Commissioned by the Guardian a while ago, but then spiked on the basis that it was too “sciencey” [sic]. I wonder what else they expected from a scientist? At any rate, here is my guide to the quirkier side of vegetables (from fresh and juicy to saggy and sorry) for a more discriminating readership. Enjoy!
A rough guide to vegetables
The vegetables in your kitchen are alive and breathing. Plucked from your garden or bought from a shop, they continue to take in oxygen to burn their sugary food reserves and produce the energy that supports living processes.
But they are slowly dying. Vegetables with high respiration rates, like asparagus, broccoli, mushrooms, peas and sweet corn, use up their reserves, and hence their nutrition and flavour values, more quickly than slower-respiring ones like garlic and onions. But all are doomed, because they cannot replenish those reserves.
Those without leaves or other green parts can no longer harvest light energy, which plants use through the process of photosynthesis to turn atmospheric carbon dioxide into sugars. Even green vegetables, like lettuce, beans and broccoli, can’t continue to produce enough sugar in the low light of the kitchen or the darkness of the refrigerator.
The green material is chlorophyll, contained in organelles called chloroplasts. Vitamin C, manufactured by the plant, is essential for their survival, but slowly degrades after a vegetable or fruit is harvested. The rate of degradation of this and other vitamins varies widely with conditions, the main culprits being oxygen, high temperatures, and rogue enzymes released from imprisonment if the vegetable is cut or otherwise damaged. Water-soluble vitamins like vitamin C may also be lost during cooking, but here again the losses can vary widely – from 5% for fresh broccoli up to 80% for spinach.
The other major problem that harvested vegetables face is loss of water. Hydroponic vegetables, with their roots intact, have the best chance (especially when placed near a window), since they can take in water to maintain their plump freshness (Hint: Keep the water shallow so that atmospheric oxygen can get through it to the roots).
Molecular pumps drive the water to high pressure, providing the “crispness” that we experience when we bite into the plant. As the plant loses its sources of energy, these pumps fail, and the plant becomes limp – a process that is speeded up by the loss of water through evaporation. Evaporation is fastest in a dry atmosphere, and interior of a refrigerator is very dry indeed. But it is also cold, and the low temperature slows down the processes of degradation that happen as the plant loses energy and the ability to maintain itself. We need to establish a balance, which can be different for each vegetable.
So how can we maximize the life of our produce, or find ways of using it even when it is limp and sad? Here are some ideas:
- Be gentle. Don’t leave fresh produce in a hot car for hours, where respiration and moisture loss will both be speeded up. Keep it cool, and store it under cool, well-ventilated conditions.
- Store correctly. Leafy vegetables are especially prone to moisture loss, so put them in a plastic bag before placing in the fridge.
- Mushrooms are a tricky case. If stored in a plastic bag, condensation will encourage surface bacteria to multiply, producing a slimy exudate. This is why they are often sold in paper bags, but then there is the opposite problem – paper bags do not keep the moisture in, so the mushrooms dry out and become leathery. The answer is to buy only the freshest mushrooms, put them in a paper bag inside a plastic bag, and put this in the fridge as soon as possible – only to take it out as soon as you can to eat the mushrooms while they are still fresh.
- In general, keep fruit and vegetables well separated (in separate refrigerators if you have that luxury!). Apples and ripe pears, for example, give off substantial quantities of the ripening gas ethylene, which makes vegetables grow old more quickly. To avoid broccoli going yellow, keep fruits like these away. Keep your fruit bowl away from your flowering house plants as well – even tiny amounts of ethylene can cause the flowers to drop off!
- If you like beautiful brown chips, keep the potatoes that you are going to use in the refrigerator for a few days beforehand. Some of the starch will be transformed into sugar, and it is the reaction of the sugar with the proteins above 150C that produces the brown colour.
- Don’t get too pernickety over vitamin loss during storage or cooking. The losses vary widely, but there is usually plenty of vitamin left, and if you have a well-balanced diet you won’t be missing out.
And if, despite your most assiduous efforts, some of your vegetables have become saggy and sorry?
- If they are just a bit past it, you can always use them to make vegetable soup.
- If they are moldy or otherwise seem infected, throw them into the compost – it’s not worth the risk. One U.S. firm has found a novel way of commercial use for such compost, by using it as a breeding ground for fly larvae, which are then fed to chickens.
In addition, my friend, the food writer Elisabeth Luard, suggests that:
- Over-the-hill bananas make great banana bread
- Soft carrots are perfect for carrot cake
- Old apples provide an excellent basis for chutney
- Saggy lettuce is fine for lettuce soup flavoured with ham-bone, with a dash of vinegar and finished with egg yolk and cream
- The outer skins of brown onions are great for adding flavour and colour to gravy.
IMAGE: M.Haller (Wikimedia Commons)