Nutrition
Life in the slow lane
by Jennifer Bowden
Crockpot cooking is enjoying a revival - but don't forget your freshly cooked greens.
Question: Is cooking food for long periods in slow cookers or crockpots less nutritious? Do you have any tips for maintaining nutrients?
Answer:
Long slow cooking is primarily for meat and lentils,” says Associate Professor Winsome Parnell from the University of Otago’s Human Nutrition department. “Vegetables are always best fresh and lightly cooked.” So yes, eight hours in a slow cooker will detrimentally affect certain nutrients in vegetables.
Water-soluble vitamins such as B, C and folate will leach into cooking water, but this isn’t a major issue because slow-cooked meats are often served with liquids or sauces. High temperatures can also damage these heat-sensitive vitamins, resulting in some losses.
On the other hand, fat-soluble nutrients such as the carotenoids and vitamin A, D, E and K aren’t affected by cooking. And mineral losses are likely to be minimal. Some nutrients, like lycopene in tomatoes, are more bioavailable after cooking.
If you want to combine meat and vegetables in a one-pot meal, minimise nutrient losses by adding the green vegetables towards the end of cooking. (Hard root vegetables must be added earlier.) In general, though, lightly cook your vegetables whenever possible.
Question: The March 21 column discussed various vegetable cooking methods. But pressure cookers weren’t covered. I cook soup in this manner. Do the elevated temperatures have a deleterious effect?
Answer:
Are those tales of dinner ending up on the ceiling an urban myth? They weren’t in past decades. Today’s pressure cookers, though, provide a fast and safe cooking method.
Pressure cookers retain the steam from food, causing the pressure inside to increase. At normal air pressure at sea level, water boils at 100°C, but at higher air pressures the boiling point increases (for example, at 15 psi water boils at 121ºC). Pressure cookers produce higher pressures, so boiling temperatures are higher, resulting in shorter cooking times.
Leaching of nutrients isn’t an issue with soup-making as cooking water isn’t discarded. The higher cooking temperatures may cause slightly more nutrient damage and losses. But a 2007 study in the Journal of Food Science suggests losses aren’t huge; researchers’ broccoli retained 90% of its vitamin C after pressure cooking. Vitamin C is often used as a marker for quality/degradation as it’s the most unstable vitamin in heat and water, says Wattie’s senior nutritionist Julie Dick. Pressure cookers are fine for soup-making, just remember to also include lightly cooked fresh vegetables regularly in your diet.
Question: A recipe book on healthy eating said microwaving destroyed 97% of the antioxidants in vegetables. A Spanish study was quoted for this claim. Is it supported by any research?
Answer:
Unfortunately, the book made some sweeping assumptions based on a 2003 study in the Journal of the Science of Food and Agriculture. This study found 97% of flavonoids (an antioxidant) were lost from 150g of broccoli that had been cut into 3cm pieces and microwaved for five minutes in 150ml of water.
Later research from Spain found phenolic compound losses (flavonoids are one example) during microwaving were considerably less and due mainly to leaching into cooking water. Nutrient losses during microwave cooking can be affected by the cooking time, power setting, quantity of water used, cutting up of vegetables, type of vegetable and type of nutrient.
The two most important factors are the quantity of cooking water and how much vegetables have been cut up, says Carolyn Lister, author of Antioxidants: A Health Revolution and a researcher at New Zealand’s Plant & Food Research. The cooking method has a lesser impact, says Lister. Overall, microwaving and steaming cause fewer nutrient losses than boiling.
So, if you’re microwaving, steaming or boiling vegetables, use a small amount of water and try to leave vegetables as intact as possible. “You’re better to chop off whole florets of broccoli rather than really chopping it up,” says Lister. “The smaller you chop things, the more surface area there is for losses.”
