July/August 2019 - 22

The Potato Association of America mission statement: The PAA and
its allied associations must strongly support the private sector through
Extension, research and commercialization efforts aimed at enhancing
the role of the potato as the premier food source for the 21st century.
Putting the squeeze on pressure bruise
Minimizing early weight loss key in keeping
potatoes sturdy during storage
By Sastry Jayanty and Nora Olsen
Potato Association of America
ressure bruise is a major defect
and source of economic losses in
potatoes for all market classes.
In bulk storage, potato tubers are
subjected to pile pressure that can
damage and deform the potato, which
over time could result in flattened or
sunken areas. These flattened areas
are considered to be a pressure bruise
if there is a grey/black discoloration
under the depression 29 (see photos).
In some cases, there is no discoloration,
and these symptoms are called pressure
flattening. Discoloration symptoms
take time (hours to days) to develop as
the damaged area becomes exposed to
oxygen. Pressure flattened and bruised
tubers can be found throughout the pile
depending upon why it occurred, but is
often found between the vent pipes or
next to the adjacent walls of the storage
bin in the bottom 4 to 5 feet of the bulk
storage. This is due to a greater force
of weight (the pile of potatoes) on less
hydrated potatoes.
The development of pressure
flattening is a result of dehydration
of the tubers. Loss of tuber hydration
can happen before and after vine kill
and prior to harvest if soils become
too dry. Moisture loss from tubers can
also occur during harvest and handling
due to wounding, skinning, cuts and
bruises - increasing exposed surface
area for evaporation to occur.
In storage, tubers lose moisture due to
a high vapor pressure deficit (function of
temperature and humidity), wounded
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Left, symptoms of pressure flattening: sunken and depressed area. Right, an area cut open to
expose black discoloration of a pressure bruise.
or poorly suberized potatoes, sprouting,
improper ventilation and humidity and
high respiration.
Measuring tuber texture with a
texture analyzer can indicate turgidity
of the tuber, cellular arrangement, skin
toughness and the tubers' overall ability
to withstand force prior to deforming -
potentially causing a pressure bruise.
Texture analysis of a tuber (y-axis)
correlates to tuber moisture content
(x-axis) and can indicate differences
between cultivars (Figure 1, Page 23).
Greater force is needed to deform tuber
tissue correlated to a more hydrated
tissue. Cultivars vary in the potential
for moisture loss and susceptibility to
pressure bruise.
Earlier research indicated that
dehydrated tubers are more susceptible
to pressure bruise in long-term storage.
New research from Colorado State
University shows that measuring tuber
texture at harvest can identify potato
fields that may have a greater risk for
pressure bruise earlier in the storage
season due to coming into the storage
dehydrated. Using this type of tool,
growers can identify which lots or fields
that may need to be managed differently
in storage or cannot be kept for long
period of time in storage due to the
potential for pressure bruise to form.
Measuring texture of potatoes at
harvest to get an idea of tuber hydration
and how the potato may respond to
the storage environment and potential
for pressure bruise is a good start to
an integrated approach to minimize
weight loss in storage.
REDUCING THE RISK
Decreasing the potential for pressure
bruise in storage is a complex matrix
of actions that encompasses many
considerations, such as status of the
potato crop, pulp temperatures, weather,
temperature set points, humidification
and ventilation capacity, delta T and pile
height. The majority of the weight loss
often occurs in the first 30 days in storage,
making early storage management the
time to focus on minimizing weight loss
and, therefore, pressure bruise.
Utilize the basic integrated principles of
postharvest storage - airflow, humidity

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July/August 2019

Table of Contents for the Digital Edition of July/August 2019