Flaxseed

I was trying to find a food science topic to write about. I have just gave my candy technology lecture to my freshman class and was going to write that up, and then realized that Exploratorium does a very good job of that already.

So…

I then thought I should do something with a Thanksgiving theme, but with being British and vegetarian I don’t really celebrate a traditional Thanksgiving. I will be eating Vegetarian moussaka if any one is interested.

Which leads to flaxseed. One of my food chemistry class projects is on flaxseed because I published an article[1] on using flaxseed meal in whole-wheat muffins. The only major difference we found between muffins with and without flaxseed meal was that the batter with the flaxseed meal was significantly thicker. I actually did this project initially as a colleague who was studying the nutritional effects of flaxseed couldn’t do a double-blinded trial as at that time it was not clear as to how stable the nutritive components of flaxseed were. Also it is hard to discuss flaxseed in a food product such as muffin, as it adds a nutty flavor.

Let us take a step back. Why would I care whether flaxseed meal could be added to muffins in the first place? Flax (Linum usitatissimum) is a versatile plant. Also known as linseed, from it we get linoleum, linen, linseed oil as well as flaxseed meal. Flaxseed is important nutritionally as it is high in polyunsaturated fatty acids, particularly alpha linolenic acid (ALA) which is an omega-3 fatty acid. For those of you going around with your eyes closed, omega-3 fatty acids are important nutrients that we mostly get from fish oil, but vegetarians like me can get them from flaxseeds and walnuts.

Flaxseed is also important as it is about 30% fiber and we always need more fiber. If that was not enough, flaxseed is the highest source of the plant lignan secoisolariciresinol diglucoside (SDG). This is converted by intestinal bacteria into the mammalian lignans, enterodiol and enterolactone. These are thought to have similar anticancer properties as soy isoflavones. SDG does have antioxidant properties and is probably the reason flaxseed oil is more stable that would be expected given its high polyunsaturated fatty acid content.

To break the nutritional facts down, 100 g flaxseed meal would give:

450 Calories, 41 g fat of which 23 g is ALA, 28 g fiber, 20 g protein and approximately 0.7- 2 g SDG.

I am interested in finding out what happens when flaxseed meal is added to a batter. Several years ago we, an undergraduate and I, did a research project on flaxseed viscosity and found that the apparent viscosity of a batter (oil, egg, water and flaxseed) increased with time in the first hour:

So last semester, another undergrad and I did a literature survey to see why this was happening. A couple of questions came to light:

1. Do flaxseed meal mixtures show Newtonian or shear-thinning behavior?
2. Was the initial rheological behavior mentioned above caused by the fiber (mucilage) absorbing water? If so, which portion of the fiber was responsible?

My food chemistry students are still working through these problems, but here is some more background.

Newtonian fluids increase in shear stress in direct proportion to the shear rate. Stirring a Newtonian fluid has no effect on its viscosity, only temperature, pressure and composition can alter its viscosity. For shear thinning fluids the apparent viscosity is reduced with the rate of shear. Thus, the faster you stir a shear thinning fluid, the thinner it gets.

Flaxseed fiber, which is most commonly called mucilage, is made up of two major moieties; rhamnogalacturonans (25%) and arabinoxylans (75%). Arabinoxylans have been shown to have shear thinning behavior but no one appears to have studied the initial change in rheology that I am interested in.

I’ll keep you posted as to what happens with the results. In the meantime you can observe this effect for yourself by making my favorite smoothie:

Blend together one mango, one banana, a handful or two of blueberries or blackberries or other red fruit with a cup and half of orange juice. Mix in a cup of plain yogurt (optional). Add two table spoons of flaxseed meal and stir. As you drink the smoothie, notice how it thickens!! Add orange juice to dilute if you don’t want a chewy smoothie.

I know what I’m making for Thanksgiving breakfast now!

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[1] Shearer, A. E. H.; Davies, C. G. A. Physicochemical properties of fresh-baked and stored whole wheat muffins with and without flaxseed meal. Journal of Food Quality 2005, 28, 50-66

 

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