Influence of the stage of ripeness on phenolic metabolism and antioxidant activity in table grapes exposed to different CO2 treatments

We have analyzed the influence of the stage of ripeness on L-phenylalanine ammonia-lyase (PAL) gene expression, accumulation of anthocyanins and total phenolics, and on antioxidant activity in the skin of table grapes treated with 20% CO₂ + 20% O₂ + 60 % N₂ for 3 or 6 d at low temperature (0 °C). The residual effect of high CO₂ treatment after transfer to air was also studied. In early harvested grapes, neither the anthocyanin content nor the accumulation of VcPAL mRNA was affected by any of the CO₂ treatments applied. However, in late harvested grapes, the duration of high CO₂ treatment determined its effect and a 6 d treatment with CO₂ sustained higher levels of total phenolics and anthocyanin accumulation, and VcPAL expression than observed in untreated late harvested grapes. The increased antioxidant capacity was correlated with the total amount of phenolics and anthocyanins. Conversely, in grapes treated for 3 d with CO₂ the phenylpropanoid pathway did not appear to be induced and a relationship between antioxidant activity and anthocyanins was not observed. Thus, further studies are needed to identify the most important antioxidants in these treated fruit.