Food Class test

what is a starch?

an energy source which is used across a range of moisture contents

starch benefits/properties in food processing

• provides interesting textures at a range of moisture contents

• can be stored dry

• is a starting material for other chemicals

• can be modified to give an even greater range of properties

• common storage carb in plants

where can starch be found naturally

• seeds

• roots

• tubers

• stems

native starch is always…

• amylopectin (waxy starch)

• amylose

high amylose starch benefits

structure functions health benefits - helices can encapsulate small enough molecules for targeted delivery and can form starch-lipid complexes which can impact quality in baking

starch macromolecules organization

semi crystalline

• amorphous amylose region

• crystalline amylopectin region

importance of water for starch

• starch is biosynthesized in an aqueous enviro.

• drying starch can cause shrinkage and cracks

• most water goes to amorphous phase or to surface of crystallites

• water absorption is reversible

starch gelatinisation

disruption of molecular orderliness within the starch granule due to water absorption

water in - granule swells - loss of some polymer

starch pasting

continuing starch transformations at temperatures exceeding gelatinisation temperature

conditions required for starch gelatinisation and pasting

• native starch granules in excess water

• temperatures above cooperative disassociation of the crystallites

heating starches in limited water

loss of helical order and crystalline state

starch conversion

wheat flour components

• protein (gluten) - forms support for final structure

• carbohydrate (starch) - sets the loaf in the oven

• damaged starch (water holding)

• enzymes (water releasing)

• lipids (fats)

• cell wall material

different flours and their purposes

• stronger - premium white products

• higher extraction where colour is less important

• weaker for standard and value white and brown bread

• wholemeal - includes endosperm, bran and germ

gluten purposes

• supplements a weak flour - improves proof height

• supports recipes that contain ingredients that damage the gluten or bubble structure e.g. vinegar, fruit, milk

• enables excess expansion

• for long fermentation processes to support dough slackened by amylases and yeast

• requires extra water and work input

water purpose in bread making

• essential for dough formation

• ‘optimum’ level is essential for good dough development, dividing, texture and colour, slice strength

yeast role in bread

• eat sugar and produce gas creating bubbles

• killed by too much salt, sugar, vinegar, preservatives, flour and heat

• need to be stored cold because they become active in warm storage

salt role in bread

• enhances flavour

• slows down yeast activity

• strengthens gluten structure by improving proof stability and giving a finer crumb texture

• increases oven spring by increasing setting temperature and delaying gas release

• slows mould growth and staling

• gives brown crust colour

sugar role in bread

• feeds yeast so saves fermentation time

• increases dough setting temperature - can lead to underbaking

• often not included commercially as yeast and amylase create their own

what are polysaccharides

natural polymers formed of repeating subunits (10+)

generic uses of polysaccharides

• thickener

• mimic full fat food textures

• create food gels

• emulsifier

• control ice crystallisation

lipids uses

• flavour

• heat transfer

• sensory qualities

• emulsification

• staling inhibition

transglycerols (TAGs)

esters derived from glycerol and 3 fatty acids making up over 90% of dietary fats

unsaturated fatty acids

• straight or kinked structure

• structure affects packing thus melting the T of TAGs

factors affecting solid fat content (SFC)

• crystal morphology

• type of vegetable oil/fat

• temperature

factors affected by solid fat content

• rheology

• melting range

• heat capacity

• density

how to modulate solid fat content

• hydrogenation

• chemical interesterification

• fractionation

hydrogenation to modulate solid fat content

• improves oxidative stability by eliminating unsaturated fatty acids

• increases solids to make more functional and useful (i.e. hardening)

• easily controlled to give a wide range of products from liquid to 100% solid

• associated with soybean and rapeseed oils

• produces trans-fatty acids therefore not a preferred apporach (bad for heart health)

interesterification to modulate solid fat content

• rearranges fatty acids on the glycerol moiety to create new TAGs

• traditional chemical process produces a random distribution of fatty acids

• enzymic process can be more controlled

• associated with palm oil, palm kernel oil and other solid fats

• does not affect fatty acid composition so cannot produce trans fatty acids

fractionation to modulate solid fat content

fractional crystallisation

1. melt oil completely

2. cool under controlled conditions until crystal nuclei form

3. allow crystals to grow and agglomerate

4. separate crystals from liquid

• long process - up to 24 hours

• environmentally friendly

• does not change fatty acids from natural state

• no health issues known

• relatively cheap

• assoicated with palm oil, palm kernel oil, milk fats

proteins

natural polymers made up of 20 different amino acids linked by peptide bonds

amino acid structure

α-carbon atom linked to a hydrogen atom, an amino group, a carboxyl group, and a side chain

high quality protein meaning

proteins containing all essential amino acids at levels above FAO/WHO/UNU reference levels

primary protein structure

amino acids linearly linked by peptide bonds to form a chain

secondary protein structure

peptide chain twists arise from near-neighbour or short-range noncovalent interactions between amino acid side chains

main structures: α-helix, β-sheet

tertiary protein structure

folding pattern of peptide chains into a 3D form due to intrachain interactions

• relocation of most of the hydrophobic amino acid residues into the interior of the protein structure away from the water environment

• relocation of most of the hydrophilic amino acid residues, especially charged residues, to the protein surface

quaternary protein structure

protein containing more than one polypeptide chain

complete protein is an oligomer

stability of the protein structure

native protein structure is stabilised by

1. intramolecular interactions emanating from forces intrinsic to protein molecule e.g. van der waals

2. intramolecular interactions affected by surrounding solvent e.g. hydrogen bonding

3. intramolecular disulfide bonds

4. intermolecular repulsive interactions between protein molecules in the same solvent

protein denaturation

changes in environment such as pH, ionic strength, temperature and solvent composition affect forces that stabilise protein structure.

level of denaturation affects protein structure

types of protein

• globular

• fibrous

• flexible

globular proteins

• polypetide chains folded into tightly packed structures

• emulsifiers or foaming agents - non-polar patches at protein surface - adsoprtion to oil-water or air-water interfaces

• gelling agents - unfolding during heat exposes hydrophobic and sulfhydryl groups promoting bonds formation between proteins and trapping solvent

fibrous proteins

• rigid molecules

• contribute to textural attributes of foods

• mainly of animal origin

• plant e.g.: glutenin

flexible proteins

• fairly disordered structures: random coils

• mainly of animal origin

• important role in emulsification

solubility in proteins

• insoluble proteins have v limited uses

• functional properties of proteins are often affected by their solubility

• solubility of a protein is the thermodynamic manifestation of the equilibrium between protein-protein and protein-solvent interactions

solubility vs pH

• amino acids have different ionisation states depending on p\h of the solution due to their acidic carboxyl group and basic amino group

• solubility curve - electrostatic repulsion and hydration of charged residues promote solubilisation of the protein

• minimum solubility occurs at about the isoelectric pH of proteins; lack of electrostatic repulsion promotes aggregation and precipitation

solubility vs ionic strength at pH values well above or below the IEP

very low salt

• proteins repel each other due to strong neg. or pos. charge of proteins

low ionic strength

• ions neutralise charges at the surface of proteins and solubility decreases due to aggregation

high salt

• anionic counter ions preferentially attach to cationic patches on the protein surfaces which increase neg. charge on the proteins and thus electrostatic repulsion and solubility increases

solubility vs temperature

at constant pH and ionic strength, solubility increases with temp. between 0 and 40C

above 40C, protein denaturation takes place, causing a solubility decrease

protein emulsification process

1. proteins form protective layer at oil-water interface, lowering interfacial tension and providing a barrier to prevent accumulation of oil droplets

2. protein molecules migrate to oil-water interface and adsorb on the oil droplets, forming a viscoelastic film that prevents destabilization

3. adsorbed protein layer lowers interfacial tension and free energy, providing electrostatic repulsion and steric barrier to prevent oil droplet accumulation

this is essential for food products such as milk, salad dressings, ice cream, butter and cakes

foaming proteins

foaming agents such as egg whites, gelatins, casein, soy and gluten influence stability and texture of food products.

foaming properties of proteins are influenced by their ability to absorb at interface and reduce surface tension

protein gelation methods

• addition of salt or acid

• enzymatic reactions

• fermentation

• shear

• high pressure

• temperature change

heat induced gelation

• native protein structure denatures under heating

• protein aggregates

• network may undergo restructuring and reinforcement upon cooling

shear stress and shear rate

types of liquid foods rates

• newtonian

• shear-thickening

• shear-thinning

viscosity

constant of proportionality between shear stress and shear stress and shear rate

temperature dependent

non-newtonian liquid foods

shear thinning

viscosity decreases with increasing shear rate

how does shear thinning occur in protein microstructures

• polymer chains disentangle and stretch

• emulsion droplets reorganise and deform

• elongated particles align with flow

• aggregated structures break down to primary particles

shear thickening

viscosity increases with increasing shear rat

yield behaviour role in foods

affects texture and ‘thickness’, important when considering flow of coatings, balance of emulsions, texture of sauces

yield stress

minimum shear stress that needs to be overcome before onset of shear flow

most simple yield stress behaviour is bingham plastic liquid

viscoelastic behaviour

• tack and stringiness

• stirring process - rod climbing effect - poor mixing result

• extrusion - extrudate swelling

shear modulus

measure of a materials ability to resist deformation when subjected to shear stress

viscoelastic moduli

oscillatory shear testing

• characterizes viscoelastic properties of fluids and soft materials

• applies sinusoidal deformation to a material and measuring its mechanical response over time

small amplitude oscillatory shear tests

• probes linear viscoelastic properties of materials

• applied strain is small enough that response remains linear, allowing for determination of storage (elastic portion) and loss (viscous portion) moduli

• commonly used for characterizing polymers, emulsions and other complex fluids

large amplitude oscillatory shear tests

• investigates nonlinear viscoelastic behaviour of materials

• as strain amplitude increases, material response becomes nonlinear, which is essential for understanding how materials behave in real-world applications where they may expereince large deformations

• useful in quality control for rubber and other materials, can reveal interactions between fillers and polymers

temperature dependence of shear

follows arrhenius type relationship

types of emulsions

• oil in water - e.g. mayo

• water in oil - e.g. butter

• water in oil in water - reduced fat and can deliver water soluble actives

emulsifier structure

• prevents coallescence

• amiphilic

emulsion microstructure size

10^-9 —> 10^-6 m colloidal systems

emulsion instability

most stable state is when oil and water are in separate layers so droplets in emulsion can coallesce to form larger droplets

why add surfactants to emulsions?

• reduces interfacial tension which facilitates droplet break-up in top-down processes

• rapid in process adsorption at interface stabilises small droplets which in turn benefits gravitational stabilisation

measuring surface/interfacial tension

• force tensiometry - Du Nouy ring and Wilhelmy plate methods

• optical methods - pendant drop technique

• capillary rise method

• maximum bubble pressure method

relationship between surface tension vs surfactant concentration

as surfactant conc. increases, surface tension decreases

de nuoy ring method

pull ring through liquid surface to cause a non-equilibrium state that affects measured surface tension

preferred for high-viscosity liquids

wilhelmy plate method

stationary

measures force required to detach a plate from the liquid surface

preferred for measuring surface tension variations over time

pendant drop technique

drop of liquid suspended from needle and image of droplet is captured using camera

software analyzes image to determine shape of droplet, which is used to calculate surface tension using young-laplace equation

high precision, so useful for development of coatings and paints

common food surfactans

• mono and diglycerides of fatty acids

• lecithin

• polysorbates

• citric acid esters

these may also be applied to process and stabilise foams or control particle aggregation and flow bheaviour

surfactant addition in bread

can enhance volume, soften crumb structure, increase shelf-life

different surfactants can strengthen or soften dough

emulsifiers in ice cream

provide smoother texture and ensure ice-cream does not melt rapidly after serving

improve freeze-thaw stability

emulsifiers in margarine

provide required stability, texture and taste

ensure water droplets are finely dispersed in oil phase and prevent splattering

polymeric emulsifiers use

better foam stabilisers than small molecular surfactants as they give stronger films

food emulsifier selection

hydrophilic lipophilic balance

• predicts behaviour of emulsifiers related to their solubility

• HLB=1-20 scale from hydrophobic to hydrophilic

bancroft rule

• hydrophobic emulsifiers stabilise water-in-oil emulsions

• hydrophilic emulsifiers stabilise oil-in-water emulsions

particulate emulsifiers

solid particles such as starch, chitin, cellulose that stabilize emulsions

offer enhanced resistance to coalescence, high stability, good repeatability, simple scalability, enhanced biocompatibility

advantages of using food powders

ease of use, processing, transport

maintain quality and stability

states of food powders

• crystalline

• amorphous

crystalline powders

non hygroscopic, stable, easy flowing e.g. salts, sugars, organic acids

amorphous powders

obtained by rapid supercooling or rapid removal of solvent

particle morphology affects/effects

affected by raw materials and process conditions

impact

• bulk density

• flowability

• ease of handling

• mixing

• compressibility

• surface activity

particle processes for liquids

• spray

• freeze

• drum

• belt

• crystallisation

particle processes for solids

• size reduction

• granulation

• mixing

water activity role in stability

water activity quantifies amount of free/bound water in a compound, so signifies availability of water for microbial processes i.e. spoilage

microorganisms require water to grow, and each has a minimum water activity threshold below which it cannot multiply

no microbial growth possible below 0.6 water activity

drying processes

• freeze

• spray

• roller and drum

• fluidised bed

roller and drum drying process

• liquid applied in thin layer to rotating steam heated drum

• dried film scraped off after ¾ of a complete rotation

• economical for small volumes

• applied to milk, oats, whey, corn and instant mash processes

freeze drying process

dehydration by sublimation of a frozen product

• primary drying - rapid sublimation occurs by controlling vacuum level in dryer and careful heat input - heat supplied by combination of conduction and radiation - ice leaves as porous dry cake

• secondary drying - warmer temp. to remove bound water, slower drying rate, moisture loss occurs only by diffusion

slow freezing leads to formation of large ice crystals while rapid freezing promotes formation of small ice crystals

cake resistance affects product temperature and speed of primary drying

main components of freeze dryer

• product chamber

• refrigeration system

• condenser

• vacuum system

• control system

pros and cons of freeze drying

• can yield high quality products

• high operational cost and long processing time restrict its usage just to high-value products

spray drying process

preparation of dry stable additives, instant food powders, functional ingredients and flavours

co-current, counter-current and mixed mode types

• convective heat and mass transfer induced by movement of air

• high pressure nozzles or centrifugal atomizers used

• rapid drying due to small size of droplets

• a range of particle morphologies can be obtained

co-current spray drying

• food droplets at room temp, air at high temp

• water removed from droplets provides an evaporative cooling effect

• when product reaches its direst state, the air has also cooled

powder separation methods

cyclone - centrifugal force causes particles to segregate from air. air flows back out the top, while particles are removed from bottom

textile filter - powdered air passes through fabric filter before being exhausted into atmosphere. fine particles are trapped by filter