To find out how the processing of produce may affect its nutrient content, I consulted Mario G. Ferruzzi, a professor in the Department of Food, Bioprocessing and Nutrition Sciences at North Carolina State University.
"The general answer is that lots of things, including processing, will affect the nutrient content, but it really depends on the nature of the product, the type and extent of processing and the actual nutrient we are talking about," he said.
Bagged greens are often pre-washed. (Isn't that why we love them?) But washing, which is intended to clean produce, can also damage plant tissues and expose them to oxygen dissolved in the washing water. This can cause a loss of vitamins that are water-soluble and sensitive to oxygen, such as vitamin C and the B vitamin folate.
All greens are washed to a certain extent, whether we're talking about a fresh bunch of spinach or the bagged version, but a triple-washed bagged spinach can create surface damage, provide opportunities for leeching or even facilitate oxidation reactions, all of which impact quality.
Still, Ferruzzi said that companies are "doing it in a way to maximize quality, and this can minimize losses" and that there is still a lot of nutrition left in the leaves after they are washed.
In general, minerals such as iron and calcium are largely stable in the plant. Losses may occur when heat is applied (as in the process of canning), but not as much through typical washing.
Vitamins tend to be more sensitive to light, heat and oxygen. Beta-carotene, for example, is not very stable in the presence of oxygen or light, Ferruzzi explained.
When it comes to chopping veggies, there may not be a great amount of nutrient loss from the cutting itself. "However, you do create damage to plant cells, potentially releasing enzymes and other factors, as well as create more surface area for oxidation to occur," Ferruzzi said. In other words, chopping may accelerate losses that one would typically see in fresh vegetables as they age. However, dramatic losses are unlikely "unless you are obliterating it."
Interestingly, chopping may actually increase the amount of beneficial compounds known as polyphenols: plant chemicals with antioxidant properties
that may help protect against the development of cancers, cardiovascular diseases, diabetes, osteoporosis and neurodegenerative diseases. "When damaged, plant tissues have the ability to release more polyphenols as a stress response," Ferruzzi said.
Bagging greens often involves a process known as modified atmosphere packaging. The amount of oxygen that typically exists in the atmosphere is reduced in the bag, replaced with an inert gas such as nitrogen. In essence, less oxygen is available to react with nutrients. "And that can enhance the retention of color and the most oxidatively sensitive nutrients, like vitamin C, folate and beta carotene," Ferruzzi said.
The main driver of this process is preventing visual changes such as browning or wilting, though nutrient loss would typically occur at the same time. The bottom line is that "bagging may help to keep what's in there there longer," Ferruzzi said.
The research on this type of packaging has shown positive results. In one study, modified atmosphere packaging in spinach helped preserve vitamin C
, whereas other research has shown retention of nutrients in similarly packaged kale
and other lettuces
For many (including myself), a bit of convenience will go a lot further than a bit more freshness -- especially since my lettuce bags may sit in the fridge for a few days before I open them.
"What the convenience factor has done to increase our consumption of vegetables in this country is massive in terms of helping our country's nutrition," Ferruzzi said. "Let's not find reasons to discourage it."
For your freshest, most nutritious produce, go with your gut. "Your intuition and your eyeballs matter," he said. Factors such as color and texture should be considered when purchasing greens, whether fresh or bagged. And with bagged, it's important to look for ones with the most recent pack date, which are put in last and are most likely fresher, Ferruzzi explained.
"You want to buy higher-quality stuff, and that usually means fresher stuff but not necessarily less processed ... and that should, in theory, be correlated to a higher nutritional value."
All of this information may cause to you to wonder: Should you buy fresh greens or bagged?
The answer may have to do with when you are planning on eating them.
"The way I think about it is that there's a typical quality and nutrient loss curve that happens from harvest to consumption. And there are steps in the middle that you can do that make it better or worse," Ferruzzi said.
"If you think about, for example, fresh lettuce or fresh spinach that's picked and harvested ... it might be washed and bundled ... and it's exposed to oxygen, light and moisture, and it's getting sprayed with water to stay cold and fresh, and there's significant nutrient loss occurring day by day."
With packaged greens, you may lose a little more up front during the initial processing, but depending on how they are packaged, you have the potential to control the rate of quality and nutritional declines when veggies are stored.
The same can be said for frozen veggies, which are initially blanched to inactivate enzymes that would otherwise break down nutrients and then frozen to stop bacterial spoilage, versus the natural degradation and rotting that occurs in fresh produce.
"When you compare fresh string beans in a store versus frozen, frozen will be almost always be higher in nutrient content, because they were picked and processed at the highest point of quality and then frozen to preserve them," Ferruzzi said.