Market lighting affects nutrients

USDA researchers have found that spinach leaves exposed to light similar to the 24-hour fluorescent light received by packages of fresh salad greens on display in grocery stores had higher levels of some nutrients than did leaves exposed to continuous dark.

BELTSVILLE, Md. —

Many people reach toward the back of the fresh-produce shelf to find the freshest salad greens with the latest expiration dates. But a new study led by Agricultural Research Service scientists may prompt consumers to instead look for packages that receive the greatest exposure to light–usually those found closest to the front.

The study was led by postharvest plant physiologist Gene Lester while in ARS’s Crop Quality and Fruit Insects Research Unit, in Weslaco, Texas. Lester and colleagues Donald Makus and Mark Hodges found that spinach leaves exposed to continuous light during storage were, overall, more nutritionally dense than leaves exposed to continuous dark. Lester is now with the USDA-ARS Food Quality Laboratory in Beltsville, Maryland.

For the study, the researchers exposed spinach leaves to light similar to the 24-hour artificial fluorescent light received by spinach in packages located at the front of the display case. A second group was enclosed in two-layer-thick brown grocery-bag paper to represent the “dark treatment.”

Both experimental groups were housed in market-type, light-transmissible polymer tubs with snap-tight lids and were kept in walk-in storage chambers at 4C–the same temperature at which markets currently display packaged spinach. The light reaction of photosynthesis is not temperature dependent and can occur at 4C in the right type of light.

The researchers found that the continuous light affected the leaves’ photosynthetic system–resulting in a significant increase in levels of carotenoids and vitamins C, E, K, and B9, or folate.

While the simulated retail light conditions actually helped the stored leaves gain in content of several human-healthy vitamins, some wilting occurred after 3 days of storage in flat-leaf but not crinkled-leaf types.

Continuous light exposure during retail display combined with specific cultivar selection (crinkled-leaf types) and leaf maturity (baby-leaf size) appears to be the strategy for preserving and enhancing the concentration of spinach-derived human-health bioactive compounds.