The edible potato quality paid attention by the contemporary Polish consumer is assessed mainly on the basis of external features (skin appearance and defects, tuber shape and regularity, mesh depth) and internal features (chemical components) as well as organoleptic features (culinary and sensory characteristics). One of the most important features determining the usefulness of potato varieties for direct consumption and for processing is potato darkening. However, this feature does not affect the taste and nutritional value.
DARKENING OF RAW POTATO
The darkening of raw tuber flesh is caused by enzymatic oxidation of phenolic compounds contained in potato tubers, mainly the aromatic amino acid tyrosine and phenolic acids, in particular chlorogenic acid and caffeic acid. This process is primarily the responsibility of the active enzyme polyphenol oxidase (PPO, phenolase), which with the participation of atmospheric oxygen, catalyzes the first stage of biochemical conversion of phenolic compounds to quinones, which form melanin compounds of dark brown color. It is melanin that causes the unfavorable coloration of potato tubers.
The darkening of the raw flesh is uneven in the tuber, which is the result of an irregular distribution of phenolic compounds in individual parts of the tuber.
It is a varietal feature and its intensity additionally depends on the duration of the oxidation reaction. The course of this reaction also depends on the presence of molecular oxygen contained in the air and on the acidity. The reaction does not take place in acidic or basic environments.
A tuber whose raw flesh reaches a rating of more than 6.5º (inverted 9-point scale) is considered to be a slight darkening.
DARKENING OF COOKED POTATO
The color of the flesh of cooked tubers also determines their aesthetic value. It is desirable that the flesh color remains stable and does not change as much as possible.
The darkening of the tubers after cooking is a result of non-enzymatic processes of oxidation of iron (II) ions bound in colorless complexes with phenols (mainly with chlorogenic acid) to iron (III), which causes gray coloration of these complexes. This is a chemical process dependent on genetic and environmental factors that affect the concentration of chlorogenic acid, citric acid, ascorbic acid and iron. These compounds determine the degree of dark coloring of the flesh, which can range from gray to almost black. The strength of the chemical darkening is determined by the ratio of chlorogenic acid to citric acid. The darkening is counteracted by organic acids present in the tuber, mainly citric acid, which binds with iron ions into colorless complexes.
Non-enzymatic darkening occurs unevenly throughout the tuber. The stolon part of the tuber has a higher pH, which favors the binding of chlorogenic acid with iron and therefore causes darker coloration than in the apical part.
RESULTS OF THE EXPERIMENTS
After conducting experiments on three varieties of edible potato, the researchers showed that the darkening of raw and cooked potato flesh depended on varieties and weather conditions. Both cooked and raw potatoes darkened the most in the coldest and wettest year. Thus, in wet and cold years, the tendency to darken is greater than in warm seasons with optimal rainfall. Varietal response to different weather conditions has been shown to vary. It should be stressed, however, that in spite of proven statistical differences, the differences within this trait were very small.
Herbicides and biostimulators applied in the experiment (i.e. preparations stimulating vital processes in plants and improving their health status) did not change the darkening of flesh of raw potato tubers, while they significantly increased the darkening of flesh of cooked potatoes.