The chemistry of perfect strawberries is key when robots learn to pick them

Nofima scientists Kjersti Aaby and Jens Petter Wold are searching for strawberries that are suitable for chemical and sensory investigations. Photo: Jon-Are Berg-Jacobsen/Nofima

The Norwegian strawberry season is underway. For the time being, all berries are picked by hand, but robots might be doing the job in just a few years. Nofima scientists are in the process of developing sensors that can measure taste.

The scientists are mapping the chemistry of ripe strawberries, getting a sensory panel to test the taste of the berries, and developing spectroscopic applications that can ‘see’ the chemistry without picking the berries.

“Norwegians prefer sweet strawberries, but a little acidity is also needed to bring out the freshness. We have already carried out two large analyses of sugar and acid content in a total of 200 strawberries, all of which were assessed as being ripe based on their colour. We are now conducting the first analyses of this year’s strawberries”, says Nofima scientist Kjersti Aaby.

What do the chemical analyses show?

Like other berries, strawberries mostly contain water, but there are large variations both in the amount of sugar and acid, and in the types of sugar and acid that dominate. Although all the berries involved in the investigation appeared ripe in terms of colour and shape, the sweetest berries contained twice as much sugar as those that were least sweet.

The variations were between 4.7 grams and 9.6 grams per 100 grams. The average was 6.6 grams per 100 grams. There was less variation in acid content, but there were some differences, from 0.6 grams to 1.1 grams per 100 grams. The average acid content was 0.9 grams.

“Another important factor to take into account when measuring the sweetness of strawberries is that strawberries are sweetest at the tip and least sweet at the hull. Therefore, we also get different results if we make juice from a berry and when we measure different parts of the berry”, Kjersti explains.

New methods of measurement are required

The average sweetness can be found approximately in the middle of the strawberry, and this is where the robot sensors have to measure in order to make sure the whole berry is sweet enough.

Trials are already underway where the aim is to get robots to select and pick strawberries, but the success rate is not good enough. To ensure that the robots pick the perfect berries, using traditional camera technology that only looks at colour and shape isn’t sufficient enough. Methods of measurement are required that also measure the chemistry of the strawberries.

This is where the so-called spectroscopic methods (e.g. NIR and Raman spectroscopy) come into play. These are methods that use light spectrum or laser to ‘see’ the chemistry of each berry and can measure the amount of sugar and acid based on this information. These methods are fast and non-destructive, i.e., they can directly measure each and every strawberry without causing any damage. Before the spectroscopic methods of measurement can be used, scientists must create models for the characteristics they wish to measure.

“We use the results from the chemical analyses and create spectroscopic models that tell us the degree of sweetness in each individual strawberry. The next step is for Nofima’s trained sensory panel to evaluate the sensory attributes and then check the results of our chemical analyses and models”, says Nofima scientist Petter Vejle Andersen.

Sensory scientists check the results of the models

Highly trained sensory assessors work at Nofima’s sensory laboratory. Their job is to describe and assess the sensory characteristics of foods.

A chemical analysis provides answers regarding the chemical composition of strawberries, i.e. how much and what types of sugar and acid they contain, but the analyses do not really say anything about how humans perceive strawberries. We use, often unconsciously, far more assessment criteria. Taste isn’t the only factor that plays a role. Smell, colour, shape and texture are also crucial elements regarding our perception of taste.

“When we assess whether the spectroscopic models are able to capture the sweetness of each strawberry, our assessors assess smell based on the criteria of acidity, sweetness, berry, fruit and total intensity. Taste is based on the criteria of acidity, sweetness, sourness, bitterness and total intensity”, explains Nofima scientist Mari Gaarder. She has analysed the taste judges’ assessments.

In the first test, Nofima’s ten taste judges tasted 20 strawberry samples, each with different degrees of sweetness and acidity. The strawberries were placed in different categories based on the results of the spectroscopy measurements, and the judges’ assessment of sweetness corresponded very well with these. This indicates that it will be possible to measure taste using spectroscopic methods, and in turn develop robotic sensors containing spectroscopic applications.

New strawberry season = new tests

In order to ensure that the applications the scientists are developing provide stable answers over time, they are now conducting new tests on this year’s strawberries. The scientists start by categorising berries using the spectroscopy applications. They then analyse the berries both chemically and sensorially to see if the applications correspond – or if the scientists need to update the models.

So far, the results indicate that some spectroscopic methods appear to be more stable than others, and the application must be fast and solid enough to work in the robot sensor. The scientists are continuing to work on this in various research projects.

It starts with method development

In order to offer technology that can be installed and used in a robot, new methods for both analyses and measurements must be developed. Much of this method development takes place in Nofima’s strategic research programme, funded by the Foundation for Research Levy on Agricultural Products (FFL).

Once the scientists have mapped how they use results from chemical and sensory analyses to calibrate rapid, non-destructive methods of measurement, it will be possible to build on the methods so that the same type of analyses and measurements can be made for other types of fruits and berries.

Research facts

The development of the various methods, both for chemical and sensory analyses and methods of measurement based on NIR and Raman Spectroscopy, takes place in Nofima’s strategic research programme, funded by the Foundation for Research Levy on Agricultural Products (FFL).

Research to further develop spectroscopic applications is carried out in SFI DigiFoods (Centre for Research-based Innovation) funded by the Research Council of Norway and participating partners, and in the innovation project called MålBær – Sensors for automatic precision picking of strawberries, funded by Research Funding for Agriculture and Food Industry and partners.

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