Agriculture Reference
In-Depth Information
temperature. Tuber responses to decreasing
temperatures are monitored by quantifying
tuber sugars. An uptick in tuber sucrose indi-
cates that reducing sugar contents are likely to
increase. If an increase in sucrose content is ob-
served, temperature ramping may be suspended
temporarily to allow sucrose contents to drop to
acceptable amounts.
The final stage of sugar management oc-
curs during long-term storage at a constant
holding temperature. The goal during this stage
is to identify problems in processing quality just
as they develop, so that appropriate action can
be taken to prevent raw product rejection by
the chip or fry processor. This is done by
tracking sucrose and reducing sugar content
during storage. An increase in sucrose may
indicate that tubers have experienced a stress,
such as a period of high CO
2
due to inadequate
ventilation, or that they are beginning to
undergo senescence sweetening. Corrective
action may be possible to reduce stress-induced
sugar accumulation, but senescence sweetening
is irreversible.
Sugar monitoring at holding temperatures
is used as an essential guide for subsequent man-
agement decisions. Prevention management of
tuber sugar accumulation includes changes in
the rate or duration of ventilation, a decrease in
allowable CO
2
content of the storage air, tem-
perature adjustment, and the timely removal
and processing of tubers. Because of the irre-
versible nature of senescence sweetening, stored
tubers should be monitored frequently, and cul-
tivar-specific guidelines should be followed if
available.
harvest, especially when tubers are harvested
during warm periods. Without cooling air, heat
produced by respiration can increase tuber tem-
perature in storage easily by more than 0.5°C
day
-
1
shortly after harvest. In addition to elevat-
ing CO
2
in the storage atmosphere and contrib-
uting to weight loss, excessive heat in storage
also aggravates disease problems by promoting
the growth of bacterial and fungal pathogens.
An abundant supply of stored energy is
available to tubers in the form of starch. This is
dissociated enzymatically into simple sugars and
sugar phosphates through the combined activ-
ities of enzymes in the amyloplast and cytosol
(see
Fig. 15.1
). Sugar phosphates enter glycoly-
sis and the mitochondrial tricarboxylic acid
(TCA) cycle, and are used to produce high-energy
compounds including ATP. This process gener-
ates CO
2
and water.
A simplified equation for the conversion of
carbohydrate to CO
2
and water during respir-
ation is:
C
6
H
12
O
6
+ 6O
2
® 6CO
2
+ 6H
2
0 +
2872 kJ mol
-
1
heat.
Although this equation does not capture
the biochemical complexity of the respiratory
process, it is adequate for understanding the sig-
nificance of respiration for potato storage. For
healthy potatoes in storage, the respiratory quo-
tient is close to unity in most cases (Burton,
1966; Isherwood and Burton, 1975). Hence, po-
tato tuber respiration is fully aerobic, and from
each mole of a
6-
carbon sugar,
6
moles of O
2
are
consumed and
6
moles of CO
2
and
6
moles of
water are produced. As a result, water produced
in respiration partially offsets water lost through
evaporation. The specific energy of glucose is ap-
proximately 2872 kJ mol
-
1
,
and approximately
this amount of energy is released as heat (Green
et al
., 1941).
15.5
Potato Respiration
Potato tubers are living entities, and like other
eukaryotic organisms, they produce energy via
aerobic respiration in mitochondria to maintain
cellular function. Respiration consumes oxygen
and produces oxidation by-products: heat, CO
2
,
and water. As described below, excessive tuber
respiration is often undesirable, as it leads to
weight loss via a reduction of stored carbon, a
build-up of CO
2
in storage, and an increase in
vital heat.
Respiration rates, and corresponding heat
production, are often highest immediately after
Respiration rates
The respiration rate of potato tubers has been
measured on many occasions (Burton, 1964,
1966; Schippers, 1977a,b; Dwelle and Stalknecht,
1978; Bethke and Busse, 2010). A typical seasonal
pattern is shown for cultivar Russet Burbank in
Fig. 15.2
(Dwelle and Stalknecht, 1978), which
