The T < Tc characteristic of the subcritical

The extraction process using supercritical
fluids is presently considered a feasible alternative for conventional extraction
methods. The solvents in supercritical state show intermediate physico-chemical
properties similar to that of liquid and gas, which increases viscosity values and intermediate values of
diffusivity coupled with the complete absence of surface tension of these
fluids allowing its rapid entry into the cells and particles of the  given sample matrix extracting the material
present in interiors (Osorio-Tobon et al.,
2016; Silva & Meireles, 2014). These characteristics enhance  process of 
inactivation  and extraction  vegetative cells. In certain cases, similar
effects can be achieved by t  the
supercritical state  at temperatures near
to its critical, the liquid state of a substance, with P > Pc and T < Tc characteristic of the subcritical state (Ceni et al., 2016). Similarly in the  conventional extraction techniques, the extraction of targeted components from plant matrix by supercritical fluid  extraction  depends on several parametersthe extracting power of the solvent. The high density of these fluids gives them a high solvation power, whereas its high diffusion and low viscosity values provide a desired penetration power in the solid matrix (Akinlua et al., 2008). Selection of supercritical fluids is very crucial for the development of a SFE technique and a wider range of substances can be used as solvents in this technique (Sihvonen et al., 1999).Neverthless  the fact that there are many compounds that can be used as SFs ( methane, ethylene, xenon nitrogen,  or fluorocarbons). Many separation systems use carbon dioxide as it is safe  and  entails low cost (Daintree et al., 2008).  Among  many SFs used for extraction, supercritical CO2 (SC-CO2) has widest  usages  used since it is  noncorrosive ,  non-flammable,  nontoxic , and  ease of  handle  allows supercritical operation at near room  temperature and low  pressures, and wide availability in bulk  with highest  degree of purity. (Saheno et al., 2009). The  advantages  of  using Carbon dioxide  have been instrumental in   ensuring minimal  changes of the bioactive compounds and to protect  their  functional or curative properties (Cavero et al., 2006).SFE using  carbon dioxide  is  viable alternative to organic solvents because it is safer to use , nontoxic, cheap  and possesses the capability  to solubilise lipophilic substances and can be easily eliminated from the  products formed (Wang & Weller 2006; Wang et al., 2008; Sahena et al., 2009). Additionally, the use of  gaseous CO2 at low pressure and room temperature making  recovery of compounds  very simple and all extracts solvent free. This molecule is eco friendly and USFDA (United states Food and Drug Administration) and EFSA (European Food Safety Authority) recognizes it as GRAS "generally recognized as safe" . In the food processing it is important to have good understanding of the solubility behavior of the components of interest in SC-CO2 . Incase of SC-CO2  ,Solubility of a solute    is highly dependent on pressure and  temperature  which influence carbon dioxide  density and consequently  solvent power. When the pressure is increased ,the SC-CO2 attains  liquid-like density resulting in enhancing the  probability of interactions between the solvent and solute  leading to a phenomenal increase in solubility. Morover enhancing  the temperature results in  reduction of density of SC-CO2, while at the same time resulting in increase in the  solute vapor pressure. The overall effect  of these two countering  factors controlling  the solubility change. When pressures are   close to the critical point, the reduction  in SC-CO2 density is more rapid such that a temperature enhancement  results in a decreased solubility, whereas at elevated pressures the vapor pressure effect takes control since the drop in  density is relatively smaller, leading to a increase in  solubility. there is well-known crossover behavior of the solubility isotherms due to The consequence of these transitions. the  solubility in SC-CO2   is  influenced by Solute properties, especially molecular weight, polarity and vapor pressure  and also by solute–solute as well as solute–solvent interactions, such as hydrogen bonding. Since  CO2    has non polar nature the non polar components are more soluble  than that of polar components with a almost similar molecular weight. When there is  increase in molecular size of a solute  there is reduction in  the solubility in the supercritical fluid. Therefore, non polar solutes having lower molecular weight and high vapor pressure are  preferentially solubilized  in SC- CO2  relatively at  low-density conditions, while high-density conditions are needed for large less volatile  and  slightly polar  solutes.So higher selectivities are obtained by simply adjusting the pressure and temperature which is the real and major advantage of the SC-CO2 extraction technology, thereby almost eliminating  additional refining if any  required  in most cases. In case the  target compound is polar,the  polarity of the supercritical solvent can be changed by  adding  the polar co-solvent, which is referred  as a  entrainer or modifier . These co-solvent through hydrogen bonding  interact with the solute thus charging transfer complex formation and dipole–dipole coupling as well as the solvent, leading to an enhancement  in the density of the solvent mixture, thereby affecting solubility positively. The  choicest co-solvent for food applications has been ethanol for obvious reasons as ethanol is  considered a GRAS (generally recognized as safe) solvent for food processing (Lesellier et al., 2015). Many research groups across globe  are using  these type of gradients with a hold at the highest modifier composition for faster SFE separations of polar substances (Ebinger  et al., 2014).