The relation between glucovanillin, beta-D-glucosidase activity and cellular compartmentation during the senescence, freezing and traditional curing of vanilla beans

The aim of this research was to improve our understanding of the mechanism of glucovanillin hydrolysis by beta-D-glucosidase activity in vanilla beans by studying their senescence, freezing and traditional curing. A batch of green pods from Madagascar was ripened at 30 degrees C until fruits turned black; another batch was frozen for few days at -18 degrees C and defrosted at 35 degrees C for 24 h and a third batch was cured using traditional methods. During treatments, samples were analysed for the yield of glucovanillin hydrolysis, and beta-glucosidase activity was measured. Cellular structures were also examined by light and transmission electron microscopy. Green fruits had a low yield of glucovanillin hydrolysis (< 5%), a high level of beta-glucosidase activity (similar to 1000 nkatal g(-1) fresh weight) and a perfect cellular integrity. Senescent fruits had a high yield of glucovanillin hydrolysis (> 95%), no measurable beta-glucosidase activity and complete cellular degradation. Similar results were observed in beans after defrosting. During curing, beans had a medium yield of glucovanillin hydrolysis (< 50%), no measurable beta-glucosidase activity and partial cellular degradation compared with senescent or defrosted beans. Results show that the mechanism of glucovanillin hydrolysis in vanilla beans is regulated by cellular compartmentation and that the beta-glucosidase activity level is not the limiting factor for complete hydrolysis. If total decompartmentation is obtained, then complete glucovanillin hydrolysis is observed even if most of the beta-glucosidase activity is lost. The beta-glucosidase activity level only has an effect on glucovanillin hydrolysis kinetics.