Do Fruit or Veg lose Nutritional Value when Cooked?

So I've heard a lot of different people saying that when you cook fruit and vegetables by boiling, steaming, frying or baking them you somehow decrease the amount of vitamins and nutrients within the fruit or vegetable. I thought that I'd research this further to see if there were any; optimum cooking methods, specific fruits or vegetables which should be eaten raw or vitamins which are very affected by heat.

There are many different molecules within fruit and vegetables that have positive effects on the body and they all respond in different ways to the cooking processes. I will be discussing examples that I found interesting within 3 groups; water soluble vitamins, fat soluble vitamins and beneficial enzymes. And then lycopene as an exception. If you want to know more about vitamins that I have not discussed in detail below please look in my bibliography for interesting articles about all kinds of nutrients and what effect cooking has on them.

First of all why do we cook fruit and vegetables? We cook fruit and vegetables to soften the cellulose fibres which makes the fruit and veg much easier to eat and to digest . We also cook fruit and veg to improve their taste.


Raw broccoli does not sound very appealing to me. However, according to recent research raw may be the best way to eat broccoli. This conclusion is based upon the fact that broccoli contains the enzyme myrosinase which, in the presence of water, chops off the glucose group of a glucosinolate (a natural component of almost all plants in the order brassicaceae (e.g. watercress, horseradish, broccoli)) The remaining molecule converts to either an isothiocynate, a nitrile or a thicyanate molecule. Isothiocynate is the standard product which is a good thing for humans because isothicynate can bind to mutated p53 proteins to increase cell death of cancer cells (p53 proteins regulate the cell cycle and therefore function as a tumour suppressant). It has also been shown to induce phase II detoxification enzymes which are involved in xenobiotic metabolism of carcinogens. Detoxification is a specific pathway which processes chemicals that are no longer needed by the body. Xenobiotic metabolism is a type of detoxification in which toxins are neutralised, solubilised and then transported to secretory glands. A carcinogen is any substance that is an agent in directly causing cancer therefore the neutralisation of these substances and expulsion from the body can only be a good thing. Inside the plant myrosinase and glucosinolates are stored in separate places and only come into contact when the plant is subjected to physical injury. Now this is the catch because this means isothicynates can only be produced when your teeth start to break down the broccoli or horseradish or watercress etc. s is why we want to preserve these enzymes. Enzymes denature at high temperatures as the hydrogen bonds break destroying the tertiary structure of the enzymes which also alters the shape of the active site, rendering the enzyme useless. This means that the effects are very different as chewing raw broccoli will convert glucosinates into isothiocynates which has beneficial affects whereas when you chew cooked broccoli the enzyme has been denatured therefore no reaction takes place myrosinase comes into contact a glucosinate :( So vegetables containing myrosinase and glucosinates are better consumed raw...


Such as:

• Broccoli
• Cabbage
• Brussels sprouts
• Cauliflower
• Kale
• Bok choy
• Rutabaga
• Collard greens
• Radishes
  

Lets move on to water soluble vitamins, such as vitamin B and C. As their name suggests, these vitamins dissolve in water and therefore when fruit or veg are boiled or steamed these vitamins are leached out. Water soluble vitamins tend to be less stable in air, water and heat which means fruit and veg containing these should not be stored too long, boiled too long or boiled in excessive water as these will all decrease the retention of vitamins.


An extremely unstable water soluble is vitamin C (which is also known as ascorbic acid). It is unstable because it is a strong reducing agent and hence rapidly oxidised to form dehydroascorbic acid. During this oxidation process the ascorbic acid loses 2 hydrogen atoms and 2 electrons. Dehydroascorbic acid is more readily transformed by a non-enzymatic reaction to a compound with no antiscorbutic activity. This non-enzyme reaction is irreversible. Therefore to preserve vitamin C our goal is to stop ascorbic acid being oxidised. The oxidisation has a few catalysts including copper, ascorbic acid oxidase and catalase. The last 2 both being enzymes. These enzymes catalyse a reaction we want don't want to happen, hence to slow this reaction as much as we can our aim is to destroy these enzymes. We can do this by heating them to a high temperatures (as I explained earlier in this post). Heating up the enzyme will break the hydrogen bonds thereby changing the shape of the active site which means the substrate (ascorbic acid) no longer fits. This means that the enzymes can no longer catalyse the oxidation, the oxidisation can still happen but it is much slower. However, as Vitamin C is water soluble it shouldn't be boiled excessively as the vitamin can leach out of the fruit or vegetable in question. A study suggests that fruit and veg that contains vitamin c should be boiled at 100 degrees for a minute and then eaten for a maximum amount of this important vitamin.


Fruit and Veg Containing Vitamin C:

• Papaya
• Bell Peppers]
• Broccoli
• Brussels Spouts
• Strawberries
• Pineapple
• Oranges
• Kiwifruit
• Cantaloupe
• Cauliflower
  

On the other end of the spectrum are fat soluble vitamins A,D,E and K which are more stable and resistant to air, water and heat.

An example of a fat soluble vitamin is beta carotene (it's converted to Vitamin A in the body). Beta carotene is embedded in a matrix with protein. If the matrix is not destroyed the body will only absorb a small percentage of the beta carotene. Chopping up the fruit and veg helps to break down this matrix and heating also breaks it up by denaturing the protein by breaking the hydrogen bonds of the tertiary structure which releases even more beta carotene. Unfortunately while the fruit and vegetables are cooked some of the beta carotene is leached out during the cooking process. Around 10-15% of beta carotene is lost by stir-frying or boiling whereas only 5% is lost when the fruit and vegetables is baked or roasted.

Lycopene ( a red pigment in fruit and veg) is an exception; it increases with cooking. Rui Hai Lui, the professor of food science at Cornell University, found that the level of cis-lycopene (a type of lycopene) rose 35% when cooking tomatoes for 30 minutes at 88 ⁰C.

A raw tomato has what is called an all trans configuration which means that the lycopene molecules are in a linear form whereas the lycopene that circulates in the human bloodstream is a cis-isomer of lycopene which has a bent form. This suggests that the cis-lycopene is absorbed more readily by the body. By heating up trans-lycopene with a bit of oil the shape of the trans-lycopene can be changed; the molecule is restructured so that it becomes bent under these conditions. The transport of cis-lycopene into the bloodstream and tissues is easier; so there will be higher levels of the antioxidant in body cells and the blood.
Fruit and Veg Containing Lycopene

• Tomatoes
• Watermelon
• Pink guava
• Red bell pepper
• Papaya

Honey Roast Peaches

Serves 4

Ingredients :
4 tbsp Butter
3 tbsp Honey
2 Peaches
4 Dollops Yoghurt/Cream/Mascarpone/Ice cream

Method :
1. Preheat the oven to 180 ⁰C.
2. Wash your peaches and then prepare them by cutting them in half and removing the stones.
3. Place them in your baking dish with the fruit flesh facing upwards.
4. Place a knob of butter on each half peach.
5. Then drizzle the peaches liberally honey
6. Put the peaches in the oven for 15-20 minutes then remove the dish from the oven and spoon the honey syrup over the peaches and place the dish back in the oven for a further 7. 10 minutes. Keep an eye on them though!!
8. Serve them with a dollop of something creamy, I used yoghurt, and drizzle some of the syrup over the top. Yum :)

Bibliography

[1] S. Subramanian, "Fact or fiction: Raw veggies are healthier than cooked ones," Scientific American, 2016. [Online]. Available: http://www.scientificamerican.com/article/raw-veggies-are-healthier/. Accessed: Jul. 25, 2016.

[2] A. L. Garcia et al., "Long-term strict raw food diet is associated with favourable plasma β-carotene and low plasma lycopene concentrations in Germans," BJN, vol. 99, no. 06, 2007. [Online]. Available: http://journals.cambridge.org/download.php?file=%2FBJN%2FBJN99_06%2FS0007114507868486a.pdf&code=d2b3d1dceeeb2eac9a01ec5fab104dcd. Accessed: Jul. 25, 2016.

[3] V. Dewanto, X. Wu, K. K. Adom, and R. H. Liu, "Thermal processing enhances the nutritional value of tomatoes by increasing total Antioxidant activity," Journal of Agricultural and Food Chemistry, vol. 50, no. 10, pp. 3010–3014, May 2002.

[4] S. T. Talcott, L. R. Howard, and C. H. Brenes, "Antioxidant changes and sensory properties of carrot Puree processed with and without Periderm tissue," Journal of Agricultural and Food Chemistry, vol. 48, no. 4, pp. 1315–1321, Apr. 2000.

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