Place 7 clean 250 ml mason jars on a rack in a boiling water canner. Cover jars with water and heat to a simmer (180°F/82°C). Set screw bands aside. Keep jars hot until ready to use.

Wash, hull and crush strawberries, one layer at a time. Measure 3-¾ cups (925 ml).

In a large, deep stainless steel saucepan, stir together prepared fruit, lemon juice, all of the sugar and ½ tsp (2 ml) butter or margarine (to reduce foaming). Over high heat, bring mixture to a full rolling boil that cannot be stirred down.

Add liquid pectin, squeezing entire contents from pouch. Return to boil; boil hard 1 minute, stirring constantly. Remove from heat and skim off foam.

Ladle hot jam into a hot jar to within ¼ inch (0.5 cm) of top of jar (headspace). Using nonmetallic utensil, remove air bubbles and adjust headspace, if required, by adding more jam. Wipe jar rim removing any food residue.

Centre sealing disc on clean jar rim. Screw band down until resistance is met, then increase to fingertip tight. Return filled jar to rack in canner. Repeat for remaining jam.

When canner is filled, ensure that all jars are covered by at least one inch (2.5 cm) of water. Cover canner and bring water to full rolling boil before starting to count processing time. At altitudes up to 1000 ft (305 m), process – boil filled jars – 10 minutes.

When processing time is complete, turn stove off, remove canner lid, wait 5 minutes, then remove jars without tilting and place them upright on a protected work surface. Cool upright, undisturbed 24 hours; DO NOT RETIGHTEN screw bands.

After cooling check jar seals. Sealed discs curve downward and do not move when pressed. Remove screw bands; wipe and dry bands and jars. Store screw bands separately or replace loosely on jars, as desired. Label and store jars in a cool, dark place. For best quality, use home canned foods within one year.