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In general, the common vegetable oils for cooking purposes are soybean, canola and sunflower, just to name a few, have low saturated fatty acids (SFA) level and a relatively high level of unsaturated fatty acids (USFA), in which the double bond is in cis configuration. Moreover, the presence of USFA such as linoleic and linolenic acid decrease the melting point of oil and more prone to autoxidation (McClements and Decker, 2008). Eventually, the vegetable oil has undesirable off-flavour and off-odour that influence the shelf life and consumer acceptability.
In order to solve this problem, the vegetable oil is usually hydrogenated to reduce the unsaturation of fatty acids (Hamm and Benjamin 2003) and modify the physical properties of the oil such as melting behavior and oxidative stability (Eskin and others 2003). Nevertheless, the downside of hydrogenation is that it produces trans fatty acids (TFA) that render coronary heart disease by raising the low-density-lipoprotein concentration and decreasing the high-density-lipoprotein concentration.
According to Jung and others (2002), the variation in hydrogenation operating conditions in temperature, pressure, agitation speed, catalyst used and catalyst concentration determines the level of TFA formed. Therefore the operating conditions need strict monitoring. Besides, using the hydrogenated vegetable oil for deep frying allows further degradation of oil and induces TFA formation upon high heating temperature (Bansal and others 2009; Xu 2000). However, during the frying process, the presence of oxygen and water from the food tends to accelerate the deterioration of frying oil by reducing the fry life, altering the free fatty acids content and increase the polar components of oil, instead of inducing the TFA formation (Innawong and others 2004).
Previous works to find the alternative method to hydrogenation, with no TFA formation but enhance the oxidative stability of frying oil, are available. The interesterification (IE) process plays the same function as hydrogenation through re-arranging the fatty acids in the glycerol backbone, either accomplished chemically or enzymatically (Eskin and others 2003). Hamm (2003) also proposed that interesterifying the completely hydrogenated oil with vegetable oil to produce zero TFA with high level of solids.
After all, the crucial consideration in frying medium is oxidative and flavor stability of the oil. In this research, the proposed null hypothesis (Ho) is that the hydrogenated soybean oil does not deplete the fry life and cause the instability in the oil when used to deep-dry food. However, the primary aim of this research is to disprove Ho by determining that there is a significant decrement of fry life and instability of the hydrogenated frying medium that produce the harmful TFA and hence, fails to meet the requirements of the health-conscious consumers.
The soybean oils undergo hydrogenation and interesterification (chemical and enzymatic) to serve as a frying medium to fry food. The quantification of TFA between the three frying medium is determined via attenuated total reflection (ATR) infrared spectroscopy, while the quality of the frying mediums is analysed thoroughly. The extent of each frying medium quality is compared with the corresponding control.
4. Materials and Methods
Cold, pressed and refined (CPR) soybean oil (SO) and CPR flaxseed oil (FO) were purchased from Oil Seed Extractions Ltd. (New Zealand). The RBD hard palm stearin (PS) was obtained from LM Wright & Co Ltd. (New Zealand). Watties Fries Shoestring (New Zealand) purchased from a local supermarket in Dunedin was used as the frying food in the frying protocol.
4.2 Hydrogenation procedure
Nysosel® 222 catalyst (22% nickel on silica support) ordered from the BASF's Catalysts in New Jersey (USA) was used to hydrogenate SO. Hydrogenation process was carried out in relation to the work of Cizmeci and others (2005; cited in Musavi and others 2008) by introducing fresh SO into the 4-L Snap-Tite reactor ordered from Autoclave Engineers (Pennsylvania, USA) under the condition of 220oC, 0.25MPa pressure and 500rpm agitation rate for 100 minutes (Karabulut and others 2003; Jung and others 2002). Approximately 10g of the SO samples were collected at 10 minutes interval during hydrogenation to observe the trend of TFA formation.
4.3 Chemical interesterification
Prior to interesterification, the blend was prepared in the proportion of 80% FO with 20% of hydrogenated SO obtained from previous hydrogenation procedure, then melted at 100oC and homogenized for 10 minutes. The blend was dried in a flask under vacuum condition and was heated to 100oC upon the addition of the 0.4% (w/w) sodium methoxide catalyst powder obtained from Sigma-Aldrich Chemical Corporation (Missouri, USA) (Andreia Schäfer De Martini Soares and others 2009). The mixture was stirred constantly for an hour to achieve complete interesterification. The interesterification reaction was brought to a halt by adding hot distilled water. The blend was dried and filtered at 80oC (Schmidt and others 1996).
4.4 Enzymatic interesterification
PS was completely melted at 60oC in oven and the blend was prepared in the volume ratio of 75:25 (fresh SO:PS) to be added into flask that contained Lipozyme TL IM of 1,3 specific Rhizomucor miehei lipase (immobilized on porous silica granulates) which was purchased from Novozymes North America Inc. (Franklinton, NC, USA). The mixture was agitated via Labotron orbital shaker (Infors HT in Switzerland) at the speed of 200rpm at 50oC for 6 hours (Shin and others 2010). The blend was filtered to remove the immobilized enzyme and was analysed for the TFA formation.
4.5 Preparation prior to frying
The frying medium prepared from the methods described above were poured into deodorizer to neutralize and deodorise the hydrogenated or interesterified vegetable oil via distillation method at 160oC under the pressure of 0.4Pa for 90 minutes (Chu and others 2001). This step involved removal of free fatty acids to prevent the oxidation of frying medium.
4.6 Frying protocol
COBRA brand deep fryer CF4 purchased from Sydney Commercial Kitchens (Australia) with the dimensions of 400Ã-800Ã-915mm and with 21litres capacity. The fryer was filled with 15litres of oil for each batch of frying mediums. Initially, the oil was heated for 1 hour and the temperature was maintained at 185 ± 5oC. The fries were put to the deep fryer to fry for about half an hour, this was denoted as the first frying cycle. A total of ten frying cycle were performed with 30 minutes break before carrying out the next frying cycle and 500g of fries were used for each frying cycle. The same frying medium was used, without replacement, for the next six consecutive days and each day, the oil was heated for 6 hours in total. The oil samples were collected every five frying cycles to quantify the TFA and assessed the oxidative stability. The collected oil samples were filtered, flushed with nitrogen gas and kept in glass bottles at -20oC (Bansal and others 2009). The frying protocol for each frying mediums was run in triplicate.
4.7 Control oil samples
Three batches of control frying medium were prepared: hydrogenated SO, 80:20 of FO and hydrogenated SO blend and 75% PS enzyme interesterified with 25% fresh unhydrogenated SO. These controls were heated without frying any fries using the frying protocol described above.
4.8 Attenuated total reflection (ATR) infrared spectroscopy
All the oil samples collected were flushed with nitrogen and kept at 4oC before analysis. The quantification of TFA for all the prepared frying medium was carried out upon completion of vegetable oil treatment and also after frying the food, by referring to the American Oil Chemists' Society (AOCS) method Cd 14d-99 (Juaneda and others 2007). The procedure described by Bansal and others (2009) was followed, which was carried out in a Spectrum One Fourier Transform Infrared (FTIR) Spectrometer purchased from PerkinElmer Inc. in Massachusetts (USA), fitted with a Michelson interferometer, a potassium bromide beam splitter, a deuterated triglycine sulphate detector and an ATR cell so that the measurements were collected at 4cm-1 resolution in the spectral range of 900-1100 cm-1 at constant temperature of 65oC (Juaneda and others 2007). The TFA standards for the purpose of calibration was done by preparing nine different concentration of T7379 glyceryl trielaidate from 0.2% to 5% (purity â‰¥ 99%) in the reference 44895-U triolein (5000 Î¼g/mL in pyridine); in which both triacylglycerols were purchased from Sigma-Aldrich Chemical Corporation (Missouri, USA). The FTIR spectrophotometer was zeroed using the fresh frying medium as reference for each batch. The spectrum for the oil samples were collected and rationed with the spectra of the corresponding reference sample. The Spectrumâ„¢ 10 FT-IR software provided by PerkinElmer Inc. (Massachusetts, USA) integrated the area of the spectrum collected between 990-945 cm-1 and compared with the calibration curve of TFA standards using linear regression equation (with R2 value â‰¥ 0.99) to quantify TFA.
4.9 Analysis of the quality of used frying medium
The methods to measure the quality of the three frying medium across seven consecutive days of frying, particularly the peroxide value, acid value, total tocopherol content, total phenolic content, total polar compounds content, oxidative index and colour index were determined according to the works of Farhoosh and others (2009) and Chu and others (2001).
4.10 Statistical analysis
All the experiments and measurements were carried out in triplicate. The data were all subjected to analysis of variance (ANOVA) using Predictive Analytics SoftWare (PASW) package. The significant differences between means (P < 0.05) were determined via Duncan's multiple range tests.