Food scientists treat the cutting, slicing, and chopping of fresh vegetables as procedures that wound the cells of the plants and trigger injury-based responses by the plant cells. As a general rule, researchers treat vegetable chopping as a form of stress that is placed on the vegetables, similar to other forms of stress like extreme heat, extreme cold, dehydration, or other physical conditions.
While the idea of wounding and injury may sound entirely negative, it is important to remember that whenever we chew a vegetable, we are wounding and injuring the cells to an even greater extent, and it's the most natural first step for us in digesting our food and becoming optimally nourished. So the question isn't really about wounding or not wounding the vegetable cells; rather, what becomes important is when this process takes place and under what circumstances. After reviewing several dozen food science studies in this area, we have several practical recommendations about the cutting and chopping of fresh vegetables:
- Studies consistently show that if you are using a knife to cut your fresh vegetables, a dull knife can cause unnecessary and unwanted damage that you can avoid with use of a sharp knife. With a blunt blade, you are likely to increase the amount of electrolyte leakage from your vegetables (including leakage of potassium and calcium) and the amount of off-odors. With a sharp blade you are likely to decrease the risk of bacterial growth and the risk of excessive softening. While you obviously need to be very careful when using a sharp kitchen knife, you're likely to get more health benefits from your chopped fresh vegetables when cutting with a sharp versus dull blade.
- Risk of discoloration, flavor loss, texture loss, dehydration, and nutrient loss generally increases as the cut vegetable pieces get smaller and smaller. In other words, shredded lettuce or shredded cabbage generally carry more risk in the above areas than coarsely chopped lettuce or coarsely chopped cabbage. In the commercial ready-to-eat fresh vegetable industry, vegetable shredding typically means a shorter shelf life. However, this general principle does not mean that your fresh vegetables should never be shredded. When shredded vegetables are marinated (for example, when shredded cabbage is made into coleslaw) or fermented (as would take place in the production of sauerkraut), there may be nutritional advantages to shredding. (In both of these cases, the marinating and fermenting steps act to eliminate the risk of dehydration and these steps also help control other factors—like acid/alkaline balance—that might otherwise damage the nutrient content of the cabbage.
As a general rule, the more finely you shred or chop your fresh vegetables, the more quickly they should be eaten. For example, shredding lettuce within one hour of serving a salad may be able to increase your potential health benefits in comparison to shredding lettuce two to three hours before the salad is consumed.
- Some of the changes involved with vegetable cutting are related to respiration rate. Respiration involves the breakdown of sugars by the vegetables through the consumption of oxygen and the release of carbon dioxide. Respiration is not a bad thing—it's a testament to the amazing vitality of plants and the fact that vegetables remain partly alive, even after we've harvested them. But the faster the respiration, the greater risk of a vegetable spoiling over a shorter period of time.
The degree to which cutting impacts respiration rate in vegetables depends on the specific vegetable involved. Respiration (as measured by oxygen consumption) increases by 20% in uncut versus shredded endive; by 40% in uncut versus chopped broccoli; and by 100% in whole versus shredded lettuce. For shredded versus whole carrots, respiration can be increased by 300-700%. At the other end of the spectrum, respiration may only increase by 1-5% in the case of cut green beans or sliced zucchini. Because these differences aren't always logical, we recommend that you adopt the following general rule: to decrease the changes of spoilage, don't cut up fresh vegetables any further ahead of time than absolutely necessary.
- Interestingly, we've seen several studies on the hand tearing of lettuce and the hand peeling of carrots (with a simple, store-bought peeler) that show less risk of bacterial contamination with hand processing than machine processing in a factory. Bacteria like E. coli and Listeria innocua were less likely to be present on hand-torn lettuce and hand-peeled carrots than on factory processed versions of these vegetables. Researchers attributed this lower contamination to reduced damage inflicted by hand processing. Based on this research, we recommend purchasing vegetables like carrots in whole form and then peeling and cutting them at home by hand rather than purchasing pre-peeled and pre-cut vegetables.
- While we are on the subject of cut carrots, we'd like to mention the "white blush" than many people see on baby carrots and pre-cut carrot slices or carrot strips. This white blush is caused by dehydration along the cut or peeled surface of the carrot. Carrots with white blush are not unsafe to eat, and we have not seen evidence that they are less nourishing. However, we do have concerns about the use of chlorine washes by manufacturers of baby carrots. These chlorine washes are used to help preserve these young, peeled-and-cut carrots. You can avoid exposure to these chlorine washes by purchasing whole carrots and cutting them at home. (If you purchase whole organic carrots and scrub them gently with a natural bristle brush under cold running water, it is also not necessary to peel them.)
- Interestingly, we have seen several studies in which the total antioxidant capacity of vegetables has been shown to increase when those vegetables are cut and chopped. While these results make it sound as if you can get more health benefits from cut versus whole vegetable, we do not believe that this conclusion is correct. When vegetables are cut, they do respond to this injury by increasing the amount of certain antioxidants called "wound-induced phenols." Some examples of the phenols include jasmonic acid, systemin, and salicylic acid. However, at the same time as these particular antioxidants may be increased, other antioxidants—for example, vitamin C and carotenoids—may be lost. So even though the total quantity of antioxidants may be increased due to a large increase in the number of wound-based molecules, the quality of our antioxidant health benefits may not be improved due to the loss of key antioxidants like carotenoids or vitamin C. Based on this research, we don't think that the cutting of vegetables should be considered as a method for increasing antioxidant health benefits.
Based on all of the research studies that we have reviewed, we are convinced that you can use some very common sense principles when you are making a decision about vegetable cutting. If you enjoy biting off a piece of fresh celery from the stalk, that approach makes very good sense to us as way to enjoy fresh celery. (Just make sure you do a good job chewing.) If you enjoy fresh chopped celery in a salad, that approach also makes sense. Just make sure to use a sharp knife in order to avoid unnecessary cell damage, and chop up the celery as close in time as possible to consumption of the fresh salad. Chopping size doesn't matter as much if you're immediately adding the vegetable to a soup, soaking it in a marinade, or fermenting it, as might be the case with shredded cabbage made into coleslaw or sauerkraut. If the chopping of a vegetable makes it easier for you to chew and digest the vegetable, then by all means do so. Fresh vegetables are meant to be enjoyed in a wide variety of ways!
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