Why do some vegetables change colour when they are cooked?
Have you ever noticed that your vegetables sometimes turn a completely different colour when you are cooking them? For example, red onion turning a greenish colour. Don’t be concerned, there is nothing wrong with your vegetables and your meal is still safe to eat (though potentially less appealing, in some cases). The change occurs when the natural colour pigments are exposed to different elements or pH.
The table below summarises the changes that occur when a vegetable is exposed to acid, alkali, heat or metal.
Table adapted from: ‘Understanding Food Principles and preparation 2014 [1]’
These different pigments all contain their individual health benefits. This is why it is encouraged to eat a range of colours!
For more on this topic, have a read of my blog on the health benefits of the different colour pigments
Why do potatoes turn green when exposed to sunlight?
When exposed to sunlight, a potato undergoes photosynthesis which makes them bitter and also produces a toxic substance known as solanine – the cause of the green tinge on sprouts and skins [1]. Whilst eating a small amount of solanine may not be too detrimental to your health, it is recommended that green potatoes be avoided all together. Large amounts may lead to poisoning [1].
Interesting fact: Onion and potato should be stored separate to each other as they will shorten the shelf life of each other!
Does microwaving vegetables present a danger to health?
Microwaving vegetables is a great method to preserve the water soluble nutrients. Minimal water is required so nutrients won’t have a liquid to leach out in to (and then be discarded – as with boiling). Additionally, the vegetables are exposed to a shorter cooking time. Nutrients are better retained when reheated quickly than held at a high heat for a longer period of time [1].
So microwaving is good in terms of nutrition but is it safe? The CSIRO and NHMRC have concluded that yes, it is [2]. There is no radiation risk to foods cooked by microwaving [2]. The microwaves do not remain in food or make is radioactive and they disappear as soon as the microwave stops [2].
Microwaving is also the best method to retain vegetables colour and texture!
Did you know that heating legumes increases their protein content?
You read right, heating legumes increases their protein availability[1]! This occurs via the destruction of enzymes called protease inhibitors [1]. Protease is needed for proteolysis which is basically the process whereby protein from food is broken down to amino acids that the body can then digest and use for many important functions.
Funky odours – the chemistry behind it
Have you noticed some unpleasant odours coming from your cooked veges? Pungent smells often come from those vegetables in the cruciferae family e.g. broccoli, cabbage or the allium genus e.g. onion and garlic [1]. These odours are released upon overheating. Let’s use cabbage as an example for the cruciferae family. Once cabbage is heated, enzymes release hydrogen sulphide gas and transform a sulphur compound known as sinigrin in to mustard oil (not to be confused with mustard itself)[2].
Allin is present in garlic – this is garlic’s sulphur compound and it is without odour until being cut [2]. After cutting, allin is converted to thiosulfinates and disulfides (which are volatile and pungent in odour)[2]. It is also this vapour that causes people to cry when cutting an onion. Fortunately the smell of onion and garlic cooking is far more pleasant than that of cabbage and broccoli.
You may have guessed that asparagus also produces some of these sulphur containing products and ammonia…
Can onion induced crying be prevented?
There are steps that can be taken to help reduce the impact of onion on your tear ducts. Trial keeping your onion in the fridge for one or so hours prior to cutting – this will slow the chemical reaction [2]. Alternatively, you could wear a diving mask which would stop the vapour from reaching your eyes and nose. This last point however seems a little extreme. I have known people to wear swimming goggles when chopping onions – this may help but the vapours will still enter through your nose!
Are frozen vegetables less nutritious than fresh?
The nutrient difference is negligible and frozen vegetables are a very convenient/ affordable way for many people to eat their vegetables. So throw out that frozen pizza and stock up with your frozen veg!
Classification of vegetables
References
- Brown A. Understanding Food Principles and Preparation. 5th Stamford: Cengage learning; 2014.
- The Safety of Microwave Ovens [internet]. 2009 [Updated 2011 Oct 14; cited 2014 Nov 11].
Contact us for results focused nutritional advice
This article was written by our dietitian Belinda Elwin who is a Dietitians Association of Australia member and Accredited Practising Dietitian and Nutritionist.
If you have questions about healthy eating, make an appointment. We‘ll provide you with a simple and effective routine targeted to your concerns. Contact us today.
