The paper highlights the use of a portable raman spectroscopy device that is able to detect changes in nutrient composition of tomato fruits in the colour transit from green to brown.
The device has the advantage of obtaining results in-situ, without cutting and handling the tomato fruits.
The team believe the same quality of information can be achieved with this device when compared to a more expensive and sophisticated confocal raman microscope.
"It is about transferring this technology, which had a specific use, to the kitchen,” said study author Josu Trebolazabala, researcher at the Department of Analytical Chemistry at the University of the Basque Country.
“Our idea was to come up with a tool that could help producers find out when their tomatoes have reached their optimum ripeness point. This is achieved by using this technique and, what is more, without destroying the fruit."
Writing in the journal Spectrochimica Acta, the analysis confirms an increase of carotenoids from an unripe to a ripe stage of these fruits, with lycopene being the characteristic carotenoid at the optimum ripening stage.
“The aim is to enable producers to go to the vegetable plot with this equipment, place the raman probe on the tomato, and find out whether it is at its optimum picking point or whether it needs to be left longer so that it can ripen properly," added Trebolazabala.
Green tomatoes exhibit less flavonoid, carotenoid, lutein, anthocyanins and terpene content when compared to red tomatoes.
Along with taste, producers place much importance in determining the content of these components in order to improve nutrient composition in current and future yields.
For the laboratory analyses, the ‘Raf’ tomato variety, native to the Basque Country in the North of Spain, was selected.
Four groups of this tomato, at different ripening stages (green, orange, red, brown (overripe)), were organised and two samples per group were analysed.
All these fruits were collected from different plants and each sample was placed in the confocal Raman microscope.
To verify that tomato structure was not damaged after raman measurements, microscopic observations were conducted before and after each raman analysis with both, the confocal raman microscope and the portable instrument.
The analysis confirmed an increase of carotenoids from an unripe to a ripe stage of these fruits, with lycopene being 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.
“When the tomato is green, the main pigments are chlorophyll (hence its green colour) and the waxy cuticles, which are on the outside," explained Trebolazabala.
“But the presence of these compounds falls as the fruit reaches its point of optimum ripeness. Once the colour changes to orange-coloured, compounds of a different type are observed; the carotenoid compounds are activated.”
Peppers and pumpkins
Trebolazabala added that the tomato gradually acquired nutrients until the optimum point was reached. It is here that the red carotenoid lycopene was at its maximum level.
“After that, the tomato begins to lose its carotenoid content, as shown by the analyses conducted on overripe tomatoes," he explained.
This innovative technique can be extrapolated to any other food that changes colour during its ripening phase.
Data on peppers and pumpkins, and their nutrient composition for example, can be obtained via this method.
Source: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Published online ahead of print: doi.org/10.1016/j.saa.2017.03.024
“Portable Raman spectroscopy for an in-situ monitoring the ripening of tomato (Solanum lycopersicum) fruits.”
Authors: Josu Trebolazabala, Maite Maguregui, Héctor Morillas, Alberto de Diego, Juan Manuel Madariaga