Solid-liquid extraction is an operation with many applications: laboratory applications (sample preparation), industrial applications (extraction from vegetable matrices) and practical applications (cleaning). It is of fundamental importance, since the successful outcome of the process depends on it.
For example, in the herbal and various other food preparation industries, when a vegetable matrix necessitates extraction for further processing, it is obviously of fundamental importance to ensure that a high degree of extraction efficiency is reached, that the extract is not oxidised, and so on.
Many laboratory procedures require solid-liquid extraction as a preliminary phase of the preparation of a sample, for example as happens in the analysis of environmental contaminants in vegetables. The operation of solid-liquid extraction must guarantee that all the analytes are retrieved completely and that they are in no way degraded during the extraction process.
Currently, in the food industry there are essentially three different solid-liquid extraction techniques in use:
Each of these techniques presents both advantages and disavantages.
“Using a suitable solvent, a negative pressure is generated between the inside and the outside of a solid matrix containing extractable material. This is followed by a stage where the initial balance is suddenly restored, inducing the forced extraction of components which are not chemically bound to the principal structure constituting the solid.”
Rapid production of lemon liqueur (“limoncello”)
The Naviglio extractor has been used to great advantage in producing limoncello in approximately two hours. At the end of the process the lemon peel is washed with drinking water, thus completely recovering the ethyl alcohol and essential oil remaining in the system; sugar is added to this enriched water, and once it dissolves, the sugar water is mixed with ethyl alcohol. The peel is dried at room temperature, and after 48 hours loses roughly 90% of its initial weight. The dry material is used as fertiliser or food supplement in animal nutrition.
Step 1. The extraction chambers are filled with the solid matrix and extraction solvent at atmospheric pressure. On the base of the two cylindrical extraction chambers are two porous membranes, which permit liquid to pass through and block the solid matrix.
Step 2. After it has been filled, the system is closed and put under pressure (generally between 7 and 9 bars). Pressure exerted by the pistons is transferred to the liquid, since the two extraction chambers are linked by a conduit.
Step 3. Once the set pressure has been reached, the system stops for the time required for the inside and outside to return to state of balance.
Step 4. After this, the pistons are set in motion, immediately lowering the pressure in the system. This starts the dynamic phase of the extraction: the substances that can be extracted from the solid are transferred to the extraction solvent by a suction effect due to the gradient of negative pressure created between the inside and the outside of the solid matrix.
Step 5. One of the functions of the dynamic phase is to re-mix the extraction liquid, avoiding the formation of concentration gradients in the immediate vicinity of exposed surfaces of the solid. The alternating static and dynamic phases constitute the extraction cycle; by repeating the cycle, the complete extraction of the solid matrix is obtained. At the end of the extraction process, the extraction solvent is expelled through an electronic valve and collected in a container.
The fast, dynamic solid-liquid extractor is an example of innovative technology for solid-liquid extraction. Compared with other extraction techniques currently in use, it allows for rapid completion of the process: the extractable substances contained in the solid matrix are transferred to the organic or inorganic solvent and mix with them. The innovation consists in its new philisophy. Current methods focus on heating the extractor system to increase yield and shorten extraction times; the Naviglio extractor® performs extraction at ambient or sub-ambient temperature, exploiting an increase in extraction liquid pressure on the solid matrix. The importance of low-temperature extraction is that this avoids thermal stress on thermolabile substances.
Lycopene Extractions from industrial tomato skin waste.
The tomato skins are treated in water using a Naviglio extractor® and the organic content is retrieved containing, amongst other substances, lycopene; it is separated by SPE (Solid Phase Extraction) and eluted with a minimum quantity of solvent. The resulting skins are dried until all traces of water contained in them is removed; the dry material is ground and used in animal feed.
Retrieval of essential oil from scraps of orange peel residue from candied peel production.
The scraps of orange peel are extracted with water using a Naviglio extractor®; the essential oil component is retrieved through centrifuge. The orange skin is dried until no traces of water remain; the dry material is finely ground and the resultant powder is employed as a food ingredient in animal nutrition.
13. Mohr method
14. Vohlard method