The Best Homemade Jam Using a Refractometer and a Thermometer

Make Your Own Perfect Preserves Using A Pocket Sized Refractometer For Jam

With strawberries around the UK ripe for the picking, we decided to grab some handheld Xylem brand instruments including an OPTi digital handheld refractometer for jam from Bellingham + Stanley, and a TLC 700 thermometer from Ebro, before harvesting some delicious fruit in order to make our very own jam.

DID YOU KNOW?  The Food Standards Agency (FSA) in the UK state that sugar content of jam, jelly and marmalades should always be measured by a refractometer. The sugar content shown on product labelling must be accurate to ±3 refractometric degrees, a requirement that the OPTi digital refractometer for jam is more than capable of meeting.

What defines a jam?

One of the most popular types of preserve in the UK, jam will normally contain both the flesh of a fruit (or occasionally a vegetable) as well as the juice. Jam has also been defined as “a cooked and jelled puree.” The term "jam" refers to a product made of whole fruit cut into pieces or crushed, then heated with water and sugar to activate its pectin before portioning out into containers - typically jars.

What is pectin?

Pectin is a soluble gelatinous polysaccharide which is present in ripe fruits.

It is made up of D-galacturonic acid connected by α (1–4) glycosidic linkages with traces of L-fructose, D-glucose, D-mannose, and D-xylose. In jams, pectin is what thickens the final product, causing the mixture to set in a jelly-like state.

What you’ll need for this jam recipe



  • 500g Strawberries with stalk removed
  • 375g Jam sugar
  • Juice of 1/2 a lemon (to help activate the pectin)




1.  Prepare your strawberries by giving them a wipe with wet kitchen towel or a quick rinse. Don’t be tempted to submerge the fruit, as this will allow the strawberries to absorb water - making the jam harder to set later on.

2.  Empty the strawberries into a large bowl and cover with the sugar. Toss gently but thoroughly, ensuring that the fruit is generously coated in the sugar. Leave uncovered at room temperature overnight. Leaving for 12hr+ will allow the sugar to dissolve, and ensures the strawberries keep their colour and don’t drastically disintegrate.

3.  Transfer to a preserving pan and add the lemon juice. Set the pan over a low heat and cook gently until all grains of sugar are dissolved. Wipe away any stray grains on the side of the pan, and check with the back of a mixing spoon whether any sugar remains.

4.  Once satisfied that the sugar has completely dissolved, steadily increase the heat and bring to the boil. Please note that the sugar must be completely dissolved before increasing the temperature else the jam will have trouble setting later on.

5.  Boil the mixture until the temperature reaches 105°C on the thermometer. This should take around 10 minutes of boiling. It’s worth noting: in our kitchen we ran 2 different thermometers whilst boiling the mixture. When our precision thermometer from Ebro read 105°C (the ideal temperature we were looking for) the cheap sugar thermometer was still some way off reading “Jam”.

6.  Use an OPTi digital refractometer for jam to measure the °Brix of the mixture. Give the prism dish a wipe with a tissue to ensure that it is clean. Take a small sample of the jam from the middle of the pan, making sure to only sample pure jam and not impurities that may have formed on the surface whilst boiling. Place the sample onto the prism and allow 15 seconds for the temperature to stabilise so that the refractometer’s automatic temperature compensation is accurate. Press the READ button. A reading of 65 °Brix indicates that the jam is ready. Clean the prism dish and store away the refractometer for next time.

7.  Remove any scum from the top of the jam using a spoon to skim the surface and discard. Leave to settle for around 20 mins and then spoon the finished jam into sterilised jars. Place a disc of greaseproof paper over the top before securing with a lid.

Get you TLC 700 thermometer and OPTi or Eclipse refractometer today!

Ready to follow the instructions but need a new refractometer and thermometer? Visit Refractometer Shop for your new ebro TLC700 thermometerOPTi Digital refractometer for jam or an Eclipse Optical refractometer for jam. Available whilst stocks last.