Chemistry in Everyday Life • Topic 3 of 3

Food & Cleansing Agents

Chemistry also shapes what we eat and how we clean. This topic covers the chemicals added to food and the agents that remove dirt and grease.

Chemicals in food

Food chemicals improve keeping quality, appearance, taste and food value. The exam focus is on three groups.

Artificial sweeteners give a sweet taste without adding calories — valuable for people with diabetes and for controlling weight. The important ones are:

Saccharin — the first artificial sweetener, about 550 times sweeter than cane sugar (sucrose). It is excreted unchanged and is safe for diabetic patients.
Aspartame — about 100 times sweeter than sucrose; widely used in cold foods and soft drinks, but it decomposes on heating, so it cannot be used in cooked food.
Sucralose — about 600 times sweeter than sucrose; it is stable to heat, has no after-taste and does not provide calories.
Alitame — about 2000 times sweeter than sucrose; a high-potency sweetener, but its sweetness is hard to control in a formulation.

Food preservatives stop spoilage by stopping the growth of micro-organisms. Common ones are table salt, sugar, vegetable oils and, chemically, sodium benzoate (C₆H₅COONa), which is metabolised and is used in soft drinks and sauces; salts of sorbic acid and propanoic acid are also used.

Antioxidants are added to fatty and oily foods to stop them turning rancid. They are more easily oxidised than the food, so they are used up first, protecting the food. The two important examples are BHT (butylated hydroxytoluene) and BHA (butylated hydroxyanisole); SO₂ and sulphite salts also act as antioxidants.

Cleansing agents

Soaps are the sodium or potassium salts of long-chain fatty acids (e.g. sodium stearate, C₁₇H₃₅COONa). They are made by saponification — boiling a fat or oil (a triester of glycerol) with sodium hydroxide:

fat/oil + 3NaOH → soap + glycerol

Types include toilet soaps (made with better grades of fat and excess alkali removed), transparent soaps (dissolved in alcohol), medicated soaps and laundry soaps. Limitation in hard water: hard water contains Ca²⁺ and Mg²⁺ ions, which react with soap to form an insoluble, sticky scum (calcium and magnesium stearate). The soap is wasted and does not lather, so soap cannot clean in hard water.

Synthetic detergents were developed to work even in hard water; their calcium and magnesium salts are soluble, so they do not form scum. They are classed by their charged part:

Anionic detergents — the cleansing part is the negative ion; sodium alkylbenzenesulphonates and sodium alkyl sulphates. Used in toothpastes and household washing.
Cationic detergents — the cleansing part is a positive ion (quaternary ammonium salts); used in hair conditioners; they are germicidal but expensive.
Non-ionic detergents — carry no charge; e.g. esters of polyethylene glycol; used in liquid dishwashing detergents.

Biodegradability: detergents with straight (unbranched) hydrocarbon chains are degraded by bacteria and are biodegradable; those with highly branched chains are not, and they cause water pollution by foaming in rivers.

Micelle and cleansing action

A soap or detergent molecule has two ends: a long hydrophobic (water-hating) hydrocarbon tail and a hydrophilic (water-loving) ionic head. In water, above a certain concentration, the molecules group into a micelle — a ball with the tails pointing inward and the heads facing the water. When cleaning, the hydrophobic tails bury themselves in the oily dirt while the hydrophilic heads stay in the water. The grease is surrounded by a micelle, lifted off the cloth, held suspended in water (emulsified) and washed away.

Solved Examples

Worked Example

What are artificial sweeteners? Why are they useful?

Solution

Artificial sweeteners are chemicals that give a sweet taste to food but add little or no calories.
They do not raise blood sugar the way sucrose does.
So they are useful for diabetic patients and for people who need to control their weight/calorie intake.

Answer: Artificial sweeteners (e.g. saccharin, aspartame) sweeten food without adding calories, making them useful for diabetics and weight control.

Worked Example

Compare aspartame and sucralose with respect to heat stability and use.

Solution

Aspartame is about 100 times sweeter than sucrose but decomposes on heating, so it can be used only in cold foods and soft drinks.
Sucralose is about 600 times sweeter than sucrose and is stable to heat.
Because it is heat-stable, sucralose can be used in cooked and baked foods, unlike aspartame.

Answer: Aspartame breaks down on heating (cold foods only); sucralose is heat-stable, so it can be used in cooking and baking.

Worked Example

What is a food preservative? Name a common chemical preservative.

Solution

A food preservative prevents spoilage of food by stopping the growth of micro-organisms.
Familiar natural preservatives are table salt, sugar and vegetable oils.
A common chemical preservative is sodium benzoate, C₆H₅COONa, used in soft drinks and sauces.

Answer: Preservatives prevent spoilage by inhibiting micro-organisms; sodium benzoate (C₆H₅COONa) is a common chemical preservative.

Worked Example

How does an antioxidant protect food? Name two.

Solution

Fatty and oily foods turn rancid because they get oxidised by air.
An antioxidant is more easily oxidised than the food itself, so it is consumed first and protects the food.
Two important antioxidants are BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene).

Answer: An antioxidant is oxidised in preference to the food, preventing rancidity; examples are BHA and BHT.

Worked Example

Why does ordinary soap fail to clean in hard water, whereas synthetic detergents do not?

Solution

Hard water contains Ca²⁺ and Mg²⁺ ions.
These ions react with soap to form an insoluble, sticky scum (calcium and magnesium salts of the fatty acid), so the soap is wasted and gives no lather.
The calcium and magnesium salts of synthetic detergents are soluble, so detergents do not form scum and clean even in hard water.

Answer: Soap forms an insoluble scum with Ca²⁺/Mg²⁺ in hard water, whereas detergents form soluble salts and so work in hard water.

Worked Example

Explain the cleansing action of soap using the idea of a micelle.

Solution

A soap molecule has a hydrophilic (water-loving) ionic head and a long hydrophobic (water-hating) hydrocarbon tail.
The tails dissolve in the oily dirt while the heads stay in the surrounding water, so the molecules arrange into a micelle around the grease.
The grease droplet, trapped inside the micelle, is lifted from the cloth, emulsified in water and washed away.

Answer: Soap molecules surround grease in a micelle — tails in the oil, heads in the water — so the dirt is emulsified, lifted off and rinsed away.

Key Points

Artificial sweeteners give sweetness without calories: saccharin (about 550x, safe for diabetics), aspartame (about 100x, decomposes on heating — cold foods only), sucralose (about 600x, heat-stable), alitame (about 2000x, high potency).
Food preservatives prevent spoilage by inhibiting micro-organisms; sodium benzoate (C₆H₅COONa), salt, sugar and salts of sorbic/propanoic acid are common.
Antioxidants (BHA, BHT) are oxidised in preference to the food and so prevent fats and oils from turning rancid.
Soaps are sodium/potassium salts of long-chain fatty acids made by saponification (fat + NaOH → soap + glycerol); they fail in hard water because Ca²⁺/Mg²⁺ form an insoluble scum.
Synthetic detergents work in hard water; they are anionic (negative cleansing ion), cationic (positive ion, in hair conditioners) or non-ionic; straight-chain ones are biodegradable. Cleansing works through micelle formation — hydrophobic tails in grease, hydrophilic heads in water.

Quick Check — 4 MCQs

Tap an option to check your answer0 / 4

Q1.Which artificial sweetener decomposes on heating and so is used only in cold foods and soft drinks?

Explanation: Aspartame is unstable to heat, so it cannot be used in cooked food; sucralose and saccharin are far more heat-stable.

Q2.Sodium benzoate (C6H5COONa) is added to food as a/an:

Explanation: Sodium benzoate is a chemical preservative that inhibits the growth of micro-organisms in soft drinks and sauces.

Q3.Soap fails to clean in hard water because Ca2+ and Mg2+ ions:

Explanation: Calcium and magnesium ions precipitate the soap as an insoluble, sticky scum, wasting it and stopping lather; detergent salts of these ions are soluble.

Q4.Cationic detergents, in which the cleansing action lies in the positive ion, are commonly used in:

Explanation: Cationic detergents (quaternary ammonium salts) are germicidal and used in hair conditioners; anionic ones are used in toothpastes and washing, non-ionic in liquid dishwashing.

Practice more MCQs → 📝 Assignment · 30 marks