Blueberries are known to have a number of health benefits such as promoting cardiovascular health, increasing insulin sensitivity, and improving memory and learning. Much of this information comes from the raw blueberry. However, what happens to blueberries when they are cooked? Along with his colleagues, Dr. Michael Grusak, professor of pediatrics at the USDA/ARS Children’s Nutrition Research Center at Baylor College of Medicine and Texas Children’s Hospital, decided to measure the levels of beneficial nutrients in blueberries after cooking them in a number of ways. They also took a look at bioactive components of wild blueberry extract after heating, using a cell culture assay.
“The point of the study was two-fold,” Grusak said. “Because blueberries are touted as such a health-beneficial food, we wanted to know the nutrient content of blueberries after consumers cook them in a number of ways. But researchers are also beginning to use blueberries in clinical trials, so the information we gained from this study will be important to help researchers design better studies.”
Grusak and his group measured anthocyanin (ANC), proanthocyanidin (PAC) and chlorogenic acid (CA) in blueberries after they exposed them to post-harvest handling practices and various cooking methods. These blueberries were of the quick-frozen type, commonly found in the freezer section of grocery stores that are often used in processed foods.
The quick-frozen blueberries were subjected to temperature fluctuations, which often occur during distribution and handling for retail sale. ANC, PAC and CA levels were measured, and they found that these antioxidant levels dropped by 8, 43 and 60 percent, respectively, relative to blueberries that were stored continuously from harvest at -80 degrees Celsius. They also measured antioxidant levels after baking, boiling and microwaving, and found that antioxidant levels were also reduced. Increasing cooking time (e.g. 3 to 5 minutes of microwaving) demonstrated the greatest drop in antioxidant levels.
Additionally, they ran cell-based assays to measure antioxidant levels on blueberry extract after various heating methods. Interestingly, microwaving for 5 minutes caused a decrease in antioxidant levels in blueberry extract, however, levels did not decrease when using shorter microwaving times, or baking or boiling.
Grusak concludes that heating — and especially microwaving — lowers the antioxidant levels, however, minimal heating would be ideal, as the blueberries still contain measurable levels, even after a number of preparations.
Grusak points out that researchers involved with conducting health-related studies such as clinical trials testing blueberries should consider blueberry preparation in their design, as this will effect clinical outcomes.
Sally Gustafson, USDA/ARS at North Carolina Research Campus; and Gad Yousef and Mary Ann Lila, Plants for Human Health Institute, North Carolina State University contributed to this research. This blueberry research was funded through USDA/ARS.