The Canon of Potato Science: 47. After-cooking Darkening

Abstract

Potato after-cooking darkening (ACD) is a tuber colour defect that occurs after cooking, including boiling, baking, frying or dehydration. The dark colour ranges from light grey to almost black, depending on cultivar. The development of the colour is caused by a non-enzymatic oxidation process during cooking, and reaches its maximum level in about 2 h after cooking is completed. After-cooking darkening is one of the most widespread and undesirable characteristics of cultivated potato. Although it does not affect the flavour or nutritional value of the potatoes, the dark colour significantly affects the appearance of products. After-cooking darkening is caused by the oxidation reaction of the ferrouschlorogenic acid complex, resulting in a bluish-grey compound ferri-dichlorogenic acid. It is suggested that tubers with higher concentration of chlorogenic acid show higher degree of ACD. Therefore these tubers have higher susceptibility to ACD. Although iron (Fe) is involved in the reaction, its concentration does not determine the degree of ACD in tubers. However, iron chelating agents, such as citric acid, ascorbic acid, malete, phosphate, and EDTA, can decrease the degree of ACD by preventing the iron to react with chlorogenic acid. The degree of the darkness in a tuber is not evenly distributed: it is always higher at the stem end and in the cortex tissue just under the skin, and lower at the bud end and the central location of the tuber. After-cooking darkening is a quantitative trait that is predominantly controlled by genetic factors. Therefore, it is cultivar dependent. Other tuber quality traits, such as skin colour, flesh colour, specific gravity and frying colour, are not related to ACD. Potato Research (2007) 50:403–406 DOI 10.1007/s11540-008-9061-x