🍎 Determination of sulfite in foodstuffs expressed as sulfur dioxide by the optimized Monier-Williams method — the test portion (50 g or a quantity containing 500–1500 µg SO2) in ethanol–water mixture is heated with refluxing hydrochloric acid 4 mol/l for 105 min, nitrogen (200 ml/min) sweeps sulfur dioxide through the condenser into 3 % hydrogen peroxide, and the generated sulfuric acid is titrated with 0.010 mol/l sodium hydroxide solution against methyl red; foodstuffs with at least 10 mg/kg sulfite, PN-EN 1988-1

Food chemistry PN-EN 1988-1

Overview

WHAT THE STANDARD COVERS. We read the text of the standard in the iTeh sample of SIST EN 1988-1:1999 (English text of EN 1988-1:1998, scan): from the title page through the contents, foreword, introduction and Clauses 1–6 to the beginning of Clause 7 (precision) on page 7 — the last visible value is the reproducibility limit for hominy. Outside the sample remain the rest of 7.3, the test report (8), Annex A (bibliography) and Annex B (precision data).

ACCORDING TO THE TEXT OF EN 1988-1:1998. The standard was prepared by CEN/TC 275 “Food analysis — Horizontal methods”, whose secretariat is held by DIN; CEN approved it on 01.01.1998, and national implementation was due by August 1998 at the latest. The standard “Foodstuffs — Determination of sulfite” has two parts: 1 — optimized Monier-Williams method and 2 — enzymatic method. According to the introduction, sulfite can be used as a food preservative, and because of possible negative health effects many countries have regulated its use, which has led to many methods for detecting and quantifying sulfite in a great variety of foods. Scope (1): a distillation method for the determination of the sulfite content, expressed as sulfur dioxide, in foodstuffs containing at least 10 mg/kg sulfite; it is applicable in the presence of other volatile sulfur compounds; it is not applicable to cabbage, dried garlic, dried onions, ginger, leeks and soy proteins — according to a footnote, analysis of isolated soy protein gives false positive results in the range 20–30 mg/kg as SO2, so for foodstuffs containing isolated soy protein the result may be proportionally enhanced and this is taken into account; products for which European Standards for the determination of sulfites exist are excluded from the scope of this horizontal standard. Reference (2): EN ISO 3696. Principle (3): free sulfite plus a reproducible portion of bound sulfites (e.g. carbonyl addition products) is measured; the test portion is heated with refluxing hydrochloric acid solution, which converts sulfite to sulfur dioxide; a stream of nitrogen introduced below the surface of the solution sweeps the sulfur dioxide through a water-cooled condenser and an attached bubbler into hydrogen peroxide solution, where it is oxidized to sulfuric acid; the acid is titrated with standardized sodium hydroxide solution, and the sulfite content is directly related to the sulfuric acid generated. Reagents (4): reagents of recognized analytical grade and water of at least grade 3 according to EN ISO 3696; hydrochloric acid 4 mol/l — for each analysis 90 ml (30 ml of concentrated 36 % acid carefully into 60 ml of water), prepared daily; sodium hydroxide standard solution 0.010 mol/l; methyl red (250 mg in 100 ml of ethanol); hydrogen peroxide 3 % (volume fraction) — just before use titrated with sodium hydroxide after adding 3 drops of methyl red to a yellow end point, and discarded if the end point is exceeded; ethanol or methylated spirit; ethanol–water mixture with 5 % ethanol; high-purity nitrogen with a regulator maintaining 200 ml/min — oxygen in the nitrogen is guarded against with an absorber as used in gas chromatography or a gas-washing bottle with alkaline pyrogallol solution (4.5 g of pyrogallol, purging with nitrogen for 2–3 min, a solution of 65 g of potassium hydroxide in 85 ml of water added under nitrogen; heat is generated on dissolution). Apparatus (5): distillation apparatus (Figure 1) — inlet adapter with rubber bulb to apply head pressure above the solution (a pressure-equalizing dropping funnel is not recommended, because condensate possibly containing sulfur dioxide is deposited in the funnel and side arm), cylindrical dropping funnel of at least 100 ml, 1 l round-bottomed flask with three leakproof joints, gas inlet tube reaching within 25 mm of the bottom of the flask, bulb condenser with 300 mm jacket, glass bubbler with dimensions according to Figure 2, vessel of about 25 mm inner diameter and 180 mm length; according to a note, keeping back pressure as low as possible avoids losses of sulfur dioxide through leaks — a thin film of stopcock grease on all joints except the joint between the dropping funnel and the flask, and clamped joints, help; according to a footnote, the description of the apparatus is informative, and equivalent glassware (e.g. the distillation apparatus described in ISO 5522:1981) may be used if it leads to the same results; 10 ml burette with overflow tube and an absorber (sodium hydroxide on silica) protecting the sodium hydroxide solution from carbon dioxide; condenser coolant, e.g. a circulating mixture of 1 part methanol and 2 parts water or water flow, at not more than 15 °C; heating mantle controllable between 20 and 120 °C; food processor or blender; oxygen absorber. Procedure (6): sample preparation and analysis are carried out as quickly as possible to avoid loss of labile forms of sulfite (6.1); according to a note, before routine use it is recommended to practise the method on foods with known amounts of sulfite — portions are fortified with a stable source of sulfite, not sodium sulfite or similar salts; sodium hydroxymethylsulfonate (HMS), a bisulfite addition product of formaldehyde structurally similar to some combined forms of sulfite in foods, is useful — 50 g of prepared sulfite-free food in the flask, an aliquot of aqueous HMS sodium salt solution, analysed immediately; HMS recoveries of more than 80 % from matrices fortified at 10 mg/kg are recommended for accurate data. Solid samples (6.2.1): 50 g of food or a quantity containing 500–1500 µg of sulfur dioxide is briefly ground with 100 ml of ethanol–water mixture only until pieces pass through the ground-glass joint of the flask; liquid samples (6.2.2): 50 g or a quantity containing 500–1500 µg of sulfur dioxide is mixed with 100 ml of ethanol–water mixture. System preparation (6.3): flask in the heating mantle, 400 ml of water, funnel stopcock closed and 90 ml of hydrochloric acid in the funnel, nitrogen flow (200 ± 10) ml/min, coolant, 30 ml of 3 % hydrogen peroxide in the vessel; after 15 min the apparatus and water are deoxygenated. Sample introduction and distillation (6.4): after removing the funnel, the test portion in aqueous ethanol is transferred quantitatively to the flask, the joint is wiped and greased, the funnel returned and every joint checked for sealing; head pressure is applied with a rubber bulb with valve and the acid let into the flask, closing the stopcock before the last 2–3 ml drain out, so that sulfur dioxide does not escape into the funnel; the power is set so that 80–90 drops of condensate per minute return from the condenser, the flask contents boil for 105 min, and then the hydrogen peroxide vessel is removed. Determination and calculation (6.5): the vessel contents are titrated immediately with sodium hydroxide solution to a yellow end point persisting longer than 20 s; the mass fraction of sulfite as sulfur dioxide in mg/kg, rounded to a whole number, is w = 32.03 × V × 1000 × N / m, where 32.03 is the milliequivalent weight of SO2, N the molarity of the sodium hydroxide solution, V the volume of 0.010 mol/l solution used to the end point in ml, m the mass of the test portion introduced into the flask in g; a reagent blank is determined by titration and results are corrected if necessary. Precision (7): interlaboratory test according to ISO 5725:1986, details in Annex B; the values may not apply to other concentration ranges and matrices; repeatability limit r (exceeded in not more than 5 % of cases): hominy at a mean of 9.2 mg/kg — 3.7 mg/kg, fruit juice at 8.1 mg/l — 3.8 mg/l, sea food at 10.4 mg/kg — 4.1 mg/kg; reproducibility limit R for hominy at 9.2 mg/kg — 4.0 mg/kg.

STATUS (as of 03.10.2026). The PKN search (query “PN-EN 1988-1”, 03.10.2026) shows PN-EN 1988-1:2001 (Polish version, “Wprowadza: EN 1988-1:1998 [IDT]”) without a withdrawal note; the other two results are PN-EN 61988-1 standards on plasma display panels. In the iTeh catalogue (read on 03.10.2026) EN 1988-1:1998 has the status “Published”, stage 90.93 (decision to confirm the standard).

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, load up to 17.09.2026, read on 03.10.2026). The number 1988-1 appears in 5 records at 5 laboratories: AB 519, AB 540, AB 565, AB 583, AB 1179. In the current PCA documents (read on 03.10.2026, issues from 03.11.2025 to 01.09.2026) the number appears in 5 rows — one at each; in each of the five documents the number of page markers equals the number of PDF pages, without repetitions; a search of all 1291 PCA documents downloaded from BIP on 03.10.2026 found no laboratories outside the database copy (17 laboratories in the register have no link to a scope document on BIP). Form at all: “PN-EN 1988-1:2001”, at AB 583 together with procedure PB-26/HŻ edition 2 of 29.09.2025. None of these items is suspended (in the AB 1179 document the suspension applies to items marked in bold italics, and the row with PN-EN 1988-1 is not so marked). The “Laboratories” tab (query “PN-EN 1988-1”) shows all 5 laboratories — checked with a tab query on the labcoda.pl production server on 03.10.2026.

WHAT THE LABORATORIES TEST (items in PCA). Test object: dried fruit (AB 519); fruit and vegetable products (AB 540); food — mushrooms and products, vegetables and vegetable products, fruit and fruit products, confectionery and bakery products, mayonnaises, sauces, mustards, alcoholic beverages, cereal products, fish, seafood and their products (AB 565); cereals and cereal products (AB 583); coffee and tea, food concentrates, meat and meat products, non-alcoholic and alcoholic beverages, fruit and vegetables and their products, sweets and confectionery, herbal raw materials and products, spices, cereals and cereal products, ready meals, food additives — except cabbage, dried garlic, dried onions, ginger, leeks and soy proteins — and environmental samples from food production and trade areas: fluids after washing and/or disinfection of production lines (AB 1179). Characteristic: sulfite and sulfur dioxide content as SO2 (AB 519, AB 1179), total sulfur dioxide content (AB 540), sulfite content (AB 565), sulfur dioxide content (AB 583). Ranges: (10–4460) mg/kg (AB 519), (5.0–3000) mg/kg or mg/l (AB 540), (10–2300) mg/kg (mg/l) SO2 (AB 565), (10–100) mg/kg (AB 583), (10–3421) mg/kg (AB 1179). Technique: titrimetric method after distillation (AB 519), titrimetric method (AB 540, AB 565), distillation-titrimetric method (AB 583), Monier-Williams distillation-titrimetric method (AB 1179).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. At the scope: the standard covers foodstuffs with at least 10 mg/kg sulfite (1) — the lower limit of the AB 540 range (5.0 mg/kg or mg/l) lies below this value. At the matrix: cabbage, dried garlic and onions, ginger, leeks and soy proteins are excluded, and isolated soy protein gives false positive results of 20–30 mg/kg (1). At time: preparation and analysis as quickly as possible, because labile forms of sulfite are lost (6.1). At tightness: the funnel stopcock is closed before the last 2–3 ml of acid drain out, and the joints must be sealed (5.1, 6.4). At oxygen: the apparatus is deoxygenated with nitrogen for 15 min, and the nitrogen is protected by an oxygen absorber (4.7, 6.3). At the peroxide: the solution is neutralized just before use and discarded if the end point is exceeded (4.4). At the check: fortification with HMS, not sodium sulfite, with recovery above 80 % at 10 mg/kg (6.1). At the result: a whole number in mg/kg as SO2, after blank correction (6.5).

WHAT WE DO NOT GIVE. We did not read further reproducibility values, the content of the test report (8) or the precision data in Annex B. We do not give maximum permitted sulfite levels in foods.

Method principle

A food test portion (50 g or a quantity with 500–1500 µg SO2) in ethanol–water mixture is boiled in a deoxygenated distillation apparatus with hydrochloric acid 4 mol/l for 105 min. A stream of nitrogen (200 ml/min) sweeps the released sulfur dioxide through the condenser and bubbler into 3 % hydrogen peroxide, where it is oxidized to sulfuric acid; the acid is titrated with 0.010 mol/l sodium hydroxide solution against methyl red, and the sulfite content is calculated as SO2 in mg/kg. Free sulfite plus a reproducible portion of bound sulfites is determined.

Applications

• Sulfite (as SO2) in foodstuffs containing at least 10 mg/kg, also in the presence of other volatile sulfur compounds (1)
• Exclusions: cabbage, dried garlic, dried onions, ginger, leeks, soy proteins and products with their own European Standards for sulfite determination (1)
• Dried fruit, fruit and vegetable products, cereal products, fish and seafood, beverages, spices, fluids after washing production lines — items in the accreditation scopes of 5 laboratories in PCA

Key parameters

Standard

Standard number

PN-EN 1988-1:2001

Title (PL)

Artykuły żywnościowe — Oznaczanie zawartości siarczynów — Część 1: Zoptymalizowana metoda Monier-Williamsa

Title (EN)

Foodstuffs — Determination of sulfite — Part 1: Optimized Monier-Williams method

Step-by-step procedure

1. Sample

   50 g (or 500–1500 µg SO2) with 100 ml ethanol–water; solid samples briefly ground; quickly, so as not to lose labile sulfites (6.1, 6.2). PN-EN 1988-1:2001 without a withdrawal note (03.10.2026).

2. System preparation

   400 ml of water in the flask, 90 ml HCl in the funnel, nitrogen (200 ± 10) ml/min, 30 ml H2O2 in the vessel, deoxygenation 15 min (6.3).

3. Introducing portion and acid

   Portion quantitatively into the flask, sealed joints; acid under head pressure, stopcock closed before the last 2–3 ml (6.4).

4. Distillation

   80–90 drops of condensate per minute, boiling 105 min, removal of the vessel (6.4).

5. Titration and result

   NaOH 0.010 mol/l to a yellow end point > 20 s; mg/kg as SO2, whole number, blank correction (6.5).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Add concentrated hydrochloric acid carefully to water (4.1)
• Dissolving potassium hydroxide for the pyrogallol solution generates heat (4.7)

Frequently asked questions