Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

POSTHARVEST FACTORS AFFECTING POTATO QUALITY AND STORABILITY 399
amino acid asparagine reacting with reducing sugars during the frying process (Mottram
et al., 2002; Stadler et al., 2002). It has been found that acrylamide levels are proportional
to the amount of reducing sugar present in the tuber (Williams, 2005). Ishihara et al. (2006)
showed that treating tuber slices with warm water before frying resulted in reduced levels of
acrylamide. In terms of food safety and from a processing standpoint, reducing free sugar
accumulation during storage is an important trait in potato tubers.
In cold-stressed tubers, the mechanism by which starch mobilization and accumulation
of reducing sugars is not well understood. The major differences observed in carbohydrate
metabolism between tubers maintained at 4 and 25 ◦ C in storage suggest that different
temperatures can influence the enzymes and associated kinetic processes. Sowokinos (2001)
suggested that sugar content in a tuber at any given time is determined by pathways involved
in carbohydrate metabolism and catabolism such as starch synthesis, starch breakdown by
glycolysis, hexogenesis, and mitochondrial respiratory pathways. Many investigators try
to understand the biochemical basis of these pathways by suppressing genes, by putting
gene in antisense orientation, or by over expressing the gene using constitutive promoters
that lead to accumulation of reducing sugars in storage at low-temperature conditions.
In the following paragraphs, important enzymatic steps and their contribution to CIS are
discussed.
19.4.1 Acid invertases
Starch degradation in the amyloplasts can occur either by hydrolytic or by phosphorolytic
processes. Products of starch degradation are exported to the cytosol in the form of hexose
phosphates (hexose-P) via the glucose phosphate–phosphate translocator or as free sugars
via the glucose and/or maltose transporters (Smith et al., 2005). Cytosolic sucrose phosphate
synthase (SPS) converts starch products to sucrose (Krause et al., 1998). Subsequently, a
proportion of the sucrose may be hydrolyzed to glucose and fructose by acid invertase
(Greiner et al., 1999). Activity of the acid invertase located in cell walls and vacuoles was
reported to be associated with the ratio between hexose and sucrose (Zrenner et al., 1996).
Acid invertase in potato is susceptible to selective splicing induced by cold stress. This
suggests that exon 2 could be the signal responding to the cold that causes an increase in
transcription of acid invertase and hence a rapid degradation of sucrose and cold sweetening
(Sturm, 1999). Hajirezaei et al. (2003) showed that cytosolic invertase could block the
phloem transport of sucrose resulting in increased reserve mobilization, which leads to a
hypothesis that metabolic signals decide the fate of starch breakdown. Starch mobilization
was accelerated in a tuber when the bacterial sucrose isomerase gene was expressed in order
to deplete sucrose content. Based on this result, the authors hypothesized that low sucrose
levels trigger starch mobilization in stored potato tubers.
19.4.2 Phosphofructokinase
One of the mechanisms suggested for cold-induced accumulation of sugars in plants is cold
sensitivity of phosphofructokinase. Phosphofructokinase converts fructose-6-phosphate to
fructose-1,6-biphosphate, which is the first committed step in glycolysis. As a result, hexose-
P sugars accumulate at low temperature and get diverted into the sucrose synthesis pathway
(Trevanion and Kruger, 1991). Hammond et al. (1990) concurred with this hypothesis