Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO 28198:2018 — Vegetable Fats and Oils — Determination of Toluene Insoluble Matter in Lecithin Formulations
Technical Article
1. Purpose and Scope of ISO 28198:2018
ISO 28198:2018 specifies a method for the determination of toluene insoluble matter (TIM) content in lecithin formulations derived from vegetable fats and oils. Lecithin (Codex Alimentarius INS No. 322) is one of the most widely used food additives globally, serving as an emulsifier, stabilizer, dispersant, and release agent in chocolate, confectionery, baked goods, margarine, dietary supplements, cosmetics, and industrial applications. The TIM content directly indicates the presence and quantity of impurities such as proteinaceous residues from incomplete extraction, carbohydrate-containing extraction residues, and other solid contaminants that affect the quality, functionality, and regulatory compliance of lecithin products. TIM values are established in the regulatory frameworks of many countries as a critical purity parameter for lecithin, making accurate determination essential for both domestic sale and international trade.
Developed by ISO/TC 34 Food products, Subcommittee SC 11 Animal and vegetable fats and oils, this second edition replaces ISO 28198:2009 with an important addition: a procedure for handling turbid samples that do not clarify within 12 hours. The key improvement is the replacement of carcinogenic benzene used in the previous method with toluene, significantly improving laboratory occupational safety while maintaining equivalent or better analytical performance. This change reflects the ongoing commitment in analytical chemistry to reduce the use of hazardous reagents while maintaining stringent analytical performance standards. The method is applicable to all lecithin formulations including fluid lecithin, de-oiled lecithin, enzymatically modified lecithin, and fractionated lecithin products.
The TIM value is a critical purity parameter specified in the regulatory frameworks of many countries worldwide. The replacement of benzene with toluene in this edition represents a significant improvement in laboratory safety.
2. Principle and Detailed Procedure
The method is based on a straightforward gravimetric principle: the sample is dissolved in toluene and filtered through a glass filter crucible of specified porosity P 40 (16 to 40 micrometres pore size). The insoluble residue retained on the filter is dried at 103 degrees C and weighed, with the TIM content expressed in grams per 100 grams of sample. The precision and accuracy of the method depend critically on careful filtration technique, precise temperature control, and rigorous quality control procedures throughout the analysis.
| Step | Description | Key Parameters |
|---|---|---|
| 1. Crucible preparation | P40 crucible dried at 103 deg C for 1 h, cooled in desiccator, weighed to 0.1 mg | P40 porosity 16-40 micrometres, 30 ml capacity; phosphate-free ultrasonic cleaning, max 10 analyses per crucible |
| 2. Sample preparation | Heat to max 60 deg C, stir vigorously for homogeneity | Record any heat pre-treatment applied |
| 3. Test portion | Weigh approximately 10.00 g to nearest 0.01 g | Reduce to about 5.00 g if TIM expected above 0.3 g/100g |
| 4. Dissolution | Dissolve in 100 ml toluene, stir with glass rod | If not clear after 12 h, filter directly per clause 8.3 |
| 5. Filtration | Vacuum filtration, rinse twice with 25 ml toluene | Pre-weighed crucible, ensure quantitative transfer |
| 6. Drying to constant mass | 103 deg C for 2 h, repeat 30-min cycles until mass change < 0.5 mg | Volatile toluene evaporates in fume hood |
Sample pre-treatment is critical: if the lecithin is not completely liquid at room temperature, it must be heated in an oven at a temperature not exceeding 60 degrees C and stirred vigorously with a glass rod immediately before weighing to ensure homogeneity. Any heat treatment applied must be recorded in the test report. For samples with TIM values expected to exceed 0.3 g/100g, the test portion may be reduced to approximately 5.00 g to avoid overloading the filter crucible and compromising filtration efficiency. The standard provides clear guidance on handling turbid or slow-settling samples, which can occur with certain lecithin types containing fine particulate matter.
Toluene is a hazardous solvent. The standard emphasizes safety measures including the use of fume hoods, local exhaust ventilation, and appropriate personal protective equipment. Operators must be trained in safe toluene handling procedures.
3. Method Precision and Interlaboratory Validation
The precision of the method was established through two comprehensive interlaboratory studies conducted in 1997 and 2007. The 1997 study involved 6-7 laboratories testing 3 crude lecithin samples, while the 2007 study was expanded to 12-14 laboratories testing 6 samples covering a broader product range: 2 crude lecithins, 2 de-oiled lecithins, 1 enzymatically hydrolyzed lecithin, and 1 fractionated lecithin. The studies conclusively demonstrated that P 100 crucibles produced significantly lower and inconsistent results, making them unsuitable for TIM determination and establishing P 40 as the mandatory porosity for this method. The repeatability standard deviation (Sr) ranges from 0.008 to 0.024 g/100g, and the reproducibility standard deviation (SR) ranges from 0.016 to 0.044 g/100g depending on the sample matrix, demonstrating good method precision across the range of lecithin product types covered by the standard.
Engineering Design Insights for Quality Control Laboratories
For quality control laboratories implementing this method, attention to filtration technique and temperature control is essential for obtaining reliable results. The crucible cleaning and maintenance protocol is critical: phosphate-free alkaline solution with ultrasonic cleaning, machine washing, and a strict limit of 10 analyses per crucible to prevent systematic errors from pore clogging. The standard provides practical guidance for handling turbid or slow-settling samples: if the solution has not sufficiently clarified after 12 hours, filtration should proceed directly and this should be noted in the report. From a method validation perspective, laboratories should establish internal quality control procedures including periodic verification using reference materials with known TIM values and participation in proficiency testing schemes to ensure ongoing analytical competence. The replacement of benzene with toluene requires laboratories to update their safety protocols and ensure proper ventilation systems are in place, as toluene although less hazardous than benzene still requires careful handling with appropriate engineering controls and personal protective equipment.
The replacement of carcinogenic benzene with toluene represents a significant advance in laboratory safety without compromising analytical performance. The establishment of P 40 as the mandatory crucible porosity ensures consistent method accuracy across laboratories.
4. FAQs
Q: What does the TIM value indicate about lecithin quality?
A: TIM measures insoluble impurities including protein residues and solid contaminants. Lower TIM values indicate higher purity. Regulatory frameworks worldwide specify maximum TIM limits for different lecithin grades.
A: TIM measures insoluble impurities including protein residues and solid contaminants. Lower TIM values indicate higher purity. Regulatory frameworks worldwide specify maximum TIM limits for different lecithin grades.
Q: Why must P 40 crucibles be used instead of P 100?
A: Interlaboratory studies proved that P 100 crucibles produce unacceptably low results because insoluble particles pass through the larger pores, making P 40 mandatory for this method.
A: Interlaboratory studies proved that P 100 crucibles produce unacceptably low results because insoluble particles pass through the larger pores, making P 40 mandatory for this method.
Q: How many times can a filter crucible be reused?
A: Each crucible may be used for a maximum of 10 determinations, after which pores become clogged with accumulated residues affecting filtration efficiency and result accuracy.
A: Each crucible may be used for a maximum of 10 determinations, after which pores become clogged with accumulated residues affecting filtration efficiency and result accuracy.
Q: How should turbid or slow-settling samples be handled?
A: If the solution has not sufficiently clarified after 12 hours, proceed with filtration directly as specified in clause 8.3 and note this in the test report.
A: If the solution has not sufficiently clarified after 12 hours, proceed with filtration directly as specified in clause 8.3 and note this in the test report.
