🔥 Total ash in food
Determination of ash (mineral substances) content in food by combustion in a muffle furnace at 550°C. Measure of mineral content.
Overview
Total ash is a measure of total inorganic (mineral) substance content in food. The determination involves burning (ashing) the sample in a muffle furnace at 550 ± 25°C until complete removal of organic matter. The inorganic residue (ash) is weighed and its mass is expressed as a percentage of the sample mass.
The method is fundamental to food composition analysis — along with protein, fat, and moisture it forms the so-called proximate analysis. Ash corresponds to the content of macro- and microelements (Ca, P, K, Na, Mg, Fe, Zn, etc.) in the form of oxides, carbonates, and phosphates. Ash determination result is required on nutritional value labels of many products, especially feed (EC Reg. 767/2009).
Standard PN-ISO 936 concerns meat and meat products, but analogous methods are used for cereals (ISO 2171), milk (ISO 5545), fruits and vegetables. Temperature of 550°C is a compromise — high enough to burn organic matter, but low enough to prevent volatilization of some minerals (e.g., sodium and potassium chlorides).
The method is simple, inexpensive, and does not require specialized equipment beyond a muffle furnace, but is time-consuming — complete ashing takes 6–12 hours (usually conducted overnight). The ash result is sometimes used to calculate carbohydrate content "by difference" (100% − protein − fat − moisture − ash = carbohydrates).
Method principle
A sample portion in a pre-ashed and weighed porcelain crucible is pre-carbonized on a hot plate or over a gas burner (avoiding ignition), then placed in a muffle furnace heated to 550 ± 25°C. The sample is ashed for minimum 3–6 hours until the ash becomes white or light gray (no black carbon particles). After cooling in a desiccator, the crucible is weighed. Ash content is calculated gravimetrically: ash [%] = (ash mass / sample mass) × 100.
Applications
- Composition analysis of meat and meat products (PN-ISO 936)
- Feed quality control (EC Reg. 767/2009)
- Total mineral determination in cereals and cereal products
- Food proximate analysis (protein + fat + moisture + ash)
- Nutritional value labeling control
- Flour quality assessment (ash as milling degree indicator)
- Testing of special dietary foods
Key parameters
| Parameter | Value |
|---|---|
| Ashing temperature | 550 ± 25°C |
| Ashing time | Min. 3 h (typically 6–12 h, overnight) |
| Sample size | 2–5 g |
| Repeatability | ±0.05–0.1% ash |
| Weighing accuracy | 0.001 g (analytical balance) |
Standard
- Standard number
- PN-ISO 936:2000
- Title (PL)
- Mięso i przetwory mięsne — Oznaczanie popiołu całkowitego
- Title (EN)
- Meat and meat products — Determination of total ash
Step-by-step procedure
1. Crucible preparation
Ash porcelain crucibles in muffle furnace at 550°C for 1 h. Cool in desiccator. Weigh with 0.001 g accuracy.
2. Sample weighing
Weigh 2–5 g homogenized sample into prepared crucible. Record mass with 0.001 g accuracy.
3. Pre-evaporation (wet samples)
High-moisture samples are pre-dried in oven at 103°C or evaporated on hot plate.
4. Carbonization
Place crucible on hot plate or over burner. Heat gradually until sample carbonizes (black color). Avoid ignition!
5. Ashing in muffle furnace
Transfer crucible to muffle furnace heated to 550°C. Ash minimum 3 h (typically overnight, 8–12 h).
6. Combustion completeness check
Ash should be white or light gray. If black particles visible — add few drops HNO₃ or H₂O₂ and repeat ashing 1 h.
7. Cooling
Turn off furnace. When temperature drops to ~200°C, transfer crucibles to desiccator. Cool to room temperature.
8. Weighing
Weigh crucible with ash with 0.001 g accuracy. Repeat ashing (1 h) and weighing until constant mass (difference < 0.001 g).
9. Calculations
Ash [%] = (ash mass / sample mass) × 100. Report result to 0.01% accuracy. Result is average of two parallel determinations.
10. Quality control
Perform duplicates. Difference between parallel determinations should not exceed 0.1% ash. Analyze CRM every series.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Muffle furnace | Nabertherm L9/11, Carbolite CWF 1100, POL-EKO FCF 5S | 8 000–25 000 PLN |
| Porcelain / quartz crucibles | Porcelain crucibles 30–50 mL (Haldenwanger, Morgan), set of 10 pcs. | 200–500 PLN |
| Analytical balance 0.1 mg | Mettler Toledo ME204, Radwag AS 220.R2 | 8 000–20 000 PLN |
| Desiccator with silica gel | Glass desiccator Ø 200 mm + indicator silica gel | 300–600 PLN |
| Hot plate / gas burner | IKA C-MAG HP 7, Bunsen burner | 1 000–3 000 PLN |
| Heat-resistant crucible tongs | Steel tongs with heat-insulated handle | 100–300 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Dilute nitric acid (optional) | 7697-37-2 | Few drops to moisten ash during re-ashing (difficult samples) |
| Hydrogen peroxide 30% (optional) | 7722-84-1 | To assist combustion of carbon residues |
| Silica gel | 112926-00-8 | Desiccant in desiccator, with moisture indicator |
Health and safety (OHS)
- Muffle furnace 550°C — extreme burn risk. Use heat-resistant gloves and tongs.
- Hot crucibles — transfer only with tongs. Never touch with bare hands.
- HNO₃ (optional) — corrosive and oxidizing. Use gloves and safety glasses.
- Ventilation — carbonization fumes may be irritating. Work under fume hood.
- Personal protection — lab coat, heat-resistant gloves, safety glasses mandatory.