Knowledge of phytochemical solubility, chemical and biochemical stability, and tissue localisation are essential in order to develop the extraction techniques and the technological processes for the preparation of high-value extracts.
As a general rule phytochemicals are poorly soluble in water, and are better extracted with organic solvents. For the more polar compounds mixtures of alcohols and water are the solvents of choice (Harborne, 1973). For the most lipophilic phytochemicals, the use of less polar solvents - such as ethyl acetate, petrol or ether - is essential. For some kinds of compounds, the solubility in water can be increased by changing the pH. Thus acidic compounds, such as polyphenols, can be extracted in water at alkaline pH
where the phenolic hydroxyls are ionised. More basic compounds, such as nitrogen-containing metabolites, can be extracted in water under acidic conditions.
Taking into consideration the nature of the residues, the extraction processes used for the preparation of these phytochemical extracts have to meet some requirements. Firstly, it is preferable to use fresh raw materials for extraction, as a drying process, although allowing storage, leads to unacceptable increases in production costs. It is also necessary to use food-compatible solvents, for example water, ethanol, methanol or a mixture of these. Thermal treatments are generally necessary to inactivate enzymes that can degrade the phytochemicals during the extraction process (Tomás-Barberán and Espín, 2001). Supercritical fluid extraction (SFE) using CO2 in supercritical state, and frequently with organic modifiers added to enhance phytochemical solubility, has been proposed as another feasible methodology for the preparation of extracts. This technology could be especially useful for the extraction of lipophilic phytochemicals. The cost of this extraction methodology is higher than the technologies referred to above, but can have applications for the preparation of high-value phyto-chemicals with the advantage that it can prevent the extraction of flavour and aroma compounds, thus widening the range of applications for the obtained extracts.
In some cases, the phytochemical yield extracted from residues can be enhanced by adding enzymes such as pectinases or cellulases (Cinar, 2005; www.vtt.fi/virtual/maxfun).
The extracts obtained usually need to be concentrated before use. Spray drying and freeze drying are feasible technologies that could be applied depending on the market price of the extract produced. In some cases, extract purification through non-ionic polymeric resins (of the type Amberlite XAD) can be used to concentrate the phytochemicals before drying (Tomás-Barberán et al., 1992). In these cases, the water extract (or a water solution of the ethanol-water extract) is filtered through the resin column and the phytochemicals are retained in the stationary phase. Then these compounds are eluted with ethanol, and this extract concentrated. The extracts obtained are generally prepared in dried form although preparation as liquid extracts and concentrates is another possibility. These extracts can then be used for the preparation of pills or other nutraceutical preparations (dried extracts) or to manufacture functional juices (Larrosa et al., 2002) or other new foods - such as soups, sauces and margarines (Llorach et al., 2005) - to which liquid or dried phytochemical extracts can be added.
Extract stabilisation or preservation is needed to prevent phytochemical degradation. Many of these compounds are antioxidants and are therefore easily oxidised by the oxygen in the atmosphere especially when stored in solution. The use of nitrogen atmospheres or addition of other antioxidants (ascorbic acid etc.) can be necessary to stabilise the extracts.
Nutraceuticals, micronutrients and functional ingredients 465 17.8 Safety issues
A topic of concern is safety. It is essential to make sure that the pesticides and other agrochemicals are not concentrated in the extracts in the same way the phytochemicals are (Tomas-Barberan et al., 2005). It is therefore essential to carry out routine analysis of pesticides in all these products. It is also important to establish the risk/benefit balance of using these phyto-chemical extracts for health-related purposes as these are biologically active compounds and therefore potentially toxic at high doses.
In addition to the chemical risks, there are also biological risks related to these phytochemical extracts. One such risk is related to mycotoxin contamination of the extracts due to fungal growth in the residues before extraction. Another is related to the presence of microbial contamination in the extracts if thermal treatments are not used during the extraction process.
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