Food Constituents: Water

Food Constituents - I

• Water, carbohydrates, lipids, and proteins are present in large quantities in foods.
• Vitamins and inorganic materials are found in smaller amounts in foods.
• Foods also contain colors, flavors, organic acids, and toxic substances.

Water (H2O)

• Composed of two hydrogen atoms and one oxygen atom.
• Molecular weight of 18.01534.
• Melting point is 0.000°C at 1 atmosphere.
• Boiling point is 100.000°C.
• Exists in gas, liquid, and solid form.
• Found in all foods except common salt, sugar, and vegetable cooking oils.
• Fruits vegetables, meat, milk, and beverages contain higher water content.
• Water molecules dissociate into H+ and OH- ions making it both a proton donor and acceptor, and therefore neutral.
• Adding an acid increases H+ and makes water acidic.
• Adding an alkali increases OH- and makes water alkaline.
• Foods contain substances including organic acids, resulting in a pH below 7.0.

Nature of Water in Food

• Exists as free (available for reactions and microorganisms) or bound water (physically or chemically).
• Physical binding predominates in emulsions and gels.
• Chemical binding involves linkages to carbohydrates and salts as water of hydration.
• Bound water reduces chemical/biochemical reactions and microbial activity.
• Foods with 0-25% equilibrium relative humidity (ERH) are stable and have unfreezable water.
• Foods with 25-80% ERH are liable to non-enzymatic browning, enzyme activity, and hydrolytic/oxidative reactions.
• Foods with 80-99% ERH have reduced freezing point and support microbial growth, enzyme activity, and browning reactions.

Nature of Food Dispersion

• Water molecules are dipoles and dissolve salts.
• Non-ionic polar compounds dissolve due to hydrogen bonding.
• Food solutions are dispersions with evenly distributed solid or semi-solid particles.
• Colloidal dispersions have particle size from 1 nm to 0.5 µm; coarse dispersions are above 0.5 µm.
• Colloidal solutions consist of macromolecules (proteins, polysaccharides, or lipids).
• Examples of colloidal solutions include solutions containing gums, pectic substances, proteins, and lipids.
• A food gel is a dispersion of interconnected particles with a continuous water phase.
• Gel properties depend on the type and concentration of gelling agent, pH, and temperature.
• Gels include jams, jellies, and marmalades.
• An emulsion is a dispersion of two immiscible liquids, typically unstable without emulsifying agents.
• Food emulsions consist of water and a plastic lipid.
• Butter and margarine are water-in-oil emulsions.
• Milk, cream, and mayonnaise are oil-in-water emulsions.

Role of Water in Foods

• Essential for all chemical reactions in living organisms.
• Important constituent governing rates of chemical reactions.
• A medium for nutrient, enzyme, and substance dispersion.
• A reactant (e.g., hydrolysis) and transport medium.
• Contributes to desirable qualities of foods.
• Determines storage life based on percentage and state.

Water in Human Body

• Constitutes 60-70% of human body.
• Obtained from drinking water, beverages, and other foods.
• Produced through the oxidation of carbohydrates, proteins, and fats.
• Normal requirements depend on age, activity, and climate.

Carbohydrates

• Contain carbon, hydrogen, and oxygen, with H:O in a 2:1 ratio.
• Formed in green plants from carbon dioxide and water via photosynthesis (energy from sun).

Classification of Carbohydrates

• Divided into simple and compound carbohydrates.
• Simple carbohydrates cannot be hydrolyzed into simpler forms.
• Simple carbohydrates include: dioses, trioses, tetroses, pentoses, hexoses, heptoses, octoses, nonoses and decoses, with hexoses (monosaccharides) being most significant.
• Compound carbohydrates can be hydrolyzed into simpler forms.

Simple Carbohydrates and Sugars

• Monosaccharides cannot be further hydrolyzed by acids or enzymes.
• Examples include glucose (dextrose), fructose (levulose), and galactose.
• Glucose and fructose are abundant in plants, especially fruits.
• Galactose exists as part of the disaccharide lactose and is released only upon hydrolysis.
• Disaccharides form when two monosaccharides condense with loss of water.
• Examples: Sucrose, maltose and lactose.
• Hydrolysis of disaccharides yields component monosaccharides.
• Monosaccharides and disaccharides are called sugars.
• Sugars are typically white, crystalline, water-soluble, and sweet tasting.
• Fructose, glucose and galactose sweetness scores are 173, 74 and 32 units respectively, relative to assigning 100 sweetness units to sucrose.
• Lactose and Maltose sweetness scores are 16 and 32 units respectively, relative to assigning 100 sweetness units to sucrose.
• Intense heating of sugars will cause them to turn brown (caramelize).

Compound Carbohydrates

• Three to seven molecules of simple sugars are called oligosaccharides, which can be formed when 3-7 molecules condense.
• More than seven monosaccharides create polysaccharides.
• Polysaccharides formula is (C5H10O5)n, with n being the number of linked monosaccharide units.
• Monosaccharides may join to create either straight (amylose) or branched (amylopectin) chain polymers.

Amylose

• Straight chain polymer containing 70-350 glucose units per chain.

Amylopectin

• Branched chain polymer containing hundreds of glucose units.

Starch

• Amylose and amylopectin are major components
• Principal nutrient in cereals, roots, and tubers

Other Polysaccharides

• Include cellulose, glycogen, pectin, agar, and alginate.
• Cellulose: structural carbohydrate in plants, unaffected by digestive enzymes in monogastric animals.
• Glycogen: stored reserve carbohydrate in animal body.
• Pectin: found mainly in fruits, with gelling properties.
• Agar and alginate: extracted from seaweeds, used as gelling agents.
• Cellulose, pectin, agar, and alginate offer no nutritive value to humans, apart from acting as bulking agents.
• Non-crystalline white powder, insoluble in water, hydrolysed into simpler molecules using acids or enzymes.

Carbohydrates in Human Nutrition

• Sugars and starches serve as main energy sources.
• Converted to monosaccharides that yield 16 kJ (3.75 kcal) of energy per gram after oxidation.
• Excess carbohydrates stored as glycogen or converted to fat.
• Major sources include cereals and their products, roots and tubers.
• Fruits and vegetables also supply carbohydrates, sugars, and fibre.

Lipids

• Soluble in organic solvents and insoluble in water.
• Include waxes, free fatty acids, carotenoids, steroids, fats, and oils.
• Contain carbon, hydrogen, and oxygen.
• Fats and oils are major food constituents.

Fats and Oils

• Esters of saturated or unsaturated fatty acids with glycerol.

Properties of Fats and Oils

• The presence of more saturated fatty acids make fats solid at ambient temperatures.
• The presence of more unsaturated fatty acids make oils liquid at ambient temperatures.
• Fats and oils can be mixed with water using emulsifiers

Applications of Fats and Oils

• Commonly used for cooking (stews, salads, frying, baking)
• Obtained from animal sources (tallow, butter) or plant materials (cotton seed, ground nut, soybean, corn, canola, sunflower)

Nutritional Significance of Fats and Oils

• Provide more than twice as much energy as carbohydrates (38 kJ or 9 kcal per gram).
• Stored as energy reserve, insulate the body, protect organs.
• Good sources of essential fatty acids and vitamins A, D, E, and K.