Measuring internal maturity of citrus

Page last updated: Wednesday, 18 March 2020 - 8:06am

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Fruit testing procedures

Three internal maturity parameters must be measured to determine the best time to harvest fruit. These are the juice content, total soluble solids (percentage sugar or °Brix) and citric acid content. The sugar-acid ratio (°Brix ÷ %Acid) and the Australian Citrus Standard ((°Brix-(%Acid x 4)) x 16.5) can then be calculated.

Step 1: Calculating juice content - percentage juice

Juice content is an important measurement of internal quality. Under- or over-ripe fruit tends to be less juicy, which directly affects eating quality.

Weighing fruit b fruit before juicingefore juicing

Weigh sample and record results; tare or zero the scales when using a tray.

Weigh your 10 fruit and record the combined weight in grams.
Weigh the empty 1 or 2 litre jug and record the weight in grams.
Juice all 10 fruit using the juicer. Apply even force and try to remove all the juice.
Strain the juice into the jug. Weigh the juice and record the weight in grams, then subtract the weight of the jug.
Calculate percentage juice content by dividing the juice weight by the total fruit weight. Multiply this by 100 to get the percentage.

Example calculation

Fruit weight = 600g
Juice weight = 300g
(Juice weight ÷ Fruit weight) x 100 = percentage (%) juice
= (300 ÷ 600) x 100 = 50% juice

Remember to subtract the weight of the jug from the weight of the juice and jug. Alternatively, place the empty jug on the scales and zero the scales prior to adding the juice.

Step 2: Calculating total soluble solids - percentage sugar or degrees Brix

Sugar levels are a common measurement in a wide range of crops. As the flesh of fruit forms it deposits nutrients as starch that transform to sugars as the fruit ripens. The percentage sugar, measured in degrees Brix (°Brix), indicates the sweetness of the fruit by measuring the soluble solids in the juice.

$Measuring the sugar content of juice using a hand held refractometer$

Direct the refractometer to a good light source.

Calibrate the refractometer with distilled water prior to use (distilled water should give a zero reading).
Ensure the refractometer prism surface (glass) is clean and dry.
Place a small amount of the fresh juice (a couple of drops is sufficient) onto the prism.
Look through the eyepiece while pointing the prism in the direction of good light (caution: not directly at the sun).
Focus the eyepiece and take the reading where the base of the blue colour sits on the scale — this reading is the sample’s Brix.
Clean the refractometer immediately with a damp tissue, and dry thoroughly.

Note: If you are using a digital refractometer place two drops of fresh juice on the prism (glass) and record the reading. Clean the prism immediately after use.

$Digital refractometer$

Digital refractometer

Step 3: Calculating citric acid content

Citric acid gives citrus its tartness and is highest early in the season, decreasing as the fruits mature. Citric acid content can be determined using two methods:

a chemical indicator called phenolphthalein
a pH meter.

Both methods involve titration, which means adding a solution of known concentration to a solution of unknown concentration until a desired reaction is achieved.

Using phenolphthalein

Draw 10mL of juice into the pipette and transfer the juice to a clean conical flask. Clean the pipette immediately after transferring the juice.
Add two to three drops of phenolphthalein indicator to the conical flask and carefully swish mixture.
Squeeze the burette fill bottle, containing 0.1 Normal sodium hydroxide (NaOH) solution, to fill the burette.
Open the burette’s tap and allow a trickle of NaOH to run into an empty beaker. This is to ensure no air is trapped in the burette prior to use.
Squeeze the burette fill bottle again until the NaOH level in the burette reads zero at the top of the scale.
Hold the conical flask containing the juice mix under the burette and while swirling the flask slowly add the NaOH to the juice by opening the burette’s tap.
Keep swirling the flask while adding NaOH, until the solution just starts to turn pink. As soon as the solution turns pink close the burette’s tap. Look at the scale on the burette and record how much sodium hydroxide you have added to the flask. Multiply the figure you have recorded by 0.064 to give you the percentage citric acid in your sample.

The first few times you do this procedure it may be difficult to see the colour-change point. If you look closely you will see the juice mix slowly lighten in colour, almost becoming clear and then change to light green. This is the point just before the end of the test and a few extra drops of the sodium hydroxide will make the solution turn pink.

If you go past this point and the solution changes from pink to a deep purple/orange, you have added too much sodium hydroxide and you will need to empty the conical flask and begin again.

Adding the 1% Normal sodium hydroxide to the solution. Note the swirl of pink in the sample

Using a pH meter

Draw 10mL of juice into the pipette and transfer to a clean beaker. Clean the pipette immediately after transferring the juice.
Carefully place pH meter probe into the solution and turn on (remember to calibrate the pH meter first if required).
Squeeze the burette fill bottle, containing 0.1 Normal sodium hydroxide (NaOH), to fill the burette.
Open burette’s tap and allow a trickle of NaOH to run into an empty beaker. This is to ensure no air is trapped in the burette prior to use.
Squeeze the burette fill bottle again until the NaOH level in the burette reads zero at the top of the scale.
Hold the beaker containing the juice mixture under the burette and while swirling the beaker, slowly add the NaOH to the juice.
Keep swirling the beaker whilst slowly adding NaOH to the solution until it reaches a pH of 8.2. Look at the scale on the burette and record how much sodium hydroxide you have added to the flask. Multiply the figure you have recorded by 0.064 — to give you the percentage citric acid in your sample.