Browning Onions and The Maillard Reaction

The Maillard reaction is one of the more interesting chemical reactions in food science, partly as it occurs almost every day in your own kitchen. Basically it is the common browning of food as it cooks: bread, meat, coffee, vegetables, chips, pastry… pretty much all of your recipes will involve this reaction to some extent. You might brown some onions before adding mince for a lasagne, or brush egg yolk on pastry before cooking it, or sauté some meat before making a stew. The reaction breaks the food down into many different flavour compounds, releasing all the different tastes the food has. Perhaps this better taste and improved aromatics are why we have evolved to cook food that has browned, despite the end products being harder to digest.

Despite its importance in cooking and for the food industry, the Maillard reaction is not particularly well characterised . The original paper describing it was published by Louis-Camille Maillard in 1912, but was largely ignored for years, as late as 1948, when it was brought to the forefront by research looking at the loss of nutritional value in heating milk powders. The reaction occurs in three stages, producing various flavour compounds from a sugar and an amino acid (often coming from protein). Some examples of the flavour compounds produced from the Maillard reaction:

Compound class and associated flavour/aroma:
Pyrazines: Cooked, roasted, toasted, baked cereals
Alkylpyrazines:  Nutty, roasted, like in coffee
Alkylpyridines:  Green, bitter, astringent, burnt, like in coffee, barley, malt (generally regarded as unpleasant)
Furans, furanones, pyranones: Sweet, burnt, pungent, caramel-like
Oxazoles: Green, nutty, sweet like in cocoa, coffee, meat
Thiofenes: Meaty

What I am going to look at here, though, is how the Maillard reaction changes under different environmental conditions- like sugar, acid level, moisture level, and so on. Since browning your food is desirable, as you’ll get a nicer taste from it, understanding how to get your food to brown- or why it isn’t browning- will improve what ends up on the plate. We know that you need heat to get food to brown, but how else can you control it?

To test these, and particularly the relative speed of food browning, I spent an evening modifying an experiment from The Kitchen As Laboratory: chopping up some onions, subjecting them to different conditions, heating them, and seeing what happened. I looked at four different conditions (as I had four different hobs): 1) A control, just regular onion, 2) Onion with added bicarbonate of soda, 3) Onion with added lemon juice, and 4) Onion with some added sugar. With the sugared onions, only a moderate amount could be put on the sugar, otherwise a caramelisation reaction might occur, which is different from the Maillard reaction, and would affect the results.

Four variations of onion mix. There are control onions (top left), onions with baking powder (top right), onions with sugar (bottom left), and onions with lemon juice (bottom right).

These choices, of course, were based on ideas of what should happen. Theoretically, the baking soda onion should be the fastest to brown, as alkaline conditions promote the reaction, followed by the sugared onion, as more sugar is available to fund the reaction. The lemon juice onion was a bit of a wild card, as there are several bits of theory working here. Acid is supposed to reduce the speed of browning, but the moderate water content (and I suppose a little sugar) is supposed to increase it. The control onions and the lemon juiced onion would then fight it out for the bottom spot. All the onions were dry-fried, that is no oil or butter was in the pan.

After seven minutes on a medium heat, I removed the onions from the pan. The baking soda onions had won by far, massively browning compared to the rest, most of which happened in the first minute of cooking. The sugared onions and lemon juiced onions had browned a similar amount, I suppose that the moisture factors helping the lemon juiced onions outweighed the detracting acidity. The control onions, though, had browned poorly, but of course were the only ones that you might actually want to eat.

Onions after browning. There are control onions (top left), baking soda onions (top right), lemon juice onions (bottom left), and sugar onions (bottom right).

So, to summarise: alkali, sugar, protein, high heat, and moderate water content all speed up the Maillard reaction, acid, lots or little water, and low heat all slow down the Maillard reaction. So if you are cooking something, especially if you are making up your own recipe, think about how you are browning food, and how you want to develop flavour. A friend once characterised all British food as “brown”… perhaps that is not such a bad thing after all.


2 Comments (+add yours?)

  1. Trackback: Cajeta and Tres Leches Cake « oxfood
  2. Trackback: Keeping Up With Food Research « oxfood

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