Portable Raman spectroscopy for an in-situ monitoring the ripening of tomato (Solanum lycopersicum) fruits

https://doi.org/10.1016/j.saa.2017.03.024Get rights and content

Highlights

  • An in-situ monitoring of tomato ripening was performed.

  • Portable Raman spectroscopy was used for the tomato ripening analyses.

  • Exhaustive Raman bands assignation of the different phases of tomato ripening was performed.

  • Phytoene/phytofluene have a key role in the tomato ripening.

Abstract

Ripening is one of the most important transformations that fruits and vegetables suffer, from an unripe to a ripe stage. In this study, it was followed up and analyzed the variations in the composition of tomato fruits at different ripening stages (green or unripe, orange or middle ripe, red or ripe and brown or overripe). The results obtained from the Raman measurements carried out showed a change in the composition of tomato fruits in the transit from green to brown. The analysis confirmed an increase of carotenoids from an unripe to a ripe stage of these fruits, being lycopene the characteristic carotenoid of the optimum ripe stage. The presence of chlorophyll and cuticular waxes decrease from the unripe to the ripe stage. Moreover, the relative intensity of phytofluene, a transition compound in the carotenoid biosynthetic pathway, is higher in the orange or middle ripening stage. The results obtained in-situ, without cutting and handling the tomato fruits, by means of a portable Raman spectrometer offered the same information that can be achieved using a more expensive and sophisticated confocal Raman microscope.

Introduction

The ripening of fruits and vegetables is a process that involves many changes in the final product [1]. Some of these changes are visible to the naked eye, e.g. the change of the colour [2]. However, there are other transformations that are not visible, such as molecular changes and reactions [3]. These transformations have been observed and described in many fruits. These ripening reactions involve changes in the texture, decrease of the firmness in the fruits and changes in the colour from green to red or yellow [4]. Furthermore, there is a change in the taste and aroma of these fruits [1]. Usually they become sweeter as the starch is transformed into sugar and the production of aromatic volatile compound is induced [5].

Among the wide variety of fruits that suffer from these ripening processes, tomatoes are one of the most important, because they have special properties that prevent many human diseases [3]. Among these properties, it can be highlighted that a high consumption of these fruits reduces the risk of suffering certain kinds of cancer [6], [7], [8], [9]. They have also been associated with the improvement of the immune system [6], [7] and the decrease in the risk of degenerative diseases like cardiovascular ones [6]. Moreover, they also decrease the risk of cataract [6], [9].

These benefits come from some compounds called phytonutrients or phytochemicals. Tomato fruits are full of these vegetable natural compounds that act as antioxidants. They are not established as essential nutrients, but these natural pigments are biologically active chemicals present in the plants [10]. Moreover, they help humans to fight against different degenerative diseases and bring beneficial effects for the health [11]. Flavonoids, carotenoids, lutein, anthocyanins and terpenes are the most important compounds attributed to this group [10]. Among them, carotenoids are one of the most valuable compounds attributed to illnesses prevention. Carotenoids are naturally present as, yellow, orange and red organic pigments. These fat-soluble organic compounds are synthesized in plants, algae and some photosynthetic microorganisms. They are consumed by humans and animals in their diets, although they are unable to perform their de novo synthesis, but they are capable to transform and assimilate them [12], [13], [14].

However, green tomatoes present other substances and have different contents in carotenoids comparing them with the red tomatoes [15]. In this sense, some authors have claimed the importance to know which the differences among the components are in the different ripening stages, to asses which tomato provides more or less content in each compound [15]. But these differences in relative compound composition can be used to monitor the ripening process if one has an experimental technique capable to identify such compounds.

Although in the literature there are spectroscopic works related to the tomato ripening using different techniques such as SERRS [16], Raman imaging for the distribution of lycopene [17] even SORS for internal tomato maturity [18]; an in-situ analysis using portable Raman spectroscopy has become a challenge. In this way, Raman spectroscopy is a promising tool to observe these differences, as it has been shown recently through direct Raman measurements on the surface of tomato fruits [19], because most of the compounds present in the fruit are clearly identified. However, it was not demonstrated that changes in the compounds that take place during the ripening can be monitored through Raman measurements on the surface of the fruits without causing any damage on it.

The aim of this study was to evaluate the use of Raman spectroscopy to identify the variations of the compounds during the ripening process of a tomato fruit without causing any damage on it. As there are several Raman set ups in the market, a second objective to this work was to estimate if the main Raman features identified as the most promising to follow the ripening process, can be clearly identified using a portable Raman spectrometer. Thus, in a first step the use of a laboratory micro-Raman spectrometer which implements a 785 nm excitation laser will be used to identify all the compounds detected through direct measurements on the surface of tomato fruits at different ripening stages (green or unripe, orange or middle ripe, red or ripe and brown or overripe). In a second step a hand held Raman spectrometer, with the same 785 nm excitation laser, will be tested on the surface of the same tomatoes, and the results will be compared to evaluate its effectiveness.

Section snippets

Samples

For the laboratory analyses, several tomato fruits were collected from an orchard located in Barrika, in the Basque Country (North of Spain). To carry out this study, “Raf” tomato variety was selected. Four groups of tomato fruits, at different ripening stages, and 2 samples per group were analyzed. In the first group green colour tomato fruits were collected, in the second group only orange colour tomato fruits, in the third group red colour tomato fruits (well ripened fruits) and in the last

Results and Discussion

The first set of Raman measurements were performed with the laboratory confocal microscope. The spectra obtained on the surface of green tomatoes showed some bands which are suggested to be attributed to chlorophyll A [20], [21]. The bands related to this natural green pigment are located ca. 744 (w), 915 (w), 985 (w) and 1325 (vw) cm 1 (see Fig. 1A and Table 1). Comparing these spectra with the ones obtained from the orange, red and brown tomatoes, it was observed a decreasing tendency of

Conclusions

Raman spectroscopy is a suitable technique to observe the changes in the composition of tomato fruits contrasted by the appearance or disappearance, and the increase or decrease, in the intensity of different compounds present in tomato fruits without cutting or removing them from the plant. Although the quality of the Raman spectra obtained in the laboratory using a confocal Raman microscope is higher than the one obtained with the portable instrument, the information obtained in-situ on the

Acknowledgements

This work has been financially supported by the Basque Government through Consolidated Research 612 Group Project 2013–2018 (Ref. IT-742-13).

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