Just sweet tomatoes, please
Norwegians love sweet tomatoes and are willing to pay a much higher price for these tomatoes. The sweet little Piccolo tomatoes grown by Wiig Gartneri in Jæren are exactly such a tomato variety, and Norwegian consumers have embraced it.
“For us, it is crucial that every Piccolo tomato gives customers exactly the taste experience they expect. At the same time, we are dealing with living plants, and we have suspected that the sugar content can vary, even on tomatoes that sit on the same cluster and have the same red color,” says Frode Ringsevjen, production manager at Wiig Gartneri.
Measuring sugar content on whole tomatoes
The sweetness in tomatoes comes from the fruit sugar, and the concentration of sugar is measured based on a so-called Brix scale. The most common way to measure Brix on fruit, berries, and vegetables is with an instrument called a refractometer, which measures the sugar concentration in the juice. This means that the juice must be squeezed out, and the fruit, berry, or vegetable is thus destroyed.
Wiig Gartneri is now investigating whether rapid, non-destructive measurement methods can be the solution to ensuring that all Piccolo tomatoes are just as sweet as the customers expect. In the first instance, the tests are carried out at Nofima. Frode Ringsevjen sent the first 200 tomatoes, with varying degrees of maturity – from dark red overripe to green very immature, to Nofima in October.
The researchers then used a prototype of an NIR sensor and developed a non-destructive measurement method for sugar concentration.
“The purpose of these first tests was to investigate whether we can get good measurements of the sugar concentration inside the tomato without touching it. SINTEF has developed an NIR sensor that can measure them at a distance of 8 cm, and it was this we used in the tests. The results were very uplifting. We could measure sugar concentration with good accuracy,” says senior researcher Jens Petter Wold who led the study.
Good calibrations require tomatoes with varying sugar content
The first 200 tomatoes were tested while lying still in their own plastic bowl. To ensure that the measurements of the sugar concentration are correct and accurate for each individual tomato, the researchers have made numerous adjustments and adaptations of both the measurement methods and the instrument.
Among the tomatoes were green, yellow, orange, red, and dark red. The large range in maturity is necessary when researchers are to test the measurement methods they develop and to know that the sensor measures the real sugar concentration and is not affected by the colors. There turned out to be large variations in sugar content even among tomatoes that were equally red.
Thoroughness is a keyword in the work of developing solid measurement methods. In November, Frode Ringsevjen sent around 100 new tomatoes to Nofima. This time, all the tomatoes were in the spectrum from orange to red, and the researchers tested again whether the measurement methods held up. They then used the measurement results to place the tomatoes in eight different sweetness classes.
Correspondence between technology and human senses
In addition to the sensor measurements, the researchers made further quality checks. This time against the senses. Nofima’s professional sensory panel, consisting of ten trained sensory assessors, tested tomatoes in the different sweetness classes and classified them based on flavour and odours.
“We were positively surprised by the correspondence between the classifications from the sensor measurements and from the sensory assessors. The differences were really tiny,” says Jens Petter Wold.
Measurements on the packaging line are the next step
The good results from the first tests create optimism. Now the researchers are investigating how temperature can affect the measurements and how the fruit sugar is distributed in the tomatoes. This is to better understand what they are measuring.
The next step will be to develop measurement methods that can also measure the sugar content in the tomatoes in motion, i.e. while they are moving, for example, on a packaging line while still attached together in the cluster.
A very useful tool
“High quality and great flavours are crucial for customers to choose our Piccolo tomatoes. We therefore already carry out thorough internal quality controls. A measuring instrument that can effectively weed out tomatoes with lower quality and less sweet taste will further strengthen this work. It will be a good tool to ensure that our loyal customers get the quality and taste they expect, without individual tomatoes spoiling, Frode Ringsevjen points out.
The production manager hopes for a measuring instrument that can be used both as a handheld device to take quality checks in greenhouses and mounted on a robot on the conveyor belt in the packaging plant.
Facts about the research
The sensor measurements methods and quality assurance carried out by Nofima’s ten professional sensory assessors have been developed in the strategic research program Precision, which is funded by the Foundation for Research Levy on Agricultural Products.
The NIR sensor tests have been carried out in collaboration at SINTEF Digital and are part of SFI DigiFoods – Centre for Research-based Innovation.
About NIR
NIR (near infrared) spectroscopy is a technique in which light is passed through a food product to measure how much light is absorbed at different wavelengths. This is a quick measurement method that is suitable for measuring various properties in food, such as fat, water, protein, carbohydrates and pigments. NIR is the most widespread on- and at-line technology in the feed and food industry.
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