🍎 Determination of crude fibre content in tea by a gravimetric method — 2–3 g of sample ground to pass a 1 mm sieve is boiled for 30 min with 200 ml sulfuric acid (c(½H2SO4) = 0,255 mol/l), filtered through a wet filter paper, transferred into a flask with 200 ml sodium hydroxide (0,313 mol/l) and boiled for 30 min, the residue is transferred to a sintered glass crucible, washed with water, hydrochloric acid, ethanol and acetone, dried at 103 °C to constant mass (m1) and ashed at 550 °C for at least 1 h (m2); crude fibre = (m1 − m2) in percent by mass of the sample on a dry matter basis (from moisture according to ISO 1573), PN-ISO 15598

Food chemistry PN-ISO 15598

Overview

WHAT THE STANDARD COVERS. We read the text of the standard in the iTeh sample of ISO 15598:1999 (English text, first edition): foreword, Clauses 1–12, Annex A (results of the interlaboratory test) and the bibliography — the sample covers the whole standard. Part of a sentence in 11.1 is covered by a watermark.

ACCORDING TO THE TEXT OF ISO 15598:1999. The standard was prepared by ISO/TC 34 “Agricultural food products”, Subcommittee SC 8 “Tea”; Annex A is informative. Scope (1): determination of crude fibre content in tea. References (2): ISO 1573:1980 (loss in mass at 103 °C), ISO 3696 (water). Definition (3.1): crude fibre — all substances insoluble and combustible under the conditions of this standard, in percent by mass of the product on a dry matter basis. Principle (4): a suitably ground sample is treated successively with boiling sulfuric acid and sodium hydroxide solutions, the residue is filtered off, washed, dried, weighed and ashed; the loss on ashing is the crude fibre content. Reagents (5): grade 3 water according to ISO 3696; sulfuric acid stock c(½H2SO4) = 2,040 ± 0,040 mol/l (100 g/l; 275 ml concentrated acid to 5 l) and working 0,255 ± 0,005 mol/l (12,5 g/l; 125 ml stock to 1 l); sodium hydroxide stock 2,504 ± 0,040 mol/l (100 g/l; 500 g to 5 l) and working 0,313 ± 0,005 mol/l (12,5 g/l); octan-1-ol as antifoaming agent; hydrochloric acid 1 % (by volume); ethanol at least 95 %; acetone; with acids and alkali — gloves and face shield. Apparatus (6): hammer or centrifugal mill with a 1 mm screen; 1 l conical flasks; dispenser for 200 ml of hot liquid; heating unit for 1 l flasks; finger condensers (or reflux condensers with ground joints); Büchner flasks with Hartley funnels for 12,5 cm filters and adapters for 70 ml crucibles; sintered glass crucibles of porosity No. 1 or P 160 (pores 100–160 µm), plate diameter 40 mm, 70 ml; oven with fan at 103 ± 2 °C; muffle furnace at 550 ± 10 °C; desiccator; filter papers 12,5 cm, hardened, ashless, porosity 20–25 µm (e.g. Whatman No. 541). Sampling (7): not part of the method, recommended procedure in ISO 1839. Preparation (8): grinding to pass a 1 mm sieve. Procedure (9): when checking repeatability — two determinations; dry matter (wD) is calculated from the loss in mass at 103 °C according to ISO 1573; test portion 2–3 g to the nearest 0,001 g (m0) in a 1 l flask; 200 ml working sulfuric acid brought to the boil; 2–3 drops of octan-1-ol, condenser, bringing to the boil within 2 min and gentle boiling for 30 min with rotation of the flask — the times are critical; the hydrolysate is poured onto a wet filter in a Hartley funnel with a shallow layer of hot water, under light suction — filtration must be complete within 10 min, otherwise the determination is repeated with a smaller test portion; the flask is rinsed twice with about 50 ml boiling water; the residue is washed from the filter into the same flask with 200 ml working sodium hydroxide brought to the boil, octan-1-ol is added and boiled for 30 min as with the acid (times critical); the residue is transferred with boiling water into a sintered glass crucible; washed successively with about 50 ml of boiling water, hydrochloric acid solution and again boiling water, twice with ethanol and three times with acetone (in the text the cross-references in 9.3.9 give hydrochloric acid as 5.6, ethanol as 5.7 and acetone as 5.8, although in Clause 5 they are numbered 5.7, 5.8 and 5.9); drying at 103 °C for 2 h, cooling in a desiccator, weighing to the nearest 0,001 g, then after 1 h each until the difference between successive weighings is not more than 0,001 g (drying overnight permitted) — m1; ashing in the furnace at 550 °C for at least 1 h, cooling and weighing — m2. Calculation (10): w = (m1 − m2)/m0 × 100 × 100/wD, in percent by mass on a dry matter basis. Precision (11): data from the interlaboratory test may not apply to other ranges and matrices; r and R in Table A.1. Test report (12): sample identification, sampling method (if known), reference to the standard, details not covered by the standard and incidents, result (or final result when repeatability was checked). Annex A: test of 1994 under ISO auspices, 11 laboratories, five black tea samples; means 28,63; 9,67; 18,95; 22,85 and 9,35 % of dry matter; repeatability limit r (2,8 × sr) from 0,430 5 to 1,214 6, reproducibility R (2,8 × sR) from 1,512 3 to 3,459 1; repeatability coefficient of variation 0,94–2,42 %, reproducibility 3,85–6,98 %; data evaluated according to the withdrawn ISO 5725:1986.

STATUS (as of 03.10.2026). In the PKN search (term “15598”, 03.10.2026) PN-ISO 15598:2002 (Polish version, “Introduces: ISO 15598:1999 [IDT]”) has no withdrawal note; the second result, PN-EN 15598:2008, concerns textiles. Card: publication date 18.10.2002, 9 pages, KT 229 Coffee, Tea and Cocoa, ICS 67.140.10, no withdrawal note. In the iTeh catalogue (read 03.10.2026) ISO 15598:1999 has the status “Published”, stage 90.20 (international standard under systematic review), dated 15.07.2026.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, load up to 17.09.2026, read 03.10.2026). The number 15598 appears in 3 records at 3 laboratories: AB 285, AB 592, AB 1179. In the current PCA documents (read 03.10.2026; AB 285 issue No. 28 of 13.01.2026, AB 592 issue No. 24 of 06.02.2026, AB 1179 issue No. 21 of 08.07.2026) the number appears in 3 lines at the same 3 laboratories; in each document the number of page markers equals the number of PDF pages, without repetitions; a search of all PCA documents downloaded from the BIP on 03.10.2026 found no laboratories outside the database copy. The form is always “PN-ISO 15598:2002”, at AB 1179 together with “Indian Standard, Tea – determination of crude fiber content, 2012”. The lines with the number are not suspended (in the AB 1179 document a suspension at the laboratory’s request from 24.04.2026 to 21.10.2026 covers other items, marked in bold italics). The “Laboratories” tab (term “PN-ISO 15598”) shows all 3 laboratories — checked by a tab query on the labcoda.pl production server on 03.10.2026.

WHAT THE LABORATORIES TEST (items in PCA). Object: tea (all three). Characteristic and range: crude fibre content (8,0–28,9) % (AB 285); on a dry matter basis (10–30) % (m/m) (AB 592); (12,0–16,0) % (AB 1179). Technique: gravimetric method.

WHERE AN APPARENTLY CORRECT RESULT IS EASY TO GET. With the basis of the result: the standard gives the result on a dry matter basis, from moisture according to ISO 1573 (9.1, 10); AB 592 states this explicitly, AB 285 and AB 1179 give “%” without this information. With time: boiling with acid and alkali for 30 min each, bringing to the boil within 2 min — the times are critical (9.3.2, 9.3.7); filtration longer than 10 min requires repetition with a smaller test portion (9.3.4). With concentrations: working solutions of 0,255 and 0,313 mol/l with a tolerance of ± 0,005 (5.3, 5.5). With washing: the cross-references in 9.3.9 are shifted by one against the numbering of the reagents — the order of washing (water, hydrochloric acid, water, ethanol, acetone) follows from the names, not from the numbers. With precision: the data concern only black tea in the range 9,35–28,63 % of dry matter (Annex A, 11.1); the laboratories’ ranges lie within or close to this interval (8,0–28,9, 10–30, 12,0–16,0 %).

WHAT WE DO NOT GIVE. We have not read ISO 1573 or the Indian standard cited by AB 1179. We do not give requirements for tea from legislation or product standards.

Method principle

Ground tea is boiled successively for 30 min with the working sulfuric acid solution and for 30 min with the working sodium hydroxide solution, which removes the components soluble under these conditions. The residue is washed with water, hydrochloric acid, ethanol and acetone, dried at 103 °C to constant mass and ashed at 550 °C; the loss in mass on ashing is the crude fibre, expressed in percent by mass of the sample on a dry matter basis.

Applications

• Crude fibre content in tea (1)
• Black tea — range in which precision was determined: 9,35–28,63 % of dry matter (Annex A)
• Tea — items in the accreditation scopes of 3 laboratories in PCA

Key parameters

Standard

Standard number

PN-ISO 15598:2002

Title (PL)

Herbata — Oznaczanie zawartości włókna surowego

Title (EN)

Tea — Determination of crude fibre content

Step-by-step procedure

1. Sample

   Grinding to 1 mm; dry matter according to ISO 1573; test portion 2–3 g (8, 9.1, 9.2). PN-ISO 15598:2002 without a withdrawal note in PKN (03.10.2026).

2. Acid hydrolysis

   200 ml boiling acid, octan-1-ol, 30 min; filtration on a wet filter ≤ 10 min, rinsing with boiling water (9.3.1–9.3.5).

3. Alkaline hydrolysis

   Washing the residue off with 200 ml boiling NaOH, 30 min; transfer to the crucible (9.3.6–9.3.8).

4. Washing and drying

   Water, HCl, water, ethanol ×2, acetone ×3; 103 °C to constant mass — m1 (9.3.9, 9.3.10).

5. Ashing and result

   550 °C ≥ 1 h — m2; w = (m1 − m2)/m0 × 100 × 100/wD (9.3.11, 10).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Boiling sulfuric acid and sodium hydroxide — gloves and face shield, beware of splashes (warnings in 5 and 9.3)
• Ethanol and acetone — flammable; muffle furnace at 550 °C — hot crucibles

Frequently asked questions