《食品化学 Food Chemistry》课程教学课件（讲稿，英文版）Water

Water

Water

Water l Major Component of most foods l Water has several effects on food stability, palatability, and overall quality l Chemical reactions, enzymatic changes, and microbial growth may occur readily in foods with high water contents

Water l Major Component of most foods l Water has several effects on food stability, palatability, and overall quality l Chemical reactions, enzymatic changes, and microbial growth may occur readily in foods with high water contents

Water Content of Foods Food Water Content (%) Meat Pork, raw, composite of lean cuts 53-60 Beef, raw, retail cuts 50-70 Chicken, raw meat without skin 74 Fish, muscle proteins 65-81 Fruits Berries, cherries, pears 80-85 Apples, peaches, oranges, grapefruit 85-90 Rhubarb, strawberries, tomatoes 90-95 Vegetables Peas (green) 74-80 Beets, broccoli, carrots, potatoes 80-90 Asparagus, beans, cabbage, 90-95 cauliflower, lettuce

Water Content of Foods Food Water Content (%) Meat Pork, raw, composite of lean cuts 53-60 Beef, raw, retail cuts 50-70 Chicken, raw meat without skin 74 Fish, muscle proteins 65-81 Fruits Berries, cherries, pears 80-85 Apples, peaches, oranges, grapefruit 85-90 Rhubarb, strawberries, tomatoes 90-95 Vegetables Peas (green) 74-80 Beets, broccoli, carrots, potatoes 80-90 Asparagus, beans, cabbage, 90-95 cauliflower, lettuce

Properties l Solvent – Dielectric properties l Mobility – Dissolve and mobilize substrate l Reactant – Protons, hydroxyl, hydronium and water itself

Properties l Solvent – Dielectric properties l Mobility – Dissolve and mobilize substrate l Reactant – Protons, hydroxyl, hydronium and water itself

General concepts l Bound water: exists in the vicinity of solutes and other non-aqueous constituents, exhibit reduced mobility and properties differing significantly from “bulk water” in the same system and does not freeze at -40°C – Constitutional water: integral part of non-aqueous constituents l < 0.03% of total water l Interstitial regions of proteins; chemical hydrates – Vicinal water: Strongly interacts with specific hydrophilic sites by water-ion and water-dipole associations - l 0.5 ± 0.4 % of total water l Optimum overall stability at the Monolayer value – Multilayer water: occupies remaining first-layer sites and forms several layers around hydrophilic groups by w-w and w-s hydrogen bonds - l 3 ± 2 % of total water l Rates of reactions increase

General concepts l Bound water: exists in the vicinity of solutes and other non-aqueous constituents, exhibit reduced mobility and properties differing significantly from “bulk water” in the same system and does not freeze at -40°C – Constitutional water: integral part of non-aqueous constituents l < 0.03% of total water l Interstitial regions of proteins; chemical hydrates – Vicinal water: Strongly interacts with specific hydrophilic sites by water-ion and water-dipole associations - l 0.5 ± 0.4 % of total water l Optimum overall stability at the Monolayer value – Multilayer water: occupies remaining first-layer sites and forms several layers around hydrophilic groups by w-w and w-s hydrogen bonds - l 3 ± 2 % of total water l Rates of reactions increase

l “Water binding” and “hydration” – Tendency of water to associate with hydrophilic substances l Water holding capacity – Ability of a matrix of molecules to entrap large amounts of water in a manner such that exudation is prevented General concepts

l “Water binding” and “hydration” – Tendency of water to associate with hydrophilic substances l Water holding capacity – Ability of a matrix of molecules to entrap large amounts of water in a manner such that exudation is prevented General concepts

Forces acting in water l Electrostatic – interactions between charges l Hydrogen bonds – hydrogen shared between two electronegative atoms l van der Waals – weak induced dipole interactions between any two atoms in close contact l Hydrophobic interactions

Forces acting in water l Electrostatic – interactions between charges l Hydrogen bonds – hydrogen shared between two electronegative atoms l van der Waals – weak induced dipole interactions between any two atoms in close contact l Hydrophobic interactions

Water Solute Interactions l Water – Ionic groups: some of the most tightly bound water in food – Strong electrostatic attraction between the permanent charge of the ion and the dipole of water l Water – Polar but uncharged solute – Weaker – Hydrogen bonding of dipole-dipole interactions – Allow polar components to dissolve

Water Solute Interactions l Water – Ionic groups: some of the most tightly bound water in food – Strong electrostatic attraction between the permanent charge of the ion and the dipole of water l Water – Polar but uncharged solute – Weaker – Hydrogen bonding of dipole-dipole interactions – Allow polar components to dissolve

l Water – non-polar molecules – Thermodynamically unfavorable _ decrease in entropy – Formation of rigid ice-like clathrate at interface – Dipole – induced dipole – Hydrophobic interactions _ association of apolar and non￾polar groups in aqueous environments Water Solute Interactions

l Water – non-polar molecules – Thermodynamically unfavorable _ decrease in entropy – Formation of rigid ice-like clathrate at interface – Dipole – induced dipole – Hydrophobic interactions _ association of apolar and non￾polar groups in aqueous environments Water Solute Interactions

l Amphiphilic molecules Water Solute Interactions

l Amphiphilic molecules Water Solute Interactions