|The oil is not the only thing that's damaged, when you're frying foods; and acrylamide is only the most prominent of these nasty compounds.|
Needless to say that this part is going to focus on purportedly healthier alternatives ranging from high-oleic acid sunflower and canola oil to another often hyped dietary fat: coconut oil.Before we delve deeper into the discussion lets briefly recap what exactly we are looking for in the "optimal" frying oil: The optimal frying oil should...
- ... either contain a low amount of readily oxidizable PUFAs or contain significant amounts of antioxidants that protect these important, but sensitive fatty acids from being oxidized,
- ... be cholesterol-free, because dietary cholesterol can be a problem, if it's consumed in large amounts in its pro-atherogenic (=tending to promote the formation of fatty deposits in the arteries) oxidized form,
- ... have a high smoke point, i.e. can be heated to very high temperatures before enough volatile compounds such as water, free fatty acids, and short-chain degradation products of oxidation emerge from the oil that a bluish smoke becomes clearly visible.
Figure 1: The consumption of soy bean oil that was heated for 3h is associated with an increase in body fat that is independent of an increased intake of food (Penumetcha. 2013)
Some scientists in fact believe that the association between the consumption of high amounts of (high omega-6) seed oil is associated with heart disease & co not due to the fatty acid composition of these oils per se, but rather due to extensive use of these products in highly processed (convenient) foods, which in turn contain high amounts of oxidized polyunsaturated fatty acids and will thus drive the development of heart disease, diabetes & co.
- Oxidized cholesterol as it occurs at relatively high amounts un highly processed foods, has the potential
cytotoxic, mutagenic, atherogenic, and possibly carcinogenic effects (Peng. 1985; Kumar. 1991; Valenzuela. 2003; Orczewska-Dudek. 2012). In that it's important to point out that not all heated animal products (e.g. eggs) will contain significant amounts of oxysterols, because the formation of these potentially unhealthy byproducts of the heating process depend on (a) temperature, (b) heating time and (c) storage & packaging.
Against that background, soft-boiled eggs are healthier than hard-boiled eggs or fried eggs and a hot chocolate milk is less of a problem than repeatedly heated and extensively processed eggs, meats & co.
Figure 2: Overview of the effects of storage, processing and heating on the levels of oxidized cholesterol in various food items (various sources).
- Figure 3: Changes occurring during deep fat frying (Fritsch. 1982).
As you can see, it's a complex series of changes and reactions that produce numerous decomposition products. As Fritsch points out, "the functional, sensory and nutritional quality of frying fats are changed and may reach a point where high quality foods can no longer be prepared," as these reactions proceed. In the end, the oil will be unpalatable.
Unfortunately, the negative effects on your health will occur way before you will be able to detect the changes in viscosity, color or taste (specifically if the oils are used in processed products, where any off tastes would be covered by (artificial) flavorings, anyways) or see the smoke. That's also why the smoke point of a given fat / oil is not exactly the best criterion to judge its suitability as "optimal frying oil".
Q: What's the optimal frying oil, now? A: It depends!
Now that we know that we want an oil that's relatively low in PUFAs, high in (natural) antioxidants, cholesterol-free and has a high smoking point, a few candidates come to mind:
- High oleic acid canola (or rapeseed) or high oleic acid sunflower oil - Both contain a low amount of PUFAs, no cholesterol, and have relatively high a smoking points of 194°C and 200°C, respectively.
Table 1: Ranking of the oils tested by Bertrand Matthäus; 1 indicates rank #1 = first place, 4 indicates rank #4 = last place (Matthäus. 2006).
If you compare the high oleic acid varieties of sunflower and rapeseed oil that are made from seeds with a low PUFA and high MUFA content, you will yet realize that the the often derided sunflower oil is actually the most stable one. The rapeseed oil, on the other hand, produced the tastiest fries (according to the full text of the study). Since "only small differences in the chemical parameters of all oils were found" and "[o]nly the total result of the assessment of PO was significantly worse than the results of the other oils (p < 0.01)" (Matthäus. 2006), the rapeseed oil comes out on top of this comparison.
|Using high MUFA rapeseed oil not just for frying, but as a major fat source in the diet leads to significant improvements in blood lipids in hypercholesterolaemic subjects (Gillingham. 2011)|
If you look objectively at the existing evidence, it is debatable whether this may not be superior to using olive oil or other high MUFA oils, but certainly superior to using other frying oils or animal fats. Plus: It works it cholesterol lowering and glucose metabolism improving magic not just in patients with high cholesterol levels (see figure on the left), but also in healthy young women (Uusitupa. 1994; Jones. 2014).
- Now, being resistant to oxidation is one thing. Having overall beneficial effects on one's health, however, is a totally different thing. A "thing" that was tested in a 2005 study, by Allman-Farinelli et al. in which the researchers were able to show that replacing saturated fats in the diet with high-oleic-acid oils like sunflower oil will favorably alter the low-density lipoprotein cholesterol, triglycerides, and factor VII coagulant activity" of healthy men and women and thus present "another useful source of MUFA for diets aimed at prevention of heart disease"(Allman-Farinelli. 2005).
Figure 4: Evidence of the effect of canola oil on health-related risk factors.
Against that background I would stick to using canola, or if you live in Europe or can acquire it overseas rapeseed oil only in its high oleic acid variety and specifically for frying foods at high temperature, when you cannot live with the taste they would acquire if you used extra virgin olive oil, instead.
- The coconut miracle not ideal for frying!? -There is little doubt that virgin coconut oil belongs to the "healthy oils" - despite, or rather due to its low PUFA content. The question is: Is it also ideal for frying? If we go through our checklist, as prevoiusly said...
- it contains only 1.7% PUFAs, 85.2% saturated and 6.6% monounsaturated fats, which means it's more or less PUFA free and thus not prone to heat induced oxidation
- it's cholesterol free, so the problem with oxidized cholesterols that is rampant in fats from animal products is not a problem,
- it's smoke point is relatively, low though, at only 163°C even the deodorized, bleeched coconut oil will form enough volatile compounds for the ugly bluish smoke to rise from your pan (Man. 1998).
Figure 5: After a 30 hours of frying (not shown) the US' favorite health killer, soybean oil, has peroxide values that exceed the measuring capacity of the assay, Yuki & Ishikawa used in their 1979 study (Yuki. 1979)
Unfortunately, its comparatively low peroxide values don't tell you the full truth about the usefulness of coconut oil. Compared to similarly bleached and deodorized palm oil, coconut oil will have higher %FFA levels, which are indicative of an increased oxditation & hydrolysis of fatty acids and are responsible for the "soapy" flavor that made the test product in a 1998 study by Man, et al. hardly palatable. From a product quality perspective, frying with palm oil would thus be (and for most companies is) the preferable choice. From a mere health perspective, though, frying with coconut oil - especially at lower heats that do not exceed the smoke point of 163°C (vs. 229° for palm oil), is yet a good and tasty choice.
- Avocado oil, the exotic alternative - There is unfortunately little research on the long-term health effects of using avocado oil for frying.
Table 2: The fatty acid composition of cold-pressed and refined avocado oil does not differ (Haiyan. 2007)
There is no cholesterol in avocado oil, so oxidized cholesterols are not an issue. Compared to olive oil, avocado oil contains an increased amount of beneficial antioxidant, is naturally lower in peroxides and free fatty acids (this does not mean that there will be necessarily less after processing) and is usually, just as olive oil, pesticide free (Reed. 2001).
Against that background it's worth taking a look at the results of a 2012 study from the University of Navarra in Spain, which found that ...
"Avocado oil was richer than olive oil in total phytosterols at time 0 h (339.64; 228.27 mg/100 g) and at 9 h (270.44; 210.30 mg/100 g) of heating. TBARs was higher in olive oil after 3 h, reaching the maximum values in both oils at 6 h of heating treatment. Vitamin E was higher in olive oil (35.52 vs. 24.5 mg/100 g) and it disappeared earlier in avocado oil (at 4 vs. 5 h). The stability of avocado oil was similar to that of olive oil." (Berasategi. 2012 | my emphasis)Since no regular consumer will fry his foods for more than 3h, the previously presented evidence clearly indicates that avocado oil is an allegedly more expensive, albeit slightly superior alternative to olive oil, the last oil on our list (see below).
- Olive oil, refined or, even better, extra virgin - If it was not for the price and the taste, the whole world would probably long have switched over to olive oil as their go-to oil for everything. I am not going to (re-)cite the plethora of epidemiological evidence in favor of its beneficial health effects, here. Instead I will briefly go through our three items from the initially presented list.
Olive oil has a relatively low omega-6 and virtually no omega-3 fatty acids (10.5g PUFA total per 100g). It's fatty acid profile, alone, does therefore tell us that it could be a suitable candidate for the "optimal" frying oil.
Figure 6: Effect of air, light and deep-frying on p-AV of olive, corn and soybean oil (Naz. 2004). Please not that improper storage will also increase the oxidative damage to your cooking oils.
- Allman-Farinelli, Margaret A., et al. "A diet rich in high-oleic-acid sunflower oil favorably alters low-density lipoprotein cholesterol, triglycerides, and factor VII coagulant activity." Journal of the American Dietetic Association 105.7 (2005): 1071-1079.
- Berasategi, Izaskun, et al. "Stability of avocado oil during heating: Comparative study to olive oil." Food Chemistry 132.1 (2012): 439-446.
- Chan, Shu-Hui, et al. "Cholesterol oxidation in whole milk powders as influenced by processing and packaging." Food chemistry 47.4 (1993): 321-328.
- De, B. K., and D. K. Bhattacharyya. "Deacidification of high-acid rice bran oil by reesterification with monoglyceride." Journal of the American Oil Chemists’ Society 76.10 (1999): 1243-1246.
- Debnath, Sukumar, et al. "Effect of frying cycles on physical, chemical and heat transfer quality of rice bran oil during deep-fat frying of< i> poori:</i> An Indian traditional fried food." Food and Bioproducts Processing 90.2 (2012): 249-256.
- Enig, M. G., et al. "Fatty acid composition of the fat in selected food items with emphasis on trans components1." Journal of the American Oil Chemists’ Society 60.10 (1983): 1788-1795.
- Fritsch, C. W. "Measurements of frying fat deterioration: a brief review." Journal of the American Oil Chemists’ Society 58.3 (1981): 272-274.
- Gillingham, Leah G., et al. "High-oleic rapeseed (canola) and flaxseed oils modulate serum lipids and inflammatory biomarkers in hypercholesterolaemic subjects." British Journal of Nutrition 105.03 (2011): 417-427.
- Haiyan, Zhong, et al. "Endogenous biophenol, fatty acid and volatile profiles of selected oils." Food chemistry 100.4 (2007): 1544-1551.
- Jones, Peter JH, et al. "High-oleic canola oil consumption enriches LDL particle cholesteryl oleate content and reduces LDL proteoglycan binding in humans." Atherosclerosis (2014).
- Osada, Kyoichi, et al. "Oxidation of cholesterol by heating." Journal of Agricultural and Food Chemistry 41.8 (1993): 1198-1202.
- Kumar, Naresh, and O. P. Singhal. "Cholesterol oxides and atherosclerosis: a review." Journal of the Science of Food and Agriculture 55.4 (1991): 497-510.
- Lercker, G., and M. T. Rodriguez-Estrada. "Cholesterol oxidation: presence of 7-ketocholesterol in different food products." Journal of Food Composition and Analysis 13.4 (2000): 625-631.
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- Man, YB Che, and WR Wan Hussin. "Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut oil." Journal of Food Lipids 5.3 (1998): 197-210.
- Matthäus, Bertrand. "Utilization of high‐oleic rapeseed oil for deep‐fat frying of French fries compared to other commonly used edible oils." European Journal of Lipid Science and Technology 108.3 (2006): 200-211.
- Morgan, J. N., and D. J. Armstrong. "Quantification of Cholesterol Oxidation Products In Egg Yolk Powder Spray‐dried with Direct Heating." Journal of food science 57.1 (1992): 43-45.
- Missler, S. R., B. A. Wasilchuk, and C. Merritt. "Separation and identification of cholesterol oxidation products in dried egg preparations." Journal of Food Science 50.3 (1985): 595-598.
- Naz, Shahina, et al. "Oxidative stability of olive, corn and soybean oil under different conditions." Food Chemistry 88.2 (2004): 253-259.
- Orczewska-Dudek, Sylwia, et al. "Cholesterol and lipid peroxides in animal products and health implications-A review." Annals of Animal Science 12.1 (2012): 25-52.
- Paniangvait, P., et al. "Cholesterol oxides in foods of animal origin." Journal of Food Science 60.6 (1995): 1159-1174.
- Peng, Shi-Kaung, et al. "Cholesterol oxidation derivatives and arterial endothelial damage." Atherosclerosis 54.2 (1985): 121-133.
- Penumetcha, Meera, et al. "A diet containing soybean oil heated for three hours increases adipose tissue weight but decreases body weight in C57BL/6 J mice." Lipids in health and disease 12.1 (2013): 26.
- Pie, Jae Eun, Khira Spahis, and Christine Seillan. "Cholesterol oxidation in meat products during cooking and frozen storage." Journal of Agricultural and Food Chemistry 39.2 (1991): 250-254.
- Reed, A. B., et al. "New Zealand extra virgin olive oils." Food New Zealand Feb/March 2001 (2001): 20-24.
- Uusitupa, Matti, et al. "Effects of two high-fat diets with different fatty acid compositions on glucose and lipid metabolism in healthy young women." The American journal of clinical nutrition 59.6 (1994): 1310-1316.
- Valenzuela, Alfonso, Julio Sanhueza, and Susana Nieto. "Cholesterol oxidation: health hazard and the role of antioxidants in prevention." Biological research 36.3-4 (2003): 291-302.
- Valsta, Liisa M., et al. "Effects of a monounsaturated rapeseed oil and a polyunsaturated sunflower oil diet on lipoprotein levels in humans." Arteriosclerosis, Thrombosis, and Vascular Biology 12.1 (1992): 50-57.
- Yuki, E., and Y. Ishikawa. "Tocopherol contents of nine vegetable frying oils, and their changes under simulated deep-fat frying conditions." Journal of the American Oil Chemists’ Society 53.11 (1976): 673-676.