REVIEW Ital. J. Food Sci., vol. 27 - 2015 277 - Keywords: molecular distillation, purification, separation, short-path, vacuum - APPLICATIONS OF MOLECULAR DISTILLATION TECHNIQUE IN FOOD PRODUCTS O. KETENOgLU* and A. TEKIN Department of Food Engineering, Ankara University, Diskapi 06110, Ankara, Turkey *Corresponding author: Tel. +90 312 2033300/3642, Fax +90 312 3178711, email: ketenoglu@eng.ankara.edu.tr AbstrAct there are several separation techniques -including conventional distillation- for extracting heat sensitive compounds from food products. However, some compounds may have high boiling points at which other compounds might be adversely affected. Vacuum application is also need- ed for such kinds of foods. Molecular distillation is an advanced vacuum distillation method per- formed by short-path evaporators. Distance between evaporator and condenser is extremely re- duced which results in minimized pressure drop. Heat sensitive material meets heat for a short time under high vacuum, thus low or no decomposition occurs. this review aims to discuss the basics and uses of molecular distillation in foods. 278 Ital. J. Food Sci., vol. 27 - 2015 IntroDuctIon Distillation is a simple physical separation process of liquid mixtures based on differenc- es in boiling points of components in the mix- ture. Very first usage of distillation dates back to 1st century (Forbes, 1970). Further experi- ments leaded to new knowledge that is known as fundamentals of distillation now. In 1830, Aeneas coffey - an Irish inventor - patented his distillation column (GAIser et al., 2002). cof- fey’s column (a.k.a “continuous still”, “patent still” or “coffey still”) achieved to reach higher concentrations of alcohol. In 20th century, some special distillation equipments and techniques were produced in correlation with increasing in- novations in petrochemical industry. especial- ly in chemical and food research, demand for extracting compounds with high purity leaded to development of computer aided systems. to- day, several distillation techniques are present for various purposes. Appropriate distillation method should be chosen depending on prop- erties of liquid mixture and distillation equip- ment. some of the specific distillation methods could be listed as: • repeated evaporation-condensation cycles, known as fractional distillation. • steam distillation of heat sensitive materials (Harwood and Moody, 1989) • Vacuum distillation of heat sensitive materi- als under reduced pressure. • reactive distillation • Azeotropic distillation • extractive distillation • catalytic distillation • Molecular distillation, an advanced vacuum distillation method. General information on molecular distillation Pure substances have certain vapor pres- sure values related to vaporization temperature. these vapor pressure-temperature data are plot- ted to a P-t diagram, which is called “phase di- agram”. Fig. 1 demonstrates a sample phase di- agram of any pure substance. As seen in Fig. 1, vaporization temperature (or boiling point) decreases when the ambient pres- sure is reduced along the vaporization curve. this principle is the basis of vacuum distilla- tion. Distillation of compounds, which may be decomposed at high boiling points and/or may be air-sensitive can be possible with vacuum dis- tillation. typically, there are two types of vacu- um distillation: • simple vacuum distillation: applied when higher vacuum levels are not needed. ex: ro- tary evaporators, Perkin triangle. • High vacuum distillation: applied when higher vacuum levels are needed for separation. Pu- rity of distillate is higher than those of other distillation techniques. ex: thin film evapora- tors (tFe) and short-path distillation equip- ment (sPD). According to sHI et al. (2007), distillation method can be called as molecular distillation if the distance between evaporator and condenser reaches to mean free path of a vapor molecule. Lei et al. (2005) described the mean free path, < λ >, with the following equation: where d (m) is the diameter of molecule, N A is Avogadro constant (6.023x1023 mol-1), P (Pa) is pressure, R is universal gas constant and T (K) is temperature. LutIšAn and cVenGroš (1995) defined molecular distillation as “the safest Fig. 1 - sample phase diagram of any pure substance. method to separate and purify thermally unsta- ble compounds”. sHI et al. (2007) pointed out that risk of thermal decomposition could be re- duced with low temperature; as well oxidation could be prevented with air removal by vacu- um. In addition, De MorAes et al. (2006) drew one’s attention to advantages of molecular dis- tillation (e.g. avoiding toxicity, protect environ- ment) that other chemical agent-based tech- niques do not have. LutIšAn and cVenGroš (1995) defined the main features of molecular distillation as; short time of exposure to heat, low evaporating temperature and a character- istic mass transfer. According to MArtIneLLo et al. (2007), “small distance between evapo- rator and condenser” can also be defined as a feature of molecular distillation. Ital. J. Food Sci., vol. 27 - 2015 279 HIGH VAcuuM AnD MoLecuLAr DIstILLAtIon equIPMent there are typically two types of evaporators used in high vacuum distillation, i.e. thin film evaporators (tFe) and short-path evaporators (sPe). these equipments have similar designs with few differences. In both evaporators, feed is agitated with a rotor-wiper system and high vac- uum is produced by vacuum pumps. In tFe, op- erating pressure can be reduced to 1-100 mbar (uIc GmbH, 2014) and there is no other unit be- tween vacuum and condenser (PILoDIst, 2014). Fig. 2 shows an illustration of a tFe. In sPe, condenser is placed in the centre of evaporator unit, so distance between boiling and condensation surface is extremely reduced and pressure drop is minimized. the operating pressure can be reduced up to 0.001 mbar. Dis- tillation performed by a short-path evaporator is also called as “molecular distillation” (buss- sMs-canzler GmbH, 2014a; buss-sMs-canzler GmbH, 2014b; PILoDIst, 2014; tecHnoForce, 2014). Fig. 3 shows an illustration of a sPe. there are many parameters that can affect distillation yield and molecular evaporation rate. Molecular evaporation rate, k i , can be cal- culated by Langmuir-Knudsen equation (ros- sI et al., 2011): where ts is evaporation temperature, R is uni- versal gas constant, M i is molecular weight of evaporating component and Pvi is vapor pres- sure of component. Xu et al. (2002) describes the most important parameters of molecular distil- lation as evaporator temperature, flow rate, vac- uum and wiper speed. Flow rate has an impor- tant effect on the contact time of the molecules with hot surface during evaporation. Higher flow rates reduce the residence times of molecules being vaporized. Wiper speed affects film thick- ness and viscosity. Feed becomes highly tur- bulent with intensive agitation, which leads to high heat transfer coefficients (buss-sMs-can- zler GmbH, 2014c). MoLecuLAr DIstILLAtIon In FooD ProcessInG: soMe eXAMPLes oF recent stuDIes Molecular distillation has many application areas in food industry. some of these applica- tions can be summarized as but not limited to: concentration of ω-3 fatty acids, distillation of monoglycerides from di- and triglycerides, con- centration of tocopherols and tocotrienols (buss- sMs-canzler GmbH, 2014d), fractionation of squalene (sun et al., 1997), recovery of carot- enoids (bAtIsteLLA and WoLF-MAcIeL, 1998). As distillation is a separation process, studies about molecular distillation generally focus on either removal of undesired compounds or con- centration of valuable compounds. Removal of undesired compounds In a study about removal of cholesterol from butter and lard by using molecular distillation (LAnzAnI et al., 1994), researchers reported that cholesterol content of lard was reduced from 988 ppm to 105 ppm in the residue after 2 hours of distillation under 10-4 torr pressure and 250oc evaporator temperature. Molecular distillation can also be used for physical deacidification. MArtIns et al. (2006) separated free fatty acids (FFA) from vegetable oil deodorizer distillate. they achieved to reduce Fig. 2 - Illustration of a tFe unit. Fig. 3 - Illustration of a sPD unit. 280 Ital. J. Food Sci., vol. 27 - 2015 FFA content to 6.4% from initial FFA content of raw material with 57.8% at 160oc evaporator temperature, under 10-6 bar pressure and 10.4 g min-1 feed flow rate. they also noted that con- centration of tocopherol in residue stream was found 18.3%, while initial tocopherol concen- tration was 8.97%. FFA elimination was 96.16% and tocopherol recovery was found 81.23%. WAnG et al. (2010) aimed to separate FFAs and diacylglycerols (DAG) from enzimatically hydro- lyzed soybean oil. they achieved to increase the removal of FFAs from 88.8% to 99.44% by in- creasing evaporator temperature from 125oc to 160oc, under 0.5-1.0 Pa process pressure, 200 mL h-1 feed rate and 300 rpm wiper speed. oLLI et al. (2013) studied removal of organic pol- lutants in fish oils. their sPD system, which has an evaporator temperature of approx. 220oc and operating pressure below 0.03 mbar, achieved to remove total amount of chlorinated pesticides (some of them are DDt and HcH) from 215.07 ng g-1 to 21.95 ng g-1, corresponding to 89% reduction. According to Meyer et al. (2011), total pesti- cide traces in rapeseed deodorizer distillate were dropped below 0.05 mg kg-1 from an initial content of 0.968 mg kg-1 by achieving more than 94.8% reduction. sPD evaporator temperature was set to 110oc, feed flow rate was 200 mL h-1 and pres- sures were between 0.006 and 0.01 mbar. re- searchers stated that it would be a mistake to affirm that all types of pesticides were removed by using sPD according to this reduction data, because many different types of pesticides might be present before distillation and analysis of ef- fects on specific compounds has to be performed. Concentration and/or fractionation of compounds bAtIsteLLA and WoLF-MAcIeL (1998) studied the recovery of carotenoids from palm oil by us- ing a molecular distillator and after a set of dis- tillation trials, they achieved to increase caro- tene concentration to 19500 ppm from an ini- tial feed concentration of 600 ppm under 9x10-5 torr pressure and 170oc evaporator temperature. sun et al. (1997) fractionated squalene from alkali-refined amaranth seed oil and their high- est recovery of squalene was 67.8% with sPD conditions of 100 mtorr pressure and 180oc dis- tillation temperature. campos et al. (2003) fractionated milk fat by sPD and recorded distillate yields (w/w) as a function of temperature. they observed that distillate yield was 0.3% at 125oc process tem- perature; however a 42.7% recovery was ob- served when process temperature was increased to 250oc, which meant a significant and posi- tive effect of temperature on process efficiency. sPD was performed on lemongrass essential oil by toVAr et al. (2011) and researchers report- ed that they were able to increase citral concen- tration in distillate stream from 17.658 mg mL-1 to 33.576 mg mL-1 when evaporator temperature was increased from 60oc to 120oc with a feed flow rate of 1.5 mL min-1 and pressure of 5 Pa. Mono and diglyceride (MDG) concentration and production are also possible with molecular distillation. FreGoLente et al. (2010) produced partial glycerides from soybean oil by using mo- lecular distillation. concentration of monoglyc- eride (MG) in distillate stream increased with el- evated evaporator temperature. At 250oc with 10 mL min-1 feed flow rate, MG concentration was increased from initial feed value of 12.75% to 80.00% in distillate stream under 24 Pa op- erating pressure. they also pointed that lower flow rate increased recovery of MG, because mol- ecules contacted with hot evaporator surface for a longer period of time. recovery for any compo- nent is defined with following equation: zHAnG et al. (2013) studied effects of evapo- ration temperature, feeding rate, feeding tem- perature and wiper speed on concentration of ω-3 fatty acids by molecular distillation and optimized these parameters with response sur- face methodology (rsM). researchers reported the optimum conditions as 110.4oc evaporator temperature, 78.7 mL h-1 feeding rate, 350 rpm wiper speed, 10 Pa operating pressure and 80oc feed temperature. concLusIons separation techniques such as extraction, evap- oration, distillation etc. are accepted as unit op- erations in food industry. Vacuum distillation is frequently used both in chemical and food indus- tries; however simple vacuum distillation might not be capable of separation of heat-sensitive ma- terials from food products. In that case, molecu- lar distillation (short-path distillation) should be used for separation of these materials. Molecular distillation has been used more in pharmaceu- tical, chemical and petrochemical applications, but nowadays importance of molecular distilla- tion has increasingly been understood in food in- dustry. separation, concentration and purifica- tion of commercially valuable food constituents can be easily performed by molecular distillation; furthermore, healthier food products can be pro- duced by removal of some health damaging com- pounds such as excess cholesterol, organic pol- lutants. Authors expect an increasing trend in us- age of molecular distillation in food industry when taking all these applications into consideration. AcKnoWLeDGeMents this review article has not been funded by any organization. Ital. J. Food Sci., vol. 27 - 2015 281 reFerences batistella c.b. and Wolf-Maciel M.r.1998. recovery of ca- rotenoids from palm oil by molecular distillation. com- puters & chemical engineering 22(supplement 1): s53- s60. buss-sMs-canzler GmbH. Molecular Distillation. Availa- ble at: http://www.sms-vt.com/en/technologies/short- path-evaporator/molecular-distillation.html (accessed 20 May 2014a). buss-sMs-canzler GmbH. short Path evaporator. Available at: http://www.sms-vt.com/en/technologies/short-path- evaporator.html (accessed 20 May 2014b). buss-sMs-canzler GmbH. General Description of thin Film Distillation. Available at: http://www.sms-vt.com/en/ technologies/thin-film-evaporator/thin-film-distillation. html (accessed 20 May 2014c). buss-sMs-canzler GmbH. typical applications of short path distillation. Available at: http://www.sms-vt.com/ en/technologies/short-path-evaporator/typical-applica- tions.html (accessed 20 May 2014d). campos r.J., Litwinenko J.W. and Marangoni A.G. 2003. Fractionation of milk fat by short-path distillation. Jour- nal of Dairy science 86(3): 735-745. De Moraes e.b., Martins P.F., batistella c.b., Alvarez M.e.t., Maciel Filho r. and Wolf-Maciel M.r. 2006. Molecular dis- tillation. Applied biochemistry and biotechnology 132(1- 3): 1066-1076. Forbes r.J. 1970.A short History of the Art of Distilla- tion: From the beginnings up to the Death of cellier blu- menthal.brill:the netherlands, pp. 57, 89. Fregolente P.b.L., Pinto G.M.F., Wolf-Maciel M.r. and Ma- ciel Filho r.2010. Monoglyceride and diglyceride produc- tion through lipase-catalyzed glycerolysis and molecu- lar distillation. Applied biochemistry and biotechnology 160(7): 1879-1887. Gaiser M., bell G.M., Lim A.W., roberts n.A., Faraday D.b.F., schulz r.A. and Grob r. 2002.computer simulation of a continuous whisky still. Journal of Food engineering 51(1): 27-31. Harwood L.M. and Moody c.J. 1989.experimental organ- ic chemistry: Principles and Practice.blackwell scientif- ic Publications:oxford, pp. 151-153. Lanzani A., bondioli P., Mariani c., Folegatti L., Venturini s., Fedeli e. and barreteau P. 1994. A new short-path dis- tillation system applied to the reduction of cholesterol in butter and lard. Journal of the American oil chemists’ society 71(6):609-614. Lei z., chen b. and Ding z. 2005.special Distillation Pro- cesses (1st ed.).elsevier b.V.: the netherlands, p. 350. Lutišan J. and cvengroš J.1995.Mean free path of mole- cules on molecular distillation. the chemical engineer- ing Journal and the biochemical engineering Journal 56(2): 39-50. Martinello M., Hecker G. and carmen Pramparo M.d. 2007. Grape seed oil deacidification by molecular distillation: Analysis of operative variables influence using the re- sponse surface methodology. Journal of Food engineer- ing 81(1): 60-64. Martins P.F., Ito V.M., batistella c.b. and Wolf-Maciel M.r. 2006. Free fatty acid separation from vegetable oil deodor- izer distillate using molecular distillation process. sepa- ration and Purification technology 48(1): 78-84. Meyer F., eggers r., oehlke K., Harbaum-Piayda b., schwarz K. and siddiqi M.A. 2011. Application of short path dis- tillation for recovery of polyphenols from deodorizer dis- tillate. european Journal of Lipid science and technol- ogy 113(11): 1363-1374. olli J.J., breivik H. and thorstad o. 2013. removal of per- sistent organic pollutants in fish oils using short-path distillation with a working fluid.chemosphere 92(3): 273-278. Pilodist. thin-Film-evaporation. Available at: http://www. pilodist.de/distillation-by-thin-film-evaporation (accessed 21 May 2014). rossi P.c., carmen Pramparo M.d., Gaich M.c., Grosso n.r. and nepote V. 2011. optimization of molecular distilla- tion to concentrate ethyl esters of eicosapentaenoic (20 : 5 ω-3) and docosahexaenoic acids (22 : 6 ω-3) using sim- plified phenomenological modeling. Journal of the science of Food and Agriculture 91(8): 1452-1458. shi J., Posada L.r., Kakuda y. and Xue s.J. 2007. Molecu- lar distillation of palm oil distillates: evaporation rates, relative volatility, and distribution coefficients of tocot- rienols and other minor components. separation science and technology 42(14): 3029-3048. sun H., Wiesenborn D., tostenson K., Gillespie J. and rayas-Duarte P.1997.Fractionation of squalene from am- aranth seed oil.Journal of the American oil chemists’ so- ciety.74(4):413–418. technoforce. short Path (Molecular) Distillation units. Avail- able at: http://www.technoforce.net/short-path-molecu- lar-distillation-units.html (accessed 18 May 2014). tovar L.P., Pinto G.M.F., Wolf-Maciel M.r., batistella c.b. and Maciel Filho r. 2011. short-path-distillation pro- cess of lemongrass essential oil: Physicochemical char- acterization and assessment quality of the distillate and the residue products. Industrial & engineering chemis- try research 50(13): 8185-8194. uIc GmbH. thin Film evaporation. Available at: http://www. uic-gmbh.de/en/basics/thin-film-evaporation.html (ac- cessed 20 May 2014). Wang y., zhao M., song K., Wang L., Han X., tang s. and Wang y. 2010. separation of diacylglycerols from enzy- matically hydrolyzed soybean oil by molecular distillation. separation and Purification technology 75(2): 114-120. Xu X., Jacobsen c., nielsen n.s., Heinrich M.t. and zhou D. 2002. Purification and deodorization of structured lip- ids by short path distillation. european Journal of Lipid science and technology 104(11): 745-755. zhang G.y., Liu J. and Liu y.F. 2013. concentration of ome- ga-3 polyunsaturated fatty acids from oil of schizochy- trium limacinum by molecular distillation: optimization of technological conditions. Industrial & engineering chemistry research 52(10): 3918-3925. Paper Received July 7, 2014 Accepted September 23, 2014