Since chlorophyll is unstable to heat, it is naturally impacted by cooking. In fact, this pigment is even more unstable to heat than its fellow carotenoid pigments—including beta-carotene and lutein! Exactly how unstable is chlorophyll? One study that we have reviewed on the blanching of asparagus and green beans has shown a decrease in chlorophyll a of 13% and a decrease in chlorophyll b of 28%. Similarly, the pressure cooking of spinach for 10 versus 5 minutes has been shown to lower its chlorophyll content by about 13%. Prolonged boiling of green beans (30 minutes) has been shown to reduce chlorophyll levels by about 60% (or sometimes more). These losses of chlorophyll due to cooking were not present, however, in one in-depth study on the cooking of broccoli florets, which held onto their chlorophyll a and chlorophyll b content fairly well after steaming and microwaving.
When trying to make sense of the research results above, we found one area of great consistency involving total cooking time. In all studies that we have reviewed, more chlorophyll is broken down as cooking times are increased, especially in prolonged cooking. In keeping with this principle, we believe that you can use your eyes to help monitor unwanted changes in chlorophyll due to cooking! When you see those bright and vibrant shades of green appearing in a cooked food, you are probably getting close to the right amount of total cooking. And when you see a food start to lose its striking green color, what you are seeing is further breakdown of its chlorophyll. At WHFoods, our minimal cooking times are designed to help preserve all heat-susceptible nutrients—including chlorophyll!
Prolonged food storage has also been found to lower chlorophyll content. For example, the chlorophyll a in frozen chard (stored in the freezer) has been found to decrease by 15% after 5 days and 21% after 10 days. At WHFoods, you will find specific storage recommendations for all 100 of our foods. With fresh chard, for example, we recommend a maximum refrigerator storage time of five days. One of the reasons for our relatively short storage time recommendations is avoidance of nutrient loss—just as this study showed for chlorophyll and chard, even when kept in the freezer.
At WHFoods, we always recommend purchase of whole vegetables from the supermarket versus pre-cut vegetable pieces. However, we also recognize that many people prefer the convenience of pre-cut vegetables, and we think that one further study on chlorophyll is worth mentioning in this regard. Chlorine, hydrogen peroxide, and ozone washes are sometimes used to help extend the shelf life of pre-cut vegetables and decrease risk of contamination with microorganisms. In one study that we reviewed, these commercial sanitizing agents were found to decrease the amount of both chlorophyll a and chlorophyll b in the pre-cut vegetables. These decreases typically fell into the range of 5-15% and became greater when higher concentrations of the sanitizing agents were used.
Our practical take-way tips: overcooking is particularly important to avoid when it comes to preservation of chlorophyll, as is prolonged storage—even in the refrigerator or freezer. Check the "Tips for Preparing and Cooking" and "How to Select and Store" sections in each of our 100 food profiles for food-specific cooking and storage recommendation that will help you retain the chlorophyll in your foods. In addition, remember that consumption of fresh vegetables in their uncooked form can eliminate cooking losses of this phytonutrient.