Agriculture Reference
In-Depth Information
used along with cool ambient air and evapora-
tive cooling to maintain temperature set points
despite ongoing heat production from the tubers.
24
22
20
15.6 Water Loss from Stored
Potatoes
18
16
Potato tubers are
75-
80% water by weight.
Water loss from stored tubers decreases product
value by decreasing the weight of tubers available
for sale. Water loss can decrease tuber quality by
increasing the incidence of pressure flattening
and pressure bruise that occurs in bulk storage,
as well as increasing tuber specific gravity, which
influences processing quality.
Water is lost from tubers by evaporation,
and the rate of water loss depends on the vapor
pressure difference (VPD) between the moist en-
vironment within the tuber and the ambient air.
Free water is contained within the cell wall ma-
trix in tubers, and in many cases it can safely be
assumed that the RH at the site of water evapor-
ation from the tuber is 100%. The outer layer of
the skin, or phellem, is the multicellular barrier
that reduces the diffusion of water from the tuber
to the atmosphere. Suberin, which is a mixture
of soluble waxes, poly(aliphatic) domains and
poly(phenolic) domains, is synthesized in the cell
wall matrix of phellem cells as they mature. The
aliphatic waxes of the phellem are primarily re-
sponsible for reducing the diffusion coefficient
for water vapor of the skin. Removal of the waxes
by chemical means increased water permeability
by
100
times (Schreiber
et al
., 2005). Burton (1966)
estimated that the presence of the mature skin
reduced the rate of diffusion from a potato tuber
by
300-
to 500-fold.
Cuts, scrapes, and physical forces that re-
sult in the skinning of tubers substantially in-
crease tuber water loss. Moderate skinning, less
than 5% of tuber surface area, can double water
loss in the short term. Subsequent wound heal-
ing and formation of a secondary periderm is
required to re-establish an effective resistance
to the diffusion of water vapor. This process
depends on the deposition of soluble waxes and
seems much less dependent on the synthesis and
deposition of the polymeric aliphatic and phen-
olic components of suberin (Lulai and Orr, 1995).
It is for this reason that all potatoes going into
14
12
10
8
6
Storage
Harvester
Windrow
Hand harvested
4
2
Oct
10
Oct
20
Date
Fig. 15.3.
Respiration rate of Katahdin tubers
1
or
11 days after harvesting by hand, or after one or
more mechanical harvest operations. (From
Pisarczyk, 1982; reprinted with permission from
Springer Science and Business Media.)
also cause an increase in tuber reducing sugars
and a darkening of fried product color (Dan-
iels-Lake
et al
., 2005b).
Computer-controlled ventilation systems
within many modern potato storages function to
minimize the adverse effects of tuber respiration.
Ventilation rates are adjusted to maintain a con-
stant small temperature differential across the
tubers, such that tubers farthest from the venti-
lation source are slightly warmer than those
close to the source. For bulk storage with a po-
tato pile
6
m deep, this temperature differential is
typically 0.25-0.8°C. Fresh, outside air is intro-
duced through external access doors as needed,
to flush out harmful concentrations of CO
2
.
Maximum allowable thresholds for CO
2
vary
with the market class of potato. They may be as
high as 3000 µl l
-
1
for seed and fresh market pota-
toes, or as low as 1200 µl l
-
1
for chip processing
potatoes. Supplemental refrigeration systems are
