Читаем Handbook of Vanilla Science and Technology полностью

Handbook of Vanilla Science and Technology

Daphna Havkin-Frenkel , Faith C. Belanger

The conversion of glucovanillin to vanillin during the curing process is shown in Figure 6.11A. After 8 days of curing at 50°C, the glucovanillin content decreased from an initial level of 14% to roughly 6% on dry weight basis. During the same period, the vanillin content, liberated from glucovanillin, rose to approximately 6%. The content of the two compounds leveled off afterward. The hydrolytic release of vanillin appears to be accompanied also by the accumulation of vanillic acid, p-hydroxybenzaldehyde, and p-hydroxy-benzoic acid (Figure 6.11B). An intriguing phenomenon is the accumulation of vanillin, whereas activity β-glucosidase, as well as other glycosyl hydrolases, declined during the same period. This occurrence casts doubt on the efficacy of the enzyme to catalyze hydrolysis of glucovanillin to vanillin. To explore this issue further, we examined the dependency of vanillin accumulation on enzymatic activity. Table 6.1 shows that application of either glucovanillin or β-glucosidase led to an increase in the vanillin content in curing fresh green beans and that vanillin content was increased further by the addition of both the substrate and the enzyme. However, when activity of endogenous β-glucosidase activity was abolished by tissue boiling, production of vanillin ceased altogether and could not be reconstituted, even by the addition of glucovanillin. Conversely, addition of β-glucosidase to the boiled tissue restored the process of hydrolytic release and accumulation of vanillin and was enhanced further by the addition of glucovanillin. Collectively, these results suggest that conversion of glucovanillin to vanillin is predicated on β-glucosidase-catalyzed action in the vanilla pod, a conclusion confirmed by Dignum et al. (2001b). Accordingly, controlled curing under laboratory conditions resulted in the disappearance of almost 95% of the glucovanillin with a potential yield of 5 to 7% vanillin on dry weight basis (Figure 6.10), suggesting sufficient enzymatic action to bring the hydrolytic release of vanillin to near completion. These results are supported by similar conclusions, suggesting that the level of β-glucosidase is not a limitation to the hydrolytic release of vanillin and merely determines the kinetics of the process (Odoux et al. 2006). Other studies (Odoux 2000) suggest, in contrast, that conversion of glucovanillin to vanillin during traditional curing in Reunion approached only 40% of the hydrolytic capacity of β-glucosidase. These results suggest that curing under traditional field conditions, yielding between 1.5 and 3% vanillin on dry weight basis, may not exploit the full potential of glycosyl hydrolases for vanillin release and accumulation. Alternatively, sub-optimal levels of vanillin may reflect losses of the formed compound during the prolonged drying and conditioning stages (Arana 1943; Broderick 1956a,b; Odoux 2000).

Fig. 6.11 Time-course changes in the content of vanillin (A) , vanillic acid, 4-hydroxybenzaldehyde and 4 hydroxybenzoic acid (B) in whole vanilla bean undergoing curing at 50°C. Reproduced with permission from Havkin-Frenkel, D., French, J.C., Graft, N.M., Pak, F.E., Frenkel, C. and Joel, D.M. (2004) Interrelation of curing and botany in vanilla (Vanilla planifolia) bean. Acta Hort. (ISHS), 629, 93-102.

Table 6.1 Vanillin content in fresh and boiled whole vanilla beans supplemented with glucovanillin (GV) and β-glucosidase

Hours of Curing | Tissue Condition | Compounds Added | Vanillin % of DW

0 | fresh tissue | none | 0.0

24 | ― | ― | 1.8

48 | ― | ― | 1.9

0 | fresh tissue | glucovanillin | 0.0

24 | ― | ― | 2.2

48 | ― | ― | 2.5

0 | fresh tissue | β-glucosidase | 0.0

24 | ― | ― | 4.2

48 | ― | ― | 3.2

0 | fresh tissue | β-glucosidase + GV | 0.0