Isomerization and increase in the antioxidant properties of lycopene from Momordica cochinchinensis (gac) by moderate heat treatment with UV–Vis spectra as a marker

Highlights

• Gac can be an interesting and stable source of all-trans-lycopene.

• Isomerization by heat treatment made gac lycopene solution more antioxidant.

• Heat process provoked isomerization to, successively, 15-, 13- and 9-cis-lycopene.

• 9-cis-Lycopene was more antioxidant than all-trans- and 13-cis-lycopene.

• Spectral ratios can be a simple marker of lycopene isomerization.

Abstract

Momordica cochinchinensis (gac) is a plant rich in lycopene. This pigment tends to solubilize in oil and get damaged during extraction. The impact of heating on cis-isomerization of oil-free lycopene in hexane was studied at 50 and 80 °C during 240 min with UV–Vis spectrometry, DAD–HPLC and TEAC test. The initial all-trans-form isomerized to the 13-cis isomer more rapidly at 80 °C. After this treatment, 16% of the lycopene compounds were in the 9-cis-form. This isomer triggered an increase in the antioxidant properties which was detectable from concentrations above 9% and resulted in a change from 2.4 to 3.7 μmol Trolox equivalent. It is thus possible to increase the bioactivity of lycopene samples by controlling heating. The evolution of ratios calculated from the global UV–Vis spectrum was representative of cis-isomerization and spectrometry can thus be a simple way to evaluate the state of isomerization of lycopene solutions.

Introduction

Carotenoids are important compounds in food as they can be used as colours, provitamin A or antioxidants and provide several interesting health benefits. Lycopene is a natural pigment that contributes to the red colour of many fresh fruits. This carotenoid is a non-provitamin A with potent antioxidant activity. In comparison with β-carotene and certain xanthophylls, lycopene is more antioxidant (Böhm et al., 2001, Cao-Hoang et al., 2011, Miller et al., 1996). The interest in lycopene and its potential cancer preventing properties are known and have been studied for 20 years (Clinton et al., 1996). However, the different isomers of lycopene behave differently, particularly concerning their bioavailability. It was suggested that cis-lycopenes may be better absorbed than the all-trans parent structure (Britton, 1995, Stahl et al., 1992). This may be explained by a greater solubility of cis-isomers as observed in bile acid micelles (Boileau, Merchen, Wasson, Atkinson, & Erdman, 1999), and this could explain why in serum and tissues, lycopene is more than 50% cis-lycopene contrasting with the composition of food sources (Clinton et al., 1996).

Momordica cochinchinensis (gac) is considered as a fruit with a high nutritional potential, a “superfruit” popular in Vietnam for its health benefits and for its use in traditional cooking. This fruit is particularly rich in lycopene (Aoki et al., 2002, Ishida et al., 2004, Vuong et al., 2006). The extraction of lycopene from this “fruit from heaven” demands moderate condition because this pigment is very prone to oxidative degradation (Cao-Hoang et al., 2011, Mortensen, 2006). Dehydration of gac aril by moderate heat causes lycopene losses exceeding 36% in comparison with freeze drying (Tran, Nguyen, Zabaras, & Vu, 2008). In gac oil also, carotenoids are quickly degraded under high temperature storage conditions (Nhung, Bung, Ha, & Phong, 2010). Natural lycopene is extracted from plant tissues with heat treatment to decrease enzymatic activity or to disrupt tissues for facilitating pigment extraction. However, heat processing may be responsible for carotenoid isomerization (Schieber & Carle, 2005) and degradation (Graziani et al., 2003). The effect of heating has been studied on tomatoes that are also a fruit rich in lycopene. It has been shown that heating leads to an improvement in nutritional quality and an increase in carotenoid bioavailability in processed tomato products. Lycopene uptake by humans is greater from heat-processed than from unprocessed tomato–oil mixtures (Stahl & Sies, 1992). Despite the fact that isomerization is the first step of carotenoid degradation, cis-isomers, which are formed during processing, are better absorbed than all-trans compounds (Achir et al., 2010, Boileau et al., 1999, Stahl and Sies, 1992). It is therefore possible that the higher lycopene absorption from cis isomer-rich products is due to higher temperature treatments (Unlua et al., 2007).

In nature, lycopene occurs mostly in the all-trans form, but cis-isomers may be more bioavailable. Controlling heating during extraction could result in lycopene with higher bioavailability and bioefficacy. Until now, the stability of lycopene from gac fruit has been mainly studied in its oil or in its crude tissues. Moreover, the antioxidant property of gac extracts has already been mainly related to the phenolic content of this fruit (Kubola & Siriamornpun, 2011). With the objective to use gac lycopene as a nutritional supplement, the current work aimed to study the isomerization of solvent-extracted lycopene from gac during mild heat treatments (50 and 80 °C) in hexane, controlling the isomeric state of lycopene, and relating this structural parameter to the antioxidant activity.