FOSC201 - Water & Carbohydrates

What is water?

2 hydrogens + 1 oxygen, polar covalent bonds, 3 states (solid, liquid, gas), 70% of earths surface.

Importance of water (% in foods)

Meat = 50-80%

Fruit = 75-85%

Vegetables = 75-95%

Determines

• physiochemical characteristics (texture, shape, stability, storage ability.)

• quality

• reactions

• degradation/shelf life

Water is polar (electrons unevenly distributed) which allows…

Hydrogen bonding - responsible for special properties

Cohesion

Water binding to itself

Unusual properties of water

High

• melting pt

• boiling pt

• heat capacity

• surface tension

Because of high amount of energy to break hydrogen bonds

Solid water

Becomes more dense until 4 degrees then becomes less dense (ice floats on water).

Molecules move slower - easier to form hydrogen bonds and form a crystalline structure - water molecules held further apart (volume increases).

Liquid water

Molecules mobile - hydrogen bonds form so relatively close together/semi-ordered (each water bonds to 4 others)

Molecules slide past each other.

Gas water

Hydrogen bonds broken (a lot of energy required)

Molecules move fast and far apart (no time for hydrogen bonds to form)

Heat of vaporization

Energy required to change liquid into gas

Vapour pressure/relative humidity

Pressure exerted by a vapour above its solid or liquid state.

Increases with temperature.

Inversely related to intermolecular forces (strong H bonds in water = lower vapour pressure)

Water boils when…

vapour pressure > atmospheric pressure

• increase pressure = higher boiling pt

• decrease pressure = lower boiling pt

Water phase diagram

Describes behaviour/states of pure water according to temp and pressure

Triple pt (below 1atm): all three phases at equilibrium

• below triple pt no liquid

Freeze drying (requires…)

Low temp - below triple pt

Low pressure

Ice sublimes to gas

Slow & expensive

Water droplets form due to…

Surface tension

• resists external force

• fluid acquires least surface area possible

• Stronger bonds at surface

Water rises in hollow tube due to…

Capillary action/adhesion

• forces between different molecules

• water adhesion to vessel wall - climbs

• concave meniscus

• narrower tube = greater effect

Solute effects on water properties

Colligative properties

• Decrease melting pt

• increase boiling pt - more energy needed to overcome atmospheric pressure

• decrease vapour pressure (more energy needed to be greater than atmospheric pressure)

• Decrease freezing pt

• Change Aw

Moisture content is…

Total quantity of water in a food

% as wet weight = 100 x weight of water/fresh weight of food

% as dry weight = 100 x weight water/weight dry solids

Water content determined by…

Drying to constant weight

• oven

• microwave

• vacuum

Water activity (Aw) - definition

Ratio of vapour pressure of water in a food to saturated vapour pressure of pure water

Free energy of water/ fraction of water available

Aw determines

Microbial growth

Rate of chemical reactions

Food stability/ shelf life/ functional properties

Relationship between Aw and moisture content described by…

Moisture sorption isotherm

• predict stability of a food

• depends on food comp and enviro conditions (temp, humidity)

• goes from high stability to fresh/perishable foods

Zone 1 (moisture sorption isotherm)

High stability

Tightly bound water (not available) - chemically bound

Immobile

Aw = 0 - 0.25

Zone 2 (moisture sorption isotherm)

Capillary water (filling in gaps in product)

Loosely bound

Reduced availability

Some mobility

difficult to freeze

Aw = 0.25-0.75

Zone 3 (moisture sorption isotherm)

Free water

Fully available/ mobile

available for enzymatic and chemical reactions and microbial growth

Freezing

Lowing binding energy - easy loss

Aw = 0.75-1

Aw<0.6

No microbial activity

Aw<0.7

most fungi and moulds inhibited

Aw<0.8

Most yeasts inhibited

Aw<0.9

Most bacteria inhibited

Enzyme activity and Aw

Zone 1: no enzyme activity due to low substrate mobility

Zone 2: slow rate of reaction (limited mobility)

Zone 3: rate increases rapidly

Lipid oxidation and Aw

Zone 1: high - free radicals (autoxidation)

Zone 2: decreased (chelating agents and antioxidants)

Zone 3: high - antioxidants diluted (Hydrolytic rancidity)

Browning and Aw

Zone 1: negligible (low mobility, sugar and amino acids are water soluble)

Zone 2: increase (mobility and high conc of reactants)

Zone 3: decrease (high moisture suppresses reaction and dilutes reactants)

Functionality and Aw (impact of characteristics)

Aw = effective water conc for hydration of materials

Impacts on characteristics

• texture

• soft vs hard

• moist vs dry

• crunch/crisp

Moisture migration (determines)

From high Aw to low Aw

Determines

• moisture absorption (hygroscopicity - low Aw foods will undergo)

• Moisture loss (dehydration/drying - high Aw foods will undergo)

Driving force for moisture migration is…

Water activity

Causes

• microbial growth, degradation

• texture/sensory change

To avoid

• separate package

• edible barriers

• modify Aw with salt, sugar, humectants

Aw measured using…

Aqualab water activity meter

• Dew point hygrometer cooled until dew point

• Find relative humidity → calculate Aw

Temperature and Aw

Aw increases with temp (at constant moisture)

• stability at lower temps

Non reversible structural changes cause…

Hysteresis

• collapse during drying

• incomplete dehydration/absorption

Adsorption isotherms: hygroscopic products

Desorption isotherms: drying products

Solutes and Aw

Decrease Aw (solutes interact with water)

Properties determined by number of solutes in water

Colligative properties

• decrease melting pt

• decrease vapour pressure

• increase boiling pt

• alter Aw

Raoult’s Law

In ideal conditions gives Aw, states when solute added vapour pressure decreases and affects Aw.

• correction factor y(gamma) used because solutes interact with each other as well

• larger molecules have greater affect on Aw (salt)

Carbs are…

most abundant organic compound in nature

made of C,H,O

• polyhydroxy aldehydes (C=O on end)

• polyhydroxy ketones (C=O in middle)

Determine functional properties of food

Biological functions of carbs

Energy source

Structural components (cellulose)

Biomolecules (DNA)

Nutritional functions of carbs

50-55% energy intake

overconsumption → obesity

Functional properties of carbs in food

Sweeteners

Precursors for flavour and odour (caramelisation and maillard reaction)

Carb classification (2 things)

Type of saccharide (Triose, pentose, hexose)

Number of saccharides (mono, di, tri, oligo, poly saccharides)

Optical activity of carbs (isomers)

optical isomers - chiral carbon

Rotation of polarised light

• left L isomer

• Right D isomer (predominant form in food)

Structure of carbs vary on

number of carbons in a unit

Position of C=O, OH

ring structures (furanose - 5 atom, pyranose 6 atom)

number and type of saccharides

Position of hydroxyl group in relation to C=O in carbs determines

Cyclic isomer

• a (OH below O from C=O)

• B (OH above O from C=O)

Reducing sugars are…

Sugar with an available carbonyl group capable of acting as a reducing agent (important for reactivity)

• glucose

• galactose

• maltose

• fructose

Caloric value of carbs

Important energy source

3.75Kcal/g

Digestibility of carbs

Available - digested and absorbed in small intestine, hydrolysed to monosaccharides by enzymes.

Partially available - lactose, depends on existence of digestive enzymes

Unavailable - dietary fibre

• insoluble fibre = cellulose, lignin

• soluble fibre = gums, fructans, pectins, pentosans

resistant starch

Sensory properties of carbs

Mono, di and oligo are sweet

Oligo sweetness decreases with increasing carbon number

Precursors for flavour and colour

• caramelisation

• maillard reaction

Monosaccharides occur as…

Pentoses (5 carbon)

• D-Xylose (wood sugar - corn, bran, cherry, peach)

• L-Arabinose (pectin sugar - beets, gum arabic)

• D-Ribose (nucleic acids, vitB2)

Hexoses (6 carbons)

• D-glucose (aldose sugar - most abundant monosaccharide)

• D-Fructose (ketose sugar - in fruits and honey

• D-Galactose (lactose in milk)

• D-Mannose

Glucose, galactose and mannose different due to…

position of OH (same chemical structure)

Disaccharides are…

2 monosaccharides joined by glycosidic bonds (condensation reaction - bond between carbonyl group and hydroxyl group)

Disaccharides - maltose

Glucose + glucose

a-1,4 glycosidic bonds

reducing sugar

Disaccharides - sucrose

Glucose + fructose

a-1,2 glycosidic bonds

non reducing (reducing ends of bond monosaccharides bonded)

Disaccharides - lactose

galactose + glucose

B -1,4 glycosidic bonds

reducing

Trisaccharides - maltotriose

3x glucose

a-1,4 glycosidic bonds

reducing

Trisaccharides - Raffinose

galactose + glucose + fructose

non reducing

a-1,2 and a-1,6 bonds

Polymers of monosaccharides…

polysaccharides (joined by glycosidic bonds)

• linear

• branched

• amorphous

• tasteless

• colourless

• viscous

Homopolysaccharides

cellulose

starch

glycogen

inulin

Homopolysaccharides - cellulose

Glucose (chains can H bond with each other to form fringed micelles)

B-1,4 glycosidic bonds

Cell wall - structure

indigestible

Homopolysaccharides - starch

Glucose - linked by a glycosidic bonds

different proportions of

• amylose - linear (a-1,4), helical structure

• amylopectin - branched (a-1,4, a-1,6 at branches every 25 glucose units)

Homopolysaccharides - glycogen

Branched chain polymer of glucose like amylopectin

a-1,4, a-1,6 at branches

branches every 12-18 glucose units

animal storage reserve for glucose and energy

Homopolysaccharides - inulin

Fructose polymer

B-1,2 bonds

unavailable carb - not digested

Heteropolysaccharides - mucilages

agar, alginates, carrageenan

gel forming

Heteropolysaccharides - Gums

Hexose + pentose + uronic acid

gum arabic

thickener, stabiliser, emulsifier

Heteropolysaccharides - pectins

a D-galacturonic acid (oxidation of D-galactose)

middle lamella of cell walls

Polysaccharide functionality (water interactions)

High affinity for water - form H bonds

Swell and hydrate

normally soluble (not cellulose)

Amylose and amylopectin arranged radially in alternating layers

Starch granule

Gelatinisation (occurs when…/results in…)

starch granules heated with water so swell as water is absorbed

Amylose leaches and creates viscous paste

Too much heat leads to implosion and complete dissolution

Gel may form when cooled

Gelation is…

process of forming a gel

• water trapped between polymers

Occurs upon cooling after gelatinisation

More amylose = stronger gel

Double helices and egg box model are…?

Models of gelation

Reactions of carbs

chemical

• oxidation (loss of electrons) reduction (gain of electrons)

• condensation

• dehydration

• isomerisation

non enzymatic browning

• caramelisation

• maillard

Reactions of carbs - oxidation

Aldehyde to aldonic acid

Uronic acids

used for blood glucose measurement

Reactions of carbs - reduction

Monosaccharide → sugar alcohols

sugar substitutes for diabetics

Reactions of carbs - condensation (loss of water)

2 molecules combine - polymerisation

Reactions of carbs - dehydration (condensation reaction)

intramolecular condensation - loss of water and bond formation within one molecule

• Furfural

• HMF

Reactions of carbs - isomerisation (starts with e)

Conversion of monosaccharides into other monosaccharides with same chemical formula

Enolisation

• 1,2 - enolisation (acid)

• 2,3 and 3,4 enolisation (alkali)

Non-enzymatic browning generate…

Brown colours and flavour compounds in food (caramel, toffee, nutty)

• caramelisation

• maillard reaction

Caramelisation is…

Pyrolysis/thermal degradation of sugar at high temp

Generates a-dicarbonyl which can react to give furfural and HMP (undesirable)

low pH = colour

neutral pH = colour and aroma

Maillard reaction

reducing sugar and amino acid → glycosylamine

polymerisation of molecules →colour change

generates a-dicarbonyl like caramelisation

Strecker degradation - maillard reaction

a-dicarbonyls react further with amino acids → strecker aldehydes + amino ketones

• strecker aldehydes are aroma active

• Amino ketones → pyrazines (roasted, nutty flavour)