High acid fruit jams and jellies are very safe to experiment with. They are naturally high in pH (a low number) and the sugar and lemon juice also are preservatives. So no worries there. It's more a matter of the pectin to sugar to fruit ratio for the thickness (or "set") of the jam.

A couple of things you might want to keep in mind:
1. The recipes using the no sugar added pectin on Bernardin's website seem to average 4 cups of fruit per package of pectin. You'd probably need to do two batches using 2 cups of cherries and 2 cups of plums per batch.
2. You'll need some liquid, which is why they call for using fruit juice. Looks like they call for about a cup of liquid per batch, so I don't think your 1/2 cup would be enough liquid to dissolve the sugar and pectin, suspend the fruit, etc. unless the fruit is super juicy, which it might be depending on the ripeness. You could use water, but I always like to add something that adds some flavor :-)
3. As far as spices, that's a matter of taste. A nice addition to cherry jam is vanilla, and I think that would be good with plums, too. You can use the seeds from a vanilla pod or vanilla bean paste. If you want to use vanilla extract, I'd stir it in after the cooking as it tends to lose its flavor when boiled.

Have fun experimenting! And do taste along the way. You can always add more sugar or spices, but you can't take them out!

As mentioned, there are no safety issues regardless. The only issue is that as you reduce sugar the keeping qualities go down (shelf life and refrigerator life once opened) so keep that in mind as you consider jar and batch sizes.

Plums have a good deal of natural pectin while cherries have little so the setting qualities of a plum-cherry jam will be greater than a plain cherry, requiring less reliance on the commercial pectin.

I have used cinnamon, cloves (oils or spices), and vanilla with plums and cherries. Liqueurs can also be good. Plums especially like orange. Brandy can be good with cherries.

I do have a couple of cautions. It's really easy to go overboard on the spices or the alcohol and then find they overtake the flavors of the fruits, especially after some time on the shelf. They can get stronger with storage time, so a light touch is best. I favor oils (clove and cinnamon are available) or whole spices which are removed before jarring up. Ground spices tend to muddy the color. It's not an issue with a butter, which tends to the dark side anyway, but if you want some of the brightness and clarity of the jam to remain, avoid the ground.

Carol

Thank you for the advice and reassurance :) I tried it last night ... turned out pretty well, if I do say so myself! It is one of my first jams so I was fairly happy that it worked.

I made 8 cups of fruit, and only used 1 package of pectin (which is only half what they recommend) but I figured that plums are a high-pectin fruit apparently and can be used without any pectin ... so I only put enough for the cherries. It "set" really well so I guess that was ok. I used basically the recipe I posted above except only 2 cups of sugar. It is certainly sweet enough like that but I was just worried because the sugar is meant to be the preservative, and I wanted there to be enough to be safe to store.

If I used 2 cups of sugar with no-sugar-needed pectin and 8 cups of fruit, how long will it last on a shelf? Regular Jams usually seems to last a couple years, would this reduced-sugar option be less?

I used allspice which was really nice with the plums and cherries. Tastes more like a fall jam than a summer one but that's ok ... it will keep till then. Next time I will try vanilla, that sounds incredible too!! Liqueurs are a great idea too, I hadn't even though of that aside from the Red Wine that was in one recipe I found. Thanks!

I only had ground allspice but I understand what you mean about the cloudy jam ... mine doesn't seem to be (yet) but I will definitely keep that in mind and look for whole spices that can be removed :)

So if I was going to use Sugar instead of Fruit Juice, would I just substitute it 1:1 Cup? The cherries seemed to be juicy enough without much added liquid so I don't think I'd need the juice for liquid.

Next up (tonight hopefully) is Raspberry - Lemon Jam. Again, high pectin fruits so will try using 8 cups of fruit and only 1 pkg of no-sugar-needed pectin with 2 cups of sugar. Raspberries are pretty juicy too, I think.

Also, you said that spices and alcohol flavours get stronger over time ... does the sweetness get more pronounced over time too or does that stay the same?

When I'm adding sugar, and tasting ... will it stay that sweet as it sits and ages, or will that also become more intense and I should go a little less sweet than I intend it to be?

There are far fewer studies of reduced-sugar jams than full sugar preserves, so it's hard to pin down a figure.

A full sugar boiling water bath preserve will last a long, long time on the shelf. Lighter color preserves tend to oxidize and become unappealing more quickly. Storage temperature and exposure to light are also issues.

A reduced sugar preserve is still safe if you're talking about an acid fruit, but the mouthfeel is different and the preservative quality of the sugar is reduced. So canning will still preserve the spread but to a lesser degree. Depending upon how great the reduction has been a lower-sugar spread might last 6 months shelved but perhaps up to a year before degradation of quality. Again, storage conditions either extend or reduce shelf life.

Deterioriation in the fridge once opened is also faster and there's a slightly higher risk of mold.

Sweetness from my perspective tends to be static. There isn't the change you may perceive with spices or alcohols.

Raspberries are not high pectin. They are low acid and low pectin so are among the trickier fruits to jam. Using 25% underripe (not green just slightly underripe) will improve the set as most of the pectin is in the cell walls of the fruit and as the fruit ripens and the cell walls collapse the pectin level goes down.

Carol

how much liquor can safely be added to jams? Is there any guidelines as to what type of liquor can be added?

I made some lower sugar elderberry jelly. It's still fine after a year on the shelf. I do notice it gets moldy quicker once opened, but it still lasts a good two months or so in the fridge.

canfan, as far as adding liquor I do it by taste. I've had it range from a tablespoon up to 1/4 cup - it just depends. The alcohol is a preservative, too, so you can safely add as much as you want for taste. As Carol said, it does get stronger so don't get carried away. It should complement the fruit, not overpower it.

You can add just about any type. I like port with plums and elderberries, cognac in orange marmalade, frangelico or Grand Marnier with peaches, chambord with raspberries, sambuca with blueberries, etc. I like those small "airline" bottles of liqueurs - enough to taste-test and use without a lot of expensive leftovers.

Carol - so you're saying that there will be a degradation of taste/quality, but still be safe to eat? Lasting a year on the shelf isn't bad, at least until the next canning time works out nicely.

Thanks for the tip on the sweet / spice / alcohol ... its hard to tell what will get stronger over time and what won't, and I didn't want to spoil a whole batch of jam to find out :)

Just for future reference, the raspberry-lemon jam needs more than 2 cups of sugar lol. With the low-sugar pectin, it set fairly well (8 cups of fruit, no-sugar-needed pectin (half the recommended amount) and then lemon pulp/juice/zest). Definitely a little on the tart side. I actually really like it, but I have a sour-tooth :) I don't want to put too much sugar because my dad is diabetic, so I think next time I'd try adding 2 cups of splenda as well, or maybe fruit juice? I'm still unclear how the fruit juice works, wouldn't it make the mixture really liquid-y and take so much longer to boil down? Maybe splenda is a safer choice, but it can sometimes have a bit of a funny taste when baking with it?

Yes, it would still be safe as long as mold isn't detected. Normally, unless there's a seal failure during shelf storage (which can happen with excessive heat in storage or sudden changes in storage temperature - i.e. storage in an uninsulated garage) you're not going to see mold.

There can be an after-taste with Splenda but I think it really depends upon how delicate or strong the fruit flavor is. I believe Linda_Lou is the forum member with the most experience with low-sugar/no-sugar jams, so you can always direct a question to her.

Fruit juice works well with a specialty pectin like Pomona. It doesn't depend upon the amount of sugar or sugar-acid-fruit ratio to set, so any combination of fruit and fruit juices could be used without the requirement of additional cooking time. (In other words, the juice sets too.)

Below is a link to the Pomona Pectin recipe page. It can be helpful to look at a range of recipes to get a sense of the many approaches to a low-sugar preserve. It's especially suitable for diabetics.

Carol

Here is a link that might be useful: Pomona Pectin Recipe Page