Organic chemistry plays a crucial role in food production, especially within the food and beverage operations sector in Kenya. Understanding how organic molecules interact helps professionals control food quality, safety, and nutritional value. This chapter explores the fundamental concepts of functional groups, which are specific groupings of atoms within molecules that determine the chemical behavior of food components. Mastery of these principles enables food technologists and chefs to manipulate ingredients effectively for desired outcomes in taste, texture, preservation, and nutrition.

3.1 Functional groups

Food is made largely of organic compounds: carbohydrates, proteins, fats, vitamins, organic acids, pigments and flavour compounds. Their behaviour in the kitchen, whether a sugar dissolves, an oil turns rancid, a meat browns or a sauce tastes sour, depends mainly on the functional groups they carry. Understanding functional groups lets food and beverage practitioners explain and control what happens to food during mixing, cooking, storage and digestion. Example: the quality officer of a fruit processing plant in Wote measures the acidity of mango pulp, which comes from the carboxyl groups of its citric and malic acids, to decide how much of the pulp can be made into nectar without adding acid.

3.1.1 Definition

A functional group is a specific atom or group of atoms attached to the carbon skeleton of an organic molecule that gives the molecule its characteristic chemical reactions and properties. The carbon and hydrogen backbone, often written as R, is fairly unreactive; the functional group is the reactive part. Molecules that share the same functional group belong to the same family of compounds and behave in similar ways: all alcohols carry the hydroxyl group, all carboxylic acids carry the carboxyl group. A single food molecule may carry several functional groups, for example glucose has five hydroxyl groups and one carbonyl group, and every amino acid has both an amine group and a carboxyl group.

3.1.2 Common functional groups

Six functional groups account for most of the chemistry of food. Each is recognised by its structure and found in particular classes of food compounds.

Hydroxyl group (-OH)

The hydroxyl group is an oxygen atom bonded to a hydrogen atom. It is the functional group of alcohols such as ethanol in beer and wine and glycerol in fats, and sugars carry several of them. Hydroxyl groups form hydrogen bonds with water, so molecules rich in them, such as sugars, dissolve easily, hold moisture and act as humectants (glycerol and sorbitol keep cakes and sweets soft). The hydroxyl groups of starch bind water during cooking, causing gelatinisation, which thickens porridge, sauces and ugali.

Carboxyl group (-COOH)

The carboxyl group is a carbon atom double bonded to one oxygen and single bonded to a hydroxyl group. It is the functional group of carboxylic (organic) acids: acetic acid in vinegar, citric acid in lemons and oranges, lactic acid in fermented milk and sour porridge, malic acid in apples and tartaric acid in grapes. Fatty acids and amino acids also carry it. Carboxyl groups release hydrogen ions in water, which gives a sour taste and lowers pH, slowing the growth of many bacteria. They react with alcohols to form esters and with bases to form salts, for example the acid in buttermilk reacting with bicarbonate of soda to release carbon dioxide that raises scones.

Amine group (-NH2)

The amine group is a nitrogen atom bonded to two hydrogen atoms. Every amino acid has one, and in proteins the amine group of one amino acid joins the carboxyl group of the next to form a peptide bond. Amine groups are basic and help buffer acidity in foods. They react with the carbonyl groups of reducing sugars in the Maillard reaction, giving the brown colour and roasted flavour of bread crust, roast meat and roasted coffee. When fish and meat spoil, bacteria break down amino acids into amines such as histamine and the fishy-smelling trimethylamine, so a strong amine smell is a sign of spoilage.

Carbonyl group (C=O)

The carbonyl group is a carbon atom double bonded to an oxygen atom. In aldehydes it sits at the end of the carbon chain (glucose in its open-chain form, vanillin in vanilla, cinnamaldehyde in cinnamon, benzaldehyde in almond essence); in ketones it sits within the chain (fructose, diacetyl which gives butter its flavour). Sugars with a free carbonyl group are reducing sugars and take part in browning reactions. Many aroma compounds are aldehydes and ketones, and some, such as hexanal, are produced when fats oxidise, giving the stale, cardboard smell of rancid oil.

Phosphate group (-PO4)

The phosphate group is a phosphorus atom bonded to four oxygen atoms. It occurs in phospholipids such as lecithin in egg yolk and soya, which are natural emulsifiers that hold mayonnaise and sauces together; in casein, the main milk protein, where phosphate groups bind calcium; in ATP, the energy carrier of living cells; and in phytic acid in cereal bran and legumes, which binds iron and zinc. Phosphates are also used as food additives, for example sodium phosphates to hold water in processed meats and to melt processed cheese smoothly, phosphoric acid in cola drinks, and acid phosphates in baking powder.

Ester group (-COO-)

The ester group is formed when a carboxyl group reacts with a hydroxyl group, releasing water (esterification). Fats and oils are triglycerides, in which glycerol is joined to three fatty acids by ester bonds. Small esters have fruity smells and give the aroma of fruits and fermented drinks, for example isoamyl acetate (banana) and ethyl butyrate (pineapple), and they are used in flavourings. Ester bonds can be broken by water, acid or the enzyme lipase (hydrolysis), releasing free fatty acids; this happens in frying oil and in stored butter and gives off-flavours.

3.1.3 Role of functional groups

Functional groups decide how food compounds behave chemically, how food tastes and feels in the mouth, and how much nutritional value the body can obtain from it. A cook who controls heat, acidity and moisture is, in effect, controlling reactions between functional groups.

Chemical properties

• Solubility and polarity: Groups containing oxygen or nitrogen (hydroxyl, carboxyl, amine, phosphate) are polar and attract water, so sugars, acids and salts dissolve; long hydrocarbon chains with few such groups, as in fats, do not, which is why oil and water separate.
• Acidity and basicity: Carboxyl and phosphate groups make foods acidic, while amine groups are basic; together they set the pH of a food, which affects its taste, colour, texture and microbial safety.
• Reactivity: Carbonyl and amine groups react in browning; double bonds and ester groups break down in rancidity; hydroxyl and carboxyl groups combine to form esters.
• Hydrogen bonding: Hydroxyl and amine groups form hydrogen bonds that hold water in starch gels and protein gels, giving body to custards, porridges and sauces.
• Emulsifying power: Molecules with a water-loving group at one end (such as the phosphate group of lecithin) and a fat-loving chain at the other hold oil droplets in water.

Taste and texture

• Sweetness: Molecules with several hydroxyl groups, such as sugars and sugar alcohols, taste sweet.
• Sourness: The carboxyl groups of organic acids give the sour taste of lemon juice, vinegar, mursik and sour porridge.
• Aroma and flavour: Esters give fruity aromas, aldehydes and ketones give the smell of vanilla, cinnamon and butter, and Maillard products from amine and carbonyl groups give roasted flavours.
• Texture: Hydroxyl groups in starch absorb water and thicken sauces; peptide bonds and side groups in proteins set in egg, cheese and meat on heating; phosphate groups help casein form the curd in cheese and yoghurt.
• Colour: Browning of bread crust, roasted groundnuts and fried onions depends on reactions of carbonyl and amine groups.

Nutritional value of food

• Protein quality: Peptide bonds between amine and carboxyl groups are broken by digestive enzymes to release amino acids, but heavy browning ties up the amine group of lysine, reducing the protein value of over-browned foods.
• Fat digestion and energy: Lipase breaks the ester bonds of triglycerides so fatty acids can be absorbed and used for energy.
• Mineral absorption: Phosphate groups in phytic acid bind iron, zinc and calcium and reduce their absorption; soaking, germinating and fermenting cereals and legumes break down phytate and improve absorption.
• Vitamin stability: Vitamin C is easily oxidised through its hydroxyl groups, so it is lost when cut vegetables are exposed to air and heat.
• Food safety and shelf life: Organic acids lower pH and help preserve pickles and fermented foods, while oxidation at double bonds and hydrolysis of ester bonds in fats cause rancidity.

Chapter Summary

This chapter explored the application of organic chemistry principles in food production, starting with the concept of functional groups. Functional groups were defined as specific groups of atoms within molecules that determine their chemical properties and reactions. The chapter identified common functional groups found in food molecules and explained their significant role in influencing the chemical behavior of these compounds during food processing. It then examined hydrocarbons in foods, detailing the different types such as alkanes, alkenes, and aromatic hydrocarbons. The discussion included how reactions involving hydrocarbons, like grilling and roasting, affect food flavor, texture, and safety. Finally, the chapter addressed the role of hydrocarbons in food preservation, highlighting their importance in maintaining food quality and extending shelf life through various chemical interactions.