Cooking vegetables: underdone or well done, which is better?

Subject: Cooking vegetables and fruit may increase their nutritional benefit.

I've always been taught that the nutritionally correct method is to undercook vegetables. Cooking destroys nutrients, so less cooked equals healthier. Raw is even better. This common wisdom might not be true.

As an undergraduate I majored in Food Science. The focus on food processing when I was a student was to preserve ascorbic acid levels.
Ascorbic acid, better known as Vitamin C, is easily damaged by heat exposure. So the less heat exposure the better. But ascorbic acid may not be the best measure of nutritional quality.

In judging a food's nutritional value there are many factors to consider. Though vitamin C content may decrease with cooking, the total antioxidant potential of the food may increase. Much of our interest in vegetables has been the anticancer effect of the phytochemicals they contain. Cooking may increase the availability of these chemicals rather than decrease it. This is not a simple matter.

A researcher at Cornell University has been looking at these questions and examining the antioxidant potential of various foods. He has determined that ascorbic acid, in many instances, plays only a small role in a food's total antioxidant potential. In fact, although cooking may lower the ascorbic acid content of the food, it may increase the total antioxidant effect of the food by releasing other chemicals.
"There is a notion that processed fruits and vegetables have a lower nutritional value than fresh produce. Those original notions seem to be false, as cooked sweet corn retains its antioxidant activity, despite the loss of vitamin C," says Dr. Rui Hai Liu, Cornell assistant professor of food science. Lui cooked batches of sweet corn kernels at a rolling boil. The kernels produced more antioxidants the longer they cooked. After 10, 25 and 50 minutes the antioxidant content was boosted by 22, 44 and 53 percent, respectively. In addition to its antioxidant benefits, cooked sweet corn unleashes a phenolic compound called ferulic acid, which provides health benefits, such as battling cancer. The ferulic acid increased by 240 percent, 550 percent and 900 percent, respectively.[1 ]

This phenomenon isn't limited to corn and other grains. It appears to occur in fruits and vegetables as well. For example in apples, ascorbic acid provides less than 0.4% of the antioxidant potential. Cooking the apples and decreasing Vitamin C does not lower the antioxidant potential. Most of the antioxidant capacity comes from other phytochemicals. Tomatoes are another example. Raw tomatoes contain about .76 micromol of vitamin C/g of tomato. Heating them for 30 minutes (88 degrees Celsius) decreased their Vitamin C level to 0.54 micromol of vitamin C/g of tomato, about a 1/3 decrease. But at the same time, heating increased available lycopene from 2.01 mg of trans-lycopene/g of tomato to 5.32 mg. The total antioxidant capacity of the tomato increased as well. The antioxidant activity of raw tomatoes increased from 4.13 micromol of vitamin C equiv/g of tomato to 6.70 micromol, more than a 50% increase. [2]

In other words, applesauce may be better for you than apples, tomato sauce better than fresh tomatoes and corn porridge better than fresh corn on the cob. Now this isn't the final story and there may be other factors to consider but clearly the research is no longer arguing against cooking fruits, vegetables or whole grains.

This is especially good news for cancer patients during chemotherapy. Chemotherapy often lowers white blood cell numbers leaving patients susceptible to infection. Common wisdom has been for them to avoid raw vegetables and salads because they may be sources of bacteria and other infectious agents. The nutritionally minded patients find themselves in a dilemma, worrying that cooking their food will lower its anti-cancer effect. The truth may be that overcooking may actually be better for them.

References:
1. Dewanto V, Wu X, Liu RH. Processed sweet corn has higher antioxidant activity.
J Agric Food Chem 2002 Aug 14;501;50(17):4959-64
2. Dewanto V, Wu X, Adom KK, Liu RH Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity .J Agric Food Chem 2002 May 8;50(10);3010-4