Elsevier

Food Chemistry

Volume 108, Issue 1, 1 May 2008, Pages 316-321
Food Chemistry

Analytical Methods
Photoacoustic spectroscopy and optothermal window as analytical tools to quantitate carotenes in margarines

https://doi.org/10.1016/j.foodchem.2007.10.021Get rights and content

Abstract

Photoacoustic spectroscopy (PAS) and optothermal window (OW) with a cw Ar ion laser used as a radiation source at 476.5, 488 and 496 nm were applied to quantify total carotenes (TC) in margarines. Both techniques, being rapid and extremely simple, allow for direct measurement without any pretreatment of the sample. The PAS has proven precise and sensitive enough to allow quantitation of TC in margarines containing 1–9 mg TC/kg, in applications such as the quality control of intermediate and final products. The sensitivity of OW is lower than that of PAS but the approach can still be used for quantitation of TC in margarine matrices at much higher concentrations.

Introduction

Beta carotene, a carotenoid commonly known as provitamin A, has in the last decades become even more important as an antioxidant, namely, as a radical scavenger and a physical scavenger of singlet oxygen (Burton, 1989, Krinsky, 1989). It is believed that β-carotene plays an important role in the inhibition of initial stages of lipid peroxidation and possibly also possesses anticancer properties (Bauernfeind, 1981, Krinsky, 1989). The protective effect of carotenes added as colorants to various foods has been the part of exhaustive epidemiological, nutritional and food quality research (Goodwin, 1986). HPLC has been often employed as a powerful technique to quantify low levels and various forms of carotenoids in foods and in human plasma (Khachik et al., 1986, Khachik et al., 1992).

Margarine compositions are coloured by the incorporation of a carotenoid pigment having a yellowish hue due to α-carotene, e.g., palm oil or saffron or carrot extract, or β-carotene, along with a natural material having a predominantly reddish hue, e.g., annatto or paprika, sufficient to produce a yellow colour. It was found by Van het Hof, Tijburg, de Boer, Wiseman, and Weststrate (1998) that the consumption of the antioxidant-fortified margarine significantly increased the levels of supplied antioxidants in plasma, with the largest increase found for α- and β-carotene.

According to Fox and Minchinton (1972) the most common colorant found in margarines is synthetic β-carotene. The same authors found 1–13 mg/kg of carotene in coloured margarine as opposed to 3.9 mg carotene/kg in margarine reported by Toyos Diaz (1968). Several methods have been used over the past decades to determine carotene colorants in margarines. Saponification and extraction were often followed by chromatographic separation and spectrophotometric/chromatographic determination (Fox and Minchinton, 1972, Maruyama et al., 1977, Toyos Diaz, 1968, Usher et al., 1968). It should be kept in mind that saponification step not only prolongs the analysis time but in addition may also cause degradation of carotenoids. RP-HPLC method was developed for separation of vitamin A and β-carotene in oil and margarine (Landen & Eitenmiller, 1979) and for quantitation of these compounds in margarine after gel permeation chromatographic separation (Chase, Akoh, Eitenmiller, & Landen, 1995). The need for availability of accurate qualitative and quantitative data on carotenoids has resulted in a development of rugged analytical techniques capable of separating, identifying and quantifying these compounds. Recently, we have proposed a direct spectrophotometric method for assaying total carotenes in margarines after a simple extraction in hexane (thereby obviating the saponification step) and discussed in-extenso its analytical performances (Luterotti, Bicanic, & Pozgaj, 2006).

On the other hand the photoacoustic spectroscopy (PAS) and optothermal window (OW) concept with laser used as the excitation source have already proven effective analytical methods for quantification of lutein in biological matrix (Bicanic, Luterotti, Becucci, Fogliano, & Versloot, 2005) and of lycopene in tomato-based products (Bicanic et al., 2003, Bicanic et al., 2004, Bicanic et al., 2005). The research study described in this paper explores the capability of PAS and OW for simple and direct (without a need for any sample pretreatment step) quantification of total carotenes (TC) in margarine matrices.

The principles of PAS and OW rely on the transfer of the periodically modulated radiation to a heat in the absorbing condensed phase sample. The heating and cooling that takes place in response to the incident radiation is in PAS converted into a pressure wave which, in a constant-volume chamber, is detected by a microphone at the modulation frequency. On the other hand, in OW, the heat generated on account of absorption in the sample causes the periodic expansion/contraction in a radial direction of a thin sapphire plate (in a good thermal contact with the sample) which is sensed at the modulation frequency by the piezoelectric device attached to the rear side of disk. For a given incident power of the selective excitation source, the intensities of PA and OW signals from microphone and piezoelectric sensor are proportional to the sample concentration. Hence PAS and OW can be conveniently applied in studies on completely opaque samples making them unique in the comparison with conventional spectroscopy where specimen’s transparency is a necessity.

Section snippets

Samples and standard

Overall, 27 margarines manufactured and purchased (some were donated by “Zvijezda”, Zagreb, Croatia) in different European countries were investigated. Eighteen among them contained an aqueous phase (water-containing margarines, abbreviated WCM, and water-containing blanks, abbreviated WCB), the remaining ones contained only the fat phase (fat margarines, abbreviated FM, and fat blanks, abbreviated FB). Overall, eight blanks (no carotene colorant added) were used: three with water added (see

Results and discussion

The absorption spectrum of β-carotene shows peaks at approximately 450 and 480 nm. Consequently, 496, 488 and 476.5 nm emission lines of Ar ion laser appear suitable for excitation of this analyte in the PAS and OW measurements.

Conclusion

In conclusion, PAS is proposed as a new analytical tools for a simple and rapid, routine control of TC concentration in commercial margarines. With the present state of art in development of strong radiation sources and required electronic equipment, it is possible to manufacture a versatile PAS-based instrument at only moderate investment cost. For proper interpretation of measured data classifying the margarines according to (i) the presence of water, (ii) total fat/water content, is a

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