SOP: Determination of Total Ash Content — Tea and Coffee
GIFSQ — Global Institute for Food Safety & Quality
The ash of a foodstuff is the inorganic residue remaining after complete burning away of the food, usually at a temperature not exceeding red heat. Ash can often serve as a measure for adulteration of a food. The higher the ash content of the sample, the higher the inorganic adulterant. Conversely, a low level of ash might indicate the food/spice with low inorganic material.
1. Principle
When tea/coffee are ignited at high temperature, in a furnace (550 ± 10°C), organic matter is burnt out and mineral and other inorganic matter remain in the ash.
2. Application
This method is applicable to measure the total inorganic material in spices and also in other food products.
3. Apparatus
- Silica or Platinum crucible
- Drying oven
- Electric balance
- Muffle furnace
- Desiccators containing an efficient desiccant such as anhydrous calcium sulfate or silica gel
4. Procedure
Place dry silica crucible in desiccators about half an hour (to avoid moisture)
↓
Take weight of moisture free crucible (W)
↓
Take 5 g of sample in the weighed crucible (A)
↓
Char on hot plate without burning
↓
Ash in muffle furnace at 550°C for 3–4 hours (till white ash is obtained or carbon free)
↓
Transfer the crucible to the oven at 100°C for 1 hr, then transfer to desiccators for cooling
↓
Repeat the drying and cooling process until the difference between two successive weighings is less than 1 mg
↓
Cool the crucible in the desiccators and weigh (B)
5. Calculation
Total Ash (% DWB) = [(B − W) ÷ (A − W)] × 100 × [100 ÷ (100 − M)]
Where,
– W = Weight of empty crucible
– A = Weight of crucible with sample
– B = Weight of crucible with Total Ash
– M = % moisture
– 100/(100 − M) = Factor for dry basis
6. Precautions
- The residue should be moistened, dried and reheated until a white grey ash remains.
- Ashing temperature in a muffle furnace must be within the prescribed range.
- Teflon or platinum tipped tongs should be used to handle the crucible.
- Direct transfer of crucible from high temperature to desiccators may damage crucible and desiccators.
7. Limit of detection
Total ash content in tea ranges from 4–7%. Thus, for total ash determination where recovery and determination of individual metals is not necessary, ashing in a silica crucible at the prescribed temperature is satisfactory.
References
FSSAI 2015, 1.3; Determination of Total Ash.