I contacted a popular brand of frozen products: Cascadian Farm. They had the following response regarding their frozen produce:
Thank you for contacting Small Planet Foods with your inquiry. After harvest, the vegetables are cleaned washed, inspected and blanched then frozen. Blanching is done by heating the vegetables quickly in very hot water. The blanching process kills harmful bacteria and deactivates certain enzymes in the vegetables so that the product will maintain its freshness.
However, Cascadian Farm blueberries, raspberries, blackberries, and strawberries are not heated during processing.
We hope you find this information helpful. Please let us know if we can help you again.