SOP: Determination of Fibre Content of Green Beans by the Blending–Screening Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
As green beans mature they become more fibrous, and the fibre contributes to the toughness of the product. In the simplified Kramer procedure the pods are blended with water, the fibrous material is washed free on a standard screen, dried and weighed; the weight of the dried fibre, expressed as a percentage of the sample weight, is the fibre content. Beans containing more than 0.15% crude fibre (by the AOAC acid–alkali digestion) are considered substandard; in this simplified method, values below 0.07% are safely within the standard level for fibrousness, whereas 0.09% indicates the limit of fibrousness when only the pod walls are used.
Apparatus required
- Blender
- 30 mesh monel metal screen, 4 inch diameter with side walls 1 inch high (previously weighed — weight A)
- Hot air oven, at 100 ± 2 °C
- Desiccator
- Balance
Reagents
a. Water.
Procedure
If raw green beans are used, first cook the material by boiling in water for about 20 min or under pressure for about 5 min (with canned products this treatment is unnecessary, since the seeds are finely comminuted in the blender — with canned products the whole beans may be comminuted)
↓
Blend 100 g of deseeded pods (or of whole beans for canned products) with 200 ml of water in a blender for 5 min
↓
Move the blended material onto the previously weighed 30 mesh monel metal screen and rinse with water until the fibrous residue runs clear
↓
Dry the screen and the fibrous material for 2 hr at 100 ± 2 °C in a hot air oven
↓
Cool in a desiccator and weigh (weight B)
Calculation
Fibre content, % = [(B − A) / sample weight] × 100
Where,
- A = weight of the empty screen, g
- B = weight of the screen plus the dried fibrous material, g
Sample weight = 100 g
Values below 0.07% are safely within the standard level for fibrousness.
- 0.09% indicates the limit of fibrousness when only the pod walls are used.
- Wax or French green beans containing more than 0.15% crude fibre (AOAC acid–alkali digestion) are considered substandard.
Precautions
- Wash until the fibrous material is completely clear; residual pulp inflates the result.
- Cook raw beans before blending so that tissues other than fibre are separated.
References
Kramer, A., et al., (simplified fibre method — the book’s citation reference is illegible in the extraction).
Association of Analytical Chemists, Official Methods of Analysis (12th edn.) (1975), AOAC — cited in the book for the crude fibre method (see Chapter 1 of the book) and for the fibrous matter in frozen French beans by cold-alkali maceration (see Chapter 27 of the book).
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.