Boiling broccoli found to sap its nutritional power

Healthy eating is not just about choosing healthy foods to eat, but also relates to how they are prepared prior to eating. Vegetables, for instance, can suffer nutritional losses during cooking. A study this week has found, for instance, that boiling broccoli in water for just 10 minutes reduced the content of plant chemicals known as glucosinolates by 40 per cent. Within the body, glucosinolates are converted into other compounds called isothiocyanates that are believed to have cancer-protective properties.

This research, conducted at Warwick Medical School in the UK, found that shorter cooking times, as expected, led to less degradation of glucosinolates: just five minutes of boiling led to losses of only 15 per cent.

This recent research reminded me of a previous piece I wrote which summarized some of the research into the nutrient losses that can occur with different methods of cooking.

Microwaving is often recommended as a preferred cooking method because it can shorten the cooking time. However, the evidence suggests that this form of cooking can lead to considerable nutrient losses.

What the research shows is that when it comes to cooking vegetables, steaming is generally best. And the shorter the cooking time, the better. For the best of health, it can help to develop a taste for al dente vegetables.

Can microwaves nuke the nutritional value from our food? ” 28th March 2004

The success of The Darkness and the return of Starsky and Hutch and the Chopper bike have made me quite nostalgic for the seventies. Mind you, not everything about this era was good. These were the days, after all, when a glass of fruit juice or half a grapefruit might be found as starter options in restaurants, and the presence of the black forest gateaux loomed large. Another portion of seventies cuisine best forgotten was a general tendency for vegetables to be cooked to within an inch of their lives. Such culinary excesses not only may result in a rather unappetising mulch, but can also rob vegetables of much of their rich stash of health-giving goodies. Many find the more recent vogue for al dente cooking to be more pleasing to the palate, and can be satisfied in that it may well help preserve a vegetable’s nutritional bite too.
Recognition of the fact that contracted cooking times mean extended benefits from the food we eat have led some to extol the rapid-fire food preparation offered by microwave ovens. Recently, this concept was subjected to a degree of scientific scrutiny. In a study published in the Journal of the Science of Food and Agriculture, researchers assessed the effects of a variety of cooking methods on nutrient levels in broccoli. While boiled broccoli was found to lose two-thirds of its original content of disease-protective nutrients known as flavonoids, this actually compared quite favourably with the whopping 97 per cent loss induced by microwave cooking. Analysis of other nutrients revealed similarly dire depreciations. Contrary to what may be expected, it seems we can microwave goodbye to a good deal of the nutritional goodness vegetables have to offer.

While this study suggests that boiling may indeed be preferable to microwave cooking, other research shows that how vegetables are boiled may also effect their nutrient status. One study found, for instance, that frozen vegetables such as spinach, peas and green beans retain appreciably more vitamin C when plunged into hot water direct from the freezer, than when thawed prior to cooking. The amount of water in which vegetables are placed also seems to have an important bearing on their nutritional status. A study published last year discovered that the smaller the volume of water used to boil vegetables, the better their retention of important nutrients known as phenolics. The evidence suggests that the one way to preserve the nutritional value of boiled vegetables is to ensure they don’t end up in deep water.

There is some evidence that there are even more benefits to be had by going off the boil and opting for steaming instead. For instance, in the study which identified hefty flavonoid losses from the microwaving and boiling of broccoli, steaming induced only an 11 per cent degradation in this type of nutrient. In another study, boiling was found to reduce the level of folate (believed to protect against both heart disease and cancer) in spinach and broccoli by more than half. In comparison, steaming had minimal effects on the level of this vitamin. For those looking to get maximum nutritional value from their cooked vegetables, I reckon it’s full steam ahead.

6 Responses to Boiling broccoli found to sap its nutritional power

  1. ben goldacre 18 May 2007 at 9:53 am #

    john, i see you are very down on microwaves, but its a shame that you dont mention the results from the study on broccoli which you are writing about, which i believe showed that microwave cooking had no effect on glucosinolate content.

    perhaps you could give the full reference to the other study that you report showed a 97% reduction in flavonoids so we can try to understand the disparity?

    i know you dislike factual issues being raised in your blog comments because you have censored my previous factual corrections but i do hope you will be able to let this pass. comment is free, after all, but facts are sacred.

  2. Mandy Sayer 19 May 2007 at 8:08 am #

    Do people in the UK really still boil vegetables in water? No wonder they eat far fewer than the rest of Western Europe! And, by the way, all the vegetables mentioned are excellent eaten raw in a salad. Stir-fry steaming is also an excellent way of quickly cooking vegetables.

  3. ally 20 May 2007 at 9:21 am #

    There is nothing new about this research – it has been widely documented for years. One also need to pursuade some people not to add bicard to veg to keep it green!

  4. Dr John Briffa 20 May 2007 at 6:32 pm #

    Ally – it seems this post is of no interest to you. However, it might be of interest to others, no?

    Ben – the reference you have asked for is: Vallejo F, et al. Phenolic compound contents in edible parts of broccoli inflorescences after domestic cooking. Journal of the Science of Food and Agriculture 2003 83(14) 1511-1516

  5. Cori H 29 November 2007 at 12:35 am #

    This link further discusses the Spanish study and concludes that microwaving for short periods with the least amount of water may be better or comparable to other recommended methods.

  6. amy 7 March 2012 at 12:26 am #

    I just read your article and it is very informative. I have a question though, would cooking broccoli in a rice steamer with rice together retain some nutrients or would that bad?


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