Sensory detection thresholds of “ladybird taint” in “Riesling” and “Pinot noir” under different fermentation and processing conditions
The Asian ladybird beetles Harmonia axyridis feed on damaged fruits in late summer and in autumn, especially on grapes. By getting harvested and processed together with the grapes, they cause an off-flavor in the wine, the so-called "ladybird taint" (LBT). Depending on fermentation conditions of red wine variety 'Pinot Noir', panelists recognized the LBT at different concentrations: Fewer beetles were required in the nonheated treatment to elicit LBT. The sensory detection threshold for LBT in must fermented wines was about 3 beetles·kg-1 of grapes and in must heated wines it was about 6 beetles·kg-1 of grapes. In the white wine variety 'Riesling', the sensory detection threshold for LBT was at 4 beetles·kg-1 of grapes. The main olfactory active compound causing LBT, 2-isopropyl-3-methoxypyrazine (IPMP), was detected by 50 % of panelists at a threshold of 1 ng·L-1 in 'Riesling' and 2 ng·L-1 in 'Pinot noir'. Thus, a threshold of five beetles with a specific amount of IPMP each in hemolymph processed within 1 kg of grapes can reach the human detection limit of 1-2 ng IPMP·L-1 of wine. Modifications in wine processing conditions can reduce the LBT in wines. The mortality of beetles in the wine press can be reduced at pressures less than 2 bar and duration of pressing shorter than 60 minutes.
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