SOP: Determination of Extraneous Material in Wet and Dried Bean Curd
GIFSQ — Global Institute for Food Safety & Quality
1. Principle
Wet and dried bean curd is examined for insect infestation, mites, rodent hairs and other extraneous material to determine compliance with Sections 4, 5 and 7 of the Food and Drugs Act. Light filth is isolated by acid digestion and defatting, separated by mineral-oil flotation in a percolator, collected on ruled filter paper and examined microscopically. Heavy filth is isolated from the retained percolator drainings by sedimentation and chloroform separation, weighed on ashless filter paper, examined microscopically, and ashed if appreciable.
2. Apparatus required
- Balance
- 2.0 L beakers with lids (watch glasses)
- Smooth magnetic stirring bar, Teflon coated (1 × 5 cm)
- Stirrer hot plates
- U.S. standard No. 230 sieve
- Shallow pan, ≥ 20 cm in diameter
- 2 L percolator, premarked at 250 and 1700 mL levels
- Rubber policeman attached to a long glass rod
- Wash bottles
- Suction filtration apparatus with Buchner or Hirsch funnel (5–7 cm perforated plate)
- Ruled filter paper — larger than the funnel plate (7–9 cm)
- Petri dishes to suit the size of filter paper used
- Stereoscopic microscope (10–30×)
- Ashless filter paper — larger than the funnel plate (7–9 cm)
- Drying oven (40–60 °C)
- Crucible
- Desiccator
- Muffle oven
3. Materials and reagents
- Concentrated HCl
- Antifoam solution — Antifoam “B”
- Detergent — 1% sodium lauryl sulphate
- To prepare: dissolve 1 g of sodium lauryl sulphate in distilled water and make up to 100 mL.
- Mineral oil — paraffin oil, white, light, 125/135 Saybolt Universal Viscosity at 38 °C, specific gravity 0.840–0.860 at 24 °C
- Alcohol — either 95% ethanol or 99% isopropanol
- Glycerol–alcohol (95%) mixture (1:1)
- To prepare: mix equal volumes of glycerol and 95% ethanol (for example, 50 mL of each).
- Chloroform (CHCl₃)
4. Sample collection and preparation
- A lot is the amount of the food (volume, weight, etc.) produced, stored and/or shipped under conditions as nearly uniform as possible, preferably designated by a common container code or marking, and in any event consisting of not more than one variety, grade or type of product from a single identifiable source.
- Scrutinize the entire lot for live infestation. If live infestation is found, do not sample until after fumigation or other effective treatment.
- Obtain three sample units selected at random from the lot, of at least 100 g each, using appropriate sampling equipment and containers. Three sample units constitute a sample.
- Keep each sample unit separate and label them 1, 2 and 3. Record complete information on the lot size, the weight of individual containers, the country of origin, the exporter, the importer or domestic manufacturer, and product and lot identification; this information accompanies the sample.
5. Procedure
If appropriate, freeze the samples prior to analysis to minimize the possibility of a live infestation of the laboratory
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Weigh 100 g of bean curd at random from a sample unit — this 100 g constitutes one analytical unit
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Repeat for the remaining two sample units
Light filth isolation
Transfer one analytical unit into a 2 L beaker. Add 1 L HCl (70 mL concentrated HCl + 930 mL water), 0.3 mL of antifoam solution and a magnetic stirring bar
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Boil with constant stirring until the mixture is finely dispersed (≥ 15 min
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If any fat material is floating on the HCl suspension, defat: wet-sieve on a No. 230 sieve with hot (50–70 °C) tap water to remove all liquid and a major proportion of fine material
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Place the sieve in a shallow pan, cover the residue to a depth of about 2 cm with hot (50–70 °C) detergent (1% sodium lauryl sulphate), leave for 5 min, drain and wet-sieve with hot tap water until all suds have washed through — be careful not to let suds foam over the sides of the sieve
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Repeat the defatting at least once, or until all visible fat has been removed. Transfer the sieve retainings to the original beaker with hot water (50–70 °C), dilute to about 1 L and bring to a boil
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Add 50 mL of mineral oil and continue heating until boiling resumes
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Transfer the beaker to a cool magnetic stirrer and stir magnetically at the maximum speed that does not produce visible or audible splashing, for 5 min
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Promptly transfer the beaker contents to a percolator containing about 250 mL of water. Rinse the beaker contents into the percolator with hot tap water, making certain to transfer all material including heavy filth. Bring the percolator volume to 1700 mL with hot tap water. After 1 min, stir the percolator contents with a glass rod
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Using wash bottles, rinse magnetic particles from the stirring bar into a beaker — first with water, then with alcohol and finally with detergent — until all particles are washed off. Filter the washings through ruled filter paper using the suction filtration apparatus. Transfer the filter paper to a petri dish and moisten with glycerol–alcohol mixture
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After 2 min, drain the oil interface in the percolator to 250 mL into a 2 L beaker. Retain the beaker contents for the heavy filth determination
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Refill the percolator by pouring hot tap water down the percolator sides to loosen adhering material, and refill to the 1700 mL mark
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After 2–3 min, drain the percolator to 250 mL, discard the drainings and refill — for 2 or more cycles. The lower layer should be almost free of suspended material after refill; if not, continue through ≥ 1 recycles
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Finally, drain the oil-water interface to the 250 mL mark and drain the remaining contents into the original beaker. Promptly, using wash bottles, wash down the sides of the percolator successively with ≥ 50 mL portions of hot tap water, alcohol and hot tap water. Use detergent if necessary
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Using the suction filtration apparatus, filter the beaker contents through ruled filter paper with a minimum of 50 mL washes each of water, alcohol, and water and/or detergent. Use a rubber policeman if necessary to clean the sides of the beaker
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Transfer the filter paper(s) to petri dish(es) and moisten with glycerol–alcohol mixture
Light filth examination and recording
Examine the filter papers microscopically at 30× for all types of extraneous material
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Record the number of whole insects and equivalent whole insects for each analytical unit separately. An equivalent whole insect is one in which more than one-half of the head, including the entire frons, is present
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Record the number and size range of insect fragments, rodent hairs and other types of light filth detected, for each analytical unit separately
Heavy filth isolation
Stir the contents of the beaker containing the drainings retained from the light filth isolation with a rubber policeman for 30 sec, and allow the suspension to stand undisturbed for 30 min
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Decant the suspended food material without disturbing the bottom contents
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Add 400–500 mL of water, stir 30 sec and let stand 10 min
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Repeat the decanting cycle until practically all plant tissue has been decanted
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If any heavy filth is present, add 100–200 mL of CHCl₃, stir 30 sec, let stand 10 min, then decant the water layer (the top layer) without disturbing or pouring off any of the CHCl₃–water interface
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Filter the beaker contents onto an oven-dried, pre-weighed ashless paper. Transfer the filter paper to a petri dish
Heavy filth examination and recording
Examine the filter paper microscopically at 30× for rodent pellets, insect excreta, insect larvae, insect eggs and other organic filth
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Record the number and size range of each type of heavy filth detected — for each analytical unit separately
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Dry the filter paper in an oven and weigh. Determine the weight of heavy filth by difference, and record
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If there is an appreciable amount of residue, transfer the filter paper with contents to a pre-weighed crucible and ash in a muffle oven at about 500 °C for two hours. Cool the material in a desiccator and weigh. Determine the ashed weight by difference, and record
6. Calculation
Weight of heavy filth = W₂ − W₁
Ashed weight = W₄ − W₃
Where,
- W₁ = weight of the oven-dried, pre-weighed ashless filter paper
- W₂ = weight of the dried filter paper with heavy filth
- W₃ = weight of the pre-weighed crucible
- W₄ = weight of the crucible with ashed material, cooled in a desiccator
7. Precautions
- If live infestation is found in the lot, do not sample until after fumigation or other effective treatment.
- If appropriate, freeze the samples prior to analysis to minimize the possibility of a live infestation of the laboratory.
- During defatting, do not let suds foam over the sides of the sieve.
References
Health Canada, Compendium of Analytical Methods, Volume 4 — Laboratory Procedure ExFLP-20, January 2019: “Determination of Extraneous Material in Wet and Dried Bean Curd” (Extraneous Matter and Food Packaging Unit, Greater Toronto Area Laboratory, Canadian Food Inspection Agency; replaces ExFLP-20 dated April 1997).
The method’s own bibliography:
- Pietrzak, E. 2009. Guidelines for the General Cleanliness of Food – An Overview. In: Volume 1. Compendium of Analytical Methods.