Why does the colour change?
It’s the chlorophyll. All green vegetables owe their colour to chlorophyll, a complex molecule present in certain plant cells. A magnesium atom is located in the centre of the molecule. When chlorophyll is in the presence of acidic molecules (compounds that easily share their hydrogen atom), the magnesium is dislodged and replaced by hydrogen atoms. This simple structural change alters the colour of chlorophyll from light green to yellowish green.
Steam cooking vegetables in a covered steamer is an excellent way to preserve their nutritive value. As a general rule, the less water used, the more vitamins and minerals are preserved where we want them: in the vegetables! Other cooking methods that achieve this are stir-fries and covered steaming in the microwave using a small amount of water.
Green vegetables mainly change colour under two circumstances: during cooking and when they come into contact with acidic ingredients. Check out the effects of these two experiments:
Experiment 1: colour and cooking
One of the main elements that affects colour is cooking time.
- Cooking too long
Vegetables turn olive green after only 10 minutes of cooking in boiling water. Why? It’s because of acidic compounds naturally present in all vegetables. Chlorophyll is protected by walls that keep these compounds at a distance. The walls are damaged during cooking and this allows acidic compounds to come into contact with chlorophyll and change its colour. What you should know is that the longer a vegetable is cooked, the more chlorophyll molecules will be altered and the more a vegetable will lose its beautiful green colour.
- Cooking just right
The solution? It’s simple: you have to reduce cooking time in order to preserve the colour. Whether in water, steamed or stir-fried, cooking green vegetables for 5-7 minutes will protect the chlorophyll against acidic damage. Do you want additional insurance? If you are preparing vegetables blanched or cooked in boiling water, immerse them in ice water to quickly stop the cooking and harmful effects of heat.
Experiment 2: colour and acidic ingredients
The acidity in some foods greatly affects the colour of green vegetables.
- Prepared in advance
To make a cold salad, green vegetables are blanched, rinsed in cold water and then mixed with pasta and vinaigrette. The salad is placed in the fridge until it’s time to serve. A few hours later, disaster strikes: the vegetables have turned olive green! What happened? The guilty party here is acidic content in the vinaigrette that attacked the surface chlorophyll in green vegetables.
- Prepared at the last minute
The solution? There are several options available so the colour does not change: choose vinaigrette with less acidity, for example, one with less lemon juice or vinegar, mix the vinaigrette in at the last minute, or use raw green vegetables that have not been blanched. However, we think the best solution is to add the green vegetables just before serving. You should also note that, unlike green vegetables, orange vegetables (pepper and carrots) consist of mostly carotene and their colour is not altered by the acidity in vinaigrette.
The case of canned vegetables
The colour of canned peas, asparagus and green beans is a lot more yellow than in their frozen counterparts. This is due to the intense heat used in the canning process, as well as the long cooking time required to ensure the product is sterilized. The combined effect considerably alters the chlorophyll molecule, which not only yields its magnesium atom, but also causes its structure to mutate. Result: an olive green colour, far from its natural green.
True or false
Do the following ingredients or tips help preserve the colour of green vegetables?
TRUE. The sodium in salt does help protect the chlorophyll molecule, but its effect is minimal. So don’t rely on a healthy dose of salt to counteract adverse effects caused by acidic ingredients or excessive cooking.
TRUE. The bicarbonate reacts with chlorophyll and transforms it into chlorophyllin, a light green-coloured molecule. But there’s a catch: the bicarbonate also attacks cell walls, weakening them. The vegetables may be greener, but softer too!
TRUE. The colour of blanched vegetables is often maintained better than in raw vegetables. This is because there are air pockets in the tissue of raw vegetables that hides their true colour. When a vegetable is immersed in boiling water, the air pockets disappear and the colour becomes brighter.
FALSE. You may have already heard that cooking green vegetables uncovered prevents them from turning yellow. This is true in theory (acids released from vegetables would escape from the pot, leaving fewer to attack the chlorophyll), but makes no difference in practice. Cooking time is much more important than cooking technique.