The lemon juice lowers the pH of the jam mixture, which also neutralizes those negative charges on the strands of pectin, so they can now assemble into a network that will “set” your jam.
Lemon juice lowers the pH of the jam mixture and neutralizes the negative charges on the strands of pectin allowing them to move together into a network to “set” the jam. The optimal pH for gelatinization is between 2.8 and 3.5. The best way to achieve this level of acidity is to use commercially bottled lemon juice.
Place jelly or jam in a saucepan and bring to a boil, stirring constantly. Quickly add the sugar, lemon juice and pectin.
Fresh fruits such a pineapple, kiwi and papaya contain enzymes which break down these protein molecules, making them smaller, so they can't tangle up, which stops the jelly setting. This is similar to how the enzymes in your stomach break down food.
Addition of citric acid to ingredients containing pectin can increase gel formation [13-15]. The more acid added, the stronger the gel is formed and the higher the water binding ability.
The Danger: The acidic treatment used to denature the collagen in animal hides leaves porcine gelatin vulnerable to solutions with a pH of 3 or below—on par with distilled white vinegar. Most desserts aren't that acidic, but ingredients like lemon, lime, passion fruit, rhubarb, and even pomegranate are.
Lemons and other citrus fruits are rich in pectin, which thickens jams. I use a lot of lemon zest, peels, and juice in my kitchen so whenever I have leftover seeds, I save them in a container in the freezer.
Acids such as vinegar, fruit juice and wine with a pH below 4 produces a weaker jelly and requires the amount of gelatine used to be increased by up to 1/3 of the original amount. Strong acids and tannins in red wine and tea can make a jelly, set with gelatine, go cloudy.
This is because the enzymes in the fruit only disrupt the gelling process if the fruits are fresh or frozen. If the fruit is heated (e.g., canning or cooking) then the enzymes are permanently inactivated, making the fruit perfectly fine for making Jell-O.
Making too large of a batch, or doubling a recipe, takes longer for the mixture to heat and can affect the gelling ability of the pectin. Moving the jelly, after it has been processed, weakens the pectin linkage that holds the gel together. Do not move the jars for 12 hours after they have been made.
Although we are not adding powdered pectin, we will add citric acid or lemon juice to the jam. This gives an additional pop of pectin but also some acidity that balances the sweetness of the jam. Citric acid provides very consistent results, which is why I like to use it.
Lemons contain a type of fiber called pectin, which may help reduce appetite and calorie intake. However, lemon water is basically diluted lemon juice, which leaves it with only trace amounts of pectin.
Taste Your Way to the Right Amount of Acid
I generally add one ounce of fresh-squeezed lemon juice for every two pounds of fruit when jamming higher-acid fruit (like tart plums), and about two ounces for lower-acid fruit (like sweet strawberries).
Adding lemon juice or any other acidic juice to condensed milk will thicken up the milk making this process an important cooking technique. The lemon juice thickens up the milk without any heat.
Lemon juice is another natural acidulant commonly used in home food preservation. To ensure safe acidity in whole, crushed, or juiced tomatoes, add 2 tablespoons of lemon juice per quart of tomatoes or 1 tablespoon per pint.
If you like making gelatin for dessert, the box often recommends not adding certain kinds of fruit, including pineapple, kiwi, mango, ginger root, papaya, figs or guava. People have a hard time getting the gelatin to solidify when they add these fruits.
Many people like to add fruit to jellies but you will not be able to make jellies with certain fresh fruit such as pineapple, kiwi fruit or papaya – the jelly will not set.
The tartness of the vinegar subtly balances the flavor of the jam, bringing out the complexities of the fruit.
The gelatin slowly releases the baking soda, so it kept on fizzing for a long time (for more than 5 minutes!) If you squeeze a handful of the gelatin, little pieces hop out of your hands. It's such a bubbly, fizzy sensory experience, we really enjoyed it as a variant on our traditional baking soda and vinegar play.
The vinegar does a couple of functions in this jelly recipe. First, peppers area low in acid and acid is needed to react with the pectin and sugar to help the jelly set. Without the vinegar, your jelly won't set up as firm. Second, the acid in the vinegar makes the jelly safe when it is canned and kept for a long time.
The secret ingredient to making jam without pectin is time. The fruit and sugar need plenty of time to cook and thicken. A long, slow boil drives the moisture out of the fruit, helping to preserve and thicken it at the same time. Fruit varies in water content as well, and some fruits may take longer to jam up.
One tablespoon of bottled lemon juice is equal to 1/4 teaspoon citric acid. This means that if a recipe instructs you to add 2 tablespoons of bottled lemon juice to each quart jar before canning, you can easily swap in 1/2 teaspoon of citric acid.
Keep in mind that it is the white pith or inner part of the citrus peel that is rich in pectin. Under-ripe fruits have more of this pith than fully ripe fruits.