Very often I stop and think (one must) of the hidden life in the cellar storeroom during the winter. I think of all those beings taken from the garden, whom the plucking has not quite killed, and who, without light, barely breathing, live like fakirs, or in limbo.
Fernand Lequenne
My Friend the Garden
If it seems odd to you to talk about how vegetables breathe, you probably have plenty of company. It’s commonly assumed, after all, that fruits and vegetables are no longer alive when they’re removed from the plant that produced them. Vegetables and fruits continue to live, however, even after they’re picked. The foods you bring in from the garden are still breathing while you hold them in your hands. If you’re counting on those foods to nourish you over the winter, then, it might be a good idea to consider how their continued respiration affects their keeping and eating qualities, and what you can do to control their life processes.
We’ve said that harvested vegetables and fruits breathe. Their respiration, like ours, involves taking in oxygen from the air, combining that oxygen with substances in their flesh, and then giving off carbon dioxide, water vapor, and heat. Perhaps at this point you find yourself picturing those apples and onions in your cellar huffing away, breathing and puffing and even conspiring to get out of the cellar. No one has ever seen an onion or a cabbage breathe, of course, because they take in oxygen through their pores, but the results of respiration — the intake of oxygen and the release of carbon dioxide, water vapor, and heat — have been measured in laboratories.
All the while your produce is breathing — and it will continue to breathe until you cook it or eat it, or until it rots — it is changing. Becoming mature. Ripening. The faster the rate of respiration, the more quickly the piece of food will age and deteriorate. Respiration uses up natural sugars in the foods, which is why vegetables sometimes change in flavor after long storage. Overripe, over-mature vegetables are less appealing to eat and they spoil more quickly.
The aging process is a natural one, of course, and it can’t be stopped. Each vegetable has its own internal time clock. Melons, tomatoes, and berries need a ripening period to attain their full flavor, sweetness, and best texture. These foods may be ripened in storage if picked immature, but must be used fairly soon after harvest. Carrots, beets, potatoes, squash, and most other vegetables don’t ripen, or improve, after picking, but they may be kept on “hold” for a much longer time.
You can see that it is to your advantage, if you’d rather not have to run to the store for carrots in January, to pack away vegetables under conditions that will retard the normal aging process and slow respiration. Lowering the temperature is the single most important thing you can do to promote vegetable longevity. Cold not only slows respiration, it also counteracts the emission of heat by the vegetable, which is a side effect of respiration. Cold is so important that increasing storage temperature by ten degrees will drastically reduce the usable life of your vegetables (see chapter 8 for exact temperature ranges preferred by different vegetables). At higher temperatures, fruits and vegetables have a fast rate of respiration, ripening, aging, and deterioration.
Fruits and vegetables contain a lot of water. Most are 80 to 90 percent water by weight, but they lose water from their tissues during storage. When water loss is marked, the foods shrivel and lose their appealing texture and some of their vitamin C. A small amount of water is given off in respiration in a complex chemical interchange between the food and the air around it. Much more tissue water is removed, though, by evaporation and transpiration. Evaporated moisture is lost from the surface of the food. Water lost by transpiration passes from the internal tissue of the vegetable through the pores and leaf surfaces into the drier surrounding air.
Water loss is highest for most fruits and vegetables during the period immediately after the harvest. Raising the humidity helps to reduce water loss by transpiration, but if the vegetable retains heat from the earth or from postharvest exposure to the sun, it will continue to lose tissue moisture even in 100 percent humidity. You can see, then, that rapid cooling of storage vegetables helps to maintain their high quality, and that keeping the storage temperature cold also helps. Because warm air can hold more water vapor than cool air, it will absorb more moisture from stored food than cold air, and water has a greater tendency to evaporate as the temperature rises.
Some cold-sensitive vegetables like sweet potatoes and peppers may suffer chilling injury from low but not freezing temperatures in storage. Chilling injury is not always immediately apparent, but it usually shows up when the vegetable is brought into a warmer place, in the form of pitting — black or corky spots in the flesh. The majority of root vegetables like to be kept as close as possible to 32 degrees F.
Some fruits and vegetables release the plant hormone ethylene gas — among them apples, tomatoes, muskmelons, pears, plums, and peaches. Ethylene gas can affect the quality of other stored vegetables. It tends to hasten the aging process, resulting in sprouting in potatoes, pale color in peppers, cucumbers, and leafy vegetables that should be green, and bitter flavor in carrots. Keeping the temperature below 45 degrees F helps to minimize ethylene’s aging effects. If you have many bushels of apples and pears, you may need to keep them separate from your vegetables, but usually a few baskets of each can coexist without serious problems.
The great majority of the vegetables we store for the winter are biennials — plants that form seed during their second growing year. Onions, leeks, the cabbage family, and root crops like carrots, beets, parsnips, and salsify are all biennials. Nature intends biennials to keep well so they can go on to bloom and bear seed after laying low all winter. It’s not just our idea, then, to keep these vegetables in good shape until spring. Winter survival is in their genes. We’re not outwitting natural laws, but cooperating with them, when we build root cellars and dig storage pits. Now that you know a bit more about how these vegetables behave after picking, you’re in a better position to take good care of them, so they can take good care of you.