Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

398 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
approaches to understand hormone action and control of crucial physiological processes
such as cell division have opened up new avenues for research on dormancy in plants. The
emerging interplay between bud dormancy status and cell division suggests that these two
fundamental processes are probably regulated by common signaling pathways.
Natural dormancy is extended in potato by controlling two essential storage parameters:
temperature and humidity. Major losses in storage are due to shrinkage, disease, and sprouting.
The sprouting factor is covered as a separate topic in the following pages. Shrinkage is
due to loss of water from the tuber. Potatoes, if stored at high temperature, will lose 7–10%
of their weight over a 3-month period. If stored at lower humidities, potato tubers will lose
more of weight—up to 20% at times. Most of the diseases affecting potatoes stop growing
around 4–6 ◦ C. Temperatures above this can result in losses. If the tubers are clean and unbruised
at harvest, problems that occur in storage are minimal. So it is important to harvest
disease-free potatoes and handle them with care. Potatoes stored in light conditions will
turn green within a week. This is due to accumulation of the alkaloid, solanin, in tubers.
Green potatoes are not marketable. Potatoes should be stored in dark, humid chambers,
maintaining low temperature to prevent sprouting.
19.4 Cold-induced sweetening
Management of temperature remains the most important factor affecting postharvest storage
life and quality of vegetable crops. Low temperatures reduce the rate of biological reactions
within the limits of tolerance of a crop. However, in addition to temperature control, reducing
water loss and other preharvest influences also play an important role in extending the storage
life and to maintain, improve, or delay the loss in quality of the product.
Storing tubers at low temperature help in reduce sprout growth and disease. However,
it results in the accumulation of reducing sugars in the tubers stored for longer periods
below 10 ◦ C. This phenomenon is called cold-induced sweetening (Sowokinos, 1990). Intermediate
storage temperatures (10–12 ◦ C) prevent this sugar accumulation. Both storage
temperature and physiological age of the tuber affect sugar accumulation (Hertog et al.,
1997). At low temperatures, starch is converted to mainly sucrose and its monosaccharide
derivatives glucose and fructose (Levitt, 1980). Some of these sugars act as osmoregulators
and cryoprotectants to help in acclamatory processes of certain plant species in order to
survive high altitude and harsh winters (Larcher, 2002; Stitt and Hurry, 2002).
Reducing sugars cause bitter taste and darken the color of potato chips when they are
fried in oil at high temperatures. This is due to Maillard reaction, the product of which is not
acceptable to the consumer. Maillard reaction is primarily a nonenzymatic reaction resulting
in the formation of dark-colored adducts when a carbonyl group of a monosaccharidereducing
sugar reacts with the amino group of a free amino acid during the frying process
of chips and french fries (Marquez and Anon, 1986; Rodriguez-Saona and Wrolstad, 1997).
Darkening of potato chip color is primarily associated with glucose concentration in the
tuber (Coleman et al., 1993). This is a serious problem for the processing industry (Dale
and Bradshaw, 2003). A reducing sugar content of 0.025–0.030% on a fresh weight basis
is the maximum concentration allowed in tubers used for chips (Sowokinos and Preston,
1988).
Recently, many publications have shown that the formation of a neurotoxic compound,
acrylamide, occurs when fried at high temperatures. Acrylamide is formed as a result of the