Science

1. Science in Archaeology. Lipids and diets by Andrew Hayes

2. 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.

3. Saturated fatty acid side chains fit together much better, so have higher melting points. Consequently animal fats are solids.

4. Plant lipids 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. Plant oils are rich in unsaturated fatty acids, 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.

5. 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.

6. Food residues After cooking food is often left on the sides of the pot. Residues easily removed from metal saucepans on washing up.

7. 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.

8. Samples are cleaned, crushed and extracted with solvents. • What kind of solvents should be used? • Non-polar. Eg; Chloroform Or a 2:1 mixture by volume of; : Methanol Dichloromethane

9. 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.

10. How to identify what was on the menu. or Lamb? Beef Start with determining the composition of fats and oils from modern animals.

11. % Distribution of fatty acids

12. Cow lipids 14:0 Linoleic Palmitic 18:1 Stearic Linoleic 14 Stearic Sheep lipids Palmitic 18:1

13. Burial can affect lipids, producing results different from the original source. Double bonds are reactive. Saturated C/C bonds are unreactive. Eg; Alkenes decolourise bromine water. Eg; Alkanes do not react with bromine water. So unsaturated fatty acids are easily oxidised. So saturated fatty acids tend to be unreactive.

14. When and where was dairying first practised? Traditionally sex/age ratii of bones provided the answer. But lipid residues absorbed by pottery can now be directly analysed. A majority of cows, with bulls killed young, is typical of dairy herds.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.