Cell wall composition is an important contributor to texture.
Obviously, growers can’t perform complex biochemical cell wall analyses in their vineyards. But it may be possible to obtain the same information using handheld spectrometers, thereby providing growers with reliable data.
Cell walls and softening
Plant cell walls are mainly made of carbohydrates, including a group of compounds collectively called pectin. Pectin transitions between liquid, gel and solid phases, depending on its esterification level (whether or not it’s charged).
Pectin is nearly ubiquitous in fruit. Commercially, pectin, extracted from mostly citrus peels, is widely used as a food stabiliser, thickener, and gelling agent. Most fruit jams owe their consistency to added pectin.
Cells deposit uncharged pectin into their walls, from where it migrates to the middle lamella (the interface between two neighbouring cells). Once it reaches the middle lamella, the pectin becomes charged and more solid, and it’s also able to bind calcium and boron ions, resulting in strong adhesion of the two adjacent cells.
In wine grapes, pectin is laid down until véraison and acts almost like cement in holding cells together. When the berry’s seeds ripen, pectin esterification levels change, the pectin starts to dissolve, and the fruit softens.
At the same time, the fruit loads sugar from the vines’ leaves. So, the ripening berry becomes sweeter, softer, and changes colour, all to attract birds, as they are the primary seed dispersers of wild grapevines.
It’s unclear whether the same processes occur in table grapes, and little has been published on the topic. In wine grapes, ripening is signalled by hormones released by seeds, whereas most modern table-grape cultivars are seedless.
It’s also unknown how the various plant growth regulators applied by table-grape growers affect pectin esterification.
Gathering table-grape data
Given the scarcity of relevant research on table grapes, I approached SATI, who agreed to fund a project exploring how firmness relates to other berry characteristics, cell wall composition, and infrared spectroscopy.
The project ran from late 2022 to the end of 2025 and involved three MSc students (Eugene Badenhorst, Miguel Henriques, and Terri-Lynne Naidoo) at different times.
Our samples included fruit from 30 table-grape cultivars and berries from two cultivars at different stages of ripening. We measured standard viticultural parameters, such as berry mass, diameter, sugars, and acids, to obtain a basic characterisation of the fruit.
We used firmness as a proxy for crunchiness. Firmness was the amount of pressure needed to cause failure when a berry was squeezed between two plates.
To obtain spectroscopic fingerprints, we did mid- and near-infrared spectroscopy on berry cell wall preparations and intact berries. Infrared spectroscopy is non-destructive and has been shown by other researchers to provide chemical information linked to, for example, berry quality indicators and cell wall polysaccharide composition.
For analysing cell wall composition, we used Comprehensive Microarray Polymer Profiling, an enzyme-linked immunosorbent assay (ELISA) that quantifies 30 cell wall components. (I had developed this technology as part of a previous SATI-funded project.)
We also combined ELISA with gel-permeation chromatography to determine changes in pectin integrity across different ripening stages.
The resulting dataset continues to be analysed using multivariate statistical methods, which highlight patterns and identify associations between specific wavelengths of infrared light and berry characteristics.
Where are we now?
As expected, a preliminary analysis of the data clearly shows variation in firmness across cultivars, likely genetically based, and ripeness stages.
We also found an association between berry mass and firmness, with larger, heavier berries tending to be firmer. In contrast, sweeter berries and berries with a higher sugar-acid ratio tended to be less firm.
The cell wall carbohydrate analysis revealed differences among the table-grape cultivars. However, I still need to conduct a correlational comparison to determine whether relationships exist between the cell wall components and physicochemical measurements, such as firmness. I also need to reanalyse the spectroscopic data.
On the other hand, we observed clear associations between pectin cell wall components and esterification across developmental stages. The question now is whether developmental differences are more important than cultivar differences, but I think both are important.
If we can establish strong correlations between particular spectroscopic fingerprints, berry cell wall profiles, and firmness, it should be possible to use handheld spectrometers to measure firmness in the vineyard. Ultimately, that would give growers another tool to monitor harvest maturity and manage grape quality.
