Abschlussbericht Projekt: „Verminderung der ... - BLE

gravity from 1.095 kg L -1 , with 0.01 increments, whilst immersing them in vessels
with demineralised water and different NaCl-concentrations. Subsequently samples
were divided into two parts. Tubers from the first part were used to determine their blackspot
index (BSI). Tubers from the second part were used for analysis. Also, nonfractionated
tubers were peeled (1-2 mm thickness) using a hand peeler to obtain periderm
and non-periderm tissue. Samples were sliced and used fresh or were shock-frozen in liquid
nitrogen and stored at -30 °C until testing or freeze-drying (Epsilon 2-40, Christ).
Analyses. Blackspot index (BSI) of whole potato tubers was determined according to the
method of the German “Bundessortenamt” (BSA) (Federal Plant Variety Office). The tubers
were cooled down to 4 to 5°C and mechanically treated for 50 s in a vegetable washing
machine with a rotating drum (Flott 18 K, Flottwerk H. J. Dames GmbH & Co. KG,
Rotenburg a. d. F., Germany) with three replications. Each replication consisted of an
amount of tubers (Σ tuber ) adequate to a volume of 6 L. Afterwards, treated tubers were
stored at room temperature for 48 hours. For BSI determination they were cut into halves
length-wise and a half tuber was evaluated visually using a scale with four scores of discolouration:
No discolouration indicated no melanin development and therefore no blackspot
susceptibility (Σ 1 ). Discolouration till quarter of length and less than 5 mm in-depth
showed slight blackspot susceptibility (Σ 2 ). Medium blackspot susceptibility corresponded
to a discolouration till quarter of length and above 5 mm in-depth or a half of the length
and maximal 5 mm in-depth (Σ 3 ). Serious blackspot susceptibility was indicated by a
stronger discolouration (Σ 4 ). The blackspot susceptibility, expressed as the percentage of
discoloured tubers was calculated from following equation:
BSI (%) = (0.3∑ + 0.5∑ 3
+ ∑ 4
) / ∑ * 100
(1)
2 tuber
The discolouration potential (DP) of tubers was determined in fresh material using a homogenization
method (14). Tubers differing in their specific gravity were analysed. According
to Lærke et al. (1), Dean et al. (3) and Delgado et al. (13) stem end and bud end of
tubers were mixed using an ultra-turrax (Janke & Kunkel, IKA lab technics, Germany).
Absorbance was measured at 475 nm with an UV-Vis spectral system (HP 8453, Germany).
Results are the mean of three replications and expressed as absorbance units
(AU 475nm ).
Investigation of the ferric reducing ability of plasma (FRAP) (20) was performed with tubers
of different specific gravities, as well as with tubers separated in periderm and nonperiderm
tissue. Sample preparation was done at 4°C by mashing the frozen samples in a
cool mortar placed in iced water. Three phases of each sample were extracted to obtain
material for the determination of free, cell wall associated compounds (8, 21, 22) and water
insoluble compounds (23), respectively. From 3 g of mashed sample the free compounds
were extracted with 0.1 M KH 2 PO 4 buffer (pH 6.0). Double extraction was performed for
cell wall associated compounds with 1M NaCl solution. Water insoluble substances were
extracted with n-hexane. Extracts were measured with an UV-Vis spectral system (HP
222