Jam, jellies & marmalade
Practical Action
In most countries, it is illegal to add preservative (such as benzoic acid, sodium or potassium
benzoate or sulphur dioxide) to jams. Besides, if the correct recipe is used, good quality
assurance procedures are in place, the method is followed accurately and the jam is made under
hygienic conditions, it is not necessary to add preservatives to jams. The only exception is in jam
that is made from fruit pulp that has been stored with chemical preservatives. In this case, a
residue of preservative (either 100ppm sulphur dioxide or 500ppm benzoic acid) is allowed in the
jam. Citric acid is not a preservative. It is added to fruit juice to adjust the pH so that the pectin
will form a good gel. Jams give a gel when there is the correct ratio of pectin to water and the pH
is between 2.5-3.45 pH. The optimum pH to give a good gel is pH 3.0.
Boiling to reach the final sugar concentration
The aim of boiling is to reduce the water content of the mixture and concentrate the fruit and
sugar in as short a time as possible. The final Total Soluble Solids (TSS) content of a jam (also
known as the “Degrees Brix” or “end-point of the jam”) should be 65 to 68%. (The TSS is a
measure of the amount of material that is soluble in water. It is expressed as a percentage -a
product with 100% soluble solids, has no water and one with 0% soluble solids is all water).
The correct sugar content is critical for proper gel formation and for preservation of the jam or
jelly. If the final TSS of jam is lower than 65-68% the shelf life will be reduced. The jam will
have a runny consistency and bacteria and moulds will be able to grow in the product. If the TSS
is higher than 68%, the jam will be very stiff and the sugar might start to form crystals in the
jam.
The end-point of boiling is measured in different ways. The most accurate method is to use a
refractometer to measure the total sugar concentration. This method is not really suitable for
home-use as a refractometer costs about US$ 150. It is only when making jam for sale that you
might consider using a refractometer to ensure consistency between different batches of the jam.
When making jam for home consumption, you can use other methods to determine the end point:
the drop test, the skin wrinkle test, or the use of a jam thermometer to test the temperature (68%
sugar corresponds to a jam temperature of 105°C).
When the jam starts to thicken, it is important to test for the end point at frequent intervals.
Remember to remove the pan from the heat source while you test or it will continue to thicken
and may burn.
Determination of the end point of a jam
a) Use a refractometer
This is the most accurate method.
Measuring the sugar content with a
refractometer
1. Take a small portion of jam from the
pan. Let it cool to 20°C. (Remember to
take the pan off the heat)
2. Place one or two drops of the jam onto
the prism and carefully close the prism.
Make sure the sample spreads evenly
over the surface of the prism.
3. Hold the refractometer near a source of
light and look through the end piece.
4. The line between the dark and light
fields will be seen through the viewer.
Read the corresponding number on the
scale, which is the percentage of sugar
in the sample.
5. Open the prism and remove the sample with a piece of tissue paper or wet cotton wool.
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