Title : Browning susceptibility due to cutting and cooking in new hybrids of water yam (Dioscorea alata) as related to their total phenolic content, phenolic profile and polyphenol oxidase activity
Abstract:
Yam is a major staple food which provides both energy and bioactive compounds. In the French West Indies, over the past ten years, yam production declined by 70%, which was partly due to a fungal disease leaf anthracnose. New hybrids of Dioscorea alata resistant to anthracnose were created but some of them have quality flaws. Some hybrids are susceptible to browning when cut[1] and all of them show an extra change in color after cooking. Regarding cutting, the new hybrids exhibited contrasted susceptibility to browning in relation to their total phenolic content (r = 0.91). The detailed polyphenol profiles of “INRA15”, highly susceptible to browning, and of “Kabusah”, with moderate susceptibility to this flaw, were achieved by HPLC coupled to UV-Visible and mass spectrometry. For the first time, total procyanidins of yam were finely characterized and quantified using HPLC after phloroglucinolysis, revealing that those compounds are by far the main polyphenols in the two cultivars. Differences in terms of browning susceptibilities of the two cultivars are clearly explained by their contrasted polyphenol profiles: - (i) absence versus presence of catechin which is a well-known substrate of polyphenol oxidase (PPO). - (ii) significant differences in procyanidin levels and in their average degree of polymerization potentially involved in PPO inhibition.
Regarding cooking darkening, it is highly undesirable in potatoes[2] but has not been previously reported in yam. We determined the influence of boiling on color attributes, total phenolic content, phenolic profile (LC-UV-MS) and polyphenol oxidase activity in three varieties of yam. Boiling during 22 min led to darkening in all varieties. It also induced an average 42 % decrease in the total phenolics, flavanols and total procyanidins contents of the pulp without any significant modification in the structure of procyanidins. The decrease in the contents of phenolic compounds after cooking could be partly due to oxidation. Polyphenol oxidase activity was the highest in « INRA15 » and was inhibited only after 11 min in boiling water when the temperature of the core of the pulp attained 60 °C. Cooling at ambient temperature during one hour induced an increase in L* value and no significant change in total phenolic content. For the first time, the phenolic profile of flavanols and the polyphenol oxidase activity in the pulp of yam were determined sequentially throughout cooking. Cooking darkening in yam occurs during boiling and not during the cooling period after boiling, unlike what was reported in potatoes[2].
