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Blanching inactivates the enzymes that cause fruits and vegetables to continue ripening and softening. If you do not blanch the fruit or vegetable it can continue to ripen, even in the freezer. So the best practice is to blanch the produce first. However, you can try freezing some without blanching and see how it turns out. Because this step is for best quality, rather than for safety, it is optional. You should also try to remove as much air from the package and use good quality freezer bags or containers in order to have the best result. The National Center for Home Food Preservation has blanching times for kohlrabi. Blanch in boiling water for 3 minutes if blanching whole or 1 minute if blanching in cubes. Transfer to an ice bath after blanching to prevent the vegetable from cooking through. See: https://nchfp.uga.edu/how/freeze/kohlrabi.html