Beach Plum Jam – A Cape and Islands Specialty
Beach plums are tiny wild plums native to the eastern coast of the US. They are prolific throughout Cape Cod and the islands of Martha’s Vineyard and Nantucket. Here on Nantucket, they can be found all over the island. They taste like conventional plums with most often dark purple and sometimes yellow skin with a white chalky bloom over it and thick tart and sweet flesh surrounding a tiny stone (seed) in the center. Some years the harvest is abundant while other years there are virtually no plums to be found come harvest time. In the spring the short shrubby trees can be easily spotted when they are flowering, their branches laden with a sea of tiny white and sometimes pink blossoms. The plums are ripe for harvest from late August to early September.
Beach plum jam (and jelly) is a hard-to-find delicacy, often only made by small-batch producers and home cooks from the areas in which the fruit grows. I used to make and sell the stuff at our local farmers’ market for a time. It was one of my best-selling products. The process of turning beach plums into jam is really no different than the basic method for making jam from any fruit: boil fruit with sugar and a bit of lemon juice until it sets. The only additional step in the process when using beach plums is to strain the fruit to remove the stones and skins. In this post, I lay out the formula for making jam from any amount of fruit you may have and give ingredient amounts for an example batch. If you’re reading this and thinking, “Well great, I don’t have access to beach plums”, don’t fret. You can use any plums instead of beach plums.
Tips for Foraging Beach Plums
The best foraging tip I have is to look for the plants when they are flowering in May. They are hard to miss when they’re in full bloom of showy white to light pink flowers. When you spot the trees pin them on your phone’s map or jot down the location in your notes so you remember it. Once you’ve seen the plants in bloom and when the fruit is ripe you will be able to spot them easily. Beach plums light sandy well-drained soil and will grow in beach dunes, but are also prevalent along roads and even inland in Nantucket’s moors. Because they are a rare fruit, most foragers will keep their favorite spots secret but if you take the time to look you can and will find them all over.
How to Make Beach Plum Jam
Step 1: Cook and strain the fruit
First, you’ll want to pick through and wash the foraged beach plums thoroughly. Note: Make sure to pick some unripe beach plums when foraging to help add natural pectin to the jam. The most accurate way to make jam is by weight, so if you have a kitchen scale measure the weight of the plums. For every pound of fruit measure out 1/2 cup of water (or for every kilogram of fruit add 260 milliliters of water). Add the beach plums and water to a large pot and bring them to a boil. Cook the fruit until the skins break open and the fruit softens, for about 10 minutes. Using a potato masher mash the plums to break them down even further and help release the flesh from the stones (seeds).
Strain the fruit by first placing a large colander over a similar-sized bowl and use a 1 or 2-cup liquid measuring cup to scoop the cooked fruit into the colander in batches. Use a rubber spatula, the back of a spoon, or a flexible bowl scraper to press the fruit mash through the colander and into the bowl, leaving the plum skins and stones behind. Discard the skins and stones and repeat the process in batches.
Step 2: Calculate how much sugar and lemon juice to add
Rinse out the pot if any plum skins or stones were left behind. Place the pot on a kitchen scale and zero out the scale then return the strained fruit mash back to the pot and record the weight of the fruit mash. You will need this weight to calculate the amount of sugar and lemon juice to add.
As a general rule jam requires anywhere from 50%-100% sugar by weight of fruit. Beach plums can be quite tart so 75% sugar to fruit or a 4:3 ratio of fruit to sugar works well to balance out the tartness of the fruit. Multiply the weight of the strained fruit mash by 0.75 to figure out the amount of sugar to add. You can adjust the percentage from anywhere from 50% to 100% of sugar to fruit mash as a matter of preference. Add the sugar to the pot.
For every 1000 g of strained fruit mash, you’ll need 30 g of lemon juice. In other words, 3% lemon juice by weight of strained fruit mash. Multiply the weight of the strained fruit mash by 0.03 to figure out the amount of lemon juice to add. Add the lemon juice to the pot.
Step 3: Add flavorings
Flavor the jam with cinnamon sticks, orange and vanilla bean or extract. The exact amount you add is negligible and can be adjusted to your personal taste. For the batch amount in the recipe below, I added 4 Ceylon cinnamon sticks, 1 cara cara orange and a generous splash of my homemade vanilla extract with seeds. Add the cinnamon sticks to the pot. Cut the orange in half and squeeze the juice into the pot then add the orange rinds as well. If the orange is seeded remove seeds first or squeeze the fruit over a strainer to catch the seeds. If using a whole vanilla bean, it can be split with a knife and added now. If using vanilla extract add it after the jam is cooked down.
Step 4: Cook the jam until it sets
Stir to dissolve the sugar as you bring the mixture to a low boil. Cook the mixture until it sets, stirring occasionally. To check if the jam has been set, chill a small dish or ramekin in the refrigerator or freezer while the jam is cooking. Spoon some of the jam onto the chilled dish and let it sit for a minute. If the spoonful of jam sets up to the consistency of jam, where it is thick and coats the spoon, then the batch is ready to can. If it is still runny, continue cooking the jam, and retest it every 10 minutes or so until it sets up. For the batch amounts in the recipe below, it will take roughly 30-40 minutes until it sets. Generally, smaller batches will take less time and large batches will take longer to cook.
Step 5: Sterilize the jars
While the jam is cooking sterilize the jars and lids. There are a few ways to do this, but I like to place the jars into my canning pot and fill it enough with hot water to cover the jars then bring it to a boil and let it simmer until the jam is ready. Remove the jars cautiously with canning tongs. To sterilize the lids I put them into a strainer and submerge them into the boiling water for 1 minute after I remove the jars.
Step 6: Fill the jars
Fill the sterilized jars with the jam leaving 1/2″ of headspace. A funnel is very handy for this step. Clean off any drips around the mouth of the jars with a damp clean cloth or paper towel. Place the lids on the jars followed by the lid rings and seal the rings to just finger tight. Do not tighten the lids too much or the air will not be able to escape as the pressure builds up while canning them.
Step 7: Process the jars in hot water
Place the filled jars back into the hot water bath and bring the water back to a rolling boil. Cover the pot with the lid cracked and boil the jars for 10 minutes.
Use canning tongs to carefully remove the jars from the hot water bath and let them sit to cool to room temperature. The jars will make a “pop” sound as they seal and the lids will depress. Once this happens remove the rings and dry any water drops on the jar lids with a towel.
Step 8: Label and store the jars
Label the jars with the date and store them at room temperature out of the sun for up to 1 year. Opened jars will need to be refrigerated and will keep for 2 weeks.
Beach Plum Jam Questions:
Can other types of plums be used in this recipe?
Yes, any type of plum can be used in this recipe. If the plums are larger than beach plums, which are about the size of grapes, you can easily just cut and remove the stones before cooking the fruit in water, which will make it easier to strain as well. Depending on the sweetness of the plum, adjust the percentage of sugar as needed. Choose a few less ripe or if you can find them unripe plums to add into the mix to help produce natural pectin as the jam cooks.
Where can I find beach plums?
Beach plum (prunus Maritima) trees grow wild along the eastern coast of the US. They are prevalent on Nantucket and can be found all over the island. Look for their clusters of white tiny blossoms in May and make note of where trees are located to forage the fruit later in the season. Beach plums are ready to harvest from late August through mid-September. The small round fruit is about the size of a grape with dark purple skin when ripe with thick sweet-tart flesh that surrounds a small stone in the center of the fruit. They are just like conventional plums at the store but miniature and a bit more tart. When harvesting the plums make sure to pick a small portion of unripe or less ripe plums, which are higher in natural pectin and will help the jam set without the need for adding commercial pectin or thickeners.
Don’t you need to add pectin?
No, and yes. Pectin is naturally occurring in fruit, and at a higher percentage in unripe tart fruit than fruit that is ripe. Therefore by adding in some unripe beach plums you will not need to add store-bought pectin for the jam to set. Along with adding some unripe fruit you also need to add a small amount of acid, often lemon juice, which will activate, for lack a better term< the pectin,. There is a chemical reaction the occurs that I am not an expert in so won’t try t explain it here, but you can google it if interested in learning the science behind it.
What equipment will I need for canning the jam?
You’ll need a canner or large pot with a lid, a tall-sided stockpot or a lobster pot will work well, to process the jars in a hot water bath. You will need jars with new lids. I like to use 8 oz. Ball jars with flat lids and rings. The rings do not need to be new, only the flat lids that will seal the jars. The other equipment you’ll need are canning tongs to safely lower and lift the jars out of the hot water, a funnel to help fill the jars, and a strainer to use when sterilizing the lids as detailed in the recipe below.
How do I store and the jam and how long does it keep?
After the jars are properly canned they will be shelf-stable for up to 1 year. Store them out of the sun at room temperature or in a cold cellar. Once you open a jar of jam it will need to be refrigerated and will keep for about 2 weeks.
For more ways to preserve summer produce check out these recipes:
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