After you’ve gone to the trouble of foraging wild grapes or picking domestic grapes, juicing them, straining the juice, and making and processing juice, syrup, or jelly/jam, the last thing you want to find are crunchy bits in the jelly or syrup or a hard crystal formation at the bottom of a jar of juice.
These crunch bits are crystals, usually of three types.
1) Tartrate Crystals – the naturally occurring components of grape juice
2) Sugar Crystals – improper cooking of the jelly or jam when the sugar is added.
3) Evaporation Crystals – loss of liquid
What are Tartrate Crystals?
Grape juice differs from many other fruit juices in that it contains naturally occurring amounts of both potassium and tartaric acid. At temperatures below 40F, these substances bind together to form crystals of potassium bitartrate better known as tartrate crystals. The crystals are benign or harmless so they pose no food safety risk but they are certainly unwanted encounters in juice, syrup or jellies. Tartrate crystals can also form in grape jam. In the wine industry, they are known as wine diamonds.
Preventing Tartrate Crystals
Regardless of the grape variety, color, or how the grapes were acquired, the problem is easy to solve with time and a fine strainer.
After juicing and straining the juice, allow the juice to sit undisturbed in covered containers for 24 – 48 hours in the refrigerator. My personal experience is that 48 hours is better than 24 hours if one has the time as crystals have continued to form in my juices after 24 hours. After the wait, slowly pour the juice through a jelly bag, cheesecloth, or very fine strainer into a clean container. Be very careful as you reach the bottom as that is where you will find the tartrate crystals; they will appear as a rough, cracked substance on the bottom of the container. Most of the crystals will be stuck to the container, but some may still be afloat.
Once the tartrate crystals have been filtered out, the juice is ready to turn into jelly, syrup, or juice without the unwanted tartrate crystals. Recipes for jelly, jam, syrup, and juice can be found at the National Center for Home Food Preservation. The juice can also be frozen to be used later for making jelly.
What are Sugar Crystals?
Sugar is a crystal in its natural state and has an affinity to return to that form. Even when dissolved in liquid as they are in jams and jellies, sugar molecules like to form into groups or crystals. All they need is a party starter like an undissolved sugar crystal as a nucleus to draw other molecules of sugar towards it, re-forming crystals. Sugar crystals are not unique to grape sweet spreads. When making a sweet spread product or syrup regardless of fruit, it is important that the sugar is completely dissolved with no traces of crystals.
Preventing Sugar Crystals
Crystals throughout the jelly may be caused by too much sugar in the jelly mixture or cooking the mixture too little, too slowly, or too long. Learn how to prevent them from this Penn State Extension video. Sweet spreads exhibiting sugar crystals are safe to eat.
Speckled crystals that form at the top of a sweet spread and scatter downward come when the product has been opened and allowed to stand; these crystals are caused by evaporation of liquid. This is more likely to happen with poorly capped, refrigerated jam or jelly. White, fluffy mold on the surface of a jelly or jam is a sign of spoilage and should be discarded.
Crystallization due to evaporation can sometimes be reversed by gently reheating. Too much heat will cause the product to break down and not reset. The jar can be placed in hot water or carefully microwaving enough to melt the crystals. If melting is successful, a fresh or clean jar should be used. Adding a small amount of lemon juice or corn syrup may also fix it. In all cases, it is a temporary fix and the product usually goes back to crystallizing shortly. A tight fitting lid is the best prevention.
With just a little patience and careful preparation, crystals of all types can be prevented in grape products.