` Making Chemistry Visible: Simple Demonstrations with Olive Oil - Olive Oil Times

Making Chemistry Visible: Simple Demonstrations with Olive Oil

Dec. 10, 2012
Patricia B. OHara and Richard A. Blatchly

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The olive pro­vides a culi­nary land­scape in which an appre­ci­a­tion of the unique chem­i­cal nature is a key to real under­stand­ing. The chem­i­cal struc­ture of the nat­ural bio­mol­e­cules of the olive can be related to the devel­op­ment of the char­ac­ter­is­tic color, taste, and smell of the oils. These unique bio­mol­e­cules give rise to the iden­ti­fi­ca­tion of olive oil as the healthy oil.

The growth in the global mar­ket for olive oil and the regional expan­sion of pro­duc­tiv­ity have neces­si­tated the draft­ing of new inter­na­tional stan­dards to pro­tect the con­sumer and pre­vent fraud­u­lent oils from get­ting to mar­ket.

The body of the olive oil lies in its com­po­nent trigly­erides, which are fatty acids bound to glyc­erin. Free fatty acids (FFAs) are nat­ural degra­da­tion prod­ucts of these triglyc­erides. The major FFA in olive oil is Oleic acid (named after the Oleo europa tree). While it can be very hard to taste the dif­fer­ence between olive oils of dif­fer­ent FFA con­tent due to the fatty acids them­selves, a high acid con­tent is usu­ally asso­ci­ated with other neg­a­tive char­ac­ter­is­tics such as improper han­dling or stor­age of the olives prior to press­ing. The cost of the oil will be deter­mined in part by whether it is clas­si­fied as an extra vir­gin (less than 0.8% FFA) or vir­gin (less than 1.5% FFA).

The stan­dard analy­sis for free fatty acids (FFA) is a titra­tion with a stan­dard base com­monly known as lye (sodium hydrox­ide-NaOH). In a pro­fes­sional analy­sis lab, the FFAs are extracted from the oil using a mixed sol­vent of equal parts ethanol and diethyl ether, the fumes of which are nox­ious and flam­ma­ble. We will demon­strate how to mea­sure the acid­ity using a sim­pler reagent to give a qual­i­ta­tive answer as to whether the oil met a cer­tain stan­dard. Briefly, the reagent uses a pH indi­ca­tor in a solu­tion con­tain­ing a known amount of NaOH. When the amount of FFA is greater than the amount of base, the indi­ca­tor changes color.


Figure 1: Alizarin Yellow test on oils with increas­ing con­cen­tra­tions of oleic acid. From left to right, 0%, 0.50%, 1.0%, 1.5%, 2.0%, 2.5%, 3.3%. The reagent was cho­sen to test for 1.5% oleic acid, explain­ing the color change begin­ning at tube 4.

Taste is one of the qual­i­ties of olive oil that is most impor­tant yet dif­fi­cult to quan­tify. Experts in the make-up of olive oil know the rela­tion­ship between spe­cific com­po­nents and the taste and prop­er­ties of the oil. To visu­ally con­nect a chem­i­cal mea­sure of these com­po­nents to the taste, we have devel­oped sev­eral sim­ple assays that demon­strate the pres­ence or absence of impor­tant con­stituents of the oil. We’ve related these tests to the fla­vor and cook­ing prop­er­ties of the oil.

One of the more con­tro­ver­sial aspects of the fla­vor of olive oil is its bit­ter­ness. During a work­shop in Turkey, we wit­nessed quite a debate between tra­di­tion­al­ists, who do not think that oil should be bit­ter, and high-end oil pro­duc­ers, who were proud of the bit­ter­ness of their oil. The bit­ter and astrin­gent com­po­nents are pri­mar­ily polyphe­no­lic com­pounds derived from hydrox­y­ty­rosol, which is made from one of the basic amino acids, tyro­sine. Other com­pounds includ­ing flavonoids such as tax­i­folin and lute­olin add depth to the fla­vor palette. All of these com­pounds are quite active antiox­i­dants, which accounts for some, but not all of their health ben­e­fits.

The antiox­i­dant qual­i­ties of olive oils can be demon­strated by using a dye known as Prussian blue. The chem­istry at work here is the same chem­istry that was used half a cen­tury ago in the pro­duc­tion of archi­tec­tural blue­prints. A mix­ture of iron salts responds to the pres­ence of an antiox­i­dant to pro­duce the deep blue col­orant, nicely demon­strat­ing the activ­ity. While this does not allow us to dis­tin­guish fine dif­fer­ences, it does show the dif­fer­ence between oils with antiox­i­dants (olive oils) and with­out (such as seed oils).

Figure 2: While min­eral oil (tube 1) and most nut oils such as sun­flower oil will not react, good qual­ity olive oils should test pos­i­tive to pro­duce a bright blue color (tubes 2 – 5). Here, an olive oil made from wild rather than cul­ti­vated olives, tube 4, pro­duced the deep­est blue color.

The pre­dom­i­nant yel­low color of olive oil is largely due to lutein, while β‑carotene lends an orange hue. Greener olive oils con­tain mol­e­cules from the chloro­phyll fam­ily. Normally, quan­ti­fy­ing the color requires a spec­tropho­tome­ter, but the basic func­tions of this expen­sive instru­ment can be mim­ic­ked by an iPhone, with an app such as Irodori, to mea­sure the Red/Green/Blue (RGB) con­tri­bu­tions to the color of the olive oils. A photo is taken with the iPhone, and the pro­gram breaks it down into color swatches. The user selects the most char­ac­ter­is­tic color for analy­sis and the RGB con­tri­bu­tion (trans­mit­ted light) for that swatch is reported. A mea­sure­ment like this needs a back­ground con­trol. Each photo was taken under iden­ti­cal illu­mi­na­tion con­di­tions, with the same vol­ume of solu­tion, and a white back­ground.

One other fea­ture of our panel of sim­ple demon­stra­tions of the chem­istry of olive oils is the inter­ac­tion of the oils with laser lights. We show how sim­ple laser point­ers can illu­mi­nate some of the finer fea­tures of the chloro­phyll-con­tain­ing greener oils.

One of the more reward­ing aspects of being a chemist is the abil­ity to get a deeper under­stand­ing of mate­ri­als we encounter every day. Olive oil is a great exam­ple of an every­day sub­stance that looks won­der­fully com­plex when you see what it’s made of. Knowing the makeup also lets us relate our gus­ta­tory enjoy­ment with the health­ful results of our con­sump­tion.

Patricia B. O’Hara is the Amanda and Lisa Cross Professor of Chemistry at the University of Massachusetts, Amherst
Richard A. Blatchly is the Organic Chemistry Department Chair at Keene State College

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