Application of flash-release, a new extraction procedure
(juice, pulp, essential oil)
Pierre Brat, Cirad-flhor
The procedure
The first part consists of a description of the process as a whole.
Stage 1
Sterilisation of the plant material at 85-90°C. Heating is by means of a vapour injection inlet. This type of blanching is necessary for implementing the procedure and also inhibits endogenous enzymatic activity (e.g. polyphenoloxidase and pectinmethylesterase) and destroys bacterial flora.
Stage 2
Release stage, with the sudden intake under vacuum (approximately 30 mbar) of previously sterilised material (fruits, vegetables, plants, etc.). Under these vacuum conditions, the boiling point of water is between 27 and 30°C. This instantaneous vacuum treatment causes the sudden evaporation of part of the water contained in the sterilised plant material (approximately 10% of the initial moisture content) and a sudden fall in temperature of the material. This instantaneous moisture loss causes fine crushing by the creation of intercellular micro-channels. Disturbance and cell bursting give the products special physicochemical, rheological and organoleptic properties. The evaporation water is recovered separately by evaporation; it is particular rich in volatile aromatic compounds and referred to as aromatic waters. It can be re-incorporated in products after 'flash release'.
Products
'Flash release' is a produce with many uses and many effects. The products obtained are treated in two categories; juices and purées and then the recovery of essential oils.
Juices and purées
The process is thus upstream of the refining stage essential for obtaining homogeneous purées and downstream of the paring stages (leaves and stems removed) and fruit washing. Two purée type applications are presented.
Passion fruit purée
Three main zones can be shown in a morphological representation of a section of purple passion fruit:
- the aril: a yellow zone traditionally used for obtaining juice;
- the mesocarp: a whitish spongy zone containing high levels of cell walls and soluble pectins and displaying low acidity and a high volatile compounds content. However, the external face of the mesocarp is a reddish purple colour and rich in anthocyanins;
- the epicarp: a strongly lignified zone closely bonded to the mesocarp.
This treatment gives an intensely purple purée with a high anthocyanin content (Table 1) whose physicochemical, rheological and organoleptic characteristics differ greatly to those of a reference juice. Following the incorporation of part of the mesocarp and the loss of cell cohesion resulting from the flash release process, the purée is an intense reddish purple colour (chromameter index a), lower acidity and higher viscosity (Table 1), allowing greater dilution for the preparation of nectars. Finally, the bursting of part of the cell material releases the aromatic compounds mainly sited in the vacuole and hence enables the preparation of a product with greater aromatic strength.
Mango purée
Only the mesocarp, consisting mainly of parenchyma cells, undergoes the cell disturbance caused by the process. The particularly lignin-rich and very rigid epicarp is not broken down by the process. The classic physico-chemical parameters (pH, titratable acidity, etc.) of a purée prepared using the 'flash release' process are therefore similar to those of a traditional purée. However, the disorganisation of parenchyma cells and the bursting of a proportion of the cells cause rheological changes. The products prepared have a distinctly firmer consistency and higher viscosity (Table 2).
The vacuum treatment (the second stage of the process) makes it possible to reduce product oxygen content considerably (1/5 of the initial level) and thus limits oxidative browning phenomena to a minimum. Mango purée prepared using 'flash-release' is therefore paler and yellower; this is correlated by an increase in chromameter indices L and b (Table 2).
Extraction of citrus essential oils
The placing of sterilised citrus peel (grapefruit, orange, mandarin and lemon) in a vacuum chamber causes the explosion of the essential oil glandes in the flavedo (the outer coloured part of the fruit). The oils are vaporised simultaneously with part of the water in the peel. The oils are recovered as an aqueous emulsion after condensation and then purified by centrifugation. The extraction yields of lemon, orange and grapefruit peel are 44, 29, 33 and 27% respectively. After terpene removal, the essential oils extracted using the 'flash release' process are of better quality than freshly pressed oils and contain higher proportions of mono- and sesquiterpene aldehydes.
A multiple-use process
Fruits
It gives more consistent, viscous purées with greater colouring intensity that enable a higher dilution ratio for the preparation of nectars (purée after dilution). Depending on the case, these particular rheological characteristics are the result of purées with higher cell wall and soluble pectin contents and also of the cell disruption called by the release effect.
After dilution, nectars are more stable and display slower phase separation. The blanching stage ensures the almost complete inhibition of endogenous enzymatic activity (polyphenoloxidase and pectinmethylesterase) while higher product viscosity reduces cloud settling phenomena in the finished product.
A very low oxygen content is observed in the products after release, limiting oxidation phenomena and hence browning. The purées still display strong colour after pasteurisation, even after storage for several months.
Higher production yields. The fine disintegration of all the parts of the fruit, and in particular of the parenchyma zones, caused by the release phase gives higher final production yields (in the case of passion fruit).
Essential oils
The essential oils obtained from citrus peel subjected to 'flash release' are qualitatively similar to those obtained by cold pressing. Yields are the same or higher as those of a classic FMC method. The process thus makes it possible to use citrus peels that are of only small economic interest today.
Subsidiary applications
Concentration process. Approximately 10% of the fruit mass placed in the heating chamber is recovered in the form of aromatic waters (condensed evaporation waters).
Deodorisation process. Some vegetables smell very unpleasant during cooking because of the volatilisation of sulphur compounds (e.g. cabbage). The prior treatment of these vegetables using the 'flash release' process would make it possible to trap all or part of the foul-smelling volatile compounds.
Facilitate the extraction of functional substances. Numerous fruits are rich in useful functional substances such as carotenes and lycopene. Fine disintegration and the bursting of part of the cells would increase the extraction yields of these substances.
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