Brazilian Journal of Chemical Engineering OBTAINING FRUCTOOLIGOSACCHARIDES FROM YACON ( Smallanthus sonchifolius ) BY AN ULTRAFILTRATION PROCESS

The objective of this study was to evaluate the separation of fructooligosaccharides (FOS) from yacon extract by an ultrafiltration process using membranes of 10 and 30 kDa. The total resistance (Rt), membrane resistance (Rm), fouling resistance (Rf), and concentration polarization (Rc) during the separation process were also assessed. The operating pressures were 1.2 and 0.75 bar for UF-10 and UF-30, respectively. The permeate flux increased upon increasing the pressure from 0.5 to 2 bar and the resistance values showed a slight increase with increasing pressure. The fouling percentages were 61.24% and 57.33% for the membranes UF-10 and UF-30, being reversible after the cleaning procedure with acidic and basic solution, resulting in high percentages of flux recovery of 76.46% and 83.56% for U-10 and UF-30, respectively. The FOS retention values were 24.48% and 6.49% for both membranes UF-10 and UF-30, corresponding to 24% and 18.4% purity.


INTRODUCTION
Nowadays, the demand for foods that promote health and well-being is increasingly evident.Thus, food that was considered for many years as a source of nutrients needed to sustain life has become the object of studies related to disease prevention and improved function of organs and tissues.
In this context, yacon roots, which originated from Andean regions, contain from 60 to 70% inulin (Vilhena et al., 2000).The native inulin is a mixture of oligomers and polymers containing long chain molecules of different degrees of polymerization (DP) from 2 to 60 units, with an average DP of about 12; the inulin present in yacon has low DP, between 3 and 10, therefore being considered fructooligosaccharides (FOS) (Goto et al., 1995;Villegas and Costell, 2007).
According to Lewis (1993), FOS are defined as a combination of three sugar molecules: 1-kestose (GF2), nystose (GF3) and frutofuranosyl nystose (GF4) where the units fructosyltransferase (F) are combined with sucrose (GF) at the β position (2 → 1).Compounds such as FOS may be used as sugar substitutes, as they are more soluble and sweeter than native and long-chain inulin (Meyer et al., 2011).
FOS have been designated as prebiotics and soluble fiber, since they pass through the stomach and small intestine without being digested, so they provide no extra calories.However, they are fermented by bacteria, such as bifidobacteria, helping to increase the microflora that is related to improving the health of the colon, affecting the general welfare (Gibson and Roberfroid, 1995).For this reason, it is important to isolate these compounds in order to Brazilian Journal of Chemical Engineering incorporate them for human consumption.An alternative is the membrane separation process (MSP) considered to be a clean technology.Ultrafiltration (UF) is a type of MSP considered to be a very attractive alternative method since it does not use heat and involves no phase change, which makes the concentration process more economical, with the advantage of not using chemicals, since only a driving force is required to separate the compounds of interest (Kamada et al., 2002b).
UF membranes have pore diameters varying between 0.01 µM and 0.001 µM, and are capable of retaining and fractionating macromolecules such as lipids, proteins and colloids present in solution whose molecular weight varies from 300 to 500,000 Daltons (Da) (Mulder, 1996).These membranes can retain solutes of different molecular weights, usually being specified by a nominal molecular weight cutoff or MWCO, which establishes the lowest molecular weight that is retained with an efficiency of at least 95% (Modler, 2000).
The use of UF to separate inulin and FOS from yacon has been reported in several studies.Gibertoni et al. (2006) evaluated the production of rich fructans syrup from clarified yacon juice using a ceramic microfiltration membrane with nominal pore size of 0.14 µm, and UF membrane with molecular weight cut off of 50 kDa.Kamada et al. (2002a) evaluated the combination of ultrafiltration and nanofiltration on the purification and concentration of FOS from yacon roots, removing most of the mono-and disaccharides and obtaining a concentrated product with DP of three or more, and 98% purity.
The MSP have some limitations, highlighting the phenomena of concentration polarization and fouling at the membrane surface.The polarization occurs in the first minutes of the process when the solute is retained at the membrane surface, making difficult the solvent permeation and resulting in a permeation flux decline, which is a reversible phenomenon.In contrast, the fouling is characterized by a permeation flux decline resulting from clogging of the membrane pores, which can be an irreversible phenomenon depending on the situation (Wu et al., 1990).
The resistance-in-series model has been used to evaluate fouling in the membrane separation process (Brião and Tavares, 2012).These resistances include membrane resistance, an external or reversible fouling, which consists of cake layer deposition and concentration polarization, and irreversible resistance (Bagci, 2014;Rezaei et al., 2014).The latter is due to particle and macromolecule deposition and adsorption of smaller-sized solutes onto the membrane pore walls (Ng et al., 2014;Rezaei et al., 2014).
The aim of this study was to evaluate the separation of FOS from yacon extract using ultrafiltration as a membrane separation process.The parameters of total resistance, membrane resistance, fouling resistance, and concentration polarization during the separation process were also investigated.

Raw Material
The yacon roots were cleaned and selected considering the absence of visual damage and infections, and peeled and sliced with a mean diameter of 4.55 ± 0.25 mm and a thickness of 1.75 ± 0.35 mm (Scher et al., 2009) and subjected to a blanching treatment for 4 minutes at 100 ºC, followed by cooling in an ice bath for 3 minutes (Fante and Noreña, 2012).The juice was extracted using a food processor.The remaining sugars were extracted from the pulp obtained after separation of the juice by addition of water at 80 ºC in a 2:1 ratio (weight water/weight pulp), maintaining the mixture at 80 ºC ± 2 ºC for one hour, according to the methodology proposed by Toneli et al. (2007).The yacon juice and the liquid solution obtained from the pulp were separately filtered under vacuum.The filtrates were mixed, which constituted the yacon extract, to which 1% (w/v) citric acid was added and filtered on Whatman filter paper Nº.01 to remove suspended solids.
Initially, the new membranes were compacted following the methodology described by Rai et al. (2007), which consists of the densification of the microstructure of the membranes using TMP of 2.5   1 00
The degree of the purity of the FOS in the permeate was 24.08% and 18.43% for UF-10 and UF-30, respectively.This degree was obtained from the relationship between FOS and sugar total concentration into the permeate.The degree of purity in the juice and yacon extract was 25.61% and 22.79%, respectively.It can be noticed that the enrichment in FOS versus simple sugars was not as high as desirable.A principal reason is that the membranes have a pore size distribution which does not allow effective separation of FOS from glucose and fructose molecules.Zhu et al. (2012) reported that the major FOS in yacon are 1-kestose (GF2), nystose (GF3) and 1fructofuranosyl nystose (GF4) whose content in oligosaccharides were 12.29%, 12.17%, 6.20%, respectively.The molecular weights of these FOS are less than 1 kDa (Kuhn et al., 2010).For this reason it is necessary to include in this process the separation by nanofiltration.Kamada et al. (2002a) used the combination of NF-UF to purify FOS present in a mixture of sugars derived from yacon and observed retention of 54.8% and the retention values of 14.0% for monosaccharides, 46.2% for disaccharides, 80.9% for trisaccharides and from 91.5 to 99.9% for sugars with DP ≥ 4, using a 1 kDa membrane and pressure of 5 bar.

CONCLUSIONS
The effects of pressure on the total resistance, membrane resistance, fouling resistance and polarization resistance were evaluated using polyethersulfone membrane with nominal molecular weight cut offs of 10 and 30 kDa.It was observed that the increase in pressure resulted in a slight increase in all resistance values.
It was found that, for both membranes, the reversible resistance was the dominant resistance which reduced the permeate flux, while irreversible fouling resistance gave less of a contribution.
A good restoration of the hydraulic permeability was obtained after the cleaning procedure with acidic and basic solution.
Fig tra (• Fig of 10 bar

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