Why did one batch homemade jam turn dark and stayed bright red?

There could be several reasons why one batch of homemade jam turned dark while the other stayed bright red:

Type of fruit: Different fruits have different natural colors and acidity levels, which can affect the final color of the jam. For example, berries like strawberries and raspberries naturally produce a darker red color, while fruits like peaches or pears may produce a lighter, golden color.

Cooking time and temperature: Overcooking the jam can cause it to darken, as the sugars in the fruit caramelize and deepen in color. Additionally, cooking the jam at too high a temperature can also contribute to darkening.

Exposure to air: Jams exposed to air during cooking or storage can oxidize, which can cause them to darken. Make sure to cover the jam while cooking and storing to minimize exposure to air.

Acid content: The acidity of the fruit or any added ingredients (like lemon juice or citric acid) can influence the color of the jam. Adding more acid can help preserve the bright red color.

Presence of metal: Reactions with metal utensils or containers can cause jams to darken. Use non-reactive cookware, such as stainless steel or glass, to avoid this issue.

Use of pectin: Commercial pectin often contains color-preserving agents that help maintain a brighter red color. If you're making jam without commercial pectin, you may want to consider using a natural source of pectin, like lemon juice or apple pectin, to help preserve the color.

Storage conditions: Jams stored in warm, light, or humid conditions can deteriorate and darken over time. Store your homemade jams in a cool, dark, and dry place to preserve their color and quality.

If you're unsure why one batch of jam turned dark while the other stayed bright red, it's best to compare the ingredients, cooking methods, and storage conditions of both batches to identify potential factors that may have influenced the color difference.