FOSC201 - Food process operations

Unit operations

Raw material selection

Storage of raw material

Cleaning

Sorting

Grading

Peeling

Processors define their requirements in terms of raw material specifications

• size

• unit per kg

• level of maturity

• sugar content

Raw material selections

Storage of raw materials governed by

Temperature

Moisture/humidity

Atmospheric composition

Spoilage mechanisms (3)

Living organisms

Biochemical activity

Physical processes including damage

removal of contaminants essential for protection of process equipment and final consumer

• Wet procedures - liquid solvents (water and detergents)

• dry procedures - lower moisture content

• can use water for fruit and veg

• should take place at earliest opportunity possible

Examples…

Cleaning

• wet - spray washers, brush washers, drum, rod washers

• Dry - air classifiers, magnetic separators

Separation of foods into categories based on measurable physical properties

Sorting

• size

• colour

• dimension

Shape and size sorting done by two methods

Flatbed screen

Drum screen

Assessment of overall quality of food using several attributes

Grading

Peeling is used to

remove unwanted or inedible material and improve appearance of final product

• flash steam

• Knife peeling

• Abrasion peeling

• Caustic peeling

• Flame peeling

Blanching is a .. heat treatment

mild

Blanching process involves

heating food rapidly to a predetermined temp, holding at temp for a specified time then cooling rapidly

Purpose of blanching is to

Inactivate enzymes

remove gas from plant tissues

• oxygen for oxidation

• helps achieve vacuum in canning

Shrinking and softening of tissue

Final cleaning and decontamination process

Significant reduction in microorganism content

Consequences of blanching

Overprocessing leads to excessive loss of texture

Weight loss from the tissue

Operation in which average size of food is reduced by grinding, compression, impact forces (shearing, tension, compression)

Size reduction

When size reduction applied to liquids it is called

Homogenisation or emulsification

benefits of size reduction to food processing

Surface area to volume ration increased - increases rate of drying, heating, cooling

Allows more complete mixing of ingredients

Control textural particle size

Control rheological properties of food (how materials deform under applied forces)

Negative impacts of size reduction on food quality

Increase in surface area to volume ratio of food means higher contact with oxygen so increased oxidation

3 types of forces used for size reduction of solids

Compression

Impact

Shearing

• in most equipments all three are used together

Stress strain diagram for foods

Internal strains are first absorbed up to elastci stress limit E

After E food permanently deformed

B is the breaking point

Size reduction of solids - amount of energy needed to fracture food depends on

Hardness of food

Moisture content

Heat sensitivity

Selection of equipment for size reduction depends on whether a food is

fibrous

smaller pieces or pulps

Dry paticulates or powders

Size reduction of fibrous foods (4)

Slicing - rotating blades

Dicing equipment - two sets of rotating knives

Shredding equipment - hammer mill with knives

Pulping equipment - compression and shearing forces for juice extraction

Size reduction of dry foods (4)

Ball mills - steel balls used to produce fine powder

Disc mills - single or double grooved disc rotating

Hammer mills - swinging hammer inside chamber

Roller mills - two or more rollers in different direction and speed

Effect of size reduction on solid foods

control textural and rheological properties of foods

Improves efficiency of mixing and heat transfer

Effect of size reduction on sensory characteristics of solid foods

Higher oxidation

• colour chnages

• off flavours

• textural changes

Effect of size reduction on nutritional characteristics of solid foods

loss of nutritional value due to oxidation of fatty acids and carotenes

Loss of vit C in chopped and sliced fruits and veg

Size reduction in liquid foods (2 types)

Emulsification

Homogenisation

Emulsification is the formation of

a stable emulsion by intimate mixing of two or more immiscible liquids so that one (dispersed phase) is formed into very small droplets within the second (continuous phase)

Homogenisation is the

reduction in size and hence increase in number of solid or liquid particles in the dispersed phase by application of intense shearing forces

Stability of an emulsion is dependent on

type and quantity of emulsifying agent

Size of the globules in the dispersed phase

interfacial forces acting on surface of globules

viscosity of continuous phase

difference in density of dispersed and continuous phase

4 equipments for homgenising

High speed mixers

Pressure homogenisers

Colloid mills

Ultrasonic homogenisers

equipment to premix emulsions of low viscosity liquids by a shearing action

high speed mixers

Equipment to make droplets even smaller

pressure homogenisers

Equipment used for high viscosity fluids

Colloid mills

Equipment that causes alternate cycles of compression and tension in low viscosity liquids and cavitation of air bubbles to form emulsions with droplet sizes

Ultrasonic homogenisers

What effects viscosity or temperature of liquid foods

type of emulsifying agent

Stabiliser

homgenisations conditions

in milk fat globule size effects cremier texture

Size reduction foods effects

Colour - milk more fat globules = greater light reflectance

aroma

nutritional value

shelf life

Dehydration/drying is the

application of heat under controlled conditions to remove majority of water normally present in a food by evaporation

main purpose of drying

Extend shelf life by reduction in water activity

• inhibits microbial growth and enzyme activity

Reduction in weight and bulk of food reduces

transport, storage and packaging costs

Negatives of drying

Highly energy intensive

Causes deterioration of eating quality and nutritional value

Examples of dried foods

Coffee, milk, raisins, pasta, flours, breakfast cereals

Examples of dried ingredients

egg powder, protein powder, flavourings, colourings, yeasts, lactose

Mechanism of drying

• hot air blown over wet food, water vapour diffuses through boundary film of air surrounding the food and carried away by moving air

• Vapour pressure gradient created as lower water vapour pressure on surface of the food due to hot air

• Gradient provides driving force for water removal from the food

3 stages of drying

Settling down period

• when drying first starts and surface is heating up

Constant rate period

• drying starts

• water moves from interior to surface at same rate as evaporation

• until critical moisture content reached

• rate of drying declines gradually

Falling rate period

• movement of water from interior to surface falls below evaporation rate

• longest part of drying

• most bound moisture removed

3 characteristics of air necessary for successful drying

moderately high dry bulb temp

low RH

high air velocity

2 equipments for drying

Hot-air driers

heated surface/contact driers

Spray driers

A fine dispersion of preconcentrated food is first atomised to form smaller droplets and then sprayed into a co or counter current flow of heated air

Effects of conventional drying on foods

Shrinkage, puffing, crystallisation

loss of fat soluble vitamins

reduction of ascorbic acid and carotenes from heat

decrease in volatile and phenolic compounds

Denaturation of proteins

production of chemical contaminants

Method used to dehydrate heat sensitive foods, process of which is based on sublimation (ice to vapour)

freeze drying

What method of drying produces the highest quality product obtainable?

Freeze drying

4 components of a freeze dryer

drying chamber

vacuum pump

heat source

a condenser

3 stages of freeze drying

initial freezing - crucial for size shape and distribution of ice crystals and hence the final structure of freeze dried foods

primary drying

secondary drying

Advantages of freeze drying

porous non- shrunken structure that can be rehydrated

Preserves sensory and some structural characteristics

Disadvantages of freeze drying

expensive

High energy consumption

Transformation of feed from a fluid state to a dried form by spraying feed into a hot drying medium

Spray drying

Spray drying parts and process

Feed is pumped to atomizing device (surface area increased)

Drying air drawn from atmosphere and filtered and heated - injected from top or bottom of sprayer

Droplets meet hot air and evaporation takes place

dried product falls to bottom of chamber

Air is passed through cyclones to remove fines (small particles)

Advantages of spray drying

Broad application and capacity

powder quality remains constant

Operation is continuous (can be done for days without stopping)

Variety of spray dryer designs

Basic summary of spray drying

Concentration

Atomization

Droplet-air contact

Droplet drying

Separation

Concentration stage of spray drying

increases the solid content and reduces amount of liquid needing to be evaporated

Atomization stage of spray drying

creates optimum conditions for spray drying to occur (increases surface area to volume ratio) - leads to a product having desired characteristics

Different nozzles

• Pressure nozzle

• Rotary nozzle (spraying and rotating)

Droplet-air contact stage of spray drying

atomized liquid brought into contact with hot air

• evaporation of 95% of liquid occurs in a few seconds

• way spray contacts air will influence behavior of droplets and dried product - position of hot air inlet

Droplet drying stage of spray drying (2 stages)

Stage 1: sufficient moisture in drop to replace liquid evaporated at surface - constant rate

Stage 2: no longer enough moisture to maintain saturated conditions - dried shell forms on surface - rate falls rapidly

Separation stage of spray drying

Particles fall to bottom of chamber

Some particles remain in air and are removed using cyclones and other separation equipment

Co-current flow spray dryer

Used for??

Product and drying air introduced in the chamber in same place

• for heat sensitive products because hottest air contacts droplets at highest moisture content

Counter current flow air dryer

Material and the air are injected at opposite locations of the drying chamber

• not suitable for heat sensitive products

Mixed flow spray dryers

combo of co current and counter current spray drying

• hot air introduced at top and material from the bottom

• lower drying rate and denser product

Spray dryers can be…and… cycle layout

open

• air put into environment after use

closed

• filters applied after feed is dried and extracted

• air put back into system

Low temperature does not destroy microorganisms or enzymes but

depresses their activity

Low temp is not a method of

permanent preservation

It cannot improve the initial quality of a product

2 low temp processes are

chilling and freezing

Freezing means a lower temp but also

depression of water activity due to water being in solid state

Arrhenius equation

describes relationship between temperature and rate of chemical reactions (higher temp = faster rate of reaction)

Low temp and enzyme activity

enzyme activity slowed but not eliminated by refrigeration

Low temp and microorganisms

psychrophiles are of biggest concern as better growth between -10-15 degrees

At low temp lag phase is longer and logarithmic phase is longer (growth is slower)

Low temp and biological activity (respiration)

control the rate of respiration

Greater preservation effects when chilling used in conjunction with

other unit operations (control of gas composition)

• reduce O2 when chilling fruit to reduce respiration

Controlled atmosphere conditions

modification of normal gas composition to control respiration rate

CAS - controlled atmosphere storage

MAS - modified atmosphere storage

MAP - modified atmosphere packaging

CAS monitors and regulates

O2, CO2 and ethylene

MAS - gas composition is

allowed to change by normal respiratory activity of the food

MAP - gas composition of packaging is

altered after food put in and before packaging sealed

Two methods of chilling (equipment)

Mechanical refrigerators

• compressor sucks gas, compresses then converts to high temp liquid which is cooled by condenser which then passed back to the refrigerator

Cryogenic systems

• cyrogen changes phase by absorbing latent heat to cool food

• solid or liquid CO2 and liquid nitrogen

Chilling causes ….in nutritional properties of food

little or no reduction

Freezing is

one of the most beneficial preservation methods

• lowers aw

Freezing curve A-B

Food cooled below freezing point - supercooling

Freezing curve B

nucleation or ice crystal formation

Freezing curve B-C

Temperature rises as ice crystals being to form and latent heat of crystallisation released

Freezing curve C-D

Ice crystallisation

• solutes become concentrated so temp must decrease

Freezing curve D-E-F

crystallisation of water and solute continues

• all water that is bound becomes ice

Freezing equipments methods

mechanical refrigerators

cryogenic systems

Two types of freezing (rates)

Slow freezing

• ice crystals grow in intercellular space and deform/rupture cell walls

• Ice crystals have lower water vapour pressure so gradient created and water moves from cells and cells become dehydrated

Fast freezing

• smallers crystals both within cells and in intercellular space

• little physical damage to cells as no water vapour gradient created

Biochemical changes in food from freezing

Degradation of pigments

loss of vitamins

residual enzyme activity

oxidation of lipids