Enzymatic blackening in potatoes: Influence of pH on dopachrome oxidation

The effects of pH on the nonenzymatic reactions of dopachrome leading to brown to black melanin formation were examined. At pH 5.0, the changes in dopachrome absorbance at 480 nm and formation of melanin were slow. After 7–8 hours the absorbance decreased to approximately one half of the original dopachrome solution. At pH 7.0 the changes were much more rapid. After 17 min the absorbance decreased by 0.15. An equivalent decrease in absorbance at pH 5.0 required almost 4 hours. At pH 7.0 the absorbance at 480 nm decreased to a minimum after 43 min and then increased for 4 hours when the absorbance of the pinkish grey solution was the same as the original orange dopachrome solution. Thus, the pH of any treatment used to inhibit enzymatic blackening is very important in determining the amount of discoloration finally obtained. A pH of 5.0 or lower which would inhibit both the nonenzymatic and enzymatic reactions in the blackening reaction would be most desirable unless flavor or texture problems arise.     

Comparisons of Inhibitors of Tyrosine Oxidation in the enzymatic blackening of potatoes

Sodium bisulfite, cysteine, dithiothreitol, and sodium diethyldithiocarbamate (Na Dieca) inhibit enzymatic blackening in potatoes by initially inhibiting the oxidation of tyrosine by polyphenol oxidase. Sodium bisulfite and Na Dieca also cause enzyme inactivation. Although cysteine and dithiothreitol inhibit tyrosine oxidation for a period of time, they do not appear to inactivate the enzyme. Dithiothreitol is a much more effective inhibitor of tyrosine oxidation than cysteine. Ascorbic and dihydroxyfumaric acids inhibit enzymatic blackening by reducing the dopaquinone to dopa. When the reducing agents are completely oxidized, blackening can occur.     

Studies on after-cooking blackening in potatoes IV. field experiments

Summary Variations in the concentration of the main biochemical factors responsible for after-cooking blackening in potatoes have been examined in a number of varieties grown on a black fen soil and a clay soil. Highly significant correlations were found between blackening and the ratio of chlorogenic acid/citric acid in the individual varieties. Blackening was usually greater in the fen grown tubers and this was generally associated with the presence of less citric acid and sometimes more clorogenic acid. The annual fluctuation in blackening at a given centre was mainly due to the different amounts of chlorogenic acid and citric acid found in the tubers under the varying climatic conditions.     

Compositional and enzymatic changes associated with the sugar-end defect in Russet Burbank potatoes

This study was initiated to determine the starchsugar composition and the activities of relevant enzymes of carbohydrate metabolism in Russet Burbank (cv.) potatoes exhibiting the sugar-end defect. The frequency of sugar-end tubers was increased by applying a single, transient, moisture-deficit stress period following tuberization. The following properties were unique to sugar-end tubers compared to normal tuber tissue. (1) Starch and total solids decreased markedly while glucose content increased 15-fold, (2) The concentration of Suc was markedly lower, (3) Pi was slightly but significantly increased, (4) The activities of UGPase and Susy decreased nearly 3 and 2-fold, respectively, (5) The activity of AGPase decreased 50%, (6) The ratio of STPLase to AGPase shifted over 3-fold in favor of starch mobilization, (7) Basal AcInv activity (assayed in the presence of inhibitor) increased 7-fold during storage, (8) Tuber Glc concentration showed a better correlation to basal Aclnv activity than to total Aclnv activity (inhibitor destroyed), (9) Kinetic analysis suggested that the level and/or effectiveness of the Aclnv inhibitor was decreased in the sugar-end tuber tissue. These results are discussed in relation to metabolic changes which occur in converting a starch storing tuber to one primarily involved with starch mobilization.     

Influence of physiological ageing of seed potatoes on yield and earliness

Summary Ageing seed potatoes at ambient temperatures after long storage (26 weeks) at 2 °C affected rate of plant emergence and yields of three potato cultivars in Cyprus. Sprout growth at ambient increased until planting, and was bigger in cv. Spunta than in cv. Arran Banner and cv. Cara. Four weeks after planting the rate of emergence was higher in physiologically younger seed; total emergence was not affected but average yields were reduced by ageing. Early yields were less affected but final yields were significantly reduced by ageing; these effects were moro pronounced in Spunta and Arran Banner, early and medium maturity cultivars respectively than in Cara, a late maturity cultivar. Ageing of seed at ambient temperature was rapid and removing seed to ambient for more than 3 weeks before planting to save storage costs adversely affected final yields.

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Zusammenfassung Eine nach verl?ngerter Lagerung bei 2 °C erfolgte Alterung der Saatkartoffeln bis zu 9 Wochen in der Umgebungstemperatur beeinflusste bei den Sorten Arran Banner, Spunta und Cara das Auflaufen der Pflanzen und die Ertr?ge. Das Keimwachstum wurde nach überführung aus dem Kaltlager in die Umgebungstemperatur bis zum Auspflanzen nicht unterbrochen; die Keime von Spunta waren l?nger als diejenigen von Arran Banner und Cara (Abb. 1). Bei physiologisch ?lterem Saatgut liefen die Pflanzen schneller auf als bei jüngerem (Tab. 1). Fünf Wochen nach dem Auspflanzen war die Auflaufrate in allen Behandlungen ?hnlich und gr?sser als 80 % (Tab. 1). Drei Wochen nach dem Auspflanzen war die Auflaufrate der Pflanzen durch die Alterung erh?ht. Die Zwischen- und Endertr?ge wurden durch die Alterung signifikant beeinflusst Saatgut, das l?nger als 3 Wochen in der Umgebungstemperatur gehalten wurde, erbrachte geringere Zwischen- und Endertr?ge. Die Frühertr?ge wurden durch die Alterungsbehandlungen weniger beeinflusst (Tabellen 2, 3 und 4). Die Saatkartoffeln alterten schneller nach überführung in die Umgebungstemperatur und ein Verbleiben des Saatgutes in der Umgebungstemperatur für mehr als 3 Wochen führt zu geringeren Ertr?gen. Wenn auch der Alterungsprozess w?hrend der Kaltlagerung fortschreitet, ist eine Verl?ngerung der Kaltlagerzeit wünschenswert, um dann das Saatgut in die Umgebungstemperatur zu übertragen und dadurch Lagerkosten zu sparen.

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Résumé Le vieillissement physiologique de plants de pomme de terre obtenu par la conservation à la température ambiante pendant des durées de 0 à 9 semaines après un maintien prolongé à 2 °C, affecte la levée et le rendement des variétés Arran Banner, Spunta et Cara. Après que les tubercules aient été sortis de chambre froide, la croissance des germes se poursuit jusqu'à la plantation. Les germes de Spunta sont plus longs que ceux d'Arran Banner et de Cara (fig. 1). La levée des plants est plus rapide chez ceux physiologiquement vieux que chez ceux physiologiquement jeunes (tableau 1). Cinq semaines après la plantation, le taux de levée est identique dans tous les traitements et dépasse 80% (tableau 1). La levée des plants trois semaines après la plantation est accrue par le vieillissement. Les rendements intermédiaires et finaux sont significativement affectés par le vieillissement. Les plants placés pendant plus de 3 semaines à température ambiante donnent de plus faibles rendements intermédiaires et finaux. La précocité des rendements est moins affectée par les traitements de vieillissement (tableaux 2, 3 et 4). Les plants vieillissent plus rapidement à la température ambiante et ceux conservés plus de trois semaines dans ces conditions donnent une réduction de rendement. Toutefois, le processus de vieillissement se poursuit également durant la période de conservation au froid aussi l'extension de la durée de cette période est préférable à cette de la mise à température ambiante lorsqu'elle est réalisée dans un but d'économie.

     

Studies on after-cooking blackening. V. Changes in after-cooking blackening and the chemistry of Majestic and Ulster Beacon tubers during the growing season

Summary Variations in the concentrations of the main biochemical factors responsible for after-cooking blackening have been followed during the growing season in two varieties with widely differing blackening behaviour (Majestic andUlster Beacon). In both varieties the intensity of blackening decreased and the distribution changed from bud to stem end during the growing season. These changes in blackening were associated with changes in the amount and distribution of citric acid and chlorogenic acid. Highly significant correlation coefficients were found between blackening and chlorogenic acid, blackening and citric acid, and between blackening and the ratio of chlorogenic acid to citric acid. At each harvestMajestic tubers blackened more thanUlster Beacon tubers and always contained larger amounts of chlorogenic acid.

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Zusammenfassung Ver?nderungen in den Konzentration der haups?chlichsten biochemischen Faktoren, die für die Schwarzverf?rbung nach dem Kochen ausschlaggebend sind, wurden w?hrend der Wachstumperiode anhand von zwei Sorten, die sich im Verhalten gegenüber Schwarzverf?rbung deutlich unterscheiden, auf ihre Auswirkungen untersucht;Majestic (eine zur Schwarzverf?rbung neigende Sorte mit hohem Gehalt an Chlorogens?ure) undUlster Beacon (eine nicht zur Schwarzverf?rbung neigende Sorte mit tiefem Chlorogens?uregehalt). W?hrend der Wachstumsperiode nahm in beiden Sorten die Intensit?t der Schwarzverf?rbung ab, und die Verteilung der Verf?rbung wechselte vom Kronen-zum Nabelende (Abb. 4). Diese Unterschiede in der Schwarzverf?rbung waren verbunden mit Ver?nderungen bezüglich Menge und Verteilung der Zitronens?ure (Abb. 1) und Chlorogens?ure (Abb. 3). Hohe signifikante Korrelationskoeffizienten wurden zwischen Schwarzverf?rbung und Chlorogens?ure, Schwarzverf?rbung und Zitronens?ure sowie zwischen Schwarzverf?rbung und dem Verh?ltnis von Chlorogens?ure zu Zitronens?ure (Tabelle 3) festgestellt. Die Verteilung der Zitronens?ure in der Knolle stand in Beziehung zur Verteilung des Kaliums (Tabelle 1). W?hrend der Wachstumsperiode blieb die Kalikonzentration—abgesehen vom Erntezeitpunkt V—in der ganzen Knolle mehr oder weniger konstant, w?hrend die Konzentration der Zitronens?ure anstieg (Tabelle 2). Bei jedem Erntezeitpunkt zeigten die Knollen der SorteMajestic st?rkere Schwarzverf?rbung und enthielten gr?ssere Mengen Chlorogens?ure als jene der SorteUlster Beacon.

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Résumé Les modifications dans les concentrations des principaux facteurs biochimiques responsables du noircissement après cuisson ont été observées pendant la période de croissance chez deux variétés présentant des comportements au noircissement largement différents:Majestic (une variété qui noircit et qui a une haute teneur en acide chlorogénique) etUlster Beacon (une variété sans noircissement et qui a une basse teneur en acide chlorogénique). Chez les deux variétés l'intensité du noireissement diminue et sa répartition varie de la couronne au hile pendant la saison de croissance (Fig. 4). Les changements dans le noircissement sont associés aux changements dans la quantité et la répartition de l'acide citrique (Fig. 1) et de l'acide chlorogénique (Fig. 3.). Des co?fficients de corrélation hautement significatifs sont trouvés entre le noircissement et l'acide chlorogénique, le noireissement et l'acide citrique, et entre le noircissement et le rapport de l'acide chlorogénique à l'acide citrique (Tableau 3). La répartition de l'acide citrique dans le tubereule est liée à la répartition du potassium (Tableau 1). Pendant la saison de croissance la concentration du potassium dans le tubercule entier restait plus ou moins constante sauf à la récolte V, tandis que la concentration en acide citrique augmentait (Tableau 2). A chaque récolte les tubercules deMajestic noircissaient davantage et contenaient de quantités plus grandes d'acide chlorogénique que les tubercules deUlster Beacon.

     

Influence of the growth retardant B 995 on growth and yield of potatoes

Summary Stem elongation in a potato crop can be reduced by spraying the growth retardant B 995 on the foliage. This treatment generally did not change the total dry matter production. Only in the last period of growth did B 995-treated plots show a higher total dry weight than the untreated ones. This was caused by a slightly later senescence of the treated plots. The B 995 treatment caused a marked change in the dry matter distribution: stem weight was decreased and tuber weight increased, but leaf weight was generally not influenced. With plants showing considerable vegetative growth, these effects resulted in an increased tuber yield. High nitrogen plots treated with B 995 combined a long productive periode with a favourable dry matter distribution, producing a very high tuber yield at the end of the growing period.