Eur Food Res Technol (2003) 217:215–218 DOI 10.1007/s00217-003-0731-2
ORIGINAL PAPER
Medeni Maskan · HalilI˙ Bag˘cı
Effect of different adsorbents on purification of used sunflower seed oil utilized for frying Received: 3 February 2003 / Revised: 14 April 2003 / Published online: 13 June 2003 Springer-Verlag 2003
Abstract The refining of used frying oils is of commercial and economic importance. The re-use of recovered oil could provide considerable savings to the food processors. In this study, 50 consecutive deep-fat fryings were done using 100-g potato samples in 1.5 l (initial) of sunflower seed oil at 170 C. The refining of used sunflower seed oil was investigated by various adsorbent treatments. Six adsorbents, CaO, MgO, Mg2CO3, magnesium silicate, activated charcoal and bentonite, as well as an available natural earth (i.e. pekmez earth, CaCO3 containing special natural white soil) were studied. Pekmez earth, magnesium silicate (florisil) and bentonite exhibited the highest abilities in viscosity, free fatty acids (FFAs) reduction and colour recovery, respectively, among the adsorbents studied. Therefore, a mixture of 2% pekmez earth, 3% bentonite and 3% magnesium silicate was found to be the best combination. The quality parameters of used sunflower seed oil (FFA content, colour values and viscosity) were improved due to adsorbent mixture application. Keywords Frying oil · Adsorbents · Free fatty acids · Viscosity · Colour
Introduction A large proportion of fats and oils in the world is used in the frying process. Fried foods are desired for their distinctive fried flavour and odour. To some extent, they provide healthy nutrients, such as essential fatty acids, vitamins and fibres in the diet. Deep-fat/oil frying is extensively used in food processing both industrially and in the home, and fried potato products are one of the largest applications. In the frying M. Maskan ()) · H. Bag˘cı Food Engineering Department, Gaziantep University, 27310 Gaziantep, Turkey e-mail:
[email protected] Tel.: +90-342-360-1200/2309 Fax: +90-342-360-1105
process of foods, the role of the fat or oil is essentially to provide an efficient heat transfer medium which is especially adapted to transmitting heat rapidly and uniformly to the surface of the food being cooked [1, 2]. During the frying process, a number of changes take place in fats and oils, involving a complex pattern of thermolytic and oxidative reactions, which depend on the type of oil used and the food fried [3, 4]. This leads to the formation of new compounds (as well as rejection of used oil) such as diacylglycerols, monoacylglycerols, free fatty acids (FFAs), monomers, polymers, and so on, which are harmful to the human body [5, 6]. The visible changes taking place in a fat or oil during frying include darkened colour, increased viscosity, decreased smoke-point and increased foaming. Current interest in the study of evaluation of used frying oil is apparent from the relatively large number of papers appearing in the last few years. There are many studies being conducted worldwide for the evaluation of used frying oil. Some of the researchers have concentrated on the utilization of this oil as an alternative fuel oil [7], for biodiesel production [8, 9, 10, 11] as alternative fuels for diesel engines. On the other hand, Jaswir et al. [12] studied the effect of addition of natural and synthetic antioxidants in frying oil in order to extend the frying life of oil. Used frying oils are generally discarded because oxidized lipid degrades the quality of fried foods. The discarded used frying oil still has a large portion of triacylglycerols. Economic considerations and the need to produce fried foods of desirable quality have stimulated an interest in the purification of frying oil. Recently, most of the researchers have focused their attention on the purification of used frying oil in order to make the oil edible by removing degradation products from the medium. For this purpose, different methods have been used. A membrane processing technique has been used by Subramanian et al. [13] in order to enhance the shelf life of used frying oils by removing oil-soluble impurities. Yoon et al. [14] have investigated the separation of triacylglycerols from used frying oil by supercritical
216
carbon dioxide extraction. However, these two systems have not yet been industrially applied, because of high investment and operating costs. Filter aid materials or their combinations have been found effective for the control of FFAs and colour [15] and for the removal of pro-oxidant transition metals [16] from used frying oils. Different adsorbents have also been used for the recovery of used frying oil [15, 17, 18]. Such studies are increasing worldwide. Therefore, the current study was conducted to test different adsorbents in terms of their ability to regenerate used frying oil. The authors wanted in particular to test the effect of pekmez earth, which is used in different processes in food industry in their country [19]. By this process, it is believed that the used frying oil can be upgraded to the level existent in freshly refined oils, thus resulting in economic savings in commercial operations. The FFA, viscosity and colour changes of used frying oil were investigated before and after adsorbent treatments to test their effectiveness.
Materials and methods Materials. The fresh potatoes and the sunflower-seed oil were purchased from a local market in Gaziantep (Turkey). The chemicals NaCl, NaOH, ethanol, CaO, MgO, Mg2CO3 and magnesium silicate were purchased from Merck (Germany), and phenolphthalein and activated charcoal were from Riedel de Haen (Germany). The bentonite, which was used for bleaching of the oil during processing, was obtained from Gvenal Oil and Soap (Gaziantep-Turkey). Pekmez earth (70% CaCO3 containing special white natural soil) was obtained from a village in Gaziantep. This is generally used for the neutralization and bleaching of natural olive oil and clarification of apple juice (unpublished data). Potato preparation. The potatoes were peeled and washed under tap water. The washed potatoes were cut into slices of size 6.0€1.237.8€1.25 49.5€4.09 mm3 using a vegetable slicer. The uniform size was essential for uniform heat transfer between the potato slices and the frying oil. The sliced potatoes were weighed as 100-g portions and soaked in a 2.5% NaCl solution for 5 min. This reduces oil absorption capacity and prevents surface darkening of the potato slices due to oxidation. Also, it positively affects the surface properties in ways such as improving the rigidity of potato slices by making a complex structure with pectin. The structure formed decreases the solubility of pectin and prevents the potato from disintegration during frying [12]. Then, following water drainage, the potato slices were blotted with a paper towel before frying. Frying and frying oil sampling. An electrical deep fat fryer (Mares de Luxe-2, France) with a frying basket and a 3 l oil capacity was used for frying of potato slices. The fryer was operated at a temperature of 170 C. Exactly 1.5 l of sunflower-seed oil were heated to 170 C at the beginning and 50 samples of potatoes, each weighing 100 g potato slices, were fried for 6 min each. The frying period was decided from the literature and this group’s preliminary studies. During the frying process, the lid of the fryer was closed whilst the basket was immersed into the hot oil. After 6 min, the fried slices were removed from the fryer. Then, the frying operation was carried out for a new potato sample. The oil was subjected to 50 consecutive fryings. In order to determine the effect of frying on the oil properties, 140 ml of frying oil was withdrawn after each 1st, 10th, 20th, 30th, 40th, and 50th frying cycle, cooled in a cool room at 13 C and filtered by using a coarse cellulosic filter paper to eliminate the suspended particles. The volume of oil was never
replenished to the original volume with fresh oil after any of the fryings. The oil samples (filtrate) were stored in a refrigerator at about 4 C in glass-stoppered flasks until use. Selection of adsorbents. Adsorbent applications have been demonstrated as being able to control fat-soluble degradation products as well as to remove insoluble particles. Many types of adsorbents which have the ability to absorb undesired frying products were studied. These were CaO, MgO, MgCO3, activated charcoal, bentonite, magnesium silicate (florisil, 0.150–0.250 mm) and pekmez earth, each with a different function. All adsorbents were uniform in size except pekmez earth. It was sieved by using an Octagon 200 Test Sieve Shaker (London, UK) with a 106 mm sized square-shaped sifter cloth. Sieving increases the surface area per volume ratio of the adsorbent, so that an increase in the adsorption of the degradation products can be achieved. Adsorbent treatments. In order to reduce viscosity (where high viscosity indicates polymer formation) and recover the colour of used frying oil, 2% (by weight) of each adsorbent (CaO, MgO, MgCO3, activated charcoal, bentonite and pekmez earth) was added to the used oil and the sample connected to a rotary evaporator (RE100-Bibby Sterilin, UK) operating at 50 C for 15 min under vacuum. Then the oil was filtered by using Whatman filter paper #42 (Whatman, Maidstone, England). The efficiency of the adsorbents was further evaluated. FFAs determination. FFA content was determined in duplicate, by the titration method of AOCS [20]. About 7 g of well-mixed oil was weighed into a 250 ml flask. Previously neutralized hot ethyl alcohol (50 ml) and 1% phenolphthalein, as indicator, were added. The mixture was titrated with 0.1 N NaOH with vigorous shaking until permanent faint pink appeared and persisted at least 1 min. The FFA content was calculated as percentage oleic acid. V N 28:2 ð1Þ m where, m is the mass of the test portion, in grams, N the normality of NaOH, and V the volume of NaOH consumed, in milliliters. %FFAðasoleicacidÞ ¼
Viscosity measurements. The viscosity measurements were carried out using a Haake Rheostress RS1 (Karslruhe, Germany) controlled stress rheometer equipped with TCP/P Peltier temperature controller unit and a Haake thermostat DC10 (Karslruhe, Germany). A cone-plate sensor (diameter 3.5 cm and angle 2) was used. The shear rate range was adjusted to 0–550 s-1. Colour measurement. Colour measurements of the oil samples were carried out using a HunterLab Colorflex (A-60–1010–615 Model Colorimeter, HunterLab, Reston, Va.). The colour values were expressed as L (whiteness or brightness/darkness), a (redness/ greenness) and b (yellowness/blueness) at any time, respectively. The total colour difference (TCD), was calculated from Eq. 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TCD ¼ ðLO LÞ2 þðaO aÞ2 þðb0 bÞ2 ð2Þ where, L0 (61.25), a0 (-3.30) and b0 (10.24) refer to reference values, i.e., colour parameters of fresh oil, and L, a and b refer to colour values at various times during frying. Statistical analysis. Statistical analysis was conducted using the SigmaPlot (Scientific Graphing Software, version 8.0). A paired ttest was applied to the properties of treated and non-treated used frying oils. Trends were considered significant when means of compared pairs differed at the P<0.05 level.
Results and discussion The formation and accumulation of non-volatile decomposition products are responsible for physical changes in
217 Table 1 Results of treatments with different adsorbents on the viscosity and colour of used oil. Values are the means of three replicates. TCD Total colour difference
Adsorbents
Viscosity at 25 C Pa sx102
L
a
b
TCD
Initial values Oil after 50th frying CaO MgO MgCO3 Activated Charcoal Bentonite Pekmez Earth
8.341d 8.945a 8.998a 8.709b 8.726b 8.577c 8.662b, c c
58.69 56.77 57.15 57.53 57.57 58.51 58.38 57.23
2.36 2.88 2.69 2.64 2.66 2.51 3.09 2.64
6.75 16.86 11.14 10.23 10.46 8.80 10.06 10.86
0.000a 10.304b 4.664c 3.678d 3.886d 2.063e 3.403d 4.370c
a, b, c, d, e
8.557
Different letters in the columns denote significant difference (P<0.05).
Table 2 Viscosity values of oil for pekmez earth treatment at 25 and 37 C. Values are the mean of three replicates Viscosity Pa s102 Earth concentration % Oil after 50th frying 0.50 1.00 1.50 2.00 2.50 3.00
25 C 8.945a 9.155a 8.771a, b 8.645b 8.530c 8.637b 8.618b
37 C 5.810a 5.783a 5.696a, b 5.728a 5.618c 5.695a, b 5.690a, b
a, b, c
Different letters in the columns denote significant difference (P<0.05)
the frying oil such as increases in viscosity, colour and foaming as well as for chemical changes such as increase in FFAs, carbonyl value, hydroxyl content and saponification value and decreases in unsaturation, with resulting increases in the formation of high molecular weight products [2]. It was reported by several authors [13, 21] that relatively simple colour, FFA content and viscosity measurements can provide a good overall estimate of frying oil quality instead of measuring all the decomposition products. Therefore, various adsorbents were selected for colour recovery, FFA and viscosity reduction (by eliminating high molecular weight compounds). Table 1 shows the viscosity and Hunter colour value results obtained from a used frying oil treated with various adsorbents. Pekmez earth and activated charcoal reduced viscosity of the oil more than the other adsorbents. Some did not change it significantly; however, CaO increased the viscosity. This may be due to a catalytic effect of this chemical resulting in production of high molecular weight materials during adsorbent treatment. Therefore, pekmez earth was selected as an adsorbent for viscosity reduction of the used oil. On the other hand, based on TCD, activated charcoal and bentonite gave reasonably good results in recovering the colour (the lower TCD values were 2.063 and 3.403 for activated charcoal and bentonite, respectively). Activated carbon was found to be very successful in recovering colour of used oil because of its powerful adsorption capability. Of these two, bentonite was selected for this purpose because of its use in the oil industry as a bleaching material. Activated charcoal can be useful in some instances, but its indiscriminate removal of good components, e.g. tocopherols [22], its high cost and its difficult separation by
Table 3 Results of effects of bentonite concentration on viscosity of 2% pekmez earth-oil mixtures Values are the mean of three replicates Bentonite in earth-oil mixture % 0.50 1.00 1.50 2.00 2.50 3.00
Viscosity Pa s102 25 C 8.645a 8.463a 8.259b 8.247b 8.179b 8.004c
37 C 5.701a 5.593a 5.510a 5.410b 5.417b 5.385c
a, b, c
Different letters in the columns denote significant difference (P<0.05) Table 4 Effect of magnesium silicate concentration on free fatty acid (FFA) reduction. Values are the mean of three replicates
Magnesium silicate % (oil after 50th frying)
FFA % 0.2212a
1.0 2.0 3.0
0.1972b 0.1716b 0.1365c
a, b, c
Different letters in the columns denote significant difference (P<0.05).
filtration limit its usefulness. For these reasons, activated charcoal was not selected for viscosity reduction and colour recovery processes of used frying oil in this study. The effect of different adsorbents on viscosity and TCD of used frying oil was statistically significant (P<0.05). After selection of the proper adsorbents, it was necessary to determine the optimum concentration of each in the oil-adsorbent mixture. Therefore, six levels of pekmez earth (0.5–3.0% by weight) were used in frying oil. Table 2 shows that 2% pekmez earth was the optimum earth concentration for viscosity reduction at both 25 and 37 C. The next step was keeping this concentration constant and trying different concentrations of bentonite (0.5–3% by weight) in earth-oil mixture. The results revealed that 3% bentonite reduced the viscosity of used oil more than the other concentration levels significantly (P<0.05) when used in combination with earth (Table 3). Finally, the optimum concentration of magnesium silicate was determined for FFA reduction studies. Three levels of magnesium silicate (1, 2, and 3% by weight) were used (Table 4). At 3%, magnesium silicate de-
218 Table 5 Oil parameters before and after adsorbent treatment. FFAs were determined as percentage oleic acid. Viscosity was measured at 25 C.
Values oil before frying Values of oil after Untreated 50th frying Treated
creased FFA content of used oil from 0.2212% to 0.1365% (significantly different, P<0.05, from the others). Therefore, this level was selected as the optimum concentration. Overall, 2% pekmez earth, 3% bentonite and 3% magnesium silicate were the desirable concentrations (optimum adsorbent mixture) that could be used to recover the used frying sunflower-seed oil. The selected percentage of adsorbent combinations used is rather high for practical purposes. However, other researchers [15, 21] reported that oil treatment with adsorbent mixtures of greater than 8% concentration had significant improvement on various parameters of used frying oil. For the adsorbent mixture treatment experiment, another brand of sunflower seed oil of the following initial values was used: FFA, 0.1756% as oleic acid; viscosity at 25 C, 0.0833 Pa s; and Hunter color values, L 61.25, a 3.30, and b 10.24. The treatment results are shown in the Table 5. As can be seen from Table 5, the adsorbent mixture reduced the FFA content of the oil sample from 0.2891 to 0.1685%, i.e., a value below the FFA content of fresh oil (0.1756%). Similarly, a reduction in viscosity of the used oil, measured at 25 C, was detected. This is due to the ability of adsorbent mixture to remove the compounds formed during frying (polar trimers, tetramers, oligomers, polymers, etc) which contribute to the increase in viscosity of the oil [2]. The improvement in colour parameters (L, a, b) indicates that the colouring materials formed during frying were eliminated by the adsorbent treatment. The TCD, which is a combination of parameters L, a and b values, is a colorimetric parameter extensively used to characterize the variation of colour in foods during processing. It was reduced from 11.38 to 2.67. This is a reasonable colour recovery. In conclusion, the quality parameters of used sunflower seed oil, utilized for frying, were improved by adsorbent treatment. The FFA content, viscosity, Hunter colour parameters (a and b), and TCD values increased during frying. However, Hunter colour parameter L decreased. Among the adsorbents studied, magnesium silicate, pekmez earth and bentonite were selected for the
FFA
Viscosity Pa s
L
A
b
TCD
0.1756 0.2891 0.1685
0.0833 0.0892 0.0861
61.25 56.34 60.16
3.30 2.62 3.81
10.24 20.49 12.63
0.00 11.38 2.67
reduction of FFA, viscosity and colour recovery, respectively. The adsorbent mixture selected (2% pekmez earth, 3% bentonite and 3% magnesium silicate) may be used for the industrial refining of used frying oils. However, to cater for the probability that trace quantities of decomposition compounds with low affinity to the adsorbents do not remain, additional work must be performed. Acknowledgements The authors thank the University of Gaziantep, Turkey, for research support.
References 1. Aguilera JM, Gloria-Hernandez H (2000) J Food Sci 65:476– 479 2. Stevenson SG, Vaisey-Genser M, Eskin NAM (1984) JAOCS 61:1102–1108 3. Demir R, Otludil B (1997) Biochem Arch 13:223–228 4. Demir R, Bas¸han M (1998) Biochem Arch 13:187–191 5. Clark WL, Serbia GW (1991) Food Technol 45:84–89 6. White PJ (1991) Food Technol 45:75–80 7. Karaosmanog˘lu F, Beker UG (1996) Energ Sources 18:637– 644 8. Alcantara R, Amores J, Canoira L, Fidalgo E, Franco MJ, Navarro A (2000) Biomass Bioenerg 18:515–527 9. Bak YC, Choi JH, Kim SB, Kang DW (1996) Korean J Chem Eng 13:242–245 10. Cvengros J, Povazanec F (1996) Biosource Technol 55:145– 150 11. Neto PRC, Rossi LFS, Zagonel GF, Ramos LP (2000) Quim Nova 23:531–537 12. Jaswir I, Che Man YB, Kitts DD (2000) Food Res Int 33:501– 508 13. Subramanian R, Nandini KE, Shella PM, Gopalakrihna AG, Raghavorao KSMS, Nakajima M, Kimura T, Maekawa T (2000) JAOCS 77:323–328 14. Yoon J, Han BS, Kang YC, Kim KH, Jung MY, Kwon YA (2000) Food Chem 71:2785–2790 15. Lin S, Akoh CC, Reynolds AE (2001) Food Res Int 34:159–166 16. Zhang WB, Addis PB (1992) J Food Sci 57:651–654 17. Lin S, Akoh CC, Reynolds AE (1998) J Food Lipid 5:1-16 18. Lin S, Akoh CC, Reynolds AE (1999) JAOCS 76:739–744 19. Maskan A, Kaya S, Maskan M (2002) J Food Eng 54:81–88 20. AOAC (1990) Official methods of analysis (15th edn). Association of Official Analytical Chemists, Washington, D.C. 21. McNeill J, Kakuda Y, Kamel B (1986) JAOCS 62:1564–1567 22. Jacobson GA (1991) Food Technol 45:72–74