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Saffron (Crocus sativus) : post harvest technology for optimising spice quality - and new product potential

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Gregory, MJ (2010) Saffron (Crocus sativus) : post harvest technology for optimising spice quality - and new product potential. PhD thesis, University of Tasmania.

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Abstract

ISO and alternative analytical techniques were applied to Australian saffron. These analysis methods were used to assess quality in comparison to the international standard and products from other major producing countries. While Australian saffron was generally shown to be of high colour quality, also identified were the potential for greatly improving aroma strength and the risk of insufficient drying leading to degradation of components during storage.
The deficiencies of the ISO method for determination of aroma strength have been referred to previously by a number of authors. In this study statistical proof of the inaccuracy of the aqueous extraction and absorbance measurement was demonstrated for the first time. Dual polarity extractions and chromatographic techniques were therefore used for all measurement of saffron components in subsequent experiments.
Experiments were conducted to investigate the optimisation of post harvest treatment of stigmas for maximizing aroma generation and colour retention. Critical to this are conditions of stigma drying during which aroma is generated. In Australia, saffron is dried at moderate temperatures (40- 50 °C) with airflow over approximately 90-120 minutes. Elsewhere a great variety of methods and conditions are used without any consensus on what constitutes optimal drying conditions. Moreover, doubts about the previously accepted chemical mechanism for safranal (the major aroma compound) generation together with proposed alternative pathways have recently surfaced.
These studies showed that drying saffron at elevated temperature (= 90 °C) produces significantly more safranal while also enhancing colour strength. This latter effect was shown to be due both to greater retention of crocin pigments (from prevention of enzymatic degradation of these pigments) and increased availability of crocins (due to changes in inter- and intra-cellular structure). The use of airflow to speed drying was shown to be detrimental to aroma content.
For the first time the heating profile of stigmas was measured to show that evaporative cooling is a significant factor during drying. Humidification of the drying chamber was shown to slow the rate of filament drying while also reducing evaporative temperature suppression. This prolongs the period that stigmas remain within an intermediate range of water activity where hydrolysis of the glycoside (picrocrocin) to form the volatile aglycon (safranal) is favoured at relatively high temperature (90 °C). It also causes the stigmas to heat more rapidly to a temperature at which this conversion occurs and also where enzymatic pigment degradation is de-activated.
Under these conditions at least 3 fold increases in safranal production were demonstrated without the need for alternative generation mechanisms via thermal crocin degradation to explain safranal gains. Drying at moderate temperatures (< 60 °C) involves the action of β-glucosidase on picrocrocin to produce an intermediate compound, followed by subsequent dehydration to form safranal. It was shown that where enzyme activity had produced a significant amount of the intermediate, the end product was not safranal when the subsequent drying was at higher temperatures. This provides a possible explanation for observations of picrocrocin loss without corresponding safranal gain that has led others to doubt the previously accepted safranal production pathway.
It was proposed that while the formation of safranal from crocins may also occur at very high temperatures (>> 100°C), this process is relevant only to the culinary use of the spice rather than as a factor during drying. Notwithstanding this, the conditions of drying could well determine the potential for subsequent aroma formation during cooking. Studies of the cellular structure and porosity of saffron demonstrated how different drying conditions affect both the availability of pigments for aqueous extraction and filament strength.
Drying at high temperature was shown to enhance the dissolution of crocins, but also increase brittleness. Elevated humidity drying prevented this brittleness by reducing intercellular spacing (porosity) while maintaining the enhanced colour availability. It is proposed that thermal cell membrane disruption (allowing rapid crocin dissolution) had occurred independent of humidity determined cell shrinkage rates.
Distillations and extractions were undertaken to assess the potential for new products from the waste flower parts (petals and stamens). The best fragrance product obtained was a non-polar extract, though it required a small content of stigmas included with the flowers extracted to give it aroma "impact". This product had a vivid yellow/orange colour derived from its content of carotenoid fatty acid esters. These carotenoids were identified as di-esters of lutein isomers. Mid-polarity extracts of waste flowers were shown to contain a high content of flavonoids, particularly glycosidic conjugates of kaempherol; compounds which are known to be potent antioxidants.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 2010 the author

Additional Information:

Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:14
Last Modified: 09 Aug 2017 03:11
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