Assessing Antioxidant Content in Tomatoes with NIR Spectroscopy

Tomatoes are laden with many healthy phytochemicals, including antioxidants, that make them a valuable food commodity. Measuring changes in the levels of phytochemicals during maturity leading to harvest and postharvest is crucial to producing nutritious tomatoes. To analyze antioxidants in the supply chain, scientists tested the effectiveness of near-infrared spectrometers to replace accurate yet laborious experimental techniques. The scientists selected the Felix F-750 Produce Quality Meter as representative of commercial tools for their experiment, deeming it the standard near-infrared spectroscopy tool.

Health Benefits of Tomatoes

Tomatoes (Solanum lycopersicum L.) have been shown to reduce the risk of cancers, cardiovascular diseases, and many other illnesses. This is attributed to tomatoes’ high antioxidant content, as well as dietary fibers, phenolic compounds, proteins, vitamins, and minerals.

Lycopene, which is responsible for the color of tomatoes, is a carotene with high antioxidant properties. The accumulation of lycopene is influenced by environmental conditions, genotypes, and growth stages. As the fruit matures, there are physiological and biochemical changes that could affect its nutritional properties.

Quantitative, instrumental measurements of these changes are preferred over sensory evaluation to get a better estimate of tomatoes’ internal quality in the supply chain. It is necessary to find a method of estimating the amount of antioxidant accumulation and flesh firmness, for the following purposes:

• Fixing harvest time to increase antioxidant levels.
• Observing levels of phytochemical change post-harvest to determine the optimal time to begin processing.

A group of scientists from Saudi Arabia and Egypt, Alenazia et al., wanted to test the best methods available to analyze internal tomato phytochemical quality onsite. They chose to test the methods on the tomato variety ‘Red Rose.’  

The scientists harvested fifty tomatoes in five maturity stages: breaker (BK), turning (TG), pink (PK), light-red (LR), and red (RD). They grew the plants in a controlled atmosphere in greenhouses until harvest and the efficiency of the estimation methods was tested at five stages to be sure they could adequately track changes.

The firmness, lycopene content, β-carotene content, total phenolic content, and total flavonoid content of tomatoes at each of the five maturity stages were tested.

Challenge: Identifying a Non-Destructive Tool

As the measurements have to be made onsite, either in the factories or in warehouses, the scientists were looking for non-destructive methods or tools that were portable. They wanted to compare the results of these methods with established laboratory procedures that are currently used to analyze antioxidant levels.

These traditional methods of quality estimation are destructive and do not give timely information on internal quality.

At present, firmness is tested by penetrometers; therefore, this was the instrument that the scientists used as the standard method.

To estimate the antioxidants, the samples were first prepared using three different chemical methods and a laboratory spectrophotometer was used to find supernatant optical densities.

Each of the four chemical separation methods used to prepare the four compounds being tested needed different chemicals and complicated procedures. These are tests that require advanced knowledge and skills in chemistry and cannot be adequately performed by farmers or processors. These methods also require a considerable amount of equipment, as well as a laboratory.

It was, therefore, not surprising that the scientists were looking for a simpler method by which to measure these parameters onsite.

Solution: The F-750 Produce Quality Meter

Figure 1. “Raw spectra curves of all tomato samples at various stages of fruit maturity (a), NIR measurements of tomatoes (b), and the five stages of fruit maturity in vertical rows (c): Vertical rows from left to right; (1) Breaker, (2) Turning, (3) Pink, (4) Light-red, (5) Red. Fresh tomato fruits were labeled and scanned using the handheld near infra-red enhanced spectrometer (F-750, Produce Quality Mater, Felix Instruments, Camas WA, USA), at wavelength range (285–1200 nm),” Alenazia et al. (Image credits: https://doi.org/10.1016/j.sjbs.2020.07.004)

For the non-invasive method, the scientists decided to use Near-Infrared Spectroscopy (NIRS), as it is widely used in the horticultural industry to monitor fruit quality. NIRS tools are non-destructive, accurate, and fast.

Among the NIR tools on the market, the scientists decided to use the portable F-750 Produce Quality Meter.

To determine quality traits, the scientists created a training set using some samples of the fruits. Then, using the Model builder provided by Felix Instruments Applied Food Science, the scientists created a model specifically for “Red Rose” using the fruit quality values from the training set.

Using the Felix F-750, the scientists tested each tomato at three equidistant positions around the equator of the fruit. They measured the reflection spectra in the range of 350–1200 nanometers (nm).

The spectral data was analyzed by F-750 Data Viewer Software to correlate reflectance between the range 285–1200 nm. The spectra generated by tomatoes at the five maturity stages are shown in Figure 1.

Benefits: A Customizable, Robust NIRS Tool

The F-750 Produce Quality Meter, manufactured by Felix Instruments Applied Food Science, proved to be a good representative of a commercial non-destructive NIR spectrometer. It had all the qualities that the scientists were looking for in a NIR spectroscopy tool: rapid and accurate analysis of several quality parameters all combined in a single device.

The single tool was able to simultaneously estimate firmness and the levels of lycopene, β-carotene, total phenolic content, and total flavonoids content. Individual readings were made in seconds and could be read from the display on the device.

The device itself is small and light enough to be operated with a single hand. It has rechargeable batteries that can perform 1600+ measurements on a single charge.

The F-750’s easy-to-follow instructions made customizing the tomato model for “Red Rose” variety simple. The usual applications of the F-750 are to analyze standard quality parameters, such as dry matter, total soluble solids content, titratable acidity, and internal and external color; however, the F-750 was versatile enough that scientists were able to create models that extended the use of the tool to also measure antioxidants with a high degree of efficiency.

NIR Spectrometers Successfully Track Quality Differences During Tomato Maturity

Figure 2. “Spectra reflectance (in the wavelength range 350–1200 nm) of tomato fruit at different maturity stages. The arrow shows the direction of ripening in progress. The reflection spectra were initiated with the spectrophotometer (Model: Felix F-750) around the equator (approximately 120°), at three equidistant positions for each fruit),” Alenazia et al. (Image credits: https://doi.org/10.1016/j.sjbs.2020.07.004)

Tomatoes at earlier maturity (BK) showed lower NIR reflectance than mature tomatoes in the stages PK-RD. The different spectra at various maturity stages are a result of a difference in the color of the tomatoes, as well as their chemical composition; see Figure 2.

The F-750 predictions had a high coefficient of determination (R2) with the quality traits of ‘Red Rose’ tomatoes.  R2 values were as follows:

• Lycopene – 0.864
• Total phenolic content – 0.834
• Total flavonoid content – 0.790
• Β-carotene – 0.708
• Fruit firmness – 0.679

The scientists concluded that the maturity at which tomatoes are harvested influences the levels of antioxidants the fruits accumulate when grown in controlled conditions. Also, there are changes in the quality of the fruits post-harvest, and this can be measured by NIR spectroscopy.

As the tomato fruits develop, the lycopene and β-carotene content increases between the second to the fourth maturity stage. Similarly, total phenolic content (TPC) and total flavonoid content (TFC) is the lowest in the breaker or first maturity stage, the highest at the mid-maturity stage, and at intermediate levels in later maturity stages.

Tomatoes are firmer at early maturity and less firm at later maturity stages.

The changes in quality as the tomato fruit matures, as detected by NIR spectroscopy, were in accordance with the laboratory tests in nearly every instance. Hence, NIR tools like F-750 Produce Quality Meter can replace complicated and cumbersome laboratory processes in the tomato production industry.

Expanding The Use of NIRS

The conclusions of the experiment allowed the scientists to extend the use of a standard commercial NIRS tool, F-750, to also measure antioxidants as a quality parameter. As more emphasis is given to the health benefits of various categories of food, producers can increase their profits and get a competitive edge if they can objectively demonstrate the nutritional superiority of their fresh produce. The analysis of antioxidants can be added to online measurements of quality for tomatoes based on this experiment. These results have the potential to improve quality control and profitability of growers, suppliers, and processors of tomatoes. Moreover, the new application can also be applied to measure antioxidants in the supply chain of other fruits and vegetables as well.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature image courtesy of Rawpixel Ltd

Sources

Alenazi, M. M., Shafiq, M., Alsadon, A. A., et al. (2020). Non-destructive assessment of flesh firmness and dietary antioxidants of greenhouse-grown tomato (Solanum lycopersicum L.) at different fruit maturity stages. Saudi Journal of Biological Sciences, 27 (10), 2839-2846. https://doi.org/10.1016/j.sjbs.2020.07.004

Surana, A.R., Kumbhare, M.R., & Wagh, R.D. (2016). Estimation of total phenolic and total flavonoid content and assessment of in vitro antioxidant activity of extracts of Hamelia patens jacq. Stems. Res. J. Phytochem., 10 (2016), pp. 67-74.
https://scialert.net/abstract/?doi=rjphyto.2016.67.74