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 779

condense on the condenser. The condenser should be in the

immediate vicinity of the evaporating surface. When an appre-

ciable number of collisions can occur in the vapor space, some

of the molecules will return to the liquid. This leads to a de-

crease in the number of molecules that reach the condensation

surface.

High-vacuum distillation may be used for certain classes of

chemical compounds that decompose, polymerize, react, or are

destroyed by conventional distillation methods. Low cost per

pound and high throughput may be obtained on certain groups of

compounds such as vitamins, epoxy resins, highly concentrated

pure fatty acids, plasticizers, fatty acid nitrogen compounds, and

a host of other heat-sensitive materials, which may require only

deodorizing and decolorizing (Spychaj 1986, Batistella et al.

2002a,b, Cermak et al. 2007, Shao et al. 2007, Compton et al.

2008). High-vacuum distillation is a safe process to separate

mixtures of organic or silicon compounds, most of which can

not withstand prolonged heating without excessive structural

change or decomposition. With short residence times and lower

distilling temperatures, thermal hazards to the organic materi-

als are greatly reduced. Purity of the distillate also depends on

the film thickness. Controlling positive pressure and supply to

the heated evaporator surface will usually provide a uniform

film throughout the distillation. The absence of air molecules in

the high-vacuum distillation column permits most of the dis-

tilling molecules to reach the condenser with relatively few

molecules returning to the liquid film surface in the evaporator.

Experimental results show a relationship between the molecular

weight and distillation temperature for a broad range of different

materials.

There are several basic design variations with short-path evap-

orators. These are the short-path falling film evaporator, the

centrifugal molecular still, and the short-path, wiped-film evap-

orator. They operate at the lowest pressure of any system and are

capable of high throughput per unit size, due to their continu-

ous nature. They also offer the shortest thermal exposure to any

process (Eckles and Benz 1992). Short-path, falling film evap-

orators can handle materials with viscosities up to 5000 cP, and

the residence time, temperature, and pressure can be controlled.

This is the simplest of the short-path stills, but more current

designs use either centrifugal force or roller-wipers to spread

the feed material on the evaporator surface. In the centrifugal

molecular still, feed material is fed into the center of a heated

spinning rotor. The material is evenly spread towards the edge

and condensed in front of the rotor.

Molecular Distillation Applications

Molecular distillation, with its important characteristics of low

pressure and low temperature, gives a high potential for this

process in the separation, purification, and concentration of nat-

ural products, which usually consist of complex and thermally

sensitive molecules. Especially under high vacuum for short op-

erating times, while using no solvents, it avoids any toxicity

problems. The effects of feed flow rate and distillation temper-

ature on the extraction of minor components are related to the

yield, purity, and rate of evaporation in terms of concentrations,

distribution coefficients, and relative volatilities. Molecular dis-

tillation is a valuable processing method that is often used to

separate or purify high-boiling materials that decompose, oxi-

dize, or polymerize at elevated temperatures. This process can

be considered for industrial uses if both distillate and residue

become high value-added products. Some molecular distillation

processes are used in the production of biologically active sub-

stances such as vitamins, sterols, and antioxidants from natural

oils and fats such as the extraction of vitamin A from fish liver

and whale oil and carotenoid recovery from esterified palm oil

(Batistella and Wolf Maciel 1998, Mutalib et al. 2003, Mar-

tins et al. 2006, Bettini 2007, Fregolente et al. 2007, Shi et al.

2007c), separation of mono and diglycerides in partially saponi-

fied fats (Holl ́o and Kurucz 1968), and the refining of crude

animal and vegetable oils (Martins et al. 2006, Behrenbruch and

Dedigama 2007, Posada et al. 2007). For instance, a Malaysian

company producesCarotino

©R

, an edible red palm oil, using

a process that involves a pretreatment of crude palm oil (i.e.,

degumming with phosphoric acid and treatment with bleach-

ing earth) followed by deacidification and deodorization using

molecular distillation (Ooi et al. 1996; Fig. 40.9). Recovery of

special components used in the nutritional, pharmaceutical, and

cosmetic areas from natural products by molecular distillation

Water phase ++ Oil phase recovery

IVIV

4

Condenser

Condenser Vent

Juice

86 °F

Scrubber

Chilled

(30% Alcohol)

water 32°F

W. phase +

O. phase

Chilled water

32 °F

Variable Chiller

pump

Condensed

water +

Residual aroma

Condensed

water

Aroma

column

Pump

Legend

Vapour

Condensed water

Steam

Juice

W. phase + O. phase

Chilled water

Liquid

seal

pump

Juice

122 °F

Figure 40.9.Recovery system of water phase and oil phase in the

orange juice industry (Bettini 2007).