Sugar, pectin, and lemon juice ratios for low-sugar to traditional jam — by fruit pectin level.
Strawberries, peaches, pears, cherries, rhubarb.
Sugar
≈ 6.80 cups granulated
3.00 lb
Commercial pectin
1 box ≈ 1.75 oz powdered pectin.
1.5 boxes
Lemon juice
Bright flavor + acid pectin needs to set.
6.0 tbsp
Setpoint temperature
Or 8°F above the boiling point of water at your altitude.
220°F
Jam sets when pectin, sugar, and acid all hit the right concentrations at the right temperature. Drop any one and you either get fruit syrup (not enough set) or a rubbery block (too much pectin / too long boiling).
Low-sugar jamsneed low- or no-sugar pectin — the standard kind relies on sugar to gel, so a 50% batch with regular pectin will not set. Pomona’s Universal pectin uses calcium to gel and works at any sugar level.
Altitude: water boils lower as you go up, so the setpoint drops too. Use 8°F above your local boiling point instead of a fixed 220°F above ~1000 ft elevation.
sugar_lb = fruit_lb × sugar_ratio
pectin_boxes = pectin_level_factor × fruit_lb ÷ 4 (high = 0.5, medium = 1.0, low = 1.5; low-sugar jams × 1.25)
lemon_tbsp ≈ 1 tbsp per lb fruit (+0.5 for low-pectin fruit, +0.5 for low-sugar ratio)
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