Blueberries have never been more popular, thanks to their widely reported health benefits, which include potentially decreasing cardiovascular risk factors, enhancing insulin sensitivity, improving cognition and more.
But what happens to blueberries after they’re cooked in a variety of different ways? In a recent study in the Journal of Berry Research, 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, and his colleagues measured the levels of the beneficial phytochemicals in blueberries after putting them through a variety of cooking methods. In addition, they examined the bioactive potential of wild blueberry extract after cooking, using a cell-based 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."
The study measured phytochemicals in blueberries that are known to have beneficial health impacts - anthocyanin (ANC), proanthocyanidin (PAC) and chlorogenic acid (CA) - after exposing the blueberries to common postharvest handling practices and culinary preparation methods. All of the blueberries used in the study were the quick-frozen type that are found in the freezer section of grocery markets and commonly used in processed foods.
The phytochemicals in individually quick frozen wild blueberries that had been subjected to temperature fluctuations, which are often encountered during distribution and handling for retail sale, were measured. The ANC, PAC and CA levels dropped by about 8, 43 and 60 percent, respectively, compared to quick-frozen blueberries that were stored continuously from harvest at -80 degrees Celsius. Baking, boiling and microwaving also reduced the levels of the compounds, with longer cooking times (e.g., 3 to 5 minutes of microwaving) leading to the biggest declines in the concentrations of these blueberry components.
Researchers also conducted cell-based assays to measure the antioxidant activity of blueberry extract after various cooking methods. Antioxidants help to fight oxidative stress, a contributor to certain human diseases. Only microwaving for five minutes had a detrimental effect on the antioxidant capacity of the blueberries. Antioxidant activity was not reduced when using shorter microwaving times, or baking or boiling.
"What we can say from this study is that cooking and heating, especially longer microwaving, does decrease the amount of beneficial compounds in the berries," Grusak said. "Minimal cooking would be ideal, but there are still measurable levels of health-beneficial components in blueberries, even after a variety of preparations."
The results also point to the need for researchers conducting future health-related studies involving blueberries to consider the methods in which they are preparing blueberries and the effects this has on the compositional attributes of the fruit at the time it is eaten.
Others who contributed to this study include 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. It was funded through the USDA/ARS.