Minerals are inorganic nutrients essential for various bodily functions. They help build bones and teeth, maintain fluid balance, support hormones, muscles, and nerves, and keep the heart and brain healthy. The body does not synthesize minerals; therefore, they must be obtained from food.

Minerals are grouped into two categories:

• Macro-minerals: Required in larger amounts (calcium, potassium, sodium)
• Trace minerals: Required in small amounts (iron, zinc, copper, manganese, selenium, iodine)

Before specific tests, food samples typically require preparation:

1. Grinding: Using a motor and pestle to obtain a fine powder increases surface area for reaction
2. Digestion: Using dilute acids (HCl or HNO₃) to release mineral compounds into solution
3. Filtration: To obtain a clear solution free from solid residues

Principle

Calcium ions react with hydroxide ions in lime water (calcium hydroxide solution) to form a white precipitate of calcium carbonate.

$\text{Ca}^{2+} + \text{CO}_3^{2-} \rightarrow \text{CaCO}_3 \downarrow \text{(white)}$

Procedure

1. Grind 5 g of food sample (e.g., eggshells, milk, or cheese) into a fine powder
2. Add a small amount of water to extract soluble calcium ions and filter
3. Pour 60 mL of lime water into the extract
4. Shake well and observe changes

Observation and Interpretation

A white precipitate indicates the presence of calcium. This method is qualitative—it shows calcium is present but does not give the exact amount.

Principle

Iron compounds are extracted using dilute hydrochloric acid. Potassium thiocyanate reacts with iron ions to form a reddish-coloured compound.

$\text{Fe}^{3+} + 3\text{SCN}^- \rightarrow \text{Fe(SCN)}_3 \text{(red)}$

Procedure

1. Cut and grind 5 g of food sample (e.g., liver)
2. Add 10 mL dilute HCl and boil for 2 minutes while stirring
3. Cool and filter the mixture
4. Add 10 mL potassium thiocyanate solution to the filtrate
5. Observe the colour change

Observation and Interpretation

A reddish colour indicates the presence of iron. The red colour forms because iron(III) ions react with thiocyanate ions to produce a complex compound.

Principle

When heated in a flame, certain metal ions produce characteristic colours:

• Potassium: Lilac/flame colour
• Sodium: Bright yellow flame

Procedure for Potassium

1. Peel and mash a ripe banana
2. Add water, shake well, and filter to obtain extract
3. Clean platinum or nichrome wire by dipping in dilute HCl
4. Dip the clean wire into the extract
5. Place the wire in a Bunsen flame and observe the colour

Observation and Interpretation

A lilac flame colour confirms the presence of potassium. A yellow flame indicates sodium contamination or presence.

Procedure for Sodium

1. Prepare a solution of the food sample (e.g., baking soda)
2. Clean the wire by dipping in dilute HCl and heating until colourless
3. Dip the wire into the sample solution
4. Place in flame and observe

Observation and Interpretation

A bright yellow flame confirms sodium is present.

Principle

Magnesium ions react with alkali (NaOH or NH₄OH) to form a white precipitate of magnesium hydroxide.

$\text{Mg}^{2+} + 2\text{OH}^- \rightarrow \text{Mg(OH)}_2 \downarrow \text{(white)}$

Procedure

1. Weigh 1 g of green leafy vegetables and grind into fine powder
2. Add 20 mL dilute HCl to digest the sample
3. Filter the mixture
4. Add 30 mL sodium hydroxide or ammonium hydroxide to the filtrate
5. Leave to settle for 5 minutes

Observation and Interpretation

A white precipitate indicates magnesium. If little or no precipitate forms, poor sample preparation may be the cause.

Principle

Chloride ions react with silver nitrate to form a white curdy precipitate of silver chloride, which dissolves in ammonia solution.

$\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl} \downarrow \text{(white)}$

Procedure

1. Prepare a solution of the food sample (e.g., table salt)
2. Add 3 drops of dilute nitric acid and mix
3. Add 5 drops of silver nitrate solution drop by drop
4. Observe any precipitate formation

Observation and Interpretation

A white curdy precipitate confirms chloride ions. The precipitate dissolves in ammonia solution, distinguishing it from other silver halides.

Principle

Sulphate ions react with barium chloride to form a white insoluble precipitate of barium sulphate.

$\text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4 \downarrow \text{(white)}$

Procedure

1. Weigh 5 g of food sample (e.g., egg white) and dissolve in 20 mL distilled water
2. Add 3 drops of dilute HCl
3. Add 5 drops of barium chloride solution while stirring
4. Observe any changes

Observation and Interpretation

A white precipitate confirms sulphate. Dilute HCl prevents formation of unwanted precipitates from other salts.

Principle

Carbonates react with acids to release carbon dioxide gas, which causes effervescence and extinguishes a glowing splint.

$\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{CO}_2 \uparrow + \text{H}_2\text{O}$

Procedure

1. Place food sample (e.g., baking soda) into a test tube
2. Add 10 mL distilled water
3. Add 10 mL dilute HCl or vinegar
4. Observe effervescence
5. Hold a lit splint near the mouth of the test tube

Observation and Interpretation

Bubbles of carbon dioxide gas indicate carbonates. The gas extinguishes a glowing splint, confirming it is not oxygen.

Principle

Phosphate ions react with ammonium molybdate to form a yellow precipitate of ammonium phosphomolybdate.

Procedure

1. Dissolve 5 g powdered milk in 30 mL distilled water
2. Shake and filter to obtain clear extract
3. Transfer 5 mL extract to a test tube
4. Add 3 drops dilute nitric acid
5. Add 3 drops ammonium molybdate solution
6. Observe the change

Observation and Interpretation

A yellow precipitate indicates phosphate. This test is useful for assessing phosphate content in cereals, milk, and eggs.

This procedure determines the total mineral content (ash percentage) of a food sample.

Procedure

1. Clean and weigh crucible: Heat gently, cool, and weigh (W₁)
2. Add sample: Put 2–5 g dry sample into crucible and weigh (W₂)
3. Pre-char: Heat on low flame until sample stops smoking and turns black
4. Ash: Increase heat until residue becomes grey or white ash
5. Cool and weigh: Allow to cool in desiccator and weigh (W₃)

Calculation

$Ash\% = \frac{W_3 - W_1}{W_2 - W_1} \times 100$

Worked Example

A student performed ash determination with the following results:

• Weight of empty crucible (W₁) = 30.0 g
• Weight of crucible + sample (W₂) = 32.0 g
• Weight of crucible + ash (W₃) = 30.4 g

Solution:

$Ash\% = \frac{30.4 - 30.0}{32.0 - 30.0} \times 100 = \frac{0.4}{2.0} \times 100 = 20\%$

Interpretation: The food sample contains 20% mineral content by mass.

When conducting mineral analysis:

• Wear safety goggles and gloves
• Heat slowly to avoid spattering
• Work in a well-ventilated area
• Do not inhale fumes
• Use tongs when handling hot crucibles
• Clean wire thoroughly between tests

• Incomplete digestion: May lead to low mineral readings
• Contaminated equipment: Affects colour observations in flame tests
• Insufficient filtering: Produces cloudy solutions causing unclear results
• Overheating during ashing: May cause loss of volatile minerals

In Tanzania, food manufacturers and quality control laboratories use these mineral analysis techniques to verify the nutritional content of fortified foods such as iodized salt and iron-fortified flour. For example, a small food processing business in Dar es Salaam testing their fortified maize flour for iron content would use the potassium thiocyanate test to ensure their product meets the Tanzania Bureau of Standards (TBS) requirements before selling it in local markets.