SOP: Examination of Heavy Extraneous Materials by Reverse Flotation

GIFSQ — Global Institute for Food Safety & Quality

Principle

Unlike insect fragments and rodent hairs, fly eggs, larvae, maggots, some nematodes, heavy rodent excreta (pellets), sand and similar materials do not float in the oil layer but settle down. The flotation procedure is therefore carried out in reverse: when petrol is mixed vigorously with the plant material and allowed to separate, most of the plant tissues rise while the heavy extraneous material settles to the bottom. A separating funnel is used instead of a trap flask, and the sediment is recovered instead of the floating material.

The method as described below is suitable for the examination of heavy extraneous material in berry fruits, cherries, spinach and other canned leafy vegetables, strained and chopped fruits and vegetables, and canned tomato products.

Apparatus required

• Separating funnel of 2-litre capacity, with a suitable stand (a 6-litre funnel for canned tomatoes)
• Stereoscopic microscope
• Filtration flask with a funnel
• Bolting cloth: silk cloth used on sieves in flour mills, woven to standard size, opening and thickness; the number of the silk specifies the number of meshes per linear inch, and “x”, “xx” or “xxx” after the number indicates the thickness of the thread from which the cloth is made. Boil a large piece of cloth in water, dry, mark the circles according to the size required, and then cut to get the discs. (Bolting cloth is used instead of filter paper because the paper might be clogged by small particles of plant tissue.)

Reagents

a. White petrol, free from lead (about 30 ml for the extraction; 125–150 ml for canned tomatoes). In place of petrol, n-heptane containing not less than 8% toluene may be used.

b. 15% pectic enzyme solution: add 10 ml to a pulpy product and allow it to stand for 15 min for clarification. If a dry enzyme preparation containing diatomaceous earth is used, dilute with water, allow it to stand and use the supernatant liquid.

Extraction

Use 100 to 150 g of sample in the case of tomato products, except canned tomatoes, in which case use 500 g of mashed tomatoes with a 6-litre capacity separating funnel and 125–150 ml of petrol

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Transfer the sample into the funnel with plenty of hot water

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If the product is pulpy, add 10 ml of 15% pectic enzyme solution and allow it to stand for 15 min for clarification (if a dry enzyme preparation containing diatomaceous earth is used, dilute with water, allow it to stand and use the supernatant liquid)

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Add about 30 ml of white petrol (free from lead) and shake for several seconds

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Cover with the stopper, invert, and open the stopcock momentarily to release the pressure

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Hold the funnel horizontally and shake vigorously for 2 min, releasing the pressure in between

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Fill the funnel with a fast stream of water and swirl; allow it to stand for 10 to 15 min

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Drain off 20 to 25 ml; repeat the swirling, standing and draining three times

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If eggs or larvae are present, repeat the process twice more

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Filter the drainings, either separately or together, through a dark coloured 10 xx silk bolting cloth or through filter paper marked with lanes as in the case of the insect fragment count

Note: in place of petrol, n-heptane containing not less than 8% toluene may be used (see Reagents).

Examination

Count the eggs and larvae using a stereoscopic microscope with the help of the probe; turn each piece and examine

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Fly eggs: tiny, symmetrical, elongated, yellow or white in colour, and covered with a transparent membrane; Drosophila eggs have a pair of pale filaments near one end

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Fly larvae: symmetrical, elongated, tapering or baggy, faintly segmented, yellow or white in colour, with no legs and only a poorly defined head; dark mouth hooks inside the pointed end are helpful in their identification

Procedure — Canned citrus juices

Filter 250 ml of juice through a Büchner funnel fitted with a piece of 10 xx bolting cloth, using a wire mesh below the bolting cloth in the funnel as it aids suction

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Pour the juice slowly to avoid accumulation of excess pulp tissue on the filter cloth; if necessary, use 2 or 3 pieces of cloth

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Examine the filter cloth microscopically

Procedure — Strained baby foods

After extraction of the sample for insect and rodent filth, transfer the residue in the Wildman trap flask to a 2-litre separating funnel

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Add 100 ml of petrol, shake vigorously and allow to settle for 2 hr, stirring the surface layer occasionally to permit eggs and maggots to settle down

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Withdraw about 200 ml from the bottom of the separating funnel and filter this material through 10 xx bolting cloth, using several pieces of cloth if there is a large accumulation of sediment

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Examine microscopically at 15–20x

Procedure — Canned mushrooms

Grind the mushrooms with water on a No. 6 sieve

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Shake the sieve while under water, remove the sieve, filter the drained liquid through a rapid filter paper and examine under the microscope

Procedure — Dried mushrooms

Soak 100 g of sample in sufficient water overnight in a 2-litre Wildman trap flask (boiling for 30 min would also suffice)

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Add 30 ml of petrol and crush the pieces of mushroom; trap off twice, filter and examine microscopically

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Transfer the remaining mushrooms in the trap flask to a No. 8 sieve partially immersed in water; rub the mushrooms over the sieve to release any insects such as maggots

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Filter through 10 xx bolting cloth and examine microscopically

Precautions

• If eggs or larvae are present, repeat the swirling, standing and draining process twice more before filtering

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Official Methods of Analysis, 11th edn., Association of Official Analytical Chemists, Washington (1970) — cited in the book.

National Canners Association, Laboratory Manual for Food Canners and Processors, Vol. 1, The AVI Publishing Co., Westport, Connecticut, USA (1968) — cited in the book.