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Week 7 & 8: Preparation of food materials for processing (Size…
Week 7 & 8: Preparation of food materials for processing
Size reduction :check:
6 Purposes
Increase surface-area-to-volume ratio of foods thus increase rate of heat transfer (drying, heating, chilling, freezing etc.) , improves efficiency and increase rate of soluble components extraction
Reduced particle size to enable more complete/uniform mixing of ingredients (e.g. salad)
Pre-determined range of particle size increase screening process (e.g. sieving) efficiency
Similar size of dispersed liquids increase product stability (e.g. emulsion)
Smaller particle size enable easier packaging
Increase product range
2 Problems
Potential increase in nutrient loss due to increase surface area
Promote degradation e.g. enzymatic deterioration, oxidation, off flavour, microbial deterioration
In Solid foods
Principle
3 types of forces, all are generated but usually only one force predominates
Impact Force ~ Short and compact
Application: Wide food variety including esp. hard and highly brittle material such as sugar crystal, spices, roast nuts
Compressive Force ~ Apply stress/force over period of time
Application: Fracture friable, brittle and crystalline foods such as grains of wheat, roasted coffee beans
Shear Force ~ Forward and backward movement resulting shearing force
Application: Fibrous or softer foods such as vegetable, fruits, bread or meat
3 Factors Affecting Selection of Size Reduction Equipment
Mechanical Properties of Feed
Crystalline & friable materials usually fracture readily along cleavage planes. Suitable for usage of roller mills
Fibrous materials relieve stress at ends of cracks so increase toughness. Suitable for usage of Disk mills, pin-disk mills or slicing device
Moisture content of Feed
If feed too moist, affect free-flowing characteristic
If feed too dry, change the manner of breaking down and dust formation
Temperature Sensitivity of Feed
Under high speed milling process, heat may be generated and increase the feed temp. resulting in loss of heat sensitive components such as flavour or aroma compounds
May be resolved by using cooling jackets
In Liquid foods: creating a stable emulsion by the intimate mixing of two of more immiscible liquids so that one (dispersed phase) is dispersed in the form of very small droplets (often sub-micro meter) within the second (continuous) phase
2 Types of dispersions
Oil-in-water (O/W): milk, ice cream
Water-in-oil (W/O): margarine, butter.
5 Influencing Factors
The type and quantity of emulsifying agent (e.g. lecithin)
The size of the globules in the dispersed phase
The interfacial tension
The viscosity of the continuous phase (e.g. usage of stabilizer such as polysaccharides and hydrocolloids)
The density difference between dispersed & continuous phases
Four main types of equipment:
Pressure homogenizers
Colloid mills
Ultrasound homogenizers
High speed mixers
Effects on Foods
Losses of nutritional compounds and oxidation of fatty acids due to exposure to surrounding environment
Dispersed particle size can affect the colour
Affect product texture
Liquid foods ~ reducing dispersed droplets size affect viscosity and mouth feeling
Solid foods ~ Increase smoothness and quick release of hydrolytic enzymes
Separation :check:
4 Purposes
Remove specific components [e.g. flavour components, essential oil, undesirable microorganism (m/o)]
Form of cleaning, sorting and grading
Retrieval of high value compounds (e.g. papain from papaya, rennet from calf stomach)
Purification of fractions (e.g. vegetable oils)
4 Main Techniques: separation of large food units ranging from few centimetres to separation of molecules or ions ranging in nanometres
Filtration
Filtration
Examples of filtration
Factors affecting Rate
Equipment
Centrifugation
Centrifugation
Separation of immiscible liquids
Liquids with different densities are introduced into cylindrical bowl rotating about a vertical axis. More dense liquid will move toward the wall while less dense liquid is displaced toward the centre of bowl under the influence of centrifugal force.
Liquid-Liquid Centrifugal Separators:
Tubular bowl centrifuge
Disk bowl centrifuge
Separation of insoluble solids from liquids
Feed containing insoluble solids particles is introduced into cylindrical bowl rotating about a vertical axis. Solid particles will move toward the wall. If particular solid particle reached wall before being swept out by liquid leaving the system, it will settle in the bowl and be separated from the liquid. If particle did not reach wall, it will be carried away by the liquid and removed as “unclarified solid”
Solid-Liquid Centrifugal Separators:
Solid bowl centrifuge (clarifier) ~ up to 2% solids
Nozzle-discharge centrifuge ~ up to 25% solids
Decanting centrifuge ~ up to 90% relatively large solids
Expression: Separation operation that involves application of physical force (force/pressure) that results in forced expulsion of fluids from semi-liquid materials
Factors Influencing Extraction Yield
Maturity and growth conditions of raw materials
Extent of disruption of cell structure
Resistance of solids to mechanical deformation
Viscosity of expressed liquid
Rate of increase in pressure
Time of pressing
Maximum pressure applied
Thickness of the pressed solids
Temperature of solid and liquid
Solvent Extraction
Processes
Mechanical/physical Separations:
Results separation of product components due to different reaction of components to forces applied
Based on Involves application of physical factors (e.g. force/pressure) to the fluid of concern
Examples
Equilibrium/Diffusion Separations:
Bring two phases together into contact for certain period of time and cause the composition of the two phases to change according to equilibrium principles
Based on: equilibrium of mixtures of different components in the solid, liquid or gas states
Examples
4 Factors Influencing Rate of Separation
Density between the liquids
Distance from centre of rotation
Speed of rotation
Diameter of particles in higher density liquid
Grading: Assessment of overall quality using several attributes or classification based on quality. Grading usually result in product upgrades :check:
More expensive than sorting due to higher costs of skilled operators
Generally use trained MANUAL operator to judge product quality and may use charted standards or even plastic models
Machine grading is feasible when food quality is associated to single physical property
Promising development of rapid and non-destructive methods for quality assessment e.g. x-rays, lasers, infrared rays
But can be costly
Raw Material Cleaning: Unit operation whereby contaminating materials are removed/separated from foods
3 purposes
Improve economics of processing
Protect consumers
Reduce food wastage
2 types
Wet Cleaning
4 Methods
Soaking: Usually preliminary stage to clean heavily contaminated materials such as root crops with large tanks(metallic or concrete) fitted with stirrer
To improve efficiency
Moving water relative to product
Moving product relative to water
Soaking in warm water (esp. increase efficiency of removing mineral oil), detergent or sanitizers
Sparging air through system for delicate produce (e.g. strawberries) or materials that trap dirt internally (e.g. celery)
Spray Washing
Factors on efficiency
Volume of water used
Water temperature
Distance of food from sprays
Exposure time of food to sprays
No. of spray jets used
Water pressure employed
2 main types
Spray drum washers: Water sprayed from centre of drum onto dirty raw materials entering the drum from end under rotation. Cleaned materials exit from the other end
2 factors to control
Speed of rotation
Angle of inclination
Spray belt washers: Raw materials moves along conveyors where water is sprayed from top
2 Variations of conveyor belts:
Roller conveyors for spherical foods
Vibratory conveyors for smaller foods
Flotation Washing: Depends on buoyancy difference between desired and undesired parts of foods to be cleaned
Applications: removal of stones, dirt and plant debris from peas, beans, dried fruits
Ultrasonic Cleaning: Use of sound waves of frequency between 20-100 kHz.
Release energy that cause violent agitation of particles and hence loosen contaminants adhered to raw material
Dirt in eggs, grease/wax on fruits
Advantages
Effective removal of firmly adhered soils
Dustless
Result in less damage to foods than dry cleaning
Allow usage of detergents and sanitizers (chlorine/citric acid may also reduced enzymatic browning)
Disadvantages
Use of large amounts of clean water
Wet surface prone to spoilage
High effluent disposal charges [high in biological oxidation demand (BOD) and chemical oxidation demand (COD)]
THEN needs Dewatering: remove excess water after washing
4 Equipments used
Vibrating screen
Dewatering reels
Dewatering centrifuges
Drying equipment
Dry Cleaning
Disadvantages
Possibility of health and explosive hazard from dust
Additional cost to control dust
Possibility of product recontamination due to dust
Less effective in cleaning efficiency
Advantages
Does not add extra moisture to raw materials
Equipment generally smaller and cheaper
No effluent disposal
Lower chemical and microbial deterioration
4 Physical methods
Abrasion: through abrasion between food unit and moving parts of cleaning machinery to loosen and remove adhering contaminants
Aspiration cleaners: a moving stream of air to separate contaminants based on their different densities
Not suitable for oxidative sensitive materials and require good dust control
Application: harvesting machines to separate stones and husks from grains, using air streams to simply blow loose contaminants from eggs/fruits
Screening: separation of materials into 2 or more size fractions using discontinuous screen or continuous screens
Drum Screen
Advantage ~ High capacity, relatively inexpensive
Disadvantage ~ Difficult to clean, possibility of recontamination
Flat bed Screen
Advantage ~ Excellent for fine materials, easily accessible for frequent cleaning
Disadvantages ~ Possibility of recontamination, damage to sensitive foods
Magnets/Electromagnets
Can be used in combination
5 Expectations
High speed efficiency
Complete removal of contaminants
Cleaned surface must be in acceptable conditions
Minimal product damage
Minimal effluent disposal
Sorting: Separation of raw material based on single measurable property (e.g. weight, size, shape, density, photometric)
:check:
4 purposes
Aesthetic and marketing advantages e.g. uniform size or colour
Ease of process control (e.g. dehydration, sterilisation, peeling, pitting)
Better control over weight-filling operations
Necessary for uniform heat transfer
3 Main types
Weight Sorting: Separated into various weight categories using spring-loaded or electronic weighing devices incorporated into conveying system
Not suitable for small items (e.g. cereals or legumes) due to relatively long process per unit
Shape Sorting: Useful when raw materials contaminated with particles of similar size and weight
Using disc/cylinders with specific indentation to pick up seeds of correct shape when the raw materials are rotated and other shapes will remain in the feed
Size Sorting: based on physical parameters
Cheap but less precise as compared to weight sorting
Essential for further heat application or heat removal processes as it will affect the rate of heat transfer
Using 3 main categories of screens
Fixed Aperture Screen (Can be)
2 types
drum screen (rotary screen)
flat bed screen (sieve)
Rate of sorting dependent on:
Shape and size distribution of product
Capacity and nature of sieve
Amplitude and frequency of shaking/vibration
Effectiveness to prevent blockage of sieves
Difficulties associated with size sorting
Excessive moisture/humidity results in lumping/caking to form larger particles and discharged as oversize
Blinding esp. when particles size are similar to screen aperture
High feed rate that results in overloading of screens and small particles being discharged with oversized particles
Variable Aperture
2 types
Continuous diverging apertures: uses pairs of diverging rollers or conveyor belts driven at varying speeds to move foods by rotation and present it to the smallest aperture
Step-wise diverging apertures: increases the apertures size by adjusting the gap between the each roller and an inclined conveyor belt
Stationary or rotating/vibrating
