At the beginning of the summer, I started making strawberry jam. Since this was my first time making jam, I didn’t have high expectations. I just wanted to try it because we had a lot of extra strawberries at the time. I found a recipe that was simply 1:1  ratio of strawberries to sugar, with a small amount of lemon juice added.1 After boiling this mixture to 220 F (I went to 223 F just to be safe), it was done. I put it in a jar, let it cool, put it in the fridge, and the next day it was a perfect consistency. Perhaps a little bit looser than a store-bought jam, but it was perfect for putting on toast and crackers.


Fast-forward to August, and I was in the same situation with some leftover grapes. I followed the same steps I used for the strawberries. I blended them, removed the skins through straining, and boiled until I reached 220 F (223 F just to be safe) and the result was… disappointing. It was basically one step thicker than grape juice. Needless to say, I was confused. Here was my understanding as of this point: That sugar and water (from the fruit juice) at a 1:1 ratio, heated to 220 F (the jam temperature) will turn into jam or jelly.2 The answer, it turns out, was a bit more complicated than that. But first, let’s differentiate between jams and jellies.


Jam vs. Jelly
The difference between jam and jelly is actually very simple: Jam is made from cooking mashed fruit whereas jelly is made from cooking fruit juice alone. This brings me to my first error: trying to make jelly instead of jam. Jelly usually relies heavily on added pectin, which helps hold the sugar and acid molecules into a gelatin-like structure. Jam, on the other hand, includes the fruit solids, which may contain naturally occurring pectin. The amount of naturally occurring pectin depends on the type of fruit used and the level of ripeness.


The fruit I used in my last jam, strawberries, tend to have low pectin. This is typically compensated for in recipes with the addition of lemon juice or lemon rinds, as citrus fruits tend to have a high concentration of pectin.3 Grapes, on the other hand, vary wildly in their pectin content.4 This means that they might contain enough pectin and acid to form a jam by themselves, or they may need added pectin from another source (natural or otherwise).


This information was helpful in getting me in the right direction, but it still didn’t answer why my first recipe didn’t form a proper jam. Like the strawberries, I still had a fruit juice and added lemon juice. But in the case of the strawberry jam, the “skins” were included in the jam. For my grape “jam”, I strained out the skins. Even though strawberries contain low pectin, they still contain naturally occurring pectin. This means that in addition to the lemon juice, I also included the naturally occurring pectin in the skin. Which leads me to my experiment: determining how much grape skins contribute to forming a grape jam, and if they can be replaced by any high-concentration pectin fruit.


The Hypothesis:
Since pectin allows jelly and jam to properly bind, can enough pectin be extracted from natural sources (i.e. grape skins or lemon rinds) to create a jam without directly adding pectin? Since there are recipes for jams and jellies without added pectin available online, this does appear to be possible. But it is hard to say exactly how much natural pectin is needed, and if this pectin can be attained from the grape skins alone or if citrus fruit is required. To attempt to determine this, I devised three samples with (in theory) increasing levels of pectin:

Sample 1: Grape juice only (strained and filtered)
Sample 2: Grape juice cooked with grape skins
Sample 3: Grape juice cooked with lemon rinds

Since lemon rinds are typically high pectin, and grape skins tend to have medium pectin levels, I believed this would create three varying levels of pectin to compare between the grape jam. The grape jam recipe that all three samples used was as follows:

1 cup grape juice (strained and filtered to remove grape skins, except for sample 2)
1 cup granulated sugar
½ lemon, squeezed
All samples were cooked on medium/medium-high heat until reaching a temperature of at least 220 F.

The Results:
As stated above, I had hoped that these three samples would create three increasing amounts of pectin, and thus, increasing amounts of “jellying”. Unfortunately, the results from all three samples were pretty much the same: all were too loose to be considered a true jelly. The flavor, on the other hand, varied greatly:

Sample 1: Plain grape flavor, no complexity
Sample 2: Fresh grape flavor, very tasty
Sample 3: Lemon flavor overpowered grape flavor, not great

One lesson learned from this experiment is that you should not leave out the grape skins when making grape jam. There was a lot of extra flavor in the grape skins that I wouldn’t have expected without trying this. Just make sure they are washed thoroughly to remove any pesticides that might have been used. If you don’t like the texture that the grape skins add, you can always filter it out at the end while the soon-to-be grape jam is still hot.

I had the highest hope for the grape jam with lemon rind added, but surprisingly this ended up being the most disappointing sample for me. The lemon flavor was overpowering and didn’t help with the jellying process at all. As a result, I was left with a lot of slightly thick grape juice. Rather than waste it, I decided I would try to salvage it the next day by continuing the boiling process. This illuminated one final variable that I had yet to consider: the water content of the fruit used.


Salvaging the Results:
To salvage the grape jam, I first did some research. From this research, it seemed like if I could decrease the water content of my current grape juice, I should be able to get it closer to a jam-like consistency.5 I combined all the samples into one (I just wanted to salvage the grape jam for myself at this point). I boiled the mixture, holding it around 220 F (I found that boiling too hot will result in forming different states of sugar, like caramel) and allowing the water to be boil off over the course of about 20 minutes. I transferred this to jars, successfully boiling off about 30% of the liquid, and then allowed it to cool. I also found a new tip for testing th the jam for doneness as it cooks: Place a drop of hot jam on a frozen plate. If it turns into your desired consistency within 15 seconds, you know it’s done.


I allowed this  mixture to cool in the fridge overnight. The result still wasn't fully the texture I desired, but it was a lot closer than any of the original samples. From making strawberry jam multiple times at the beginning of the summer I thought it would be a breeze to easily make a grape jam too. In the end, I learned a lot about jams and jellies, and how pectin, water content and temperature control all play a role in creating a great jam or jelly. So, my recommendation for if you want to make grape jam is to be aware of all these factors. Jams and jellies strike a delicate balance between all these factors and any variation can drastically change the consistency.



My grape jam without added pectin might not have been a complete success, but it has illuminated these complicating factors I was not aware of from making strawberry jam successfully. As you’ve probably heard before, you can learn a lot more a from a failure than a success, and that is why I wanted to share the results here. Going into this, I believed that simply combing juice, sugar, acid, and heat would result in jam. I now realize how the additional variables of pectin concentration, water content, and temperature control all factor into creating that perfect jam consistency we know and love. Here is my recipe for grape jam that got as close to my desired consistency as possible. I will likely circle back to this recipe in the future as I optimize these other variables, but for now this is the one that I am most satisfied with.


Recipe:
1 lb. grapes (yields roughly 1 cup juice after blending)
1 cup granulated sugar
½ lemon, juiced (or 1.5 Tbsp lemon juice)

Directions:
Blend grapes for 30 seconds to a minute, until no large pieces of grape are visible.
Add the blended grape juice, sugar, and lemon juice to a saucepan over medium-high heat.
Heat to a boil, then hold the boil between medium to medium-high heat. Make sure the mixture does not heat up beyond 125 F.
Boil the mixture between 15-30 minutes. (To check if done, place a drop on a frozen saucer plate. If it solidifies into a firm, but still mailable texture, it’s done.)
Remove from the heat and allow to cool slightly.
If planning to store at room temperature, sanitize a glass jar by boiling it in a pot of water for 10 minutes (if refrigerating for immediate use this is not necessary).
Strain grape jam mixture through a fine mesh filter into a bowl.
Pout the grape jam mixture into the jar, sealing while still hot to create a vacuum seal.
Refrigerate for at least 24 hours, serve, and enjoy!
References:
Butter with a Side of Bread - Strawberry Jam without Pectin
National Center for Home Food Preservation - Jelly Point
The Kitchn - 5 Mistakes to Avoid When Making Strawberry Jam
The Spruce Eats - The Best Fruits High in Pectin for Jellies, Jams, and Preserves
The Kitchn - How to Make a Basic Fruit Jam Without Pectin