Abschlussbericht Projekt: „Verminderung der ... - BLE

various possible mechanisms like singlet-oxygen-quenching capacity, transition-metalchelating
activity and free-radical scavenging action (8). Potato tissue contains several enzymes
scavenging ROS [several types of superoxide dismutase (SOD), catalase (CAT),
peroxidase (POD)] and a network of low molecular metabolites like phenolic compounds
and vitamins (6, 8, 9). Also the pigment melanin possesses antioxidative properties. According
to Hung et al. (10) numerous studies refer to the participation of melanin in freeradical
reactions. At the stage of initiating chain reactions the function of the pigment
might be tapping transitional metals or inactivating O - 2 . Well, it is also known that the
process of melanogenesis requires O - 2 (11) or even molecular oxygen (O 2 ) to oxidise phenolic
substances to dopaquinone catalysed by the enzyme polyphenoloxidase (PPO) and
finally polymerise to melanin (2, 12). In potato tubers dark melanin pigments develop under
the intact skin after impact resulting in blackspot formation (2). This internal discolouration
is a major quality problem for fresh and processing market worldwide.
To quantify the effect of harvesting and handling operations or of culture practice on
melanogenesis different methods are described, e.g. pendulum, falling bolt and shaking
procedures, which provide information about tissue firmness (3, 12, 13). In the present
study a mechanical load test was used. This test is similar to commercial situations but the
control of the mechanical force is limited compared to other methods. Moreover, this
method is time consuming because there is a time slot of 48 hours between mechanical
treatment and visual evaluation. More rapid are methods which use photometric grading (1,
3, 13, 14). In general, these tests require homogenized material and the obtained results
allow the quantification of the final optical density of the studied sample solutions or the
rate of colour change over time. Via homogenization method the determination of the discolouration
potential (DP) of potato tubers was practiced in the past (3, 13, 14) and in the
present study. With respect to the measurement of antioxidative compounds potentially
responsible to prevent melanogenesis it is possible to measure all components individually,
but as far as known antioxidants work additively and synergistically (15). Therefore single
substance tests may not reflect the antioxidant functionality. Several analyses were developed
to determine the combined antioxidant activity (16-19).
The objectives of this study were to test and compare several methods for the determination
of blackspot susceptibility and to study the contribution of antioxidants (excepted
melanin) in the blackspot susceptibility in potato tubers.
Materials and methods
Material. Eight cultivars of table potatoes (Adretta, Afra, Gala, Granola, Lolita, Marabel,
Nicola and Renate) were grown near Dethlingen, Germany with conventional farming
methods during two vegetation periods in 2005 and 2006. From harvested plants tubers
with a similar size of 40 to 50 mm were selected to get comparable results. Analyses were
performed in tubers directly after harvest and after five and eight months of storage at 4°C
and 95 % relative humidity, respectively. Tubers were divided into six groups of specific
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