Agriculture Reference
In-Depth Information
melanoidin pigments and numerous other com-
pounds that contribute positively or negatively
to cooked product taste and aroma. Tubers with
elevated amounts of reducing sugars produce
unacceptably dark-colored chips or fries that
may have a bitter flavor that is undesirable to
consumers (Kumar
et al
., 2004). Processors may
reject tubers if the reducing sugar content exceeds
the commercially acceptable range. Acrylamide
is another product of the Maillard reaction, and
it is formed when reducing sugars react with free
asparagine. Acrylamide consumption is cur-
rently being investigated as a potential health
risk; maintaining low reducing sugars in process
and chip potatoes reduces acrylamide, which, in
turn, reduces the potential health risk (Bethke
and Bussan, 2013).
Cold-induced sweetening (CIS), also referred
to as low-temperature sweetening, typically de-
velops when potato tubers are stored at temperat-
ures at or below 10°C (Sowokinos, 2001b; Malone
et al
., 2006). The effects of CIS on reducing sugar
accumulation are typically observed
1-3
weeks
after tubers are transferred to excessively cool
temperatures, with higher accumulations devel-
oping at lower storage temperatures. Cultivars
differ widely in their susceptibility to CIS. Some
chip and fry processing cultivars resist CIS and
can therefore be stored successfully under 5.5°C.
Storage temperatures of less than 4°C almost in-
evitably result in unacceptable chip and fry color,
regardless of cultivar.
Sugar-end defect is a serious physiological
defect and quality concern of fry processing
cultivars (Thompson
et al
., 2008). The defining
characteristic of sugar-end defect appears when
the tubers are cut into fries and cooked. One
end of the fry will be darker in color than the
other end. It is usually the basal or stem end of
tubers that fries dark because it has high con-
centrations of reducing sugars relative to the
apical or bud end (Sowokinos
et al
., 2000; Mat-
suura-Endo
et al
., 2004). Sugar-end defects are
typically not observed at harvest, but develop
following weeks of storage. Unlike CIS, sugar-
end defects are not caused by postharvest con-
ditions and do not require low-temperature
storage to develop. Instead, sugar-end defects re-
sult from transient periods of heat or water stress
during early tuber development (Iritani
et al
.,
1973; Sowokinos
et al
., 1985; Shock
et al
., 1992,
1993; Kincaid
et al
., 1993; Eldredge
et al
., 1996).
Sugar-end defects may not become apparent
until tubers have been in storage for several
weeks, after which time defects become increas-
ingly severe. Unlike CIS, sugar- end defects cannot
be removed by reconditioning at warm temper-
atures (Thompson
et al
., 2008). The significance
of this is that once tubers develop sugar-end
defects, their value as fry processing material
may be reduced substantially.
Stem-end chip defect is similar to sugar-end
defect in that products made from defective
tubers have dark color, and reducing sugars are
high in tissues located at the stem-end of the
tuber. However, in stem-end chip defect, the de-
fect is more specifically localized at or near the
vascular regions (Wang
et al
., 2012). Stem-end
chip defects tend to develop in storage, but low
temperatures are not required for defect forma-
tion. Unlike sugar-end defects, stem-end chip de-
fects for many potato cultivars become less severe
following extended storage periods. Stem-end
defect is thought to be caused by environmental
or pathogen stress during tuber development.
Specifically, high temperatures during growth
and infection with the vascular pathogen,
Verti-
cillium dahliae
, are thought to contribute to stem-
end chip defect formation (Wang
et al
., 2012; Wang
and Bethke, 2013).
Senescence sweetening describes a develop-
mentally controlled accumulation of reducing
sugars that occurs several months after harvest
(Burton, 1966). Senescence sweetening is char-
acterized by a blush of dark color in the center of
chips and fries that appears after they are fried.
Pronounced end-to-end asymmetry is not a fea-
ture of senescence sweetening, and this helps to
differentiate it from sugar-end defect or stem-
end chip defect. Cultivars vary widely in how
long it takes before they begin senescence sweet-
ening; the sweetening begins to occur anywhere
from a few months to 9 months or more after
harvest. Conditions in storage that promote
higher rates of respiration tend to promote earl-
ier senescence sweetening (Herrman
et al
., 1996).
Senescence sweetening is an irreversible process
that can develop rapidly in storage; warming the
tubers will not improve chip or fry processing
quality. Moreover, increasing the storage tem-
perature in an attempt to recondition tubers
(decrease reducing sugars) may actually exacer-
bate the issue by increasing sugar accumulation
in the tuber.
