3.1. Changes in sugars during ripening transition stages.
The date of harvest has a pronounced influence on the storage potential
of fruit. To determine optimum harvest dates, it is essential to know
the changes that occur in sugar levels during ripening transition
stages. This is important as they influence and determine storage life
duration and fruit quality, especially sweetness. We performed
chromatographic determinations of sucrose, glucose, fructose and other
sugars associated with hexose metabolism, such as myo-inositol and
trehalose. These are compatible solutes involved in osmotic adjustment
and protect fruit properties when subjected to changing environmental
conditions. In addition, FOS and RFOS were also characterized and
quantified by MS during ripening transitions stages. In ‘Mara des Bois,’
glucose and fructose accounted for the vast majority of change in
sugars, followed by sucrose. Myo-inositol and trehalose were found to
contribute to a lesser extent (Figure 1). In AR strawberries (66 %
redness), sucrose content was similar to that found in FR strawberries
but significantly lower in comparison with DR fruit (Figure 1A). Sucrose
content was approximately half that of glucose and fructose, and
demonstrated marked increases during the DR stage, reaching values of
11.18±1.10 mg/g fresh weight. A significantly high level of glucose
(Figure 1B) and fructose (Figure 1C) was also quantified at the DR
stage, relative to the AR and FR stages. The increase in sucrose,
glucose and fructose was similar in all cases, approximately doubling in
quantity. Similarly, higher levels of myo-inositol and trehalose were
quantified at the DR stage as compared to the AR and FR stages (Figure
1D and E). With respect to FOS, the main polymer is 1-kestose (DP3)
(Figure 1F). This showed levels that were six times more abundant than
levels of the isomer 6G-kestose (Figure 1G), and more than sixty times
more abundant than nystose (DP4) (Figure 1H). In the case of 1-kestose,
6G-kestose and nystose, levels peak at the FR stage and decline at the
DR stage. In comparison, Kestopentaose (Figure 1I) is found at much
lower concentrations generally, with levels declining progressively
during the ripening transition and being lowest at the DR stage. With
respect to RFOS content, no clear trends can be observed in raffinose at
the different ripening transition stages (Figure 1J). On the other hand,
estaquiose, which is 25 times less abundant, showed a minor decrease in
concentration following the AR stage (Figure 1K).