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Raw Material Preparation (6. Dehydration (b. Water in food (Water…
Raw Material Preparation
1. Cleaning
a. Purpose of cleaning:
- Protect consumers (presence of foreign matter, pathogenic microorganisms)
- Improve economics of processing
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2. Sorting & Grading
Sorting
a. Purpose of sorting
- Aesthetic and marketing advantages
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- Better control over weight-filling operations
- Necessary for uniform heat transfer
b. Size sorting
Flat bed screen (sieve) - most widely used, single or multi-deck screens
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- Variable aperture screen - gentler hence suitable for larger, more delicate materials
Continuous diverging apertures - uses pairs of diverging or conveyor belts driven at varying speeds to move foods by rotation and present it to the smallest aperture
Step-wise diverging apertures - increases aperture size by adjusting the gap between each roller and an inclined conveyor belt
- Stationary or rotating/vibrating
c. Shape sorting
- Raw materials separated by shape
- Useful when raw materials contaminated with particles of similar size and weight
- Applicable: grains contaminated with other seeds
- Using disc/cylinders with specific indentation to pick up seeds of correct shape when raw materials are rotated and other shapes will remain in the feed
d. Weight sorting
- Most precise method - independent of product geometry
- Separated into various weight categories using spring-loaded or electronic weighing devices incorporated into conveying system
- Not suitable for small items due to relatively long process per unit
- Aspiration & flotation sorting based on density differences
Grading
- Classification on basis of overall quality, requires judgement on food acceptability
- More expensive than sorting due to higher costs of skilled operators
- Generally use trained manual operators to judge product quality and use charted standards or even plastic model
- Machine grading is feasible when food quality is associated to single physical property
3. Size Reduction
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c. Solid foods
In size reduction:
- Material is first stressed by mechanical actions
- Stress is absorbed internally by the material as strain energy
- Local strain energy > critical level, fracture occurs along weakest lines
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Fracture of food:
Compression force - Apply stress force over a period of time (Fracture friable, brittle and crystalline foods such as grains of wheat, roasted coffee beans)
Impact force - Short and compact (hard & highly brittle material such as sugar crystal, spices)
Shearing force - Forward & backward movement resulting in shearing force (Fibrous or softer food such as vegetables, fruits, bread or meat)
- Deformation - Result in permanent deformation of food if exceeds critical level
- Flow - If stress continues, food will flow once strain reaches a yield point
- Break - Fracture alone along weakness lines
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d. Liquid food
In size reduction:
- It is also known as emulsification and homogenisation
- A process of creating a stable emulsion by mixing two or more immiscible liquids so that one is dispersed in the form of very small droplets within the second phase
- Homogenization is often the most severe size reduction of liquid particles in the dispersed phase
- Result in change of functional properties, texture/sensory alteration and prevent phase separation
Types of Dispersions
Oil in water (O/W) - milk, ice cream
Water in oil (W/O) - margarine, butter
Influencing factors:
- Type and quantity of emulsifying agent
- The size of the globules in the dispersed phase
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- Viscosity of the continuous phase
- Density difference between dispersed and continuous phases
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e. Effects on food
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- Loss of nutritional compounds and oxidation of fatty acids due to exposure to surrounding environment
- Dispersed particle size can affect the colour
4. Separation
a. Purpose of separation
- Remove specific components e.g. flavor components, essential oil, undesirable microorganism
- 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
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c. Filtration
Purpose:
- Remove insoluble solids from feed slurry (suspension) by passing through a filter medium (porous material), resulting liquid is filtrate and retained solids are filter cake
- Clarify liquids by removing small amounts of solid particles (e.g. wines, beer, edible oil etc.) or to separate liquids from the solid part of a food (fruit juice)
- Separate components based on particle/molecular size
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Filtration equipment:
- Pressure filters (e.g. plate and frame filter press) - application of high pressure to maintain flow. As pressure reaches the pre-determined value, the plates are backwashed with water. The press is dismantled and the cake is removed, ready for to start the next cycle.
d. Centrifugation
Purpose:
- Use of centrifugal force to separate materials
- Separation of immiscible liquids
- Separation of insoluble solids from liquids
Equipment:
- Liquid-liquid centrifugal separators
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- 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
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Applications:
- Production of cream and/or skimmed milk from separation of milk. Fat content of skimmed milk can be reduced to <0.05%. Disk bowl centrifuges are commonly used.
- Fruit juice processing - removal of pulp in citrus juices or production of oils from citrus fruits. Tubular bowl centrifuges commonly used to clarify apple juice but increasing trends to use nozzle discharge centrifuge
e. Expression
Purpose:
- It involves application of physical force (force/pressure) that results in forced expulsion of fluids from semi-liquid materials
- Primary application includes extraction of plant components for subsequent processing or direct consumption
Applications:
- Oil extraction from oil seeds - suitable to produce high quality oil from seeds containing more than 25% oil, press cake (solid residue) may still contain about 10% oil. It may be further separated by solvent extraction technique.
- Juice extraction from fruit - use lower pressure and fewer pressings to avoid solids or phenolic compounds contamination. Phenolic compounds from fruit skins can cause bitterness and browning.
- De-watering to pre-treat semi-solid food prior to dehydration via heat - the energy required to remove water by mechanical means is lesser compared to dehydration using heat.
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Equipment:
- Batch press e.g. hydraulic ram press
f. Solvent Extraction
Purpose:
- Equilibrium separation between solid and solvent that leads to extraction of target components within solid
- Driving force for extraction is the concentration difference between solid and solvent
- Residual solvents in both the extracted phase and the remaining solid must be remove by some evaporation technique
Application:
- Manufacture of instant coffee, instant tea, sugar from sugar beet
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5.Evaporation
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c. Basic components
- Source of steam & heat exchanger to transfer heat from steam to food
- Separator to separate vapour produced
- Condenser to condense vapour removed from evaporator
- Vacuum pump to create a vacuum
d. Equipments
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- Heated in a steam jacketed spherical vessel
- Open to atmosphere or connected to a condenser and vacuum
- Small heat-transfer area per unit volume
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- Low heat-transfer coefficients
- Suitable for low viscosity food
- Tubes (10-15m long) in contact with steam
- Liquid rises inside tubes by vapour formed near the bottom of the heating tubes; and a thin liquid film quickly moves upward
- At least 14 degreeC temperature difference across the tube to obtain a well-developed film
- High convective heat-transfer coefficients
- Residence time ~ 3 - 4 min
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e. Effects on food
- Loss of volatile and heat sensitive components e.g. aroma compounds
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- Reduction in water activity
- Darkening of colour e.g. maillard reaction
- Increase shelf life e.g. reduction of microbial counts
- Some volatiles may be recovered or retained
- Lower product yield if foaming occurs
f. Application
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- Vegetable juice or purees
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6. Dehydration
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b. Water in food
- Bound water: Water molecules are tightly bound to the food matrix. Properties of bound water different from bulk water
Moisture content:
- Wet-based
- Dry-based
- Partly bound water: Water molecules binds less firmly than those of bound water, a transition from bound to free water
- Free water: Those mechanically trapped in the void spaces of food, having nearly all properties similar to those of bulk water
Water activity:
- A measure of the availability of water for microbial, enzymatic or chemical activity within a food system
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Water movements:
- Liquid movement by capillary forces
- Diffusion of liquids, caused by differences in the concentration of solutes in different region of the food
- Diffusion of liquids which are absorbed in layers at the surfaces of solid components of the food
- Water vapour diffusion in air spaces within the food caused by vapour pressure gradients
c. How it works
- A process of moisture removal due to simultaneous heat and mass transfer
- Heat transfer - moisture removed by evaporation, heat must be transferred to equalize the heat of vapourisation. Heating can be by convection, conduction and radiation
- Mass transfer - removal of moisture vapour from surface by moving air through convection. Diffusion of moisture from interior toward the surface
d. Stages of dehydration
- Initial unsteady condition
- Constant rate period: Vapourization at the product surface and free water with water activity of 1 is always available at the surface to vaporize. The rate is limited by surface evaporation.
- First falling rate period: rate is controlled by moisture diffusion toward the surface. Most of the water is still free water with water activity of 1. However, diffusion toward the surface is necessary for vaporization to occur. Slight increase in the temperature of the product.
- Second falling rate period: dehydration of bound water (held by multilayer absorption and capillary condensation) Vaporization occurs in the interior and water vapor has to diffuse to the surface. Heat of vaporization at this stage is higher than heat of vaporization of pure water. Water activity is less than 1
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g. Equipments
- Hot-air dryers: sun/solar dryer, kiln dryer, tray or cabinet dryer, tunnel dryer, belt or conveyer dryer, fluidised-bed dryer, spray dryer
Advantages: Most energy efficient, Environment friendly (no pollution)
Disadvantages: Weather dependent, low productivity (long processing time), large space requirement, product degradation by biochemical and microbiological reaction, insect infestation
- Extremely high surface contact area between food and hot air
- Very high rate of mass transfer
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- Mostly suited for low viscosity fluid materials
- Heated-surface dryers: drum dryer
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Disadvantages: damage to heat sensitive components, high capital cost
- Novel drying technologies: infrared dryer, microwave dryer, osmotic drying, freeze drying, supercritical fluid extration
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