The total of soluble sugars present in a fruit or vegetable changes as it matures, which is why the time of harvest is often critical.
Sugar content can be assessed by destructive methods, for example by expressing cell sap into a refractometer (oBrix), but near-infrared (NIR) spectroscopy can offer a more reliable, non-destructive method.
Sugar content is rarely influenced by agronomic or cultural practices in the field, but is sensitive to environmental factors. These include nutrition (low N increases sugar accumulation), solar radiation receipt and, in protected environments, continuous light and CO2 concentration. Many post-harvest treatments, including exposure to low temperature or to acetaldehyde, can enhance the sweetness of some fruits, as can the application of synthetic plant growth regulators.
The most reliable way to increase sweetness, however, appears to be through conventional plant breeding. An unconventional approach would be to encourage fructose accumulation (fructose is sweeter than glucose), while another would be to introduce the taste-modifying proteins that create the sensation of sweetness when eaten.
Review: Molecular Approaches for Enhancing Sweetness in Fruits and Vegetables by Nookaraju, Upadhyaya, Pandey, Young, Hong, Park and Park (2010). Scientia Horticulturae 127 (1): 1-15. Scientia Horticulturae is available online at www.elsevier.com/locate/scihorti
Dr Ken Cockshull is associate fellow, Warwick Crop Centre, University of Warwick