14:0 16:0 18:0 20:0 16:1 18:1 18:2 18:3 20:1
Cow
6.3
27.4 14.1
Pig
1.8
21.8 8.9
Sheep
4.6
24.6 30.5
36
2.1
25.5 28.1 2.4
38.4
Goat
3.5
49.6 2.5
0.8
4.2
53.4 6.6
4.3
0.8
0.8
Cow lipids
14:0
Linoleic
2.5
6.3
Palmitic
27.4
49.6
18:1
14.1
0
Stearic
14
Stearic
Sheep lipids
4. 3
Linoleic
4. 6
Palmitic
24. 6
36
0
30. 5
18:1
Burial can affect lipids, producing results different
from the original source.
Double bonds are
reactive.
Eg; Alkenes
decolourise
bromine
water.
So unsaturated fatty
acids are easily
oxidised.
Saturated C/C bonds
are unreactive.
Eg; Alkanes do
not react with
bromine water.
So saturated fatty
acids tend to be
unreactive.
When and where was
dairying first practised?
A majority of cows, with
bulls killed young, is typical
of dairy herds.
Traditionally
sex/age ratii
of bones
provided the
answer.
But lipid residues
absorbed by pottery
can now be directly
analysed.
Milk and lipids
• Milk is highly nutritious,
rich in protein, minerals
and fats.
• Including c2.5 – 5.5%
lipids.
• 98% of which are
triglycerides.
• Palmitic acid, stearic
acid and oleic acid
together make up 50%+
of milk lipids.
• Milk also contains
unique fats not to be
found in adipose tissue.
Many unique fatty acids are made by bacteria in
the stomachs of ruminants, including sheep
and cattle.
Eg; Short chain fatty
acids, C4 –C14, and
branched chain fatty
acids.
But during burial smaller fatty acids are easily
hydrolysed as there is less stearic hindrance.
Once released they are more volatile and
soluble and so tend to be lost.
Altering the lipid composition so that it
becomes almost identical to other animal
lipids, dominated by C16/C18.
How to distinguish milk from meat lipids.
• Adipose tissue makes
Stearic Acid (C18:0)
from carbohydrate.
• But mammary glands
are unable to do this
and must obtain it from
a dietary source.
• In the process it is
depleted in the stable
isotope C13.
• Allowing milk and meat
lipids to be
distinguished.
What about the veg?
• Leaves have a coating
of waxes to reduce
water loss via
transpiration.
• Like animal lipids they
can be absorbed by
porous pottery.
• Extracted using
solvents, then identified
using GLC/MS.