636 Produce Degradation: Reaction Pathways and their Prevention
periderm beneath the suberized parenchyma cells. The wound is healed when the
wound periderm is three to seven layers thick.
As can be seen in Table 20.7, both suberization and formation of the periderm
are temperature-dependent. Optimal curing conditions for potatoes are around 20°C
with RH of 80 to 100%.^54 With an RH of 90%, periderm formation took 6 d at 18°C,
10 d at 15°C, and 12 d at 13°C. Similarly, a RH less than 80% delays curing.^20
Curing at 15°C is commonly recommended to minimize decay. The processes of
suberization and periderm formation are similar for sweet potatoes and yams; how-
ever, higher temperatures (27^ to 29°C for sweet potatoes, 35^ to 40°C for yams) are
generally recommended.
Frequently, potatoes are harvested in the fall in temperate climates where tem-
peratures are colder than the optimal for curing.^55 However, respiration in storage
will heat the tubers and raise the RH. Temperature and RH control during this
“sweating” or curing time is managed by using fans to bring outside air into the
storage room, depending on the need, during the night or day. Once cured, a process
requiring 1 to 2 weeks, the tuber temperature is lowered by 1 to 2°C per day until
the desired maintenance temperature and RH are reached. In some cases, two
additional steps must be employed: drying of wet potatoes upon entry into storage
and warming tubers before removal from storage. Forced movement of air is used
to ensure uniform temperature throughout the storage pile. Maintaining 95 to 99%
RH is required at all times to minimize shrink and pressure bruising in storage.
Following curing, optimum storage temperatures are determined by the end use
of the tubers.^55 The respiration rate of potato tubers is lowest at 2 to 3°C, so this
temperature is recommended for long-term storage. Lower temperatures, 0 to 2°C,
increase the risk of freezing or chilling injury. Sprouting accelerates at temperatures
greater than 4 to 5°C, so seed tubers are commonly stored at these temperatures.
Tubers for fresh consumption are stored at 7 to 10°C, to minimize conversion of
nonreducing sugars such as starch to reducing sugars such as glucose, which darken
TABLE 20.7
Effect of Temperature on Rate of Suberization and
Periderm Formation in PotatoesTemperature (°C)Days to
SuberizationDays to Periderm Formation
after Suberization≥ 21 1 Data not given
15 2 1
10 3 3
5 5–8 > 10
Note:Average values; actual values vary with cultivar.
Source:derived from data in Thompson, J.F., Postharvest treatments, in
Postharvest Technology of Fruits and Vegetables, Blackwell Science,
Oxford, 1996, Chap. 4.