Food Biochemistry and Food Processing (2 edition)

BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come

40 Separation Technology in Food Processing 777

Water

reservoir

Pressure regulator

pump and valve

controller

Supply

valve (1)

Heater (2)

Oven

Micro

filters

Eluent

valve (4)

Sample

collector

(5)

Micro

filters

Pump

Temperature controller

Extraction cell

(3)

Figure 40.7.Diagram of pressurized low-polarity water extractor. The electrical connections are marked by dashed lines, while the path of

subcritical water is shown by solid line with arrow. The high-pressured water passes through a supply vale (1) into a heating coil (2) and into

an extraction cell (3). The microfilters are placed before and after an eluent valve (4). The extract is collected in a sample collector (5).

organic compounds in superheated water, this medium can be

considered for the extraction and other processes, to replace

conventional organic solvents. But some additional reactions of

the compounds being processed may also occur, by hydrolysis,

oxidation, etc.

Industrial Applications

Using pressurized low-polarity water provides a number of ad-

vantages over traditional extraction techniques (i.e., hydrodis-

tillation, organic solvents, solid–liquid extraction). These are

mainly shorter extraction times, higher quality of the extracts

(mostly for essential oils), lower costs of the extracting agent,

and an environmentally compatible technique. Since water is

perhaps the most environmentally friendly solvent available in

high purity and at low cost, it has been exploited for the ex-

traction of avoparcin in animal tissue (Curren and King 2001),

fungicides in agricultural commodities (Pawlowski and Poole

1998), fragrances from cloves (Rovio et al. 1999), antioxidative

components from sage (Ollanketo et al. 2002), anthocyanins and

total phenolics from dried red grape skin (Ju and Howard 2003),

saponins from cow cockle seed (Guc ̈ ̧lu- ̈Ust ̈ undag et al. 2007), ̈

and other bioactive components from plant materials (Ong and

Len 2003). Some additional successful applications of this tech-

nique are for the extraction of essential oils from various plant

materials (Khajenoori et al. 2009, Mortazavi et al. 2010), extrac-

tion of sweet components fromSiraitia grosvernorii, extraction

of lactones from kava roots, extraction of antioxidant compounds

from microalgaeS. platensis(Iba ́ ̃nez et al. 1999, Ib ́anez et al. ̃

2003, Herrero et al. 2004), extraction ofGinkgo biloba, and

of biophenols from olive leaves (Japon-Luj ́ ́ana and Luque de

Castro 2006).

The quality of the oil obtained is therefore better than that

from steam distillation, as it contains more of the oxygenated

compounds and lower terpene content. The yield is also slightly

higher than from steam distillation, in spite of the fact that all the

terpenes are not extracted. This may be because, at the higher

temperatures and under pressure, the plant material is more ef-

fectively penetrated. However, about twice the amount of water

is required than for steam distillation. Energy costs are much less

than for steam distillation. The energy required to heat a given

mass of water from 30◦C to 150◦C under pressure is one-fifth

of that needed to boil water at atmospheric pressure from 30◦C.

Furthermore, it is possible to recycle most (three-quarters) of the

heat, whereas it is difficult to recycle heat in steam distillation.

Thus, in spite of the fact that twice as much water is needed,

only one-tenth of the energy of a steam distillation is required.

Pressurized low-polarity water extraction has been suggested as

a method to extract valuable health-promoting compounds from

plant materials.

Molecular Distillation

Molecular distillation is a unit operation that is a peculiar case of

evaporation, which happens under extremely low pressures and

low temperatures and is used for the separation of constituents

from mixtures by partial evaporation. It is based on the fact that

the vapor is relatively richer in the component with the highest

vapor pressure, that is, the more volatile component. Distilla-

tion is a process of heating a substance until the most volatile

constituents change into the vapor phase, and then cooling the

vapors to recover the constituents in liquid form by condensa-

tion. The main purpose of distillation is to separate a mixture

into individual components by taking advantage of their differ-

ent level of volatilities. Distillation is one of the main methods of

extracting essential oils from plants. It can be carried out either

as simple distillation or fractional distillation (rectification). In