Elsevier

Scientia Horticulturae

Volume 168, 26 March 2014, Pages 249-257
Scientia Horticulturae

Determining the harvest maturity of vanilla beans

https://doi.org/10.1016/j.scienta.2014.02.002 Get rights and content

Highlights

  • Judging bean maturity is difficult as they reach full size soon after pollination.

  • Glucovanillin accumulates from 20 weeks, maximum about 40 weeks after pollination.

  • Mature green beans have 20% dry matter but less than 2% glucovanillin.

  • The accumulation of dry matter and glucovanillin are highly correlated.

  • Dry matter measured by NIR spectroscopy is a reliable surrogate for glucovanillin.

Abstract

Current methods for determining the maturity of vanilla (Vanilla planifolia Andrews) beans are unreliable. Yellowing at the blossom end, the current index, occurs before beans accumulate maximum glucovanillin concentrations. Beans left on the vine until they turn brown have higher glucovanillin concentrations but may split and have low quality. To find a better index, changes in bean dimensions, dry matter (DM) and glucovanillin accumulation were followed over four seasons. Beans reached their maximum length and width before glucovanillin and DM accumulation began, and there was no clear point when their weight ceased to increase. Beans harvested when the blossom end turned yellow had lower glucovanillin concentrations than beans harvested later; glucovanillin accumulation continued until beans started to turn brown on the vine. Therefore, bean colour, dimensions, weights and glucovanillin content are not useful indicators of maturity, and the lack of visual changes until beans senesce explains why commercially cured beans vary in quality. DM accumulation reached a maximum before browning and was highly correlated with glucovanillin content; the central portion was the most representative of the entire bean. Therefore, optimum harvest time occurs when DM accumulation slows and should be measured in the central portion of beans. Two near infrared spectrometers using interactance geometry were trialled for non-invasive assessment of DM. Cross validation r and RMSECV values of 0.87 and 1.76, respectively, for a unit using wavelengths between 1100 and 2300 nm, and 0.82 and 1.05 for a portable unit using wavelengths between 800 and 1050 nm were obtained from the second derivative of absorbance spectra. The latter unit allows infield monitoring of maturation.

Introduction

Vanilla is an expensive and popular flavouring used by food, pharmaceutical and cosmetic industries and has considerable potential as a food preservative. The annual world production of cured beans was estimated to be approximately 9000 T in 2011 (FAO, 2011). Natural, cured vanilla is a complex product, and its distinctive flavour and aroma comes mainly from the phenolic compound, vanillin, and other aromatic compounds that comprise less than 2% by weight of fresh vanilla beans. Glucovanillin is the most important storage form of vanillin; it is odourless, and mature, green pods do not generate their characteristic flavour or aroma until they are cured during which time glucovanillin is converted to vanillin.

If vanilla beans are left on the vine, they turn yellow, and then brown and eventually split in two (dehisce) from the blossom end. Overripe beans may also split during curing (Purseglove et al., 1981), which results in loss of aroma, requiring them to be sold at lower prices than whole beans. In practice, because flowering and fruit set are protracted, many beans are harvested immature resulting in a large variation in vanillin and other aroma compounds among individual cured beans (Van Dyk et al., 2010). Research by Van Dyk et al. (2010) showed that growing conditions (water supply and weather) and the physiological age of the beans at harvest are important factors affecting the quality of cured vanilla beans. However, the concentration of vanillin in the cured beans was highly variable, suggesting that levels of this phenolic are not suitable for estimating harvest maturity.

The appearance of developing vanilla beans remains unchanged for many months, and they are not considered commercially mature until 8–9 months after pollination. One of the traditional indicators of commercial maturity is the change in the colour of the blossom end of the bean from green to yellow. Sagrero-Nieves and Schwartz (1988) investigated the influence of harvest time on the concentrations of the major phenolic constituents of V. planifolia and found that phenolics increased as the beans matured; however, no correlation was observed between the colour of the beans and their glucovanillin content. These findings agree with earlier studies by Jones and Vicente (1949) and Arana (1944). Thus, colour changes do not appear to be a good indicator of harvest maturity, and a better indicator is required.

Near infrared spectroscopy (NIRS; involving the spectral range 750–2500 nm) is suited to the assessment of water and other constituents in plant material, due either to a strong spectral signature from the constituent of interest, or through a secondary relationship of the constituent of interest to another that can be assessed using NIRS. Lower cost, silicon detector-based NIRS instrumentation operating in the short wave near infrared region (750–1050 nm) has found application in agriculture. For example, Walsh et al. (2004) report the use of NIRS for assessment of dry matter (DM) content of several fruit types. In addition, Subedi et al. (2007) demonstrated the use of NIRS for on-tree measurement of the dry matter content of mango fruit and the prediction of total soluble solids after ripening. The typical root mean square error of prediction (RMSEP) reported in these studies was around 1% DM. Consequently, there may be potential for a NIRS-based measurement to be used for the judgement of vanilla bean maturity and, consequently, the production of high quality vanilla beans. This measurement would be especially valuable if it could be made in the field. The objective of this study was, therefore, to test the preceding hypothesis.

Section snippets

Plant materials

Experiments following the pattern of accumulation of glucovanillin and dry matter during growth and development of the beans were conducted over four seasons.

Experiment 1 Queensland (QLD) 2009 and Experiment 2 QLD 2010: Flowers (300–350) of Vanilla planifolia Andrews (Asparagales: Orchidaceae) were hand-pollinated and tagged on 1st November, 2008 (Expt. 1) or 28th October, 2009 (Expt. 2) at the plantation of Daintree Vanilla and Spice, Daintree, Queensland. Beans were collected at 14 different

Growth, total vanillin concentration and dry matter content during maturation

Experiment 1, 2009 (data not shown): The harvest weight of the beans was highly variable during the assessment period (20–35 weeks after pollination), with bean weights of varying from 10 to 15 g. However, during weeks 20–35, the dry matter percentage increased linearly (P < 0.0001, r2 = 0.98). Total vanillin also increased linearly (P < 0.0001, r2 = 0.94) and was highly correlated with the increase in dry matter percentage (P < 0.0001, r2 = 0.99). No yellowing or browning of the beans was observed.

Discussion

This study has followed the phenology of vanilla beans as they ripen and the concomitant changes in dry matter and total vanillin content over four seasons in beans obtained from a commercial plantation and from a planting in a controlled environment greenhouse. The critical events in the development and maturation of beans observed in the greenhouse (Experiment 3) are summarised in Table 6. This experiment enabled more detailed observations during growth and development than was possible at

Acknowledgements

This project was financially supported by Mr Paul Squires, Australian Vanilla Plantations, New South Wales, Australia. The help of Daintree Vanilla & Spice, Queensland, Australia is also acknowledged for access to his plantation and supplying developing vanilla beans for this project.

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