Correlation of fatty acid composition of vegetable oils with rheological behaviour and oil uptake
Introduction
Frying has been widely used for food industrial and household purposes as a typical way to prepare for foods. Frying is a very complex process where a variety of chemical and physical phenomena take place such as starch retrogradation, Maillard reactions, surface porosity, oil degradation, and etc. (Mellema, 2003). Specially, in terms of heat and mass transfer, heat is transferred from hot oils to foods and water is evaporated from the surface of the foods. Consequently, the pores caused by water evaporation are occupied by oils, thus causing a significant increase in the oil content of the product. Therefore, it seems that frying belongs to a dehydration process (Budzaki & Seruga, 2005).
Frying provides good sensory properties such as crisp texture, brownish colour, and juicy taste. Nonetheless, a negative aspect of fried foods has recently come to the attention of consumers since such products may contain undesirable components such as acrylamide, trans/saturated fatty acids, and a high amount of oil, even up to 50% of the total weight (Funami, Funami, Tawada, & Nakao, 1999). Specifically, since trans fatty acids are widely reported to be a causative factor in health problems such as coronary heart disease (Van de Vijver et al., 2000, Willett and Ascherio, 1994), non-hydrogenated vegetable oils have been receiving great attractions, consequently causing a dramatic increase in its use as an edible frying medium. This comprehensive trend is also supported by the new food guide pyramid of the United States Department of Agriculture (Goldie, 2005), which recommends to consume vegetable oils high in mono- or polyunsaturated fats and low in saturated fats. Therefore, a variety of natural oils from vegetable sources have been used, which include soybean, corn, palm, sunflower, canola, olive oils, and etc.
Basically, frying is the immersion and cooking of foods in hot oils. Therefore, the first step to understand the frying process would be to characterise the quality of the frying oils being used. Thus, a number of previous studies on vegetable oils have been found where a focus was mainly placed on their chemical (Abdulkarim et al., 2007, Dobarganes et al., 2000) and rheological (Erhan et al., 2002, Wan Nik et al., 2005) properties. However, detailed correlations between such properties have not yet been analysed and only limited sources of vegetable oils have been utilised in preceding studies.
In addition, few reports are available on the amount of absorbed oils to foods depending on the type of vegetable oils, even though the oil uptake is a critical issue in a frying process. A couple of studies demonstrated that oil uptake was related to the oxidative degradation of vegetable oils (Dobarganes et al., 2000, Tseng et al., 1996). However, to our knowledge, there was only a report in 2007 (Kita, Lisinska, & Golubowska, 2007) in which the oil uptake and texture of foods fried in sunflower, soybean, peanut, rapeseed, olive, palm, and modified oils were compared as a function of frying temperature. Unfortunately, this study did not provide detailed information on the effect of the oil type on the oil uptake.
Therefore, in this study, the flow behaviours of seven vegetable oils were investigated and then correlated with their fatty acid composition. Also, the effect of the type of vegetable oils on the oil uptake of fried potato strips was investigated.
Section snippets
Experimental materials
Seven edible vegetable oils (canola, corn, grapeseed, hazelnut, olive, soybean, and sunflower oils) were purchased from a commercial source. All chemicals used in this study were of analytical grade.
Measurement of fatty acid composition
Fatty acid composition of fresh vegetable oils was investigated by using GC–MS (6890N, Agilent Technologies, DE, USA). After each oil sample was methylated with boron trifluoride (Seiichi, Akiko, Yasue, & Chiemi, 2007), the fatty acid methyl esters were injected into a capillary DB-23 column (0.25 mm
Results and discussion
The fatty acid compositions of seven different vegetable oils were analysed as shown in Table 1. For all of the oils tested in this study, the percent amount of total saturated and unsaturated fatty acids ranged from 12% to 22% and from 78% to 88%, respectively. Specially, canola and hazelnut oils contained a great amount of unsaturated fatty acids, relatively compared to the other oil samples. The predominant component of olive, canola, and hazelnut oils was oleic acid (18:1) whilst the others
Conclusions
As a principal and essential frying component, edible vegetable oils were characterised in terms of their fatty acid composition, flow behaviours, and oil uptake. The results demonstrated that there was a high correlation between oil viscosity and fatty acid composition, suggesting that the oils with more double bonds appeared to have lower viscosity due to their loosely-packed structure. Also, their flow behaviours over temperature could be well characterised by the Arrhenius model.
Acknowledgements
Thanks go to Mr. Hong Gyoon Oh for his great support during frying experiments. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-331-F00046).
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