**6. Diffuse-light absorption spectroscopy**

The scattered colorimetry technique allows for assessing the olive oil by considering both color and turbidity. Indeed, although the intrinsic turbidity of the oil can be regarded as a peculiar characteristic, it has an unstable and non-reproducible influence on absorption measurements because of its time dependent nature. In fact, suspended particles created during production of the olive oil usually settle down in a non-reversible way, because they tend to aggregate at the bottom of the container, creating a sort of sludge. Absorption spectroscopy in the UV-VIS-NIR of filtered samples demonstrated effectiveness to achieve wider quality information. However, sample filtering is not only a time-consuming procedure, but is also an action that alters the composition of the sample. In fact, turbidity is also due to the presence of water, and water removal causes a serious loss of water-soluble compounds–such as polyphenols–that are responsible for the unusual character and authenticity of olive oil.

(% oleic acid) 0.12 - 1.555 780 - 2500 3 0.8407

(meq O / kg. oil) 3.76 - 13.98 1000 – 2333 2 0.9628

Table 1. Prediction of quality parameters of the extra virgin olive oil collection of Figure 9

**Fatty acids Calibration range (%) Spectroscopic** 

Table 2. Prediction of fatty acids of the Sicilian extra virgin olive oils of Figure 9

The scattered colorimetry technique allows for assessing the olive oil by considering both color and turbidity. Indeed, although the intrinsic turbidity of the oil can be regarded as a peculiar characteristic, it has an unstable and non-reproducible influence on absorption measurements because of its time dependent nature. In fact, suspended particles created during production of the olive oil usually settle down in a non-reversible way, because they tend to aggregate at the bottom of the container, creating a sort of sludge. Absorption spectroscopy in the UV-VIS-NIR of filtered samples demonstrated effectiveness to achieve wider quality information. However, sample filtering is not only a time-consuming procedure, but is also an action that alters the composition of the sample. In fact, turbidity is also due to the presence of water, and water removal causes a serious loss of water-soluble compounds–such as polyphenols–that are responsible for the unusual character and

**6. Diffuse-light absorption spectroscopy** 

authenticity of olive oil.

K232 0.922 - 1.548 1333 – 2222 3 0.9942 K270 0.062 - 0.1178 1333 – 2222 3 0.9825 ΔK -0.004 - 0.01 1333 - 2222 2 0.4344

Oleic 65.847 - 76.334 1333 - 2222 1 0.9986 Palmitic 9.62 - 17.113 300 - 2300 2 0.9847 Linoleic 4.469 - 10.95 1333 - 2222 1 0.9553 Stearic 2.565 - 4.046 780 - 2500 2 0.9942 Palmiticoleic 0.367 - 1.457 1333 - 2222 2 0.9504 Linolenic 0.646 - 1.066 1000 - 2300 1 0.9822 Arachiric 0.382 - 0.642 1000 - 2222 1 0.9896 Eicosenoic 0.212 - 0.431 1000 - 2300 2 0.9821 Behenic 0.042 - 0.411 300 - 2300 2 0.8892 Heptadecenoic 0.053 - 0.356 300 - 2300 2 0.8081 Heptadecanoic 0.025 - 0.29 1000 - 2300 2 0.8337 Lignoceric 0.026 - 0.205 1333 - 2222 1 0.8532

**range (nm)**

**Spectroscopic range (nm)**

**Number of PLS regressors**

> **# PLS regressors**

**R²** 

**R²** 

**Quality parameters Calibration** 

Oleic acidity

Peroxide value

**range**

Diffuse-light absorption spectroscopy, that is, spectroscopy carried out by means of an integrating cavity, is an alternative spectroscopic technique which allows to achieve scattering-free absorption spectra, that is, without caring about the intrinsic turbidity of the olive oil. It has been proposed in the literature as an effective method for overcoming scattering problems in process control (Fecht et al., 1999) and biological applications (Merzlyak et al., 2000), as well as for more general quantitative spectrophotometry (Jàvorfi et al., 2006).

Diffuse-light absorption spectroscopy makes use of an integrating sphere that contains the sample under test. The source and the detector are butt-coupled to the sphere, as shown in Figure 10.

Fig. 10. Setup for diffuse-light absorption spectroscopy by means of an integrating sphere

Almost all the light shining on the sphere surface is diffusely reflected, and the detector can be placed anywhere in the sphere in order to gather the average flux (Elterman, 1970; Fry et al., 1992; Nelson et al., 1993; Kirk, 1995). By inserting an absorbing medium in the cavity, a reduction of the radiance in the sphere occurs. The reduction is related to the absorption of the medium and to its volume, and is independent of non-absorbing objects within it, such as suspended scattering particles. The light intensity detected by means of this measuring setup is described by Equation 4:

$$I = \frac{R \ I\_0 \ A\_d}{S} \frac{1}{1 - \frac{R}{S} \left(S - A\_s - a \ V\right)}\tag{4}$$

where I0 : source power; I: detected power; =C: sample absorption coefficient; V: sample volume; Ad: detector area; As: source area; R: cavity power reflectivity; S: cavity surface area.

This technique was used to detect the adulteration of high quality extra virgin olive oils produced in Tuscany caused by lower quality olive oils such as olive pomace, refined olive pomace, refined olive, and deodorized olive oils (Mignani et al., 2011). Mixtures of four original extra virgin olive oils and the four types of adulterants were artificially created at different adulterant concentration. Figure 11-top shows the diffuse-light absorption spectra of high quality oils (left) and the others used as adulterants (right). Then, chemometrics was applied for achieving adulterant discrimination and prediction of relative concentration. Figure 11-bottom shows the discriminating maps: PCA was capable of discriminating

Optical Absorption Spectroscopy for Quality Assessment of Extra Virgin Olive Oil 59

Other optical spectroscopic techniques are emerging for the quality assessment of extra virgin olive oils, especially the fluorescence and the Raman spectroscopies. The interested readers can have a look at the most recent bibliography (Ross Kunz et al., 2011; El-Abassy et al., 2010; Paiva-Martins et al., 2010; El-Abassy et al., 2009; Tena et al., 2009; Zou et al., 2009;

Many sponsors and people contributed to the scientific work of the authors of this chapter.



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The following deserve a special acknowledgement:

**8. Acknowledgments** 

67987-C02-01 projects.

**9. References** 

337.

samples adulterated by means of deodorized olive oil (left), while a deeper LDA processing was needed for discriminating the other adulterants (right).

Fig. 11. Top: Diffuse-light absorption spectra of original extra virgin olive oils (left) and other lower quality olive oils commonly used as adulterants (right). Bottom: Discriminating maps obtained by chemometric data processing of absorption spectra (with kind permission of Springer Science+Business Media)
