SOP: Determination of Total Solids in Tomato Products by Drying

GIFSQ — Global Institute for Food Safety & Quality

Principle

Water is removed from the tomato product by drying under controlled conditions, and the residue that remains is the total solids. Drying in vacuo at 70°C is the accurate reference method used by public analysts for control work. Drying at atmospheric pressure at the temperature of boiling water is less accurate but quicker; its results are related to the vacuum-dried results by a constant factor. Total solids may also be obtained from the specific gravity or the refractive index of the pulp or filtrate, which are dealt with in separate SOPs.

Apparatus required

• Flat-bottom metallic dish with tight-fitting lid
• Oven (110°C) — for preparing the tared dish
• Boiling water bath
• Oven with forced draft (70°C), or vacuum oven (70°C) with the release cock adjustable
• Desiccator

Reagents

a. Filter aid — spread at the rate of 15 mg per sq. cm of dish surface.

b. Distilled water — for facilitating uniform distribution of the sample, if necessary.

Sample preparation

Thoroughly mix the sample in its container by stirring. Transfer a portion to a large china or glass dish and mix again.

Procedure — Drying in vacuo

Spread filter aid into the flat-bottom metallic dish at the rate of 15 mg per sq. cm

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Dry the dish with its lid in the oven at 110°C for 30 min; cool and note the tare weight

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Spread the mixed sample on the tared dish in such a quantity that the dry residue is between 9 and 30 mg per sq. cm of the drying surface

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Mix the sample with the filter aid and distribute uniformly over the surface, using water if necessary to facilitate uniform distribution

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Dry the sample to apparent dryness (approximately to more than 50% dry solids) on a boiling water bath, or in an oven with forced draft at 70°C, or in a vacuum oven at 70°C with the release cock partially left open so that the vacuum is not more than 17.5 in.

↓

Dry the partially dried sample in the vacuum oven at 70°C for 4 hr; cool in a desiccator and weigh

Calculation

Total solids % = [(Z − X) / (Y − X)] × 100

Where,

• X = weight of the tared dish with filter aid
• Y = weight of the dish with the sample spread on it
• Z = weight of the dish with the dried residue

Procedure — Drying at atmospheric pressure (alternate method)

Dry the sample at the temperature of boiling water at atmospheric pressure for 4 hr, following the same spreading and weighing steps as above.

The book states that moisture determined by drying at atmospheric pressure, multiplied by 1.085, agrees quite closely (±0.1%) with the per cent obtained by drying in vacuo. The book’s sentence comparing the two results directly is garbled in the available copy and is not reconstructed here.

Precautions

• The sample spread on the dish must be uniform over the whole drying surface; an uneven spread gives uneven drying.
• Do not exceed the stated dry-residue limits (9 to 30 mg per sq. cm); too thick a layer dries incompletely.
• When using partial vacuum for the preliminary drying, keep the vacuum at not more than 17.5 in.

References

Official Methods of Analysis, Association of Official Analytical Chemists, 11th edn. (1970).

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).