Water: Properties and Structure

• Water is vital for life, influencing food stability, perishability, and consumer acceptance
• The properties of water are linked to its chemical structure
• Water is a polar molecule, acting as a di-pole with both positive and negative charges
• A water molecule can form hydrogen bonds with up to four other water molecules, called nearest neighbors

Hydrogen Bonds and Water Structure

• Hydrogen bonds between water molecules last about 10^-11 seconds
• The rapid formation and breaking of hydrogen bonds leads to complexes known as flickering clusters
• A snapshot of water at the molecular level would reveal randomly sized clusters linked by hydrogen bonds
• High internal cohesion between water molecules is responsible for surface tension

Heat and Thermal Expansion

• High internal cohesion influences how water reacts when heated
• Heating and evaporation are critical in food manufacturing and cooking
• As temperature increases, the amount of energy in the system also increases
• Increased energy promotes rotation, vibration, and overall movement of water molecules
• Liquid water exists as random clusters forming every 10^-11 seconds via hydrogen bonds
• As temperature (energy) increases, the distance between water molecules increases, known as thermal expansion
• Boiling requires enough heat to break hydrogen bonds and counteract atmospheric vapor pressure

Vapor Pressure

• Vapor pressure is a key component in liquid-to-gas conversion; it is the pressure exerted against the atmosphere
• Liquids with vapor pressures exceeding atmospheric pressure evaporate easily and tend to have strong smells like acetone
• Water requires energy to evaporate because its vapor pressure is not greater than the atmosphere
• Increasing the temperature of a liquid system increases its vapor pressure against the atmosphere
• Changing atmospheric pressure alters water's boiling point
• Flicker cluster properties remain valid in initial vapor state
• Steam moves and dissipates accordingly

Solutes impact & Table salt

• Solutes affect the energy required to convert solid or liquid water into a gas
• Table salt (NaCl) dissociates into Na⁺ and Cl⁻ ions in water
• Charged components interact with water molecules through hydrogen bonds
• This requires more energy to convert liquid water to gas, as water must first be separated from solutes and each other
• Substances with charges like carbohydrates and proteins can interact with water, increasing the energy needed for conversion

Specific Heat, Latent Heat of Fusion and Vaporization

• Specific Heat of Water: 1 cal/g; the energy to raise 1g of water by 1°C
• Latent Heat of Fusion: 80 cal; the energy needed to convert 1g of ice to water at 0°C
• Latent Heat of Vaporization: 540 cal; the energy needed to convert 1g of water to vapor at 100°C

Freezing Process

• Freezing water involves removing energy, allowing water molecules to assemble via nucleation
• Nucleation: The assembly of water molecules through energy removal and increased hydrogen bond strength
• Following nucleation, other water molecules attach, forming an ice crystal
• Ice crystal number and size depend on the freezing rate/temperature
• Water thermodynamically favors binding to existing ice crystals
• Slow freezing, like in home freezers, promotes the formation of a few large ice crystals
• Commercial blast freezers promote the formation of numerous small ice crystals
• Blast/Flash Freezing (≤ -150°C/-238°F) is an extremely rapid freezing method that results in small ice crystals
• Household Freezers (≈0°C/32°F) employ slow freezing, leading to larger ice crystals

Ice Crystal Changes

• During transit from the supermarket, ice starts to melt and forms micro rivers
• Accretion: The process by which ice crystals travel and merge to form large crystals
• Household freezers use a thaw cycle to minimize ice accumulation
• During the thaw cycle, ice crystals constantly thaw and freeze, reaching temperatures up to ≈7°C/45°F every 6-12 hours
• Large numbers of small ice crystals are gradually converted into fewer large ones, resulting in "internal" freezer burn
• Internal freezer burn causes gritty ice cream and runny meats, which results from the destruction of cellular membranes
• Visible ice crystals on frozen food are from humidity inside the freezer

Sublimation & Ice quality

• During thaw, some ice converts to gas via sublimation
• The water vapor binds to existing ice crystals during the freeze cycle to increase their size
• Water in vapor or gas phase has a stronger tendency to bind to already existing ice crystals than to generate new ones

Water Impact

• Water affects food quality and storage
• Measuring water content is essential for predicting food quality and understanding chemical processes during storage
• Water activity (Aw), not % moisture, is measured by food scientists
• % moisture indicates the water content, but not the state of the water
• Free water is water available as a liquid that can participate in reactions, support microorganisms and easy to remove during heating
• Bound water is water bound to other chemical components and cannot act as free water
• Bound water limits its ability to act as a solvent, mobility, taking part in chemical reactions and availability to microorganisms

Water activity

• Water activity (Aw) is measured on a scale from 0-1
• Understanding free water quantity enables predictions of product perishability and chemical stability
• High Aw foods are more perishable
• Low Aw foods are less perishable, they tend to undergo rancidity and other oxidative chemical reactions

Ice Crystal Steps

• Energy is removed when water freezes, which causes water molecules to stop rotating and vibrating
• The cease in vibration promotes nuclei formation
• Water more easily bonds with existing nuclei
• Quality degrades following ice formation
• Sublimation results in physical freezer burn on external crystals
• Internal freezer burn takes place on the inside of food

Fat Crystallization

• Slowing the process causes nuclei to form
• Accumulation occurs given that it's more thermodynamically favorable
• More thermodynamically stability due to packing
• Getting closer together by way of tightening

Packing & Polymorphism

• Softest with the lowest melting point
• Highest degree of packing and highest melting temp
• As water is forced out, it becomes increasingly packed tighter
• Sugar on chocolate is similar to Syneresis in carb protein

Staling

• In bread, similar to packing, retrogradation can be observed
• Olive oil will not result in packing due to chain reaction
• Chain length dictates the melting point
• Oil under lights gets light sediment

Carbohydrates

Overview

• Carbohydrates are essential organic compounds
• They provide 70-80% of a human's daily caloric intake at 4 calories/gram
• Carbohydrates are in 3 broad groups: monosaccharides, di-/oligosaccharides, and polysaccharides
• The smallest carbohydrate units that can't be hydrolyzed are monosaccharides such as D- or L-sugars
• Sugars' D or L form is related to the position of the hydroxyl group on the highest numbered carbon
• This hydroxyl group is the reference group

Carbon Chain Length

• The length of carbon chain classifies monosaccharides, where:
• A three-carbon chain is a triose
• A four-carbon chain is a tetrose
• A five-carbon chain is a pentose
• A six-carbon chain is a hexose
• Etc...

Monosaccharide Classification

• Monosaccharides further classify by function groups, such as aldehyde and ketone groups
• Monosaccharides with aldehyde groups are aldoses
• Monosaccharides with ketone groups are ketoses
• For nomenclature:
• The suffix -ose denotes an aldehyde group
• The suffix -ulose denotes a ketone group

Glucose

• A carbon chain of 6 combined with an aldehyde makes it, classifiable as hexose
• Fructose on the other hands is classify as hexulose
• The D- or L- designation is based on the position of the reference hydroxyl group
• In organic chemistry, monosaccharides are often drawn in linear form, however, they exist in ring structures

Equilibrium & Structures

• In solution, monosaccharides form kept in equilibrium between linear and cyclic structure