Cutting edge Biochemistry – How cutting changes your food’s aroma

The fact that cutting food into different shapes and sizes makes them cook, marinate or dry in different ways is somewhat obvious: a thin slice of potato will cook much faster than a whole one. What’s less obvious is that cutting food starts chemical reactions within it. The fact that there’s a difference between cutting with or against the grain of meat is pretty common knowledge nowadays, but recent (and not so recent) data suggest that this is true for fruits and vegetables, too. The cutting angle changes the rate of browning in bananas and can even influence the aroma of lettuce. To understand how this comes about, let’s have a quick look at how plant cells are organised.

Plant cells: Water balloons in a shell

Like any other cell, a plant cell’s contents are closed off from the outside by a flexible membrane. Plant cells additionally form a rigid cell wall around their outer membrane which determines their shape, like a water balloon pressing against a solid shell. In very broad terms, a cell’s central job is to maintain order, more specifically, to create the optimal environment for the various biochemical reactions it needs to keep going to survive. Different reactions require different conditions and might even compete so that complex cells possess membrane-enclosed spaces, so-called compartments, to contain them. This compartmentalisation is a fundamental principle in complex cells.

**Figure 1** This is the skin of a red onion under a microscope. The red colour comes from anthocyanins stored in the cells’ vacuoles. In the upper section, a couple of cells are losing water and shrink. As they do so, their membranes slowly detach from the cell wall and intensify in colour as the anthocyanin concentration in the vacuole increases.

Usually, the biggest compartment by far in a plant cell is its vacuole (Figure 1). The vacuole is basically a big storage compartment where the cell stores waste, supplies and some other compounds serving specialised purposes. Usually, the vacuole takes up almost the entire volume of the cell. The rest of the cell, the cytoplasm, is reduced to a thin layer of liquid between the vacuole and cell membrane. This is where most of the cell’s metabolism takes place and where additional compartments like the nucleus and organelles such as mitochondria are located.¹

Cutting bursts cells open

When we cut into a vegetable (or any part of a (formerly) living thing), we essentially rip apart a thin sliver of cells. Their contents get mixed up and (usually) exposed to air. Now, all sorts of chemical reactions can take place that the intact cell kept in check. The more cells burst, the larger the scale of these reactions. This is where the cutting angle comes in. Return to the image of onion skin under the microscope: its cells are much longer than they are wide; cutting parallel to the long side will pop much fewer cells than cutting perpendicular to it.²

With that settled, let’s now take a look at the actual reactions taking place.

Onions violate the Geneva Protocol

Onions don’t care about international law – cut one open and face a retaliative gas attack. This is how it works: the onion cells’ cytoplasm holds sulfur-containing isoalliin while the vacuoles store enzymes that can transform this into reactive volatile compounds, which, in turn, form sulfuric acid with water in your eyes. Chopping the onion exposes isoalliin to the enzymes, and the ensuing reactions yield the tear-inducing counterattack. However, the initial reaction also produces the spoils of war: the breakdown products of isoalliin are so reactive that they form a whole range of tasty compounds that are characteristic for plants of the allium family (leek, chives, garlic and so on; the reactions are slightly different between species, though). The mechanism here is really interesting but beyond the scope of this article – I might cover this in a separate article someday.

Make your lettuce cry!

According to a 2011 study,³ lettuce cut parallel to the middle rib produces more “green” or “leaf-like” compounds.⁴ These compounds come from the same reaction that gives vegetables their “green” aroma: oxidation of fatty acids by lipid peroxidases (LOXs).

**Figure 2** Oxidation of fatty acids by LOXs to leafy-smelling compounds. I drew a free fatty acid here but more likely would be a fatty acid bound in a lipid.

LOXs peroxidise unsaturated fatty acids, which then break down into more or less volatile aldehydes that can undergo further enzymatic reactions yielding more volatile compounds (Figure 2). Some of them are responsible for the familiar smell of leaves, and the two most prominent are fittingly referred to as leaf aldehyde and leaf alcohol. Another product of this LOX-catalysed breakdown is traumatin which acts as a plant hormone signalling tissue damage. So, in a way, leafy aroma is actually a plant’s chemical cry of pain…⁵

Are you cutting your bananas correctly?

Finally, let’s get to answering the question of who’d cut up a banana end-to-end: the answer, of course, is scientists. But relax, science doesn’t want you to cut bananas lengthwise; the scientists in question found that the bananas sliced at 0° (into circles) browned slower than those cut at more obtuse angles, with those cut lengthwise (90°) browned the most.⁶ Admittedly, most of this change is only really significant after a very long time: they ran all their experiments for four days at 20°C and measured the first time at 10 or 20h after the start of the experiment. That’s reasonable for you and me, who did some meal prep the night before or stored some leftovers for a day. But I don’t think most of us don’t (intentionally) keep sliced bananas around as long as they did. It might be relevant for industry, though.
As for the mechanism, the browning reaction itself is due to polyphenol oxidases that turn colourless polyphenolic compounds into brown(ish) oxidised ones. I’m not sure what’s going on with the angle dependence, though; unfortunately, except for the figures, the article is in Japanese so I’m not aware of what the authors themselves made of their data.

Concluding remarks

This post is probably not going to change anyone’s approach to cooking – I’m here for the fun of it and I’m happy to share what I find interesting. – And if you enjoy reading about that that’s really all I can hope for :).

Footnotes

1
So when you eat plant matter, most of what you get is actually vacuoles, or, put differently: fruit juice is mostly vacuole contents, add cell wall fragments, and you get a smoothie. I always found it quite interesting to think about food in terms of cell components.
2
In a full-sized plant organ like a fruit, the cells would point in different directions, especially in curved ones like bananas. Still, you can reasonably expect some regularity in the cells’ orientation and thus that different cutting angles crush different amounts of cells.
3
Deza-Durand, Karla M., and Mikael Agerlin Petersen. ‘The Effect of Cutting Direction on Aroma Compounds and Respiration Rate of Fresh-Cut Iceberg Lettuce (Lactuca Sativa L.)’. Postharvest Biology and Technology 61, no. 1 (July 2011): 83–90. https://doi.org/10.1016/j.postharvbio.2011.02.011
4
I’m not sure if these changes are detectable by humans, though.
5
A 2010 paper offered another perspective. It was found that the saliva of a certain species of caterpillar reacts with a compound from the plant it feeds on to form another compound that attracts predators. These would then flock to the damaged plant and attack the caterpillars. The authors thus went with “Modified SOS calls” as a description, which stretches my analogy to a somewhat messed-up picture. – I guess the way we humanise the plants’ reaction here is a matter of perspective.
6
Link to the paper (mostly in Japanese).