Combine strawberries, sugar, orange concentrate and zest in a large pot set over high heat. Bring to rapid boil. (To safely boil, pot should be no more than a 1/3 full). Boil hard for 2 minutes. Remove from heat, mix in Certo and stir occasionally for 5 minutes to prevent fruit from floating. Skim away foam. Pour into warm, sterilized canning jars within a 1/4-inch of the rim. Wipe rims clean, and then seal. Process sealed jars by submerging them in boiling water for 5 minutes. Remove jars and cool, undisturbed, for 24 hours. Check seal.
The sealed lids should curve downward. (Consult Bernardin canning box for instructions on how to safely sterilize, seal and process canning jars).
Makes 14 cups (enough to fill 12 250ml jars, plus 2 cups of jam you can enjoy right away).
*Note: To prepare fruit, wash, hull, and then crush 6 pints of strawberries using short pulses in a food processor or by hand with a potato masher, one layer at a time.