There are a lot of different brands of pectin out there and each brand may have different types of pectin for different types of jam: Instant pectin for freezer jam, regular pectin which requires a lot of sugar, low/no sugar pectin, liquid pectin.
In the past few years, pectin has widely become available in bulk packaging, making it easier than ever to make small batches of jam requiring less pectin than contained in the traditional, three-tablespoon envelopes of pectin. Bulk pectin is also handy for recipes with low-pectin fruit like strawberries and nectarines. Instead of splitting a new envelope, you can just add an extra tablespoon or two of pectin from the jar.
Be aware that not all pectin brands are alike. Each company has its own formula and they cannot be substituted blindly for each other. If you switch brands, watch your first batch carefully and note whether it jells as well at that amount as your other pectin did. In addition, liquid pectin and dry pectin cannot be substituted without changing the whole jam procedure.
I regularly switch between Ball and Dutch Jell low sugar brands. For four cups of fruit, the Ball Low Sugar recipe calls for 3 tablespoons of pectin; the Dutch Jell Lite recipe calls for 1/3 cup rounded of pectin. That’s between 5 and 6 tablespoons and quite a difference from the amount of pectin Ball calls for. In practice, I have found that I can use much less Dutch Jell than they recommend, but I usually need a bit more Ball pectin than they recommend.
How do you know how much to use? Follow the guidelines in the recipes included with your pectin. If you like a softer jell, then use less pectin next time. If you like a harder jell, then use more. Take notes or write them on the side of your pectin container with a pen or marker.
Robin's Rule of Thumb for Pectin: Use 1 Tablespoon of pectin for every cup of fruit in the recipe. Don't use more than 6 cups of fruit in a batch. This generally works for even lower pectin fruit like pears. If you aren't sure, then make a small batch, see what happens and adjust accordingly for your next batches. Use more pectin for low pectin fruit. Use less pectin for high pectin fruit.
Low Pectin Fruits require the addition of a full measure of pectin or a very long cooking time. Don't skimp on pectin when you're using these fruits for jam.
- Italian plums
High Pectin Fruits can often be turned into jam or jelly without adding extra pectin. Note: the riper the fruit, the less pectin in it. Use 20-25% under-ripe fruit if you want to make jam with no added pectin.
- citrus peels: limes, lemons, etc. with the white pith and seeds. [You can put the pith and seeds in a tea ball or bag during cooking so you can remove them from the finished jam.]
- elaeagnus [autumn olive]
- Eastern Concord grapes
- plums, not Italian