The last things that anyone wants to find in homemade grape jelly or syrup are granular or crunchy bits or a hard crystal formation at the bottom of a jar of juice or intermingled in jelly. After the effort of foraging wild grapes or picking domestic grapes, juicing the grapes, straining the juice, and making and processing juice, syrup, or jelly/jam, these finds are discouraging. The hard formation or crunch bits are crystals—crystals formed from tartrate acid, sugar, or evaporation.
What Are Tartrate Crystals?
Grapes differ from many other fruits in that they contain tartaric acid, a unique, natural compound. When juice is extracted from grapes and cooled, the tartaric acid reacts with potassium, the mineral of highest concentration in most grape juices, to form harmless potassium bitartrate crystals, also known as tartrate crystals or tartrates. While the crystals pose no food safety risk, they are certainly unwanted when preparing grape juice, syrup, jams, jellies, and even wine. Tartrates in wine are known as wine diamonds and may appear as salt crystals, glass shards, or fine dust in a bottle or glass of wine—visual defects that the wine industry works hard to prevent. Tartaric acid is used in the making of cream of tartar.
Preventing Tartrate Crystals
Regardless of the grape variety or color, preventing tartrates is easy to solve with time and filtering materials.
Begin by extracting the juice from the grapes by traditional methods or a steam canner. Let the freshly extracted juice sit undisturbed overnight or for 24 hours in the refrigerator. Oregon State University Extension suggests two to five days. My personal experience is that longer is better if one has the time and space, as crystals have formed in my juices after sitting overnight and even after 24 hours. When the juice is warm, the tartaric acid is suspended; as the juice cools, the tartaric acid binds with the potassium to form solid crystals that sink to the bottom of the container along with any other sediment.
Remove the juice from the refrigerator, being careful not to disturb the juice. Clarify the juice by pouring the juice into a clean container through cheesecloth, a jelly bag, or a fine strainer before beginning any further preparation. As you near the bottom of the container, avoid pouring the sediment and crystals into the container. The sediment and crystals should be discarded. It is also an option to filter the juice a second time, if desired.
Free of tartrates and sediment, the juice is ready to be used for jelly, syrup, or juice. Recipes for jelly, 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 or enjoying as juice.
What are Sugar Crystals?
Sugar is made up of tiny sucrose crystals which prefer to remain in their dry, undisturbed crystalline state. Even when sugar is dissolved in liquid, as when making jams and jellies, the sugar molecules like to congregate into groups or crystals. All they need to start the process is a “seed,” such as a lone undissolved sugar crystal as a nucleus to draw other sugar molecules towards it. Sugar crystals are not unique to grape sweet spreads. Regardless of the fruit, the sugar must be completely dissolved with no traces of crystals to provide a “seed” when making sweet spreads or syrups.
Preventing Sugar Crystals
Crystals in sweet spreads may be caused by excessive sugar, undissolved sugar sticking to the sides of the kettle, or a mixture cooked too slowly, too long, or too little. To prevent crystals from forming:
- Follow the recipe exactly. Do not add or decrease the sugar or double the recipe.
- Wipe the side of the pan free of crystals with a damp cloth before filling the jars. This will prevent a seed point from forming.
- Cook with a rapid boil. Remove from heat immediately when the jellying point is reached.
- Cook until the sugar has completely dissolved and mixed with the fruit juice.
Crystals that form at the top of a jar of jelly or sweet spread that has been opened and allowed to stand are caused by evaporation of liquid. Over time, these crystals may also work their way down through the spread. This is usually due to long-term refrigeration or a poorly capped jar kept in the refrigerator. Fruit spreads exhibiting evaporation crystals are safe to eat. White, fluffy mold on the surface of a sweet spread is a sign of spoilage and should be discarded.
Sweet spreads that have crystallized due to evaporation can sometimes be saved with a gentle rewarming to melt the crystals, as one might reheat honey that has crystallized. Place the jar in a pan of hot water or carefully microwave until the crystals melt. Stir as needed. If melting is successful, a fresh jar should be used to prevent recrystallization from the crystals that may remain on the walls of the jar. Adding a small amount of lemon juice or corn syrup may also fix it. In all cases, it is usually a temporary fix as the product may crystallize again if not used in a timely manner. 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.
- Grapes 101, Cornell University
- Wine Fault Series: Tartrate Crystal Precipitation, Iowa State University Extension and Outreach, Midwest Grape and Wine Institute
- Jams and Jellies: Problems and Solutions (SP 50-746), Oregon State University Extension Service
- Selecting, Preparing and Canning Fruit – Grape Juice, National Center for Home Food Preservation
- Preventing Crystal Formation when Making Jam and Jelly, PennState Extension