I like blackberry preserves on toast for breakfast, but I'm not
terribly keen on the high fructose corn syrup, so yesterday I decided
to make some blackberry preserves/jam at home for the first time. My
previous experience was limited to making grapefruit marmelade without
pectin, adding in a couple of lemons, then cooking until 218 degrees.

My marmelades have always set nicely, no problem.

Yesterday I looked up a recipe on the Internet for blackberry
preserves.

It said equal weight of fruit and sugar, a bit of lemon juice, simmer
for an hour, mix in 1 sachet of pectin, and away you go. This sounded
nice and simple so I got 2 lbs of frozen blackberries and made the
preserves.

However, I find today that it is not completely set. It is very nice,
but it runs a bit more than I would like.

I also made grapefruit marmelade yesterday in a slightly larger
quantity. I found that my candy thermometer was broken but boiled it
down until it looked right, then threw in a sachet of pectin for good
luck. The marmelade came out perfect. Positively orgasmic.

So why is the blackberry jam less set? I followed the recipe exactly.

The recipe seemed to suggest that if you used pectin there was no need
to cook to 218 degrees. Or is it because I used frozen blackberries
and not fresh? Or do I need more pectin? As pectin is rather
expensive, this would definitely add to the cost of a batch.

Of course the jam is perfectly good, as long as you don't mind it
running a bit, but I would like to know for next time.