SOP: Determination of Total Solids in Tomato Products from Refractive Index

GIFSQ — Global Institute for Food Safety & Quality

Principle

The refractive index of the filtered tomato serum is closely related to its soluble solids content. An Abbe refractometer reading, taken at 25°C and corrected for temperature and for any added salt, is converted to total solids by means of the tomato pulp and paste tables (Tables 30-1 to 30-4 of the book). The method is extremely rapid and is used during manufacture for production control. A refractometer reading accurate to 0.0001 corresponds to 0.14% tomato solids.

Apparatus required

• Abbe refractometer (or any other refractometer)
• Cotton cloth or piece of muslin (for straining)
• 10-ml pipette with a fairly large opening at the tip (for cooling hot pulp)
• Can of cold water (for the hot-pulp cooling method)
• Ultra-speed centrifuge capable of producing approximately 150,000 × g (for clear serum of heavy concentrates)
• Low-speed blender (for enzyme treatment, if used)
• Filter paper or filter funnel (for enzyme-treated samples)

Reagents

a. Distilled water — for checking the adjustment of the refractometer scale.

b. Pectic enzyme (clarifying pectic enzyme, or dry pectic enzyme preparation) — for treating tomato paste to enable easy filtration.

c. Enzyme solution — about 4 to 5 g of clarifying pectic enzyme dissolved in 1 litre of water (filter the solution if specific gravity is also to be determined).

Sample preparation — choose the treatment that fits the product

Rapidly filtering products (juice, light puree, hot sauce): strain through cotton or a piece of muslin.

Hot pulp (factory control): cool rapidly by the pipette method described in the procedure.

Tomato paste: dilute with 3 parts of water, mix thoroughly, and filter (or treat with pectic enzyme as described below).

Heavy puree and paste for the paste tables: treat with pectic enzyme, or obtain clear serum by ultracentrifugation.

Procedure — Checking the refractometer

Periodically verify the adjustment of the refractometer scale by taking the reading of distilled water, and compare with the following values:

• At 20°C (68°F): 1.3330
• At 22°C (71.5°F): 1.3328
• At 24°C (75°F): 1.3326
• At 26°C (79°F): 1.3324

Check the adjustment of the scale periodically in this way.

Procedure — Reading the filtrate of juice, light puree or hot sauce

Strain the product through cotton or a piece of muslin

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Place a drop of the filtrate on the refractometer prism

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Note the refractive index or Brix reading; correct for temperature (see below) and read the total solids from the appropriate table

Procedure — Cooling hot pulp rapidly (NCA method, for factory control)

With a 10-ml pipette having a fairly large opening at the tip, withdraw about 10 ml of the hot pulp from the sampling cup; hold the sample within the pipette with the forefinger over the top

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Immerse the length of the pipette in a can of cold water for a few seconds to cool the sample

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Take the pipette out of the water and wipe the outside surface dry

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Discard the bottom portion of the column (it may have been diluted by contact with the cooling water); take a drop or two from the middle portion of the column for examination with the Abbe or any other refractometer

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Read immediately, since the drop quickly assumes the temperature of the prism

Procedure — Tomato paste by dilution

Dilute the tomato paste, if necessary, with 3 parts of water

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Mix thoroughly; filter

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Allow at least 1 ml of the filtrate to collect; place a drop on the prism and read (do not wait too long, as evaporation may occur)

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Find the total solids of the diluted sample from the tables; multiply by the dilution factor, i.e. 4, to obtain the actual solids of the paste

Procedure — Tomato paste by pectic enzyme treatment (with dilution)

Add about 4 to 5 g of clarifying pectic enzyme to 1 litre of water and make into solution (filter the enzyme solution if specific gravity is to be determined)

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To 100 g of enzyme solution, add 100 g of well-mixed sample; mix thoroughly either by hand or by blending in a low-speed blender for 15 to 30 sec

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If need be, keep aside for some time for the enzymes to act; then filter

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After a few millilitres of solution have filtered, allow a drop to fall on the prism of the refractometer and rapidly note the reading; take further readings with fresh drops at 1 or 2 min intervals until the readings do not vary more than 0.0001 after temperature correction

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Subtract from the observed refractive index the enzyme correction obtained from the expression:

(RI of enzyme solution at 20°C − RI of water at 25°C) / 2

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Apply temperature correction (Table 30-5) and read the total solids from Table 30-4

Procedure — Tomato paste by direct pectic enzyme treatment (up to about 32% solids, no dilution)

To 100 g of paste, mix thoroughly 0.2 to 0.4 g of dry pectic enzyme with the help of a spatula

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Allow the enzyme to act for 0.5 to 1 hr in a warm place; filter and note the refractive index

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To correct for the added enzyme, mix an equal weight of enzyme in 100 ml of water and note the refractive index of the enzyme solution as well as that of water; apply temperature correction if necessary to obtain the refractive index at 25°C

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Subtract the refractive index of water from that of the enzyme solution; subtract the value so obtained from the refractive index of the tomato paste

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Find the total solids by referring to Table 30-2

Procedure — Tomato paste of 33 to 50% total solids (clear serum by ultracentrifugation)

Centrifuge the paste in an ultra-speed centrifuge capable of producing a force of approximately 150,000 × g (a lesser force may be sufficient for some samples) to obtain a clear serum

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Read the refractive index of the clear serum directly without dilution; read the total solids from Table 30-3

Temperature and salt corrections

• If the reading is made at any temperature other than 25°C (77°F), apply the correction from Table 30-5 (refractive index readings) or Table 30-6 (Brix scale readings).
• If added salt is present, apply the correction to the observed refractive index by referring to Table 30-7, since salt affects both refractive index and specific gravity more than an equal concentration of tomato solids does.

Precautions

• Do not wait too long between filtration and reading; evaporation changes the reading.
• Record only readings that do not vary more than 0.0001 between successive drops.
• Read immediately when examining hot pulp; the drop quickly takes the temperature of the prism.
• The refractometer scale adjustment must be checked periodically with distilled water.

References

Bigelow, W.D., H.R. Smith & C.A. Greenleaf, Tomato Products, Pulp, Paste, Catsup, Chilli Sauce and Juice, Bull. 27-L, Revised, National Canners Association Research Laboratory (1950).

National Canners Association Research Laboratories, Laboratory Manual for Food Canners and Processors, Vol. II (1968).