SOP: Determination of Specific Gravity of Tomato Products

GIFSQ — Global Institute for Food Safety & Quality

Principle

When the procedure is followed properly, the specific gravity method gives an accurate and quite rapid estimate of total solids. The specific gravity of whole juice, pulp or filtrate bears a reasonably close relationship to total solids, which are ascertained by referring to the tomato pulp and paste tables. The specific gravity bottle procedure is used for juice, pulp and puree; the filtrate may also be measured with a hydrometer.

Apparatus required

• Specific gravity bottle of Pyrex glass, about 100 ml capacity, of the NCA type (see the book’s Fig. 30.1 for dimensions)
• Water bath at 25°C (± 0.1°C)
• Centrifuge (a Babcock centrifuge may be used) — 2,000 rpm
• Aluminium foil
• Thermometer
• Sugar sack or piece of muslin cloth, 24 in. square (for the filtrate method)
• Clean dry funnel
• Cylinder (1.5 × 8 in.) (for the filtrate method)
• Set of 3 special specific-gravity hydrometers of range 1.005 to 1.02, 1.02 to 1.04, and 1.04 to 1.06 (for the filtrate method)

Reagents

a. Distilled water — for calibrating the bottle.

b. Pectic enzyme solution — for clarifying diluted tomato paste (filter the enzyme solution before adding to the paste); see the reagent preparation in the SOP “Determination of Total Solids in Tomato Products from Refractive Index”.

Procedure — Calibration of the bottle

Clean the bottle thoroughly, dry, and take the empty weight

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Fill the bottle with an excess of distilled water at a temperature slightly below 25°C and place the stopper; wipe the top of the stopper with a filter paper, remove the stopper, add 0.1 ml of water, and place in the water bath at 25°C ± 0.1°C

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When the water in the bottle has reached the temperature, remove the bottle from the water bath and immediately place the stopper; wipe the moisture from the outside of the bottle and weigh immediately

Procedure — Juice, pulp or puree

Fill the calibrated bottle with juice, pulp or puree slightly above the mark; cover with aluminium foil

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Centrifuge for about 5 min at 2,000 rpm; after centrifugation there should be no air bubbles (refill the bottles and recentrifuge if necessary)

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Place the bottles in a water bath at 25° ± 0.1°C for 15 min, or allow the temperature to come to 25°C; check the temperature with a thermometer

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Adjust the volume, either by filling with the material of the same temperature, or by removing if in excess (a layer of fluid at the top after centrifugation permits easy manipulation)

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When the temperature of the material is 25°C, place the stopper gently to allow the liquid to fill the hole in the stopper; then press down firmly

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Wipe the top of the stopper and the neck with a filter paper; dry the outside of the bottle with a piece of cloth and weigh accurately to read to 0.1 mg

Procedure — Tomato paste (after dilution with an equal quantity of water)

Dilute the tomato paste with an equal quantity of water, following the dilution and enzyme-treatment procedure given in the SOP “Determination of Total Solids in Tomato Products from Refractive Index” (enzyme solution may be used in place of water to help clarification and centrifugation; filter the enzyme solution before adding to the paste)

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Determine the specific gravity of the diluted paste by the bottle procedure above

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To correct for the added enzyme, mix an equivalent weight of enzyme with water equal to the weight of tomato pulp taken, filter, and note the specific gravity of this enzyme solution

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Calculate the actual specific gravity of the tomato paste using the expression:

Specific gravity of tomato paste = Actual (measured) specific gravity − 1/2 × Specific gravity of enzyme solution

Flag: the printed layout of this expression is heavily garbled in the available copy; the transcription above is the best literal reading of the visible characters. Verify against the printed book before use.

Procedure — Filtrate by hydrometer

On a clean dry funnel, place a sugar sack (or a piece of muslin cloth) 24 in. square; pour hot pulp and strain; refilter the first portion

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Cool the filtrate to 20°C; transfer to the cylinder

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Check the temperature and measure the specific gravity with the appropriate specific-gravity hydrometer

Calculation

Specific gravity = [(Wt of pulp + flask) − Wt of flask] / [(Wt of water + flask) − Wt of flask]

Where,

• Wt of pulp + flask = weight of the bottle filled with the pulp sample
• Wt of water + flask = weight of the bottle filled with distilled water
• Wt of flask = weight of the empty dry bottle

Temperature correction

If specific gravity is determined at a temperature other than 25°C, apply temperature correction according to Table 30-8 of the book. The specific gravity should be accurate to 0.0001, which corresponds to 0.05% of tomato solids.

No procedure has so far been developed for determining the specific gravity of tomato paste without dilution.

Precautions

• Note the temperature accurately in the determination of specific gravity: an error of 1°C (1.8°F) in the temperature corresponds to an error of 0.07% in tomato solids, or to 0.003 in specific gravity.
• There must be no air bubbles in the centrifuged sample.
• Weigh the bottle immediately after drying the outside; delays change the weight.
• Read the bottle weight to 0.1 mg.

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).