Produce loses its appearance when kept in the refrigerator too long, and experts say the nutritional value decreases as well.

A recent study led by Luke LaBorde, associate professor of food science, found that nutrient loss in spinach is related to storage time and temperature of the product.

"In general, nutrients are lost at a faster rate when temperature increases," LaBorde said. "The quality visibly declined as well."

After eight days, spinach stored at 39 degrees Fahrenheit lost 53 percent of its folate, which is a vitamin B compound that helps prevent birth defects, according to the study. The average temperature of a refrigerator is about 40 degrees.

Half of the carotenoids were lost after eight days at this temperature as well, LaBorde said. Carotenoids are pigments that are converted to vitamin A in the body when eaten, he added.

Nutrient loss in fruits and vegetables is a result of a process called senescence, which means a slow death, LaBorde said.

LaBorde said all cells have enzymes that are capable of degrading vitamins, but in living cells, they are kept separate and confined to certain reactions.

"When you pick the vegetable, senescence begins, and part of it is the breakdown of cell membranes," he said. "The enzymes are released and find their way to the vitamins and start breaking them down."

LaBorde said nutrient loss in fruits and vegetables is irreversible.

"Some people will take spinach and put it in cold water to rehydrate it," he said. "It will look nice again, but vitamins are lost and can't be recovered."

While vitamins and disease-preventing compounds called phytochemicals might degrade after a product is stored too long, mineral nutrients in produce will not change at all, said Kathleen Brown, professor of postharvest physiology.

She said the types of produce that lose nutrients quickly are the ones that spoil faster, such as leafy vegetables and broccoli.

"Most fruits tend to last longer than vegetables, except for berries, which are extremely perishable," Brown said. "You could keep winter squash for months and it would still be very nutritious."

Brown said vitamin C is very sensitive to any factor that causes a product to overage, such as temperature.

"It declines even in the freezer in frozen food," she said.

William Lamont Jr., professor of vegetable crops, said that one can never improve the quality of a fruit or vegetable once it is harvested.

"All you can hope to do in your storage is to slow the degradation of the product," Lamont said. "If you don't have a good quality product going into storage, then you won't have a good quality product coming out of it."

Peter Ferretti, professor of vegetable crops, said that people should separate fruits that ripen, such as peppers and plums, from root crops and leafy greens in the refrigerator.

"Fruits release ethylene, which can make root crops bitter and make leafy greens yellow and prematurely start to decay," Ferretti said. "When they start doing that, they lose nutrients too."

Another factor that speeds up nutrient loss is tissue damage, LaBorde said.

"Anything that's cut or chopped will deteriorate faster," he said. "The less damage you do to it, the longer it will last."

LaBorde said that people should consume fresh produce as quickly as possible and should not assume the vitamins are at their peak just because the food is fresh.

"The advantage of canned and frozen food is that, while they do tend to lose some nutrients in processing, they're more stable," LaBorde said. "Heating them deactivates the enzymes that cause vitamins to degrade."

With fresh produce, refrigeration is the only thing that slows those reactions, he said.