Porosity maps give an ‘airy’ image of fruit and vegetables

See the internal structure of your product through 3D X-ray based porosity mapping.  

The porosity of a fruit or vegetable quantifies the amount of air spaces inside the product. The porosity strongly determines to what extent and how it dries out, or how well the product responses to modified or controlled atmopshere conditions.  

A horticultural product transports required oxygen and many waste materials such as carbon dioxide and water through the pores. The porosity will therefore also have an influence on all kinds of changes  that are associated with defective disposal of these waste materials, or a lack of oxygen. Examples are internal brown discolouration of apple or celeriac.  

Under standing porosity

The transport of gases in fruit and vegetables is more difficult in those parts of the fruit that contain small pores. Porosity is very heterogeneous throughout fruit and vegetables with sometimes very open and very dense parts. That is why it is important to quantitatively visualize porosity throughout the fruit in order to better understand the transport of respiratory gases. 

KU Leuven demonstrated that X-ray CT (computed tomography) is a good method to map the porosity distribution of a whole fruit non-destructively and accurately, based on a simple correlation model between shades of gray (on a CT image) and porosity. The correlation is valid for different products, demonstrating wide application potential.  

By locating the dense tissues in a product, we can better understand why certain patterns of abnormalities / symptoms occur in particular products or cultivars. This will help in optimizing storage conditions, understand storage and shelf life and design appropriate MA packaging.  

A non-destructive method for porosity mapping is also a first step towards sensor development for inline measurement of porosity on sorting lines for specific quality assessment in relation to storability.  

Porosity maps of Jonagold apples, Purple-globe eggplants, Purple-top turnips and Conference pears confirm that fruits and vegetables cannot be seen as a uniform structure (Figure 1). Differences in porosity in specific tissues can be accurately predicted and visualized. On average, eggplant has the most porous structure (41.8 ± 1.0% porosity), followed by turnip (23.3 ± 3.4%), apple (19.7 ± 1.1%) and pear (4.0 ± 1.6%). 

The highest porosity is in the core of the eggplant and turnip (45 – 65%). More towards the surface of the vegetables, the porosity of the flesh lowers to 30 to 45%. Certain zones have a porosity of less than 10%. In apple the porosity increases from core to surface, varying between 10 and 30%. The porosity of pear is very low and below 10% with a more dense tissue in the core. 
The new porosity measurement technique is more convenient than many previously used methods, because only a juice reference scan and a homogeneous water sample are needed to draw up porosity maps of other horticultural products. Based on the simple linear correlation, expansion to other products can therefore be done fairly quickly and easily.  

The porosity maps will be of considerable value to comprehensively understand transport phenomena of metabolic gasses and water during postharvest handling and storage. Furthermore, this may lead to non-destructive online measurement methods for porosity in the framework of internal quality inspection.  

Transaxial (X-Y, top) and coronal (X-Z, bottom) slices of eggplant (a), turnip (b), apple (c), and pear (c) porosity maps translated from grayscale CT images

By: Pieter Verboven