[Joanna Savage]

 A Treatise on Adulteration of Food, and Culinary Poisons  by Frederick Accum, London 1820 p.318
seen on Public Domain website

Poisonous Oive Oil
This commodity is sometimes contaminated with lead, because the fruit which yields the oil is submitted to the action of the press between leaden plates; a practice (particularly in Spain) to suffer the oil to become clear in leaden cisterns, before it is brought to the market for sale. The French and Italian oil is usually free from this impregnation.
Olive oil is sometimes mixed with oil of poppy seeds: but, by exposing the mixture to freezing temperature, the olive oil freezes, while that of the poppy seeds remains fluid;  and as oils which freeze with most difficulty are most apt to become rancid, olive oil is deteriorated by the mixture of poppy oil.
Good olive oil should have a pale yellow colour, somewhat inclining to green; a bland taste, without smell; and should congeal at 38 degrees Fahrenheit. In this country it is frequently  met with rancid.
The presence of lead is detected by shaking, in a stoppered vial, one part of the suspected oil, with two or three parts of water, impregnated with sulphuretted hydrogen. This agent will render the oil of a dark brown or black colour, if any metal, deleterious to health, be present. The practice of keeping the oil in pewter or leaden cisterns as is often the case, is  objectionable; because the oil acts upon the metal. The dealers in this commodity assert, that it prevents the oil from becoming rancid and hence some retailers often suffer a pewter measure to remain immersed in the oil.

[Joanna Savage]

I kept my comment separate from the above quote because I had no idea how much space the quote would take.

Can anyone tell me what rancid actually means? I am sure we all know it when we taste it, but what has happened to the oil?

There is some fascinating info on the public domain web site and I am managing to waste(?) a lot of time reading. I am wondering if there is an easy way of transferring text from the articles to the MGS Forum, rather than typing it all out which I did for the above as it is so interesting how differently oil is treated these days. Or is it?

[Fleur Pavlidis]

I think all our scientific members are away on holiday so no reply to you rancid question. I looked at the original paper and it seems as if you could cut and paste the text, but I can't test it because I'm away too with only my husband's iPad to play with. what kind of olive oli was this man testing if he thinks it should have a bland taste and be pale yellow?

[Trevor Australis]

I'm not scientific either but I did once direct an institution that grew olives and made oil and won prizes for it. As I recall the rancidity is caused by slow oxidisation of the oil. To slow the process oil is best kept in dark glass bottles in a dark, cool place and it is best used within a year of being made.

[Joanna Savage]

Thanks to Fleur and Trevor for comments. The oil under discussion sounds horribly like those bottles of olive oil which used to be kept for rare medicinal use, it was very  pale colour and kept in a bottle covered with dust which adhered to the oil spilt last time it was used. It was marginally less revolting than castor oil.

[JTh]

I found this information on a website (http://www.livestrong.com/article/426889-how-to-identify-bad-olive-oil/ ): 'How to Identify Bad Olive Oil... Bad olive oil, also termed as rancid, is affected by a variety of factors....'

So, rancid seems to be used as a synonym for bad olive oil.

[Alisdair]

Very interesting quote, Joanna - thanks!

To add a little footnote, the oxidisation which as Trevor says is caused by oxidisation (and therefore slowest if air and heat as well as light are excluded as much as possible), is of that relatively small proportion of the oil which consists of polyunsaturated fatty acids - perhaps around 10%. And it's slowed by various antioxidants that olive oil contains naturally, and that tend to be most concentrated in extra virgin olive oil.