How Roasting Changes Seeds, and Why Rancidity Happens: The Chemistry in Plain Language

Toasting seeds takes a few minutes and changes them completely. The same oils that make seeds delicious also make them vulnerable to going off. Here is the science, without the jargon.

What happens in the pan

When seeds are heated, water in the surface evaporates and the temperature rises. Above roughly 140 degrees Celsius, sugars and amino acids start to react in what is called the Maillard reaction. This creates hundreds of new aroma and flavour compounds, giving the toasted, nutty smell and golden colour. The seed also becomes drier and crisper.

Cooking at home is the best way to control the roast, since packaged roasted seeds can vary a lot in how long ago they were made. Check dates on labels and prefer sealed bags.

Timing and temperature

Small seeds like sesame toast in minutes, while sunflower and pumpkin take a little longer. Use medium heat and stir constantly in a dry pan, or spread seeds on a tray in a moderate oven. Pull them off when they smell fragrant and the colour has just begun to change, as they continue to cook in the hot pan. Burnt seeds taste bitter and produce unwanted compounds, so discard them.

What rancidity is

Rancidity is the process by which fats and oils break down and develop stale, sharp or paint-like smells. The main cause in seeds is oxidation, where oxygen reacts with unsaturated fatty acids, producing smaller molecules such as aldehydes that have strong odours. Light, heat and metal traces speed up the reaction.

You can test freshness at home with a simple taste check. Chew a few seeds before using a whole batch. Fresh seeds taste sweet, nutty or mild. Old ones may taste flat, bitter or like old crayons. Because heating speeds oxidation, avoid toasting more seeds than you plan to use within a week or so, and let them cool completely before storing, since trapped steam can cause damp and mould in the jar.

Why seeds go off at different speeds

Seeds high in polyunsaturated fats, such as flax, hemp and walnuts, oxidise faster than those with more stable fats. Grinding exposes much more surface area to air, so ground flax or seed meal goes stale much sooner than whole seeds. Natural antioxidants like vitamin E and sesame lignans slow the process, which helps explain why sesame is relatively stable.

Keeping seeds fresh

Store seeds in airtight containers away from heat and light, and keep larger amounts in the refrigerator or freezer. Buy smaller quantities, label your jars with dates, and taste a few seeds before adding them to a recipe. A bitter, soapy or crayon-like taste means they have turned. Rancid seeds are not generally a serious acute danger, but they taste unpleasant and it is best to throw them away.

A little knowledge of this chemistry makes you a better cook and helps your seeds taste as good as they should.

This article is general information, not medical advice.