Discolouration of cooked potatoes: Proteomics points to a solution

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  • Published: Jan 10, 2011
  • Author: Steve Down
  • Channels: Proteomics & Genomics
thumbnail image: Discolouration of cooked potatoes: Proteomics points to a solution

After-cooking darkening: the affliction of cooked potatoes

The discolouration that blights cooked potatoes has plagued the potato industry for many years. This phenomenon, known as after-cooking darkening (ACD), is one of the most undesirable characteristics and has been reported from every potato-growing area in the world. It takes place, as the term implies, when cooked potatoes are exposed to air and produces coloured areas ranging from grey though blue and purple to black.

Although ACD has no effect on the flavour or nutritional value of potatoes, it bestows an unpleasant appearance and is an unwelcome side effect of cooking. Boiled and steamed potatoes are most affected but fried, dehydrated and reconstituted-rehydrated potatoes are also marred.

The mechanism of ACD is entirely different to the discolouration of cut surfaces of raw potatoes, which is produced by an enzymatic reaction. Rather, ACD occurs when the complex between chlorogenic acid and ferrous ions that is formed during cooking is subsequently oxidised in air in a non-enzymatic reaction to the coloured ferric form.

Potato tubers with higher amounts of chlorogenic acid appear to turn darker, but other factors are also involved. For instance, citric acid reduces the degree of darkening, probably by forming a colourless complex itself with iron.

Within the industry, processed potatoes are treated with sodium acid pyrophosphate to capture iron and ease the darkening but this introduces an added step and increased costs. A better solution would be to use potato cultivars that tend to produce less darkening.

Potato proteomics: wound-response proteins correlated with after-cooking darkening

Research has already established a strong genetic trait to ACD but the underlying molecular mechanisms are unclear. So, scientists from Canada have compared the proteomic profiles of potato tubers to try and identify proteins that are implicated in ACD.

Gefu Wang-Pruski and Fanming Kong from Nova Scotia Agricultural College, Truro, with Devanand Pinto from the NRC-Institute for Marine Biosciences, Halifax, and Patrick Murphy from Dalhousie University, Halifax, examined six sets of potatoes from a breeding family clonally propagated by the Agriculture and Agri-Food Canada Potato Research Centre.

Three clones exhibiting a high susceptibility to ACD and three with a low susceptibility were studied, all with a highly segregated degree of darkening. ACD is more prevalent at the stem ends of the tubers, so tissue from these portions of the steamed potatoes was removed for analysis. The apical ends of the high ACD clones were also examined.

The proteins were extracted from each group and digested with trypsin. The resultant peptides were modified by reductive methylation which labelled the N-termini of any lysine residues with either formaldehyde-H2O, formaldehyde-d2-D2O or [13C]formaldehyde-d2-D2O. By pooling the differentially labelled peptides from the different clones and subjecting them to LC-tandem-MS analysis, the relative amounts of the peptides and their precursor proteins could be estimated.

A subsequent clustering analysis led to a set of 30 proteins which correlated with ACD, including protease inhibitors. Three aspartase protease in this group implicated enhanced wound-response mechanisms involving protease inhibitors in potatoes with high ACD.

This conclusion was supported by the increased abundances of patatins and lipoxygenases. Patatins are storage proteins which also act as lipid acyl hydrolases and are involved in wound response. Lipoxygenases are also implicated in wound response, helping to express protease inhibitors as part of the plant systemic response to wounding.

Five of the proteins were validated at the transcript level using real-time quantitative PCR.

The research team proposed that wound response took place in conjunction with an increase in polyphenols (chlorogenic acid) synthesis, leading to darkening.

Although this is a small sample set, knowledge of these proteins that are associated with ACD will help in the development of potato varieties that will be more resistant to darkening without affecting the other desirable properties.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The wound response proteins associated with the black discolouration of potatoes following cooking have been identified by quantitative proteomics and cluster analysis, which should help to produce varieties with a reduced darkening tendency

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