Agriculture Reference
In-Depth Information
Table 16.2
Average Constituents of Potato Tubers.
Constituents
destined for processing into chips and crisps are stored at
higher temperatures to avoid the formation of dark brown
colouration due to caramelisation on cooking. Thus there is
a wide range of storage conditions used depending on
final use (Snowden 1991); seed potatoes are stored at
2-4°C and then exposed to warmth and light for “chitting”,
or sprouting before planting. 5-6°C is used for fresh market
potatoes, 6-8°C for chipping potatoes and 7-10°C for
crisping potatoes. This has important implications for
sprout control (see below).
Percentage (by weight)
Moisture 50-81
Protein 1.0-2.4
Fat 1.8-6.4
Starch 8-29
Nonstarch Carbohydrates 0.5-7.5
Reducing sugar 0.5-2.5
Ash 0.9-1.4
Carotene (average) 4 mg /100 g
Thiamine 0.10 mg /100 g
Riboflavin 0.06 mg /100 g
Ascorbic acid 12 mg /100 g
Source: From Diop and Calverley (1998).
Greening of stored tubers
If tubers are exposed to light, greening occurs in the
periderm and in the outer parenchyma cells of the cortex.
This is caused by the synthesis of chlorophyll and the
formation of toxic glycoalkaloids, including solanin, and
results in the development of a bitter taste. The glyco-
alkaloid content of potato tubers, but can be as high as
210 mg/100 g (fresh weight) (Rastovski
et al
. 1981). The
highest levels of glycoalkaloids are found in the periderm
and cortex; hence about 60% will be removed by peeling.
Potatoes containing amounts greater than 1 mg/100 g are
generally considered unsuitable for human consumption.
1983). Suberin acts both as a barrier to pathogen invasion,
and due to its characteristic as a highly hydrophobic com-
pound, prevents water loss through the wound. However,
there is evidence that it is the waxes associated with the
suberin polymer, rather than the polymer itself which con-
stitute the major diffusion barrier in native and wound
periderm (Espelie
et al
. 1980; Soliday
et al
. 1979; Vogt
et al
. 1983). Some studies suggest suberin has a directly
anti-fungal role (Kolattukudy 1984). This is followed by
cell division in a meristematic layer, the phellogen, that
develops below the wound, to form a new periderm (wound
periderm) or phellem. The process has been reviewed in
detail by Thomson
et al
. (1995).
Given that potato tubers tend to get damaged during
harvest, successful strategies for storing potato tubers
depend on keeping the tubers under conditions that promote
wound-healing for a few days immediately following
harvest. This process is called curing.
Curing is highly temperature dependent, increasing with
temperature, and requires high humidity to prevent water
loss which can inhibit the initial suberization stage. At 20°C
curing may take three to six days, while at 10°C it will take
seven to 14 days. There is variation among cultivars in the
efficiency of wound-healing (Wiggington 1974), and it has
also been noted that the ability to wound heal decreases
with storage time, with final periderm thickness decreasing
from 10 cell layers down to to two or three.
Tuber dormancy and sprouting
When harvested, potato tubers are in a state of dormancy
which may last weeks or months depending on the cultivar,
until the tubers sprout. In addition to the physical develop-
ment of the sprouts themselves, tuber sprouting is associ-
ated with further quality loss due to an increase in reducing
sugar, water loss and an increase in glycoalkaloid content
(Burton 1989; Suttle 2004a). For tubers destined for
processing, therefore, maintenance of tuber dormancy is a
critical aspect of successful potato storage. On the other
hand, rapid termination of tuber dormancy is desirable for
certain sectors of the potato industry such as for seed
certification trials and same-season use of seed potatoes for
southern markets.
Although the use of low-temperature storage to extend
shelf-life and prolong dormancy for ware potatoes is
widespread, the efficiency of this is less where tempera-
tures used for storage must be kept above a certain level to
prevent low temperature conversion of starch to sugar and
consequent poor processing quality. Further, in some situa-
tions (e.g. African highlands, and some parts of Central
America) controlled temperature storage is not feasible.
Extensive studies of the control of potato tuber dormancy
have been undertaken to identify strategies and chemicals
(both natural and artificial) for commercial sprout control.
More recently, work on the molecular biology of dormancy
Low-temperature sweetening
It has been known for decades that potatoes sweeten at low
temperature (Burton 1989) with the conversion of starch to
sugars. This is generally reversible, except after long-term
storage. As a result of low-temperature sweetening, tubers
