To Microwave or not to Microwave? Fish Thrives, While Extra Virgin Olive Oil Deteriorates in the Microwave Oven

The effect of microwave steaming, i.e. steaming with the microwave by the means of one of those microwave steaming bags has not been tested in the study at hand, but it should be relatively harmless... assuming your steaming bag is BPA free.

I know that people all around the world still argue that microwaving your foods would modify them in ways that render them at least less, if not simply downright unhealthy. In reality, however, the safety of microwaving is well established; and there's evidence suggesting that it is rather the way of heating you should prefer than the way to heat you should avoid.

With the latest study from the Unidad de Investigación y Desarrollo de Alimentos the previously discussed and often (falsely) ignored formation of oxysterols, i.e. oxidized cholesterol that's bad for your metabolic and cardiovascular health microwave (MW) foods or I should say fish just got even more attractive.

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As Leal-Castañeda et al. point out in the introduction to their paper, "cholesterol oxidation products (COPs) formation depends on the conditions of MW heating and the composition of the food matrix" (Leal-Castañeda. 2016).

It is thus only logical that the Mexican researchers compared different oils / foods, namely palm, extra virgin olive, soybean and fish oils. All oils were heated in a two different microwaves:

• a Panasonic, Model NN-6653, 900 W, operating at in multimode
• Thermo CEM DISCOVER series SP-D, operating in monomode

But wait, where's the cholesterol in palm, olive ad soybean oils? Obviously, there is none. Accordingly, the scientists added a standardized amount of 2.5 mg of cholesterol/g of oil to all four test oils before MW heating them.

Table 1: Temperature reached by in lipid systems made of oils and cholesterol during heating in multimode MW oven; the monomode MW was set to heat until the temperature was 180°C  (Leal-Castañeda. 2016).

Next to the temperature and oxysterol formation, the scientists also monitored the changes in peroxide value and fatty acid (FA) profiles.

"MW heating of edible oils may cause its degradation by oxidation, hydrolysis and polymerization. It has been suggested that the unsaturated components of the oils are usually the most susceptible to these degradation processes, altering their physicochemical properties and FA profile. However, it has been reported that heating time and temperature, and the presence of natural antioxidants (tocopherols, chlorophylls, carotenoids and phenolic compounds) have a significant influence on the extent of the oxidative processes. [In the study at hand, the] most relevant change was the decrease of PUFAs presen[t] in fish oil (Figure 1)" (Leal-Castañeda. 2016).).

As Leal-Castañeda, et al point out (2016), the significant decrease of PUFAs in fish oil heated in MW (both unimode and multimode) and conventional oven is in agreement with Weber et al. (2008) and Zhang, et al. (2013), who observed a slight decrease in the PUFA content in silver catfish fillets and grass carp (ctenopharynyodon idellus) fillet (this is important, because this is a whole food) baked in conventional oven, and a greater reduction when MW oven was employed.

• Overall, the maximum total COPs contents "largely varied (46.4-250.4 μg/g lipids), depending on the type of heating system and oil matrix" (Leal-Castañeda. 2016).
• In that, the multimode MW heating caused greater COPs formation than the unimode MW. 
• On the other hand, the COP formation in the allegedly healthier was significantly higher - albeit only for fish oil - than during either of the two microwaving treatments.

If you scrutinize the data in Figure 1 you will see that another not exactly expected result of the study at hand was that that "soybean oil, compared with the other oils tested, did not promote cholesterol oxidation during MW heating" (Leal-Castañeda. 2016).

The contents of vitamin C and total carotenoids, aliphatic and indole glucosinolate in broccoli cooked by different methods Effects of  cooking methods (1. raw; 2. boiled; 3. steamed; 4. microwaved; 5. stir-fried; 6. stir-fried/boiled | Yuan. 2016).

Steaming and microwaving have been repeatedly shown to produce the least oxidative damage / detorioration of vitamins and other healthy molecules to cholesterol- and non-cholesterol containing food products, such potatoes (Tian. 2016), broccoli and cauliflower (Yuan. 2009; Mansour. 2016), other legumes and vegetables (Fabbri. 2016).

In that, the effects may differ not just based on the macronutrient composition of the food, it may also differ for individual micronutrients such as vitamins or indoles.

That's much in contrast to the allegedly healthier extra-virgin olive oil and palm oil, where the formation of oxysterols skyrocketed, in spite of the presence of natural antioxidants during MW heating.

Figure 1: Oxid. of cholesterol in lipid systems made of oils and cholesterol after 20 min of heating in unimode (UMO), multimode MW (MMO) oven and conventional (CO) oven (Leal-Castañeda. 2016) - 5,6α-CE, 5,6α-epoxycholesterol; 5,6β-CE, 5,6β-epoxycholesterol; 7α-HC, 7α hydroxycholesterol; 7β-HC, 7β-hydroxycholesterol; 7-KC, 7-ketocholesterol

In view of these important differences, it should be obvious that "the type and composition of the lipid medium should be considered for formulation of food products to be cooked in MW for long time periods (>10 min)" (Leal-Castañeda. 2016) - with high PUFA oils being more susceptible to oxidative damage than low PUFA oils and a suspicious lack of effect of natural anti-oxidants on the extents of oxidative degradation, and thus the overall nutritional quality and safety of food products.

After being absorbed in the gut, exogenous oxysterols are incorporated into cyclomicrons and will subsequently have the same ill (heart-)health effects as their endogenously produced (by inflammation) cousins (Otaegui-Arrazola. 2010)

It's not just about fish... If you revisit the data in Figure 1 you will realize that the likewise relatively PUFA rich extra-virgin olive oil showed a highly significant increase in oxysterols, especially 7α hydroxycholesterol, when it was heated in a classic (older technology) multimode microwave with its characteristically random temperature differences in different part of whatever it is you are cooking.

Whether and why the local temperature distribution is highly relevant for olive oil, but neither palm, soy or fish oil is something future studies will have to determine... in fact, we'd even have to confirm that it is the temperature difference, nothing else that explains the significant differences.

But do you even care? Well, it depends. Oftentimes you may not even have cholesterol in your EVOO. "Often", however, is "not always" and whenever you microwave animal products in EVOO, you will end up having sign. amounts of oxidizable cholesterol around - cholesterol that has been shown to be absorbed and "could represent a significant health risk" (Leal-Castañeda. 2016 | cf. Sottero, 2009 & Staprans, 2003). Comment on Facebook!

References:

• Fabbri, Adriana DT, and Guy A. Crosby. "A Review of the Impact of Preparation and Cooking on the Nutritional Quality of Vegetables and Legumes." International Journal of Gastronomy and Food Science 3 (2016): 2-11.
• Mansour, Abd Allah, et al. "Effect of Domestic Processing Methods on the Chemical Composition and Organoleptic Properties of Broccoli and Cauliflower." American Journal of Food and Nutrition 3.5 (2016): 125-130.
• Otaegui-Arrazola, Ana, et al. "Oxysterols: a world to explore." Food and Chemical Toxicology 48.12 (2010): 3289-3303.
• Sottero, Barbara, et al. "Cholesterol oxidation products and disease: an emerging topic of interest in medicinal chemistry." Current medicinal chemistry 16.6 (2009): 685-705.
• Staprans, Ilona, et al. "Oxidized cholesterol in the diet is a source of oxidized lipoproteins in human serum." Journal of lipid research 44.4 (2003): 705-715.
• Tian, Jinhu, et al. "Domestic cooking methods affect the phytochemical composition and antioxidant activity of purple-fleshed potatoes." Food chemistry 197 (2016): 1264-1270.
• Yuan, Gao-feng, et al. "Effects of different cooking methods on health-promoting compounds of broccoli." Journal of Zhejiang University Science B 10.8 (2009): 580-588.
• Zhang, Jinjie, et al. "Effect of cooking styles on the lipid oxidation and fatty acid composition of grass carp (ctenopharynyodon idellus) fillet." Journal of Food Biochemistry 37.2 (2013): 212-219.