612 Produce Degradation: Reaction Pathways and their Prevention
content is less desirable in other vegetables with high carbohydrate content such as
tubers like potatoes, sweet potatoes, and yams and swollen tap roots like carrots,
parsnips, horseradish, and turnips.
Low temperature sweetening (LTS) occurs in roots and tubers when some of
the breakdown products of starches normally destined for respiration accumulate as
excess sugar.^13 Wismer and coworkers^18 provided an in-depth discussion of this
phenomenon and proposed mechanisms for its occurrence.
In potatoes, LTS occurs at temperatures lower than 10°C that are often used to
minimize respiration and sprouting during storage.^18 The LTS is characterized by a
breakdown of starch to reducing sugars. Thus, the sugars that accumulate plus those
consumed in respiration equal the amount of starch metabolized. Structural polysac-
charides (e.g., cellulose and pectin) are not degraded and do not contribute to the
excess sugar. Processed products such as chips made from potatoes that have under-
gone LTS tend to be dark brown due to the presence of reducing sugars that
participate in Maillard browning during frying. Due to the increase in potato tuber
sugar content that occurs early in cold storage, and over 2 to 3 months at storage
temperatures of 1 to 3°C, tubers can lose as much as 30% of their starch content.
Pollock and ap Rees^19 reported that both sucrose and reducing sugars increased
within 5 d in tubers stored at 2°C, and after 20 d in storage the sugar content was
approximately six times greater than at d 0.
Reconditioning tubers by storing them at 18°C after cold storage may be used
to decrease the sugar content and raise the starch levels.^18 However, the response to
reconditioning is not consistent or completely restorative and tends to be cultivar-
dependent.
20.3 REMOVAL OF FIELD HEAT
20.3.1 WHY PRECOOL?
Temperature control of produce begins in the field. At harvest, the temperature of
fruits and vegetables is close to that of ambient air, which, depending on the location
and the time of the year, may be as high as 40°C.^2 In order to ensure the lowest possible
temperature at harvest, it is generally recommended that harvest of most fruits and
vegetables occur in the coolest part of the day, usually early morning. Exceptions to
this recommendation is produce such as citrus fruit, which is damaged if handled in
the morning when it is turgid, and situations when fruit is harvested late in the afternoon
so that it can be transported to a local market during the cooler night hours.^20
After harvest, produce should be handled to keep it as cool as possible until
removed from the field. Exposure of picked fruit to direct sun can result in a
significant rise in fruit temperature and an associated loss of quality. Nelson^11
reported that the temperature of grapes held in the sun was as much as 7°C above
air temperature while that of shaded fruit remained at least 3°C below air tempera-
ture. This temperature range can mean the difference between fruit that is acceptable
and fruit that has lost a significant amount of quality.
Time between picking and cooling should be kept to a minimum. Rapid cooling
is beneficial because lower temperatures lead to reduced metabolic rates, decreased